Relative contributions of cytochrome CYP3A4 versus CYP3A5 for CYP3A-cleared drugs assessed in vitro using a CYP3A4-selective inactivator (CYP3cide)

Intracellular Penetration of Atazanavir, Ritonavir and Dolutegravir With Concomitant Rifampicin: A Dose Escalation Study

Ritonavir-boosted atazanavir is a victim of drug-drug interaction with rifampicin, a key component of antitubercular treatment. In a recent dose escalation clinical trial, we showed that increasing atazanavir/ritonavir to 300/100 mg b.i.d. compensates for reduced drug exposure in plasma due to rifampicin, but the intracellular effects remained unexplored. This sub-study investigated the intracellular penetration of atazanavir/ritonavir and dolutegravir into peripheral blood mononuclear cells (PBMC). Twenty-six healthy volunteers living with HIV, virologically suppressed, and taking atazanavir/ritonavir containing regimens were enrolled. The trial consisted of four sequential periods: PK1, participants were on atazanavir/ritonavir 300/100 mg q.d.; at PK2, rifampicin 600 mg q.d. and dolutegravir 50 mg b.i.d. were added (2 weeks); at PK3, atazanavir/ritonavir dose was increased to 300/100 mg b.i.d. (1 week); at PK4, rifampicin dose was doubled (1 week). Atazanavir, ritonavir, and dolutegravir were quantified in plasma and PBMC using LC-MS/MS methods to evaluate steady-state concentrations at the end of each period. Atazanavir/ritonavir dose escalation successfully restored intracellular concentrations comparable to those observed without rifampicin, with a geometric mean ratio of 0.99 (CI90 0.72-1.41) for atazanavir at PK3 compared with PK1. The intracellular concentration of dolutegravir increased significantly with atazanavir/ritonavir dose escalation, similar to plasma. Finally, further, increasing the rifampicin dose did not show an additional impact on atazanavir/ritonavir concentrations in PBMC. The study confirms that increasing the ATV/r dose can be an effective strategy for compensating rifampicin effects even at the intracellular level, supporting its use in clinical settings.

Decoding the selective chemical modulation of CYP3A4

Drug-drug interactions associate with concurrent uses of multiple medications. Cytochrome P450 (CYP) 3A4 metabolizes a large portion of marketed drugs. To maintain the efficacy of drugs metabolized by CYP3A4, pan-CYP3A inhibitors such as ritonavir are often co-administered. Although selective CYP3A4 inhibitors have greater therapeutic benefits as they avoid inhibiting unintended CYPs and undesirable clinical consequences, the high homology between CYP3A4 and CYP3A5 has hampered the development of such selective inhibitors. Here, we report a series of selective CYP3A4 inhibitors with scaffolds identified by high-throughput screening. Structural, functional, and computational analyses reveal that the differential C-terminal loop conformations and two distinct ligand binding surfaces disfavor the binding of selective CYP3A4 inhibitors to CYP3A5. Structure-guided design of compounds validates the model and yields analogs that are selective for CYP3A4 versus other major CYPs. These findings demonstrate the feasibility to selectively inhibit CYP3A4 and provide guidance for designing better CYP3A4 selective inhibitors.

In vitro-in vivo scaling of cytochrome P450-mediated metabolic clearance using a relative activity factor approach

Quantitative prediction of hepatic clearance is a key element in predicting the human pharmacokinetic profile in the nonclinical stages. In the present study, we focused on the major cytochrome P450 (P450) isoforms (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4) and tested a relative activity factor (RAF) method to quantitatively predict in vivo hepatic intrinsic clearance (CLh,int,vivo) and fraction metabolized (fm) by the P450 isoforms directly from an in vitro recombinant P450 system. We selected multiple probe substrates for CYP1A2 (caffeine, tizanidine, phenacetin), CYP2C9 ((S)-acenocoumarol, glimepiride, lornoxicam, tolbutamide, (S)-warfarin), CYP2C19 ((S)-lansoprazole, omeprazole, pantoprazole), CYP2D6 (desipramine, metoprolol, nebivolol, tolterodine), and CYP3A4 (alprazolam, felodipine, midazolam, nisoldipine, sildenafil, triazolam) to calculate the representative RAF value for each P450 isoform based on the in vivo-to-in vitro clearance ratio of the multiple probe substrates. The most pronounced substrate dependency of the RAF values was noted for CYP3A4 (2698 [alprazolam] to 19073 [nisoldipine] pmol P450/kg). Using the geometric mean of the RAF values for each isoform, a within 3-fold prediction of the CLh,int,vivo was obtained for all the 11 test drugs, except glibenclamide, which is a known substrate of hepatic uptake transporters. The fm values of the responsible P450 isoform(s) could be well predicted for mexiletine, tamsulosin, risperidone, celecoxib, and glibenclamide. This simple, practical RAF method can be one of the useful nonclinical methods to estimate the CLh,int,vivo and fm mediated by the major P450 isoforms, which would promote earlier understanding of the impact of genetic polymorphisms and drug-drug interactions on the human pharmacokinetics of the substrate compounds. SIGNIFICANCE STATEMENT: The relative activity factor method has been used for extrapolating in vitro clearance from recombinant systems to liver microsomes, but this study utilized this method to predict in vivo hepatic clearance and fraction metabolized values. By applying relative activity factor values obtained from multiple probe substrates, this study was able to quantitatively predict the in vivo clearances mediated by CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. This simple, practical method will help optimize metabolic clearances via the major cytochrome P450 isoforms in the nonclinical stages.

Brain Cytochrome P450: Navigating Neurological Health and Metabolic Regulation

Human cytochrome P450 (CYP) enzymes in the brain represent a crucial frontier in neuroscience, with far-reaching implications for drug detoxification, cellular metabolism, and the progression of neurodegenerative diseases. The brain's complex architecture, composed of interconnected cell types and receptors, drives unique neuronal signaling pathways, modulates enzyme functions, and leads to distinct CYP gene expression and regulation patterns compared to the liver. Despite their relatively low levels of expression, brain CYPs exert significant influence on drug responses, neurotoxin susceptibility, behavior, and neurological disease risk. These enzymes are essential for maintaining brain homeostasis, mediating cholesterol turnover, and synthesizing and metabolizing neurochemicals, neurosteroids, and neurotransmitters. Moreover, they are key participants in oxidative stress responses, neuroprotection, and the regulation of inflammation. In addition to their roles in metabolizing psychotropic drugs, substances of abuse, and endogenous compounds, brain CYPs impact drug efficacy, safety, and resistance, underscoring their importance beyond traditional drug metabolism. Their involvement in critical physiological processes also links them to neuroprotection, with significant implications for the onset and progression of neurodegenerative diseases. Understanding the roles of cerebral CYP enzymes is vital for advancing neuroprotective strategies, personalizing treatments for brain disorders, and developing CNS-targeting therapeutics. This review explores the emerging roles of CYP enzymes, particularly those within the CYP1-3 and CYP46 families, highlighting their functional diversity and the pathological consequences of their dysregulation on neurological health. It also examines the potential of cerebral CYP-based biomarkers to improve the diagnosis and treatment of neurodegenerative disorders, offering new avenues for therapeutic innovation.

Impact of variation in CYP3A and CYP2C8 on tucatinib metabolic clearance in human liver microsomes

Tucatinib is a small molecule tyrosine kinase inhibitor indicated for HER2-positive breast cancer. This recently approved drug is primarily metabolized by cytochrome P450 (P450) 2C8 and CYP3A. Given the interindividual variability in the pharmacokinetics of some kinase inhibitors, the present study explored how variability in CYP2C8 and CYP3A activities and concentrations can influence variability in overall tucatinib metabolic clearance in vitro. Tucatinib depletion, P450 activities, and P450 concentrations were measured in human liver microsomes from 21 donors (males n = 11, females n = 10). CYP2C8 and CYP3A activities were quantitated by liquid chromatography-tandem mass spectrometry analysis using the following marker reactions: amodiaquine N-deethylation and midazolam 1'-hydroxylation, respectively. CYP2C8, CYP3A4, and CYP3A5 protein concentrations were measured using quantitative targeted absolute proteomics. The minimum clearance rate was 2.01 μL/mg/min, and the maximum clearance rate was 28.9 μL/mg/min, indicating a 14.3-fold variation in the apparent tucatinib clearance between donors. Tucatinib clearance was significantly correlated with both CYP2C8 and CYP3A enzyme activities and protein concentrations in this donor cohort (r = 0.781, r = 0.904, r = 0.907, and r = 0.882, respectively). A multiple linear regression model was developed to determine the most significant parameters influencing tucatinib clearance. Overall, we found that CYP2C8 and CYP3A activities were significant predictors of tucatinib apparent clearance in human liver microsomes from individual donors. Proteomics data are available with identifier PXD057282 via ProteomeXchange. SIGNIFICANCE STATEMENT: The results from this study demonstrate a strong relationship between CYP2C8 and CYP3A phenotypes and interindividual variability in tucatinib metabolism. By elucidating how variability in CYP2C8 and CYP3A phenotypes influence tucatinib pharmacokinetics, this study has the potential to provide the framework for future studies that could inform dosing to minimize adverse events and improve therapeutic outcomes. Ultimately, understanding how individual cytochrome P450 phenotypes influence the clearance of cancer therapeutics will aid in the development of tailored regimens for diverse patient populations.

Innovation in cancer pharmacotherapy through integrative consideration of germline and tumor genomes

Precision cancer medicine is widely established, and numerous molecularly targeted drugs for various tumor entities are approved or are in development. Personalized pharmacotherapy in oncology has so far been based primarily on tumor characteristics, for example, somatic mutations. However, the response to drug treatment also depends on pharmacological processes summarized under the term ADME (absorption, distribution, metabolism, and excretion). Variations in ADME genes have been the subject of intensive research for >5 decades, considering individual patients' genetic makeup, referred to as pharmacogenomics (PGx). The combined impact of a patient's tumor and germline genome is only partially understood and often not adequately considered in cancer therapy. This may be attributed, in part, to the lack of methods for combined analysis of both data layers. Optimized personalized cancer therapies should, therefore, aim to integrate molecular information, which derives from both the tumor and the germline genome, and taking into account existing PGx guidelines for drug therapy. Moreover, such strategies should provide the opportunity to consider genetic variants of previously unknown functional significance. Bioinformatic analysis methods and corresponding algorithms for data interpretation need to be developed to integrate PGx data in cancer therapy with a special meaning for interdisciplinary molecular tumor boards, in which cancer patients are discussed to provide evidence-based recommendations for clinical management based on individual tumor profiles. SIGNIFICANCE STATEMENT: The era of personalized oncology has seen the emergence of drugs tailored to genetic variants associated with cancer biology. However, the full potential of targeted therapy remains untapped owing to the predominant focus on acquired tumor-specific alterations. Optimized cancer care must integrate tumor and patient genomes, guided by pharmacogenomic principles. An essential prerequisite for realizing truly personalized drug treatment of cancer patients is the development of bioinformatic tools for comprehensive analysis of all data layers generated in modern precision oncology programs.

Effects of Berberis vulgaris, and its active constituent berberine on cytochrome P450: a review

The cytochrome P450 (CYP450) family is crucial for metabolizing drugs and natural substances. Numerous compounds, such as pharmaceuticals and dietary items, can influence CYP activity by either enhancing or inhibiting these enzymes, potentially leading to interactions between drugs or between drugs and food. This research explores the impact of barberry and its primary component "berberine" on key human CYP450 enzymes. The text discusses the effects of this plant on the 12 primary human CYP450 enzymes, with summarized data presented in tables. Berberine exerts an influence on the function of various CYP450 isoforms, including CYP3A4/5, CYP2D6, CYP2C9, CYP2E1, CYP1A1/2, and most isoforms within the CYP2B subfamily. Given the significant role of these CYP450 isoforms in metabolizing commonly used drugs and endogenous substances, as well as activating procarcinogens into carcinogenic metabolites, the influence of barberry and its active constituent on these enzymes may impact the pharmacokinetics and toxicity profiles of various compounds. More specifically, regarding the crucial role of CYP2D6 and CYP3A4 in metabolizing clinically used drugs, and the inhibitory effects of berberine on these two CYP450 isoforms, it seems that the most important drug interaction of berberine that should be considered is related to its inhibitory effect on CYP2D6 and CYP3A4. In conclusion, due to the impact of barberry on multiple CYP450 isoforms, healthcare providers should conduct thorough consultations and investigations to ensure patient safety and prevent any potential adverse interactions before recommending the consumption of these herbs. Additional research, particularly clinical trials is crucial for preventing any potentially adverse interactions in patients who consume this herb.

Pharmacokinetics, Pharmacodynamics, and Side Effects of Midazolam: A Review and Case Example

Midazolam, a short-acting benzodiazepine, is widely used to alleviate patient anxiety, enhance compliance, and aid in anesthesia. While its side effects are typically dose-dependent and manageable with vigilant perioperative monitoring, serious cardiorespiratory complications, including fatalities and permanent neurological impairment, have been documented. Prolonged exposure to benzodiazepines, such as midazolam, has been associated with neurological changes in infants. Despite attempts to employ therapeutic drug monitoring for optimal sedation dosing, its efficacy has been limited. Consequently, efforts are underway to identify alternative predictive markers to guide individualized dosing and mitigate adverse effects. Understanding these factors is crucial for determining midazolam's suitability for future administration, particularly after a severe adverse reaction. This article aims to elucidate the factors influencing midazolam's pharmacokinetics and pharmacodynamics, potentially leading to adverse events. Finally, a case study is presented to exemplify the complex investigation into the causative factors of midazolam-related adverse events.

Characterization of CYP3A5 Selective Inhibitors for Reaction Phenotyping of Drug Candidates

CYP3A is one of the most important classes of enzymes and is involved in the metabolism of over 70% drugs. While several selective CYP3A4 inhibitors have been identified, the search for a selective CYP3A5 inhibitor has turned out to be rather challenging. Recently, several selective CYP3A5 inhibitors have been identified through high-throughput screening of ~ 11,000 compounds and hit expansion using human recombinant enzymes. We set forth to characterize the three most selective CYP3A5 inhibitors in a more physiologically relevant system of human liver microsomes to understand if these inhibitors can be used for reaction phenotyping studies in drug discovery settings. Gomisin A and T-5 were used as selective substrate reactions for CYP3A4 and CYP3A5 to determine IC50 values of the two enzymes. The results showed that clobetasol propionate and loteprednol etabonate were potent and selective CYP3A5 reversible inhibitors with selectivity of 24-fold against CYP3A4 and 39-fold or more against the other major CYPs. The selectivity of difluprednate in HLM is much weaker than that in the recombinant enzymes due to hydrolysis of the acetate group in HLM. Based on the selectivity data, loteprednol etabonate can be utilized as an orthogonal approach, when experimental fraction metabolized of CYP3A5 is greater than 0.5, to understand CYP3A5 contribution to drug metabolism and its clinical significance. Future endeavors to identify even more selective CYP3A5 inhibitors are warranted to enable accurate determination of CYP3A5 contribution to metabolism versus CYP3A4.

Hydrocodone, Oxycodone, and Morphine Metabolism and Drug-Drug Interactions

Awareness of drug interactions involving opioids is critical for patient treatment as they are common therapeutics used in numerous care settings, including both chronic and disease-related pain. Not only do opioids have narrow therapeutic indexes and are extensively used, but they have the potential to cause severe toxicity. Opioids are the classical pain treatment for patients who suffer from moderate to severe pain. More importantly, opioids are often prescribed in combination with multiple other drugs, especially in patient populations who typically are prescribed a large drug regimen. This review focuses on the current knowledge of common opioid drug-drug interactions (DDIs), focusing specifically on hydrocodone, oxycodone, and morphine DDIs. The DDIs covered in this review include pharmacokinetic DDI arising from enzyme inhibition or induction, primarily due to inhibition of cytochrome p450 enzymes (CYPs). However, opioids such as morphine are metabolized by uridine-5'-diphosphoglucuronosyltransferases (UGTs), principally UGT2B7, and glucuronidation is another important pathway for opioid-drug interactions. This review also covers several pharmacodynamic DDI studies as well as the basics of CYP and UGT metabolism, including detailed opioid metabolism and the potential involvement of metabolizing enzyme gene variation in DDI. Based upon the current literature, further studies are needed to fully investigate and describe the DDI potential with opioids in pain and related disease settings to improve clinical outcomes for patients. SIGNIFICANCE STATEMENT: A review of the literature focusing on drug-drug interactions involving opioids is important because they can be toxic and potentially lethal, occurring through pharmacodynamic interactions as well as pharmacokinetic interactions occurring through inhibition or induction of drug metabolism.

Differences of Atomic-Level Interactions between Midazolam and Two CYP Isoforms 3A4 and 3A5

CYP 3A4 and CYP 3A5 are two important members of the human cytochrome P450 family. Although their overall structures are similar, the local structures of the active site are different, which directly leads to obvious individual differences in drug metabolic efficacy and toxicity. In this work, midazolam (MDZ) was selected as the probe substrate, and its interaction with two proteins, CYP 3A4 and CYP 3A5, was studied by molecular dynamics simulation (MD) along with the calculation of the binding free energy. The results show that two protein-substrate complexes have some similarities in enzyme-substrate binding; that is, in both complexes, Ser119 forms a high occupancy hydrogen bond with MDZ, which plays a key role in the stability of the interaction between MDZ and the enzymes. However, the complex formed by CYP 3A4 and MDZ is more stable, which may be attributed to the sandwich structure formed by the fluorophenyl group of the substrate with Leu216 and Leu482. Our study interprets the binding differences between two isoform-substrate complexes and reveals a structure-function relationship from the atomic perspective, which is expected to provide a theoretical basis for accurately measuring the effectiveness and toxicity of drugs for individuals in the era of precision medicine.

Pregnancy related hormones increase CYP3A mediated buprenorphine metabolism in human hepatocytes: a comparison to CYP3A substrates nifedipine and midazolam

Introduction: Pregnancy increases the clearance of CYP3A4 substrate drugs and pregnancy-related hormones (PRHs) induce hepatic CYP3A4 expression and metabolism. However, it remains unclear to what extent the magnitude of PRH-evoked changes in hepatic CYP3A metabolism varies across multiple substrates. This study quantified the impact of PRHs on CYP3A protein concentrations and buprenorphine metabolism in human hepatocytes, and compared the magnitude of these effects to nifedipine and midazolam metabolism. Methods: Sandwich-cultured human hepatocytes (SCHH) from female donors were exposed to PRHs, administered in combination across a range of physiologically relevant concentrations, for 72 h. Absolute protein concentrations of CYP3A4, CYP3A5, and CYP3A7 in SCHH membrane fractions were quantified by nanoLC-MS/MS, and norbuprenorphine (nor-BUP), dehydro-nifedipine (dehydro-NIF), and 1-hydroxy-midazolam (1-OH-MDZ) formation was evaluated. Results: Compared to control, PRH exposure increased CYP3A4, CYP3A7, and total CYP3A protein concentrations, but not CYP3A5 concentrations, and increased nor-BUP, dehydro-NIF, and 1-OH-MDZ formation in a concentration-dependent manner. The formation of nor-BUP, dehydro-NIF, and 1-OH-MDZ each positively correlated with PRH-mediated changes in total CYP3A protein concentrations. The PRH-evoked increase in nor-BUP formation was evident in all donors; however, the PRH induction of dehydro-NIF and 1-OH-MDZ formation was diminished in a hepatocyte donor with high basal CYP3A5 expression. Discussion: These findings demonstrate that PRHs increase buprenorphine, nifedipine, and midazolam metabolism in SCHH via induction of CYP3A4 and total CYP3A protein concentrations, and the magnitude of these effects vary across hepatocyte donors in a substrate-specific manner. These data provide insight into the contribution of PRH induction of CYP3A4 metabolism to increased buprenorphine clearance during pregnancy.

5-ALA treatment increases intracellular heme levels and enhances CYP3A4 activity in genome-edited Caco-2 cells

In nonclinical studies, models that can predict the metabolism of drug candidates by cytochrome P450 (CYP), including Cytochrome P450 family 3 subfamily A member 4 (CYP3A4) are helpful. CYP3A4-overexpressing human cells have been used universally to evaluate whether CYP3A4 metabolizes drug-candidate compounds. However, CYP3A4-overexpressing human cell lines are problematic because their activity levels are lower than that of in vivo human CYP3A4. Heme plays a paramount role in CYP activity. The rate-limiting step in heme biosynthesis is the generation of 5-aminolevulinic acid (5-ALA). In this study, we examined whether treatment with 5-ALA to CYP3A4-POR-UGT1A1-CES2 knockin and CES1 knockout (genome-edited) Caco-2 cells enhances CYP3A4 activity. A 7-day 5-ALA treatment increased intracellular heme levels in genome-edited Caco-2 cells without cytotoxicity. Moreover, consistent with the increase in intracellular heme content, 5-ALA treatment increased CYP3A4 activity in genome-edited Caco-2 cells. The results of this research are expected to be applied to pharmacokinetic studies using CYP-overexpressing human cells containing CYP3A4.

Disposition of Nirmatrelvir, an Orally Bioavailable Inhibitor of SARS-CoV-2 3C-Like Protease, across Animals and Humans

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 3C-like protease inhibitor PF-07321332 (nirmatrelvir), in combination with ritonavir (Paxlovid), was recently granted emergency use authorization by multiple regulatory agencies for the treatment of coronavirus disease 2019 (COVID-19) in adults and pediatric patients. Disposition studies on nirmatrelvir in animals and in human reagents, which were used to support clinical studies, are described herein. Plasma clearance was moderate in rats (27.2 ml/min per kg) and monkeys (17.1 ml/min per kg), resulting in half-lives of 5.1 and 0.8 hours, respectively. The corresponding oral bioavailability was moderate in rats (34%-50%) and low in monkeys (8.5%), primarily due to oxidative metabolism along the gastrointestinal tract in this species. Nirmatrelvir demonstrated moderate plasma protein binding in rats, monkeys, and humans with mean unbound fractions ranging from 0.310 to 0.478. The metabolism of nirmatrelvir was qualitatively similar in liver microsomes and hepatocytes from rats, monkeys, and humans; prominent metabolites arose via cytochrome P450 (CYP450)-mediated oxidations on the P1 pyrrolidinone ring, P2 6,6-dimethyl-3-azabicyclo[3.1.0]hexane, and the tertiary-butyl group at the P3 position. Reaction phenotyping studies in human liver microsomes revealed that CYP3A4 was primarily responsible (fraction metabolized = 0.99) for the oxidative metabolism of nirmatrelvir. Minor clearance mechanisms involving renal and biliary excretion of unchanged nirmatrelvir were also noted in animals and in sandwich-cultured human hepatocytes. Nirmatrelvir was a reversible and time-dependent inhibitor as well as inducer of CYP3A activity in vitro. First-in-human pharmacokinetic studies have demonstrated a considerable boost in the oral systemic exposure of nirmatrelvir upon coadministration with the CYP3A4 inhibitor ritonavir, consistent with the predominant role of CYP3A4 in nirmatrelvir metabolism. SIGNIFICANCE STATEMENT: The manuscript describes the preclinical disposition, metabolism, and drug-drug interaction potential of PF-07321332 (nirmatrelvir), an orally active peptidomimetic-based inhibitor of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 3CL protease, which has been granted emergency use authorization by multiple regulatory agencies around the globe for the treatment of coronavirus disease 2019 (COVID-19) in COVID-19-positive adults and pediatric patients who are at high risk for progression to severe COVID-19, including hospitalization or death.

Structural characterization of the homotropic cooperative binding of azamulin to human cytochrome P450 3A5

Cytochrome P450 3A4 and 3A5 catalyze the metabolic clearance of a large portion of therapeutic drugs. Azamulin is used as a selective inhibitor for 3A4 and 3A5 to define their roles in metabolism of new chemical entities during drug development. In contrast to 3A4, 3A5 exhibits homotropic cooperativity for the sequential binding of two azamulin molecules at concentrations used for inhibition. To define the underlying sites and mechanisms for cooperativity, an X-ray crystal structure of 3A5 was determined with two azamulin molecules in the active site that are stacked in an antiparallel orientation. One azamulin resides proximal to the heme in a pose similar to the 3A4-azamulin complex. Comparison to the 3A5 apo structure indicates that the distal azamulin in 3A5 ternary complex causes a significant induced fit that excludes water from the hydrophobic surfaces of binding cavity and the distal azamulin, which is augmented by the stacking interaction with the proximal azamulin. Homotropic cooperativity was not observed for the binding of related pleuromutilin antibiotics, tiamulin, retapamulin, and lefamulin, to 3A5, which are larger and unlikely to bind in the distal site in a stacked orientation. Formation of the 3A5 complex with two azamulin molecules may prevent time-dependent inhibition that is seen for 3A4 by restricting alternate product formation and/or access of reactive intermediates to vulnerable protein sites. These results also contribute to a better understanding of sites for cooperative binding and the differential structural plasticity of 3A5 and 3A4 that contribute to differential substrate and inhibitor binding.

Intersystem Extrapolation Factors Are Substrate-Dependent for CYP3A4: Impact on Cytochrome P450 Reaction Phenotyping

The use of intersystem extrapolation factors (ISEF) is required for the quantitative scaling of drug metabolism data generated in individually expressed cytochrome P450 (CYP) enzymes when estimating fractional contribution (fm) to metabolism by P450 enzymes in vivo. For successful prediction of fm, ISEF values must be universal across all substrates for any individual enzyme. In this study, ISEF values were generated for ten CYP3A4 selective substrates using a common source of recombinant heterologously expressed CYP3A4 (rCYP) and a pool of human liver microsomes. The resulting ISEF values for CYP3A4 were substrate-dependent and ranged 8-fold, with the highest value generated from intrinsic clearance of midazolam depletion (0.36) and the lowest from quinidine depletion (0.044). Application of these ISEF values for estimation of the fractional contribution of CYP3A4 and CYP2C19 to omeprazole clearance yielded values that ranged from 0.21-0.63 and 0.37-0.79, respectively, as compared with back-extrapolated in vivo fm values of 0.27 (CYP3A4) and 0.85 (CYP2C19) from clinical pharmacokinetic data. For risperidone, estimated fm values for CYP3A4 and CYP2D6 ranged from 0.87-0.98 and 0.02-0.13, respectively, as compared with in vivo values of 0.36 (CYP3A4) and 0.63-0.88 (CYP2D6), showing that the importance of CYP3A4 was overestimated, and the importance of CYP2D6 underestimated. Overall, these findings suggest that ISEF values for CYP3A4 can vary with the marker substrate used to derive them, thereby reducing the effectiveness of the approach of using metabolism data from rCYP3A4 with ISEF values for the prediction of fraction metabolized values in vivo. SIGNIFICANCE STATEMENT: Intersystem extrapolation factors are utilized for assigning fractional contributions of individual enzymes to drug clearance (fm) from drug metabolism data generated in recombinant P450s. The present data shows that intersystem extrapolation factors values for cytochrome P4503A4 vary with the substrate. This can lead to variable and erroneous prediction of fm.

Broad Application of CYP3A4 Liquid Chromatography-Mass Spectrometry Protein Quantification in Hepatocyte Cytochrome P450 Induction Assays Identifies Nonuniformity in mRNA and Protein Induction Responses

Screening for cytochrome P450 (CYP) induction potential is routine in drug development. Induction results in a net increase in CYP protein and is assessed typically by measuring indirect endpoints, i.e., enzyme activity and mRNA in vitro. Recent methodological advancements have made CYP protein quantification by liquid chromatography-mass spectrometry in vitro induction studies more accessible and amenable to routine testing. In this study, we evaluated CYP3A4 concentration dependence of induction response for 11 compounds (rifampin, rifabutin, carbamazepine, efavirenz, nitrendipine, flumazenil, pioglitazone, rosiglitazone, troglitazone, pazopanib, and ticagrelor) in plated hepatocytes from two or three donors incorporating in the assessment all three endpoints. In addition, the time-dependence of the induction was examined over 1, 2, or 3 days of treatment. For most compounds, mRNA, enzyme activity, and protein endpoints exhibited similarity in induction responses. Pazopanib and ticagrelor were notable exceptions as neither protein nor enzyme activity were induced despite mRNA induction of a magnitude similar to efavirenz, pioglitazone, or rosiglitazone, which clearly induced in all three endpoints. Static modeling of clinical induction responses supported a role for protein as a predictive endpoint. These data highlight the value of including CYP protein quantification as an induction assay endpoint to provide a more comprehensive assessment of induction liability. SIGNIFICANCE STATEMENT: Direct, liquid chromatography-mass spectrometry (LC-MS)-based quantification of cytochrome P450 (CYP) protein is a desirable induction assay endpoint; however such application has been limited due to inefficient workflows. Here, we incorporate recent advancements in protein quantitation methods to efficiently quantify CYP3A4 protein in in vitro induction assays with 11 compounds in up to 3 donors. The data indicate induction responses from mRNA do not always align with those of protein suggesting assessment of induction liability is more complex than thought previously.

Leveraging Human Plasma-Derived Small Extracellular Vesicles as Liquid Biopsy to Study the Induction of Cytochrome P450 3A4 by Modafinil

Preparations of plasma-derived small extracellular vesicles (sEVs) were deployed as liquid biopsy to study cytochrome P450 (CYP) 3A4 (CYP3A4) induction following modafinil 400 mg once daily × 14 days (young healthy volunteers, N = 10 subjects). Induction was confirmed using the 4β-hydroxycholesterol-to-cholesterol (4βHC/C) ratio, a plasma CYP3A4/5 biomarker, with a mean 2.1-fold increase (Day 15 vs. Day 1; 90% confidence interval (CI) = 1.8-2.3; P value = 0.0004). Proteomic analysis revealed the induction (mean Day 15 vs. Day 1 fold-increase (90% CI)) of both liver (1.3 (1.1-1.5), P value = 0.014) and nonliver (1.9 (1.6-2.2), P value = 0.04) sEV CYP3A4 protein expression. In CYP3A5 nonexpresser subjects, the baseline (pre-dose) 4βHC/C plasma ratio was more highly correlated with liver sEVs (r = 0.937, P value = 0.001) than nonliver sEVs (r = 0.619, P value = 0.101) CYP3A4 protein expression. When CYP3A5 expressers (CYP3A5*1/*3) were included, the correlation with liver sEVs (r = 0.761, P value = 0.011) and nonliver sEVs (r = 0.391, P value = 0.264) CYP3A4 protein was weaker. Although modafinil-induced changes in plasma 4βHC/C ratio did not correlate with sEVs CYP3A4 protein expression, the individual subject sEVs proteomic data were used successfully to predict victim drug (midazolam, triazolam, dextromethorphan, 17α-ethinylestradiol, and abemaciclib) area under the plasma concentration-time curve (AUC) ratios (AUCRs) following modafinil. Based on the AUCR values, modafinil was classified as a weak to moderate CYP3A4 inducer (vs. rifampicin). For the first time, it was possible to deploy plasma-derived sEVs to study CYP3A4 induction beyond rifampicin, a more potent CYP3A4 inducer.

Pharmacogenomics insights into precision pediatric oncology

PURPOSE OF REVIEW

Pharmacogenomic insights provide an opportunity to optimize medication dosing regimens and patient outcomes. However, the potential for interindividual genomic variability to guide medication dosing and toxicity monitoring is not yet standard of care. In this review, we present advances for the thiopurines, anthracyclines and vincristine and provide perspectives on the actionability of pharmacogenomic guidance in the future.

RECENT FINDINGS

The current guideline on thiopurines recommends that those with normal predicted thiopurine methyltransferase and NUDT15 expression receive standard-of-care dosing, while 'poor metabolizer' haplotypes receive a decreased 6-mercaptopurine starting dose to avoid bone marrow toxicity. Emerging evidence established significant polygenic contributions that predispose to anthracycline-induced cardiotoxicity and suggest this knowledge be used to identify those at higher risk of complications. In the case of vincristine, children who express CYP3A5 have a significantly reduced risk of peripheral neuropathy compared with those expressing an inactive form or the CYP3A4 isoform.

SUMMARY

The need for adequately powered pediatric clinical trials, coupled with the study of epigenetic mechanisms and their influence on phenotypic variation and the integration of precision survivorship into precision approaches are featured as important areas for focused investments in the future.

Trans-eQTLs of the CYP3A4 and CYP3A5 associated with tacrolimus trough blood concentration in Chinese renal transplant patients

This study aimed to systematically investigate trans-eQTLs of CYP3A4 and CYP3A5 affecting tacrolimus trough blood concentrations in Chinese renal transplant patients. We used Plink v1.90 to perform data quality control and linear regression analysis on GTEx v8 data. SNPs with p-value < 0.05 were selected and the GTEx eQTL Calculator was used to further prioritize the eQTLs of CYP3A4 and CYP3A5 in the liver and small intestine. The eQTLs with a p-value < 5 × 10^-5 and MAF≥ 0.05 in the CHB population were selected as candidate eQTLs. The genotyping of candidate eQTLs was performed using high-resolution melting (HRM) assays and Sanger DNA sequencing. This study included 845 Chinese renal transplant patients who received tacrolimus as an immunosuppressive agent. Association between 103 candidate eQTLs and log-transformed tacrolimus concentration/dose ratio (log (C₀/D)) in this cohort was conducted using the SNPassoc package of R software. In the end, a total of 75,632 liver eQTLs of CYP3A4, 69,558 liver eQTLs of CYP3A5, 48,596 small intestine eQTLs of CYP3A4 and 28,616 small intestine eQTLs of CYP3A5 were obtained using the GTEx v8 eQTL Calculator. Of the 103 candidate eQTLs, rs75727207, rs181294422 and rs28522676 were significantly associated with tacrolimus log(C₀/D) in different genetic models. We discovered a substantial number of novel eQTLs of CYP3A4 and CYP3A5 in liver and small intestine, also found that rs75727207, rs181294422 and rs28522676 may affect tacrolimus trough blood concentrations in Chinese renal transplant patients.

Predicting Drug-Drug Interactions between Rifampicin and Ritonavir-Boosted Atazanavir Using PBPK Modelling

Objectives

The aim of this study was to simulate the drug–drug interaction (DDI) between ritonavir-boosted atazanavir (ATV/r) and rifampicin (RIF) using physiologically based pharmacokinetic (PBPK) modelling, and to predict suitable dose adjustments for ATV/r for the treatment of people living with HIV (PLWH) co-infected with tuberculosis.

Methods

A whole-body DDI PBPK model was designed using Simbiology 9.6.0 (MATLAB R2019a) and verified against reported clinical data for all drugs administered alone and concomitantly. The model contained the induction mechanisms of RIF and ritonavir (RTV), the inhibition effect of RTV for the enzymes involved in the DDI, and the induction and inhibition mechanisms of RIF and RTV on the uptake and efflux hepatic transporters. The model was considered verified if the observed versus predicted pharmacokinetic values were within twofold. Alternative ATV/r dosing regimens were simulated to achieve the trough concentration (C_trough) clinical cut-off of 150 ng/mL.

Results

The PBPK model was successfully verified according to the criteria. Simulation of different dose adjustments predicted that a change in regimen to twice-daily ATV/r (300/100 or 300/200 mg) may alleviate the induction effect of RIF on ATV C_trough, with > 95% of individuals predicted to achieve C_trough above the clinical cut-off.

Conclusions

The developed PBPK model characterized the induction-mediated DDI between RIF and ATV/r, accurately predicting the reduction of ATV plasma concentrations in line with observed clinical data. A change in the ATV/r dosing regimen from once-daily to twice-daily was predicted to mitigate the effect of the DDI on the C_trough of ATV, maintaining plasma concentration levels above the therapeutic threshold for most patients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40262-021-01067-1.

Cytochrome P450-Catalyzed Metabolism of Cannabidiol to the Active Metabolite 7-Hydroxy-Cannabidiol

Cannabidiol (CBD) is a naturally occurring nonpsychotoxic phytocannabinoid that has gained increasing attention as a popular consumer product and for its use in Food and Drug Administration-approved Epidiolex (CBD oral solution) for the treatment of Lennox-Gastaut syndrome and Dravet syndrome. CBD was previously reported to be metabolized primarily by CYP2C19 and CYP3A4, with minor contributions from UDP-glucuronosyltransferases. 7-Hydroxy-CBD (7-OH-CBD) is the primary active metabolite with equipotent activity compared with CBD. Given the polymorphic nature of CYP2C19, we hypothesized that variable CYP2C19 expression may lead to interindividual differences in CBD metabolism to 7-OH-CBD. The objectives of this study were to further characterize the roles of cytochrome P450 enzymes in CBD metabolism, specifically to the active metabolite 7-OH-CBD, and to investigate the impact of CYP2C19 polymorphism on CBD metabolism in genotyped human liver microsomes. The results from reaction phenotyping experiments with recombinant cytochrome P450 enzymes and cytochrome P450-selective chemical inhibitors indicated that both CYP2C19 and CYP2C9 are capable of CBD metabolism to 7-OH-CBD. CYP3A played a major role in CBD metabolic clearance via oxidation at sites other than the 7-position. In genotyped human liver microsomes, 7-OH-CBD formation was positively correlated with CYP2C19 activity but was not associated with CYP2C19 genotype. In a subset of single-donor human liver microsomes with moderate to low CYP2C19 activity, CYP2C9 inhibition significantly reduced 7-OH-CBD formation, suggesting that CYP2C9 may play a greater role in CBD 7-hydroxylation than previously thought. Collectively, these data indicate that both CYP2C19 and CYP2C9 are important contributors in CBD metabolism to the active metabolite 7-OH-CBD. SIGNIFICANCE STATEMENT: This study demonstrates that both CYP2C19 and CYP2C9 are involved in CBD metabolism to the active metabolite 7-OH-CBD and that CYP3A4 is a major contributor to CBD metabolism through pathways other than 7-hydroxylation. 7-OH-CBD formation was associated with human liver microsomal CYP2C19 activity, but not CYP2C19 genotype, and CYP2C9 was found to contribute significantly to 7-OH-CBD generation. These findings have implications for patients taking CBD who may be at risk for clinically important cytochrome P450-mediated drug interactions.

Characterization of Hepatic UDP-Glucuronosyltransferase Enzyme Abundance-Activity Correlations and Population Variability Using a Proteomics Approach and Comparison with Cytochrome P450 Enzymes

The expression of ten major drug-metabolizing UDP-glucuronosyltransferase (UGT) enzymes in a panel of 130 human hepatic microsomal samples was measured using a liquid chromatography-tandem mass spectrometry-based approach. Simultaneously, ten cytochromes P450 and P450 reductase were also measured, and activity-expression relationships were assessed for comparison. The resulting data sets demonstrated that, with the exception of UGT2B17, 10th to 90th percentiles of UGT expression spanned 3- to 8-fold ranges. These ranges were small relative to ranges of reported mean UGT enzyme expression across different laboratories. We tested correlation of UGT expression with enzymatic activities using selective probe substrates. A high degree of abundance-activity correlation (Spearman's rank correlation coefficient > 0.6) was observed for UGT1As (1A1, 3, 4, 6) and cytochromes P450. In contrast, protein abundance and activity did not correlate strongly for UGT1A9 and UGT2B enzymes (2B4, 7, 10, 15, and 17). Protein abundance was strongly correlated for UGTs 2B7, 2B10, and 2B15. We suggest a number of factors may contribute to these differences including incomplete selectivity of probe substrates, correlated expression of these UGT2B isoforms, and the impact of splice and polymorphic variants on the peptides used in proteomics analysis, and exemplify this in the case of UGT2B10. Extensive correlation analyses identified important criteria for validating the fidelity of proteomics and enzymatic activity approaches for assessing UGT variability, population differences, and ontogenetic changes. SIGNIFICANCE STATEMENT: Protein expression data allow detailed assessment of interindividual variability and enzyme ontogeny. This study has observed that expression and enzyme activity are well correlated for hepatic UGT1A enzymes and cytochromes P450. However, for the UGT2B family, caution is advised when assuming correlation of expression and activity as is often done in physiologically based pharmacokinetic modeling. This can be due to incomplete probe substrate specificities, but may also be related to presence of inactive UGT protein materials and the effect of splicing variations.

Differential Reversible and Irreversible Interactions between Benzbromarone and Human Cytochrome P450s 3A4 and 3A5

Mounting evidence has revealed that despite the high degree of sequence homology between cytochrome P450 3A isoforms (i.e., CYP3A4 and CYP3A5), they have the propensities to exhibit vastly different irreversible and reversible interactions with a single substrate. We have previously established that benzbromarone (BBR), a potent uricosuric agent used in the management of gout, irreversibly inhibits CYP3A4 via mechanism-based inactivation (MBI). However, it remains unelucidated if CYP3A5-its highly homologous counterpart-is susceptible to inactivation by BBR. Using three structurally distinct probe substrates, we consistently demonstrated that MBI was not elicited in CYP3A5 by BBR. Our in silico covalent docking models and molecular dynamics simulations suggested that disparities in the susceptibilities toward MBI could be attributed to the specific effects of BBR covalent adducts on the F-F' loop. Serendipitously, we also discovered that BBR reversibly activated CYP3A5-mediated rivaroxaban hydroxylation wherein apparent V max increased and K m decreased with increasing BBR concentration. Fitting data to the two-site model yielded interaction factors α and β of 0.44 and 5.88, respectively, thereby confirming heterotropic activation of CYP3A5 by BBR. Furthermore, heteroactivation was suppressed by the CYP3A inhibitor ketoconazole in a concentration-dependent manner and decreased with increasing preincubation time, implying that activation was incited via binding of parent BBR molecule within the enzymatic active site. Finally, noncovalent docking revealed that CYP3A5 can more favorably accommodate both BBR and rivaroxaban in concert as compared with CYP3A4, which further substantiated our experimental observations. SIGNIFICANCE STATEMENT: Although it has been previously demonstrated that benzbromarone (BBR) inactivates CYP3A4, it remains uninterrogated whether it also elicits mechanism-based inactivation in CYP3A5, which shares ∼85% sequence similarity with CYP3A4. This study reported that BBR exhibited differential irreversible and reversible interactions with both CYP3A isoforms and further unraveled the molecular determinants underpinning their diverging interactions. These data offer important insight into differential kinetic behavior of CYP3A4 and CYP3A5, which potentially contributes to interindividual variabilities in drug disposition.

Classification of drugs for evaluating drug interaction in drug development and clinical management

During new drug development, clinical drug interaction studies are carried out in accordance with the mechanism of potential drug interactions evaluated by in vitro studies. The obtained information should be provided efficiently to medical experts through package inserts and various information materials after the drug's launch. A recently updated Japanese guideline presents general procedures that are considered scientifically valid at the present moment. In this review, we aim to highlight the viewpoints of the Japanese guideline and enumerate drugs that were involved or are anticipated to be involved in evident pharmacokinetic drug interactions and classify them by their clearance pathway and potential intensity based on systematic reviews of the literature. The classification would be informative for designing clinical studies during the development stage, and the appropriate management of drug interactions in clinical practice.

An automated cocktail method for in vitro assessment of direct and time-dependent inhibition of nine major cytochrome P450 enzymes - application to establishing CYP2C8 inhibitor selectivity

We developed an in vitro high-throughput cocktail assay with nine major drug-metabolizing CYP enzymes, optimized for screening of time-dependent inhibition. The method was applied to determine the selectivity of the time-dependent CYP2C8 inhibitors gemfibrozil 1-O-β-glucuronide and clopidogrel acyl-β-D-glucuronide. In vitro incubations with CYP selective probe substrates and pooled human liver microsomes were conducted in 96-well plates with automated liquid handler techniques and metabolite concentrations were measured with quantitative UHPLC-MS/MS analysis. After determination of inter-substrate interactions and K_m values for each reaction, probe substrates were divided into cocktails I (tacrine/CYP1A2, bupropion/CYP2B6, amodiaquine/CYP2C8, tolbutamide/CYP2C9 and midazolam/CYP3A4/5) and II (coumarin/CYP2A6, S-mephenytoin/CYP2C19, dextromethorphan/CYP2D6 and astemizole/CYP2J2). Time-dependent inhibitors (furafylline/CYP1A2, selegiline/CYP2A6, clopidogrel/CYP2B6, gemfibrozil 1-O-β-glucuronide/CYP2C8, tienilic acid/CYP2C9, ticlopidine/CYP2C19, paroxetine/CYP2D6 and ritonavir/CYP3A) and direct inhibitor (terfenadine/CYP2J2) showed similar inhibition with single substrate and cocktail methods. Established time-dependent inhibitors caused IC₅₀ fold shifts ranging from 2.2 to 30 with the cocktail method. Under time-dependent inhibition conditions, gemfibrozil 1-O-β-glucuronide was a strong (>90% inhibition) and selective (<< 20% inhibition of other CYPs) inhibitor of CYP2C8 at concentrations ranging from 60 to 300 μM, while the selectivity of clopidogrel acyl-β-D-glucuronide was limited at concentrations above its IC₈₀ for CYP2C8. The time-dependent IC₅₀ values of these glucuronides for CYP2C8 were 8.1 and 38 µM, respectively. In conclusion, a reliable cocktail method including the nine most important drug-metabolizing CYP enzymes was developed, optimized and validated for detecting time-dependent inhibition. Moreover, gemfibrozil 1-O-β-glucuronide was established as a selective inhibitor of CYP2C8 for use as a diagnostic inhibitor in in vitro studies.

In vivo evaluation of intestinal human CYP3A inhibition by macrolide antibiotics in CYP3A-humanised mice

It is important to predict drug-drug interactions via inhibition of intestinal cytochrome P450 3A (CYP3A) which is a determinant of bioavailability of orally administered CYP3A substrates. However, inhibitory effects of macrolide antibiotics on CYP3A-mediated metabolism are not entirely identical between humans and rodents.We investigated the effects of macrolide antibiotics, clarithromycin and erythromycin, on in vitro and in vivo metabolism of triazolam, a CYP3A substrate, in CYP3A-humanised mice generated by using a mouse artificial chromosome vector carrying a human CYP3A gene.Metabolic activities of triazolam were inhibited by macrolide antibiotics in liver and intestine microsomes of CYP3A-humanised mice.The area under the plasma concentration-time curve ratios of 4-hydroxytriazolam to triazolam after oral dosing of triazolam were significantly decreased by multiple administration of macrolide antibiotics. The plasma concentrations ratios of α-hydroxytriazolam and 4-hydroxytriazolam to triazolam in portal blood were significantly decreased by multiple administration of clarithromycin in CYP3A-humanised mice.These results suggest that intestinal CYP3A activity was inhibited by macrolide antibiotics in CYP3A-humanised mice in vitro and in vivo. The plasma concentrations of triazolam and its metabolites in the portal blood of CYP3A-humanised mice would be useful for direct evaluation of intestinal CYP3A-mediated drug-drug interactions.

Investigation of the Impact of CYP3A5 Polymorphism on Drug-Drug Interaction between Tacrolimus and Schisantherin A/Schisandrin A Based on Physiologically-Based Pharmacokinetic Modeling

Wuzhi capsule (WZC) is commonly prescribed with tacrolimus in China to ease drug-induced hepatotoxicity. Two abundant active ingredients, schisantherin A (STA) and schisandrin A (SIA) are known to inhibit CYP3A enzymes and increase tacrolimus’s exposure. Our previous study has quantitatively demonstrated the contribution of STA and SIA to tacrolimus pharmacokinetics based on physiologically-based pharmacokinetic (PBPK) modeling. In the current work, we performed reversible inhibition (RI) and time-dependent inhibition (TDI) assays with CYP3A5 genotyped human liver microsomes (HLMs), and further integrated the acquired parameters into the PBPK model to predict the drug–drug interaction (DDI) in patients with different CYP3A5 alleles. The results indicated STA was a time-dependent and reversible inhibitor of CYP3A4 while only a reversible inhibitor of CYP3A5; SIA inhibited CYP3A4 and 3A5 in a time-dependent manner but also reversibly inhibited CYP3A5. The predicted fold-increases of tacrolimus exposure were 2.70 and 2.41, respectively, after the multidose simulations of STA. SIA also increased tacrolimus’s exposure but to a smaller extent compared to STA. An optimized physiologically-based pharmacokinetic (PBPK) model integrated with CYP3A5 polymorphism was successfully established, providing more insights regarding the long-term DDI between tacrolimus and Wuzhi capsules in patients with different CYP3A5 genotypes.

In vitro inhibitory effects of ganoderic acid A on human liver cytochrome P450 enzymes

Context: Ganoderic acid A (GAA) is usually used to prevent cancers or other diseases, which make it likely to be used with other drugs metabolized by cytochromes P450.Objective: This study investigates the effect of GAA on eight major cytochrome P450 isoforms in human liver microsomes.Material and method: The effects of GAA (100 μM) on eight human liver CYP isoforms (i.e., 1A2, 3A4, 2A6, 2E1, 2D6, 2C9, 2C19, and 2C8) were investigated in vitro using human liver microsomes (HLMs) with specific substrates for the CYPs, and the enzyme kinetic parameters were calculated.Results: The results showed that GAA inhibited the activity of CYP3A4, 2D6, and 2E1, but did not affect other isoforms. The inhibition of CYP3A4, 2D6, and 2E1 was concentration-dependent with IC₅₀ values of 15.05, 21.83, and 28.35 μM, respectively. Additionally, GAA was not only a non-competitive inhibitor of CYP3A4, but also a competitive inhibitor of CYP2D6 and 2E1, with Ki values of 7.16, 10.07, and 13.45 μM. Meanwhile, the inhibition of CYP3A4 was time-dependent, with the K_I/K_inact value of 7.91/0.048 μM/min.Discussion and conclusion: The in vitro study indicated that GAA has the potential to result in drug-drug interactions with other drugs metabolized by CYP3A4, 2D6, and 2E1. Further clinical studies are needed for the identification of this interaction.

In Vitro Characterization of Ertugliflozin Metabolism by UDP-Glucuronosyltransferase and Cytochrome P450 Enzymes

Ertugliflozin is primarily cleared through UDP-glucurosyltransferase (UGT)-mediated metabolism (86%) with minor oxidative clearance (12%). In vitro phenotyping involved enzyme kinetic characterization of UGTs or cytochrome P450 enzymes catalyzing formation of the major 3-O-β-glucuronide (M5c) and minor 2-O-β-glucuronide (M5a), monohydroxylated ertugliflozin (M1 and M3), and des-ethyl ertugliflozin (M2) metabolites in human liver microsomes (HLMs). Fractional clearance (fCL) from HLM intrinsic clearance (CLint) indicated a major role for glucuronidation (fCL 0.96; CLint 37 µl/min per milligram) versus oxidative metabolism (fCL 0.04; CLint 1.64 µl/min per milligram). Substrate concentration at half-maximal velocity (Km), maximal rate of metabolism (Vmax), and CLint for M5c and M5a formation were 10.8 µM, 375 pmol/min per milligram, and 34.7 µl/min per milligram and 41.7 µM, 94.9 pmol/min per milligram, and 2.28 µl/min per milligram, respectively. Inhibition of HLM CLint with 10 µM digoxin or tranilast (UGT1A9) and 3 µM 16β-phenyllongifolol (UGT2B7/UGT2B4) resulted in fraction metabolism (fm) estimates of 0.81 and 0.19 for UGT1A9 and UGT2B7/UGT2B4, respectively. Relative activity factor scaling of recombinant enzyme kinetics provided comparable fm for UGT1A9 (0.86) and UGT2B7 (0.14). Km and Vmax for M1, M2, and M3 formation ranged 73.0-93.0 µM and 24.3-116 pmol/min per milligram, respectively, and was inhibited by ketoconazole (M1, M2, and M3) and montelukast (M2). In summary, ertugliflozin metabolism in HLMs was primarily mediated by UGT1A9 (78%) with minor contributions from UGT2B7/UGT2B4 (18%), CYP3A4 (3.4%), CYP3A5 (0.4%), and CYP2C8 (0.16%). Considering higher ertugliflozin oxidative metabolism (fCL 0.12) obtained from human mass balance, human systemic clearance is expected to be mediated by UGT1A9 (70%), UGT2B7/UGT2B4 (16%), CYP3A4 (10%), CYP3A5 (1.2%), CYP2C8 (0.5%), and renal elimination (2%). SIGNIFICANCE STATEMENT: This manuscript describes the use of orthogonal approaches (i.e., enzyme kinetics, chemical inhibitors, and recombinant enzymes) to characterize the fraction of ertugliflozin metabolism through various UDP-glucuronosyltransferase (UGT) and cytochrome P450 (CYP) enzyme-mediated pathways. Phenotyping approaches routinely used to characterize CYP hepatic fractional metabolism (fm) to estimate specific enzymes contributing to overall systemic clearance were similarly applied for UGT-mediated metabolism. Defining the in vitro metabolic disposition and fm for ertugliflozin allows risk assessment when considering potential victim-based drug-drug interactions perpetrated by coadministered drugs.

New insights into quetiapine metabolism using molecular networking

Metabolism is involved in both pharmacology and toxicology of most xenobiotics including drugs. Yet, visualization tools facilitating metabolism exploration are still underused, despite the availibility of pertinent bioinformatics solutions. Since molecular networking appears as a suitable tool to explore structurally related molecules, we aimed to investigate its interest in in vitro metabolism exploration. Quetiapine, a widely prescribed antipsychotic drug, undergoes well-described extensive metabolism, and is therefore an ideal candidate for such a proof of concept. Quetiapine was incubated in metabolically competent human liver cell models (HepaRG) for different times (0 h, 3 h, 8 h, 24 h) with or without cytochrom P450 (CYP) inhibitor (ketoconazole as CYP3A4/5 inhibitor and quinidine as CYP2D6 inhibitor), in order to study its metabolism kinetic and pathways. HepaRG culture supernatants were analyzed on an ultra-high performance liquid chromatography coupled with tandem mass spectrometry (LC-HRMS/MS). Molecular networking approach on LC-HRMS/MS data allowed to quickly visualize the quetiapine metabolism kinetics and determine the major metabolic pathways (CYP3A4/5 and/or CYP2D6) involved in metabolite formation. In addition, two unknown putative metabolites have been detected. In vitro metabolite findings were confirmed in blood sample from a patient treated with quetiapine. This is the first report using LC-HRMS/MS untargeted screening and molecular networking to explore in vitro drug metabolism. Our data provide new evidences of the interest of molecular networking in drug metabolism exploration and allow our in vitro model consistency assessment.

Physicochemical Properties, Biotransformation, and Transport Pathways of Established and Newly Approved Medications: A Systematic Review of the Top 200 Most Prescribed Drugs vs. the FDA-Approved Drugs Between 2005 and 2016

BACKGROUND

Enzyme-mediated biotransformation of pharmacological agents is a crucial step in xenobiotic detoxification and drug disposition. Herein, we investigated the metabolism and physicochemical properties of the top 200 most prescribed drugs (established) as well as drugs approved by the US Food and Drug Administration (FDA) between 2005 and 2016 (newly approved).

OBJECTIVE

Our objective was to capture the changing trends in the routes of administration, physicochemical properties, and prodrug medications, as well as the contributions of drug-metabolizing enzymes and transporters to drug clearance.

METHODS

The University of Washington Drug Interaction Database (DIDB^®) as well as other online resources (e.g., CenterWatch.com, Drugs.com, DrugBank.ca, and PubChem.ncbi.nlm.nih.gov) was used to collect and stratify the dataset required for exploring the above-mentioned trends.

RESULTS

Analyses revealed that ~ 90% of all drugs in the established and newly approved drug lists were administered systemically (oral or intravenous). Meanwhile, the portion of biologics (molecular weight > 1 kDa) was 15 times greater in the newly approved list than established drugs. Additionally, there was a 4.5-fold increase in the number of compounds with a high calculated partition coefficient (cLogP > 3) and a high total polar surface area (> 75 Å²) in the newly approved drug vs. the established category. Further, prodrugs in established or newly approved lists were found to be converted to active compounds via hydrolysis, demethylases, and kinases. The contribution of cytochrome P450 (CYP) 3A4, as the major biotransformation pathway, has increased from 40% in the established drug list to 64% in the newly approved drug list. Moreover, the role of CYP1A2, CYP2C19, and CYP2D6 were decreased as major metabolizing enzymes among the newly approved medications. Among non-CYP major metabolizers, the contribution of alcohol dehydrogenases/aldehyde dehydrogenases (ADH/ALDH) and sulfotransferases decreased in the newly approved drugs compared with the established list. Furthermore, the highest contribution among uptake and efflux transporters was found for Organic Anion Transporting Polypeptide 1B1 (OATP1B1) and P-glycoprotein (P-gp), respectively.

CONCLUSIONS

The higher portion of biologics in the newly approved drugs compared with the established list confirmed the growing demands for protein- and antibody-based therapies. Moreover, the larger number of hydrophilic drugs found in the newly approved list suggests that the probability of toxicity is likely to decrease. With regard to CYP-mediated major metabolism, CYP3A5 showed an increased involvement owing to the identification of unique probe substrates to differentiate CYP3As. Furthermore, the contribution of OATP1B1 and P-gp did not show a significant shift in the newly approved drugs as compared to the established list because of their broad substrate specificity.

Application of Azamulin to Determine the Contribution of CYP3A4/5 to Drug Metabolic Clearance Using Human Hepatocytes

Early determination of CYP3A4/5 contribution to the clearance of new chemical entities is critical to inform on the risk of drug-drug interactions with CYP3A inhibitors and inducers. Several in vitro approaches (recombinant P450 enzymes, correlation analysis, chemical and antibody inhibition in human liver microsomes) are available, but they are usually labor-intensive and/or suffer from specific limitations. In the present study, we have validated the use of azamulin as a specific CYP3A inhibitor in human hepatocytes. Azamulin (3 µM) was found to significantly inhibit CYP3A4/5 (>90%), whereas other P450 enzymes were not affected (less than 20% inhibition). Because human hepatocytes were used as a test system, the effect of azamulin on other key drug-metabolizing enzymes (aldehyde oxidase, carboxylesterase, UGT, flavin monooxygenase, and sulfotransferase) was also investigated. Apart from some UGTs showing minor inhibition (∼20%-30%), none of these non-P450 enzymes were inhibited by azamulin. Use of CYP3A5-genotyped human hepatocyte batches in combination with CYP3cide demonstrated that azamulin (at 3 µM) inhibits both CYP3A4 and CYP3A5 enzymes. Finally, 11 compounds with known in vivo CYP3A4/5 contribution have been evaluated in this human hepatocyte assay. Results showed that the effect of azamulin on the in vitro intrinsic clearance of these known CYP3A4/5 substrates was predictive of the in vivo CYP3A4/5 contribution. Overall, the study showed that human hepatocytes treated with azamulin provide a fast and accurate estimation of CYP3A4/5 contribution in metabolic clearance of new chemical entities. SIGNIFICANCE STATEMENT: Accurate estimation of CYP3A4/5 contribution in drug clearance is essential to anticipate risk of drug-drug interactions and select the appropriate candidate for clinical development. The present study validated the use of azamulin as selective CYP3A4/5 inhibitor in suspended human hepatocytes and demonstrated that this novel approach provides a direct and accurate determination of the contribution of CYP3A4/5 (fraction metabolized by CYP3A4/5) in the metabolic clearance of new chemical entities.

HIV-1 Sanctuary Sites-the Role of Membrane-Associated Drug Transporters and Drug Metabolic Enzymes

Despite significant advances in the treatment of human immunodeficiency virus-1 (HIV) infection with highly active antiretroviral drug therapy, the persistence of the virus in cellular and anatomic reservoirs is a major obstacle preventing total HIV eradication. Viral persistence could result from a variety of contributing factors including, but not limited to, non-adherence to treatment and adverse drug reactions, latently infected cells carrying replication-competent virus, drug-drug interactions, and inadequate antiretroviral drug (ARV) concentrations reached in several anatomic sites such as the brain, testis, and gut-associated lymphoid tissues. The distribution of ARVs at specific sites of infection is primarily dependent on drug physicochemical properties and drug plasma protein binding, as well as drug efflux, influx, and metabolic processes. A thorough understanding of the functional roles of drug transporters and metabolic enzymes in the disposition of ARVs in immune cell types and tissues that are characterized as HIV reservoirs and sanctuaries is critical to overcome the challenge of suboptimal drug distribution at sites of persistent HIV infection. This review summarizes the current knowledge related to the expression and function of drug transporters and metabolic enzymes in HIV cellular and anatomic reservoirs, and their potential contribution to drug-drug interactions and insufficient drug concentration at these sites.

Ethosome-Derived Invasomes as a Potential Transdermal Delivery System for Vardenafil Hydrochloride: Development, Optimization and Application of Physiologically Based Pharmacokinetic Modeling in Adults and Geriatrics

Aim

The aim of the current work was to develop vardenafil hydrochloride (VRD)-loaded ethosome-derived invasomes as a possible transdermal system which could be used for patients suffering from pulmonary arterial hypertension.

Methods

VRD-loaded ethosomes were developed at three concentrations of phosphatidylcholine (5, 10 and 15 mg/mL) and three percentages of ethanol (20%, 30% and 40%, v/v). The best achieved VRD-loaded ethosomes (ETH9) were optimized to invasomes via incorporation of terpenes (limonene, cineole and a 1:1 mixture) at three concentrations (0.5%, 1% and 2%, v/v). All systems were evaluated for vesicle size, zeta potential, drug entrapment efficiency (EE%), cumulative drug permeated percentages after 0.5hrs (Q_0.5h) and 12hrs (Q_12h) and steady-state flux (J_ss). The optimized system (ETH9-INV8) was further characterized for morphology, histopathology and confocal laser scanning microscopy (CLSM). Physiologically based pharmacokinetic (PBPK) modeling was employed to estimate VRD pharmacokinetic parameters from the optimized transdermal system and an oral aqueous drug dispersion, in adults and geriatrics.

Results

The optimized invasomal system (ETH9-INV8) was characterized with spherical vesicles (159.9 nm) possessing negative zeta potential (-20.3 mV), promising EE% (81.3%), low Q_0.5h (25.4%), high Q_12h (85.3%) and the largest steady-state flux (6.4 µg.cm^-2h^-1). Following a leave-on period of 12hrs in rats, it showed minor histopathologic changes. CLSM studies proved its ability to deeply permeate rat skin. Lower C_max values, delayed T_max estimates and greater AUC_0-24h folds in adults and geriatrics (≈ 2.18 and 1.69, respectively) were estimated following the transdermal application of ETH9-INV8 system.

Conclusion

ETH9-INV8 is a promising transdermal system for VRD.

Nuances to precision dosing strategies of targeted cancer medicines

Selecting the dose of a targeted cancer medicine that is most appropriate for a specific individual is a rational approach to maximize therapeutic outcomes and minimize toxicity. There are many different options for optimizing the dose of targeted cancer medicines and the purpose of this review is to provide a comprehensive comparison of the main options explored in prospective studies. Precision initial dose selection of targeted cancer therapies has been minimally explored to date; however, concentration, toxicity, and therapeutic outcome markers are used to guide on-therapy dose adaption of targeted cancer therapies across several medicines and cancers. While a specific concentration, toxicity, or therapeutic outcome marker commonly dominates an investigated precision on-therapy dose adaption strategy, greater attention to simultaneously account for exposure, toxicity, therapeutic outcomes, disease status, time since treatment initiation and patient preferences are required for optimal patient outcomes. To enable successful implementation of precision dosing strategies for targeted cancer medicines into clinical practice, future prospective studies aiming to develop strategies should consider these elements in their design.

Group-based pharmacogenetic prediction: is it feasible and do current NHS England ethnic classifications provide appropriate data?

Inter-individual variation of drug metabolising enzymes (DMEs) leads to variable efficacy of many drugs and even adverse drug responses. Consequently, it would be desirable to test variants of many DMEs before drug treatment. Inter-ethnic differences in frequency mean that the choice of SNPs to test may vary across population groups. Here we examine the utility of testing representative groups as a way of assessing what variants might be tested. We show that publicly available population information is potentially useful for determining loci for pre-treatment genetic testing, and for determining the most prevalent risk haplotypes in defined groups. However, we also show that the NHS England classifications have limitations for grouping for these purposes, in particular for people of African descent. We conclude: (1) genotyping of hospital patients and people from the hospital catchment area confers no advantage over using samples from appropriate existing ethnic group collections or publicly available data, (2) given the current NHS England Black African grouping, a decision as to whether to test, would have to apply to all patients of recent Black African ancestry to cover reported risk alleles and (3) the current scarcity of available genome and drug effect data from Africans is a problem for both testing and treatment decisions.

In vitro effects of peimine on the activity of cytochrome P450 enzymes

Peimine is a major component of Fritillaria ussuriensis, which is a widely used herb in pediatric. It is very common in Chinese traditional medicine to combine with two or more herbs in the clinic. To investigate the effect of peimine on the activity of cytochrome P450 enzymes (CYP450) is necessary for the clinical application of peimine.The effects of peimine on eight human liver CYP isoforms (i.e., 1A2, 3A4, 2A6, 2E1, 2D6, 2C9, 2C19 and 2C8) were investigated in vitro in human liver microsomes (HLMs) with the specific inhibitors as positive control and without peimine or inhibitors as negative control. The enzyme kinetic parameters were calculated.It was found that peimine inhibited the activity of CYP3A4, 2E1, and 2D6 in a concentration-dependent manner with the IC₅₀ values of 13.43, 21.93, and 22.46 μM, respectively. The inhibition of CYP3A4 was performed in a non-competitive manner with the Ki value of 6.49 μM, and the inhibition of CYP2E1 and 2D6 was performed in a competitive manner with Ki values of 10.76 and 11.95 μM. Additionally, peimine inhibited the activity of CYP3A4 in a time-dependent manner with the K_I/K_inact value of 6.17/0.049 min^-1μM^-1.Peimine inhibited the activity of CYP3A4, 2E1, and 2D6, which indicated the potential interaction between peimine and drugs metabolized by CYP3A4, 2E1, and 2D6. Further studies are needed to verify the drug-drug interaction and the in vivo effects.

In vitro-in vivo extrapolation of metabolic clearance using human liver microsomes: factors showing variability and their normalization

In vitro-in vivo extrapolation (IVIVE) using human liver microsomes has been widely used to predict metabolic clearance, but some of the factors used in the process of prediction show variability for the same compound: notably, microsomal intrinsic clearance values corrected by the unbound fraction (CL_{int, u}), physiological parameters used for scale-up, and the source of in vivo clearance data.The purpose of this study was to assess the correlation between in vitro and in vivo CL_int with a focus on factors showing variability using four cytochrome P450 (CYP)3A substrates.We surveyed in vivo clearance values in literature and also determined the microsomal CL_{int, u} values. A scaling factor (SF_direct) was defined as in vivo CL_int divided by the microsomal CL_{int, u}, which ranged from 1190 to 2310 (mg protein per kg body weight). The application of a mean SF_direct of 1600 (mg protein per kg body weight) and further normalization by the microsomal CL_{int, u} values of midazolam, the most commonly used substrate, resulted in improved prediction accuracy for CL_{int, u} values from various microsomal batches.The results suggest the normalization of variability might be useful for predicting the in vivo CL_int.

Clobetasol Propionate Is a Heme-Mediated Selective Inhibitor of Human Cytochrome P450 3A5

The human cytochrome P450 (CYP) enzymes CYP3A4 and CYP3A5 metabolize most drugs and have high similarities in their structure and substrate preference. Whereas CYP3A4 is predominantly expressed in the liver, CYP3A5 is upregulated in cancer, contributing to drug resistance. Selective inhibitors of CYP3A5 are, therefore, critical to validating it as a therapeutic target. Here we report clobetasol propionate (clobetasol) as a potent and selective CYP3A5 inhibitor identified by high-throughput screening using enzymatic and cell-based assays. Molecular dynamics simulations suggest a close proximity of clobetasol to the heme in CYP3A5 but not in CYP3A4. UV-visible spectroscopy and electron paramagnetic resonance analyses confirmed the formation of an inhibitory type I heme-clobetasol complex in CYP3A5 but not in CYP3A4, thus explaining the CYP3A5 selectivity of clobetasol. Our results provide a structural basis for selective CYP3A5 inhibition, along with mechanistic insights, and highlight clobetasol as an important chemical tool for target validation.

Tacrolimus treatment in childhood refractory nephrotic syndrome: A retrospective study on efficacy, therapeutic drug monitoring, and contributing factors to variable blood tacrolimus levels

Tacrolimus, an immunosuppressive drug, was recommended by the 2012 KDIGO guidelines to treat nephrotic syndrome (NS) in children and adults. However, it has high interpatient pharmacokinetic variability and exposure levels should be monitored, although there are no specified target concentrations. This retrospective study aimed to review efficacy and safety after concomitant treatment with tacrolimus and prednisone, and to identify factors that contribute to the variable blood-trough-concentration-to-dose (C₀/Dose) ratio in children with refractory NS (RNS). A 6-month therapy induced complete or partial remission in 95% of patients. One-year follow-up indicated a high remission rate and low nephrotoxicity. Under maintenance dosages, approximately 95% of the C₀ values were 2-7 ng/mL. Body weight (BW), age, CYP3A5 polymorphisms were the factors affecting the C₀/Dose ratio. The C₀/Dose ratio in patients with a BW of <20 kg was 1.5-fold than that in patients with BW of ≥40 kg. Moreover, the C₀/Dose ratio in patients aged 1-≤6 and 6-≤12 years was significantly lower than that in patients aged 12-≤18 years, by 25% and 48%, respectively. There were no significant association between CYP3A5 genotyping and C₀/Dose ratio in younger children (1-≤6 years), rather than older children (6-≤18 years). In conclusion, routine CYP3A5 genotyping should be considered in children aged over 6 years and exposure levels (C₀) of 2-7 ng/mL may be feasible when tacrolimus is combined with low-dose prednisone to treat childhood RNS.

A dual functional probe for assessing human CYP450 3A5 and 3A enzymes bioactivities

Aim: CYP3A5 plays a vital role in the drug metabolism, it displays varied expression levels among individuals and is easily influenced by genetic polymorphisms and some diseases. Methodology & results: A dual function probe isobutyryl-11-keto-β-boswellic acid (IKBA) was developed; it possessed a high selectivity toward CYP3A5 and CYP3A enzymes for its two individual metabolites, respectively. The probe has the high accuracy and wide applicability in measuring the real activity of CYP3A5. Finally, IKBA was successfully used for the evaluation of the activity of CYP3A5 and CYP3A enzymes in various bio samples. Conclusion: IKBA could serve as a useful tool for exploring the physiology and pathology functions of CYP3A5 and give some useful guidance for the rational use of clinical drugs.

Interindividual Variation in CYP3A Activity Influences Lapatinib Bioactivation

Lapatinib is a dual tyrosine kinase inhibitor associated with rare but potentially severe idiosyncratic hepatotoxicity. We have previously shown that cytochromes P450 CYP3A4 and CYP3A5 quantitatively contribute to lapatinib bioactivation, leading to formation of a reactive, potentially toxic quinone imine. CYP3A5 is highly polymorphic; however, the impact of CYP3A5 polymorphism on lapatinib metabolism has not been fully established. The goal of this study was to determine the effect of CYP3A5 genotype and individual variation in CYP3A activity on the metabolic activation of lapatinib using human-relevant in vitro systems. Lapatinib metabolism was examined using CYP3A5-genotyped human liver microsomes and cryopreserved human hepatocytes. CYP3A and CYP3A5-selective activities were measured in liver tissues using probe substrates midazolam and T-5 (T-1032), respectively, to evaluate the correlation between enzymatic activity and lapatinib metabolite formation. Drug metabolites were measured by high-performance liquid chromatography-tandem mass spectrometry. Further, the relative contributions of CYP3A4 and CYP3A5 to lapatinib O-debenzylation were estimated using selective chemical inhibitors of CYP3A. The results from this study demonstrated that lapatinib O-debenzylation and quinone imine-GSH conjugate formation were highly correlated with hepatic CYP3A activity, as measured by midazolam 1'-hydroxylation. CYP3A4 played a dominant role in lapatinib bioactivation in all liver tissues evaluated. The CYP3A5 contribution to lapatinib bioactivation varied by individual donor and was dependent on CYP3A5 genotype and activity. CYP3A5 contributed approximately 20%-42% to lapatinib O-debenzylation in livers from CYP3A5 expressers. These findings indicate that individual CYP3A activity, not CYP3A5 genotype alone, is a key determinant of lapatinib bioactivation and likely influences exposure to reactive metabolites. SIGNIFICANCE STATEMENT: This study is the first to examine the effect of CYP3A5 genotype, total CYP3A activity, and CYP3A5-selective activity on lapatinib bioactivation in individual human liver tissues. The results of this investigation indicate that lapatinib bioactivation via oxidative O-debenzylation is highly correlated with total hepatic CYP3A activity, and not CYP3A5 genotype alone. These findings provide insight into the individual factors, namely, CYP3A activity, that may affect individual exposure to reactive, potentially toxic metabolites of lapatinib.

Baihe Zhimu formula attenuates the efficacy of tamoxifen against breast cancer in mice through modulation of CYP450 enzymes

Background

Major depression is an important complication in patients with breast cancer, but is an underrecognized and undertreated condition in this population. The Baihe Zhimu Tang (BZ formula) is a traditional Chinese formula consisting of Lilium brownii var. viridulum Baker (L. brownii) and Anemarrhena asphodeloides (A. asphodeloides) Bunge that is used for the treatment of depression. However, the interaction between tamoxifen and BZ formula is frequently overlooked by traditional and alternative medical doctors. In the present study, the influence of BZ formula on the effectiveness of tamoxifen in breast cancer in mice and the effects of tamoxifen on the antidepressant effect of BZ formula and its major components mangiferin and timosaponin BII in mice were investigated.

Methods

Identification of the major components of BZ formula was performed using fast HPLC-tandem mass spectrometry (HPLC-MS/MS). The main flavonoids and saponins in A. asphodeloides were determined by HPLC-UV and HPLC-ELSD, separately. The antidepressant efficacy of BZ formula was evaluated using a mouse tail-suspension test. The effects of BZ formula on the antineoplastic activity of tamoxifen were performed in a mouse xenograft model of human breast cancer MCF-7 cells. P450 activity was determined using microsomal incubations by HPLC-MS/MS. Measurement of serum concentrations of tamoxifen and its metabolites was used by HPLC-MS/MS.

Results

BZ formula attenuated the effectiveness of tamoxifen treatment of breast cancer and reduced the concentrations of endoxifen and 4-OH-tamoxifen in tumor-bearing mice. Of two of the major components of BZ formula, the antidepressant effect of mangiferin, but not timosaponin BII, was significantly inhibited by tamoxifen in mice. BZ formula and its component mangiferin also significantly inhibited CYP450 enzyme activity in rat liver microsomes.

Conclusion

BZ formula attenuated the effectiveness of tamoxifen in treatment of breast cancer in mice by influencing CYP450 enzymes. The present study laid a foundation for the treatment of patients with breast cancer and depression by BZ formula or other Chinese herbal formulas containing A. asphodeloides.

A Pediatric Covariate Function for CYP3A-Mediated Midazolam Clearance Can Scale Clearance of Selected CYP3A Substrates in Children

Recently a framework was presented to assess whether pediatric covariate models for clearance can be extrapolated between drugs sharing elimination pathways, based on extraction ratio, protein binding, and other drug properties. Here we evaluate when a pediatric covariate function for midazolam clearance can be used to scale clearance of other CYP3A substrates. A population PK model including a covariate function for clearance was developed for midazolam in children aged 1–17 years. Commonly used CYP3A substrates were selected and using the framework, it was assessed whether the midazolam covariate function accurately scales their clearance. For eight substrates, reported pediatric clearance values were compared numerically and graphically with clearance values scaled using the midazolam covariate function. For sildenafil, clearance values obtained with population PK modeling based on pediatric concentration-time data were compared with those scaled with the midazolam covariate function. According to the framework, a midazolam covariate function will lead to systemically accurate clearance scaling (absolute prediction error (PE) < 30%) for CYP3A substrates binding to albumin with an extraction ratio between 0.35 and 0.65 when binding < 10% in adults, between 0.05 and 0.55 when binding > 90%, and with an extraction ratio ranging between these values when binding between 10 and 90%. Scaled clearance values for eight commonly used CYP3A substrates were reasonably accurate (PE < 50%). Scaling of sildenafil clearance was accurate (PE < 30%). We defined for which CYP3A substrates a pediatric covariate function for midazolam clearance can accurately scale plasma clearance in children. This scaling approach may be useful for CYP3A substrates with scarce or no available pediatric PK information.

Identification of the caffeine to trimethyluric acid ratio as a dietary biomarker to characterise variability in cytochrome P450 3A activity

PURPOSE

Cytochrome P450 (CYP) 3A plays an important role in the metabolism of many clinically used drugs and exhibits substantial between-subject variability (BSV) in activity. Current methods to assess variability in CYP3A activity have limitations and there remains a need for a minimally invasive clinically translatable strategy to define CYP3A activity. The purpose of this study was to evaluate the potential for a caffeine metabolic ratio to describe variability in CYP3A activity.

METHODS

The metabolic ratio 1,3,7-trimethyluric acid (TMU) to caffeine was evaluated as a biomarker to describe variability in CYP3A activity in a cohort (n = 28) of healthy 21 to 35-year-old males. Midazolam, caffeine, and TMU concentrations were assessed at baseline and following dosing of rifampicin (300 mg daily) for 7 days.

RESULTS

At baseline, correlation coefficients for the relationship between apparent oral midazolam clearance (CL/F) with caffeine/TMU ratio measured at 3, 4, and 6 h post dose were 0.82, 0.79, and 0.65, respectively. The strength of correlations was retained post rifampicin dosing; 0.72, 0.87, and 0.82 for the ratios at 3, 4, and 6 h, respectively. Weaker correlations were observed between the change in midazolam CL/F and change in caffeine/TMU ratio post/pre-rifampicin dosing.

CONCLUSION

BSV in CYP3A activity was well described by caffeine/TMU ratios pre- and post-induction. The caffeine/TMU ratio may be a convenient tool to assess BSV in CYP3A activity, but assessment of caffeine/TMU ratio alone is unlikely to account for all sources of variability in CYP3A activity.

Active-site differences between substrate-free and ritonavir-bound cytochrome P450 (CYP) 3A5 reveal plasticity differences between CYP3A5 and CYP3A4

Cytochrome P450 (CYP) 3A4 is a major contributor to hepatic drug and xenobiotic metabolism in human adults. The related enzyme CYP3A5 is also expressed in adult liver and has broader age and tissue distributions. However, CYP3A5 expression is low in most Caucasians because of the prevalence of an allele that leads to an incorrectly spliced mRNA and premature termination of translation. When expressed, CYP3A5 expands metabolic capabilities and can augment CYP3A4-mediated drug metabolism, thereby reducing drug efficacy and potentially requiring dose adjustments. The extensive role of CYP3A4 in drug metabolism reflects in part the plasticity of the substrate-free enzyme to enlarge its active site and accommodate very large substrates. We have previously shown that the structure of the CYP3A5-ritonavir complex differs substantially from that of the CYP3A4-ritonavir complex. To better understand whether these differences are conserved in other CYP3A5 structures and how they relate to differential plasticity, we determined the X-ray crystallographic structure of the CYP3A5 substrate-free complex to 2.20 Å resolution. We observed that this structure exhibits a much larger active site than substrate-free CYP3A4 and displays an open substrate access channel. This reflected in part a lower trajectory of the helix F-F' connector in CYP3A4 and more extensive π-CH interactions between phenylalanine residues forming the roof of the active-site cavity than in CYP3A5. Comparison with the CYP3A5-ritonavir complex confirmed conserved CYP3A5 structural features and indicated differences in plasticity between CYP3A4 and CYP3A5 that favor alternative ritonavir conformations.

CYP3A4/5 Activity Probed with Testosterone and Midazolam: Correlation between Two Substrates at the Microsomal and Enzyme Levels

Testosterone (TST) and midazolam (MDZ) are widely used as probes to detect CYP3A4/5 activity, but the data acquired with these two substrates do not correlate well at the microsomal level (per milligram of microsomal protein), and the reason is unclear. In this study, CYP3A4/5 activity was probed with TST and MDZ at the microsomal and enzyme levels (per picomole of CYP3A4/5) in 72 human liver samples. Correlation coefficients were lower in V_max and CL_int at the microsomal level, as compared with those at the enzyme level ( V_max 0.658 vs 0.883; CL_int no correlation vs 0.796). Compared with TST, MDZ was found to correlate better with the content of CYP3A4/5 (no correlation vs 0.431) and CYP3A5 (no correlation vs 0.447), and huge variations in enzyme content existed among different genotypes, which explained the lower degree of correlation at the microsomal level. In addition, different genotypes had varying effects on activity at the enzyme level, whereas the difference between activity at the enzyme level probed with TST and that probed with MDZ was not obvious ( P > 0.05), indicating that the effect of gene polymorphisms on correlation between activity probed with these two substrates was limited at the enzyme level. In conclusion, our study demonstrates a high degree of correlation between CYP3A4/5 activity probed with TST and MDZ at the enzyme level but not at the microsomal level and allows us to correctly understand the influence of gene polymorphisms on the correlations.