HLM chip - A microfluidic approach to study the mechanistic basis of cytochrome P450 inhibition using immobilized human liver microsomes

Cytochrome P450 (CYP) system is a critical elimination route to most pharmaceuticals in human, but also prone to drug-drug interactions arising from the fact that concomitantly administered pharmaceuticals inhibit one another's CYP metabolism. The most severe form of CYP interactions is irreversible inhibition, which results in permanent inactivation of the critical CYP pathway and is only restored by de novo synthesis of new functional enzymes. In this study, we conceptualize a microfluidic approach to mechanistic CYP inhibition studies using human liver microsomes (HLMs) immobilized onto the walls of a polymer micropillar array. We evaluated the feasibility of these HLM chips for CYP inhibition studies by establishing the stability and the enzyme kinetics for a CYP2C9 model reaction under microfluidic flow and determining the half-maximal inhibitory concentrations (IC50) of three human CYP2C9 inhibitors (sulfaphenazole, tienilic acid, miconazole), including evaluation of their inhibition mechanisms and nonspecific microsomal binding on chip. Overall, the enzyme kinetics of CYP2C9 metabolism on the HLM chip (KM = 127 ± 55 µM) was shown to be similar to that of static HLM incubations (KM = 114 ± 14 µM) and the IC50 values toward CYP2C9 derived from the microfluidic assays (sulfaphenazole 0.38 ± 0.09 µM, tienilic acid 3.4 ± 0.6 µM, miconazole 0.54 ± 0.09 µM) correlated well with those determined using current standard IC50 shift assays. Most importantly, the HLM chip could distinguish between reversible (sulfaphenazole) and irreversible (tienilic acid) enzyme inhibitors in a single, automated experiment, indicating the great potential of the HLM chip to simplify current workflows used in mechanistic CYP inhibition studies. Furthermore, the results suggest that the HLM chip can also identify irreversible enzyme inhibitors, which are not necessarily resulting in a time-dependent inhibition (like suicide inhibitors), but whose inhibition mechanism is based on other kind of covalent or irreversible interaction with the CYP system. With our HLM chip approach, we could identify miconazole as such a compound that nonselectively inhibits the human CYP system with a prolonged, possibly irreversible impact in vitro, even if it is not a time-dependent inhibitor according to the IC50 shift assay.

Metabolic activation and cytochrome P450 inhibition of piperlonguminine mediated by CYP3A4

Piperlonguminine (PLG) is a major alkaloid found in Piper longum fruits. It has been shown to possess a variety of biological activities, including anti-tumor, anti-hyperlipidemic, anti-renal fibrosis and anti-inflammatory properties. Previous studies have reported that PLG inhibits various CYP450 enzymes. The main objective of this study was to identify reactive metabolites of PLG in vitro and assess its ability to inhibit CYP450. In rat and human liver microsomal incubation systems exposed to PLG, two oxidized metabolites (M1 and M2) were detected. Additionally, in microsomes where N-acetylcysteine was used as a trapping agent, N-acetylcysteine conjugates (M3, M4, M5 and M6) of four isomeric O-quinone-derived reactive metabolites were found. The formation of metabolites was dependent on NADPH. Inhibition and recombinant CYP450 enzyme incubation experiments showed that CYP3A4 was the primary enzyme responsible for the metabolic activation of PLG. This study characterized the O-dealkylated metabolite (M1) through chemical synthesis. The IC50 shift assay showed time-dependent inhibition of CYP3A4, 2C9, 2E1, 2C8 and 2D6 by PLG. This research contributes to the understanding of PLG-induced enzyme inhibition and bioactivation.

Consideration of Nevirapine Analogs To Reduce Metabolically Linked Hepatotoxicity: A Cautionary Tale of the Deuteration Approach

Idiosyncratic drug reactions (IDRs) in their most deleterious form can lead to serious medical complications and potentially fatal events. Nevirapine (NVP), still widely used in developing countries for combinatorial antiretroviral and prophylactic therapies against HIV infection, represents a prototypical example of IDRs causing severe skin rashes and hepatotoxicity. Complex metabolic pathways accompanied by production of multiple reactive metabolites often complicate our understanding of IDR's origin. While assessment of NVP analogs has helped characterize the pathways involved in IDRs for NVP, which are largely driven by metabolism at the 12-methyl position, it has yet to be investigated if some of these analogs could be valuable replacement drugs with reduced reactive metabolite properties and drug-drug interaction (DDI) risks. Here, we evaluated a set of eight NVP analogs, including the deuterated 12-d3-NVP and two NVP metabolites, for their efficacy and inhibitory potencies against HIV reverse transcriptase (HIV-RT). A subset of three analogs, demonstrating >85% inhibition for HIV-RT, was further assessed for their hepatic CYP induction-driven DDI risks. This led to a closer investigation of the inactivation properties of 12-d3-NVP for hepatic CYP3A4 and a comparison of its propensity in generating reactive metabolite species. The metabolic shift triggered with 12-d3-NVP, increasing formation of the 2-hydroxy and glutathione metabolites, emphasized the importance of the dynamic balance between induction and metabolism-dependent inactivation of CYP3A4 and its impact on clearance of NVP during treatment. Unfortunately, the strategy of incorporating deuterium to reduce NVP metabolism and production of the electrophile species elicited opposite results, illustrating the great challenges involved in tackling IDRs through deuteration.

The impact of CYP2C19 genotype on phenoconversion by concomitant medication

Introduction: Pharmacogenetics-informed drug prescribing is increasingly applied in clinical practice. Typically, drug metabolizing phenotypes are determined based on genetic test results, whereupon dosage or drugs are adjusted. Drug-drug-interactions (DDIs) caused by concomitant medication can however cause mismatches between predicted and observed phenotypes (phenoconversion). Here we investigated the impact of CYP2C19 genotype on the outcome of CYP2C19-dependent DDIs in human liver microsomes. Methods: Liver samples from 40 patients were included, and genotyped for CYP2C19*2, *3 and *17 variants. S-mephenytoin metabolism in microsomal fractions was used as proxy for CYP2C19 activity, and concordance between genotype-predicted and observed CYP2C19 phenotype was examined. Individual microsomes were subsequently co-exposed to fluvoxamine, voriconazole, omeprazole or pantoprazole to simulate DDIs. Results: Maximal CYP2C19 activity (V_max) in genotype-predicted intermediate metabolizers (IMs; *1/*2 or *2/*17), rapid metabolizers (RMs; *1/*17) and ultrarapid metabolizers (UMs; *17/*17) was not different from V_max of predicted normal metabolizers (NMs; *1/*1). Conversely, CYP2C19*2/*2 genotyped-donors exhibited V_max rates ∼9% of NMs, confirming the genotype-predicted poor metabolizer (PM) phenotype. Categorizing CYP2C19 activity, we found a 40% concordance between genetically-predicted CYP2C19 phenotypes and measured phenotypes, indicating substantial phenoconversion. Eight patients (20%) exhibited CYP2C19 IM/PM phenotypes that were not predicted by their CYP2C19 genotype, of which six could be linked to the presence of diabetes or liver disease. In subsequent DDI experiments, CYP2C19 activity was inhibited by omeprazole (-37% ± 8%), voriconazole (-59% ± 4%) and fluvoxamine (-85% ± 2%), but not by pantoprazole (-2 ± 4%). The strength of CYP2C19 inhibitors remained unaffected by CYP2C19 genotype, as similar percental declines in CYP2C19 activity and comparable metabolism-dependent inhibitory constants (K_inact/K_I) of omeprazole were observed between CYP2C19 genotypes. However, the consequences of CYP2C19 inhibitor-mediated phenoconversion were different between CYP2C19 genotypes. In example, voriconazole converted 50% of *1/*1 donors to a IM/PM phenotype, but only 14% of *1/*17 donors. Fluvoxamine converted all donors to phenotypic IMs/PMs, but *1/*17 (14%) were less likely to become PMs than *1/*1 (50%) or *1/*2 and *2/*17 (57%). Conclusion: This study suggests that the differential outcome of CYP2C19-mediated DDIs between genotypes are primarily dictated by basal CYP2C19 activity, that may in part be predicted by CYP2C19 genotype but likely also depends on disease-related factors.

In vitro metabolic characterization of the SARS-CoV-2 papain-like protease inhibitors GRL0617 and HY-17542

The SARS-CoV-2 pandemic requires a new therapeutic target for viral infection, and papain-like protease (Plpro) has been suggested as a druggable target. This in-vitro study was conducted to examine the drug metabolism of the GRL0617 and HY-17542, Plpro inhibitors. Metabolism of these inhibitors was studied to predict the pharmacokinetics in human liver microsomes. The hepatic cytochrome P450 (CYP) isoforms responsible for their metabolism were identified using recombinant enzymes. The drug-drug interaction potential mediated by cytochrome P450 inhibition was estimated. In human liver microsomes, the Plpro inhibitors had phase I and phase I + II metabolism with half-lives of 26.35 and 29.53 min, respectively. Hydroxylation (M1) and desaturation (-H2, M3) of the para-amino toluene side chain were the predominant reactions mediated with CYP3A4 and CYP3A5. CYP2D6 is responsible for the hydroxylation of the naphthalene side ring. GRL0617 inhibits major drug-metabolizing enzymes, including CYP2C9 and CYP3A4. HY-17542 is structural analog of GRL0617 and it is metabolized to GRL0617 through non-cytochrome P450 reactions in human liver microsomes without NADPH. Like GRL0617 and HY-17542 undergoes additional hepatic metabolism. The in-vitro hepatic metabolism of the Plpro inhibitors featured short half-lives; preclinical metabolism studies are needed to determine therapeutic doses for these inhibitors.

Towards the Elucidation of the Pharmacokinetics of Voriconazole: A Quantitative Characterization of Its Metabolism

The small-molecule drug voriconazole (VRC) shows a complex and not yet fully understood metabolism. Consequently, its in vivo pharmacokinetics are challenging to predict, leading to therapy failures or adverse events. Thus, a quantitative in vitro characterization of the metabolism and inhibition properties of VRC for human CYP enzymes was aimed for. The Michaelis-Menten kinetics of voriconazole N-oxide (NO) formation, the major circulating metabolite, by CYP2C19, CYP2C9 and CYP3A4, was determined in incubations of human recombinant CYP enzymes and liver and intestine microsomes. The contribution of the individual enzymes to NO formation was 63.1% CYP2C19, 13.4% CYP2C9 and 29.5% CYP3A4 as determined by specific CYP inhibition in microsomes and intersystem extrapolation factors. The type of inhibition and inhibitory potential of VRC, NO and hydroxyvoriconazole (OH-VRC), emerging to be formed independently of CYP enzymes, were evaluated by their effects on CYP marker reactions. Time-independent inhibition by VRC, NO and OH-VRC was observed on all three enzymes with NO being the weakest and VRC and OH-VRC being comparably strong inhibitors of CYP2C9 and CYP3A4. CYP2C19 was significantly inhibited by VRC only. Overall, the quantitative in vitro evaluations of the metabolism contributed to the elucidation of the pharmacokinetics of VRC and provided a basis for physiologically-based pharmacokinetic modeling and thus VRC treatment optimization.

In Vitro Inhibitory Effects of Agarwood Tea (Aquilaria malaccensis Lamk) Aqueous Extract on Human Cytochrome P450 (CYP) Enzyme Activities

BACKGROUND

Agarwood tea derived from Aquilaria malaccensis Lamk is becoming an increasingly popular herbal drink that is said to have multiple health benefits. Co-administration of this tea and clinical used drugs is possible, but it increases the risk of drug-herb interactions.

OBJECTIVE

This in vitro study investigated the inhibitory effects of agarwood tea aqueous extract on the eight major human drug-metabolising cytochrome P450 (CYP) enzyme activities.

METHODS

High-throughput fluorescence-based Vivid® CYP450 screening kits were employed to obtain the enzyme activities before and after incubation with agarwood tea aqueous extract.

RESULTS

Agarwood aqueous extract potently inhibited CYP2C9, CYP2D6, and CYP3A4 activities with Ki values of 5.1, 34.5, and 20.3μg/ml, respectively. The most likely inhibition mode responsible for these inhibitions was non-competitive inhibition. On the other hand, at 1000μg/ml, agarwood tea aqueous extract negligibly inhibited CYP1A2, CYP2B6, CYP2C19, CYP2E1, and CYP3A5 activities.

CONCLUSION

These findings can be used to design additional in vitro investigations using clinical relevant drug substrates for CYP2C9, CYP2D6, and CYP3A4. Subsequently, future studies can be conducted to determine potential interactions between agarwood tea aqueous extract and CYP using in vivo models.

Cannabinoid Metabolites as Inhibitors of Major Hepatic CYP450 Enzymes, with Implications for Cannabis-Drug Interactions

The legalization of cannabis in many parts of the United States and other countries has led to a need for a more comprehensive understanding of cannabis constituents and their potential for drug-drug interactions. Although (-)-trans-Δ9-tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN) are the most abundant cannabinoids present in cannabis, THC metabolites are found in plasma at higher concentrations and for a longer duration than that of the parent cannabinoids. To understand the potential for drug-drug interactions, the inhibition potential of major cannabinoids and their metabolites on major hepatic cytochrome P450 (P450) enzymes was examined. In vitro assays with P450-overexpressing cell microsomes demonstrated that the major THC metabolites 11-hydroxy-Δ9-tetra-hydrocannabinol and 11-nor-9-carboxy-Δ9-THC-glucuronide competitively inhibited several major P450 enzymes, including CYP2B6, CYP2C9, and CYP2D6 (apparent Ki,u values = 0.086 ± 0.066 µM and 0.90 ± 0.54 µM, 0.057 ± 0.044 µM and 2.1 ± 0.81 µM, 0.15 ± 0.067 µM and 2.3 ± 0.54 µM, respectively). 11-Nor-9-carboxy-Δ9- tetrahydrocannabinol exhibited no inhibitory activity against any CYP450 tested. THC competitively inhibited CYP1A2, CYP2B6, CYP2C9, and CYP2D6; CBD competitively inhibited CYP3A4, CYP2B6, CYP2C9, CYP2D6, and CYP2E1; and CBN competitively inhibited CYP2B6, CYP2C9, and CYP2E1. THC and CBD showed mixed-type inhibition for CYP2C19 and CYP1A2, respectively. These data suggest that cannabinoids and major THC metabolites are able to inhibit the activities of multiple P450 enzymes, and basic static modeling of these data suggest the possibility of pharmacokinetic interactions between these cannabinoids and xenobiotics extensively metabolized by CYP2B6, CYP2C9, and CYP2D6. SIGNIFICANCE STATEMENT: Major cannabinoids and their metabolites found in the plasma of cannabis users inhibit several P450 enzymes, including CYP2B6, CYP2C9, and CYP2D6. This study is the first to show the inhibition potential of the most abundant plasma cannabinoid metabolite, THC-COO-Gluc, and suggests that circulating metabolites of cannabinoids play an essential role in CYP450 enzyme inhibition as well as drug-drug interactions.

Cytochrome P450 enzymes: a review on drug metabolizing enzyme inhibition studies in drug discovery and development

Assessment of drug candidate's potential to inhibit cytochrome P450 (CYP) enzymes remains crucial in pharmaceutical drug discovery and development. Both direct and time-dependent inhibition of drug metabolizing CYP enzymes by the concomitant administered drug is the leading cause of drug-drug interactions (DDIs), resulting in the increased toxicity of the victim drug. In this context, pharmaceutical companies have grown increasingly diligent in limiting CYP inhibition liabilities of drug candidates in the early stages and examining risk assessments throughout the drug development process. This review discusses different strategies and decision-making processes for assessing the drug-drug interaction risks by enzyme inhibition and lays particular emphasis on in vitro study designs and interpretation of CYP inhibition data in a stage-appropriate context.

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.

Metabolic Activation of Aegeline Mediated by CYP2C19

1. Aegeline (AGL) is a natural alkaloidal amide mainly isolated from the leaves and fruits of tropical plant Aegle marmelos, with multiple pharmacological activities.2. As one component of several dietary supplements, AGL caused a series of acute and chronic liver injuries. Nevertheless, the mechanisms of AGL-induced hepatotoxicity remain unclear. This study was conducted to identify reactive metabolite(s), to determine related metabolic pathways, and define the possible association of the bioactivation with AGL cytotoxicity.3. A demethylation metabolite (M1) and a GSH conjugate (M2) were detected in rat liver microsomal incubations containing AGL and GSH. The two metabolites were both found in bile of rats and rat primary hepatocytes after AGL administration.4. Recombinant P450 enzyme incubations showed that CYP2C19 was the principal enzyme catalyzing this metabolic activation.5. Ticlopidine, a selective inhibitor of CYP2C19, decreased the formation of M1 and M2 in hepatocytes and attenuated the susceptibility of hepatocytes to the cytotoxicity of AGL. The results suggest that AGL was metabolized to a p-quinone methide intermediate which could in part participate in AGL-induced cytotoxicity.

Assessment of the in vitro cytochrome P450 (CYP) inhibition potential of nafithromycin, a next generation lactone ketolide antibiotic

Nafithromycin is a next generation lactone ketolide antibiotic slated to enter phase III clinical development in India for the treatment of CABP as a shorter 800 mg-OD X3 day therapeutic regimen. Nafithromycin exhibits potent activity against MDR Streptococcus pneumoniae including erythromycin and telithromycin-resistant resistant strains. Older macrolides/ketolides are reported to be potent inhibitors of CYP3A4/5. To facilitate comparative assessment of drug-drug interaction potential, CYP inhibitory activities of nafithromycin was evaluated in comparison with clarithromycin, telithromycin, cethromycin and solithromycin. CYP inhibitory activities were assessed against key CYP isoforms (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6 and CYP3A4/5) using human liver microsomes. Additionally, time-dependent inhibition (TDI), metabolism-based inhibition (MBI) and k _inact /K _I activities were also investigated for CYP3A4/5. Nafithromycin did not inhibit key CYP enzymes and was found to be a weak inhibitor of CYP3A4/5. Comparator antibiotics were found to be potent inhibitors with 2- to 50-fold leftward shifts in CYP3A4/5 IC50 values, while such shift was not noted for nafithromycin. k _inact /K _I ratio of nafithromycin was 3- to 153-fold lower than comparator drugs, further substantiating its lower affinity for CYP3A4/5. In sum, weaker inhibition and lower k _inact /K _I ratio for CYP3A4/5, points towards nafithromycin's lower propensities towards clinical drug-drug interactions as compared to other macrolides/ketolides antibiotics.

CYP4F13 is the Major Enzyme for Conversion of alpha-Eleostearic Acid into cis-9, trans-11-Conjugated Linoleic Acid in Mouse Hepatic Microsomes

Our previous studies have shown that α-eleostearic acid (α-ESA; cis-9, trans-11, trans-13 (c9,t11,t13)-conjugated linolenic acid (CLnA)) is converted into c9,t11-conjugated linoleic acid (CLA) in rats. Furthermore, we have demonstrated that the conversion of α-ESA into CLA is a nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enzymatic reaction, which occurs mostly in the rat liver. However, the precise metabolic pathway and enzyme involved have not been identified yet. Therefore, in this study we aimed to determine the role of cytochrome P450 (CYP) in the conversion of α-ESA into c9,t11-CLA using an in vitro reconstitution system containing mouse hepatic microsomes, NADPH, and α-ESA. The CYP4 inhibitors, 17-ODYA and HET0016, performed the highest level of inhibition of CLA formation. Furthermore, the redox partner cytochrome P450 reductase (CPR) inhibitor, 2-chloroethyl ethyl sulfide (CEES), also demonstrated a high level of inhibition. Thus, these results indicate that the NADPH-dependent CPR/CYP4 system is responsible for CLA formation. In a correlation analysis between the specific activity of CLA formation and Cyp4 family gene expression in tissues, Cyp4a14 and Cyp4f13 demonstrated the best correlations. However, the CYP4F substrate prostaglandin A₁ (PGA₁) exhibited the strongest inhibitory effect on CLA formation, while the CYP4A and CYP4B1 substrate lauric acid had no inhibitory effect. Therefore, we conclude that the CYP4F13 enzyme is the major enzyme involved in CLA formation. This pathway is a novel pathway for endogenous CLA synthesis, and this study provides insight into the potential application of CLnA in functional foods.

Refined Prediction of Pharmacokinetic Kratom-Drug Interactions: Time-Dependent Inhibition Considerations

Preparations from the leaves of the kratom plant (Mitragyna speciosa) are consumed for their opioid-like effects. Several deaths have been associated with kratom used concomitantly with some drugs. Pharmacokinetic interactions are potential underlying mechanisms of these fatalities. Accumulating in vitro evidence has demonstrated select kratom alkaloids, including the abundant indole alkaloid mitragynine, as reversible inhibitors of several cytochromes P450 (CYPs). The objective of this work was to refine the mechanistic understanding of potential kratom-drug interactions by considering both reversible and time-dependent inhibition (TDI) of CYPs in the liver and intestine. Mitragynine was tested against CYP2C9 (diclofenac 4'-hydroxylation), CYP2D6 (dextromethorphan O-demethylation), and CYP3A (midazolam 1'-hydroxylation) activities in human liver microsomes (HLMs) and CYP3A activity in human intestinal microsomes (HIMs). Comparing the absence to presence of NADPH during preincubation of mitragynine with HLMs or HIMs, an ∼7-fold leftward shift in IC₅₀ (∼20 to 3 μM) toward CYP3A resulted, prompting determination of TDI parameters (HLMs: K _I , 4.1 ± 0.9 μM; k _inact , 0.068 ± 0.01 min^-1; HIMs: K _I , 4.2 ± 2.5 μM; k _inact , 0.079 ± 0.02 min^-1). Mitragynine caused no leftward shift in IC₅₀ toward CYP2C9 (∼40 μM) and CYP2D6 (∼1 μM) but was a strong competitive inhibitor of CYP2D6 (K _i , 1.17 ± 0.07 μM). Using a recommended mechanistic static model, mitragynine (2-g kratom dose) was predicted to increase dextromethorphan and midazolam area under the plasma concentration-time curve by 1.06- and 5.69-fold, respectively. The predicted midazolam area under the plasma concentration-time curve ratio exceeded the recommended cutoff (1.25), which would have been missed if TDI was not considered. SIGNIFICANCE STATEMENT: Kratom, a botanical natural product increasingly consumed for its opioid-like effects, may precipitate potentially serious pharmacokinetic interactions with drugs. The abundant kratom indole alkaloid mitragynine was shown to be a time-dependent inhibitor of hepatic and intestinal cytochrome P450 3A activity. A mechanistic static model predicted mitragynine to increase systemic exposure to the probe drug substrate midazolam by 5.7-fold, necessitating further evaluation via dynamic models and clinical assessment to advance the understanding of consumer safety associated with kratom use.

Lack of Correlation between In Vitro and In Vivo Studies on the Inhibitory Effects of (‒)-Sophoranone on CYP2C9 is Attributable to Low Oral Absorption and Extensive Plasma Protein Binding of (‒)-Sophoranone

(‒)-Sophoranone (SPN) is a bioactive component of Sophora tonkinensis with various pharmacological activities. This study aims to evaluate its in vitro and in vivo inhibitory potential against the nine major CYP enzymes. Of the nine tested CYPs, it exerted the strongest inhibitory effect on CYP2C9-mediated tolbutamide 4-hydroxylation with the lowest IC₅₀ (K_i) value of 0.966 ± 0.149 μM (0.503 ± 0.0383 μM), in a competitive manner. Additionally, it strongly inhibited other CYP2C9-catalyzed diclofenac 4′-hydroxylation and losartan oxidation activities. Upon 30 min pre-incubation of human liver microsomes with SPN in the presence of NADPH, no obvious shift in IC₅₀ was observed, suggesting that SPN is not a time-dependent inactivator of the nine CYPs. However, oral co-administration of SPN had no significant effect on the pharmacokinetics of diclofenac and 4′-hydroxydiclofenac in rats. Overall, SPN is a potent inhibitor of CYP2C9 in vitro but not in vivo. The very low permeability of SPN in Caco-2 cells (P_app value of 0.115 × 10⁻⁶ cm/s), which suggests poor absorption in vivo, and its high degree of plasma protein binding (>99.9%) may lead to the lack of in vitro–in vivo correlation. These findings will be helpful for the safe and effective clinical use of SPN.

Time-dependent enzyme inactivation: Numerical analyses of in vitro data and prediction of drug-drug interactions

Cytochrome P450 (CYP) enzyme kinetics often do not conform to Michaelis-Menten assumptions, and time-dependent inactivation (TDI) of CYPs displays complexities such as multiple substrate binding, partial inactivation, quasi-irreversible inactivation, and sequential metabolism. Additionally, in vitro experimental issues such as lipid partitioning, enzyme concentrations, and inactivator depletion can further complicate the parameterization of in vitro TDI. The traditional replot method used to analyze in vitro TDI datasets is unable to handle complexities in CYP kinetics, and numerical approaches using ordinary differential equations of the kinetic schemes offer several advantages. Improvement in the parameterization of CYP in vitro kinetics has the potential to improve prediction of clinical drug-drug interactions (DDIs). This manuscript discusses various complexities in TDI kinetics of CYPs, and numerical approaches to model these complexities. The extrapolation of CYP in vitro TDI parameters to predict in vivo DDIs with static and dynamic modeling is discussed, along with a discussion on current gaps in knowledge and future directions to improve the prediction of DDI with in vitro data for CYP catalyzed drug metabolism.

Propranolol is a mechanism-based inhibitor of CYP2D and CYP2D6 in humanized CYP2D6-transgenic mice: Effects on activity and drug responses

BACKGROUND AND PURPOSE

Genetics and drug interactions contribute to large interindividual variation in human CYP2D6 activity. Here, we have characterized propranolol inhibition of human and mouse CYP2D using transgenic (TG) mice, which express both mouse CYP2D and human CYP2D6, and wild-type (WT) mice. Our purpose was to develop a method for in vivo manipulation of CYP2D6 enzyme activity which could be used to investigate the role of CYP2D6 in drug-induced behaviours.

EXPERIMENTAL APPROACH

Dextromethorphan metabolism to dextrorphan was used to measure CYP2D activity and to characterize propranolol inhibition in vitro and in vivo. Effects of propranolol pretreatment (24 hr) on serum levels of the CYP2D6 substrate haloperidol and haloperidol-induced catalepsy were also studied.

KEY RESULTS

Dextrorphan formation velocity in vitro was threefold higher in liver microsomes of TG compared to WT mice. Propranolol acted as a mechanism-based inhibitor (MBI), inactivating CYP2D in liver microsomes from TG and WT mice, and humans. Pretreatment (24 hr) of TG and WT mice with 20 mg·kg^-1 intraperitoneal propranolol reduced dextrorphan formation in vivo and by liver microsomes in vitro. Serum haloperidol levels and catalepsy were increased.

CONCLUSIONS AND IMPLICATIONS

Propranolol was a potent MBI of dextrorphan formation in liver microsomes from TG and WT mice, and humans. The inhibition parameters in TG overlapped with those in WT mice and in humans. Inhibition of CYP2D with propranolol in vivo in TG and WT mice altered drug responses, allowing further investigation of variations in CYP2D6 on drug interactions and drug responses.

Dimensionally Enhanced Antibacterial Library Screening

The emergence and spread of antimicrobial resistance is a major public health threat, and there is an urgent need to develop new strategies to address the issue. In this study, the possibility of enhancing a whole cell based antibacterial library screen by increasing the dimensionality of the screening effort is explored using methicillin-resistant Staphylococcus aureus (MRSA) as the target organism. One dimension involved generating and screening a human liver microsome metabolized FDA approved drug library. Comparative screening of the un-metabolized (UM) and pre-metabolized (PM) libraries allows identification of intrinsically active agents from the UM library screen and of agents with active metabolites from the PM library screen. To further enhance this screening effort, it was combined with a -/+ resistant to antibiotic screen (-/+ cefoxitin; Cef). This allows the identification of agents that can act synergistically with the resistant to antibiotic. This approach revealed five compounds with substantially improved activity after metabolism and four compounds with substantial synergistic activity with cefoxitin. Capecitabine in particular only had significant antibacterial activity after metabolism. Its metabolites were isolated, identified, and characterized for spectrum of activity along with several other anticancer drugs with anti-MRSA activity. Floxuridine, gemcitabine, novobiocin, and rifaximin were identified as having substantial synergy with cefoxitin from the -/+Cef screens. Checkerboard assays verified synergy for these agents. Floxuridine demonstrated a particularly high degree of synergy with cefoxitin (FIC = 0.14). This study demonstrates how a dimensionally enhanced comparative screening effort can identify new antibacterial agents and strategies for countering antibacterial agent resistance.

Piperine Is a Mechanism-Based Inactivator of CYP3A

Piperine (PPR) is the representative alkaloid component of the piper species (family: Piperaceae). Our rapid screening study found PPR caused time-dependent inhibition of cytochrome P450s (CYP) 3A and 2D6, and CYP3A was inactivated the most. Further study demonstrated that PPR is a time-, concentration-, and NADPH-dependent inhibitor of CYP3A, and significant loss (49.5% ± 3.9%) of CYP3A activity was observed after 20minute incubations with 80 μM PPR at 37°C. The values of K _I and k _inact were 30.7 μM and 0.041 minutes^-1, respectively. CYP3A competitive inhibitor ketoconazole showed protective effect against the enzyme inactivation. Superoxide dismutase/catalase and GSH displayed minor protection against the PPR-caused enzyme inactivation. Ferricyanide partially reduced the enzyme inhibition by PPR. Additionally, NADPH-dependent formation of reactive metabolites from PPR were found in human liver microsomal incubation mixtures. An ortho-quinone intermediate was trapped by NAC in microsomal incubations with PPR. DM-PPR, demethylene metabolite of PPR, showed weak enzyme inactivation relative to that caused by PPR. It appears that both carbene and ortho-quinone intermediates were involved in the inactivation of CYP3A caused by PPR. SIGNIFICANCE STATEMENT: CYP3A subfamily members (mainly CYP3A4 and CYP3A5) play a critical role in drug metabolism. Piperine (PPR), a methylenedioxyphenyl derivative combined with an unsaturated ketone, is the major active ingredient of pepper. PPR revealed time-, concentration-, and NADPH-dependent inhibitory effect on CYP3A. Carbene and quinone metabolites were both involved in the observed CYP3A inactivation by PPR. Apparently, the unsaturated ketone moiety did not participate in the enzyme inactivation. The present study sounds an alert of potential risk for food-drug interactions.

Impact of Lipid Partitioning on the Design, Analysis, and Interpretation of Microsomal Time-Dependent Inactivation

Nonspecific drug partitioning into microsomal membranes must be considered for in vitro-in vivo correlations. This work evaluated the effect of including lipid partitioning in the analysis of complex TDI kinetics with numerical methods. The covariance between lipid partitioning and multiple inhibitor binding was evaluated. Simulations were performed to test the impact of lipid partitioning on the interpretation of TDI kinetics, and experimental TDI datasets for paroxetine (PAR) and itraconazole (ITZ) were modeled. For most kinetic schemes, modeling lipid partitioning results in statistically better fits. For MM-IL simulations (KI,u = 0.1 µM, kinact = 0.1 minute-1), concurrent modeling of lipid partitioning for an fumic range (0.01, 0.1, and 0.5) resulted in better fits compared with post hoc correction (AICc: -526 vs. -496, -579 vs. -499, and -636 vs. -579, respectively). Similar results were obtained with EII-IL. Lipid partitioning may be misinterpreted as double binding, leading to incorrect parameter estimates. For the MM-IL datasets, when fumic = 0.02, MM-IL, and EII model fits were indistinguishable (δAICc = 3). For less partitioned datasets (fumic = 0.1 or 0.5), the inclusion of partitioning resulted in better models. The inclusion of lipid partitioning can lead to markedly different estimates of KI,u and kinact A reasonable alternate experimental design is nondilution TDI assays, with post hoc fumic incorporation. The best fit models for PAR (MIC-M-IL) and ITZ (MIC-EII-M-IL and MIC-EII-M-Seq-IL) were consistent with their reported mechanism and kinetics. Overall, experimental fumic values should be concurrently incorporated into TDI models with complex kinetics, when dilution protocols are used.

An Assessment of the In Vitro Inhibition of Cytochrome P450 Enzymes, UDP-Glucuronosyltransferases, and Transporters by Phosphodiester- or Phosphorothioate-Linked Oligonucleotides

Oligonucleotides represent an expanding class of pharmacotherapeutics in development for various indications. Typically, oligonucleotides are developed with phosphorothioate linkages for the improvement of biologic stability; however, limited data are available on the potential of these molecules to cause drug-drug interactions (DDIs). In this study, four nontherapeutic oligonucleotides with either a phosphodiester or phosphorothioate linkage and partial sequences towards glutathione peroxidase or β-actin (PD-GP and PD-Ac or PT-GP and PT-Ac, respectively) were evaluated in vitro for their potential to inhibit cytochrome P450 (P450) enzymes and UGP-glucuronosyltransferases (UGTs) in both human liver microsomes (HLMs) and cryopreserved human hepatocytes (CHHs) and to inhibit select transporters in expression systems. PD-GP and PD-Ac had little to no inhibitory effect on any P450 or UGT enzymes in HLMs and CHHs, except for PD-Ac in HLMs for CYP2C19 (IC₅₀ = 29 μM). Conversely, PT-GP and PT-Ac caused direct inhibition of almost all P450 and UGT enzymes, with CYP1A2 (IC₅₀ values of 0.8-4.2 μM), CYP2C8 (IC₅₀ values of 1.1-12 μM), and UGT1A1 (IC₅₀ values of 4.5-5.4 μM) inhibited to the greatest extent. There was evidence of possible time-dependent inhibition (TDI) of P450 enzymes with PT-GP and PT-Ac for CYP2B6, CYP2C8, CYP2C19, CYP2C9, CYP2D6, and CYP3A4/5; however, this TDI was reversible. In contrast to HLMs, there was little to no direct P450 inhibition by any oligonucleotide in CHHs [except for PD-Ac with CYP2C19 (IC₅₀ = 36 μM) and TDI by PT-GP with CYP2C8], demonstrating test system-dependent outcomes. Inhibition was observed for the organic anion uptake transporters, including organic anion-transporting polypeptide OATP1B1 and OATP1B3, organic anion transporters OAT1 and OAT3, and organic cation transporter OCT2 (IC₅₀ values of 12-29 μM), but not OCT1 or the efflux transporters breast cancer resistance protein and P-glycoprotein by the phosphorothioate oligonucleotides.

Comparative Study of Effects of Vonoprazan and Esomeprazole on Antiplatelet Function of Clopidogrel or Prasugrel in Relation to CYP2C19 Genotype

Drug-drug interaction between antiacid and antiplatelet agents has not been fully elucidated. Vonoprazan, a new potassium competitive acid blocker, has been available in Japan. CYP2C19 and CYP3A4 are involved in the metabolism of clopidogrel, prasugrel, esomeprazole, and vonoprazan. Using a P2Y12 assay, we compared the effects of vonoprazan and esomeprazole on the antiplatelet functions of clopidogrel or prasugrel in 31 healthy Japanese volunteers (14 CYP2C19 homo-extensive (homo-EMs), nine hetero-extensive (hetero-EMs), and eight poor metabolizers (PMs)). Vonoprazan decreased the median inhibition of platelet aggregation (IPA) values of clopidogrel and prasugrel more potently than esomeprazole (P < 0.001 for clopidogrel and P = 0.011 for prasugrel). The same tendencies were observed when stratified by CYP2C19 genotype groups (P = 0.004 in homo-EMs, 0.033 in hetero-EMs, and 0.043 in PMs). Vonoprazan attenuated the antiplatelet function of clopidogrel more potently than esomeprazole. Esomeprazole did not affect that of prasugrel irrespective of CYP2C19 genotype.

Transcriptional Profile for Detoxification Enzymes AeaGGT1 and AaeGGT2 From Aedes aegypti (Diptera: Culicidae) in Response to Larvicides

Aedes aegypti (L.) is the vector responsible for transmitting dengue, chikungunya, yellow fever, and Zika viruses, as well as other pathogens. Microbial larvicides based on Bacillus thuringiensis Berliner israelensis (Bti) and Saccharopolyspora spinosa Mertz and Yao, such as VectoBac 12AS and Natular 2EC, have been shown to be effective in reducing larval populations of Ae. aegypti. We examined the gene expression of two detoxification enzymes, glucosyl and glucuronosyl transferases (AaeGGT1 and AaeGGT2), through developmental stages and a time course study in response to larvicide exposure using qualitative real-time polymerase chain reaction (qPCR). AaeGGT1 and AaeGGT2 gene expressions were differentially regulated during development of the immature stages. We also found that male adults had higher expression than female adults after controlling for age effects. AaeGGT1 and AaeGGT2 gene expression were both upregulated in response to VectoBac 12AS and Natular 2EC treatments with the maximum level of expression occurring 24 h after treatment applications. This information sheds light on larvicide-induced changes in the physiology of Ae. aegypti with implications for development of mosquito control strategies.

Single concentration loss of activity assay provides an improved assessment of drug-drug interaction risk compared to IC50-shift

1. The utility of two abbreviated, higher-throughput assays [IC50-shift and the loss of activity (LOA) assay] to evaluate time-dependent inhibition (TDI) of 24 structurally related compounds was compared. 2. Good correlation (R(2)  = 0.90) between % inhibition and kinact/KI suggested that the LOA assay has utility as an indicator of TDI potential. Weaker correlation was observed for the shifted IC50 (IC50(T = 30)) (R(2) = 0.61) and the fold-shift in IC50 (R(2) = 0.17). 3. Primary mechanism for poor correlation was depletion of active enzyme at concentrations > 1 μM leading to greater than predicted inhibition in the IC50-shift assay. 4. Previously reported strong correlations between IC50(T = 30) and kinact/KI were found to be dependent on potent TDI compounds with kinact/KI > 30; correlation was reduced for moderate inhibitors (kinact/KI < 30). LOA assay maintained good correlation even when strong TDI compounds were excluded. 5. LOA assay (% Inhibition at 30 min, 10 μM) was a good predictor of in vivo DDI (AUCr), providing a graded response with low potential for false negatives or positives. IC50-shift assay had bias for over-predicting in vivo DDI and was more likely to identify false positives.

Development and validation of an in vitro, seven-in-one human cytochrome P450 assay for evaluation of both direct and time-dependent inhibition

INTRODUCTION

Direct and time-dependent inhibition (TDI) of cytochrome P450 enzymes (CYP) raises drug safety concerns and has major implications in drug development. This study describes the development of a liquid chromatography-tandem mass spectrometry (LC-MS/MS) based screening tool to simultaneously assess both the direct and the time-dependent inhibitory potential of xenobiotics on the seven major CYPs using a two-step approach.

METHODS

The in vitro cocktail of FDA recognized model substrates was incubated with human liver microsomes (HLM) and consisted of caffeine (CYP1A2), bupropion (CYP2B6), rosiglitazone (CYP2C8), tolbutamide (CYP2C9), omeprazole (CYP2C19), dextromethorphan (CYP2D6) and midazolam (CYP3A4). Direct and time-dependent inhibitory profiles of direct and time-dependent reference inhibitors for each CYP were studied. For validation, the results were compared to those obtained with the traditional single substrate approach. Statistical uncertainty was quantified using the bootstrap method.

RESULTS

The direct inhibition assay showed an acceptable fold bias of 1.35 (geometric mean fold absolute deviation, range 1.01-2.61) in the IC50 values for the cocktail assay compared to the single substrate results with no trend for under- or overestimation. Using a single point inactivation assay to assess TDI, we were able to identify all seven tested time-dependent reference inhibitors, without any false negatives.

DISCUSSION

The presented design enhances throughput by assessing the seven major CYPs simultaneously and allows for detection of and discrimination between direct and time-dependent CYP inhibition via IC50 and single point inactivation experiments. For the latter, a threshold of 10% TDI is proposed for carrying out more detailed inactivation kinetic experiments.

The Reliability of Estimating Ki Values for Direct, Reversible Inhibition of Cytochrome P450 Enzymes from Corresponding IC50 Values: A Retrospective Analysis of 343 Experiments

In the present study, we conducted a retrospective analysis of 343 in vitro experiments to ascertain whether observed (experimentally determined) values of Ki for reversible cytochrome P450 (P450) inhibition could be reliably predicted by dividing the corresponding IC₅₀ values by two, based on the relationship (for competitive inhibition) in which Ki = IC₅₀/2 when [S] (substrate concentration) = Km (Michaelis-Menten constant). Values of Ki and IC₅₀ were determined under the following conditions: 1) the concentration of P450 marker substrate, [S], was equal to Km (for IC₅₀ determinations) and spanned Km (for Ki determinations); 2) the substrate incubation time was short (5 minutes) to minimize metabolism-dependent inhibition and inhibitor depletion; and 3) the concentration of human liver microsomes was low (0.1 mg/ml or less) to maximize the unbound fraction of inhibitor. Under these conditions, predicted Ki values, based on IC₅₀/2, correlated strongly with experimentally observed Ki determinations [r = 0.940; average fold error (AFE) = 1.10]. Of the 343 predicted Ki values, 316 (92%) were within a factor of 2 of the experimentally determined Ki values, and only one value fell outside a 3-fold range. In the case of noncompetitive inhibitors, Ki values predicted from IC₅₀/2 values were overestimated by a factor of nearly 2 (AFE = 1.85; n = 13), which is to be expected because, for noncompetitive inhibition, Ki = IC₅₀ (not IC₅₀/2). The results suggest that, under appropriate experimental conditions with the substrate concentration equal to Km, values of Ki for direct, reversible inhibition can be reliably estimated from values of IC₅₀/2.

P450-Based Drug-Drug Interactions of Amiodarone and its Metabolites: Diversity of Inhibitory Mechanisms

In this study, IC50 shift and time-dependent inhibition (TDI) experiments were carried out to measure the ability of amiodarone (AMIO), and its circulating human metabolites, to reversibly and irreversibly inhibit CYP1A2, CYP2C9, CYP2D6, and CYP3A4 activities in human liver microsomes. The [I]u/Ki,u values were calculated and used to predict in vivo AMIO drug-drug interactions (DDIs) for pharmaceuticals metabolized by these four enzymes. Based on these values, the minor metabolite N,N-didesethylamiodarone (DDEA) is predicted to be the major cause of DDIs with xenobiotics primarily metabolized by CYP1A2, CYP2C9, or CYP3A4, while AMIO and its N-monodesethylamiodarone (MDEA) derivative are the most likely cause of interactions involving inhibition of CYP2D6 metabolism. AMIO drug interactions predicted from the reversible inhibition of the four P450 activities were found to be in good agreement with the magnitude of reported clinical DDIs with lidocaine, warfarin, metoprolol, and simvastatin. The TDI experiments showed DDEA to be a potent inactivator of CYP1A2 (KI = 0.46 μM, kinact = 0.030 minute(-1)), while MDEA was a moderate inactivator of both CYP2D6 (KI = 2.7 μM, kinact = 0.018 minute(-1)) and CYP3A4 (KI = 2.6 μM, kinact = 0.016 minute(-1)). For DDEA and MDEA, mechanism-based inactivation appears to occur through formation of a metabolic intermediate complex. Additional metabolic studies strongly suggest that CYP3A4 is the primary microsomal enzyme involved in the metabolism of AMIO to both MDEA and DDEA. In summary, these studies demonstrate both the diversity of inhibitory mechanisms with AMIO and the need to consider metabolites as the culprit in inhibitory P450-based DDIs.

Further Characterization of the Metabolism of Desloratadine and Its Cytochrome P450 and UDP-glucuronosyltransferase Inhibition Potential: Identification of Desloratadine as a Relatively Selective UGT2B10 Inhibitor

Desloratadine (Clarinex), the major active metabolite of loratadine (Claritin), is a nonsedating antihistamine used for the treatment of seasonal allergies and hives. Previously we reported that the formation of 3-hydroxydesloratadine, the major human metabolite of desloratadine, involves three sequential reactions, namely N-glucuronidation by UGT2B10 followed by 3-hydroxylation by CYP2C8 followed by deconjugation (rapid, nonenzymatic hydrolysis of the N-glucuronide). In this study we assessed the perpetrator potential of desloratadine based on in vitro studies of its inhibitory effects on cytochrome P450 and UDP-glucuronosyltransferase (UGT) enzymes in human liver microsomes (HLM). Desloratadine (10 µM) caused no inhibition (<15%) of CYP1A2, CYP2C8, CYP2C9, or CYP2C19 and weak inhibition (32-48%) of CYP2B6, CYP2D6, and CYP3A4/5. In cryopreserved human hepatocytes (CHH), which can form the CYP2C8 substrate desloratadine N-glucuronide, desloratadine did not inhibit the CYP2C8-dependent metabolism of paclitaxel or amodiaquine. Assessment of UGT inhibition identified desloratadine as a potent and relatively selective competitive inhibitor of UGT2B10 (Ki value of 1.3 μM). Chemical inhibition of UGT enzymes in HLM demonstrated that nicotine (UGT2B10 inhibitor) but not hecogenin (UGT1A4 inhibitor) completely inhibited the conversion of desloratadine (1 µM) to 3-hydroxydesloratadine in HLM fortified with both NADPH and UDP-glucuronic acid. 3-Hydroxydesloratadine formation correlated well with levomedetomidine glucuronidation (UGT2B10 marker activity) with a panel of individual CHH (r(2) = 0.72). Overall, the results of this study confirm the role of UGT2B10 in 3-hydroxydesloratadine formation and identify desloratadine as a relatively selective in vitro inhibitor of UGT2B10.

Inhibition of cytochrome P450 by ethambutol in human liver microsomes

Although cytochrome P450 inhibition is the major drug-drug interaction (DDI) mechanism in clinical pharmacotherapy, DDI of a number of well-established drugs have not been investigated. Rifampicin, isoniazid, pyrazinamide and ethambutol combination therapy inhibits clearance of theophylline in patients with tuberculosis. We determined the inhibitory effects of ethambutol on the activities of nine CYP isoforms including CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4 in pooled human liver microsomes (HLM). As measured by liquid chromatography-electrospray ionization tandem mass spectrometry, ethambutol exhibited strong inhibitory potential against CYP1A2 and CYP2E1, moderate against CYP2C19 and CYP2D6 and weak against CYP2A6, CYP2C9 and CYP3A4, based on the IC50 values. The K(i) value of ethambutol for CYP1A2 was 1.4 μM and for CYP2E1 was 2.9 μM. Inhibition of CYP1A2 and CYP2E1 was not increased by preincubation with ethambutol and β-nicotinamideadenine dinucleotide phosphate (NADPH), suggesting that the ethambutol-induced CYP inhibition may not be metabolism-dependent. Kinetic analysis showed that the inhibition of CYP1A2 and CYP2E1 by ethambutol was best fit to a competitive inhibition model. Formation of 1-methylxanthene and 1,3-dimethyluric acid from theophylline in HLM was decreased to 47% and 36%, respectively, by 3.0 μM ethambutol, which is comparable to its IC50 value against CYP1A2. Considering its maximal plasma concentrations of ~10 μM and long half-life of ~22 h, our findings raise the possibility that ethambutol causes significant DDIs in clinical situations with drugs with narrow therapeutic index, such as theophylline, in clinical situations.

Irreversible enzyme inhibition kinetics and drug-drug interactions

This chapter describes the types of irreversible inhibition of drug-metabolizing enzymes and the methods commonly employed to quantify the irreversible inhibition and subsequently predict the extent and time course of clinically important drug-drug interactions.

Cocktail-substrate approach-based high-throughput assay for evaluation of direct and time-dependent inhibition of multiple cytochrome P450 isoforms

Avoiding drug-drug interactions (DDIs) mediated through inhibition of cytochrome P450 (CYP) activity is highly desirable. Direct inhibition (DI) of CYP through new chemical entities (NCEs) or time-dependent inhibition (TDI) through reactive metabolites should be elucidated at an early stage of drug discovery research. In particular, TDI of CYP occurring through reactive metabolites may be irreversible and even sustained, causing far more serious DDIs for TDIs than for DIs. Furthermore, it is important to ascertain whether an NCE inhibits multiple CYP isoforms. Hence, using a cocktail-substrate approach that we previously established (in which the activity of 8 CYP isoforms is simultaneously evaluated in a single run), we evaluated the IC50 values of direct inhibitors and TDI parameters (kobs, shifted IC50, KI and kinact) of time-dependent inhibitors that affect multiple CYP isoforms. The IC50 values for 8 CYP isoforms obtained using the cocktail-substrate approach were nearly identical to values previously reported. The TDI parameters for CYP1A2, 2C9, 2C19, 2D6, and CYP3A4/5 obtained using the cocktail-substrate approach were also nearly identical to those obtained using a single-substrate approach. Thus, the cocktail-substrate approach is useful for evaluating DI and TDI in the early stages of drug discovery and development processes.

Lysosomal sequestration (trapping) of lipophilic amine (cationic amphiphilic) drugs in immortalized human hepatocytes (Fa2N-4 cells)

Lipophilic (logP > 1) and amphiphilic drugs (also known as cationic amphiphilic drugs) with ionizable amines (pKa > 6) can accumulate in lysosomes, a process known as lysosomal trapping. This process contributes to presystemic extraction by lysosome-rich organs (such as liver and lung), which, together with the binding of lipophilic amines to phospholipids, contributes to the large volume of distribution characteristic of numerous cardiovascular and central nervous system drugs. Accumulation of lipophilic amines in lysosomes has been implicated as a cause of phospholipidosis. Furthermore, elevated levels of lipophilic amines in lysosomes can lead to high organ-to-blood ratios of drugs that can be mistaken for active drug transport. In the present study, we describe an in vitro fluorescence-based method (using the lysosome-specific probe LysoTracker Red) to identify lysosomotropic agents in immortalized hepatocytes (Fa2N-4 cells). A diverse set of compounds with various physicochemical properties were tested, such as acids, bases, and zwitterions. In addition, the partitioning of the nonlysosomotropic atorvastatin (an anion) and the lysosomotropics propranolol and imipramine (cations) were quantified in Fa2N-4 cells in the presence or absence of various lysosomotropic or nonlysosomotropic agents and inhibitors of lysosomal sequestration (NH4Cl, nigericin, and monensin). Cellular partitioning of propranolol and imipramine was markedly reduced (by at least 40%) by NH4Cl, nigericin, or monensin. Lysosomotropic drugs also inhibited the partitioning of propranolol by at least 50%, with imipramine partitioning affected to a lesser degree. This study demonstrates the usefulness of immortalized hepatocytes (Fa2N-4 cells) for determining the lysosomal sequestration of lipophilic amines.

In vitro assessment of drug-drug interaction potential of boceprevir associated with drug metabolizing enzymes and transporters

The inhibitory effect of boceprevir (BOC), an inhibitor of hepatitis C virus nonstructural protein 3 protease was evaluated in vitro against a panel of drug-metabolizing enzymes and transporters. BOC, a known substrate for cytochrome P450 (P450) CYP3A and aldo-ketoreductases, was a reversible time-dependent inhibitor (k(inact) = 0.12 minute(-1), K(I) = 6.1 µM) of CYP3A4/5 but not an inhibitor of other major P450s, nor of UDP-glucuronosyltransferases 1A1 and 2B7. BOC showed weak to no inhibition of breast cancer resistance protein (BCRP), P-glycoprotein (Pgp), or multidrug resistance protein 2. It was a moderate inhibitor of organic anion transporting polypeptide (OATP) 1B1 and 1B3, with an IC(50) of 18 and 4.9 µM, respectively. In human hepatocytes, BOC inhibited CYP3A-mediated metabolism of midazolam, OATP1B-mediated hepatic uptake of pitavastatin, and both the uptake and metabolism of atorvastatin. The inhibitory potency of BOC was lower than known inhibitors of CYP3A (ketoconazole), OATP1B (rifampin), or both (telaprevir). BOC was a substrate for Pgp and BCRP but not for OATP1B1, OATP1B3, OATP2B1, organic cation transporter, or sodium/taurocholate cotransporting peptide. Overall, our data suggest that BOC has the potential to cause pharmacokinetic interactions via inhibition of CYP3A and CYP3A/OATP1B interplay, with the interaction magnitude lower than those observed with known potent inhibitors. Conversely, pharmacokinetic interactions of BOC, either as a perpetrator or victim, via other major P450s and transporters tested are less likely to be of clinical significance. The results from clinical drug-drug interaction studies conducted thus far are generally supportive of these conclusions.

Assessment of the time-dependent inhibition (TDI) potential of test compounds with human liver microsomes by IC50 shift method using a nondilution approach

Time-dependent inhibition (TDI) of hepatic cytochrome P450 (CYP) enzymes is increasingly implicated in the majority of clinically relevant drug-drug interactions (DDIs). A time-dependent inhibitor or its reactive metabolite irreversibly inactivates CYP enzymes, thereby inhibiting the metabolism of other drugs. As the majority of marketed drugs are metabolized by CYP enzymes, their inhibition has important clinical consequences, such as in decreasing the metabolic clearance of a co-administered drug (victim drug). This could be life threatening, as such an effect narrows the therapeutic index for drugs such as warfarin and other potentially toxic agents. Therefore, it is essential to examine new chemical entities for their potential to cause TDI to minimize adverse drug reactions during human studies and use. This unit presents an in vitro procedure utilizing a nondilution method in human liver microsomes for determining the TDI potential of test compounds.