Hepatic and intestinal metabolism of indinavir, an HIV protease inhibitor, in rat and human microsomes. Major role of CYP3A

An Integrative Approach to Elucidate Mechanisms Underlying the Pharmacokinetic Goldenseal-Midazolam Interaction: Application of In Vitro Assays and Physiologically Based Pharmacokinetic Models to Understand Clinical Observations

The natural product goldenseal is a clinical inhibitor of CYP3A activity, as evidenced by a 40%-60% increase in midazolam area under the plasma concentration versus time curve (AUC) after coadministration with goldenseal. The predominant goldenseal alkaloids berberine and (-)-β-hydrastine were previously identified as time-dependent CYP3A inhibitors using human liver microsomes. Whether these alkaloids contribute to the clinical interaction, as well as the primary anatomic site (hepatic vs. intestinal) and mode of CYP3A inhibition (reversible vs. time-dependent), remain uncharacterized. The objective of this study was to mechanistically assess the pharmacokinetic goldenseal-midazolam interaction using an integrated in vitro-in vivo-in silico approach. Using human intestinal microsomes, (-)-β-hydrastine was a more potent time-dependent inhibitor of midazolam 1'-hydroxylation than berberine (KI and kinact: 8.48 μM and 0.041 minutes-1, respectively, vs. >250 μM and ∼0.06 minutes-1, respectively). Both the AUC and Cmax of midazolam increased by 40%-60% after acute (single 3-g dose) and chronic (1 g thrice daily × 6 days) goldenseal administration to healthy adults. These increases, coupled with a modest or no increase (≤23%) in half-life, suggested that goldenseal primarily inhibited intestinal CYP3A. A physiologically based pharmacokinetic interaction model incorporating berberine and (-)-β-hydrastine successfully predicted the goldenseal-midazolam interaction to within 20% of that observed after both chronic and acute goldenseal administration. Simulations implicated (-)-β-hydrastine as the major alkaloid precipitating the interaction, primarily via time-dependent inhibition of intestinal CYP3A, after chronic and acute goldenseal exposure. Results highlight the potential interplay between time-dependent and reversible inhibition of intestinal CYP3A as the mechanism underlying natural product-drug interactions, even after acute exposure to the precipitant. SIGNIFICANCE STATEMENT: Natural products can alter the pharmacokinetics of an object drug, potentially resulting in increased off-target effects or decreased efficacy of the drug. The objective of this work was to evaluate fundamental mechanisms underlying the clinically observed goldenseal-midazolam interaction. Results support the use of an integrated approach involving established in vitro assays, clinical evaluation, and physiologically based pharmacokinetic modeling to elucidate the complex interplay between multiple phytoconstituents and various pharmacokinetic processes driving a drug interaction.

Overnight 1-mg Dexamethasone Suppression Test for Screening Cushing Syndrome and Mild Autonomous Cortisol Secretion (MACS): What Happens when Serum Dexamethasone Is Below Cutoff? How Frequent Is it?

OBJECTIVE

To determine the frequency of "invalid" 1-mg overnight dexamethasone (Dex) suppression tests (DSTs) (1-mg DST) on a large series of patients investigated for hypercortisolism and examine the interference of substances and clinical conditions that may explain low serum Dex levels.

METHODS

A retrospective analysis of 1300 Dex-controlled 1-mg DST applied to patients screened for Cushing syndrome or mild autonomous cortisol secretion in a single center for which there were identified invalid tests and distinctive characteristics that may have interfered with the outcome.

RESULTS

Among all tests, 146 (11.2%) were considered invalid (serum Dex levels <140 ng/dL, 36 [24.7%] of which were undetectable [<19.5 ng/dL]). In the Dex-undetectable group, 17% failed to take Dex correctly, 25% were on glucocorticoids (GCs), and 20% were on anticonvulsants and moderate CYP3A4 inducers. In the remaining 110 tests (serum Dex 20-140 ng/dL), 6.5% did not take Dex or were using GC, 22% were on anticonvulsants or CYP3A4 inducers, and another 13% had previous gastrointestinal tract abnormalities impairing drug absorption.

CONCLUSION

Inappropriately low serum Dex levels during the 1-mg DST may lead to false-positive results. This is associated with recurrent use of CYP3A4-inducing drugs and/or gastrointestinal abnormalities. When serum Dex is undetectable, the key reason is failure to take the medication or the use of GC (when cortisol is suppressed). Simultaneous measurement of serum cortisol and Dex allows for DST validation, improving its accuracy and avoiding unnecessary repetitions. Adherence to verbal/written recommendations and actual use of medication are critical for interpreting the test.

Involvement of CYP3A4 and MDR1 in altered metabolism and transport of indinavir in 1,25(OH)2D3-treated Caco-2 cells

Altered drug concentrations may induce unexpected toxicity or treatment failure; thus, understanding the factors that alter the pharmacokinetic profiles of drugs is crucial for optimal disease treatment. Vitamin D receptor (VDR), a nuclear receptor, regulates the expression of cytochrome P450 3A4 (CYP3A4) and multidrug resistance protein 1 (MDR1), which are crucial determinants of drug pharmacokinetics. In this study, we investigated the effects of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], a VDR ligand, on the metabolism, transport, and pharmacokinetics of indinavir, a dual substrate of CYP3A4 and MDR1. 1,25(OH)₂D₃ treatment for three days upregulated the expression levels of CYP3A4 and MDR1 in Caco-2 cells and consequently led to an increase in the level of a metabolite formed via CYP3A4 (indinavir M6) and the efflux ratio of indinavir in transport study. The increase in the metabolic reaction was also confirmed through a metabolism assay performed using the lysate of 1,25(OH)₂D₃-treated Caco-2 cells. In the Ussing chamber study conducted with the rat intestine, 1,25(OH)₂D₃ treatment did not alter the transport of indinavir into the basolateral side but increased indinavir M6 formation. Similarly, plasma levels of the metabolite increased in 1,25(OH)₂D₃-treated rats; however, systemic exposure to indinavir led to insignificant alterations. Considering the overlapping substrate specificities for CYP3A4 and MDR1 and their significant roles in drug pharmacokinetics, VDR may play an important role in drug interactions of CYP3A4 and MDR1 substrates for accessing more effective and safe disease treatments.

Atypical kinetics of cytochrome P450 enzymes in pharmacology and toxicology

Atypical kinetics are observed in metabolic reactions catalyzed by cytochrome P450 enzymes (P450). Yet, this phenomenon is regarded as experimental artifacts in some instances despite increasing evidence challenging the assumptions of typical Michaelis-Menten kinetics. As P450 play a major role in the metabolism of a wide range of substrates including drugs and endogenous compounds, it becomes critical to consider the impact of atypical kinetics on the accuracy of estimated kinetic and inhibitory parameters which could affect extrapolation of pharmacological and toxicological implications. The first half of this book chapter will focus on atypical non-Michaelis-Menten kinetics (e.g. substrate inhibition, biphasic and sigmoidal kinetics) as well as proposed underlying mechanisms supported by recent insights in mechanistic enzymology. In particular, substrate inhibition kinetics in P450 as well as concurrent drug inhibition of P450 in the presence of substrate inhibition will be further discussed. Moreover, mounting evidence has revealed that despite the high degree of sequence homology between CYP3A isoforms (i.e. CYP3A4 and CYP3A5), they have the propensities to exhibit vastly different susceptibilities and potencies of mechanism-based inactivation (MBI) with a common drug inhibitor. These experimental observations pertaining to the presence of these atypical isoform- and probe substrate-specific complexities in CYP3A isoforms by several clinically-relevant drugs will therefore be expounded and elaborated upon in the second half of this book chapter.

Multiresponse Optimization of HPLC Method: Simultaneous Estimation of Protease Inhibitors and NNRTI in Human Plasma

Multiresponse optimization approach to develop a simple isocratic, highly sensitive and accurate HPLC method for the simultaneous determination of Efavirenz, Atazanavir, Lopinavir and Ritonavir in human blood plasma along with carvedilol as an internal standard. Optimized the factors (ACN, buffer concentration and flow rate) effecting and interacting with the responses (k1, Rs2,1, Rs3,2 and tR5) applying Central Composite Design a chemometric tool. All the mathematical models as well as response surfaces were defined and derived for the separation using this strategy. Chromatography was performed on Thermo Hypersil C18 column using mobile phase comprising of ACN: 10 mM KH2PO4 (51.2:48.8) with 1 mL min-1 flow rate and detection wavelength was fixed at 210 nm. The analysis time was within 9 min. The method developed was validated by following "Bioanalytical method validation" [USFDA-CDER, 2001]. The developed method can be applied for bioavailability and pharmacokinetic studies.

The in vitro toxicity of nitrile and epithionitrile derivatives of glucosinolates from swedes (Brassica napus) in human and bovine liver cells

AIM

To investigate the direct in vitro toxicity in human and bovine liver cells, and inhibition of activity of ATP-binding cassette transporter G2 (ABCG2) and cytochrome P450 3A4 (CYP3A4) by five nitrile and epithionitrile derivatives from swede (Brassica napus).

METHODS

The following compounds were investigated: 1-cyano-2-hydroxy-3-butene (CHB, epithionitrile derivative of progoitrin), 1-cyano-2-hydroxy-3,4-epithiobutane (epithionitrile derivative of progoitrin), 3-butenenitrile (nitrile from sinigrin), 4-pentenenitrile (nitrile from gluconapin), and 5-hexenenitrile (nitrile from glucobrassicanapin). Direct cytotoxicity was assessed by incubating the compounds (at 100 mM, 200 mM, 2 M) with human (HepG2) hepatocellular carcinoma cells or bovine primary hepatocytes for 24 hours. Cell viability was then assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Cytotoxicity in Hep2G cells was also assessed after incubation for 72 hours at sub-chronic concentrations (1, 2.5, 5, 10, 15, 20 µM) and for combinations of compounds (20 µM). The ability of compounds to inhibit activity of the ABCG2 transporter and the CYP3A4 enzyme were assessed using human ABCG2 vesicles and demethylation of erythromycin by rat liver microsomes, respectively.

RESULTS

No reduction of cell viability compared to control assays was observed when the tested compounds were incubated with Hep2G cells or bovine liver cells at concentrations up to 2 mM for 24 hours or with Hep2G cells at concentrations up to 20 µM for 72 hours. None of the five tested compounds inhibited the ability of the ABCG2 transporter to transport the fluorescent substrate at concentrations up to 2 mM. Furthermore, no inhibition of CYP3A4 activity (measured as N-demethylation of erythromycin) was observed for CHB up to 2 mM.

CONCLUSION

This study suggests that under these conditions, the selected nitrile or epithionitrile derivatives of glucosinolates are not hepatotoxic in vitro.

Antitumor activity of DMAKO-05, a novel shikonin derivative, and its metabolism in rat liver microsome

The antitumor activity of shikonin derivatives is largely dependent on the generation of superoxide radicals and the alkylation activity of their naphthoquinone moiety. However, our recent study showed that 1,4-dioxime-5,8-dimethoxynaphthalene (DMAKO-05), a novel shikonin derivative, displayed more potential antitumor activity and less toxicity compared to fluorouracil (5-FU) both in vitro and in vivo, even though the hydroxyl and carbonyl groups of its naphthoquinone structure were shielded. To understand the underlying mechanisms, we investigated the metabolism of DMAKO-05 in rat liver microsomes. The kinetic analysis indicated that DMAKO-05 underwent a biphasic metabolism in rat liver microsomes. The inhibition experiments showed that CYP1A and CYP3A were the major enzymes in the metabolism of DMAKO-05, along with partial contribution from CYP2A. In addition, the structures of eight DMAKO-05 metabolites, which were characterized by accurate mass and MS/MS fragmentograms, implied that DMAKO-05 was mainly metabolized through the oxygenation of its naphthoquinone nucleus and the hydrolysis of its side chain, instead of the oxidation of hydroxyimine to ketone. Therefore, DMAKO-05 should not be considered as a traditional naphthoquinone prodrug.

Safety and pharmacokinetics of the HIV-1 protease inhibitor TMC310911 coadministered with ritonavir in healthy participants: results from 2 phase 1 studies

OBJECTIVES

To evaluate safety, tolerability, and pharmacokinetics of TMC310911, a novel human immunodeficiency virus type-1 protease inhibitor.

METHODS

Healthy participants aged 18-55 years with body mass index 18-30 kg/m were enrolled in 2 phase 1 studies. In the first-in-human, single-dose study, 18 participants received placebo or TMC310911 (75-2000 mg) in the double-blind phase and 8 participants received 300 or 600 mg of TMC310911 [administered alone or with 100 mg ritonavir twice daily (bid)] in the subsequent open-label phase. The multiple-dose double-blind study included 5 successive treatment sessions wherein healthy participants received placebo or TMC310911 [300 mg bid, 600 mg once daily or 150 mg bid (plus 100 mg ritonavir bid), 900 mg bid (alone) or 300 mg bid (plus ritonavir 50 mg bid)]; in all sessions, TMC310911 and ritonavir were administered for 6 and 9 days, respectively.

RESULTS

In the single-dose study, no dose-limiting toxicity was observed up to 2000 mg of TMC310911. Systemic exposure to TMC310911 generally increased in a dose-proportional manner after the single- or multiple-dose administrations. Coadministration of ritonavir increased the systemic exposure to TMC310911. The mean Cmax and area under plasma concentration-time curve values (single-dose: 1200 mg TMC310911) were higher under fasted conditions than in fed condition. In both studies, most treatment-emergent adverse events were related to gastrointestinal system.

CONCLUSIONS

TMC310911 exhibited a linear pharmacokinetic profile after the single- (up to 2000 mg) and multiple-dose (up to 900 mg) administrations; ritonavir improved the pharmacokinetic profile of TMC310911. TMC310911 was generally safe and tolerable when administered with or without ritonavir.

Selection of suitable prodrug candidates for in vivo studies via in vitro studies; the correlation of prodrug stability in between cell culture homogenates and human tissue homogenates

PURPOSE

To determine the correlations/discrepancies of drug stabilities between in the homogenates of human culture cells and of human tissues.

METHODS

Amino acid/dipeptide monoester prodrugs of floxuridine were chosen as the model drugs. The stabilities (half-lives) of floxuridine prodrugs in human tissues (pancreas, liver, and small intestine) homogenates were obtained and compared with ones in cell culture homogenates (AcPC-1, Capan-2, and Caco-2 cells) as well as human liver microsomes. The correlations of prodrug stability in human small bowel tissue homogenate vs. Caco-2 cell homogenate, human liver tissue homogenate vs. human liver microsomes, and human pancreatic tissue homogenate vs. pancreatic cell, AsPC-1 and Capan-2, homogenates were examined.

RESULTS

The stabilities of floxuridine prodrugs in human small bowel homogenate exhibited the great correlation to ones in Caco-2 cell homogenate (slope = 1.0-1.3, r2 = 0.79-0.98). The stability of those prodrugs in human pancreas tissue homogenate also exhibited the good correlations to ones in AsPC-1 and Capan-2 cells homogenates (slope = 0.5-0.8, r2 = 0.58-0.79). However, the correlations of prodrug stabilities between in human liver tissue homogenates and in human liver microsomes were weaker than others (slope = 1.3-1.9, r2 = 0.07-0.24).

CONCLUSIONS

The correlations of drug stabilities in cultured cell homogenates and in human tissue homogenates were compared. Those results exhibited wide range of correlations between in cell homogenate and in human tissue homogenate (r2 = 0.07 - 0.98). Those in vitro studies in cell homogenates would be good tools to predict drug stabilities in vivo and to select drug candidates for further developments. In the series of experiments, 5'-O-D-valyl-floxuridine and 5'-O-L-phenylalanyl-L-tyrosyl-floxuridine would be selected as candidates of oral drug targeting delivery for cancer chemotherapy due to their relatively good stabilities compared to other tested prodrugs.

Impact of the herbal medicine Sophora flavescens on the oral pharmacokinetics of indinavir in rats: the involvement of CYP3A and P-glycoprotein

Sophora flavescens is a Chinese medicinal herb used for the treatment of gastrointestinal hemorrhage, skin diseases, pyretic stranguria and viral hepatitis. In this study the herb-drug interactions between S. flavescens and indinavir, a protease inhibitor for HIV treatment, were evaluated in rats. Concomitant oral administration of Sophora extract (0.158 g/kg or 0.63 g/kg, p.o.) and indinavir (40 mg/kg, p.o.) in rats twice a day for 7 days resulted in a dose-dependent decrease of plasma indinavir concentrations, with 55%-83% decrease in AUC(0-∞) and 38%-78% reduction in C(max). The CL (Clearance)/F (fraction of dose available in the systemic circulation) increased up to 7.4-fold in Sophora-treated rats. Oxymatrine treatment (45 mg/kg, p.o.) also decreased indinavir concentrations, while the ethyl acetate fraction of Sophora extract had no effect. Urinary indinavir (24-h) was reduced, while the fraction of indinavir in faeces was increased after Sophora treatment. Compared to the controls, multiple dosing of Sophora extract elevated both mRNA and protein levels of P-gp in the small intestine and liver. In addition, Sophora treatment increased intestinal and hepatic mRNA expression of CYP3A1, but had less effect on CYP3A2 expression. Although protein levels of CYP3A1 and CYP3A2 were not altered by Sophora treatment, hepatic CYP3A activity increased in the Sophora-treated rats. All available data demonstrated that Sophora flavescens reduced plasma indinavir concentration after multiple concomitant doses, possibly through hepatic CYP3A activity and induction of intestinal and hepatic P-gp. The animal study would be useful for predicting potential interactions between natural products and oral pharmaceutics and understanding the mechanisms prior to human studies. Results in the current study suggest that patients using indinavir might be cautioned in the use of S. flavescens extract or Sophora-derived products.

Species differences in the pharmacokinetics of KW-7158 [(2S)-(+)-3,3,3-Trifluoro-2-hydroxy-2-methyl-N-(5,5,10-trioxo-4,10-dihydrothieno[3,2-c][1]benzothiepin-9-yl)propanamide]: formation of hydrolyzed metabolite in human and animals

Species differences in the pharmacokinetics of KW-7158 [(2S)-(+)-3,3,3-Trifluoro-2-hydroxy-2-methyl-N-(5,5,10-trioxo-4,10-dihydrothieno[3,2-c][1]benzothiepin-9-yl)propanamide] were studied in in vivo and in vitro experiments. The exposure ratio of hydrolyzed metabolite (M2, primary metabolite in human plasma)/KW-7158 was higher than the ratio of thiophen-to-furan converted metabolite (M1)/KW-7158 in human subjects after oral administration, but the mouse, rat and dog studies gave opposite results. M2 was produced in the highest amount by the 9000g supernatant of small intestine, followed by that of liver and kidney in human subjects. After correction for protein contents, the results obtained suggested that the small intestine plays a major role in the metabolism to M2 for the first pass effect after oral administration of KW-7158. The formation of M2 was independent of the presence of NADPH and was inhibited by various esterase inhibitors. These observations suggested that the predominant enzymes or isozymes involved in the formation of M2 are esterases, which differ between humans and animals. Such differences may be one of the reasons for the species differences in the pharmacokinetics of KW-7158 between humans and animals.

Twenty-six years of HIV science: an overview of anti-HIV drugs metabolism

From the identification of HIV as the agent causing AIDS, to the development of effective antiretroviral drugs, the scientific achievements in HIV research over the past twenty-six years have been formidable. Currently, there are twenty-five anti-HIV compounds which have been formally approved for clinical use in the treatment of AIDS. These compounds fall into six categories: nucleoside reverse transcriptase inhibitors (NRTIs), nucleotide reverse transcriptase inhibitors (NtRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (PIs), cell entry inhibitors or fusion inhibitors (FIs), co-receptor inhibitors (CRIs), and integrase inhibitors (INIs). Metabolism by the host organism is one of the most important determinants of the pharmacokinetic profile of a drug. Formation of active or toxic metabolites will also have an impact on the pharmacological and toxicological outcomes. Therefore, it is widely recognized that metabolism studies of a new chemical entity need to be addressed early in the drug discovery process. This paper describes an overview of the metabolism of currently available anti-HIV drugs.

Grapefruit-drug interactions

Grapefruit juice and grapefruit product consumption have potential health benefits; however, their intake is also associated with interactions with certain drugs, including calcium channel blockers, immunosuppressants and antihistamines. The primary mechanism through which interactions are mediated is mechanism-based intestinal cytochrome P450 3A4 inhibition by furanocoumarins resulting in increased bioavailability of administered medications that are substrates. Grapefruit products have also been associated with interactions with P-glycoprotein (P-gp) and uptake transporters (e.g. organic anion-transporting polypeptides [OATPs]). Polyphenolic compounds such as flavonoids have been proposed as the causative agents of the P-gp and OATP interactions. The mechanisms and magnitudes of the interactions can be influenced by the concentrations of furanocoumarins and flavonoids in the grapefruit product, the volume of juice consumed, and the inherent variability of specific enzymes and transporter components in humans. It is therefore challenging to predict the extent of grapefruit product-drug interactions and to compare available in vitro and in vivo data. The clinical significance of such interactions also depends on the disposition and toxicity profile of the drug being administered. The aim of this review is to outline the mechanisms of grapefruit-drug interactions and present a comprehensive summary of those agents affected and whether they are likely to be of clinical relevance.

Identification and validation of novel human pregnane X receptor activators among prescribed drugs via ligand-based virtual screening

Human pregnane X receptor (hPXR) plays a key role in regulating metabolism and clearance of endogenous and exogenous substances. Identification of novel hPXR activators among commercial drugs may aid in avoiding drug-drug interactions during coadministration. We applied ligand-based computational approaches for virtual screening of a commonly prescribed drug database (SCUT). Bayesian classification models were generated with a training set comprising 177 compounds using Fingerprints and 117 structural descriptors. A cell-based luciferase reporter assay was used for evaluation of chemical-mediated hPXR activation in HepG2 cells. All compounds were tested at 10 μM concentration with rifampicin and dimethyl sulfoxide as positive and negative controls, respectively. The Bayesian models showed specificity and overall prediction accuracy up to 0.92 and 0.69 for test set compounds. Screening the SCUT database with this model retrieved 105 hits and 17 compounds from the top 25 hits were chosen for in vitro testing. The reporter assay confirmed that nine drugs, i.e., fluticasone, nimodipine, nisoldipine, beclomethasone, finasteride, flunisolide, megestrol, secobarbital, and aminoglutethimide, were previously unidentified hPXR activators. Thus, the present study demonstrates that novel hPXR activators can be efficiently identified among U.S. Food and Drug Administration-approved and commonly prescribed drugs, which should lead to detection and prevention of potential drug-drug interactions.

HIV protease inhibitors and obesity

PURPOSE OF REVIEW

To review the current scientific literature and recent clinical trials on HIV protease inhibitors and their potential role in the pathogenesis of lipodystrophy and metabolic disorders.

RECENT FINDINGS

HIV protease inhibitor treatment may affect the normal stimulatory effect of insulin on glucose and fat storage. Further, chronic inflammation from HIV infection and protease inhibitor treatment trigger cellular homeostatic stress responses with adverse effects on intermediary metabolism. The physiologic outcome is such that total adipocyte storage capacity is decreased, and the remaining adipocytes resist further fat storage. This process leads to a pathologic cycle of lipodystrophy and lipotoxicity, a proatherogenic lipid profile, and a clinical phenotype of increased central body fat distribution similar to the metabolic syndrome.

SUMMARY

Protease inhibitors are a key component of antiretroviral therapy and have dramatically improved the life expectancy of HIV-infected individuals. However, they are also associated with abnormalities in glucose/lipid metabolism and body fat distribution. Further studies are needed to better define the pathogenesis of protease inhibitor-associated metabolic and body fat changes and their potential treatment.

Metabolism of ramelteon in human liver microsomes and correlation with the effect of fluvoxamine on ramelteon pharmacokinetics

Ramelteon is a melatonin receptor agonist used as a treatment for insomnia. It is subject to a remarkably large drug-drug interaction (DDI) caused by fluvoxamine coadministration, resulting in a more than 100-fold increase in exposure. The objective of this study was to determine whether the DDI could be estimated using in vitro metabolism data. Ramelteon was shown to undergo hydroxylation in human liver microsomes to eight metabolites via six pathways. The main routes of metabolism included hydroxylation on the ethyl side chain and the benzylic position of the cyclopentyl ring, as assessed through enzyme kinetic measurements. Hydroxylation at the other benzylic position was observed in human intestinal microsomes. Ramelteon metabolism was catalyzed by CYP1A2, CYP2C19, and CYP3A4 as shown through the use of recombinant human cytochrome P450 enzymes and specific inhibitors. In liver, CYP1A2, CYP2C19, and CYP3A4 were estimated to contribute 49, 42, and 8.6%, respectively, whereas in intestine only CYP3A4 contributes. The in vitro data were used to estimate the magnitudes of DDI caused by ketoconazole, fluconazole, and fluvoxamine. The DDIs caused by the former were reliably estimated (1.82-fold estimated versus 1.82-fold actual for ketoconazole; 2.99-fold estimated versus 2.36-fold actual for fluconazole), whereas for fluvoxamine it was underestimated (11.4-fold estimated versus 128-fold actual). This suggests that there may be a limit on the magnitude of DDI that can be estimated from in vitro data. Nevertheless, the example of the fluvoxamine-ramelteon DDI offers a unique example wherein one drug can simultaneously inhibit multiple enzymatic pathways of a second drug.

Pharmacogenomics of CYP3A: considerations for HIV treatment

The understanding of the cytochrome P450 3A SNP in antiretroviral therapy is important, because it is highly inducible, extremely polymorphic and metabolizes many of the drugs that are key components of highly active antiretroviral therapy regimens. This enzyme is prolific and promiscuous towards drug and xenobiotic substrate selection and it is also unpredictable among individuals, having a 5- to 20-fold variability in its ability to contribute to drug clearance. The importance of human CYP3A pharmacogenetics is also gaining attention in other established areas of pharmacotherapy as it may contribute to the goal of predicting efficacy and/or toxicity, specifically with the discovery of null allele CYP3A4*20. This review summarizes the current understanding, implications of genetic variation in the CYP3A enzymes, the central role of CYP3A in linking human genetics, the pharmacokinetics and resulting pharmacodynamic responses to certain antiretroviral drugs, and their eventual place in applied clinical pharmacotherapy.

Central nervous system distribution kinetics of indinavir in rats

The central nervous system (CNS) distribution kinetics of indinavir were extensively evaluated using a combinational in-vivo model comprising the integration plot method (a single-passage approach) and neuropharmacokinetic method (a multiple-passage approach). A 5 mg kg−1 dose of indinavir was administered intravenously to rats. Blood and cerebrospinal fluid (CSF) samples and whole brain were collected from the animals at specified time points and the drug concentration in each sample was determined using a high-performance liquid chromatography method. For the neuropharmacokinetic study, the simultaneous plasma, CSF and brain concentrations were fitted to an integrated model, which resulted in the estimation of the influx (Kin) and efflux (Kout) rate constants of the drug to/from CSF and brain parenchyma. The integration plot method involved plotting the brainplasma or CSF-plasma concentration ratios (Kp,app) against AUC0r&#x030a;t/Cp(t), and estimating the uptake clearance of the drug by brain/CSF from the slope of the initial linear portion of the plot. The Kin and Kout values of the drug to/from CSF were estimated to be 2.42 times 10−2 and 13.26 times 10−2min−1, respectively, and the corresponding values for brain parenchyma were 1.02 times 10−2 and 1.32 times 10−2 min−1, respectively. The uptake clearances of indinavir by CSF and brain parenchyma were 8.89 and 8.38 μLmin−1 g−1, respectively. The permeability surface area products of the drug for the blood-brain barrier and blood-CSF barrier were estimated as 1.05 times 10−2 and 2.45 times 10−2 mL min−1 g−1, respectively. The estimated kinetic parameters indicated limited CNS entry of the drug because of the limited blood-brain barrier permeability and the efficient drug efflux from CNS, particularly from CSF.

Does the well-stirred model assess the intestinal first-pass effect well?

The pre-systemic intestinal extraction ratio (Eg) has been estimated by an equation based on the well-stirred model, which does not have a term of membrane transport. In this report, we have identified the application limitations of the well-stirred model equation to assess the pre-systemic intestinal extraction ratio. The Eg of metoprolol (CYP2D6 substrate) was assessed by three methods. Intrinsic clearances for metoprolol metabolism in hepatic and gastrointestinal microsomes were from a published report. Method 1 (model-independent method): the Eg of 0.228 was obtained according to the equation, F = Ff × (1 — Eg) × Fh, where F, Ff and Fh were the bioavailability, the fraction entering the intestinal tissue and the hepatic availability, respectively. Method 2: the Eg of 0.0071 was calculated according to the well-stirred model equation, and was much lower than the value of 0.228. Method 3: the Eg of 0.213 was obtained by the transport-metabolism-flow (TMF) model equation, and was much closer to the value of 0.228 obtained by the model-independent method than the Eg of 0.0071 calculated by the well-stirred model equation. Therefore, we propose that the factor of membrane transport process be incorporated into the pharmacokinetic model for the assessment of the pre-systemic intestinal extraction ratio.

Bioactivation of a novel 2-methylindole-containing dual chemoattractant receptor-homologous molecule expressed on T-helper type-2 cells/D-prostanoid receptor antagonist leads to mechanism-based CYP3A inactivation: glutathione adduct characterization and prediction of in vivo drug-drug interaction

The 2-methyl substituted indole, 2MI [2-(4-(4-(2,4-dichlorophenylsulfonamido)-2-methyl-1H-indol-5-yloxy)-3-methoxyphenyl)acetic acid] is a potent dual inhibitor of 1) chemoattractant receptor-homologous molecule expressed on T-helper type-2 cells and 2) d-prostanoid receptor. During evaluation as a potential treatment for asthma and allergic rhinitis, 2MI was identified as a mechanism-based inactivator of CYP3A4 in vitro. The inactivation was shown to be irreversible by dialysis and accompanied by an NADPH-dependent increase in 2MI covalent binding to a 55- to 60-kDa microsomal protein, consistent with irreversible binding to CYP3A4. Two glutathione (GSH) adducts, G1 and G2, were identified in vitro, and the more abundant adduct (G1) was unambiguously determined via NMR to be GSH adducted to the 3-position of the 2-methylindole moiety. The potential for a clinical drug-drug interaction arising from mechanism-based inactivation of CYP3A4 by 2MI was predicted using a steady-state model, and a 4.3- to 7.5-fold increase in the exposure of midazolam was predicted at anticipated therapeutic concentrations. To better assess the potential for in vivo drug-drug interactions, the Sprague-Dawley rat was used as an in vivo model. An excellent in vitro-in vivo correlation was observed for the reduction in enzyme steady-state concentration (E'(ss/Ess)) as well as the change in the exposure of a prototypical CYP3A substrate, indinavir (area under the curve (AUC) for indinavir/AUC). In summary, 2MI was identified as a potent mechanism-based inactivator of CYP3A and was predicted to elicit a clinically relevant drug-drug interaction in humans at an anticipated therapeutic concentration.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Synthesis and evaluation of inhibitors of cytochrome P450 3A (CYP3A) for pharmacokinetic enhancement of drugs

The HIV protease inhibitor ritonavir (RTV) is also a potent inhibitor of the metabolizing enzyme cytochrome P450 3A (CYP3A) and is clinically useful in HIV therapy in its ability to enhance human plasma levels of other HIV protease inhibitors (PIs). A novel series of CYP3A inhibitors was designed around the structural elements of RTV believed to be important to CYP3A inhibition, with general design features being the attachment of groups that mimic the P2-P3 segment of RTV to a soluble core. Several analogs were found to strongly enhance plasma levels of lopinavir (LPV), including 8, which compares favorably with RTV in the same model. Interestingly, an inverse correlation between in vitro inhibition of CYP3A and elevation of LPV was observed. The compounds described in this study may be useful for enhancing the pharmacokinetics of drugs that are metabolized by CYP3A.

Effect of intestinal glucuronidation in limiting hepatic exposure and bioactivation of raloxifene in humans and rats

Raloxifene (Evista) is a second generation selective estrogen receptor modulator used in the treatment of osteoporosis and for chemoprevention of breast cancer. It is bioactivated to reactive intermediates, which covalently bind to proteins and form GSH conjugates upon incubation with NADPH and GSH-supplemented human and rat liver microsomes. Despite these in vitro findings, no major raloxifene-related toxic events have been reported upon its oral administration to humans. This disconnect between safety of raloxifene and its in vitro bioactivation is attributed to its presystemic metabolism via glucuronidation. Current studies investigated the effect of hepatic and intestinal glucuronidation in modulating hepatic availability of raloxifene and its subsequent bioactivation, in vitro. The study design involved preincubation of raloxifene with intestinal microsomes followed by a sequential incubation with liver microsomes. The degree of bioactivation of raloxifene was assessed from the percentage of GSH conjugate formed in liver microsomal incubations or the amount of covalent binding of raloxifene-related material to liver microsomal proteins. The results indicated that human intestinal glucuronidation limited the hepatic exposure of raloxifene that underwent bioactivation in the liver. Similar experiments with rat microsomal preparations showed very little effect of intestinal glucuronidation. This effect of intestinal glucuronidation and the observed species difference were explained by comparing the efficiency (Cl(int)) of glucuronidation and oxidation in the two species. These findings suggested that even though the rate of bioactivation in the two species was similar, the Cl(int) of glucuronidation was 7.5-fold higher in the human intestine as compared to rats. These results support the hypothesis that intestinal glucuronidation modulates the amount of raloxifene undergoing bioactivation by liver and corroborate the importance of assessing other competitive metabolic pathways and species differences in metabolism prior to extrapolation of bioactivation results from rats to humans.

Population-based mechanistic prediction of oral drug absorption

The bioavailability of drugs from oral formulations is influenced by many physiological factors including gastrointestinal fluid composition, pH and dynamics, transit and motility, and metabolism and transport, each of which may vary with age, gender, race, food, and disease. Therefore, oral bioavailability, particularly of poorly soluble and/or poorly permeable compounds and those that are extensively metabolized, often exhibits a high degree of inter- and intra-individual variability. While several models and algorithms have been developed to predict bioavailability in an average person, efforts to accommodate intrinsic variability in the component processes are less common. An approach that incorporates such variability for human populations within a mechanistic framework is described together with examples of its application to drug and formulation development.

Novel therapeutic biosensor for indinavir-a protease inhibitor antiretroviral drug

An amperometric drug metabolism biosensor consisting of cytochrome P450-3A4 (CYP3A4) encapsulated in a didodecyldimethylammonium bromide (DDAB) vesicular system on a Pt disk electrode was developed for the determination of indinavir, a protease inhibitor antiretroviral drug. Cyclic, square wave and pulse voltammetric responses of the bioelectrode showed quasi-reversible electrochemistry of the Fe(3+)/Fe(2+) redox species of the heme thiolate CYP3A4 enzyme under aerobic and anaerobic conditions. The biosensor exhibited excellent response to indinavir with a detection limit and response time of 6.158 x 10(-2)mgL(-1), and 40s, respectively. The detection limit is well below the plasma concentration of indinavir (8h after intake) which range from 0.13 to 8.6mgL(-1).

Species Differences in In Vitro and In Vivo Small Intestinal Metabolism of CYP3A Substrates

Intestinal first-pass metabolism has a great impact on the bioavailability of CYP3A substrates in humans, and the in vivo impact has quantitatively been evaluated using CYP3A inhibitors or inducers. In vitro and in vivo preclinical investigations for intestinal metabolism are essential in clarifying pharmacokinetic behavior in animal species and predicting the effect of intestinal metabolism in the human. In this review, we will discuss species differences in intestinal CYP3A enzymes, and CYP3A-mdediated intestinal elimination. Identical CYP3A4 enzyme is expressed in human intestine and liver, but different CYP3A enzymes in both tissues of the mouse and rat are found, that is, respective intestinal enzyme is considered as cyp3a13 and CYP3A62. There is little information on CYP3A enzymes in the monkey and dog intestine, unlike the liver. In vitro metabolic activities of midazolam and nisoldipine are higher in the human and monkey than in the rat. In vivo assessment of cyclosporine, midazolam, nifedipine, tacrolimus, and verapamil has been reported in various species (monkey, rat, mouse, and/or dog) including the human. For midazolam, the monkey shows significant in vivo intestinal metabolism, as evidenced in the human. The monkey might be an appropriate animal model for evaluating small intestinal first-pass metabolism of CYP3A substrates.

Application of rat in situ single-pass intestinal perfusion in the evaluation of presystemic extraction of indinavir under different perfusion rates

BACKGROUND/PURPOSE

First-pass effect has been an important concern for oral pharmaceuticals. An in vivo system was developed for measuring different concentrations of pharmaceuticals in the portal vein and hepatic vein (via the inferior vena cava) for delineating presystemic metabolism under different perfusion rates by using indinavir as an exemplary agent.

METHODS

An in situ single-pass intestinal perfusion technique was modified from previous studies to concomitantly obtain portal and hepatic venous bloods. Portal and hepatic venous samples were simultaneously taken from rats at appropriate time points using the perfusion model of 1 mg/mL indinavir at flow rates of 0.05, 0.1, 0.5 and 1.0 mL/min. The indinavir concentrations were assayed by binary-gradient high-pressure liquid chromatography with UV detection.

RESULTS

The mean indinavir concentrations in portal vein concentration-time profiles at different perfusion times under various flow rates were all higher than those obtained for hepatic veins. At flow rates of 0.5 and 1.0 mL/min, in particular, the area under the curve (AUC) and maximal concentration (Cmax) of indinavir absorption were significantly different between portal veins and hepatic veins (p < 0.05), indicating considerable hepatic involvement in the presystemic extraction of indinavir. The system also has potential for use when estimating the hepatic extraction ratio (E(H)) and hepatic clearance (Cl(H)).

CONCLUSION

This in vivo approach could provide another useful tool for improving our basic understanding of the absorption kinetics and hepatic metabolism of pharmaceuticals under development and facilitating the clinical application of such.

Pharmacokinetic properties of TDP4815 after single intravenous and oral administrations to rat, rabbit, monkey, dog and in vitro drug metabolism

The pharmacokinetics of TDP4815 was evaluated in rats, rabbits, dogs and monkeys. After intravenous administration, TDP4815 achieved C(O) of 3255 ng/ml in rats at 5 mg/kg, 9066 ng/ml in rabbits and 7858 ng/ml in monkeys at 6 mg/kg, and 4457 ng/ml in dogs at 3 mg/kg. The clearance (C(L)) was 3105, 1692, 835 and 640 ml/h/kg in rats, rabbits, monkeys and dogs, respectively. The volume of distribution (V(Z)) was more than 3861 ml/kg in all species, except 1915 ml/kg in monkeys. The oral bioavailability was rabbit >rat> monkey compared at 100 mg/kg, but it was much higher in dogs (>64%) after oral administrations. The calculated intrinsic clearance data suggested that the clearance of dog and human was restricted by binding to the plasma protein, and the clearance of rat and monkey was dependent on both the free fraction of plasma protein binding and the liver blood flow rate. The unbound hepatic intrinsic clearance of monkey was close to its C(L) suggesting that the hepatic clearance was an important excretion in monkeys. The poor oral bioavailability in the monkey may be related to the extensive glucuronidation. The V(Z).kg and C(L).kg in test species showed good correlation with the animal body weights (R(2)=0.87 and 0.96).

Low Oral Bioavailability and Pharmacokinetics of Senkyunolide A, a Major Bioactive Component in Rhizoma Chuanxiong, in the Rat

The pharmacokinetics of senkyunolide A, one of the major bioactive ingredients in the traditional Chinese medicinal herb Rhizoma Chuanxiong, which is commonly used for the treatment of cardiovascular diseases, was studied in rats. After intravenous (IV) administration, senkyunolide A was extensively distributed (Vd/F: 6.74 +/- 0.73 L/kg) and rapidly eliminated from the plasma (CL/F: 7.20 +/- 0.48 L/h per kilogram and t1/2: 0.65 +/- 0.06 hr). Hepatic metabolism was suggested as the major route of senkyunolide A elimination as indicated by the results of in vitro S9 fraction study. After intraperitoneal (IP) administration, senkyunolide A exhibited dose-independent pharmacokinetics. The absorption after IP administration was rapid (Tmax: 0.04 +/- 0.01 hours), and the bioavailability was 75%. After oral administration, senkyunolide A was also absorbed rapidly (Tmax: 0.21 +/- 0.08 hours); however, its oral bioavailability was low (approximately 8%). The contributing factors were determined to be instability in the gastrointestinal tract (accounting for 67% of the loss) and hepatic first-pass metabolism (accounting for another 25%). Pharmacokinetics of senkyunolide A were unaltered when Chuanxiong extract was administered, which suggests that components in the extract have insignificant effects on senkyunolide A pharmacokinetics.

Metabolism and disposition of the HIV-1 protease inhibitor lopinavir (ABT-378) given in combination with ritonavir in rats, dogs, and humans

PURPOSE

The objective of this study was to examine the metabolism and disposition of the HIV protease inhibitor lopinavir in humans and animal models.

METHODS

The plasma protein binding of [14C]lopinavir was examined in vitro via equilibrium dialysis technique. The tissue distribution of radioactivity was examined in rats dosed with [14C]lopinavir in combination with ritonavir. The metabolism and disposition of [14C]lopinavir was examined in rats, dogs, and humans given alone (in rats only) or in combination with ritonavir.

RESULTS

The plasma protein binding of lopinavir was high in all species (97.4-99.7% in human plasma), with a concentration-dependent decrease in binding. Radioactivity was extensively distributed into tissues, except brain, in rats. On oral dosing to rats, ritonavir was found to increase the exposure of lopinavir-derived radioactivity 13-fold. Radioactivity was primarily cleared via the hepato-biliary route in all species (>82% of radioactive dose excreted via fecal route), with urinary route of elimination being significant only in humans (10.4% of radioactive dose). Oxidative metabolites were the predominant components of excreted radioactivity. The predominant site of metabolism was found to be the carbon-4 of the cyclic urea moiety, with subsequent secondary metabolism occurring on the diphenyl core moiety. In all the three species examined, the primary component of plasma radioactivity was unchanged lopinavir (>88%) with small amounts of oxidative metabolites.

CONCLUSIONS

Lopinavir was subject to extensive metabolism in vivo. Co-administered ritonavir markedly enhanced the pharmacokinetics of lopinavir-derived radioactivity in rats, probably due to inhibition of presystemic and systemic metabolism, leading to an increased exposure to this potent HIV protease inhibitor.

Automated, fast, and sensitive quantification of drugs in human plasma by LC/LC-MS: quantification of 6 protease inhibitors and 3 nonnucleoside transcriptase inhibitors

An analytic assay based on automated sample preparation and liquid chromatography (LC) coupled with electrospray mass spectrometry (ESI-MS) was developed for the quantification of 6 protease inhibitors (PIs) and 3 nonnucleoside reverse transcriptase inhibitors (NNRTIs). The 6 PIs, amprenavir, indinavir, ritonavir, lopinavir, nelfinavir, and saquinavir, as well as the three NNRTIs, nevirapine, efavirenz, and delavirdine, require a succinct analysis technique for therapeutic drug monitoring in HIV/AIDS patients. After protein precipitation, samples were loaded on a C8, 10 x 4-mm extraction column, washed, and, after activation of the column-switching valve, backflushed onto the 30 x 2.1 mm C8 analytic column. [M+H] ions were detected in the selected ion mode. A nonlinear fit (y(-1) = a + b/x, all r2 > 0.999) for amprenavir, indinavir, ritonavir, lopinavir, nelfinavir, and saquinavir and a linear fit (y = ax + b, all r2 > 0.999) for nevirapine, efavirenz, and delavirdine led to best regression. Absolute recoveries were as follows: PIs > 81%; NNRTIs > 76%. Interday and intraday precision were <12.5% for the PIs and <11.7% for the NNRTIs. Interday and intraday accuracy were <12.2% for the PIs and <14.9% for the NNRTIs. Limits of quantification were 20, 40, 50, 40, 40, 20, and 100 microg/L for amprenavir, indinavir, ritonavir, lopinavir, nelfinavir, saquinavir, and the NNRTIs, respectively. The assay allows fast analysis of patient samples for therapeutic drug monitoring (TDM) and has successfully been used for TDM and pharmacokinetic drug-drug interactions studies.

Prediction of Human Drug Clearance from in Vitro and Preclinical Data Using Physiologically Based and Empirical Approaches

PURPOSE

The aim of this study is to compare the accuracy of five methods for predicting in vivo intrinsic clearance (CL(int)) and seven for predicting hepatic clearance (CL(h)) in humans using in vitro microsomal data and/or preclinical animal data.

METHODS

The human CL(int) was predicted for 33 drugs by five methods that used either in vitro data with a physiologic scaling factor (SF), with an empirical SF, with the physiologic and drug-specific (the ratio of in vivo and in vitro CL(int) in rats) SFs, or rat CL(int) directly and with allometric scaling. Using the estimated CL(int), the CL(h) in humans was calculated according to the well-stirred liver model. The CL(h) was also predicted using additional two methods: using direct allometric scaling or drug-specific SF and allometry.

RESULTS

Using in vitro human microsomal data with a physiologic SF resulted in consistent underestimation of both CL(int) and CL(h). This bias was reduced by using either an empirical SF, a drug-specific SF, or allometry. However, for allometry, there was a substantial decrease in precision. For drug-specific SF, bias was less reduced, precision was similar to an empirical SF. Both CL(int) and CL(h) were best predicted using in vitro human microsomal data with empirical SF. Use of larger data set of 52 drugs with the well-stirred liver model resulted in a best-fit empirical SF that is 9-fold increase on the physiologic SF.

CONCLUSIONS

Overall, the empirical SF method and the drug-specific SF method appear to be the best methods; they show lower bias than the physiologic SF and better precision than allometric approaches. The use of in vitro human microsomal data with an empirical SF may be preferable, as it does not require extra information from a preclinical study.

Mechanism-Based Inactivation of CYP3A by HIV Protease Inhibitors

Human immunodeficiency virus (HIV) protease inhibitors (PIs) are inhibitors of CYP3A enzymes, but the mechanism is poorly defined. In this study, time- and concentration-dependent decreases in activity as defined by maximum rate of inactivation (k(inact)) and inhibitor concentration that gives 50% maximal inactivation (K(I)) of CYP3A by amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir were quantified using testosterone 6beta-hydroxylation as a marker for CYP3A activity with recombinant CYP3A4(+b(5)), recombinant CYP3A5, and pooled human liver microsomes (HLMs). All the PIs, except indinavir, displayed inactivation with CYP3A4(+b(5)) and HLMs. Ritonavir was the most potent (K(I) = 0.10 and 0.17 microM) and demonstrated high k(inact) values (0.32 and 0.40 min(-1)) with both CYP3A4(+b(5)) and HLMs. Ritonavir was not significantly depleted by high-affinity binding with CYP3A4(+b(5)) and confirmed that estimation of reversible inhibition was confounded with irreversible inhibition. For CYP3A5, nelfinavir exhibited the highest k(inact) (0.47 min(-1)), but ritonavir was the most potent (K(I) = 0.12 microM). Saquinavir and indinavir did not show time- and concentration-dependent decreases in activity with CYP3A5. Spectrophototmetrically determined metabolic intermediate complex formation was observed for all of the PIs with CYP3A4(+b(5)), except for lopinavir and saquinavir. The addition of nucleophilic and free aldehyde trapping agents and free iron and reactive oxygen species scavengers did not prevent inactivation of CYP3A4(+b(5)) by ritonavir, amprenavir, or nelfinavir, but glutathione decreased the inactivation by saquinavir (17%) and catalase decreased the inactivation by lopinavir (39%). In conclusion, all the PIs exhibited mechanism-based inactivation, and predictions of the extent and time course of drug interactions with PIs could be underestimated if based solely on reversible inhibition.

Conversion of the HIV protease inhibitor nelfinavir to a bioactive metabolite by human liver CYP2C19

Antiretroviral therapy for human immunodeficiency virus (HIV) infection includes treatment with both reverse transcriptase inhibitors and protease inhibitors, which markedly suppress viral replication and circulating HIV RNA levels. Cytochrome P450 (P450) enzymes in human liver, chiefly CYP3A4, play a pivotal role in protease inhibitor biotransformation, converting these agents to largely inactive metabolites. However, the protease inhibitor nelfinavir (Viracept) is metabolized mainly to nelfinavir hydroxy-t-butylamide (M8), which exhibits potent antiviral activity, and to other minor products (termed M1 and M3) that are inactive. Since indirect evidence suggests that CYP2C19 underlies M8 formation, we examined the role of this inducible, polymorphic P450 enzyme in nelfinavir t-butylamide hydroxylation by human liver. Rates of microsomal M8 formation were 50.6 +/- 28.3 pmol of product formed/min/nmol P450 (n = 5 subjects), whereas kinetic analysis of the reaction revealed a KM of 21.6 microM and a Vmax of 24.6 pmol/min/nmol P450. In reconstituted systems, CYP2C19 catalyzed nelfinavir t-butylamide hydroxylation at a turnover rate of 2.2 min(-1), whereas CYP2C9, CYP2C8, and CYP3A4 were inactive toward nelfinavir. Polyclonal anti-CYP2C9 (cross-reactive with CYP2C19) and monoclonal anti-CYP2C19 completely inhibited microsomal M8 production, whereas monoclonal CYP2C9 and polyclonal CYP3A4 antibodies were without effect. Similarly, the CYP2C19 substrate omeprazole strongly inhibited (75%) hepatic nelfinavir t-butylamide hydroxylation at a concentration of only 12.5 microM. Our study shows that CYP2C19 underlies formation in human liver of M8, a bioactive nelfinavir metabolite. The inducibility of CYP2C19 by agents (e.g., rifampicin) often taken concurrently with nelfinavir, together with this P450's known polymorphic nature, may thus be important determinants of nelfinavir's antiviral potency.

Nanoparticles with specific bioadhesive properties to circumvent the pre-systemic degradation of fluorinated pyrimidines

The aim was to evaluate the potential of specific bioadhesive nanoparticles to increase the oral bioavailability of pre-systemic degraded drugs, using 5-fluorouridine (FURD) as model. For this purpose, poly(methylvinylether-co-maleic anhydride) nanoparticles (NP), NP coated with albumin (BSA-NP) and NP treated with albumin and 1,3-diaminopropane (BD-NP) were used. All the formulations displayed a similar size and drug loading. However, BSA-NP showed a tropism for the stomach, NP developed adhesive interactions with both the stomach and middle portions of the small intestine and BD-NP with the distal regions of the small intestine. These formulations were orally administered to laboratory animals and the FURD levels in plasma, tissues and urine were quantified at different times. From the urine data, the FURD bioavailability when loaded in either BSA-NP or NP was about 79% and 21%, respectively. For the control oral solution and BD-NP this parameter was 11% and 2%, respectively. FURD metabolism in gut was assessed by HPLC analysis of the lumen content. A FURD metabolite was found. Comparing the three nanoparticle formulations, the presence of the metabolite in the lumen contents was significantly higher for BD-NP than for NP and BSA-NP. In summary, the use of bioadhesive nanoparticles with tropism for the stomach mucosa may be considered as an adequate alternative to increase the bioavailability of some pre-systemic metabolised drugs.

Expression profiles of drug-metabolizing enzyme CYP3A and drug efflux transporter multidrug resistance 1 subfamily mRNAS in small intestine

The purpose of this study is to examine the expression profiles of CYP3A1, CYP3A2, CYP3A9, and CYP3A18 mRNAs as well as multidrug resistance (mdr)1a and mdr1b mRNAs in the liver and small intestine of normal male Wistar rats using a reverse transcription-polymerase chain reaction (PCR). In the rat liver, the PCR products for CYP3A1, CYP3A2, and CYP3A18 were readily detectable, whereas CYP3A9 was slightly and mdr1a and mdr1b barely detected. Surprisingly, no PCR products for CYP3A1 and CYP3A2 were detected in the small intestine, but those for CYP3A9, CYP3A18, and mdr1a were readily detectable, and a faint band for mdr1b was also observed. Both CYP3A9 and CYP3A18 levels were found to be high in the duodenum and decreased from the top to bottom of the gut, indicating regional differences in both CYP3A9 and CYP3A18 expression in the small intestine. In contrast, mdr1a expression increased gradually from the upper to lower intestine. Consequently, it was suggested that drug metabolism in the small intestine of normal rats was mediated by CYP3A9 and CYP3A18 rather than CYP3A1 and CYP3A2. Also, regional differences of CYP3A9, CYP3A18, and mdr1a expression were found in the small intestine. The distributions of CYP3A9 and CYP3A18 were different from the distribution of mdr1a, suggesting the cooperative action of drug clearance pathways. This information is important to drug metabolism research based on ex vivo and in vivo studies using rats.

In vitro metabolism of clindamycin in human liver and intestinal microsomes

Incubations with human liver and gut microsomes revealed that the antibiotic, clindamycin, is primarily oxidized to form clindamycin sulfoxide. In this report, evidence is presented that the S-oxidation of clindamycin is primarily mediated by CYP3A. This conclusion is based upon several lines of in vitro evidence, including the following. 1) Incubations with clindamycin in hepatic microsomes from a panel of human donors showed that clindamycin sulfoxide formation correlated with CYP3A-catalyzed testosterone 6beta-hydroxylase activity; 2) coincubation with ketaconazole, a CYP3A4-specific inhibitor, markedly inhibited clindamycin S-oxidase activity; and 3) when clindamycin was incubated across a battery of recombinant heterologously expressed human cytochrome P450 (P450) enzymes, CYP3A4 possessed the highest clindamycin S-oxidase activity. A potential role for flavin-containing monooxygenases (FMOs) in clindamycin S-oxidation in human liver was also evaluated. Formation of clindamycin sulfoxide in human liver microsomes was unaffected either by heat pretreatment or by chemical inhibition (e.g., methimazole). Furthermore, incubations with recombinant FMO isoforms revealed no detectable activity toward the formation of clindamycin sulfoxide. Beyond identifying the drug-metabolizing enzyme responsible for clindamycin S-oxidation, the ability of clindamycin to inhibit six human P450 enzymes was also evaluated. Of the P450 enzymes examined, only the activity of CYP3A4 was inhibited (approximately 26%) by coincubation with clindamycin (100 microM). Thus, it is concluded that CYP3A4 appears to account for the largest proportion of the observed P450 catalytic clindamycin S-oxidase activity in vitro, and this activity may be extrapolated to the in vivo condition.