CYP isoforms involved in the metabolism of clarithromycin in vitro: comparison between the identification from disappearance rate and that from formation rate of metabolites

Acceleration of infectious disease drug discovery and development using a humanized model of drug metabolism

A key step in drug discovery, common to many disease areas, is preclinical demonstration of efficacy in a mouse model of disease. However, this demonstration and its translation to the clinic can be impeded by mouse-specific pathways of drug metabolism. Here, we show that a mouse line extensively humanized for the cytochrome P450 gene superfamily ("8HUM") can circumvent these problems. The pharmacokinetics, metabolite profiles, and magnitude of drug-drug interactions of a test set of approved medicines were in much closer alignment with clinical observations than in wild-type mice. Infection with Mycobacterium tuberculosis, Leishmania donovani, and Trypanosoma cruzi was well tolerated in 8HUM, permitting efficacy assessment. During such assessments, mouse-specific metabolic liabilities were bypassed while the impact of clinically relevant active metabolites and DDI on efficacy were well captured. Removal of species differences in metabolism by replacement of wild-type mice with 8HUM therefore reduces compound attrition while improving clinical translation, accelerating drug discovery.

Mechanism Underlying Conflicting Drug-Drug Interaction Between Aprepitant and Voriconazole via Cytochrome P450 3A4-Mediated Metabolism

Background

Voriconazole is an antifungal drug for which therapeutic monitoring is recommended to prevent side effects. Temporary administration of the antiemetic drug fosaprepitant remarkably decreases the plasma concentration of voriconazole from the therapeutic range. The ratio of the major metabolite voriconazole N-oxide to voriconazole exceeded that at any other time for a patient who started chemotherapy during voriconazole therapy. We attributed this unpredictable result to cytochrome P450 3A4 induced by aprepitant that was converted from fosaprepitant in vivo.

Methods

Concentrations of voriconazole and voriconazole N-oxide were measured using liquid chromatography-mass spectrometry/mass spectrometry in primary human hepatocytes after incubation with aprepitant. Aprepitant suppressed voriconazole N-oxide formation within 24 h, followed by a continuous increase. Levels of drug-metabolizing cytochrome P450 mRNA were measured using real-time PCR in primary human hepatocytes incubated with aprepitant.

Results

Cytochrome P450 3A4 and 2C9 mRNA levels increased ~4- and 2-fold, respectively, over time. Cytochrome P450 3A4 induction was confirmed using reporter assays. We also assessed L-755446, a major metabolite of aprepitant that lacks a triazole ring. Both compounds dose-dependently increased reporter activity; however, induction by L-755446 was stronger than that by aprepitant.

Conclusion

These results indicate that aprepitant initially inhibited voriconazole metabolism via its triazole ring and increased cytochrome P450 3A4 induction following L-755446 formation. The decrease in plasma voriconazole concentration 7 days after fosaprepitant administration was mainly attributed to cytochrome P450 3A4 induction by L-755446.

Assessments of CYP‑inhibition‑based drug-drug interaction between vonoprazan and poziotinib in vitro and in vivo

CONTEXT

Poziotinib and vonoprazan are two drugs mainly metabolized by CYP3A4. However, the drug-drug interaction between them is unknown.

OBJECTIVE

To study the interaction mechanism and pharmacokinetics of poziotinib on vonoprazan.

MATERIALS AND METHODS

In vitro experiments were performed with rat liver microsomes (RLMs) and the contents of vonoprazan and its metabolite were then determined with UPLC-MS/MS after incubation of RLMs with vonoprazan and gradient concentrations of poziotinib. For the in vivo experiment, rats in the poziotinib treated group were given 5 mg/kg poziotinib by gavage once daily for 7 days, and the control group was only given 0.5% CMC-Na. On Day 8, tail venous blood was collected at different time points after the gavage administration of 10 mg/kg vonoprazan, and used for the quantification of vonoprazan and its metabolite. DAS and SPSS software were used for the pharmacokinetic and statistical analyses.

RESULTS

In vitro experimental data indicated that poziotinib inhibited the metabolism of vonoprazan (IC₅₀ = 10.6 μM) in a mixed model of noncompetitive and uncompetitive inhibition. The inhibitory constant K_i was 0.574 μM and the binding constant αK_i was 2.77 μM. In vivo experiments revealed that the AUC_(0-T) (15.05 vs. 90.95 μg/mL·h) and AUC_(0-∞) (15.05 vs. 91.99 μg/mL·h) of vonoprazan increased significantly with poziotinib pretreatment. The MRT_(0-∞) of vonoprazan increased from 2.29 to 5.51 h, while the CLz/F value decreased from 162.67 to 25.84 L/kg·h after pretreatment with poziotinib.

CONCLUSIONS

Poziotinib could significantly inhibit the metabolism of vonoprazan and more care may be taken when co-administered in the clinic.

Identification of Antibiotics in Surface-Groundwater. A Tool towards the Ecopharmacovigilance Approach: A Portuguese Case-Study

Environmental monitoring, particularly of water, is crucial to screen and preselect potential hazardous substances for policy guidance and risk minimisation strategies. In Portugal, extensive data are missing. This work aimed to perform a qualitative survey of antibiotics in surface- groundwater, reflecting demographic, spatial, consumption and drug profiles during an observational period of three years. A passive sampling technique (POCIS) and high-resolution chromatographic system were used to monitor and analyse the antibiotics. The most frequently detected antibiotics were enrofloxacin/ciprofloxacin and tetracycline in surface-groundwater, while clarithromycin/erythromycin and sulfamethoxazole were identified only in surface water. The detection of enzyme inhibitors (e.g., tazobactam/cilastatin) used exclusively in hospitals and abacavir, a specific human medicine was also noteworthy. North (Guimarães, Santo Tirso and Porto) and South (Faro, Olhão and Portimão) Portugal were the regions with the most significant frequency of substances in surface water. The relatively higher detection downstream of the effluent discharge points compared with a low detection upstream could be attributed to a low efficiency in urban wastewater treatment plants and an increased agricultural pressure. This screening approach is essential to identify substances in order to perform future quantitative risk assessment and establishing water quality standards. The greatest challenge of this survey data is to promote an ecopharmacovigilance framework, implement measures to avoid misuse/overuse of antibiotics and slow down emission and antibiotic resistance.

Comparable Intestinal and Hepatic First-Pass Effect of YL-IPA08 on the Bioavailability and Effective Brain Exposure, a Rapid Anti-PTSD and Anti-Depression Compound

YL-IPA08, exerting rapid antidepressant-like and anxiolytic-like effects on behaviors by translocator protein (TSPO) mediation, is a novel compound that has been discovered and developed at our institute. Fit-for-purpose pharmacokinetic properties is urgently needed to be discovered as early as possible for a new compound. YL-IPA08 exhibited low bioavailability (∼6%) during the preliminary pharmacokinetics study in rats after oral administration. Our aim was to determine how metabolic disposition by microsomal P450 enzymes in liver and intestine limited YL-IPA08’s bioavailability and further affected brain penetration to the target. Studies of in vitro metabolic stability and permeability combined with in vivo oral bioavailability, panel CYP inhibitor co-administration via different routes, and double cannulation rats were conducted to elucidate the intestinal and hepatic first-pass effect of YL-IPA08 on bioavailability. Unbound brain-to-plasma ratio (K_p,uu) in rats was determined at steady state. Results indicated that P450-mediated elimination appeared to be important for its extensive first-pass effect with comparative contribution of gut (35%) and liver (17%), and no significant species difference was observed. The unbound concentration of YL-IPA08 in rat brain (6.5 pg/ml) was estimated based on K_p,uu (0.18) and was slightly higher than in vitro TSPO-binding activity (4.9 pg/ml). Based on the onset efficacy of YL-IPA08 toward TPSO in brain and K_p,uu, therapeutic human plasma concentration was predicted to be ∼27.2 ng/ml would easily be reached even with unfavorable bioavailability.

In Vitro Assessment of Potential for CYP-Inhibition-Based Drug-Drug Interaction Between Vonoprazan and Clopidogrel

BACKGROUND AND OBJECTIVES

It was recently proposed that CYP-mediated drug-drug interactions (DDIs) of vonoprazan with clopidogrel and prasugrel can attenuate the antiplatelet actions of the latter two drugs. Clopidogrel is metabolized to the pharmacologically active metabolite H4 and its isomers by multiple CYPs, including CYP2C19 and CYP3A4. Therefore, to investigate the possibility of CYP-based DDIs, in vitro metabolic inhibition studies using CYP probe substrates or radiolabeled clopidogrel and human liver microsomes (HLMs) were conducted in this work.

METHODS

Reversible inhibition studies focusing on the effects of vonoprazan on CYP marker activities and the formation of the [¹⁴C]clopidogrel metabolite H4 were conducted with and without pre-incubation using HLMs. Time-dependent inhibition (TDI) kinetics were also measured.

RESULTS

It was found that vonoprazan is not a significant direct inhibitor of any CYP isoforms (IC₅₀ ≥ 16 μM), but shows the potential for TDI of CYP2B6, CYP2C19, and CYP3A4/5. This TDI was weaker than the inhibition induced by the corresponding reference inhibitors ticlopidine, esomeprazole, and verapamil, based on the measured potencies (k_inact/K_I ratio and the R2 value). In a more direct in vitro experiment, vonoprazan levels of up to 10 µM (a 100-fold higher concentration than the plasma C_max of 75.9 nM after taking 20 mg once daily for 7 days) did not suppress the formation of the active metabolite H4 or other oxidative metabolites of [¹⁴C]clopidogrel in a reversible or time-dependent manner. Additionally, an assessment of clinical trials and post-marketing data suggested no evidence of a DDI between vonoprazan and clopidogrel.

CONCLUSIONS

The body of evidence shows that the pharmacodynamic DDI reported between vonoprazan and clopidogrel is unlikely to be caused by the inhibition of CYP2B6, CYP2C19, or CYP3A4/5 by vonoprazan.

Contributions of Hepatic and Intestinal Metabolism to the Disposition of Niclosamide, a Repurposed Drug with Poor Bioavailability

Niclosamide, an antiparasitic, has been repositioned as a potential therapeutic drug for systemic diseases based on its antiviral, anticancer, and anti-infection properties. However, low bioavailability limits its in vivo efficacy. Our aim was to determine whether metabolic disposition by microsomal P450 enzymes in liver and intestine influences niclosamide's bioavailability in vivo, by comparing niclosamide metabolism in wild-type, liver-Cpr-null (LCN), and intestinal epithelium-Cpr-null (IECN) mice. In vitro stability of niclosamide in microsomal incubations was greater in the intestine than in liver in the presence of NADPH, but it was much greater in liver than in intestine in the presence of UDPGA. NADPH-dependent niclosamide metabolism and hydroxy-niclosamide formation were inhibited in hepatic microsomes of LCN mice, but not IECN mice, compared with wild-type mice. In intestinal microsomal reactions, hydroxy-niclosamide formation was not detected, but rates of niclosamide-glucuronide formation were ∼10-fold greater than in liver, in wild-type, LCN, and IECN mice. Apparent Km and V _max values for microsomal niclosamide-glucuronide formation showed large differences between the two tissues, with the intestine having higher Km (0.47 μM) and higher V _max (15.8) than the liver (0.09 μM and 0.75, respectively). In vivo studies in LCN mice confirmed the essential role of hepatic P450 in hydroxy-niclosamide formation; however, pharmacokinetic profiles of oral niclosamide were only minimally changed in LCN mice, compared with wild-type mice, and the changes seem to reflect the compensatory increase in hepatic UDP-glucuronosyltransferase activity. SIGNIFICANCE STATEMENT: These results suggest that efforts to increase the bioavailability of niclosamide by blocking its metabolism by P450 enzymes will unlikely be fruitful. In contrast, inhibition of niclosamide glucuronidation in both liver and intestine may prove effective for increasing niclosamide's bioavailability, thereby making it practical to repurpose this drug for treating systemic diseases.

Antimalarial interaction of quinine and quinidine with clarithromycin

Quinine (QN) and quinidine (QND) have been commonly used as effective and affordable antimalarials for over many years. Quinine primarily is used for severe malaria treatment. However, plasmodia resistance to these drugs and poor patient compliance limits their administration to the patients. The declining sensitivity of the parasite to the drugs can thus be dealt with by combining with a suitable partner drug. In the present study QN/QND was assessed in combination with clarithromycin (CLTR), an antibiotic of the macrolide family. In vitro interactions of these drugs with CLTR against Plasmodium falciparum (P. falciparum) have shown a synergistic response with mean sum fractional inhibitory concentrations (ΣFICs) of ≤1 (0.85 ± 0.11 for QN + CLTR and 0.64 ± 0.09 for QND + CLTR) for all the tested combination ratios. Analysis of this combination of QN/QND with CLTR in mouse model against Plasmodium yoelii nigeriensis multi-drug resistant (P. yoelii nigeriensis MDR) showed that a dose of 200 mg/kg/day for 4 days of QN or QND produces 100% curative effect with 200 mg/kg/day for 7 days and 150 mg/kg/day for 7 days CLTR respectively, while the same dose of individual drugs could produce only up to a maximum 20% cure. It is postulated that CLTR, a CYP3A4 inhibitor, might have caused reduced CYP3A4 activity leading to increased plasma level of the QN/QND to produce enhanced antimalarial activity. Further, parasite apicoplast disruption by CLTR synergies the antimalarial action of QN and QND.

Clarithromycin enhances bortezomib-induced cytotoxicity via endoplasmic reticulum stress-mediated CHOP (GADD153) induction and autophagy in breast cancer cells

The specific 26S proteasome inhibitor, bortezomib (BZ) potently induces apoptosis as well as autophagy in metastatic breast cancer cell lines such as MDA-MB-231 and MDA-MB-468. The combined treatment of clarithromycin (CAM) and BZ significantly enhances cytotoxicity in these cell lines. Although treatment with up to 100 μg/ml CAM alone had little effect on cell growth inhibition, the accumulation of autophagosomes and p62 was observed after treatment with 25 μg/ml CAM. This result indicated that CAM blocked autophagy flux. However, the combined treatment of BZ and CAM resulted in more pronounced autophagy induction, as assessed by increased expression ratios of LC3B-II to LC3B-I and clearance of intracellular p62, than treatment with BZ alone. This combination further enhanced induction of the pro-apoptotic transcription factor CHOP (CADD153) and the chaperone protein GRP78. Knockdown of CHOP by siRNA attenuated the death-promoting effect of BZ in MDA-MB-231 cells. A wild-type murine embryonic fibroblast (MEF) cell line also exhibited enhanced BZ-induced cytotoxicity with the addition of CAM, whereas a Chop knockout MEF cell line completely abolished this enhancement and exhibited resistance to BZ treatment. These data suggest that endoplasmic reticulum (ER)-stress mediated CHOP induction is involved in pronounced cytotoxicity by combining these reagents. Simultaneously targeting two major intracellular protein degradation pathways such as the ubiquitin-proteasome system by BZ and the autophagy-lysosome pathway by CAM may improve the therapeutic outcome in breast cancer patients via ER-stress mediated apoptosis.

Clarithromycin, a cytochrome P450 inhibitor, can reverse mefloquine resistance in Plasmodium yoelii nigeriensis- infected Swiss mice

During the last 2 decades there have been numerous reports of the emergence of mefloquine resistance in Southeast Asia and nearly 50% resistance is reported in Thailand. A World Health Organization report (2001) considers mefloquine as an important component of ACT (artesunate+mefloquine) which is the first line of treatment for the control of uncomplicated/multi-drug resistant (MDR) Plasmodium falciparum malaria. In view of the emergence of resistance towards this drug, it is proposed to develop new drug combinations to prolong the protective life of this drug. Prior studies have suggested that mefloquine resistance can be overcome by a variety of agents such as ketoconazole, cyproheptadine, penfluridol, Icajine and NP30. The present investigation reports that clarithromycin (CLTR), a new macrolide, being a potent inhibitor of Cyt. P450 3A4, can exert significant resistance reversal action against mefloquine resistance of plasmodia. Experiments were carried out to find out the curative dose of CLTR against multi-drug resistant P. yoelii nigeriensis. Mefloquine (MFQ) and clarithromycin (CLTR) combinations have been used for the treatment of this MDR parasite. Different dose combinations of these two drugs were given to the infected mice on day 0 (prophylactic) and day 1 with established infection (therapeutic) to see the combined effect of these combinations against the MDR malaria infection. With a dose of 32 mg/kg MFQ and 225 mg/kg CLTR, 100% cure was observed, while in single drug groups, treated with MFQ or CLTR, the cure was zero and 40% respectively. Therapeutically, MFQ and CLTR combinations 32+300 mg/kg doses cleared the established parasitaemia on day 10. Single treatment with MFQ or CLTR showed considerable suppression of parasitaemia on day 14 but neither was curative. Follow-up of therapeutically treated mice showed enhanced anti-malarial action as reflected by their 100% clearance of parasitaemia. The present study reveals that CLTR is a useful antibiotic to be used as companion drug with mefloquine in order to overcome mefloquine resistance in plasmodia.

Physiologically based pharmacokinetic model of mechanism-based inhibition of CYP3A by clarithromycin

The prediction of clinical drug-drug interactions (DDIs) due to mechanism-based inhibitors of CYP3A is complicated when the inhibitor itself is metabolized by CYP3Aas in the case of clarithromycin. Previous attempts to predict the effects of clarithromycin on CYP3A substrates, e.g., midazolam, failed to account for nonlinear metabolism of clarithromycin. A semiphysiologically based pharmacokinetic model was developed for clarithromycin and midazolam metabolism, incorporating hepatic and intestinal metabolism by CYP3A and non-CYP3A mechanisms. CYP3A inactivation by clarithromycin occurred at both sites. K(I) and k(inact) values for clarithromycin obtained from in vitro sources were unable to accurately predict the clinical effect of clarithromycin on CYP3A activity. An iterative approach determined the optimum values to predict in vivo effects of clarithromycin on midazolam to be 5.3 microM for K(i) and 0.4 and 4 h(-1) for k(inact) in the liver and intestines, respectively. The incorporation of CYP3A-dependent metabolism of clarithromycin enabled prediction of its nonlinear pharmacokinetics. The predicted 2.6-fold change in intravenous midazolam area under the plasma concentration-time curve (AUC) after 500 mg of clarithromycin orally twice daily was consistent with clinical observations. Although the mean predicted 5.3-fold change in the AUC of oral midazolam was lower than mean observed values, it was within the range of observations. Intestinal CYP3A activity was less sensitive to changes in K(I), k(inact), and CYP3A half-life than hepatic CYP3A. This semiphysiologically based pharmacokinetic model incorporating CYP3A inactivation in the intestine and liver accurately predicts the nonlinear pharmacokinetics of clarithromycin and the DDI observed between clarithromycin and midazolam. Furthermore, this model framework can be applied to other mechanism-based inhibitors.

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.

Modeling the autoinhibition of clarithromycin metabolism during repeated oral administration

Clarithromycin decreases CYP3A4 activity and thus gradually inhibits its own metabolism as well as that of coadministered drugs. The aim of this study was to obtain an understanding of the time course of these changes. The plasma concentration-time profiles of clarithromycin and its active metabolite, 14(R)-hydroxy-clarithromycin, in 12 young healthy volunteers after oral administration of a clarithromycin suspension (500 mg twice a day [b.i.d.] for seven doses) were modeled by population pharmacokinetic analysis in the NONMEM program. The nonlinearity of clarithromycin metabolism was considered during model development, and the metabolite disposition kinetics were assumed to be linear. The absorption kinetics of clarithromycin were best described by a Weibull function model. The pharmacokinetics of clarithromycin and its 14(R)-hydroxyl metabolite were adequately described by a one-compartment model each for clarithromycin and its metabolite as well as an inhibition compartment that reflects the autoinhibition of clarithromycin metabolism. Up to 90% of the apparent total clarithromycin clearance (60 liters/h) was susceptible to reversible autoinhibition, depending on the concentration in the inhibition compartment. The proposed semimechanistic population pharmacokinetic model successfully described the autoinhibition of clarithromycin metabolism and may be used to adjust the doses of other drugs that are metabolized by CYP3A4 and that are coadministered with clarithromycin. Simulations showed that for the standard dose of 500 mg b.i.d., no further increase in the level of exposure occurs after approximately 48 h of treatment. For a 1,000-mg b.i.d. dose, the achievement of steady state is expected to take several days and to achieve a 3.6-fold higher level of clarithromycin exposure than the 500-mg b.i.d. dose. This evaluation provides a rationale for safer and more effective therapy with clarithromycin.

Identification and relative contributions of human cytochrome P450 isoforms involved in the metabolism of glibenclamide and lansoprazole: evaluation of an approach based on thein vitrosubstrate disappearance rate

1. The identification and relative contributions of human cytochrome P450 (CYP) enzymes involved in the metabolism of glibenclamide and lansoprazole in human liver microsomes were investigated using an approach based on the in vitro disappearance rate of unchanged drug. 2. Recombinant CYP2C19 and CYP3A4 catalysed a significant disappearance of both drugs. When the contribution of CYPs to the intrinsic clearance (CL(int)) of drugs in pooled human microsomes was estimated by relative activity factors, contributions of CYP2C19 and CYP3A4 were determined to be 4.6 and 96.4% for glibenclamide, and 75.1 and 35.6% for lansoprazole, respectively. 3. CL(int) of glibenclamide correlated very well with CYP3A4 marker activity, whereas the CL(int) of lansoprazole significantly correlated with CYP2C19 and CYP3A4 marker activities in human liver microsomes from 12 separate individuals. Effects of CYP-specific inhibitors and anti-CYP3A serum on the CL(int) of drugs in pooled human liver microsomes reflected the relative contributions of CYP2C19 and CYP3A4. 4. The results suggest that glibenclamide is mainly metabolized by CYP3A4, whereas lansoprazole is metabolized by both CYP2C19 and CYP3A4 in human liver microsomes. This approach, based on the in vitro drug disappearance rate, is useful for estimating CYP identification and their contribution to drug discovery.

Usefulness of hepatocytes for evaluating the genetic polymorphism of CYP2D6 substrates

The usefulness of human hepatocytes for assessing CYP2D6-related genetic polymorphisms was investigated. Propranolol and propafenone, which undergo phase I and II biotransformations, were used as model substrates alongside metoprolol, which is only metabolized via oxidative pathways. The contributions of CYP2D6 to the primary metabolisms of the substrates were estimated from the quinidine-mediated inhibition of their depletion rate constants in human hepatocytes and liver microsomes. The contributions in hepatocytes were 19.2% for propranolol at 0.05 microM and 36.7--76.3% for propafenone at 0.05--1.0 microM, and smaller than the contribution in microsomes, unlike the case for metoprolol. The differences between microsomes and hepatocytes were attributable to conjugate formation. The CYP2D6 contributions in hepatocytes reflected the in vivo data. The relevance of the concentration-dependent involvement of CYP2D6 in propafenone metabolism in hepatocytes to the in vivo polymorphic profile and the applicability of hepatocytes for evaluating these polymorphisms are discussed.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

IN VITRO STUDIES ON QUETIAPINE METABOLISM USING THE SUBSTRATE DEPLETION APPROACH WITH FOCUS ON DRUG-DRUG INTERACTIONS

The metabolism of the atypical antipsychotic quetiapine was investigated by in vitro methods. Pharmacokinetic parameters were determined in human liver microsomes and recombinant cytochrome P450 measuring substrate depletion and product formation. The cytochrome P450 isozymes CYP3A4 and CYP2D6 displayed activity towards quetiapine. The isozyme CYP2D6 played a minor role in the metabolism of quetiapine as CYP3A4 contributed 89% to the overall metabolism. A Km value of 18 microM was determined by substrate depletion, suggesting linear kinetics under therapeutic conditions. Drugs known to inhibit CYP3A4, such as ketoconazole and nefazodone, displayed almost complete inhibition at low concentrations, whereas inhibitors of CYP2D6 do not seem to have a clinically relevant effect.