Potent CYP3A4 Inhibitor Research Articles

Possible pharmacokinetic interaction involving ritonavir and docetaxel in a patient with Kaposi's sarcoma

A 40-year-old patient, known to be HIV-infected since 1990 but who had always been a poor adherent to follow-up and treatment, was referred for Pneumocystis jiroveci pneumonia in March 2005. In addition to fever (38.2°C), dyspnea and diffuse crackles raises, a 1-cm purplish tumefaction of the left lower eyelid was noticed, which had appeared a few days earlier, without any visual impairment. The rest of the cutaneous examination was normal. CD4 count was 11/μl (2%) and viral load was 190 000 RNA copies/ml (5.28 log). The patient received cotrimoxazole for his pneumonia, and an antiretroviral therapy was started combining lamivudine, tenofovir disoproxil fumarate and ritonavir-boosted lopinavir. The patient was cured of his pneumonia and was discharged from the hospital. He missed the follow-up appointments and neglected the eyelid lesion. Two months later, the lesion had evolved to a 3-cm tumor. An orbital invasion was documented by computed tomography scan (Fig. 1). The hypothesis of a Kaposi's sarcoma of the eyelid was raised. The patient refused orbital exenteration, and subsequently conservative surgery was performed. The pathologic examination of the tumor confirmed the diagnosis of Kaposi's sarcoma. As the surgical treatment was not optimal, especially regarding margins of resection, the patient was started with adjuvant chemotherapy consisting in weekly docetaxel 25 mg/m2, according to previously described regimens [1].Fig. 1: Orbital computed tomography scan, showing the invasion of the orbital cavity.Eight days after the first infusion of docetaxel, the patient developed neutropenic fever, with a neutrophil count of 450/μl and a C-reactive protein of 196 mg/mlL. He was rehospitalized and received intravenous antibiotics. Microbiological tests were negative, and the neutropenia resolved in the following week. We hypothesized that ritonavir-induced inhibition of CYP3A4, the main enzyme that metabolizes docetaxel [1,2], was responsible for an increased plasma exposure to docetaxel, and thus may have caused this febrile neutropenia. Hence, docetaxel was stopped, and the patient remained under highly active antiretroviral therapy (HAART). During the following 6 months, the CD4 count reached 130/μl (10%), and the viral load remained below 40 RNA copies/ml. No local recurrence of the Kaposi's sarcoma of the eyelid was noticed during the following 2 years. Kaposi's sarcoma of the eyelid is a rare manifestation of AIDS, which was first reported in 1989 in the setting of HIV infection [3], while other ocular manifestations of AIDS were known since the very beginning of the pandemia [4]. Before the HIV era, this manifestation of Kaposi's sarcoma was scarcely reported in the English literature [5,6], with a poorer prognosis than in other sites [7]. Some HIV-infected patients, even with a good immune restoration after the introduction of HAART, still display aggressive Kaposi's sarcoma, and therefore require chemotherapy. Anthracyclines and taxanes are active in this setting. In patients with Kaposi's sarcoma of the eyelid, the first line of treatment should be orbital exenteration [3]. However, in this patient, a conservative surgical treatment was decided, and we assumed that adjuvant chemotherapy together with HAART could improve disease control. The patient developed a febrile neutropenia while undergoing a HAART containing ritonavir, concomitantly to weekly docetaxel. Weekly docetaxel is a well tolerated and feasible regimen in Kaposi's sarcoma patients, with limited toxicities: in a recent phase II study in 12 patients [1], no grade 4 neutropenia or neutropenic fever was observed. It is worth noting that patients included in this study received HAART without ritonavir pharmacoenhancement. Ritonavir is a protease inhibitor that potently inhibits CYP3A4, and is subsequently prescribed in HAART as a booster for other protease inhibitors metabolized by CYP3A4 [8]. Moreover, preclinical data indicate that the inhibition of CYP3A4 indiced by ritonavir can cause a significant increase in exposure to docetaxel [9]. In addition, the concomitant administration of docetaxel and ketoconazole, another potent inhibitor of CYP3A4, resulted in a major reduction in plasma clearance of docetaxel [10]. Hence, we postulate that ritonavir may cause a decrease in docetaxel plasma clearance through inhibition of CYP3A4, and therefore induce acute toxicity of docetaxel, including febrile neutropenia as described in the present case. According to the drug interaction probability scale [11], the interaction between ritonavir and docetaxel was possible (score = 2). In conclusion, we hypothesize that ritonavir may decrease the plasma clearance of docetaxel through inhibition of CYP3A4. Therefore, we suggest avoiding the concomitant administration of ritonavir and docetaxel. Hence, Kaposi's sarcoma patients undergoing a HAART including ritonavir should receive anticancer drugs that are not metabolized by CYP3A4 instead of docetaxel. For instance, paclitaxel would be a valuable option since it is mainly metabolized by CYP2C8 [12] with a minor contribution of CYP3A4.

Differential effects of ketoconazole on exposure to temsirolimus following intravenous infusion of temsirolimus

Intravenous (i.v.) temsirolimus, a novel inhibitor of mammalian target of rapamycin, is approved for the treatment of advanced renal cell carcinoma and is being studied in patients with mantle cell lymphoma. Because temsirolimus and its primary metabolite, sirolimus, are metabolised by the cytochrome P450 3A4 pathway (CYP3A4), the potential exists for pharmacokinetic (PK) drug interactions with the numerous agents that modulate CYP3A4 isozyme activity. We investigated the effects of ketoconazole, a potent CYP3A4 inhibitor, on the PK profile of i.v. temsirolimus in healthy adults. Coadministration of 400 mg oral ketoconazole with 5 mg i.v. temsirolimus had no significant effect on temsirolimus maximum concentration (Cmax) or area under the concentration curve (AUC). However, mean AUC increased 3.1-fold and AUCsum (sum of temsirolimus plus sirolimus AUCs) increased 2.3-fold compared with temsirolimus alone. A single 5-mg dose of temsirolimus with ketoconazole was well tolerated, and there were no unexpected safety results. Therefore, in cancer patients receiving 25 mg i.v. temsirolimus, concomitant treatment with agents that have strong CYP3A4 inhibition potential should be avoided. If a concomitant strong CYP3A4 inhibitor is necessary, a temsirolimus dose reduction to 12.5 mg weekly should be considered.

Prediction of Pharmacokinetic Drug-Drug Interactions Using Human Hepatocyte Suspension in Plasma and Cytochrome P450 Phenotypic Data. II. In Vitro-in Vivo Correlation with Ketoconazole

Traditional cytochrome P450 (P450) based drug-drug interaction (DDI) predictions are based on the ratio of an inhibitor's physiological concentration [I] and its inhibition constant K(i). Determining [I] at the enzymatic site, although critical for predicting clinical DDIs, remains a technical challenge. In our previous study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was investigated to mimic the in vivo concentration of ketoconazole at the enzymatic site (Lu et al., 2007), effectively eliminating the estimation of the elusive [I] value. P450 inhibition in this system appears to model that in vivo. Using the ketoconazole inhibition information in a human hepatocyte-plasma suspension together with quantitative P450 phenotypic information, we successfully predicted the pharmacokinetic DDIs for a small set of drugs, such as theophylline, tolbutamide, omeprazole, desipramine, midazolam, loratadine, cyclosporine, and alprazolam, as well as an investigational compound. For the applicability of this model on a wider scale the in vitro-in vivo correlation data set needed to be expanded. However, for most drugs in the literature there is not enough quantitative information on the involvement of individual P450s to predict DDIs retrospectively. To facilitate that, in this study we determined quantitative P450 phenotyping for seven marketed drugs: budesonide, buprenorphine, loratadine, sirolimus, tacrolimus, docetaxel, and methylprednisolone. Augmentation of the new data set with the one generated previously produced broader a database that provided further support for the wider applicability of this approach using ketoconazole as a potent CYP3A inhibitor. This application is predicted to be equally effective with other P450 inhibitors that are not substrates of efflux pumps.

Hydroxyitraconazole, Formed During Intestinal First-Pass Metabolism of Itraconazole, Controls the Time Course of Hepatic CYP3A Inhibition and the Bioavailability of Itraconazole in Rats

Itraconazole (ITZ) is a substrate of CYP3A and both ITZ and hydroxyitraconazole (OH-ITZ), a major metabolite formed by CYP3A, are potent inhibitors of CYP3A. The concentration- and time-dependent changes in the hepatic availability (F(H)) of ITZ were evaluated in rats after oral doses of 5 and 40 mg/kg. Simultaneous blood samples were obtained from the aorta, portal vein, and hepatic vein for 24 h following duodenal ITZ administration, and concentrations of ITZ and OH-ITZ determined by LC/MS. During the absorption phase, the F(H) of ITZ increased from 0.2 to 1.0, reflecting the time course of hepatic CYP3A inhibition. A counterclockwise hysteresis was observed between ITZ concentrations entering the liver (C(IN,ITZ)) and F(H), whereas there was no time delay observed between the change in F(H) and the OH-ITZ concentrations entering the liver (C(IN,OH-ITZ)). The direct relationship between C(IN,OH-ITZ) and F(H) suggested that OH-ITZ was mainly responsible for the inhibition of CYP3A. A positive portal venous-aortic gradient for OH-ITZ was measured after duodenal administration of ITZ, indicating intestinal formation of OH-ITZ. The in vivo Ki for OH-ITZ (38 +/- 3 nM) was estimated from C(IN,OH-ITZ) versus F(H) of ITZ, and is similar to values obtained from inhibition of midazolam hydroxylation in CYP3A4 supersomes (Drug Metab Dispos 32:1121-1131, 2004). The data suggest that OH-ITZ, formed by intestinal CYP3A, controls the time course of hepatic CYP3A inhibition and is mainly responsible for the observed increase in F(H) of ITZ.

Effects of CYP3A4 inhibitors on the pharmacokinetics of maraviroc in healthy volunteers

To evaluate the influence of cytochrome P450 (CYP) 3A4 inhibitors on the clinical pharmacokinetics of maraviroc, a novel CCR5 antagonist. Four open-label, randomized, placebo-controlled studies were conducted in healthy subjects to assess the effect of separate and distinct combinations of CYP3A4 inhibitors on the steady-state pharmacokinetics of maraviroc. Study 1 was a two-way crossover study investigating the influence of saquinavir (SQV; 1200 mg t.i.d.) and ketoconazole (400 mg q.d.) on the pharmacokinetics of maraviroc (100 mg b.i.d.). All subjects received maraviroc for 7 days in both study periods. Cohort 1 subjects also received SQV or placebo and cohort 2 subjects also received ketoconazole or placebo. Study 2 was a parallel-group study including four treatment groups investigating the effects of ritonavir-boosted lopinavir (LPV/r; 400 mg/100 mg b.i.d.), ritonavir-boosted saquinavir (SQV/r; 1000 mg/100 mg b.i.d.), and low-dose ritonavir (RTV; 100 mg b.i.d.) on the steady-state pharmacokinetics of maraviroc (100 mg b.i.d.), and exploring whether maraviroc dose adjustment can compensate for interaction effects. Treatment lasted 28 days and comprised three distinct phases: (i) maraviroc alone on days 1-7; (ii) maraviroc + interactant on days 8-21; and (iii) maraviroc (adjusted dose) + interactant on days 22-28. Study 3 was a two-way crossover study investigating the effects of atazanavir (ATZ; 400 mg q.d.) and ritonavir-boosted atazanavir (ATZ/r; 300 mg/100 mg b.i.d.) on the pharmacokinetics of maraviroc (300 mg b.i.d.). All subjects received maraviroc on days 1-14 of both study periods. Subjects also received ATZ on days 1-7 and ATZ/r on days 8-14 of one treatment period, and placebo on days 1-14 of the other treatment period. Study 4 was a two-way crossover study investigating the effects of ritonavir-boosted tipranavir (TPV/r; 500 mg/200 mg b.i.d.) on the pharmacokinetics of maraviroc (150 mg b.i.d.). Subjects received maraviroc plus TPV/r or placebo on days 1-8. All of the drugs/drug combinations tested (except for TPV/r) increased maraviroc exposure, albeit to different degrees of magnitude. SQV/r caused the largest increase in maraviroc exposure (8.3-fold increase in AUC(tau)), whereas RTV caused the smallest increase in maraviroc exposure (2.6-fold increase in AUC(tau)). Downward adjustment of the maraviroc dose in study 2 during co-administration of HIV protease inhibitors was able to compensate for the interactions. TPV/r had no clinically relevant effect on maraviroc exposure at steady state. There were no treatment-related serious adverse events or discontinuations due to adverse events in any of the studies, and most adverse events were mild or moderate in severity and resolved without intervention. Potent CYP3A4 inhibitors, including ketoconazole and protease inhibitors (except TPV/r), increase maraviroc exposure. Downward adjustment of the maraviroc dose during co-administration with protease inhibitors can compensate for the interaction. TPV/r does not affect the steady-state pharmacokinetics of maraviroc, and hence no dose adjustment would be warranted.

Effect of Itraconazole on the Pharmacokinetics of Imidafenacin in Healthy Subjects

The effect of itraconazole, a potent inhibitor of the CYP3A isoenzyme family, on the pharmacokinetics of imidafenacin, a novel synthetic muscarinic receptor antagonist, was investigated. Twelve healthy subjects participated in this open-label, self-controlled study. In period I, subjects received a single oral dose of 0.1 mg imidafenacin. In period II, they received multiple oral doses of 200 mg itraconazole for 9 days and a single oral dose of 0.1 mg imidafenacin on day 8. Plasma concentrations of imidafenacin and M-2, the major metabolite of imidafenacin metabolized by CYP3A4, were determined. Analytes were measured by liquid chromatography tandem mass spectrometry. Following coadministration with itraconazole, the maximum plasma concentration (C(max)) of imidafenacin increased 1.32-fold (90% confidence intervals [CIs]: 1.12-1.56), and the area under the plasma concentration-time curve from time 0 to infinity (AUC(0-infinity)) increased 1.78-fold (90% CI: 1.47-2.16). In conclusion, itraconazole increases the plasma concentrations of imidafenacin by inhibiting CYP3A4. Therefore, itraconazole or potent CYP3A4 inhibitors should be carefully added to imidafenacin drug regimens.

Effect of Ginkgo biloba extract on lopinavir, midazolam and fexofenadine pharmacokinetics in healthy subjects

ABSTRACTObjective: Animal and in vitro data suggest that Ginkgo biloba extract (GBE) may modulate CYP3A4 activity. As such, GBE may alter the exposure of HIV protease inhibitors metabolized by CYP3A4. It is also possible that GBE could alter protease inhibitor pharmacokinetics (PK) secondary to modulation of P-glycoprotein (P‑gp). The primary objective of the study was to evaluate the effect of GBE on the exposure of lopinavir in healthy volunteers administered lopinavir/ritonavir. Secondary objectives were to compare ritonavir exposure pre- and post-GBE, and assess the effect of GBE on single doses of probe drugs midazolam and fexofenadine.Methods: This open-label study evaluated the effect of 2 weeks of standardized GBE administration on the steady-state exposure of lopinavir and ritonavir in 14 healthy volunteers administered lopinavir/ritonavir to steady-state. In addition, single oral doses of probe drugs midazolam and fexofenadine were administered prior to and after 4 weeks of GBE (following washout of lopinavir/ritonavir) to assess the influence of GBE on CYP3A and P‑gp activity, respectively.Results: Lopinavir, ritonavir and fexofenadine exposures were not significantly affected by GBE administration. However, GBE decreased midazolam AUC0–∞ and Cmax by 34% ( p = 0.03) and 31% ( p = 0.03), respectively, relative to baseline. In general, lopinavir/ritonavir and GBE were well tolerated. Abnormal laboratory results included mild elevations in hepatic enzymes, cholesterol and triglycerides, and mild-to-moderate increases in total bilirubin.Conclusions: Our results suggest that GBE induces CYP3A metabolism, as assessed by a decrease in midazolam concentrations. However, there was no change in the exposure of lopinavir, likely due to ritonavir's potent inhibition of CYP3A4. Thus, GBE appears unlikely to reduce the exposure of ritonavir-boosted protease inhibitors, while concentrations of unboosted protease inhibitors may be affected. Limitations to our study include the single sequence design and the evaluation of a ritonavir-boosted protease inhibitor exclusively.

Celecoxib is a CYP1A2 inhibitor in vitro but not in vivo

We recently discovered that rofecoxib is a potent mechanism-based inhibitor of CYP1A2. The effect of the widely used cyclo-oxygenase-2 selective non-steroidal anti-inflammatory drug celecoxib on CYP1A2 activity has not been reported. The effect of celecoxib on CYP1A2 activity (phenacetin O-deethylation) was first studied in vitro using human liver microsomes. This was followed by a randomized, placebo-controlled, cross-over study in which 12 healthy volunteers were given celecoxib (200 mg twice daily) or placebo for 4 days. On day 3, a caffeine test was performed. On day 4, the subjects ingested 2 mg tizanidine. Plasma samples for the measurement of the concentrations of tizanidine, its metabolites and celecoxib were collected up to 24 h post-administration. Pharmacodynamic variables (e.g. blood pressure, subjective drowsiness and drug effect) were recorded up to 12 h post-adm. Celecoxib was found to be a moderately potent competitive inhibitor of CYP1A2 in vitro with a K(i) (inhibitor constant) of 25.4 microM. However, in vivo, celecoxib did not affect the caffeine test, or the peak concentration, time to peak concentration, area under the concentration-time curve or half-life of tizanidine. The pharmacodynamic variables of tizanidine also remained unchanged. Unlike rofecoxib, celecoxib does not clinically to significantly inhibit CYP1A2. The lack of significant in vivo inhibition of CYP1A2 can be correctly predicted on the basis of in vitro K(i) data and the free peripheral or portal plasma concentration of celecoxib.

In Vitro and In Vivo Evaluations of Cytochrome P450 1A2 Interactions with Duloxetine

To determine whether duloxetine is a substrate, inhibitor or inducer of cytochrome P450 (CYP) 1A2 enzyme, using in vitro and in vivo studies in humans. Human liver microsomes or cells with expressed CYP enzymes and specific CYP inhibitors were used to identify which CYP enzymes catalyse the initial oxidation steps in the metabolism of duloxetine. The potential of duloxetine to inhibit CYP1A2 activity was determined using incubations with human liver microsomes and phenacetin, the CYP1A2 substrate. The potential for duloxetine to induce CYP1A2 activity was determined using human primary hepatocytes treated with duloxetine for 72 hours. Studies in humans were conducted using fluvoxamine, a potent CYP1A2 inhibitor, and theophylline, a CYP1A2 substrate, as probes. The subjects were healthy men and women aged 18-65 years. Single-dose duloxetine was administered either intravenously as a 10-mg infusion over 30 minutes or orally as a 60-mg dose in the presence or absence of steady-state fluvoxamine (100 mg orally once daily). Single-dose theophylline was given as 30-minute intravenous infusions of aminophylline 250 mg in the presence or absence of steady-state duloxetine (60 mg orally twice daily). Plasma concentrations of duloxetine, its metabolites and theophylline were determined using liquid chromatography with tandem mass spectrometry. Pharmacokinetic parameters were estimated using noncompartmental methods and evaluated using mixed-effects ANOVA. Safety measurements included vital signs, clinical laboratory tests, a physical examination, ECG readings and adverse event reports. The in vitro results indicated that duloxetine is metabolized by CYP1A2; however, duloxetine was predicted not to be an inhibitor or inducer of CYP1A2 in humans. Following oral administration in the presence of fluvoxamine, the duloxetine area under the plasma concentration-time curve from time zero to infinity (AUC(infinity)) and the maximum plasma drug concentration (C(max)) significantly increased by 460% (90% CI 359, 584) and 141% (90% CI 93, 200), respectively. In the presence of fluvoxamine, the oral bioavailability of duloxetine increased from 42.8% to 81.9%. In the presence of duloxetine, the theophylline AUC(infinity) and C(max) increased by only 13% (90% CI 7, 18) and 7% (90% CI 2, 14), respectively. Coadministration of duloxetine with fluvoxamine or theophylline did not result in any clinically important safety concerns, and these combinations were generally well tolerated. Duloxetine is metabolized primarily by CYP1A2; therefore, coadministration of duloxetine with potent CYP1A2 inhibitors should be avoided. Duloxetine does not seem to be a clinically significant inhibitor or inducer of CYP1A2; therefore, dose adjustment of CYP1A2 substrates may not be necessary when they are coadministered with duloxetine.

Effect of Clarithromycin on the Pharmacokinetics of Pranlukast in Healthy Volunteers

Pranlukast is a cysteinyl leukotriene receptor antagonist that has been used to treat bronchial asthma and allergic rhinitis. In vitro data suggest that pranlukast is a substrate of CYP3A4. Thus, the effect of clarithromycin, a potent CYP3A4 inhibitor, on the pharmacokinetics of pranlukast was examined in an open-label, randomized, two-way crossover study in 16 healthy male volunteers. In treatment A, volunteers received a single, 225 mg dose of pranlukast. In treatment B, 200 mg of clarithromycin was administered twice daily for 7 days and a single, 225 mg dose of pranlukast was coadministered on day 7. Blood samples were collected up to 24 hours after treatment, and pranlukast concentrations in the plasma were measured. The geometric mean ratios [GMR] (90% confidence intervals [CIs]) for pranlukast AUC(0-infinity) and C(max) (with/without clarithromycin) were 1.06 (0.91, 1.24) and 1.17 (0.95, 1.45), respectively. In conclusion, clarithromycin and pranlukast could be coadministered without dose adjustment because clarithromycin minimally affected the pharmacokinetics of pranlukast.

Inhibition of metabolism of [ 11C]Loperamide in mouse by the potent CYP3A4 inhibitor, ketoconazole

Inhibition of metabolism of [ 11C]Loperamide in mouse by the potent CYP3A4 inhibitor, ketoconazole

General Framework for the Prediction of Oral Drug Interactions Caused by CYP3A4 Induction from In Vivo Information

Induction of cytochrome P450 (CYP) 3A4 potentially reduces the blood concentrations of substrate drugs to less than one-tenth, which results in ineffective pharmacotherapy. Although the prediction of drug-drug interactions (DDIs) that are mediated by induction of CYP3A4 has been performed mainly on the basis of in vitro information, such methods have met with limited success in terms of their accuracy and applicability. Therefore, a realistic method for the prediction of CYP3A4-mediated inductive DDIs is of major clinical importance. The objective of the present study was to construct a robust and accurate method for the prediction of CYP3A4-mediated inductive DDIs. Such a method was developed on the basis of the principle applied for prediction of inhibitory DDIs in a previous report. A unique character of this principle is that the extent of alterations in the area under the plasma concentration-time curve (AUC) is predicted on the basis of in vivo information from minimal clinical studies without using in vitro data. The analysis is based on 42 DDI studies in humans reported in 37 published articles over the period 1983-2007. Kinetic analysis revealed that the reduction in the AUC of a substrate of CYP3A4 produced by consecutive administration of an inducer of CYP3A4 could be approximated by the equation 1/(1 + CRCYP3A4 * ICCYP3A4), where CRCYP3A4 is the ratio of the apparent contribution of CYP3A4 to the oral clearance of a substrate and ICCYP3A4 is the apparent increase in clearance of a substrate produced by induction of CYP3A4. Using this equation, the ICCYP3A4 was calculated for seven inducers (bosentan, carbamazepine, efavirenz, phenytoin, pioglitazone, rifampicin [rifampin], and St John's wort [hypericum]) on the basis of the reduction in the AUC of a coadministered standard substrate of CYP3A4, such as simvastatin, in ten DDI studies. The CRCYP3A4 was calculated for 22 substrates on the basis of the previously reported method from inhibitory DDI studies using a potent CYP3A4 inhibitor such as itraconazole or ketoconazole. The proposed method enabled the prediction of AUC reduction by CYP3A4 induction with any combination of these substrates and inducers (total 154 matches). To assess the accuracy of the prediction, the AUC reductions in 32 studies were analysed. We found that the magnitude of the deviation between the mean values of the observed and predicted AUCs of all substrate drugs was <20% of the AUCs of the respective substrate drugs before administration of the inducers. In addition, rifampicin was found to be the most potent inducer among the compounds analysed in the present study, with an ICCYP3A4 value of 7.7, followed by phenytoin and carbamazepine, with values of 4.7 and 3.0, respectively. The ICCYP3A4 values of the other CYP3A4 inducers analysed in the present study were approximately 1 or less, which suggests that the AUCs of coadministered drugs may not be reduced to less than approximately half, even if the drug is metabolized solely by CYP3A4. By using the method reported in the present study, the susceptibilities of a substrate drug of CYP3A4 to inductive DDIs can be predicted quantitatively. It was indicated that coadministration of rifampicin, phenytoin and carbamazepine may reduce plasma AUCs to less than half for a broad range of CYP3A4 substrate drugs, with CRCYP3A4 values greater than 0.13, 0.21 and 0.33, respectively.

Pharmacokinetic Comparison of the Potential Over-the-Counter Statins Simvastatin, Lovastatin, Fluvastatin and Pravastatin

HMG-CoA reductase inhibitors (statins) dose-dependently lower both the level of low-density lipoprotein cholesterol and risk of cardiovascular disease. In 2004, the UK approved a low-dose over-the-counter (OTC) simvastatin, but the US has rejected applications for non-prescription preparations of statins. The pharmacokinetics and interaction potentials of the possible OTC candidate statins simvastatin, lovastatin, fluvastatin and pravastatin are clearly different. Simvastatin and lovastatin are mainly metabolized by cytochrome P450 (CYP) 3A, fluvastatin is metabolized by CYP2C9, and pravastatin is excreted largely unchanged. Several cell membrane transporters can influence the disposition of statins, e.g. the organic anion transporting polypeptide (OATP) 1B1 enhances their hepatic uptake. The c.521T>C (p.Val174Ala) genetic polymorphism of SLCO1B1 (encoding OATP1B1) considerably increases the plasma concentrations of simvastatin acid and moderately increases those of pravastatin but seems to have no significant effect on fluvastatin. Strong inhibitors of CYP3A (itraconazole, ritonavir) greatly (up to 20-fold) increase plasma concentrations of simvastatin, lovastatin and their active acid forms, thus enhancing the risk of myotoxicity. Weak or moderately potent CYP3A inhibitors such as verapamil, diltiazem and grapefruit juice can be used cautiously with low doses of simvastatin or lovastatin, but their concomitant use needs medical supervision. Potent inducers of CYP3A can greatly decrease plasma concentrations of simvastatin and simvastatin acid, and probably those of lovastatin and lovastatin acid. Although fluvastatin is metabolized by CYP2C9, its concentrations are changed less than 2-fold by inhibitors or inducers of CYP2C9. Pravastatin plasma concentrations are not significantly affected by any CYP inhibition and only slightly affected by inducers. Ciclosporin inhibits CYP3A, P-glycoprotein and OATP1B1. Gemfibrozil and its glucuronide inhibit CYP2C8 and OATP1B1. Ciclosporin and gemfibrozil increase plasma concentrations of statins and the risk of their myotoxicity, but fluvastatin seems to carry a smaller risk than other statins. Inhibitors of OATP1B1 may decrease the benefit-risk ratio of simvastatin, lovastatin and pravastatin by interfering with their (active acid forms) entry into hepatocytes. Understanding the differences in the pharmacokinetics and interaction potential of various statins helps in their selection for possible non-prescription status. On the pharmacokinetic basis, fluvastatin and pravastatin can be better choices than simvastatin or lovastatin for an OTC statin.

Influence of CYP3A4 Inhibition on the Steady-State Pharmacokinetics of Imatinib

To evaluate the effects of ritonavir, a potent inhibitor of CYP3A4, on the steady-state pharmacokinetics of imatinib. Imatinib pharmacokinetics were evaluated in cancer patients receiving the drug for at least 2 months, after which ritonavir (600 mg) was administered daily for 3 days. Samples were obtained on the day before ritonavir (day 1) and on the third day (day 4). The in vitro metabolism of imatinib with or without ritonavir and the effect of imatinib on 1-OH-midazolam formation rate, a probe for CYP3A4 activity, were evaluated with human CYP3A4 and pooled liver microsomes. In 11 evaluable patients, the geometric mean (95% confidence interval) area under the curve of imatinib on days 1 and 4 were 42.6 (33.0-54.9) microg.h/mL and 41.2 (32.1-53.1) microg.h/mL, respectively (P = 0.65). A population analysis done in NONMEM with a time-dependent covariate confirmed that ritonavir did not influence the clearance or bioavailability of imatinib. In vitro, imatinib was metabolized to the active metabolite CGP74588 by CYP3A4 and CYP3A5 and, to a lesser extent, by CYP2D6. Ritonavir (1 micromol/L) completely inhibited CYP3A4-mediated metabolism of imatinib to CGP74588 but inhibited metabolism in microsomes by only 50%. Imatinib significantly inhibited CYP3A4 activity in vitro. At steady state, imatinib is insensitive to potent CYP3A4 inhibition and relies on alternate elimination pathways. For agents with complex elimination pathways that involve autoinhibition, interaction studies that are done after a single dose may not be applicable when drugs are administered chronically.

Pharmacokinetic interaction between ketoconazole and praziquantel in healthy volunteers

Praziquantel, a broad-spectrum anthelminthic, has been reported to undergo extensive first-pass metabolism by cytochrome P450 (CYP) enzymes in vivo. Ketoconazole, a potent CYP3A4 inhibitor, is known to markedly increase plasma concentrations of many co-administered drugs. However, no data are available on the potential pharmacokinetic drug interaction between ketoconazole and praziquantel in humans. To investigate the potential pharmacokinetic interaction of ketoconazole with praziquantel in healthy adult Thai male volunteers. In an open-label, randomized two-phase crossover study, separated by a 2-week period, 10 healthy adult Thai male volunteers ingested a single dose of 20 mg/kg praziquantel alone or with co-administration of 400-mg ketoconazole orally daily for 5 days. Venous blood samples were collected at specific times for a 24-h period. Plasma concentrations of praziquantel were determined using high-performance liquid chromatography. A non-compartmental model was applied for pharmacokinetic parameter analysis of praziquantel. Concurrent administration of ketoconazole with praziquantel significantly increased the mean area under the curve from time zero to infinity (AUC(0-alpha)) and maximum plasma concentration (Cmax) of praziquantel by 93% (955.94 +/- 307.74 vs. 1843.10 +/- 336.39 ng h/mL; P < 0.01) and 102% (183.38 +/- 43.90 vs. 371.31 +/- 44.63 ng/mL; P < 0.01), respectively, whereas the mean total clearance (Cl/F) of praziquantel was significantly decreased by 58% (2.65 +/- 0.64 vs. 1.11 +/- 0.35 mL/h/kg; P < 0.01). Ketoconazole co-administration alters the pharmacokinetics of praziquantel in humans, possibly through inhibition of CYP3A, particularly CYP3A4, first-pass metabolism of praziquantel. Our data suggest that when praziquantel is co-administered with ketoconazole, the dose of praziquantel could be reduced to half the standard dose of praziquantel to reduce the cost of therapy.

The effects of CYP3A4 inhibition on erlotinib pharmacokinetics: computer-based simulation (SimCYP™) predicts in vivo metabolic inhibition

Erlotinib is an orally active antitumor agent. Analyses in vitro using human liver microsomes and recombinant enzymes showed that erlotinib was metabolized primarily by CYP3A4, with a secondary contribution from CYP1A2. A computer-based simulation model, SimCYP, predicted that CYP3A4 contributed to approximately 70% of the metabolic elimination of erlotinib, with CYP1A2 being responsible for the other approximately 30%. A drug-drug interaction study was therefore conducted for erlotinib and a potent CYP3A4 inhibitor, ketoconazole, in healthy male volunteers to evaluate the impact of CYP3A4 inhibition on erlotinib exposure. Ketoconazole caused an almost two-fold increase in erlotinib plasma area under the concentration curve and in maximum plasma concentration. This is consistent with the SimCYP prediction of a two-fold increase in erlotinib AUC, further validating a primary (approximately 70%) role of CYP3A4 in erlotinib elimination. Prediction of clinically important drug-drug interaction with SimCYP using in vitro human metabolism data can be a powerful tool during early clinical development to ensure safe administration of anticancer drugs, which are often co-administered at maximum tolerated doses with other drugs as part of a palliative treatment regimen.

The Effect of Silymarin on Oral Nifedipine Pharmacokinetics

Silibinin, the main component of silymarin (a milk thistle extract used for treatment of liver injury), has been shown to inhibit CYP3A4 in human liver microsomes. The present study was conducted to examine whether inhibition of CYP3A4 by silymarin is also present IN VIVO. Immediate release nifedipine (10 mg) was administered as a CYP3A4 test drug either alone or with co-administration of silymarin (280 mg administered 10 hours and 1.5 hours prior to the administration of nifedipine) to 16 healthy male volunteers (mean age 27 years, mean body weight 77 kg). Nifedipine and silibinin concentrations were quantified by HPLC, heart rate and blood pressure were monitored for safety reasons. Pharmacokinetic parameters were calculated by non-compartmental methods, and the potential interaction by silymarin was handled as an equivalence problem. We found that nifedipine AUC was 1.13-fold higher (90 % CI, 0.97- to 1.32-fold) in the silymarin period, C (max) values were 0.70-fold (90 % CI, 0.39- to 1.27-fold) of those of the reference period, with a trend to delayed absorption in the silymarin period. Intraindividual variability especially for C (max) (intrasubject CV 120 %) was unexpectedly high. There was no meaningful effect on hemodynamic parameters. In conclusion, our data suggest that co-administration of silymarin does not considerably change the extent of absorption or metabolism of nifedipine but may decrease the absorption rate. Silymarin thus is not a potent CYP3A4 inhibitor IN VIVO.

Imazalil modulates CYPs 1A1 and 3A4 activities in the human Caco-2 cells as an intestinal model to assess food safety

There are increasing evidences that ingestion of contaminated food provokes interactions at the intestinal level of major concern for food safety and appearance of drug interactions. We currently use human intestinal Caco-2 cells to evaluate interactions between chemicals and intestinal xenobiotic-metabolizing enzymes, mainly CYPs 1A1 and 3A4 isoforms, as well as apical efflux transporters. Imazalil (IMA) is a widely used post-harvest fungicide and, as such, a relevant food contaminant. IMA strongly induced CYP1A1 activity in a dose- and time-dependent manner. At low, but realistic intestinal concentrations (25 ppb), CYP1A1 activity increased rapidly, but only transiently, suggesting IMA biotransformation and/or (intralysosomal) accumulation by Caco-2 cells. In these cells, IMA was not an aryl hydrocarbon receptor (AhR) agonist, as revealed by reporter gene assay. In TCDD-induced cells, IMA showed no antagonistic effect towards CYP1A1 activity. Finally, some polyphenols reduced the IMA-induced CYP1A1 activity, whereas an additive effect was observed upon coincubation of IMA with benzo(a)pyrene. IMA also appeared as a CYP3A4 inhibitor as potent as ketoconazole, a classical pharmacological inhibitor. In addition, IMA seems to be without effect on either PgP or MRP-2 efflux pumps. HPLC analyses were carried out in order to evaluate IMA absorption by Caco-2 cells. These data reveal that IMA is a potent inducer of CYP1A1 activity in human intestinal Caco-2 cells, occurring by an AhR-independent mechanism, as well as a potent inhibitor of CYP3A4. These CYPs interactions may potentially cause adverse effects on human health by activating procarcinogenic food contaminants or by modulating drug bioavailability.

Tolfenamic acid is a potent CYP1A2 inhibitor in vitro but does not interact in vivo: correction for protein binding is needed for data interpretation

Our aim was to correlate the in vitro and in vivo CYP1A2 inhibition potential of tolfenamic acid, an NSAID highly (99.7%) bound to plasma proteins, to study the significance of protein binding of inhibitor in metabolic drug interactions. The effect of tolfenamic acid on CYP1A2 (phenacetin O-deethylation) was studied using human liver microsomes, with and without albumin (0-10 mg/ml). In a randomized, crossover study, 10 volunteers took 200 mg tolfenamic acid or placebo t.i.d. for 3 days. On day 2, a caffeine test was performed. On day 3, each ingested 4 mg of the CYP1A2 substrate tizanidine. Plasma tizanidine, its metabolites (M) and tolfenamic acid, and pharmacodynamic variables were measured. Tolfenamic acid strongly inhibited phenacetin-O-deethylation in vitro (IC(50) 1.8 microM without albumin). Albumin decreased its inhibitory effect in a concentration-dependent manner; the IC(50) exceeded 100 microM with 10 mg/ml of albumin. Tolfenamic acid had no effect on the area under the concentration-time curve (AUC(0-oo)), peak concentration, time of peak concentration or half-life of tizanidine or M-3; only the AUC(0-oo) of secondary metabolite M-4 was slightly decreased (13%, P = 0.004). The caffeine test and the pharmacodynamic effects of tizanidine were unchanged. Tolfenamic acid potently inhibits CYP1A2 in vitro when studied without albumin, but not in vivo. This apparent discrepancy is due to the high protein binding of tolfenamic acid. To avoid overestimation of the interaction potential, the inhibitory effect of highly albumin-bound compounds should also be studied in vitro with albumin, or their exact unbound plasma concentration should be used in predictions.

Ritonavir 100 mg Does Not Cause QTc Prolongation in Healthy Subjects: A Possible Role as CYP3A Inhibitor in Thorough QTc Studies

To assess the QTc prolongation by ritonavir (RTV) 100 mg and explore its potential use as CYP3A inhibitor in thorough QTc (TQT) studies. Randomized, crossover study of single-dose RTV 100 mg, placebo, and moxifloxacin (MFLX) 400 mg in 65 healthy subjects with serial triplicate electrocardiograms obtained for 12 h post-dose. Largest mean placebo-adjusted QTcF increase from baseline (90% confidence interval (CI)) for RTV 100 mg was noninferior to placebo (0.16 ms (-1.38, 1.69)). Study sensitivity was validated by detecting the largest mean placebo-adjusted QTcF increase from baseline (90% CI) for MFLX of 8.31 ms (6.44, 10.18). A single dose of RTV 100 mg does not cause QTc prolongation in healthy subjects. Based on the potent CYP3A4 inhibition, lack of QTc effect and better safety profile, RTV 100 mg could replace ketoconazole as the CYP3A4 inhibitor in TQT studies.