THE METABOLIC DISPOSITION OF APREPITANT, A SUBSTANCE P RECEPTOR ANTAGONIST, IN RATS AND DOGS

Simple Liquid Chromatography-Tandem Mass Spectrometry Method for Quantitation of Total and Free Aprepitant and Its Active N-Dealkylated Metabolites in Human Plasma

BACKGROUND

Aprepitant, an antiemetic selective neurokinin-1 receptor antagonist, is primarily metabolized to the active N-dealkylated form (ND-AP) and then converted to its carbonyl form (ND-CAP) in humans. This study developed a simple liquid chromatography-tandem mass spectrometry method using electrospray ionization for the quantitation of plasma total and free aprepitant and its N-dealkylated metabolites and used them to analyze patient plasma.

METHODS

Free aprepitant and ND-AP in plasma were fractionated using centrifugal ultrafiltration. The analytes in plasma or their ultrafiltered specimens treated with triethylamine/acetonitrile were isocratically separated using a 3-μm octadecylsilyl column with a total run time of 10 minutes and scanned using positive ion electrospray ionization.

RESULTS

The calibration curves of total aprepitant, ND-AP, and ND-CAP were prepared at concentration ranges of 50-2500, 20-1000, and 5-250 ng/mL, respectively, whereas that of free aprepitant and ND-AP were at a concentration range of 2-150 ng/mL. The intraassay and interassay accuracy and imprecision values were 93.5%-107.7% and 94.6%-103.3%, and 2.1%-7.5% and 1.0%-8.9%, respectively. Aprepitant and its metabolites did not exhibit any matrix effects or instabilities in the plasma specimens. In cancer patients receiving oral aprepitant, the plasma concentration ranges of total aprepitant, ND-AP, and ND-CAP, and free aprepitant and ND-AP were 137-2170, 104-928, 22.4-97.6, 8.11-60.0, and 3.53-56.0 ng/mL, respectively. The median plasma free fraction proportion of aprepitant and ND-AP was 4.14% and 4.90%, respectively.

CONCLUSIONS

The present developed method showed an acceptable analytical performance and can be used to evaluate total and free aprepitant and its N-dealkylated metabolites in patient plasma.

Determination and validation of aprepitant in rat plasma using LC-MS/MS

Aim: The assessment of efficacy should be paralleled with extensive pharmacokinetic parameters, and a valid bioanalytical method is a pre-condition for accurate plasma concentration. Materials & methods: A simple, specific, rapid and sensitive LC-MS/MS method has been developed for quantitative analysis of aprepitant in rat plasma. A C18 column was used as stationary phase and the mobile phase consisted of a mixture of formic acid in water and formic acid in acetonitrile. Quantification was performed using multiple reaction monitoring mode. Results: The selectivity, linearity, accuracy, precision, robustness and ruggedness of the method were evaluated in accordance with bioanalytical method validation guideline of ICH and all results were within the acceptable range. Conclusion: The validated LC-MS/MS method was found to be useful for the quantitative analysis of aprepitant in rat plasma samples.

Neurogenic tachykinin mechanisms in experimental nephritis of rats

We demonstrated earlier that renal afferent pathways combine very likely “classical” neural signal transduction to the central nervous system and a substance P (SP)–dependent mechanism to control sympathetic activity. SP content of afferent sensory neurons is known to mediate neurogenic inflammation upon release. We tested the hypothesis that alterations in SP-dependent mechanisms of renal innervation contribute to experimental nephritis. Nephritis was induced by OX-7 antibodies in rats, 6 days later instrumented for recording of blood pressure (BP), heart rate (HR), drug administration, and intrarenal administration (IRA) of the TRPV1 agonist capsaicin to stimulate afferent renal nerve pathways containing SP and electrodes for renal sympathetic nerve activity (RSNA). The presence of the SP receptor NK-1 on renal immune cells was assessed by FACS. IRA capsaicin decreased RSNA from 62.4 ± 5.1 to 21.6 ± 1.5 mV s (*p < 0.05) in controls, a response impaired in nephritis. Suppressed RSNA transiently but completely recovered after systemic administration of a neurokinin 1 (NK1-R) blocker. NK-1 receptors occurred mainly on CD11⁺ dendritic cells (DCs). An enhanced frequency of CD11c⁺NK1R⁺ cell, NK-1 receptor⁺ macrophages, and DCs was assessed in nephritis. Administration of the NK-1R antagonist aprepitant during nephritis reduced CD11c⁺NK1R⁺ cells, macrophage infiltration, renal expression of chemokines, and markers of sclerosis. Hence, SP promoted renal inflammation by weakening sympathoinhibitory mechanisms, while at the same time, substance SP released intrarenally from afferent nerve fibers aggravated immunological processes i.e. by the recruitment of DCs.

The Significance of NK1 Receptor Ligands and Their Application in Targeted Radionuclide Tumour Therapy

To date, our understanding of the Substance P (SP) and neurokinin 1 receptor (NK1R) system shows intricate relations between human physiology and disease occurrence or progression. Within the oncological field, overexpression of NK1R and this SP/NK1R system have been implicated in cancer cell progression and poor overall prognosis. This review focuses on providing an update on the current state of knowledge around the wide spectrum of NK1R ligands and applications of radioligands as radiopharmaceuticals. In this review, data concerning both the chemical and biological aspects of peptide and nonpeptide ligands as agonists or antagonists in classical and nuclear medicine, are presented and discussed. However, the research presented here is primarily focused on NK1R nonpeptide antagonistic ligands and the potential application of SP/NK1R system in targeted radionuclide tumour therapy.

Relative bioavailability of an extemporaneously prepared aprepitant oral suspension in healthy adults

Purpose

Use of aprepitant for chemotherapy-induced nausea and vomiting prophylaxis in patients unable to swallow capsules is hindered by the lack of a commercially available oral liquid formulation in many jurisdictions. A stable oral suspension can be extemporaneously prepared using commercially available capsules. We aimed to determine the bioavailability of this aprepitant suspension relative to the capsule.

Methods

This two-period crossover study enrolled 17 healthy adult volunteers. Volunteers received a single 125 mg aprepitant dose during each study period. Order of formulation presentation (capsule vs suspension first) was randomized. Thirteen blood samples were collected over a 48-h period. Aprepitant plasma concentrations were determined using liquid chromatography-mass spectroscopy. Relative bioavailability was defined as the geometric least squares mean ratio for area under the concentration versus time curve (AUC) from time zero to infinity of the aprepitant suspension versus the capsule. Bioequivalence, defined as per Health Canada guidelines, was assessed as a secondary aim.

Results

Relative bioavailability of the aprepitant suspension was 82.3% (90% CI: 69.09-98.00%). Bioequivalence was not established: geometric least squares mean ratios (suspension/capsule) for AUC time zero to 48 h and maximum concentration were 87.8% (90% CI: 75.48–102.16%) and 86.1% (90% CI: 75.59–98.16%), respectively. No serious adverse events were observed.

Conclusions

With a relative bioavailability of 82.3%, the extemporaneous aprepitant oral suspension was well-absorbed relative to the capsule. Though not bioequivalent to the oral capsule, the clinical use of this aprepitant oral suspension in adult and pediatric patients unable to swallow capsules is likely to be effective and safe.

Antiinflammatory effects of aprepitant coadministration with cART regimen containing ritonavir in HIV-infected adults

BACKGROUND

HIV-infected individuals, even well controlled with combined antiretroviral therapy (cART), have systemic inflammation and comorbidities. Substance P (SP) is an undecapeptide, which mediates neurotransmission and inflammation through its cognate neurokinin 1 receptor (NK1R). Plasma SP levels are elevated in HIV-infected individuals. The FDA-approved antiemetic aprepitant, an NK1R antagonist, has anti-HIV effects and antiinflammatory actions. We evaluated the safety, pharmacokinetics, and antiinflammatory properties of aprepitant in HIV-positive individuals receiving cART.

METHODS

We conducted a phase 1B study of 12 HIV-positive individuals on a ritonavir-containing regimen (HIV viral load less than 40 copies/ml and CD4 > 400 cells/μl). Participants received open-label aprepitant 375 mg per day for 28 days and were followed for an additional 30 days. Changes in plasma levels of proinflammatory markers were assessed using flow cytometry, ELISA, luminex, and SOMAscan assays.

RESULTS

The mean peak aprepitant plasma concentration was 30.7 ± 15.3 μg/ml at day 14 and 23.3 ± 12.3 μg/ml at day 28. Aprepitant treatment resulted in decreased plasma SP levels and affected 176 plasma proteins (56 after FDR) and several metabolic pathways, including inflammation and lipid metabolism. No change in soluble CD163 was observed. Aprepitant treatment was associated with a moderate increases in total and HDL cholesterol and affected select hematologic and metabolic markers, which returned to baseline levels 30 days after aprepitant treatment was stopped. There were 12 mild and 10 moderate adverse events (AE).

CONCLUSIONS

Aprepitant is safe and well tolerated. The antiinflammatory properties of aprepitant make it a possible adjunctive therapy for comorbid conditions associated with HIV infection.

TRIAL REGISTRATION

ClinicalTrials.gov (NCT02154360).

FUNDING

This research was funded by NIH UO1 MH090325, P30 MH097488, and PO1 MH105303.

Pharmacologic rationale for the NK1R antagonist, aprepitant as adjunctive therapy in HIV

BACKGROUND

Many HIV infected individuals with suppressed viral loads experience chronic immune activation frequently developing neurological impairment designated as HIV associated neurocognitive disorder (HAND). Adjunctive therapies may reduce HIV associated inflammation and therefore decrease the occurrence of HAND.

METHODS

We have conducted in vitro, animal and clinical studies of the neurokinin 1 receptor (NK1R) antagonist aprepitant in HIV/SIV infection.

RESULTS

Aprepitant inhibits HIV infection of human macrophages ex vivo with an ED50 ~ 5 µM. When administered at 125 mg once daily for 12 months to SIV-infected rhesus macaques, aprepitant reduced viral load by approximately tenfold and produced anti-anxiolytic effects. The anti-viral and anti-anxiolytic effects occur at approximately the third month of dosing; and the effects are sustained throughout the duration of drug administration. Protein binding experiments in culture media and animal and human plasma indicate that the free fraction of aprepitant is lower than previously reported supporting usage of higher doses in vivo. The analysis of blood samples from HIV positive individuals treated for 2 weeks with aprepitant at doses up to 375 mg demonstrated reduced levels of pro-inflammatory cytokines including G-CSF, IL-6, IL-8 and TNFα. Decreased pro-inflammatory cytokines may reduce HIV comorbidities associated with chronic inflammation.

CONCLUSIONS

Our results provide evidence for a unique combination of antiretroviral, anti-inflammatory and behavioral modulation properties of aprepitant in vitro and in vivo. These results provide robust support for a clinical exposure target above that recommended for chemotherapy-induced nausea and vomiting. Doses up to 375 mg once daily in HIV-infected patients still elicit sub-therapeutic exposure of aprepitant though effective plasma concentrations can be achievable by proper dose modulation.

In vitro glucuronidation of aprepitant: a moderate inhibitor of UGT2B7

1. Aprepitant, an oral antiemetic, commonly used in the prevention of chemotherapy-induced nausea and vomiting, is primarily metabolized by CYP3A4. Aprepitant glucuronidation has yet to be evaluated in humans. The contribution of human UDP-glucuronosyltransferase (UGT) isoforms to the metabolism of aprepitant was investigated by performing kinetic studies, inhibition studies and correlation analyses. In addition, aprepitant was evaluated as an inhibitor of UGTs. 2. Glucuronidation of aprepitant was catalyzed by UGT1A4 (82%), UGT1A3 (12%) and UGT1A8 (6%) and Kms were 161.6 ± 15.6, 69.4 ± 1.9 and 197.1 ± 28.2 µM, respectively. Aprepitant glucuronidation was significantly correlated with both UGT1A4 substrates anastrazole and imipramine (rs = 0.77, p < 0.0001 for both substrates; n = 44), and with the UGT1A3 substrate thyroxine (rs = 0.58, p < 0.0001; n = 44). 3. We found aprepitant to be a moderate inhibitor of UGT2B7 with a Ki of ∼10 µM for 4-MU, morphine and zidovudine. Our results suggest that aprepitant can alter clearance of drugs primarily eliminated by UGT2B7. Given the likelihood for first-pass metabolism by intestinal UGT2B7, this is of particular concern for oral aprepitant co-administered with oral substrates of UGT2B7, such as zidovudine and morphine.

Fluorine in medicinal chemistry

Since its first use in the steroid field in the late 1950s, the use of fluorine in medicinal chemistry has become commonplace, with the small electronegative fluorine atom being a key part of the medicinal chemist's repertoire of substitutions used to modulate all aspects of molecular properties including potency, physical chemistry and pharmacokinetics. This review will highlight the special nature of fluorine, drawing from a survey of marketed fluorinated pharmaceuticals and the medicinal chemistry literature, to illustrate key concepts exploited by medicinal chemists in their attempts to optimize drug molecules. Some of the potential pitfalls in the use of fluorine will also be highlighted.

Preclinical in vivo ADME studies in drug development: a critical review

INTRODUCTION

The last two decades have brought many fundamental changes to the drug development process. One such change is the importance of preclinical pharmacokinetics, which has become an essential part of early drug discovery. Furthermore, bioanalytical methods have become more sensitive and the identification and quantitation of metabolites can now be carried out on limited amount of biological material. There has also been a change in regulatory expectations, which are now particularly focused on the safety of human metabolites.

AREAS COVERED

The focus of this paper is on some 'traditional' in vivo ADME studies: excretion balance, metabolic profile and WBA in the toxicological species. These studies, performed with radiolabeled material, have a long history: and are a regular presence in submission dossiers. This paper reviews their value in the perspective of the contemporary drug development process.

EXPERT OPINION

These experiments may sometimes still be relevant to explain toxicological findings or for other special purposes but should not be considered required pieces of the registration dossiers. An appropriate investigation of samples coming from safety evaluation and human Phase I studies and the knowledge generated during the lead optimization phase provide, in most instances, all the DMPK information needed to take decisions in the drug development process.

Use of high-dose cisplatin with aprepitant in an outpatient setting

Chemotherapy-induced nausea and vomiting (CINV) and nephrotoxicity are adverse events induced by cisplatin administration. These effects can be reduced by treatment regimens with low-dose cisplatin, but high-dose cisplatin is still used. In Japan, high-dose cisplatin is usually administered in an inpatient setting to permit management of CINV. However, with use of new-generation antiemetic agents such as aprepitant, CINV and nephrotoxicity are controllable in an outpatient setting. Here, we discuss issues related to the management of high-dose cisplatin administration in outpatients. Grade 2 or worse CINV induced by high-dose cisplatin occurs in more than 40% of patients without treatment with aprepitant, but is controllable by administration of a 5-HT3 receptor antagonist, steroids and aprepitant. Moreover, prevention of CINV using these drugs is cost-effective, since outpatient settings have advantages with regard to health economics and patient quality of life. These findings suggest that shifting high-dose cisplatin administration to the outpatient setting may be achieved with co-administration of aprepitant. Available facilities and the status of the patient should be considered when selecting whether an outpatient setting is suitable for administration of cisplatin, but the use of aprepitant and adequate oral hydration should allow use of cisplatin in this setting.

Safety evaluation of aprepitant for the prevention of chemotherapy-induced nausea and vomiting

INTRODUCTION

Aprepitant is the only neurokinin (NK(1)) receptor antagonist (RA) approved for prevention of chemotherapy-induced nausea and vomiting (CINV). Aprepitant is co-administered with a 5-HT(3) RA and a corticosteroid. Although aprepitant is safe, in most clinical settings potential drug-drug interactions need to be considered before prescription.

AREAS COVERED

This article thoroughly reviews aprepitant and, in particular, clinically relevant safety aspects of the drug. The literature review was performed using Medline with the following search terms: adverse events, aprepitant, chemotherapy, CYP3A4, MK-0869, neurokinin(1) receptor antagonist, safety and tolerability.

EXPERT OPINION

The recommended antiemetic regimen of aprepitant, a 5-HT(3) RA and a corticosteroid is safe. The combination of aprepitant, a 5-HT(3) RA and dexamethasone is now the gold standard of antiemetic treatment in prevention of CINV induced by HEC, or by the combination of an anthracycline and cyclophosphamide. The intravenous formulation of aprepitant used as a single dose is expected to be of benefit to cancer patients.

In vitro metabolism of indomethacin morpholinylamide (BML-190), an inverse agonist for the peripheral cannabinoid receptor (CB(2)) in rat liver microsomes

The in vitro metabolism of an inverse agonist of the peripheral cannabinoid receptor (CB(2)), indomethacin morpholinylamide (BML-190), has been characterized using rat liver microsomal incubation. BML-190 was found to yield at least 15 metabolic products as identified by HPLC-MS/MS analysis. Four major phase one metabolic pathways either individually, or in combination, were proposed to account for the identified metabolic products: (1) loss of the p-chlorobenzyl group, (2) hydroxylation on the indole or on the morpholine ring, (3) morpholinyl ring opening, and (4) demethylation of the methoxyl group on the indole ring.

Aprepitant: a review of its use in the prevention of nausea and vomiting

Aprepitant (Emend) is a neurokinin-1 (NK(1)) receptor antagonist that is able to alleviate the emetic effects of substance P. When combined with a standard regimen of a corticosteroid (dexamethasone) and a serotonin 5-HT(3) receptor antagonist (ondansetron), oral aprepitant (125 mg on day 1 then 80 mg once daily on days 2 and 3) was effective in the prevention of acute and delayed chemotherapy-induced nausea and vomiting (CINV) associated with single or multiple cycles of highly emetogenic chemotherapy (HEC). This aprepitant regimen was also effective in the prevention of CINV in patients treated with single or multiple cycles of moderately emetogenic chemotherapy (MEC). A single oral dose of aprepitant 40 mg administered prior to patients undergoing abdominal surgery was also effective in the prevention of postoperative nausea and vomiting (PONV). Aprepitant was generally well tolerated. Aprepitant is a recommended option for the treatment of PONV, and when combined with a corticosteroid and 5-HT(3) receptor antagonist is a recommended regimen for the treatment of CINV.

Efficacy of injectable maropitant (Cerenia) in a randomized clinical trial for prevention and treatment of cisplatin-induced emesis in dogs presented as veterinary patients

Chemotherapy-induced nausea and vomiting (CINV) is a common side-effect of cisplatin therapy. Maropitant (Cerenia), a novel neurokinin-1 receptor antagonist, was evaluated for prevention and treatment of cisplatin-induced emesis in tumour-bearing dogs. Dogs (n = 122) were randomly allocated to three treatment groups: T01, placebo before and after cisplatin; T02, placebo before and maropitant after cisplatin; or T03, maropitant before and placebo after cisplatin. Maropitant treatment (T02) following a cisplatin-induced-emetic event resulted in significantly fewer subsequent emetic events (P = 0.0005) than in placebo-treated dogs (T01). In placebo-treated (T01) dogs, 56.4% were withdrawn from the study because of treatment failure compared with 5.3% in group T02. When maropitant was administered prior to cisplatin treatment (T03) in a prevention regime, 94.9% did not vomit compared with only 4.9% of placebo-treated dogs, and significantly fewer emetic events (P < 0.0001) were observed in those dogs that did vomit. In summary, maropitant was safe and highly effective in reducing or completely preventing cisplatin-induced emesis.

Fosaprepitant dimeglumine (MK-0517 or L-785,298), an intravenous neurokinin-1 antagonist for the prevention of chemotherapy induced nausea and vomiting

BACKGROUND

This paper reviews the existing literature on fosaprepitant, an intravenous neurokinin-1 anatgonist for the prevention of chemotherapy induced nausea and vomiting.

OBJECTIVES

To describe the development of fosaprepitant and to situate the intravenous form of aprepitant in the current market of available antiemetics.

METHODS

Literature was screened and selected in order to compare the intravenous form of the already commonly used NK-1 receptor antagonist aprepitant.

RESULTS

Aprepitant is the first and still the only marketed neurokinin-1 (NK-1) antagonist. Interestingly, the first studies were performed with fosaprepitant dimeglumine (MK-0517 or L-785,298), the water-soluble prodrug of aprepitant. Fosaprepitant is converted into aprepitant within 30 min after intravenous administration. Based on equivalence studies, 115 mg fosaprepitant seems to be the substitute for 125 mg orally administrated aprepitant. Tolerability of the prodrug is no different from the active drug. The number of efficacy studies with fosaprepitant is very limited and most data are derived from existing aprepitant results. Fosaprepitant has recently been approved by FDA and EMEA as an intravenous substitute for oral aprepitant on day 1 of the standard 3-day CINV prevention regimen, which also includes dexamethasone and a 5-HT3 antagonist.

Preclinical pharmacokinetics and metabolism of a novel prototype DNA-PK inhibitor NU7026

In this study we investigated the in vitro time dependence of radiosensitisation, pharmacokinetics and metabolism of NU7026, a novel inhibitor of the DNA repair enzyme DNA-dependent protein kinase (DNA-PK). At a dose of 10 μM, which is nontoxic to cells per se, a minimum NU7026 exposure of 4 h in combination with 3 Gy radiation is required for a significant radiosensitisation effect in CH1 human ovarian cancer cells. Following intravenous administration to mice at 5 mg kg⁻¹, NU7026 underwent rapid plasma clearance (0.108 l h⁻¹) and this was largely attributed to extensive metabolism. Bioavailability following interperitoneal (i.p.) and p.o. administration at 20 mg kg⁻¹ was 20 and 15%, respectively. Investigation of NU7026 metabolism profiles in plasma and urine indicated that the compound undergoes multiple hydroxylations. A glucuronide conjugate of a bis-hydroxylated metabolite represented the major excretion product in urine. Identification of the major oxidation site as C-2 of the morpholine ring was confirmed by the fact that the plasma clearance of NU7107 (an analogue of NU7026 methylated at C-2 and C-6 of the morpholine ring) was four-fold slower than that of NU7026. The pharmacokinetic simulations performed predict that NU7026 will have to be administered four times per day at 100 mg kg⁻¹ i.p. in order to obtain the drug exposure required for radiosensitisation.

Effect of aprepitant on the pharmacokinetics and pharmacodynamics of warfarin

OBJECTIVE

To examine the effect of aprepitant on the pharmacokinetics and pharmacodynamics of warfarin. Aprepitant is a neurokinin-1 (NK1)-receptor antagonist developed as an antiemetic for chemotherapy-induced nausea and vomiting.

METHODS

This was a double-blind, placebo-controlled, randomized, two-period, parallel-group study. During period 1, warfarin was individually titrated to a stable prothrombin time (expressed as international normalized ratio, INR) from 1.3 to 1.8. Subsequently, the daily warfarin dose remained fixed for 10-12 days. During period 2, the warfarin dose was continued for 8 days, and on days 1-3 administered concomitantly with aprepitant (125 mg on day 1, and 80 mg on days 2 and 3) or placebo. At baseline (day -1 of period 2) and on day 3, warfarin pharmacokinetics was investigated. INR was monitored daily. During period 2, warfarin trough concentrations were determined daily.

RESULTS

The study was completed by 22 healthy volunteers (20 men, 2 women). On day 3, steady-state pharmacokinetics of warfarin enantiomers after aprepitant did not change, as assessed by warfarin AUC(0-24 h) and C(max). However, compared with placebo, trough S(-) warfarin concentrations decreased on days 5-8 (maximum decrease 34% on day 8, P<0.01). The INR decreased after aprepitant with a mean maximum decrease on day 8 of 11% versus placebo (P=0.011).

CONCLUSION

These data are consistent with a significant induction of CYP2C9 metabolism of S(-) warfarin by aprepitant. Subsequently, in patients on chronic warfarin therapy, the clotting status should be monitored closely during the 2-week period, particularly at 7-10 days, following initiation of the 3-day regimen of aprepitant with each chemotherapy cycle.

CYTOCHROME P450 3A4 IS THE MAJOR ENZYME INVOLVED IN THE METABOLISM OF THE SUBSTANCE P RECEPTOR ANTAGONIST APREPITANT

The contribution of human cytochrome P450 (P450) isoforms to the metabolism of aprepitant in humans was investigated using recombinant P450s and inhibition studies. In addition, aprepitant was evaluated as an inhibitor of human P450s. Metabolism of aprepitant by microsomes prepared from baculovirus-expressed human P450s was observed only when CYP1A2, CYP2C19, or CYP3A4 was present in the expression system. Incubation with CYP1A2 and CYP2C19 yielded only products of O-dealkylation, whereas CYP3A4 catalyzed both N- and O-dealkylation reactions. The metabolism of aprepitant by human liver microsomes was inhibited completely by ketoconazole or troleandomycin. No inhibition was observed with other P450 isoform-selective inhibitors. Aprepitant was evaluated also as a P450 inhibitor in human liver microsomes. No significant inhibition of CYP1A2, CYP2B6, CYP2C8, CYP2D6, and CYP2E1 was observed in experiments with isoform-specific substrates (IC50 > 70 microM). Aprepitant was a moderate inhibitor of CYP3A4, with Ki values of approximately 10 microM for the 1'- and 4-hydroxylation of midazolam, and the N-demethylation of diltiazem, respectively. Aprepitant was a very weak inhibitor of CYP2C9 and CYP2C19, with Ki values of 108 and 66 microM for the 7-hydroxylation of warfarin and the 4'-hydroxylation of S-mephenytoin, respectively. Collectively, these results indicated that aprepitant is both a substrate and a moderate inhibitor of CYP3A4.