CYP3A5 polymorphism, amlodipine and hypertension

Epigenetic and transcriptional regulation of the human angiotensinogen gene by high salt

Hypertension is caused by a combination of genetic and environmental factors. Angiotensinogen (AGT) is a component of RAAS, that regulates blood pressure. The human angiotensinogen (hAGT) gene has -6A/-6G polymorphism and -6A variant is associated with human hypertension. In this study, we have investigated the epigenetic regulation of the hAGT. To understand transcriptional regulation of the hAGT, we have made transgenic animals containing -6A. We show that HS affects DNA methylation and modulates transcriptional regulation of this gene in liver and kidney. High salt (HS) increases hAGT gene expression in -6A TG mice. We have observed that the number of CpG sites in the hAGT promoter is decreased after HS treatment. In the liver, seven CpG sites are methylated whereas after HS treatment, only three CpG sites remain methylated. In the kidney, five CpG sites are methylated, whereas after HS treatment, only three CpG sites remain methylated. These results suggest that HS promotes DNA demethylation and increasing AGT gene expression. RT-PCR and immunoblot analysis show that hAGT gene expression is increased by HS. Chip assay has shown that transcription factors bind strongly after HS treatment. RNA-Seq identified differentially expressed genes, novel target genes associated with hypertension, top canonical pathways, upstream regulators. One of the plausible mechanisms for HS induced up-regulation of the hAGT gene is through IL-6/JAK/STAT3/AGT axis.

Cinnamon modulates the pharmacodynamic & pharmacokinetic of amlodipine in hypertensive rats

The objective of this study was to investigate the effects of cinnamon on the pharmacodynamic (PD) & pharmacokinetic (PK) of amlodipine in hypertensive rats. The hypertensive control group of Wistar rats received L-NAME (40 mg/kg, daily, orally) only. The cinnamon group of rats was treated with cinnamon (200 mg/kg, daily, orally) along with L-NAME. Following 14 days treatment period, blood pressures of rats were monitored at designated intervals over 24 h utilizing a tail-cuff system for measuring blood pressure. To assess the oral PK; amlodipine was administered as a single oral dose of 1 mg/kg to rats and blood samples were collected at specified intervals over 24 h and analysed by UPLC-LC MS/MS. Synergistic decreased in rat's blood pressure was observed in presence of cinnamon + amlodipine. Simultaneous administration of cinnamon ameliorates the Cmax and AUC0-t of amlodipine, the Cmax and AUC0-t was 11.04 ± 1.01 ng/ml and 113.76 ± 5.62 ng h/ml for the cinnamon + amlodipine group as compared to 4.12 ± 0.49 ng/ml and 48.59 ± 4.28 ng h/ml for the amlodipine alone group. The study demonstrates that the use of cinnamon considerably decreases the blood pressure levels and enhances the PK parameters of amlodipine in hypertensive rats.

Identification of Potential Cytochrome P450 3A5 Inhibitors: An Extensive Virtual Screening through Molecular Docking, Negative Image-Based Screening, Machine Learning and Molecular Dynamics Simulation Studies

Cytochrome P450 3A5 (CYP3A5) is one of the crucial CYP family members and has already proven to be an important drug target for cardiovascular diseases. In the current study, the PubChem database was screened through molecular docking and high-affinity molecules were adopted for further assessment. A negative image-based (NIB) model was used for a similarity search by considering the complementary shape and electrostatics of the target and small molecules. Further, the molecules were segregated into active and inactive groups through six machine learning (ML) matrices. The active molecules found in each ML model were used for in silico pharmacokinetics and toxicity assessments. A total of five molecules followed the acceptable pharmacokinetics and toxicity profiles. Several potential binding interactions between the proposed molecules and CYP3A5 were observed. The dynamic behavior of the selected molecules in the CYP3A5 was explored through a molecular dynamics (MD) simulation study. Several parameters obtained from the MD simulation trajectory explained the stability of the protein–ligand complexes in dynamic states. The high binding affinity of each molecule was revealed by the binding free energy calculation through the MM-GBSA methods. Therefore, it can be concluded that the proposed molecules might be potential CYP3A5 molecules for therapeutic application in cardiovascular diseases subjected to in vitro/in vivo validations.

Adverse Drug Reactions of Antihypertensives and CYP3A5*3 Polymorphism Among Chronic Kidney Disease Patients

Chronic kidney disease (CKD) patients may be more susceptible to adverse drug reactions (ADRs), given their complex medication regimen and altered physiological state driven by a decline in kidney function. This study aimed to describe the relationship between CYP3A5*3 polymorphism and the ADR of antihypertensive drugs in CKD patients. This retrospective, multi-center, observational cohort study was performed among adult CKD patients with a follow-up period of up to 3 years. ADRs were detected through medical records. CYP3A5*3 genotyping was performed using the direct sequencing method. From the 200 patients recruited in this study, 33 (16.5%) were found to have ADRs related to antihypertensive drugs, with 40 ADRs reported. The most frequent ADR recorded was hyperkalemia (n = 8, 20.0%), followed by bradycardia, hypotension, and dizziness, with 6 cases (15.0%) each. The most common suspected agents were angiotensin II receptor blockers (n = 11, 27.5%), followed by angiotensin-converting enzyme inhibitors (n = 9, 22.5%). The CYP3A5*3 polymorphism was not found to be associated with antihypertensive-related ADR across the genetic models tested, despite adjustment for other possible factors through multiple logistic regression (p > 0.05). After adjusting for possible confounding factors, the factors associated with antihypertensive-related ADR were anemia (adjusted odds ratio [aOR] 5.438, 95% confidence interval [CI]: 2.002, 14.288) and poor medication adherence (aOR 3.512, 95% CI: 1.470, 8.388). In conclusion, the CYP3A5*3 polymorphism was not found to be associated with ADRs related to antihypertensives in CKD patients, which requires further verification by larger studies.

Cytochrome P450 3A5 polymorphism affects the metabolism of sorafenib and its toxicity for hepatocellular carcinoma cells in vitro

OBJECTIVE

Cytochrome P450 3A5 (CYP3A5) is a highly polymorphic gene and the encoded protein variants differ in catalytic activity, leading to inter-individual variation in metabolic ability. The aim of the current study was to investigate the effects of seven allelic variants on the ability of CYP3A5 to metabolize sorafenib in vitro and further explore the impacts of CYP3A5 polymorphism on the proliferation and apoptosis of hepatocellular carcinoma cell line (HepG2) induced by sorafenib.

METHODS

Wild-type and variant CYP3A5 enzymes were expressed in Spodoptera frugiperda insect cells using a baculovirus dual-expression system, and protein expression was checked by western blot. The enzymes were incubated with sorafenib at 37°C for 30 min, and formation of the major metabolite sorafenib N-oxide was assayed using ultra-performance liquid chromatography and tandem mass spectrometry. Intrinsic clearance values (V_max/K_m) were calculated for each enzyme. Additionally, recombinant HepG2 cells transfecting with CYP3A5 variants were used to investigate the effects of sorafenib on the proliferation of HepG2 cells.

RESULTS

Intrinsic clearance of the six variants CYP3A5*2, CYP3A5*3A, CYP3A5*3C, CYP3A5*4, CYP3A5*5, and CYP3A5*7 was 26.41-71.04% of the wild-type (CYP3A5*1) value. In contrast, the clearance value of the variant CYP3A5*6 was significantly higher (174.74%). Additionally, the decreased ATP levels and cell viability and the increased cell apoptosis in HepG2 cells transfected with CYP3A5*2, CYP3A5*3A, CYP3A5*3C, CYP3A5*4, CYP3A5*5, and CYP3A5*7 were observed, whereas, the increased ATP levels and cell viability and the reduced cell apoptosis in HepG2 cells transfected with CYP3A5*6 were also investigated when compared to CYP3A5*1.

CONCLUSION

Our results suggest that CYP3A5 polymorphism influences sorafenib metabolism and pharmacotherapeutic effect in hepatic carcinomas. These data may help explain differential response to drug therapy for hepatocellular carcinoma, and they support the need for individualized treatment.

Cytochrome P450 3A5 polymorphism affects the metabolism of sorafenib and its toxicity for hepatocellular carcinoma cells in vitro

OBJECTIVE

Cytochrome P450 3A5 (CYP3A5) is a highly polymorphic gene and the encoded protein variants differ in catalytic activity, leading to inter-individual variation in metabolic ability. The aim of the current study was to investigate the effects of seven allelic variants on the ability of CYP3A5 to metabolize sorafenib in vitro and further explore the impacts of CYP3A5 polymorphism on the proliferation and apoptosis of hepatocellular carcinoma cell line (HepG2) induced by sorafenib.

METHODS

Wild-type and variant CYP3A5 enzymes were expressed in Spodoptera frugiperda insect cells using a baculovirus dual-expression system, and protein expression was checked by western blot. The enzymes were incubated with sorafenib at 37°C for 30 min, and formation of the major metabolite sorafenib N-oxide was assayed using ultra-performance liquid chromatography and tandem mass spectrometry. Intrinsic clearance values (V_max/K_m) were calculated for each enzyme. Additionally, recombinant HepG2 cells transfecting with CYP3A5 variants were used to investigate the effects of sorafenib on the proliferation of HepG2 cells.

RESULTS

Intrinsic clearance of the six variants CYP3A5*2, CYP3A5*3A, CYP3A5*3C, CYP3A5*4, CYP3A5*5, and CYP3A5*7 was 26.41-71.04% of the wild-type (CYP3A5*1) value. In contrast, the clearance value of the variant CYP3A5*6 was significantly higher (174.74%). Additionally, the decreased ATP levels and cell viability and the increased cell apoptosis in HepG2 cells transfected with CYP3A5*2, CYP3A5*3A, CYP3A5*3C, CYP3A5*4, CYP3A5*5, and CYP3A5*7 were observed, whereas, the increased ATP levels and cell viability and the reduced cell apoptosis in HepG2 cells transfected with CYP3A5*6 were also investigated when compared to CYP3A5*1.

CONCLUSION

Our results suggest that CYP3A5 polymorphism influences sorafenib metabolism and pharmacotherapeutic effect in hepatic carcinomas. These data may help explain differential response to drug therapy for hepatocellular carcinoma, and they support the need for individualized treatment.

Effects of CYP3A5 Polymorphism on Rapid Progression of Chronic Kidney Disease: A Prospective, Multicentre Study

Personalised medicine is potentially useful to delay the progression of chronic kidney disease (CKD). The aim of this study was to determine the effects of CYP3A5 polymorphism in rapid CKD progression. This multicentre, observational, prospective cohort study was performed among adult CKD patients (≥18 years) with estimated glomerular filtration rate (eGFR) ≥30 mL/min/1.73 m2, who had ≥4 outpatient, non-emergency eGFR values during the three-year study period. The blood samples collected were analysed for CYP3A5*3 polymorphism. Rapid CKD progression was defined as eGFR decline of >5 mL/min/1.73 m2/year. Multiple logistic regression was then performed to identify the factors associated with rapid CKD progression. A total of 124 subjects consented to participate. The distribution of the genotypes adhered to the Hardy-Weinberg equilibrium (X2 = 0.237, p = 0.626). After adjusting for potential confounding factors via multiple logistic regression, the factors associated with rapid CKD progression were CYP3A5*3/*3 polymorphism (adjusted Odds Ratio [aOR] 4.190, 95% confidence interval [CI]: 1.268, 13.852), adjustments to antihypertensives, young age, dyslipidaemia, smoking and use of traditional/complementary medicine. CKD patients should be monitored closely for possible factors associated with rapid CKD progression to optimise clinical outcomes. The CYP3A5*3/*3 genotype could potentially be screened among CKD patients to offer more individualised management among these patients.

The rs776746 variant of CYP3A5 is associated with intravenous midazolam plasma levels and higher clearance in critically ill Mexican paediatric patients

WHAT IS KNOWN AND OBJECTIVE

Midazolam is a drug that is metabolized by cytochrome P450 (CYP450) enzymes, particularly CYP3A4 and CYP3A5. The presence of single-nucleotide polymorphisms (SNPs) in the genes encoding these enzymes, such as CYP3A4*1B which is associated with low enzyme expression and activity and CYP3A5*3, has been associated with decrease in enzymatic activity and reduced drug clearance, with potential effects on drug levels and/or toxicity. The present study was conducted to determine the frequencies of the allelic variants of the CYP3A4 (rs2740574) and CYP3A5 (rs776746) genes and their effects on the plasma levels and clearance of intravenous midazolam in critically ill Mexican paediatric patients.

METHODS

Seventy-two DNA samples were genotyped by real-time PCR with TaqMan probes. Plasma midazolam levels were determined at 3 and 24 h post infusion by high-performance liquid chromatography.

RESULTS AND DISCUSSION

The allelic variant rs776746 (CYP3A5*3) was associated with high midazolam plasma levels; the median concentration in patients with the normal genotype (CC) <0.01 ng/ml (Q₂₅ 0.01-Q₇₅ 196.09), whereas patients with the allelic variant (TT+TC) had a median midazolam concentration of 320.3 ng/ml (Q₂₅ 37.51-Q₇₅ 529.51), p = 0.001. The median pharmacokinetic clearance rates were 0.10 L/kg/h (Q₂₅ 0.01-Q₇₅ 0.34) in patients with the allelic variant (TT+TC) and 0.03 L/kg/h (Q₂₅ 0.002-Q₇₅ 0.13) in patients with the normal genotype (CC), p = 0.042.

WHAT IS NEW AND CONCLUSION

This is the first study that reports the frequency of the rs776746 polymorphism in critically ill paediatric patients, which is relevant, since carriers of the *1 allele synthesizing a functional enzyme may need higher doses to achieve adequate sedation. Our results show that compared with carriers of the normal allele, patients with the CYP3A5*3 allelic variant (rs776746) had increased plasma midazolam levels at 3 h after infusion discontinuation (320.3 ng/ml) and greater clearance (0.10 L/kg/h) of the drug.

Antisense oligonucleotide development for the selective modulation of CYP3A5 in renal disease

CYP3A5 is the primary CYP3A subfamily enzyme expressed in the human kidney and its aberrant expression may contribute to a broad spectrum of renal disorders. Pharmacogenetic studies have reported inconsistent linkages between CYP3A5 expression and hypertension, however, most investigators have considered CYP3A5*1 as active and CYP3A5*3 as an inactive allele. Observations of gender specific differences in CYP3A5*3/*3 protein expression suggest additional complexity in gene regulation that may underpin an environmentally responsive role for CYP3A5 in renal function. Reconciliation of the molecular mechanism driving conditional restoration of functional CYP3A5*3 expression from alternatively spliced transcripts, and validation of a morpholino-based approach for selectively suppressing renal CYP3A5 expression, is the focus of this work. Morpholinos targeting a cryptic splice acceptor created by the CYP3A5*3 mutation in intron 3 rescued functional CYP3A5 expression in vitro, and salt-sensitive cellular mechanisms regulating splicing and conditional expression of CYP3A5*3 transcripts are reported. The potential for a G-quadruplex (G4) in intron 3 to mediate restored splicing to exon 4 in CYP3A5*3 transcripts was also investigated. Finally, a proximal tubule microphysiological system (PT-MPS) was used to evaluate the safety profile of morpholinos in proximal tubule epithelial cells, highlighting their potential as a therapeutic platform for the treatment of renal disease.

Association of CYP3A5 Gene Polymorphisms and Amlodipine-Induced Peripheral Edema in Chinese Han Patients with Essential Hypertension

Background

Amlodipine is one of the most used members of calcium channel blockers (CCB), available to treat hypertension. It is mainly metabolized by the Cytochrome P450 3A4/5 (CYP3A4/5) in the liver. Peripheral edema emerges as the major adverse drug reaction to amlodipine and is the primary reason for discontinuation of amlodipine therapy. However, genetic changes in CYP3A5 may lead to changes in the tolerability of amlodipine.

Purpose

In this study, we were interested whether variants in CYP3A5 have a role to play in amlodipine-induced peripheral edema.

Methods

A total number of 240 Chinese Han patients that have experienced hypertension were included in the study. Sixty-four patients had experienced amlodipine-induced peripheral edema, while the remaining 176 patients with no history of edema formed the control group. Twenty-four single-nucleotide polymorphisms (SNPs) of CYP3A5 gene were sequenced by targeted region sequencing method. The relationship of these genetic variants with amlodipine-induced peripheral edema risk was assessed using logistic regression.

Results

The allele frequencies of CYP3A5*1D (rs15524), CYP3A5*1E (rs4646453) and CYP3A5*3 (rs776746) were significantly different between cases and controls (P<0.05). The CYP3A5 *3/*3 (CC) or CYP3A5 *1D/*1D (AA) carriers showed an increased risk of amlodipine-induced peripheral edema in dominant model. Meanwhile, patients carrying CYP3A5 *1E (AC/AA) showed a reduced risk of peripheral edema. Furthermore, we found a strong linkage disequilibrium among rs15524, rs4646453 and rs776746.

Conclusion

Our study reveals for the first time that CYP3A5 *1D, *1E and *3 were associated with amlodipine-induced peripheral edema in Chinese Han patients with hypertension. However, further studies comprising larger number of samples, more related genes and other factors are wanted.

Pharmacogenomics of amlodipine and hydrochlorothiazide therapy and the quest for improved control of hypertension: a mini review

Blood pressure (BP) is a complex trait that is regulated by multiple physiological pathways and include but is not limited to extracellular fluid volume homeostasis, cardiac contractility, and vascular tone through renal, neural, or endocrine systems. Uncontrolled hypertension (HTN) has been associated with an increased mortality risk. Therefore, understanding the genetics that underpins and influence BP regulation will have a major impact on public health. Moreover, uncontrolled HTN has been linked to inter-individual variation in the drugs’ response and this has been associated with an individual’s genetics architecture. However, the identification of candidate genes that underpin the genetic basis of HTN remains a major challenge. To date, few variants associated with inter-individual BP regulation have been identified and replicated. Research in this field has accelerated over the past 5 years as a direct result of on-going genome-wide association studies (GWAS) and the progress in the identification of rare gene variants and mutations, epigenetic markers, and the regulatory pathways involved in the pathophysiology of BP. In this review we describe and enhance our current understanding of how genetic variants account for the observed variability in BP response in patients on first-line antihypertensive drugs, amlodipine and hydrochlorothiazide.

Genetics and ESKD Disparities in African Americans

African Americans have a 2- to 4-fold greater incidence of end-stage kidney disease (ESKD) than whites, which has long raised the possibility of a genetic cause for this disparity. Recent advances in genetic studies have shown a causal association of polymorphisms at the apolipoprotein L1 gene (APOL1) with the markedly increased risk for the nondiabetic component of the overall disparity in ESKD in African Americans. Although APOL1-associated kidney disease is thought to account for a substantial proportion of ESKD in African Americans, not all the increased risk for ESKD is accounted for, and a complete cataloging of disparities in genetic causes of ESKD eludes our current understanding of genetic-associated kidney disease. Genetic testing aids the screening, diagnosis, prognosis, and treatment of diseases with a genetic basis. Widespread use of genetic testing in clinical practice is limited by the small number of actionable genetic variants, limited health literacy of providers and patients, and underlying complex ethical, legal, and social issues. This perspective reviews racial and ethnic differences associated with genetic diseases and the development of ESKD in African Americans and discusses potential uncertainties associated with our current understanding of penetrance of genetically linked kidney disease and population-attributable risk percent.

Pharmacogenomics And Hypertension: Current Insights

Hypertension is a multifactorial disease that affects approximately one billion subjects worldwide and is a major risk factor associated with cardiovascular events, including coronary heart disease and cerebrovascular accidents. Therefore, adequate blood pressure control is important to prevent these events, reducing premature mortality and disability. However, only one third of patients have the effective control of blood pressure, despite several classes of antihypertensive drugs available. These disappointing outcomes may be at least in part explained by interpatient variability in drug response due to genetic polymorphisms. To address the effects of genetic polymorphisms on blood pressure responses to the antihypertensive drug classes, studies have applied candidate genes and genome wide approaches. More recently, a third approach that considers gene-gene interactions has also been applied in hypertension pharmacogenomics. In this article, we carried out a comprehensive review of recent findings on the pharmacogenomics of antihypertensive drugs, including diuretics, β-blockers, angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers, and calcium channel blockers. We also discuss the limitations and inconsistences that have been found in hypertension pharmacogenomics and the challenges to implement this valuable approach in clinical practice.

A dual functional probe for assessing human CYP450 3A5 and 3A enzymes bioactivities

Aim: CYP3A5 plays a vital role in the drug metabolism, it displays varied expression levels among individuals and is easily influenced by genetic polymorphisms and some diseases. Methodology & results: A dual function probe isobutyryl-11-keto-β-boswellic acid (IKBA) was developed; it possessed a high selectivity toward CYP3A5 and CYP3A enzymes for its two individual metabolites, respectively. The probe has the high accuracy and wide applicability in measuring the real activity of CYP3A5. Finally, IKBA was successfully used for the evaluation of the activity of CYP3A5 and CYP3A enzymes in various bio samples. Conclusion: IKBA could serve as a useful tool for exploring the physiology and pathology functions of CYP3A5 and give some useful guidance for the rational use of clinical drugs.

Construction and verification of CYP3A5 gene polymorphisms using a Saccharomyces cerevisiae expression system to predict drug metabolism

The present study evaluated the ability of a Saccharomyces cerevisiae expression system to predict the pharmacokinetic (PK) activity of a calcium channel blocker in patients with distinct cytochrome P450 3A5 (CYP3A5) polymorphisms. The blood pressure lowering activity of amlodipine in 57 hypertensive patients with CYP3A5*1/*1, CYP3A5*1/*3, CYP3A5*4 and CYP3A5*6 polymorphisms was evaluated by the current study. Subsequently, a Saccharomyces cerevisiae expression system for CYP3A5 gene polymorphisms was constructed to examine the PK activity of CYP3A5*1/*1, CYP3A5*4 and CYP3A5*6 polymorphisms. This system was used to predict the PK of amlodipine and was compared with the in vivo data from different gene polymorphism groups. The blood pressure lowering activity of amlodipine in hypertensive patients varied among CYP3A5 polymorphisms. The in vivo results demonstrated that CYP3A5*6 exhibited the highest metabolic rate, followed by CYP3A5*1/*1, CYP3A5*4 and CYP3A5*1/*3. The difference between CYP3A5*6 and CYP3A5*1/*1 was not statistically significant (P=0.5). In accordance with in vivo data, CYP3A5*1/*1 exhibited the highest in vitro metabolic rate, followed by CYP3A5*6 and CYP3A5*4. With the exception of the comparison between CYP3A5*6 and CYP3A5*1/*1, polymorphisms exhibited statistically significant differences compared with CYP3A5*1/*1 (P<0.05). The Saccharomyces cerevisiae expression system may be a cost effective and potentially useful tool for assessing the PK activity of drugs that are metabolized by CYP3A5.

Randomized placebo-controlled clinical trial of a chewable formulation of amlodipine for the treatment of hypertension in client-owned cats

Background

There is an unmet clinical need for a cat‐specific formulation of amlodipine to treat hypertensive cats.

Objectives

To assess the efficacy of chewable amlodipine tablets in reducing systolic blood pressure (SBP) in cats diagnosed with systemic arterial hypertension.

Animals

Seventy‐seven client‐owned cats with systemic hypertension were included (median age 14 years).

Methods

The study was randomized, double‐blinded, and placebo‐controlled. Forty‐two cats received 0.125–0.50 mg/kg amlodipine once daily for 28 days; 35 cats received placebo. After 28 days all cats continued with amlodipine for 2–3 months in an open‐label phase. Blood pressure was measured using high definition oscillometry. A responder was defined as a cat showing a decrease of SBP to <150 mmHg at 28 days or a decrease from baseline ≥15%.

Results

Sixty‐one cats completed the study. The responder rate was 63% in amlodipine group and 18% in placebo group. Cats receiving amlodipine were 7.9 (95% CI 2.6–24.1) times more likely to be classified as responders when compared to those receiving placebo (P < .001). From a mean (±SD) baseline value of 181 (±12) mmHg, SBP decreased to 154 (±17) mmHg with amlodipine and to 170 (±21) mmHg with placebo (P < .001). The voluntary acceptance rate of amlodipine formulation was 73%.

Conclusions and Clinical Importance

The chewable amlodipine tablet effectively reduced SBP compared with placebo in hypertensive cats, and was well‐tolerated. It can be used concomitantly with angiotensin‐converting enzyme inhibitors and in cats with chronic kidney disease.

Amlodipine metabolism in human liver microsomes and roles of CYP3A4/5 in the dihydropyridine dehydrogenation

Amlodipine is a commonly prescribed calcium channel blocker for the treatment of hypertension and ischemic heart disease. The drug is slowly cleared in humans primarily via dehydrogenation of its dihydropyridine moiety to a pyridine derivative (M9). Results from clinical drug-drug interaction studies suggest that CYP3A4/5 mediate metabolism of amlodipine. However, attempts to identify a role of CYP3A5 in amlodipine metabolism in humans based on its pharmacokinetic differences between CYP3A5 expressers and nonexpressers failed. Objectives of this study were to determine the metabolite profile of amlodipine (a racemic mixture and S-isomer) in human liver microsomes (HLM), and to identify the cytochrome P450 (P450) enzyme(s) involved in the M9 formation. Liquid chromatography/mass spectrometry analysis showed that amlodipine was mainly converted to M9 in HLM incubation. M9 underwent further O-demethylation, O-dealkylation, and oxidative deamination to various pyridine derivatives. This observation is consistent with amlodipine metabolism in humans. Incubations of amlodipine with HLM in the presence of selective P450 inhibitors showed that both ketoconazole (an inhibitor of CYP3A4/5) and CYP3cide (an inhibitor of CYP3A4) completely blocked the M9 formation, whereas chemical inhibitors of other P450 enzymes had little effect. Furthermore, metabolism of amlodipine in expressed human P450 enzymes showed that only CYP3A4 had significant activity in amlodipine dehydrogenation. Metabolite profiles and P450 reaction phenotyping data of a racemic mixture and S-isomer of amlodipine were very similar. The results from this study suggest that CYP3A4, rather than CYP3A5, plays a key role in metabolic clearance of amlodipine in humans.