Variation in the Response of Clozapine Biotransformation Pathways in Human Hepatic Microsomes to CYP1A2‐ and CYP3A4‐selective Inhibitors

Abstract

The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to conventional antipsychotic drugs. Cytochromes P450 (CYPs) 1A2 and 3A4 oxidize CLZ to norCLZ and CLZ N-oxide in human liver. Concurrent treatment with inducers and inhibitors of CYP1A2 modulates CLZ elimination that disrupts therapy. Drug-drug interactions involving CYP3A4 are also significant but less predictable. To further characterize the factors underlying these interactions, we used samples from a cohort of human livers to assess variation in CLZ oxidation pathways in relation to intrinsic CYP3A4 and CYP1A2 activities and the effects of the corresponding selective inhibitors ketoconazole (0.2 and 2 μM) and fluvoxamine (1 and 10 μM). The CYP3A4-selective inhibitor ketoconazole (2 μM) impaired CLZ N-oxide formation in all 14 of the livers used in inhibition studies (≥50% inhibition) while the CYP1A2-selective inhibitor fluvoxamine (10 μM) decreased norCLZ formation in nine. Ketoconazole effectively inhibited CLZ metabolism in five of seven livers that catalysed CYP3A4-dependent testosterone 6β-hydroxylation at or above the median rate and in four other livers with lower intrinsic CYP3A4 activity. Similarly, fluvoxamine (10 μM) readily inhibited CLZ oxidation in seven livers with high CYP1A2-mediated 7-ethoxyresorufin O-deethylation activity (at or above the median) and three livers with lower intrinsic CYP1A2 activity. In three livers, CLZ biotransformation was impaired by both ketoconazole and fluvoxamine, consistent with a major role for both CYPs. These findings suggest that the intrinsic activities of CYPs 1A2 and 3A4 are unrelated to the response to CYP-selective inhibitors and that assessment of the activities in vivo may not assist the prediction of drug-drug interactions.

Population Pharmacokinetics of Clozapine: A Systematic Review

“…However, the concomitant use of benzodiazepine was investigated even though only a pharmacodynamic interaction with clozapine was reported in the literature [63]. On the other hand, clozapine levels were found to be higher in patients receiving CYP1A2 inhibitors, such as fluvoxamine [39,64,65], CYP2D6 inhibitors as in paroxetine and fluoxetine [64], and CYP3A4 inhibitors like valproate [64,66]. Ketoconazole (CYP3A4 inhibitor) impaired the clozapine N-oxide metabolite formation [65].…”

“…On the other hand, clozapine levels were found to be higher in patients receiving CYP1A2 inhibitors, such as fluvoxamine [39,64,65], CYP2D6 inhibitors as in paroxetine and fluoxetine [64], and CYP3A4 inhibitors like valproate [64,66]. Ketoconazole (CYP3A4 inhibitor) impaired the clozapine N-oxide metabolite formation [65]. On the other hand, lower clozapine levels were observed with CYP3A4 inducers like phenobarbital [64] and carbamazepine [67].…”

CYP3A4/5 mediates the metabolic detoxification of humantenmine, a highly toxic alkaloid from Gelsemium elegans Benth.

“…The metabolism of HMT in HLMs was further determined in the presence or absence of specific CYP chemical inhibitors. Fulvoxamine maleate (10 μ m ) (Brosen, Skjelbo, Rasmussen, Poulsen, & Loft, ; Murray, Zhang, & Edwards, ), 8‐methoxypsoralen (20 μ m ) (Tiong et al, ), clopidogrel (20 μ m ) (Jiang et al, ), gemfibrozil (75 μ m ) (Eagling, Tjia, & Back, ; Tornio, Neuvonen, Niemi, & Backman, ), amiodarone hydrochloride (20 μ m ) (Eagling et al, ; Kumar, Locuson, Sham, & Tracy, ), omeprazole (20 μ m ) (Eagling et al, ; Zvyaga et al, ), quinidine (20 μ m ) (Wu et al, ), diethyldithiocarbamic acid (20 μ m ) (Eagling et al, ; Y. Hu et al, ; Sapkota, Hottor, DeVasure, Wyatt, & McCaskill, ) and Keto (1 μ m ) (Eagling et al, ; Y. Hu et al, ; Lopez‐Barcons, Maurer, Kang, & Reynolds, ) for CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4/5 were respectively utilized as corresponding inhibitors. According to the reported inhibition constant (Ki) of each inhibitor, the chosen concentrations were considerably higher than their Ki (Ye et al, ).…”

PharmGKB summary

“…An in vitro study using human liver microsomes investigated production of N-desmethylclozapine and clozapine n-oxide in response to cotreatment with fluvoxamine [51]. The authors concluded that caffeine phenotyping was not reliable in predicting CYP1A2-related clozapine-fluvoxamine DDI [51].…”

“…However, in a study of 21 patients receiving clozapine and ketoconazole, no significant changes in clozapine and metabolites were observed [33]. Also in a series of human liver microsome experiments using CYP3A4 phenotyping with testosterone 6β‐hydroxylation, the CYP3A4 DDI of clozapine with ketoconazole was not predictable [51].…”