Modulatory effects of valproic acid and UGT polymorphisms on norclozapine plasma levels in clozapine-treated patients with schizophrenia

Glucuronidation, catalysed by UGT enzymes, plays a key role in the metabolism of clozapine (CLZ) and its active metabolite, norclozapine (NCLZ). Genetic polymorphisms in UGT1A4 and UGT2B10 can influence NCLZ plasma levels. Co-administration of valproic acid (VPA), commonly used in Treatment Resistant Schizophrenia, has been linked to decreased NCLZ concentrations and increased neutropenia risk. Additionally, CYP1A2, the main enzyme responsible for CLZ N-demethylation, may affect CLZ and indirectly NCLZ levels, specially considering genetic variability and smoking status. A total of 296 plasma samples from 61 CLZ-treated patients were retrospectively analysed. CLZ and NCLZ levels were quantified using Liquid Chromatography-Tandem masses (LC-MS/MS). VPA levels were measured by Ultraviolet–Visible spectrophotometric immunoassay. Genotyping for CYP1A2*1F (rs762551) and UGT2B10*2 (rs61750900) polymorphisms was performed using Real-Time PCR and UGT1A4*3 (rs2011425) polymorphism was conducted through direct sequencing. VPA co-administration significantly reduced NCLZ levels (–49.20%, p < 0.001) with minimal impact on CLZ. CYP1A2*1F/*1F and UGT1A4*1/*3 or *3/3 were associated with lower NCLZ levels and dose-adjusted ratios (C/D). These associations were not significant in the VPA group. Absolute neutrophil count (ANC) was 15.96% lower in VPA co-treated patients (p < 0.001) and positively correlated with NCLZ levels. These results emphasize the importance of integrating pharmacogenetic and pharmacokinetic data when prescribing VPA and CLZ concomitantly, to optimize therapeutic efficacy and minimize adverse effects.