Carbamazepine exhibits significant inter-individual variability in its efficacy and safety, which leads to unpredictable therapy outcomes for the majority of patients. Although its complex biotransformation depends on CYP3A5 activity, evidence of association between carbamazepine treatment outcomes and CYP3A5 functional variations remains inconclusive. The aim of the present study was to investigate the distribution of two of the functionally important CYP3A5 variants *2 and *3 as well as their effects on carbamazepine dose requirements, plasma concentrations and clearance in a Serbian population. The study involved 40 paediatric epileptic patients on steady-state carbamazepine treatment. Genotyping was conducted using the PCR-RFLP method, and carbamazepine plasma concentrations were determined using the HPLC method. CYP3A5*2 and *3 polymorphisms were found at frequencies of 0.0% and 97.5%, respectively, which corresponds well to previously published data for Caucasians. No differences in CYP3A5*3 allele frequencies were detected among epileptic patients in comparison to healthy volunteers within similar ethnic populations (p>0.08), indicating that CYP3A5 polymorphism does not represent a risk factor for epilepsy development. There was an observed tendency towards lower dosage requirements (mean±SD: 15.06±4.45 mg/kg vs. 18.74±5.55 mg/kg; p=0.26), higher plasma concentrations (mean±SD: 0.45±0.13 mg/kg vs. 0.38±0.03 mg/kg; p=0.47) and lower clearance (mean±SD: 0.14±0.05 mg/kg vs. 0.15±0.01 mg/kg; p=0.79) of carbamazepine in homozygous carriers of CYP3A5*3/*3 compared to heterozygous CYP3A5*1A/*3 Serbians. Because these genotype groups did not differ significantly in terms of their carbamazepine pharmacokinetics parameters, the proposed effects of CYP3A5*3 on carbamazepine metabolism could not be confirmed.
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