RESUMEN
The concomitant use of herbal products and synthetic drugs necessitates the assessment of their interaction potentials. The herbal hepatoprotective medicine, silybin A inhibits cytochrome P450 (CYP) 2C9 and 3A4 enzymes, thus, may interact with the drugs that are substrates of CYP2C9 and 3A4, such as losartan. The three most prominent genotypes, expressed by CYP2C9 are the CYP2C9*1/*1, CYP2C9*1/*2 and CYP2C9*1/*3. This study aimed to assess silybin A-losartan interaction in different CYP2C9 genotypes using physiological-based pharmacokinetic (PBPK) model approach. The individual PBPK models for silybin A and losartan were developed using PK-Sim®. Losartan pharmacokinetics was predicted with or without co-administration of silybin A in individuals of different CYP2C9 genotypes to find herbal-drug interaction. The predicted drug plasma curves and pharmacokinetic parameters were optimized using parameter identification tool and were compared with reported pharmacokinetic parameters from the published clinical studies for model validation. The silybin-losartan interactions were predicted by change in area under the curve (AUC) and peak systemic concentration (Cmax). The co-treatment of silybin A, 420 mg/24 h (140 mg/8 h) with losartan 50 mg/24 h, exhibited a genotype-dependent change in the losartan's AUC and Cmax. In CYP 2C9*1/*1 genotype, AUC and Cmax of losartan were increased 1.16 and 1.37 folds, respectively falling in a range stipulated for negligible interaction. Increase in AUC and Cmax by 0.873 and 0.294 folds, respectively in CYP2C9*1/*3 after co-administration of silybin A exhibited a minor interaction with losartan. However, in individuals with CYP2C9*1/*2 genotype, the losartan's AUC and Cmax were decreased by 0.01 folds, manifesting a moderate interaction. Hence, in CYP2C9*1/*1 and CYP2C9*1/*3 genotypes, silybin A is a weak CYP inhibitor for losartan while in CYP2C9*1/*2 genotype, the co-administration of silybin consequents into a moderate pharmacokinetic interaction with losartan.