Impact of CYP2C19 genetics on pharmacokinetic variability of escitalopram and sertraline - a study based on therapeutic drug monitoring data

Escitalopram and sertraline are among the most widely used drugs in the treatment of depression in Norway. Both drugs show substantial pharmacokinetic variability. Previous studies have indicated that the drug metabolising enzyme cytochrome P450 2C19 (CYP2C19), which exhibits extensive variability in activity due to genetic polymorphism, is involved in the metabolism of escitalopram and sertraline. The aim of this thesis was therefore to investigate the impact of CYP2C19 genetics on the pharmacokinetic variability of escitalopram and sertraline in psychiatric patients. By use of data from therapeutic drug monitoring, CYP2C19 genotype was shown to be a major determinant of the pharmacokinetics of escitalopram. On average, doseadjusted serum concentration of escitalopram differed 9.7-fold between CYP2C19 poor metabolisers (PMs) and CYP2C19 ultrarapid metabolisers (UMs). Compared to CYP2C19 extensive metabolisers (EMs), the effect was more pronounced for CYP2C19 PMs than for UMs (5.7-fold higher vs. 42% lower mean dose-adjusted serum concentration, respectively). It was further identified that CYP2C19, besides catalysing the well known N-desmethylation of escitalopram, was able to catalyse formation of the propionic acid metabolite. The differences in serum concentration of escitalopram between CYP2C19 genotypes were most likely caused by a combined effect on the two metabolic pathways. Genetic variability in CYP2C19 was an important determinant of the pharmacokinetics of sertraline as well. Dose-adjusted serum concentration of sertraline was on average 3.2-fold higher in CYP2C19 PMs compared to EMs, but did not differ between CYP2C19 UMs and EMs. The substantial differences in pharmacokinetics of escitalopram and sertraline between CYP2C19 genotypes are of potential importance for the clinical response during treatment with these drugs. CYP2C19 UMs might constitute a subgroup of patients at increased risk of therapeutic failure, whereas CYP2C19 PMs are possibly at higher risk of dose-dependent side effects. Although further studies are needed to investigate the value of CYP2C19 genotyping in the prevention of therapeutic failure and side effects during treatment with escitalopram and sertraline, the findings of the present thesis may provide a fundament for individual dosing to limit variability in exposure of these drugs.