Identification of a novel CYP2C19-mediated metabolic pathway of S-citalopram in vitro.

Systemic exposure of the antidepressant S-citalopram (escitalopram, SCIT) differs several-fold according to variable cytochrome P450 2C19 activity, demonstrating the importance of this enzyme for the metabolic clearance of SCIT in vivo. However, previous studies have indicated that the involvement of CYP2C19 in formation of the metabolite N-desmethyl S-citalopram (SDCIT) is limited. Therefore, the purpose of the present in vitro study was to investigate to what extent the CYP2C19-mediated clearance of SCIT was due to a metabolic pathway different from N-desmethylation and to identify the product(s) of this possible alternative metabolic reaction. CYP2C19-mediated metabolism of SCIT was investigated using recombinant Supersomes expressing human CYP2C19. Initial experiments showed that approximately half of the CYP2C19-mediated clearance of SCIT was accounted for by the N-desmethylation pathway. Subsequent experiments identified that, in addition to SDCIT, the propionic acid metabolite of SCIT (SCIT PROP) was formed by CYP2C19 in vitro. Formation of SCIT PROP accounted for 35% of total CYP2C19-mediated clearance of SCIT (calculated as the ratio between metabolite formation rate and substrate concentration at low substrate concentration). Moreover, analysis of samples from six CYP2C19-genotyped patients treated with SCIT indicated that differences in serum concentrations of SCIT between CYP2C19 genotypes may be due to a combined effect on SCIT PROP and SDCIT formation. Identification of SCIT PROP as a metabolic pathway catalyzed by CYP2C19 might explain why impaired CYP2C19 activity has a substantially larger effect on SCIT exposure than estimated from in vitro data based solely on formation of SDCIT.

Serum concentrations of sertraline and N-desmethyl sertraline in relation to CYP2C19 genotype in psychiatric patients.

OBJECTIVE: To investigate the impact of CYP2C19 genotype on serum concentrations of sertraline and N-desmethyl sertraline in psychiatric patients. METHODS: Patients treated with sertraline (n = 121) were divided into six subgroups according to CYP2C19 genotype: CYP2C19*17/*17, CYP2C19*1/*17, CYP2C19*1/*1, CYP2C19*17/def, CYP2C19*1/def and CYP2C19def/def (def = allele encoding defective CYP2C19 metabolism, i.e. *2 and *3). Dose-adjusted serum concentrations were compared by linear mixed model analyses using the CYP2C19*1/*1 subgroup as reference. RESULTS: Subgroups carrying one or two alleles encoding defective CYP2C19 metabolism achieved significantly higher mean dose-adjusted serum concentrations of sertraline and N-desmethyl sertraline compared to the CYP2C19*1/*1 subgroup (P < 0.05). The effect of CYP2C19 genotype was expressed as 3.2-fold (sertraline) and 4.5-fold (N-desmethyl sertraline) higher dose-adjusted serum concentrations in the CYP2C19def/def subgroup compared to the CYP2C19*1/*1 subgroup (P < 0.01). The CYP2C19*17 allele had no influence on the dose-adjusted serum concentrations of sertraline and N-desmethyl sertraline. CONCLUSION: The significantly higher serum concentrations associated with alleles encoding defective CYP2C19 metabolism might be of relevance for the clinical outcome of sertraline treatment.

Impact of the ultrarapid CYP2C19*17 allele on serum concentration of escitalopram in psychiatric patients.

Recently, a novel allelic variant of cytochrome P450 2C19 encoding ultrarapid enzyme activity was described (denoted CYP2C19*17). The objective of this study was to evaluate the impact of CYP2C19*17 on serum concentration of escitalopram in psychiatric patients. One hundred and sixty-six patients treated with escitalopram were divided into the following subgroups according to CYP2C19 genotype: CYP2C19*17/*17 (n=7), CYP2C19*1/*17 (n=43), CYP2C19*1/*1 (n=60), CYP2C19*17/def (n=16), CYP2C19*1/def (n=34), and CYP2C19def/def (n=6) (def=defective allele, i.e., CYP2C19*2 or *3). Dose-adjusted serum concentrations of escitalopram were compared using the CYP2C19*1/*1 subgroup as reference. Geometric mean of the escitalopram serum concentration was 42% lower in patients homozygous for CYP2C19*17 (P<0.01) and 5.7-fold higher in subjects homozygous for defective CYP2C19 alleles (P<0.001). Of the heterozygous subgroups, only CYP2C19*1/def was significantly different from CYP2C19*1/*1 (P<0.001). In conclusion, a homozygous CYP2C19*17 genotype is associated with lower serum concentration of escitalopram, which might imply increased risk of therapeutic failure.

The complexity of active metabolites in therapeutic drug monitoring of psychotropic drugs.

In therapeutic drug monitoring (TDM) practice of psychotropic agents, it is common to summarize plasma concentrations of parent drugs and metabolites when these are considered equipotent. However, there is no clear definition of the term equipotent and one should be aware that metabolites referred to as equipotent in the literature could display several-fold differences in affinities toward target proteins. The fact that the parent drug and metabolite may have different abilities to penetrate the blood-brain-barrier further complicates the picture. Potential differences in brain distribution imply that various metabolite/drug ratios representing the same total concentration in plasma reflect different active concentrations in the brain. Plasma metabolite/drug ratios could differ extensively according to metabolic phenotype and administration route. An example is risperidone where the plasma metabolite/drug ratio is 30-fold lower in cytochrome P450 2D6 poor metabolizers compared to ultrarapid metabolizers, and four-fold lower after intramuscular compared to oral administration. As risperidone is more lipophilic and less effluxed by P-glycoprotein in the blood-brain-barrier than the active metabolite 9-hydroxyrisperidone, one might speculate that patients with high plasma metabolite/drug ratios obtain lower active concentrations in the brain. However, the relative drug-metabolite brain distribution needs to be quantified in humans to clarify to what degree drug and metabolite plasma levels reflect active brain concentrations. The present review illustrates the complexity of active metabolites in TDM with focus on amitriptyline, clomipramine, doxepin, imipramine, fluoxetine, venlafaxine and risperidone, all psychotropic drugs where target plasma concentration ranges are based on the sum of parent drug and metabolite. In addition, perspectives on the possibility of using distribution- and activity-weighted plasma concentrations are provided.