CYP2D6 genotype and phenotype relationship in South Indians

Background : Genotypes of the drug-metabolizing enzyme CYP2D6 influence plasma levels of 25% of commonlyprescribed drugs. This is the first study in India to investigate the genotype-phenotype relationship of CYP2D6.
Aim : To study the influence of some CYP2D6 genotypes on the metabolism of its substrate dextromethorphanin healthy South Indian volunteers and to assess the contribution of the CYP2D6*10 and CYP2D6*4 alleles.
Materials and Methods : Twenty-six subjects from a previous CYP2D6 genotyping study of healthy volunteerswere included for phenotyping in this study. Selected volunteers belonged to any one of three genotype groups:Group I - two normal activity alleles, Group II - one reduced activity allele and one normal activity allele andGroup III - one loss of function allele along with either a wild type or reduced activity allele. Volunteers werephenotyped for the CYP2D6 enzyme using dextromethorphan as probe drug. Concentrations of the parent drugand metabolite dextrorphan were estimated using high performance liquid chromatography. Metabolic ratioswere calculated as the ratio of parent drug to metabolite in 0-8h urine samples.
Statistical Analysis : Metabolic ratios from each genotype group were compared using the Mann-Whitney testat 5% significance, to observe their difference between genotype groups.
Results : The mean metabolic ratios±SD in Groups I, II and III were 0.0039±0.0031, 0.0032±0.0017 and0.0391±0.0331 respectively. The mean metabolic ratio of Group III was significantly higher when comparedwith Groups I or II. In heterozygous individuals, the *1 or *2 alleles compensated for the reduced enzymeactivity due to the *10 allele. However, if a heterozygous individual had a *4 allele, the reduced enzyme activitycould not be compensated by the *1 or *2 alleles.
Conclusions : The CYP2D6 enzyme activity was found to be decreased in individuals carrying *4 or *5 alleles.The *1 or *2 allele could compensate for the reduced function due to *10 allele, but not for the loss of functiondue to *4 allele.