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Risperidone (R) is metabolized to 9-hydroxyrisperidone (9-OHR) by cytochrome P450 2D6 (CYP2D6). The main objective of this naturalistic study was to investigate the variables associated with two plasma ratios: the plasma R:9-OHR concentration ratio and the total concentration-to-dose (C:D) ratio. These ratios were studied as continuous measures by linear regression analyses and as three dichotomous variables in logistic regression analyses: R:9-OHR ratio >1 (indicative of lack of CYP2D6 activity), C:D ratio >14 (indicative of diminished R elimination), and C:D ratio <3.5 (indicative of increased R elimination). Plasma R levels; genotypes for CYP2D6, CYP3A5; and ABCB1 genes, and co-medication, including CYP inhibitors and CYP3A inducers, were studied in 277 patients. Almost all CYP2D6 poor metabolizers (PMs) had an inverted R:9-OHR ratio (>1). Having a CYP2D6 PM phenotype was strongly associated with a C:D ratio >14 (OR=8.2; 95% confidence interval [CI]=2.0-32.7), indicating diminished R elimination. CYP2D6 ultrarapid metabolizers (UMs) did not exhibit an increased R elimination. Some ABCB1 (or MDR1) variants were significantly associated with increased R:9-OHR ratios and decreased C:D ratios, but the results were neither consistent nor robust. Taking CYP inhibitors was significantly associated with a C:D ratio >14 (OR=3.8; CI=1.7-8.7) and with an inverted R:9-OHR ratio. Taking CYP3A inducers was significantly associated with a C:D ratio <3.5 (OR=41.8; CI=12.7-138), indicating increased R elimination. Female gender and old age appeared to be associated with a lower R elimination. Our study indicated that the CYP2D6 PM phenotype may have a major role in personalizing R doses, whereas the CYP3A5 PM phenotype probably has no role. CYP inducers and inhibitors appear to be relevant to R dosing. New studies are needed, particularly to further assess the role of the CYP2D6 UM phenotype and ABCB1 variants in R pharmacokinetics.

This study tested the association between tardive dyskinesia (TD) and polymorphic variations in (a) 2 cytochrome P450 (CYP) genes (CYP2D6 or CYP3A5), (b) 2 DRD2 variants (Ser311Cys and -141C Ins/del) and the Ser9Gly DRD3 variants, (c) 2 glutathione S-transferases (GSTT1 and GSTM1), and (d) variations in the PgP gene, MDR1. The study sample included 516 severely mentally ill patients from Central Kentucky facilities. Logistic regression models that included clinical variables associated with TD were developed. Gene variants were added to these clinical models. The total sample included 31% (162/516) with TD where 30% (49/162) of those had severe TD. Polymorphisms in DRD2, MDR1, and GSTT1 were never significant. Two gene variants appeared to be significant after adding them to the clinical regression models: (1) Ser9Gly DRD3 polymorphism was associated with severe TD (odds ratio for patients with 1 mutant allele when compared with individuals with 2 wild types was 2.5, 95% confidence interval 1.1-5.6, whereas the odds ratio for patients with 2 mutant alleles when compared with individuals with 1 mutant was 2.8, 95% confidence interval 1.0-7.4), and (2) GSTM1 absence was associated with TD (odds ratio 1.7, 95% confidence interval 1.2-2.4) particularly in white women. The CYP2D6 and CYP3A5 absence showed potential for significant associations in larger samples, particularly in white men. New studies need to replicate whether these or other genes could be used conjointly with clinical variables to identify subjects at risk for TD in clinical settings.

AmpliChip CYP450 prototype microarray assay, along with allele-specific-PCR and PCR restriction fragment length polymorphism methods. No significant difference was noted between controls and psychiatric patients in any CYP2D6 allele frequencies. Three subjects were genotyped as poor metabolizers (1.4%; 0.0-2.9%, 95% confidence intervals (CI)), and 10 were classified as ultrarapid metabolizers (4.5%; 1.8-7.2%, 95% CI). A new CYP2D6 allele (*58) and two new duplicated CYP2D6 alleles (*17xn and *2Lxn) not previously reported were also identified. The frequency of the CYP2D6 overexpression in African-Americans may represent a greater therapeutic challenge than its deficiency based on these results. The most common alleles found in African-Americans including CYP2D6*1, *17 and *41 need to be investigated more closely for race-specific allelic variations and the mechanism responsible for differences in allele function more closely examined. The diversity of CYP2D6 alleles suggests that nucleotide arrays or similar methods are needed to efficiently test for the most prominent/ relevant CYP2D6 alleles in humans.

ABSTRACT:Genotype/phenotype analysis with human hepatocytes has identified a new inactive CYP2D6 allele, CYP2D6*56. Cryopreserved human hepatocytes from 51 livers were evaluated for CYP2D6 activity with dextromethorphan as the probe substrate. Hepatocyte lots that lacked CYP2D6 activity were further evaluated for CYP2D6 expression and known genetic variations, including CYP2D6*2, *3, *4, *5, *6, *7, *8, *9, *10, *11, *14, *15, *17, *18, *19, *20, *25, *26, *29, *30, *35, *40, *41, *43, and various multiple copy CYP2D6 alleles (*1xn, *2xn, and *4xn) by the AmpliChip CYP450 prototype microarray (Roche Molecular Systems, Inc., Branchburg, NJ). Two discrepancies were uncovered between the CYP2D6 genotype and activity by this approach. In one sample, a previously unreported 3201C 224 T transition in exon 7 resulted in Arg344(CGA) being replaced by a stop codon (TGA), resulting in a CYP2D6 enzyme lacking the terminal 153 amino acids. This allele was given the designation of CYP2D6*56 and the GenBank accession number DQ282162. The lack of CYP2D6 activity in cryopreserved hepatocytes and microsomes found in the second sample, despite a normal level of CYP2D6 expression and a genotype (*10/*1) predictive of normal CYP2D6 activity, was attributed to enzyme inactivation by an unknown metabolite. The identification and characterization of the CYP2D6*56 allele indicates that commercial cryopreserved human hepatocytes may provide a valuable means to rapidly identify genetic variations with functional relevance. This integrated approach of identifying alleles and examining allele relationships to gene expression and function could be of tremendous value to understanding the mechanism responsible for functional differences in gene variation. The commercial availability of human cryopreserved hepatocytes also makes this potential readily available to any who are interested in it, not just those with access to private liver banks.

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