Drug treatment of psychiatric disorders is troubled by severe adverse effects, low compliance, and a lack of efficacy in about 30% of patients. Consequently, much research has been performed on metabolizing enzymes, such as the CYP enzymes and the effect, of their variation on the efficacy and tolerability of commonly used antipsychotic and antidepressant, drugs. Twelve families of CYP enzymes have been described, of which four (CYP1 to CYP4) are directly involved in drug metabolism.39 They constitute the best-studied family of xenobiotic-metabolizing enzymes. Mutations in the genes CYP2D6, CYP2C9, and CYP2C19

have already Inhibitors,research,lifescience,medical been shown to be the cause of altered drug pharmacokinetics:40-42 Possibly the most-studied drug-metabolizing enzyme is CYP2D6, which may be involved in the metabolism of up to 25% of commonly used drugs.43 Mutations in the CYP2D6 gene have been found to be responsible for phenotypic variation in the metabolism of debrisoquine, and individuals Inhibitors,research,lifescience,medical can be classified as poor metabolizers (PMs), intermediate metabolizers (IMs), extensive metabolizers (EMs), or ultrarapid metabolizers (UMs). medical Ninety-five percent of the PMs are generally homozygous for two of the mutations

or the deletion of the entire CYP2D6 gene. Polymerase chain reaction (PCR) methods are available for the rapid detection of these mutations as well as mutations in other drugmetabolizing enzyme genes, Inhibitors,research,lifescience,medical such as CYP2C9, Inhibitors,research,lifescience,medical CYP2C19, and cytosolic N-acetyltransferase 2 gene (NAT2), in order to facilitate the prediction of an individual’s metabolizing rates. Due to the high frequency of mutations in metabolizing enzymes in the general population, they will probably remain important in the success of therapeutic treatment. It, has been proposed that, variation in metabolizing enzymes, and variation in drug targets Inhibitors,research,lifescience,medical or receptors, combine to fully explain the heterogeneity in response to psychiatric treatment. DNA chips

(see below) for the detection of CYP2D6 and CYP2C19 mutations have already been developed for the identification of PMs44-45 and these will be combined with the pharmacogenetic single nucleotide polymorphism (SNP) profiles described in the next section to predict, with a high degree of accuracy, individuals who are likely to have an ADR to a medication, even without specific knowledge of the metabolism of the drug Methisazone or of the specific alleles that modulate responses to it. SNPs and the testing for common complex disorders If a region of the human genome is sequenced from two randomly chosen individuals, 99% of the examined DNA will be identical. Of the 0.1 % that differs, more than 80% will be SNPs.46 SNPs represent a single bp variation (for example, a C to T transition) between individuals in the population, where each version of the variant, (in the above example, C or T) is observed in the general population at a frequency of more than 1%.