The Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE) was a systematic

The antipsychotic drug, olanzapine, one of the most widely used drugs in clinical medicine, has a high rate of discontinuation due to inefficacy and/or adverse effects. We identified a SNP in the drug metabolizing enzyme cytochrome, P450 3A43 (CYP3A43; rs472660) that highly significantly predicted olanzapine clearance in the CATIE trial (p=5.9e−7). Moreover, at standard antipsychotic doses, 50 % of individuals with the high clearance genotype (AA) have trough blood levels below the therapeutic range. Interestingly, a much higher proportion of African Americans carry the A allele compared to Caucasians (allele frequency 67 % vs. 14%). After accounting for CYP3A43 genotype, race is no longer a significant predictor of olanzapine clearance. Olanzapine clearance was associated with measures of clinical response. Patients with greater clearance had higher symptom ratings and were more likely to discontinue treatment due to an inadequate response. Our data identify a genetic mechanism for variation in olanzapine response and demonstrate that blood level monitoring of olanzapine treatment is advisable