Pimozide is often coprescribed with serotonin reuptake inhibitor (SSRI) antidepressants to treat depression in patients with Tourette's syndrome. In human liver microsomes (HLMs), the inhibition of the primary route of pimozide metabolism, N-dealkylation to 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI), by four SSRIs (fluoxetine, sertraline, paroxetine, and fluvoxamine) and azithromycin was tested. Inhibition constants (K(i) values) were estimated from Dixon plots (three HLMs for each inhibitor) using the appropriate enzyme inhibition model by nonlinear regression. At 10 microM paroxetine, sertraline, fluoxetine, or fluvoxamine, the formation of DHPBI from pimozide (10 microM) in HLMs was inhibited by an average (three HLMs) of 7%, 7.7%, 8%, and 16%, respectively, whereas this inhibition did not exceed 55% at the maximum concentrations (100 microM) of the SSRIs tested. Azithromycin had negligible effect on pimozide (10 microM) N-dealkylation (19% at 100 microM azithromycin). These inhibition data were compared with ketoconazole, which was included as a positive control of CYP3A inhibition. At 0.1 microM and 0.5 microM ketoconazole, the formation of DHPBI from 10 microM pimozide was inhibited by 32% and 62%, respectively. The K(i) values (+/- SD) of ketoconazole, sertraline, fluvoxamine, azithromycin, fluoxetine, and paroxetine were 0.07 microM, 89 +/- 44 microM, 89 +/- 24 microM, 103 +/- 52 microM, 117 +/- 27 microM, and 129 +/- 33 microM, respectively. These values are least 100-fold higher than the expected plasma concentrations after the usual daily doses of the SSRIs and azithromycin, suggesting that coadministration of SSRIs and azithromycin are unlikely to markedly diminish the elimination of pimozide in patients. However, in vivo predictions from in vitro data are not always perfect. In vivo, the SSRIs or azithromycin may concentrate in the liver relative to plasma. In addition, the possibility that these drugs could alter pimozide disposition through effects on transport proteins or via promoter repression cannot be ruled out.