The effect of ritonavir on the pharmacokinetics of a single dose of olanzapine (10 mg) was determined in 14 healthy volunteers (13 male, 1 female). Ritonavir was titrated to 500 mg twice daily over 7 days with 500 mg bd adminstered for 4 days prior to olanzapine administration. Ritonavir administration resulted in a 53% reduction in olanzapine AUC (501 to 253 ng/ml.h) and a 40% reduction in olanzapine Cmax (15 to 9 ng/ml). The geometric mean ritonavir trough concentration was 5100 ng/ml (range 2500-12000 ng/ml).The data suggest that some patients receiving these drugs in combination may experience reduced therapuetic benefit from olanzapine. Patients stabilised on a regimen containing olanzapine and ritonavir, who have the ritonavir discontinued should be monitored closely for new side effects potentially arising from increased olanzapine exposure.
Influence of ritonavir on olanzapine pharmacokinetics in healthy volunteers. Penzak SR, Yi Hon Y, Lawhorn WD, et al. J Clin Psychopharmacol, 2002, 22:366-370.