Antipsychotic-induced priapism in an HIV patient: a cytochrome P450-mediated drug interaction

BACKGROUND With upwards of 48% of human immunodeficiency virus (HIV)-infected persons having a probable psychiatric disorder, the possibility of cross-class drug interactions causing adverse effects or fatalities exists. AIMS This report discusses an emergent case of low-flow priapism caused by an interaction between a previously prescribed combination protease inhibitor (PI) and newly added antipsychotic medications. METHODS A 50-year-old HIV-positive man on highly active antiretroviral therapy (HAART), including the combination PI, lopinavir/ritonavir (Kaletra((R))), experienced an episode of priapism hours after beginning two new antipsychotic medications. Quetiapine (Seroquel((R))) and perphenazine (Trilafon((R))) were added to treat a diagnosed schizoaffective disorder. RESULTS The patient presented to the emergency department complaining of a constant, painful erection lasting approximately 42 h. Treatment with intracavernous ephedrine, irrigation, and aspiration helped achieve detumescence. CONCLUSION This case displays the immediate and detrimental effects due to the addition of antipsychotic medications to previously altered cytochrome P450 (CYP450) enzyme levels. The inhibition of CYP450 enzymes 3A4 and 2D6 by the combination PI, lopinavir/ritonavir, was likely the major culprit in causing greater than expected free levels of perphenazine and quetiapine resulting in priapism.