Int J Emerg Med (2010) 3:81–84 DOI 10.1007/s12245-010-0175-y ORIGINAL RESEARCH ARTICLE Antipsychotic-induced priapism in an HIV patient: a cytochrome P450-mediated drug interaction Matthew J. Geraci & Stacey L. McCoy & Paul M. Crum & Rajnikant A. Patel Received: 20 April 2009 / Accepted: 25 February 2010 / Published online: 26 May 2010 # Springer-Verlag London Ltd 2010 Abstract Background With upwards of 48% of human immunodefi- ciency 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®), experienced an episode of priapism hours after beginning two new antipsychotic medications. Quetiapine (Seroquel®) and perphenazine (Trilafon®) were added to treat a diagnosed schizoaffective disorder. Results The patient presented to the emergency department complaining of a constant, painful erection lasting approx- imately 42 h. Treatment with intracavernous ephedrine, irrigation, and aspiration helped achieve detumescence. Conclusion This case displays the immediate and detri- mental effects due to the addition of antipsychotic medi- cations to previously altered cytochrome P450 (CYP450) The views expressed in this paper are those of the author(s) and not those of the editors, editorial board or publisher. M. J. Geraci (*) : S. L. McCoy : R. A. Patel Emergency Department, Baptist Medical Center Downtown, 800 Prudential Drive, Jacksonville, FL 32207, USA e-mail: matthew.geraci@bmcjax.com P. M. Crum McIver Urological Clinic, Riverside Office, 710 Lomax St, Jacksonville, FL 32204, USA 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. Keywords Drug interaction . Cytochrome P450 . Antipsychotic . HIV. Priapism . Protease inhibitor Introduction The employment of highly active antiretroviral therapy (HAART) has raised interest and awareness in the human immunodeficiency virus (HIV) psychopharmacology arena due to potential drug-drug interactions. There is also a lack of data and documented clinical studies with regards to the wide range of adverse effects and combinations of these medications. As upwards of 48% of HIV-infected persons have a probable psychiatric disorder, the possibility of cross-class drug interactions causing adverse effects or fatalities exists [1, 2]. Undesirable symptoms or peculiar presentations may be an indication that pharmaceutical treatments may be counterproductive, requiring healthcare professionals to investigate drug therapy. The following describes a case of drug-induced low-flow priapism provoked by the addition of new antipsychotic medications to an HIV patient previously on HAART. The cytochrome P450 (CYP450) family of enzymes has been implicated in a large number of preventable, adverse drug interactions when individually discussing HIV and psychopharmacology. HAART can pose significant side effects for patients and alter CYP450 metabolism exten- sively; however, when combined with antipsychotic drugs that require CYP450 enzymes for transport and metabo-