The Effect of the Protease Inhibitors Ritonavir on the Rate of Metabolism of Midazolam

Abstract : Midazolam (MDZ) is a benzodiazepine administered for preoperative sedation.Protease inhibitors (PI) such as ritonavir (RIT) are drugs used in the treatment of the human immunodeficiency virus (HIV) and acquired immune deficiency syndrome (AIDS). They act as inhibitors of HIV-1 protease, a major enzyme responsible for production and maturation of infectious viral progeny. A recent case study discussed prolonged hypnotic effects of MDZ when administered to a patient on a PI regimen. MDZ and RIT biotransformation is accomplished by the P450 3A4 enzyme, suggesting a drug interaction may occur. The purpose of this study was to assess the in vitrometabolic reactions of MDZ in the presence of RIT. Human liver microsomes (HLM) from four cadaver samples were prepared and pooled to provide a homogenous mixture of P450 isozymes. They were incubated with MDZ alone and then in combination with RIT. Therapeutic concentrations of MDZ were used (0.5, 1, 3, 6, 12 M). Subtherapeutic RIT concentrations of 0.005, 0.01, 0.05, 0.1 M were used, as pilot studies demonstrated complete inhibition above 0.1 M. Through the use of high performance liquid chromotography (HPLC), the ratio between MDZ's major metabolite and the internal standard were obtained. Significant inhibition of MDZ metabolism was demonstrated at even subtherapeutic concentrations of RIT. Kinetic analysis showed RIT with an apparent Ki (constant of inhibition) value of 0.00956. Percent inhibition was as high as 95%. These values suggest the possibility of prolonged sedation and respiratory depression in the clinical setting.