Therapeutic hypothermia has demonstrated considerable benefit in patients experiencing cardiac arrest. Despite increasing clinical use, there is a paucity of information regarding the effect of hypothermia on the disposition of medications, specifically cytochrome P450-mediated drug metabolism. The objective was to determine the effect of hypothermia after cardiac arrest on the in vivo kinetics of a cytochrome P450 (CYP2E1) probe drug, chlorzoxazone, and to investigate the mechanism of these alterations.
University pharmacy school and animal research facility.
Sixteen male Sprague-Dawley rats.
An asphyxial arrest rat model was used and moderate hypothermia was induced immediately postinsult via surface cooling. Chlorzoxazone was administered as an intravenous bolus, and plasma concentrations were analyzed using high-performance liquid chromatography methods. Protein binding was analyzed using rat control plasma, and Michaelis-Menten enzyme kinetic analysis was performed at 37°C and 30°C using control rat microsomes at varying concentrations of chlorzoxazone.
Measurements and Main Results:
Moderate hypothermia after cardiac arrest in rats markedly decreased the systemic clearance of the CYP2E1 substrate, chlorzoxazone, when compared with normothermia after cardiac arrest, 1.26 ± 0.34 mL/min vs. 0.580 ± 0.37 mL/min (p < .001). No changes in chlorzoxazone protein binding were observed at 37°C and 30°C, and CYP2E1 enzyme capacity (maximum velocity) was not altered at these different incubation temperatures. However, Michaelis-Menten constant was significantly increased at 30°C (551 ± 150 μM) compared with incubations at 37°C (255 ± 52 μM, p < .01).
Moderate hypothermia markedly reduces the systemic clearance of chlorzoxazone in cardiac arrest rats. This results from hypothermia-induced decreases in the CYP2E1 enzyme affinity for the substrate chlorzoxazone. This is the first systematic mechanistic investigation of the effect of hypothermia on CYP2E1-mediated metabolism.