Objectives Therapeutic hypothermia is widely-employed for neuroprotection after cardiac arrest(CA). However, concern regarding elevated drug concentrations during hypothermia and increased adverse drug reaction risk complicates concurrent pharmacotherapy. Many commonly used medications in critically ill patients rely on the cytochrome P450(CYP) 3A isoform for their elimination. Therefore, our study objectives were to determine the effect of mild hypothermia on the in vivo pharmacokinetics of fentanyl and midazolam, two clinically-relevant CYP3A substrates, after CA and to investigate the mechanisms of these alterations. Design Prospective, randomized, controlled study Setting University research laboratory Subjects Thirty two adult male Sprague-Dawley rats Interventions An asphyxial CA rat model was used and mild hypothermia(33 °C) was induced 1h post injury by surface cooling and continued for 10 hours to mimic the prolonged clinical application of hypothermia accompanied by intensive care interventions. Fentanyl and midazolam were independently administered by intravenous infusion and plasma and brain concentrations were analyzed using ultra-performance liquid chromatography tandem mass spectrometry. Cyp3a2 protein expression was measured and a Michaelis-Menten enzyme kinetic analysis was performed at 37°C and 33°C using control rat microsomes. Measurements and Main Results Mild hypothermia decreased the systemic clearance of both fentanyl (61.5±11.5 to 48.9±8.95 mL/min/kg;p < 0.05) and midazolam (89.2±12.5 to 73.6±12.1 mL/min/kg;p < 0.05) after CA. The elevated systemic concentrations did not lead to parallel increased brain exposures of either drug. Mechanistically, no differences in Cyp3a2 expression was observed, but the in vitro metabolism of both drugs was decreased at 33 °C versus 37 °C through reductions in enzyme metabolic capacity rather than substrate affinity. Conclusions Mild hypothermia reduces the systemic clearances of fentanyl and midazolam in rats after CA through alterations in Cyp3a metabolic capacity rather than enzyme affinity as observed with other CYPs. Contrasting effects on blood and brain levels further complicates drug dosing. Consideration of the impact of hypothermia on medications whose clearance is dependent on CYP3A metabolism is warranted.

Adenosine triphosphate-binding cassette (ABC) transport proteins ABCC1 and ABCB1 (also known as multidrug resistance-associated protein 1 and p-glycoprotein, respectively), are key membrane efflux transporters of drugs and endogenous substrates, including in the brain. The impact of traumatic brain injury (TBI) on ABCC1 and ABCB1 expression in humans is unknown. We hypothesized that ABCC1 and ABCB1 expression would be altered in brain tissue from patients acutely after severe TBI. Archived TBI samples (n = 10) from our Brain Trauma Research Center and control samples (n = 7) from our Alzheimer Disease Research Center were obtained under Institutional Review Board approval. Protein was extracted from fresh frozen cortical brain tissue for Western blot analysis and sections were obtained from fixed cortical tissue for immunohistochemistry. Relative abundance of ABCC1 was increased in samples from TBI versus controls (2.8 -2.5 fold; p = 0.005). ABCC1 immunohistochemistry was consistent with Western blot data, with increased immunoreactivity in cerebral blood vessel walls, as well as cells with the morphological appearance of neurons and glia in TBI versus controls. Relative abundance of ABCB1 was similar between TBI and controls ( p = 0.76), and ABCB1 immunoreactivity was primarily associated with cerebral blood vessels in both groups. These human data show that TBI increases ABCC1 expression in the brain, consistent with possible implications for both patients receiving pharmacological inhibitors and/or substrates of ABCC1 after TBI.

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