Therapeutic hypothermia-induced pharmacokinetic alterations on CYP2E1 chlorzoxazone-mediated metabolism in a cardiac arrest rat model*

Treatment of Cardiovascular Dysfunction With PDE5-Inhibitors - Temperature Dependent Effects on Transport and Metabolism of cAMP and cGMP

Introduction

Cardiovascular dysfunction is a potentially lethal complication of hypothermia. Due to a knowledge gap, pharmacological interventions are not recommended at core temperatures below 30°C. Yet, further cooling is induced in surgical procedures and survival of accidental hypothermia is reported after rewarming from below 15°C, advocating a need for evidence-based treatment guidelines. In vivo studies have proposed vasodilation and afterload reduction through arteriole smooth muscle cGMP-elevation as a favorable strategy to prevent cardiovascular dysfunction in hypothermia. Further development of treatment guidelines demand information about temperature-dependent changes in pharmacological effects of clinically relevant vasodilators.

Materials and Methods

Human phosphodiesterase-enzymes and inverted erythrocytes were utilized to evaluate how vasodilators sildenafil and vardenafil affected cellular efflux and enzymatic breakdown of cAMP and cGMP, at 37°C, 34°C, 32°C, 28°C, 24°C, and 20°C. The ability of both drugs to reach their cytosolic site of action was assessed at the same temperatures. IC₅₀- and K_i-values were calculated from dose–response curves at all temperatures, to evaluate temperature-dependent effects of both drugs.

Results

Both drugs were able to reach the intracellular space at all hypothermic temperatures, with no reduction compared to normothermia. Sildenafil IC₅₀ and K_i-values increased during hypothermia for enzymatic breakdown of both cAMP (IC₅₀: 122 ± 18.9 μM at 37°C vs. 269 ± 14.7 μM at 20°C, p < 0.05) and cGMP (IC₅₀: 0.009 ± 0.000 μM at 37°C vs. 0.024 ± 0.004 μM at 32°C, p < 0.05), while no significant changes were detected for vardenafil. Neither of the drugs showed significant hypothermia-induced changes in IC₅₀ and K_i–values for inhibition of cellular cAMP and cGMP efflux.

Conclusion

Sildenafil and particularly vardenafil were ableto inhibit elimination of cGMP down to 20°C. As the cellular effects of these drugs can cause afterload reduction, they show potential in treating cardiovascular dysfunction during hypothermia. As in normothermia, both drugs showed higher selectivity for inhibition of cGMP-elimination than cAMP-elimination at low core temperatures, indicating that risk for cardiotoxic side effects is not increased by hypothermia.

Reperfusion using lactate Ringer's mixture partially eliminates IGF II receptor involved cardiac damage caused by hemorrhagic shock in diabetic rats

Diabetes is a metabolic disorder that damages many organs. We investigated the effects of reperfusion using lactate Ringer's solution (LR) in a diabetic animal model. Eight-week-old rats were divided into groups: control, hemorrhagic shock induced (HS), diabetes mellitus (DM), DM plus HS (DM + HS) and DM rats that received LR after HS (DM + HS + LR). HS was induced by withdrawing blood from the femoral artery and arterial pressure was maintained at 40 mm Hg for 1 h. Animals were perfused with either withdrawn blood or LR. Rats were sacrificed and hearts were collected from all groups. Histopathological studies were performed using left ventricles and western blotting analysis was performed using protein extracted from the left ventricle. Using the TUNEL assay, we found more apoptotic cells in the DM + HS group compared to the control group, whereas in animals resuscitated with LR, the number of apoptotic cells was reduced. Western blotting showed a significant reduction in apoptotic markers, cyt c, cas 9 and cas 3, and increased survival markers, pPI3K and pAKT, in the DM + HS + LR group. Reperfusion with LR may have therapeutic effects on trauma induced HS by blocking the IGF II R facilitated apoptosis pathway in diabetic rats.

The influence of targeted temperature management on the pharmacokinetics of drugs administered during and after cardiac arrest: a systematic review

OBJECTIVE

Pharmacokinetic parameters of drugs are widely investigated under normothermic conditions and normal hemodynamic parameters. The European Resuscitation Council recommends the use of targeted temperature management (TTM) with a target temperature of 34 °C in cardiac arrest (CA) patients. The aim of this literature review is to investigate the influence of CA combined with TTM on the pharmacokinetics of drugs. Results of preclinical and clinical studies are compared with each other. Only the most important drugs, administered during CA in emergency setting, were studied.

METHODS

A literature search was conducted within PubMed and Google Scholar. The search terms included 'therapeutic hypothermia', 'TTM', 'drug metabolism', 'pharmacokinetics during hypothermia', 'cardiac arrest/etiology'. In Pubmed, MeSH-terms were also included: 'myocardial infarction/therapy', 'heart arrest/complications' and 'hypothermia'. To search for preclinical studies: the search terms 'pigs' and 'swine' were used. After the primary shift of relevant findings, further articles were found through references of these (snowballing method), as well as through related articles as suggested by the databases.

RESULTS

Due to the reduced cardiac output during TTM, most of the distribution volume ([Formula: see text]) of drugs included in this literature study is decreased. Only the [Formula: see text] of chlorzoxazone in CA rats and midazolam in non-CA patients are significantly increased during respectively deep and mild hypothermia. The renal, hepatic and biliary clearance of drugs administered during CA/TTM/hypothermia are decreased.

DISCUSSION

The combination of a decreased [Formula: see text] and a decrease in the metabolization/excretion of drugs during CA/TTM result in higher plasma concentrations compared to the plasma concentrations during CA without TTM.

Adverse drug reactions in therapeutic hypothermia after cardiac arrest

BACKGROUND

Therapeutic hypothermia (TH) improves survival and neurologic function in comatose survivors of cardiac arrest. Many medications used to support TH have altered pharmacokinetics and pharmacodynamics during this treatment. It is unknown if or at what frequency the medications used during TH cause adverse drug reactions (ADRs).

METHODS

A retrospective chart review was conducted for patients admitted to an intensive care unit (ICU) after cardiac arrest and treated with TH from January 2009 to June 2012 at two urban, university-affiliated, tertiary-care medical centres. Medications commonly used during TH were screened for association with significant ADRs (grade 3 or greater per Common Terminology Criteria for Adverse Events) using three published ADR detection instruments.

RESULTS

A total of 229 patients were included, the majority being males with median age of 62 presenting with an out-of-hospital cardiac arrest in pulseless electrical activity or asystole. The most common comorbidities were hypertension, coronary artery disease, and diabetes mellitus. There were 670 possible ADRs and 69 probable ADRs identified. Of the 670 possible ADRs, propofol, fentanyl, and acetaminophen were the most common drugs associated with ADRs. Whereas fentanyl, insulin, and propofol were the most common drugs associated with a probable ADR. Patients were managed with TH for a median of 22 hours, with 38% of patients surviving to hospital discharge.

CONCLUSIONS

Patients undergoing TH after cardiac arrest frequently experience possible adverse reactions associated with medications and the corresponding laboratory abnormalities are significant. There is a need for judicious use and close monitoring of drugs in the setting of TH until recommendations for dose adjustments are available to help prevent ADRs.

Pharmacokinetic changes of unbound theophylline are due to plasma protein binding displacement and CYP1A2 activity inhibition by baicalin in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Baicalin is one of the major bioactive constituents of Scutellariae Radix, the root of Scutellariae baicalensis Georgi and possesses a wide variety of pharmacological properties.

AIM OF THE STUDY

To elucidate the effect of baicalin on the pharmacokinetics of theophylline in rats, focusing on plasma protein binding displacement and inhibition effect on CYP1A2 in vivo and in vitro.

MATERIALS AND METHODS

The study was a randomized, three-period crossover design. Nine rats were given saline (control) or 450 mg/kg baicalin (dosage regimen A or B). Dosage regimen A was administered once at 0 h. Dosage regimen B was divided into three dosages (225,112.5, 112.5 mg/kg) and was given at 0, 2 and 4 h, respectively. Then theophylline (5 mg/kg, i.v.) was administered immediately. The effect of baicalin on CYP1A2 activity was determined by metabolism of phenacetin in vitro and plasma protein binding of theophylline was determined by ultrafiltration.

RESULTS

C(max) decreased from (12.4 ± 1.6) to (8.7 ± 0.9) and (8.6 ± 2.0) mg/L, T(1/2) increased by 116 and 96%, V(d) increased by 51 and 49% for total theophylline in rats treated with dosage regimen A and B of baicalin, respectively. Cmax was significantly increased, V(d) decreased by 43 and 29% for unbound theophylline in rats treated with dosage regimen A and B of baicalin, respectively (P < 0.01). T(1/2) of unbound theophylline increased by 104% only in rats treated with dosage regimen B. No significant effects on the CL and AUC of both total and unbound theophylline were observed in the rats treated with dosage regimen A, but the CL decreased and AUC increased for total theophylline and CL decreased for unbound theophylline in the group treated with dosage regimen B (P < 0.05). Correlation analysis showed that the mean unbound theophylline (%) and mean baicalin concentration was in good correlation (P < 0.01). Baicalin decreased metabolism of phenacetin and exhibited a mixed-type inhibition in rat liver microsomes, with a K(i) value of 88.1 μM in vitro. Moreover baicalin was a competitive displacer of theophylline from plasma protein in vitro.

CONCLUSIONS

The changes in Cmax, T(1/2), CL and AUC of theophylline due to baicalin may be attributed to two mechanisms, plasma protein binding displacement and CYP1A2 activity inhibition.

Evaluation for effect of hypothermia on the disposition of 4-nitrophenol in rats by in-vitro metabolism study and rat liver perfusion system

Objectives

The aim of this study was to evaluate the effect of hypothermia on the in-vivo pharmacokinetics of 4-nitrophenol (4NP) using rat liver homogenate and rat liver perfusion system.

Methods

Rat liver homogenate was incubated with 4NP, which is mainly metabolized by cytochrome P450 2E1, at 37, 34, 32 or 28°C. The Michaelis constant (Km) and maximum elimination velocity (Vmax) of 4NP were calculated by a Hanes–Woolf plot. The hepatic extraction ratio (Eh) of 4NP was evaluated in a rat liver perfusion study at 37, 34, 32 or 28°C. Moreover, the plasma concentration profiles of 4NP after its intravenous (i.v.) administration to rats were analysed by the moment theory and were compared with in-vitro parameters.

Key findings

While the Km of 4NP was not changed, the Vmax and Eh were reduced at low temperatures. The plasma concentrations of 4NP after its i.v. administration to rats were significantly increased at 28°C.

Conclusion

Changes in the pharmacokinetics of 4NP under hypothermic conditions were caused by alterations in Vmax and Eh. We may be able to predict the disposition of a drug by in-vitro studies.

Population pharmacokinetics of phenobarbital in infants with neonatal encephalopathy treated with therapeutic hypothermia

OBJECTIVE

Phenobarbital is the first-line treatment for neonatal seizures. Many neonates with hypoxic ischemic encephalopathy are treated with therapeutic hypothermia, and about 40% have clinical seizures. Little is known about the pharmacokinetics of phenobarbital in infants with hypoxic ischemic encephalopathy who undergo therapeutic hypothermia. The objective of this study was to determine the effect of therapeutic hypothermia on phenobarbital pharmacokinetics, taking into account maturational changes.

SETTING

Level 3 neonatal ICU.

PATIENTS

Infants with hypoxic ischemic encephalopathy and suspected seizures, all treated with phenobarbital. Some of these infants also received treatment with therapeutic hypothermia.

INTERVENTIONS

None.

DESIGN

A retrospective cohort study of 39 infants with hypoxic ischemic encephalopathy treated with phenobarbital (20 were treated with therapeutic hypothermia and 19 were not).

MEASUREMENTS AND MAIN RESULTS

Data on phenobarbital plasma concentrations were collected in 39 subjects with hypoxic ischemic encephalopathy with or without therapeutic hypothermia. Using nonlinear mixed-effects modeling, population pharmacokinetics of phenobarbital were developed with a total of 164 plasma concentrations. A one-compartment model best described the pharmacokinetics. The clearance of phenobarbital was linearly related to body weight and matured with increasing age with a maturation half-life of 22.1 days. Therapeutic hypothermia did not influence the pharmacokinetic parameters of phenobarbital.

CONCLUSIONS

Therapeutic hypothermia does not influence the clearance of phenobarbital after accounting for weight and age. Standard phenobarbital dosing is appropriate for the initial treatment of seizures in neonates with hypoxic ischemic encephalopathy treated with therapeutic hypothermia.

Concentration-dependent inhibitory effect of Baicalin on the plasma protein binding and metabolism of chlorzoxazone, a CYP2E1 probe substrate, in rats in vitro and in vivo

Some of the components found in herbs may be inhibitors or inducers of cytochrome P450 enzymes, which may therefore result in undesired herb-drug interactions. As a component extracted from Radix Scutellariae, the direct effect of baicalin on cytochrome P450 has not been investigated sufficiently. In this study, we investigated concentration-dependent inhibitory effect of baicalin on the plasma protein binding and metabolism of chlorzoxazone (CZN), a model CYP2E1 probe substrate, in rats in vitro and in vivo. Animal experiment was a randomized, three-period crossover design. Significant changes in pharmacokinetic parameters of CZN such as C_max, t_1/2 and V_d were observed after treatment with baicalin in vivo (P<0.05). C_max decreased by 25% and 33%, whereas t_1/2 increased by 34% and 53%, V_d increased by 37% and 50% in 225 mg/kg and 450 mg/kg baicalin-treated rats, respectively. The AUC and CL of CZN were not affected (P>0.05). Correlation analysis showed that the changes in CZN concentrations and baicalin concentrations were in good correlation (r>0.99). In vitro experiments, baicalin decreased the formation of 6-OH-chlorzoxazone in a concentration-dependent manner and exhibited a competitive inhibition in rat liver microsomes, with a Ki value of 145.8 µM. The values of C_max/Ki were 20 and 39 after treatment with baicalin (225 and 450 mg/kg), respectively. Protein binding experiments in vivo showed that the plasma free-fraction (fu) of CZN increased 2.6-fold immediately after baicalin treatment (450 mg/kg) and in vitro showed that baicalin (125–2500 mg/L) increased the unbound CZN from 1.63% to 3.58%. The results indicate that pharmacokinetic changes in CZN are induced by inhibitory effect of baicalin on the plasma protein binding of CZN and CYP2E1 activity.

Mild hypothermia decreases fentanyl and midazolam steady-state clearance in a rat model of cardiac arrest

OBJECTIVES

Therapeutic hypothermia is widely employed for neuroprotection after cardiac arrest. 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 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 cytochrome P450 3A substrates, after cardiac arrest 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 cardiac arrest rat model was used and mild hypothermia (33°C) was induced 1 hr post injury by surface cooling and continued for 10 hrs 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 ultraperformance liquid chromatography tandem mass spectrometry. Cytochrome P450 3a2 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 < .05) and midazolam (89.2 ± 12.5 to 73.6 ± 12.1 mL/min/kg; p < .05) after cardiac arrest. The elevated systemic concentrations did not lead to parallel increased brain exposures of either drug. Mechanistically, no differences in cytochrome P450 3a2 expression was observed, but the in vitro metabolism of both drugs was decreased at 33°C vs. 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 cardiac arrest through alterations in cytochrome P450 3a2 metabolic capacity rather than enzyme affinity as observed with other cytochrome P450s. Contrasting effects on blood and brain levels further complicates drug dosing. Consideration of the impact of hypothermia on medications whose clearance is dependent on P450 3A metabolism is warranted.

Cardiac arrest and therapeutic hypothermia decrease isoform-specific cytochrome P450 drug metabolism

Mild therapeutic hypothermia is emerging clinically as a neuroprotection therapy for individuals experiencing cardiac arrest (CA); however, its effects combined with disease pathogenesis on drug disposition and response have not been fully elucidated. We determined the activities of four major hepatic-metabolizing enzymes (CYP3A, CYP2C, CYP2D, and CYP2E) during hypothermia after experimental CA in rats by evaluating the pharmacokinetics of their probe drugs as a function of altered body temperature. Animals were randomized into sham normothermia (37.5-38°C), CA normothermia, sham hypothermia (32.5-33°C), and CA hypothermia groups. Probe drugs (midazolam, diclofenac, dextromethorphan, and chlorzoxazone) were given simultaneously by intravenous bolus after temperature stabilization. Multiple blood samples were collected between 0 and 8 h after drug administration. Pharmacokinetic (PK) analysis was conducted using a noncompartmental approach and population PK modeling. Noncompartmental analysis showed that the clearance of midazolam (CYP3A) in CA hypothermia was reduced from sham normothermia rats (681.6 ± 190.0 versus 1268.8 ± 348.9 ml · h(-1) · kg(-1), p < 0.05). The clearance of chlorzoxazone (CYP2E) in CA hypothermia was also reduced from sham normothermia rats (229.6 ± 75.6 versus 561.89 ± 215.9 ml · h(-1) · kg(-1), p < 0.05). Population PK analysis further demonstrated the decreased clearance of midazolam (CYP3A) was associated with CA injury (p < 0.05). The decreased clearance of chlorzoxazone (CYP2E1) was also associated with CA injury (p < 0.01). Hypothermia was found to be associated with the decreased volume of distribution of midazolam (V(1)), dextromethorphan (V(1)), and peripheral compartment for chlorzoxazone (V(2)) (p < 0.05, p < 0.05, and p < 0.01, respectively). Our data indicate that hypothermia, CA, and their interaction alter cytochrome P450-isoform specific activities in an isoform-specific manner.

The effect of therapeutic hypothermia on drug metabolism and response: cellular mechanisms to organ function

INTRODUCTION

Therapeutic hypothermia is being employed clinically due to its neuro-protective benefits. Both critical illness and therapeutic hypothermia significantly affect drug disposition, potentially contributing to drug-therapy and drug-disease interactions. Currently, there is limited information on the known alterations in drug concentration and response during mild hypothermia treatment, and there is a limited understanding of the specific mechanisms that underlie alterations in drug concentrations and the potential clinical importance of these changes.

AREAS COVERED

A systemic review of the effect of therapeutic hypothermia on drug metabolism, disposition and response is provided. Specifically, the clinical and preclinical evidence of the effects of therapeutic hypothermia on blood flow, specific hepatic metabolism pathways, transporter function, renal excretion, pharmacodynamics and the effects during rewarming are reviewed.

EXPERT OPINION

Available evidence demonstrates that mild hypothermia decreases the clearance of a variety of drugs with apparently little change in drug-protein binding. Recent evidence suggests that the magnitude of the change is elimination route specific. Further research is needed to determine the impact of these alterations on both drug concentration and response in order to optimize the therapeutic hypothermia in this vulnerable patient population.

Phenobarbital for neonatal seizures in hypoxic ischemic encephalopathy: a pharmacokinetic study during whole body hypothermia

PURPOSE

Therapeutic hypothermia has recently been introduced to treat term newborns with hypoxic-ischemic encephalopathy, of whom more than half have seizures. Phenobarbital is widely used to treat neonatal seizures, but it is unknown whether its pharmacokinetics is affected by hypothermia. We evaluated the influence of hypothermia on phenobarbital pharmacokinetics in asphyxiated newborns.

METHODS

Nineteen term asphyxiated newborns treated with mild whole body hypothermia, started within 6 h after birth and protracted for 72 h, received phenobarbital for clinical seizures. Treatment schedule consisted of a loading dose of 20 mg/kg, titrated to response, up to a maximum dose of 40 mg/kg, followed by a maintenance dose of 2.5 or 1.5 mg/kg every 12 h. Phenobarbital concentrations were measured on 28 dried blood spots in each newborn.

KEY FINDINGS

Eighteen newborns showed plasma concentrations within the reference range after receiving a loading dose of 20 mg/kg. In the remaining newborn, who had received a loading dose of 35 mg/kg, phenobarbital concentrations exceeded the upper reference limit. Phenobarbital concentrations reached a virtual steady state in all newborns. Pharmacokinetic parameters were then calculated. Minimum and maximum concentration (24.7 ± 8.8 and 30.63 ± 10.3 mg/L), average plasma concentration (27.37 ± 9.4 mg/L), and half-life (173.9 ± 62.5 h) were considerably higher than reported in literature for normothermic newborns. Pharmacokinetic parameters did not differ significantly between infants receiving different maintenance doses.

SIGNIFICANCE

Phenobarbital administered to newborns under whole body hypothermia results in higher plasma concentrations and longer half-lives than expected in normothermic newborns.

Therapeutic hypothermia: the Safar vision

At the 2(nd) International Brain Hypothermia conference, in Miami, the late Dr. Peter Safar was honored for his many contributions to the field of therapeutic hypothermia. Therapeutic hypothermia played a central role in his overall vision for optimized resuscitation and neurointensive care, across a large number of potential insults. The successful use of therapeutic hypothermia in comatose patients after cardiac arrest, for example, was already included in the historic first "ABCs" of resuscitation, published by Safar in 1964. This review addresses key historical events in the development and implementation of therapeutic hypothermia across a number of central nervous system insults. A discussion of future potential uses of this therapy in a variety of applications as part of the Safar vision is also presented.

Moderate hypothermia prevents cardiac arrest-mediated suppression of drug metabolism and induction of interleukin-6 in rats

OBJECTIVE

Therapeutic hypothermia is being clinically used to reduce neurologic deficits after cardiac arrest (CA). Patients receiving hypothermia after CA receive a wide-array of medications. During hypothermia, drug metabolism is markedly reduced. Little, however, is known about the impact of hypothermia on drug metabolism after rewarming. The objective of this study was to examine the effect of CA and hypothermia on the functional regulation of two major drug metabolizing cytochrome P450 (CYP) isoforms.

DESIGN

Laboratory investigation.

SETTING

University pharmacy school and animal research facility.

SUBJECTS

Thirty-six male Sprague-Dawley rats.

INTERVENTIONS

Hypothermia was induced via surface cooling in a rat CA model and maintained for 3 hrs. Animals were killed at 5 or 24 hrs and liver was analyzed for hepatic activity and mRNA expression of CYP3A2 and CYP2E1. Plasma interleukin-6 (IL-6) concentrations were determined. The effect of IL-6 on pregnane X receptor-mediated transcription of the rat CYP3A2 promoter was evaluated via luciferase reporter in HepG2 cells.

MEASUREMENTS AND MAIN RESULTS

At 24 hrs after CA a decrease in CYP3A2 and CYP2E1 activity was observed, 55.7% +/- 12.8% and 46.8% +/- 29.7% of control, respectively (p < 0.01). CA decreased CYP3A2 mRNA (p < 0.05), but not CYP2E1 mRNA. Expression of other pregnane X receptor target enzymes and transporter genes were similarly down-regulated. CA also produced an approximately ten-fold increase in plasma IL-6. CA-mediated inhibition of CYP3A2 and CYP2E1 was attenuated by hypothermia, as was the increase in IL-6. Furthermore, IL-6 attenuated pregnane X receptor-mediated transcription of the CYP3A2 promoter.

CONCLUSIONS

CA produces CYP3A2 down-regulation at 24 hrs, potentially via IL-6 effects on pregnane X receptor-mediated transcription. Also, hypothermia attenuates the CA-mediated down-regulation, thereby normalizing drug metabolism after rewarming.

Effects of hypothermia on drug disposition, metabolism, and response: A focus of hypothermia-mediated alterations on the cytochrome P450 enzyme system

OBJECTIVES

Therapeutic hypothermia has been shown to decrease neurologic damage in patients experiencing out-of-hospital cardiac arrest. In addition to being treated with hypothermia, critically ill patients are treated with an extensive pharmacotherapeutic regimen. The effects of hypothermia on drug disposition increase the probability for unanticipated toxicity, which could limit its putative benefit. This review examines the effects of therapeutic hypothermia on the disposition, metabolism, and response of drugs commonly used in the intensive care unit, with a focus on the cytochrome P450 enzyme system.

DATA SOURCES AND STUDY SELECTION

A MEDLINE/PubMed search from 1965 to June 2006 was conducted using the search terms hypothermia, drug metabolism, P450, critical care, cardiac arrest, traumatic brain injury, and pharmacokinetics.

DATA EXTRACTION AND SYNTHESIS

Twenty-one studies were included in this review. The effects of therapeutic hypothermia on drug disposition include both the effects during cooling and the effects after rewarming on drug metabolism and response. The studies cited in this review demonstrate that the addition of mild to moderate hypothermia decreases the systemic clearance of cytochrome P450 metabolized drugs between approximately 7% and 22% per degree Celsius below 37degreesC during cooling. The addition of hypothermia decreases the potency and efficacy of certain drugs.

CONCLUSIONS

This review provides evidence that the therapeutic index of drugs is narrowed during hypothermia. The magnitude of these alterations indicates that intensivists must be aware of these alterations in order to maximize the therapeutic efficacy of this modality. In addition to increased clinical attention, future research efforts are essential to delineate precise dosing guidelines and mechanisms of the effect of hypothermia on drug disposition and response.