**3. The effects of therapeutic hypothermia on drug pharmacokinetics**

Therapeutic Hypothermia: Implications on Drug Therapy 139

**Concentration & PK Parameters** 

concentrations, ↓ CLs, ↓ Vd, ↑ half-life,

concentrations, ↓ CL 70%,t1/2<sup>β</sup> ↑ 2-fold, Vd↓

Total CL↓ 42%, plasma AUC↑ 2-fold, renal

↑ Plasma

secretion ↓

Vd↑ 83%, CL↓, Ke↓

AUC↑ 180%, CL↓ 67%

and Ke↓ 50%

50%

CL↓

**Drug Route of** 

CA rats/30C Chlorzoxazone CYP2E1 ↓ CLs, t1/2, ke. ↑ Vd

Koren *et al.* [45] Piglets/31.6C Fentanyl CYP3A4 ↑ Plasma

Dog/30C Morphine UGT, CYP2C,

Rats/32°C PSP Renal Tubular

HIE Infants/33-34C Morphine UGT, CYP2C,

Liu X. *et al*. [20] HIE Infants/33.5°C Gentamicin Renal Filtration No change in CL

Abbreviations: CL: Systemic clearance; AUC: Area under curve; Ke: Elimination rate; Vd: Volume of distribution; T1/2:

**Table 2.** Summary of the findings of clinical studies evaluating the effects of therapeutic hypothermia

**Elimination**

CYP3A4

Secretion

Rocuronium CYP2D6/Renal CL↓ to 51%

CYP3A4

CYP2C19

Midazolam CYP3A4 CL↓ 11% per degree

Vecuronium CYP450s CL↓ 11.3% per degree

TBI Patients/32-34C Midazolam CYP3A4 Plasma concentration↑,

Phenytoin CYP2C9 &

Gentamicin Renal Filtration No change in CL

Digoxin Renal Filtration Direction from B to A ↓

**Study Group Subject Population/**

Satas S. *et al*. [40] Hypoxia newborn pig/35°C

Jin J *et al*. [39] In vitro kidney

epithelial cell/32C

Neurosurgical Patients/30.4C

volunteers/35.5-36.5°C

Healthy

Patients/34°C

Volunteers/<35, 35- 35.9,36-36.9°C

Iida Y. *et al*. [25] Brain Damage

*Preclinical Studies* 

[26]

Tortorici *et al*.

Bansinath M*. et* 

Nishida K. *et al*.

*Clinical Studies*  Fukuoka N. *et al*.

Beaufort A. M. *et al*. [46]

Roka A. *et al*.

Hostler D. *et al*.

Caldwell J. E. *et* 

on drug disposition.

*al*. [35]

Half- life.

*al*. [38]

[19]

[32]

[37]

[33]

**Temperature Cooled**

In general, hypothermia can affect drug disposition in various ways. We have previously discussed the physiological changes induced by hypothermia. These effects generally include decreases in active transport processes of drug absorption and excretion, no alteration in passive processes of drug disposition, and a general reduction in the overall rate of drug metabolism. Although these are general alterations, it is important to note that each of these alterations have been shown to be drug specific and requires particular evaluations of drug disposition in the cooled patient. In addition, hypothermia is also known to alter the different phases of drug pharmacokinetics. These phases can be broken up into absorption, distribution, metabolism and transport, and excretion. This section will highlight the effect of therapeutic hypothermia on each of these four phases, and the current research in the area. A summary of the current clinical studies on drug disposition is given in Table 2. In addition, Figure 1 summarizes the known physiologic and drug disposition effects of hypothermia and provides a statement of the level of evidence that currently exists in the published literature.

#### a. Drug absorption effects

Most drugs in the ICU are administered intravenously. However, some drugs are given non-intravenously, typically via oral administration. Drugs that are administered orally are subject to many factors that influence the rate and amount of drug that can be absorbed before it reaches the bloodstream. Some of these factors, such as disintegration and dissolution, are drug dependent and will vary among drugs based on their dosage form (tablet, capsule, etc) as well as the components that make up the drug (active ingredient, excipients, etc). Physiochemical properties of the drug, such as the pKa, lipophilicity, and solubility, will also influence the total amount of drug absorbed.

As previously addressed in the physiology section, gastrointestinal motility is known to decrease with mild hypothermia. Furthermore, a decrease in temperature can decrease blood flow at the site of absorption, and increase or decrease the gastric and duodenal pH, all factors that will ultimately affect a drug's absorption.

Pre-clinical studies investigated the effects of moderate hypothermia on these physiological factors. Hypothermia is associated with a decrease in passive transport via ABCB1. Results demonstrated a 30-44% decrease in the absorption rate constant, ka, of pentobarbital, levodopa and uracil. However, these pre-clinical studies induced moderate or severe hypothermia. Therefore, the decrease in drug absorption may be more pronounced than what would be observed clinically under mild hypothermia.

Overall, the effect of hypothermia on drug absorption may lead to a decreased rate and prolonged time to reach maximal concentration for some drugs. Furthermore, the time of onset may be delayed and the magnitude of the pharmacological response, due to these reduced concentrations, may be diminished. However, current studies do not accurately reflect the range of temperature cooling *in vivo* and further clinical studies need to be done to determine if the magnitude of alterations in drug absorption is clinical relevant.


in the published literature. a. Drug absorption effects

**3. The effects of therapeutic hypothermia on drug pharmacokinetics** 

In general, hypothermia can affect drug disposition in various ways. We have previously discussed the physiological changes induced by hypothermia. These effects generally include decreases in active transport processes of drug absorption and excretion, no alteration in passive processes of drug disposition, and a general reduction in the overall rate of drug metabolism. Although these are general alterations, it is important to note that each of these alterations have been shown to be drug specific and requires particular evaluations of drug disposition in the cooled patient. In addition, hypothermia is also known to alter the different phases of drug pharmacokinetics. These phases can be broken up into absorption, distribution, metabolism and transport, and excretion. This section will highlight the effect of therapeutic hypothermia on each of these four phases, and the current research in the area. A summary of the current clinical studies on drug disposition is given in Table 2. In addition, Figure 1 summarizes the known physiologic and drug disposition effects of hypothermia and provides a statement of the level of evidence that currently exists

Most drugs in the ICU are administered intravenously. However, some drugs are given non-intravenously, typically via oral administration. Drugs that are administered orally are subject to many factors that influence the rate and amount of drug that can be absorbed before it reaches the bloodstream. Some of these factors, such as disintegration and dissolution, are drug dependent and will vary among drugs based on their dosage form (tablet, capsule, etc) as well as the components that make up the drug (active ingredient, excipients, etc). Physiochemical properties of the drug, such as the pKa, lipophilicity, and

As previously addressed in the physiology section, gastrointestinal motility is known to decrease with mild hypothermia. Furthermore, a decrease in temperature can decrease blood flow at the site of absorption, and increase or decrease the gastric and duodenal pH,

Pre-clinical studies investigated the effects of moderate hypothermia on these physiological factors. Hypothermia is associated with a decrease in passive transport via ABCB1. Results demonstrated a 30-44% decrease in the absorption rate constant, ka, of pentobarbital, levodopa and uracil. However, these pre-clinical studies induced moderate or severe hypothermia. Therefore, the decrease in drug absorption may be more pronounced than

Overall, the effect of hypothermia on drug absorption may lead to a decreased rate and prolonged time to reach maximal concentration for some drugs. Furthermore, the time of onset may be delayed and the magnitude of the pharmacological response, due to these reduced concentrations, may be diminished. However, current studies do not accurately reflect the range of temperature cooling *in vivo* and further clinical studies need to be done

to determine if the magnitude of alterations in drug absorption is clinical relevant.

solubility, will also influence the total amount of drug absorbed.

all factors that will ultimately affect a drug's absorption.

what would be observed clinically under mild hypothermia.

Abbreviations: CL: Systemic clearance; AUC: Area under curve; Ke: Elimination rate; Vd: Volume of distribution; T1/2: Half- life.

**Table 2.** Summary of the findings of clinical studies evaluating the effects of therapeutic hypothermia on drug disposition.

Therapeutic Hypothermia: Implications on Drug Therapy 141

between *in vitro* and *in vivo* results could be the difference in cooling temperature. The *in vitro* studies cool to a much lower temperature than is possible *in vivo* (17C and 24C versus 31C) and therefore may demonstrate a greater change in protein binding. To date, studies have not reported altered protein binding over the mild therapeutic hypothermia

Another factor that is influenced by hypothermia is the pH of the blood. As temperature decreases, the partial pressure of carbon dioxide decreases and the pH increases. For every 10 degree change in temperature, the blood pH increases from 7.40 to 7.55. Depending on the pKa of the drug, more or less of the drug will be ionized after the shift in pH. Consequently, more or less of the drug will be able to pass through permeable membranes. Theroretically, drugs like Lidocaine (pKa 7.9) that have a pKa between 7 and 8 may be most susceptible by these slight changes in blood pH [50]. *In vivo* cooling is usually no more than a 6 - 7C change. Thus, blood pH would be expected to change in small increments and the

Finally, hypothermia may alter the lipid solubility and tissue binding of drugs. Preliminary studies demonstrate that hypothermia induced a decrease in transfer processes in water/noctanol systems of atenolol and pindolol. Furthermore, phenytoin was shown to have increased tissue binding in rats at higher temperatures potentially due to temperaturemediated changes in protein conformation, leading to an altered tissue binding capacity

Although hypothermia has been shown to have mixed effects on protein binding, blood pH, and lipophilicity at moderate to severe hypothermia, more studies are needed to determine the clinical magnitude and effects during mild hypothermia in patients. A change in any of these factors during mild hypothermia has the potential to alter the Vd of the drug. The limited number of published studies to date suggest no significant alteration in drug disposition during mild cooling, however, only a small number of drugs have been

Many drugs that are administered to critically ill patients undergo extensive hepatic metabolism. These drugs are predominately metabolized by cytochrome P (CYP) enzymes. Various isoforms of the CYP450 enzyme family are involved in metabolism to varying degrees. These isoforms include CYP3A, CYP2C9 and CYP2C19, CYP2D6, and CYP2E1. Of these isoforms, CYP3A is one of the most important in hepatic drug metabolism in part due to its broad substrate specificity which allows for it to metabolize a wide range of compounds. Drugs commonly used in the ICU that are metabolized by CYP3A include

Midazolam is a well-known CYP3A4 substrate that has been most extensively studied in therapeutic hypothermia. One clinical study looked at the effect of cooling on midazolam pharmacokinetics in patients with TBI. The normothermic group achieved a steady state

clinical effects of these changes remain to be elucidated.

evaluated with respect to changes in distribution.

midazolam, fentanyl, lidocaine, and vecuronium.

c. Hepatic drug metabolism

temperature range.

[50].

**Figure 1.** This figure depicts the known effects of therapeutic hypothermia on drug abosrption, distribution, metabolism, excretion and response. Also depicted is the quality of the current data with respect to each of these processes.

#### b. Drug distribution effects

When a drug is absorbed into the bloodstream, it distributes throughout the body into various tissues and organs. Generally, the space that the drug distributes into the body, or the volume of distribution (Vd), is important for drug dosing since it affects important pharmacokinetic parameters such as the loading dose and the half-life (t1/2) of the drug. The factors that influence drug distribution include protein binding, blood pH and lipophilicity. As previously stated, many of the drugs used in the ICU have relatively large volumes of distribution (Table 1), which implies that the drug compounds preferentially distribute into the tissues over the blood. With drugs that have large volumes of distribution it is common for this distribution to first occur into the easily perfused tissues, followed by a more delayed distribution into more difficult to perfuse tissues.

Much of the effect of hypothermia on plasma protein binding is still largely unknown. Two *in vivo* studies (chlorzoxazone in rats and phenytoin in humans) showed unchanged plasma protein binding during hypothermia, whereas *in vitro* studies of sulfanilamide and lidocaine did show changes in the plasma protein binding. Sulfanilamide showed a 65% increase in plasma protein binding when cooled to 17C while lidocaine showed a 24% decrease in plasma protein binding when cooled to 24C [50]. A possible explanation for the discrepancy between *in vitro* and *in vivo* results could be the difference in cooling temperature. The *in vitro* studies cool to a much lower temperature than is possible *in vivo* (17C and 24C versus 31C) and therefore may demonstrate a greater change in protein binding. To date, studies have not reported altered protein binding over the mild therapeutic hypothermia temperature range.

Another factor that is influenced by hypothermia is the pH of the blood. As temperature decreases, the partial pressure of carbon dioxide decreases and the pH increases. For every 10 degree change in temperature, the blood pH increases from 7.40 to 7.55. Depending on the pKa of the drug, more or less of the drug will be ionized after the shift in pH. Consequently, more or less of the drug will be able to pass through permeable membranes. Theroretically, drugs like Lidocaine (pKa 7.9) that have a pKa between 7 and 8 may be most susceptible by these slight changes in blood pH [50]. *In vivo* cooling is usually no more than a 6 - 7C change. Thus, blood pH would be expected to change in small increments and the clinical effects of these changes remain to be elucidated.

Finally, hypothermia may alter the lipid solubility and tissue binding of drugs. Preliminary studies demonstrate that hypothermia induced a decrease in transfer processes in water/noctanol systems of atenolol and pindolol. Furthermore, phenytoin was shown to have increased tissue binding in rats at higher temperatures potentially due to temperaturemediated changes in protein conformation, leading to an altered tissue binding capacity [50].

Although hypothermia has been shown to have mixed effects on protein binding, blood pH, and lipophilicity at moderate to severe hypothermia, more studies are needed to determine the clinical magnitude and effects during mild hypothermia in patients. A change in any of these factors during mild hypothermia has the potential to alter the Vd of the drug. The limited number of published studies to date suggest no significant alteration in drug disposition during mild cooling, however, only a small number of drugs have been evaluated with respect to changes in distribution.

c. Hepatic drug metabolism

140 Therapeutic Hypothermia in Brain Injury

respect to each of these processes.

b. Drug distribution effects

delayed distribution into more difficult to perfuse tissues.

**Figure 1.** This figure depicts the known effects of therapeutic hypothermia on drug abosrption, distribution, metabolism, excretion and response. Also depicted is the quality of the current data with

When a drug is absorbed into the bloodstream, it distributes throughout the body into various tissues and organs. Generally, the space that the drug distributes into the body, or the volume of distribution (Vd), is important for drug dosing since it affects important pharmacokinetic parameters such as the loading dose and the half-life (t1/2) of the drug. The factors that influence drug distribution include protein binding, blood pH and lipophilicity. As previously stated, many of the drugs used in the ICU have relatively large volumes of distribution (Table 1), which implies that the drug compounds preferentially distribute into the tissues over the blood. With drugs that have large volumes of distribution it is common for this distribution to first occur into the easily perfused tissues, followed by a more

Much of the effect of hypothermia on plasma protein binding is still largely unknown. Two *in vivo* studies (chlorzoxazone in rats and phenytoin in humans) showed unchanged plasma protein binding during hypothermia, whereas *in vitro* studies of sulfanilamide and lidocaine did show changes in the plasma protein binding. Sulfanilamide showed a 65% increase in plasma protein binding when cooled to 17C while lidocaine showed a 24% decrease in plasma protein binding when cooled to 24C [50]. A possible explanation for the discrepancy Many drugs that are administered to critically ill patients undergo extensive hepatic metabolism. These drugs are predominately metabolized by cytochrome P (CYP) enzymes. Various isoforms of the CYP450 enzyme family are involved in metabolism to varying degrees. These isoforms include CYP3A, CYP2C9 and CYP2C19, CYP2D6, and CYP2E1. Of these isoforms, CYP3A is one of the most important in hepatic drug metabolism in part due to its broad substrate specificity which allows for it to metabolize a wide range of compounds. Drugs commonly used in the ICU that are metabolized by CYP3A include midazolam, fentanyl, lidocaine, and vecuronium.

Midazolam is a well-known CYP3A4 substrate that has been most extensively studied in therapeutic hypothermia. One clinical study looked at the effect of cooling on midazolam pharmacokinetics in patients with TBI. The normothermic group achieved a steady state

concentration of midazolam which was maintained during the 216 hours. Conversely, the hypothermic group never reached a steady state concentration and midazolam concentrations were about five-fold higher than the normothermic group. Further studies by Hostler *et al.* also saw a reduction in the clearance of midazolam during hypothermia. In this study normal, healthy volunteers were infused with cold saline and plasma samples were obtained to determine midazolam levels and clearance. A significant difference was observed in the overall metabolism of midazolam under mild hypothermic conditions. Furthermore, this study determined that midazolam clearance is reduced by 11% per degree Celsius change in temperature. Similarly, another preclinical study reported about a 17% decrease in midazolam clearance at steady state in hypothermic rats versus normothermic rats after cardiac arrest.

Therapeutic Hypothermia: Implications on Drug Therapy 143

Digoxin is a calcium channel blocker used to treat arrhythmias in the ICU. A pre-clinical study of ABCB1 transport of digoxin showed that during mild hypothermia the rate of active transport was decreased. No difference in passive diffusion or tight junction activity was seen. The same group also studied the ABCB1-mediated transport of quinidine, another antiarrhythmic drug. In this study, no net effect was seen on quinidine transport during cooling. The authors propose that quinidine is also a substrate for the OATP transporter which may have influenced the results of temperature effects. Although these studies indicate that hypothermia may alter the active transport of drugs by ABCB1, further studies need to be completed to determine the *in vivo* relevance of these changes and explore the

To date, most of the clinical and pre-clinical studies demonstrate a decrease in hepatic metabolism particularly with the CYP enzyme system during therapeutic hypothermia. Although there is a general reduction in drug metabolism, the magnitude of these alterations appears to be pathway specific and therefore, not all hepatically eliminated drugs will have reduced metabolism. In addition, many of these current clinical studies are small and underpowered. Additional studies still need to be performed to determine the extent of hepatic metabolism on drug concentrations and how clinicians can best dose patients

Renal drug elimination is a common route of elimination for hydrophilic drugs. Renal elimination can be divided into filtration, tubular secretion and reabsorption. Filtration is a passive process, whereas tubular secretion is an active process of renal elimination. To date, few clinical studies exist that investigate the effect of hypothermia on renal drug elimination. A small number of preclinical studies have explored how cooling affects renal

Gentamicin is a commonly administered drug in the ICU to treat infections, and predominately eliminated via passive filtration with little to no tubular secretion. Liu *et al.* showed that gentamicin concentrations remained unchanged in hypothermic neonates with HIE compared to normothermic neonates. This demonstrated that the clearance of gentamicin was not changed during mild hypothermia. Another study investigated the pharmacokinetics of gentamicin in piglets during mild hypothermia. They observed no change in gentamicin pharmacokinetics in hypoxic piglets versus normothermic piglets. These combined gentamicin studies coupled with the aforementioned evidence indicating no alterations in creatinine clearance suggest that mild hypothermia does not affect the

In conclusion, these studies suggest that the passive processes of renal filtration are unaffected by mild hypothermia, whereas the active processes of renal tubular secretion may be decreased. However, these conclusions are based off of a single preclinical study in rats that investigated the active process of tubular secretion (previously discussed in renal physiology section). To accurately assess the effect of hypothermia on renal excretion,

effects on other drug transporters.

receiving therapeutic hypothermia.

d. Renal drug excretion

filtration and secretion.

passive process of renal filtration.

further studies in humans are needed.

Vecuronium, which is given as a muscle relaxant in the ICU, is another CYP3A4 substrate. The effect of hypothermia on vecuronium was studied in healthy human volunteers. Similarly to midazolam, the clearance of vecuronium was also decreased during cooling. Similarly, these studies demonstrated that an 11% reduction in vecuronium clearance is observed per degree Celsius change in body temperature. Furthermore, a preclinical study by Zhou *et al* demonstrated that hypothermia alters CYP3A activity, however the significant changes in CYP450 activity were isoform specific with significant alterations in CYP3A and CYP2E1 with no significant alteration in CYP2D or CYP2C probe metabolism. Collectively, these studies indicate that drugs which rely on CYP3A metabolism have decreased clearance during mild hypothermia, however, the reduced P450 activity appears to be isoform and potentially drug specific.

In addition to CYP450 enzymes, Phase II enzymes also play an important role in the metabolism of many drugs used in critical care. Phase II enzymes include UDPglucuronosyltransferases (UGT), glutathione S-transferases, methyltransferases, sulfotransferases, and N-acetyltransferases. Of these enzymes, UGT is one of the only studied phase II enzymes and metabolizes a large number of drugs given in the ICU, such as morphine, propofol, phenobarbital, propranolol, aspirin, and acetaminophen. Of these, the effects of hypothermia on morphine have been most extensively studied.

Morphine, a commonly administered analgesic in the ICU, is predominately metabolized by UGT2B7 with almost no metabolism by Phase I enzymes. One study measured morphine concentrations in neonates with hypoxic-ischemic encephalopathy (HIE). This randomized study compared peak serum morphine concentrations in neonates with HIE who were randomly assigned to either a hypothermic or normothermic group. After 72 hours, six of the seven neonates in the hypothermic group had morphine concentrations greater than 300 ng/mL compared to one of six neonates in the normothermic group. Further, the clearance of morphine in the hypothermic group was significantly decreased. As previously mentioned, neonates undergo a longer, 72 hour duration of cooling. A pre-clinical animal study also showed a significant decrease in morphine clearance in the hypothermic model as compared to the normothermic model. These studies demonstrate a reduced clearance of midazolam during cooling. One possible explanation could be a decrease in UGT activity. Additional studies are needed on other UGT substrates to validate these results.

Digoxin is a calcium channel blocker used to treat arrhythmias in the ICU. A pre-clinical study of ABCB1 transport of digoxin showed that during mild hypothermia the rate of active transport was decreased. No difference in passive diffusion or tight junction activity was seen. The same group also studied the ABCB1-mediated transport of quinidine, another antiarrhythmic drug. In this study, no net effect was seen on quinidine transport during cooling. The authors propose that quinidine is also a substrate for the OATP transporter which may have influenced the results of temperature effects. Although these studies indicate that hypothermia may alter the active transport of drugs by ABCB1, further studies need to be completed to determine the *in vivo* relevance of these changes and explore the effects on other drug transporters.

To date, most of the clinical and pre-clinical studies demonstrate a decrease in hepatic metabolism particularly with the CYP enzyme system during therapeutic hypothermia. Although there is a general reduction in drug metabolism, the magnitude of these alterations appears to be pathway specific and therefore, not all hepatically eliminated drugs will have reduced metabolism. In addition, many of these current clinical studies are small and underpowered. Additional studies still need to be performed to determine the extent of hepatic metabolism on drug concentrations and how clinicians can best dose patients receiving therapeutic hypothermia.

#### d. Renal drug excretion

142 Therapeutic Hypothermia in Brain Injury

isoform and potentially drug specific.

concentration of midazolam which was maintained during the 216 hours. Conversely, the hypothermic group never reached a steady state concentration and midazolam concentrations were about five-fold higher than the normothermic group. Further studies by Hostler *et al.* also saw a reduction in the clearance of midazolam during hypothermia. In this study normal, healthy volunteers were infused with cold saline and plasma samples were obtained to determine midazolam levels and clearance. A significant difference was observed in the overall metabolism of midazolam under mild hypothermic conditions. Furthermore, this study determined that midazolam clearance is reduced by 11% per degree Celsius change in temperature. Similarly, another preclinical study reported about a 17% decrease in midazolam clearance at steady state in hypothermic rats versus normothermic rats after cardiac arrest.

Vecuronium, which is given as a muscle relaxant in the ICU, is another CYP3A4 substrate. The effect of hypothermia on vecuronium was studied in healthy human volunteers. Similarly to midazolam, the clearance of vecuronium was also decreased during cooling. Similarly, these studies demonstrated that an 11% reduction in vecuronium clearance is observed per degree Celsius change in body temperature. Furthermore, a preclinical study by Zhou *et al* demonstrated that hypothermia alters CYP3A activity, however the significant changes in CYP450 activity were isoform specific with significant alterations in CYP3A and CYP2E1 with no significant alteration in CYP2D or CYP2C probe metabolism. Collectively, these studies indicate that drugs which rely on CYP3A metabolism have decreased clearance during mild hypothermia, however, the reduced P450 activity appears to be

In addition to CYP450 enzymes, Phase II enzymes also play an important role in the metabolism of many drugs used in critical care. Phase II enzymes include UDPglucuronosyltransferases (UGT), glutathione S-transferases, methyltransferases, sulfotransferases, and N-acetyltransferases. Of these enzymes, UGT is one of the only studied phase II enzymes and metabolizes a large number of drugs given in the ICU, such as morphine, propofol, phenobarbital, propranolol, aspirin, and acetaminophen. Of these, the

Morphine, a commonly administered analgesic in the ICU, is predominately metabolized by UGT2B7 with almost no metabolism by Phase I enzymes. One study measured morphine concentrations in neonates with hypoxic-ischemic encephalopathy (HIE). This randomized study compared peak serum morphine concentrations in neonates with HIE who were randomly assigned to either a hypothermic or normothermic group. After 72 hours, six of the seven neonates in the hypothermic group had morphine concentrations greater than 300 ng/mL compared to one of six neonates in the normothermic group. Further, the clearance of morphine in the hypothermic group was significantly decreased. As previously mentioned, neonates undergo a longer, 72 hour duration of cooling. A pre-clinical animal study also showed a significant decrease in morphine clearance in the hypothermic model as compared to the normothermic model. These studies demonstrate a reduced clearance of midazolam during cooling. One possible explanation could be a decrease in UGT activity. Additional

effects of hypothermia on morphine have been most extensively studied.

studies are needed on other UGT substrates to validate these results.

Renal drug elimination is a common route of elimination for hydrophilic drugs. Renal elimination can be divided into filtration, tubular secretion and reabsorption. Filtration is a passive process, whereas tubular secretion is an active process of renal elimination. To date, few clinical studies exist that investigate the effect of hypothermia on renal drug elimination. A small number of preclinical studies have explored how cooling affects renal filtration and secretion.

Gentamicin is a commonly administered drug in the ICU to treat infections, and predominately eliminated via passive filtration with little to no tubular secretion. Liu *et al.* showed that gentamicin concentrations remained unchanged in hypothermic neonates with HIE compared to normothermic neonates. This demonstrated that the clearance of gentamicin was not changed during mild hypothermia. Another study investigated the pharmacokinetics of gentamicin in piglets during mild hypothermia. They observed no change in gentamicin pharmacokinetics in hypoxic piglets versus normothermic piglets. These combined gentamicin studies coupled with the aforementioned evidence indicating no alterations in creatinine clearance suggest that mild hypothermia does not affect the passive process of renal filtration.

In conclusion, these studies suggest that the passive processes of renal filtration are unaffected by mild hypothermia, whereas the active processes of renal tubular secretion may be decreased. However, these conclusions are based off of a single preclinical study in rats that investigated the active process of tubular secretion (previously discussed in renal physiology section). To accurately assess the effect of hypothermia on renal excretion, further studies in humans are needed.
