The effect of pentobarbitone anaesthesia and hypothermia on the hepatic clearance of indocyanine green and S(-)-acenocoumarol in the rat

Application of tail vein serial microsampling for plasma or dried plasma spots in toxicokinetic assessment in rats using acetaminophen as the model compound

In the current study, two groups of rats (five per group) were administered a single oral dose of 500 mg/kg acetaminophen. For toxicokinetic assessment, the Group 1 animals were bled via conventional sparse (two animals/time point) sublingual vein bleeding (~0.5 ml) with anesthesia, while the Group 2 animals were bled via serial tail vein microsampling (~0.075 ml) without anesthesia. All collected blood was processed for plasma. Each Group 2 plasma sample (~30 μl) was divided into 'wet' and 'dried' (dried plasma spots). All plasma samples were analyzed by LC-MS/MS for acetaminophen and its major metabolites acetaminophen glucuronide and acetaminophen sulfate. In addition, plasma and urine samples were collected for analysis of corticosterone and creatinine to assess stress levels. Comparable plasma exposure to acetaminophen and its two metabolites was observed in the plasma obtained via conventional sparse sublingual vein bleeding and serial tail vein microsampling and between the 'wet' and 'dried' plasma obtained by the latter. Furthermore, comparable corticosterone levels or corticosterone/creatinine ratios between the two groups suggested that serial microsampling without anesthesia did not increase the levels of stress as compared with conventional sampling with anesthesia, confirming the utility of microsampling for plasma or dried plasma spots in rodent toxicokinetic assessment.

Anticonvulsant treatment of asphyxiated newborns under hypothermia with lidocaine: efficacy, safety and dosing

BACKGROUND

Lidocaine is an antiarrythmicum used as an anticonvulsant for neonatal seizures, also during therapeutic hypothermia following (perinatal) asphyxia. Hypothermia may affect the efficacy, safety and dosing of lidocaine in these patients.

OBJECTIVE

To study the efficacy and safety of lidocaine in newborns with perinatal asphyxia during moderate hypothermia, and to develop an effective and safe dosing regimen.

METHODS

Hypothermic newborns with perinatal asphyxia and lidocaine for seizure control were included. Efficacy was studied using continuous amplitude-integrated electroencephalography. Safety was assessed using continuous cardiac monitoring. An optimal dosing regimen was developed with simulations using data from a pharmacokinetic model. Plasma samples were collected during hypothermia on consecutive mornings.

RESULTS

A total of 22 hypothermic and 26 historical normothermic asphyxiated newborns with lidocaine were included. A response of 91% on epileptiform activity on the amplitude-integrated EEG was observed for lidocaine add-on therapy. No relationship between lidocaine or MEGX plasma concentrations and heart frequency could be identified. None of the newborns experienced cardiac arrythmias. Hypothermia reduced lidocaine clearance by 24% compared with normothermia. A novel dosing regimen was developed an initial bolus loading dose of 2 mg/kg, for patients with body weight 2.0-2.5 kg followed by consecutive continuous infusions of 6 mg/kg/h (for 3.5 h), 3 mg/kg/h (for 12 h), 1.5 mg/kg/h (for 12 h), or for patients with body weights 2.5-4.5 kg 7 mg/kg/h (for 3.5 h), 3.5 mg/kg/h (for 12 h), 1.75 mg/kg/h (for 12 h), before stopping.

CONCLUSIONS

Lidocaine can be assumed to be an effective antiepileptic drug during hypothermia in asphyxiated neonates.

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.

Pharmacokinetic comparisons of tail-bleeding with cannula- or retro-orbital bleeding techniques in rats using six marketed drugs

INTRODUCTION

The evaluation of drug disposition properties of chemical entities in drug discovery research typically involves the conduct of pharmacokinetic studies in rodents that requires blood sampling over several time points, preferably without disrupting the physiological status of the animals. Several blood withdrawal methods have been employed throughout the industry, yet these methods have not been comprehensively evaluated with regard to their effects on pharmacokinetic profiles of the drug investigated to recommend best practices.

METHODS

In this paper, the pharmacokinetics of six marketed drugs from four distinct therapeutic classes were compared using tail-vein, femoral-artery cannula-, and retro-orbital sinus bleeding techniques. The marketed drugs used in these studies were pentoxifylline, gemfibrozil, glipizide, methotrexate, clonidine, and fluoxetine.

RESULTS

Following oral administration, peak plasma concentration (C(max)), and area under the curve (AUC(0-24)) values for all compounds were not significantly different with the tail-vein method when compared to cannula- or retro-orbital sinus bleeding, except for fluoxetine and gemfibrozil for which minor, but statistically significant differences were observed. The effect of arterial versus venous tail-bleeding on the pharmacokinetics of pentoxifylline indicated no statistical differences in either C(max) or AUC(0-24) values. However, for fluoxetine, higher exposures were observed with tail arterial than venous sampling (2-fold with respect to C(max) and 1.7-fold with respect to AUC(0-24), p<0.05).

DISCUSSION

The observed differences with fluoxetine may be due to its pharmacological effects on thermoregulatory responses that influence tail blood flow, a hypothesis that remains to be tested. Based on these observations, we recommend the tail-bleeding technique for pharmacology or toxicology exposure and F% studies, particularly in early discovery work. Retro-orbital bleeding is controversial and is no longer considered a humane method. Cannula-bleeding, especially coupled with automated blood-collection techniques, has become the most efficient way for pharmaceutical industry to perform rat bioavailability studies.

The survival and function of baboon red blood cells, platelets, and plasma proteins: a review of the experience from 1972 to 2002 at the Naval Blood Research Laboratory, Boston, Massachusetts

The studies reported in this monograph were performed between 1972 and 2002 when it was possible to study healthy male and female baboons. A colony of baboons was maintained for 30 years without any adverse events observed in these baboons in the numerous studies that were performed. These protocols were reviewed and approved by the institutional animal care and use committees (IACUC) at the sites where the studies were performed and by the veterinarian services of the U.S. Navy's Bureau of Medicine and Surgery, the Office of Naval Research, and the Department of Defense. The physiology of red blood cells (RBCs), platelets (PLTs), and plasma proteins in the baboon was investigated together with the viability and function of preserved RBCs, PLTs, and plasma proteins. These studies in the baboon could not have been performed in normal volunteers and patients. The data obtained have provided critical information to explain the clinical observations reported in normal volunteers and patients after transfusion of fresh and preserved blood products. These studies were supported by the U.S. Navy's Bureau of Medicine and Surgery and the Office of Naval Research. In addition, the support of the late Congressman J. Joseph Moakley from Massachusetts is acknowledged because without his support many of these studies could not have been performed. The authors acknowledge the contributions of the numerous research collaborators identified in the 52 peer-reviewed publications that cite other funding agencies that supported the research that is reported, the editorial assistance of Ms Cynthia Ann Valeri, and the assistance of Ms Deborah Tattersall who prepared the figures and tables reported in this publication.

Quantitative structure-pharmacokinetics relationships: I. Development of a whole-body physiologically based model to characterize changes in pharmacokinetics across a homologous series of barbiturates in the rat

As part of an overall program to develop a framework for evaluating the contribution of structural and physicochemical properties to pharmacokinetics, the distribution kinetics of nine 5-n-alkyl-5-ethyl barbituric acids in arterial blood and 14 tissues (lung, liver, kidney, stomach, pancreas, spleen, gut, muscle, adipose, skin, bone, heart, brain, testes) was examined after i.v. bolus administration in rats. The barbituric acids studied form a true homologous series; therefore any differences in pharmacokinetics, noted between congeners, can be directly linked to the increase in lipophilicity, resulting from the addition of a methylene group. A whole-body physiologically based pharmacokinetic model has been developed, assuming most of the tissues to be well-stirred compartments. Brain and testes, in which distribution for the lower homologues was permeability rate-limited, were represented by two compartments. For each homologue, the model parameters have been optimized, using the tissue concentration-time data. The initial distribution processes in the system were very rapid, making it quite stiff, and essentially over before the first samples were taken. A progressively increasing redistribution from lean tissues into adipose on ascending the homologous series was observed, characterized by a tendency for a progressive decrease in the magnitude of the concentration-time profiles for some of the lean and well-perfused tissues, an increase in the adipose concentration-time profile, and an increase in the time to reach the maximum adipose concentration. A shift from permeability rate limitation to perfusion rate limitation of the distribution processes for brain and testes, as well as an increase in the intrinsic hepatic clearance and decrease in the renal clearance with the increase of lipophilicity of the homologues, were quantified. An increase in the total unbound volume of distribution on ascending the homologous series was also observed. Muscle was found to be the major drug depot at steady state, accounting for approximately 50% of the total unbound volume of distribution, regardless of the lipophilicity of the homologue; the unbound volume of distribution of adipose increases more than 10-fold with the increase of lipophilicity.

Influence of pentobarbitone on in-vivo local disposition of diclofenac in rat liver

Because the liver is the main organ eliminating many drugs from the body and because pentobarbitone and other analogues can inhibit biliary secretion, the influence of pentobarbitone on hepatic local disposition of diclofenac has been investigated. Diclofenac was infused into the portal and femoral veins of non-anaesthetized rats (group A) and rats anaesthetized with pentobarbitone (group B) and the plasma concentration of diclofenac and the total amount of diclofenac excreted in the bile (in both cases intact diclofenac plus its glucuronide) were simultaneously monitored by HPLC at appropriate time intervals. The time-courses of plasma concentration and amount excreted in the bile were evaluated by moment analysis with trapezoidal integration. The hepatic recovery ratio (FH) was calculated by comparing the area under the curve (AUC) of plasma concentration after intravenous infusion with that after intraportal infusion. The mean biliary transit time (tb) was estimated by subtracting the mean residence time (MRT) of the plasma data from the mean biliary residence time (MRTb) of the biliary excretion data. The FH values of diclofenac were 0.664 in group A and 0.643 in group B. The biliary excretion ratio (Fb) of total diclofenac after intravenous administration was 27.0% in group A and 14.1% in group B. The tb values for total diclofenac were estimated to be 0.192 h (intravenous) and 0.159 h (intraportal) in group A, and 0.174 h and 0.238 in group B. Analysis of variance showed that differences among these four tb values were insignificant at the 5% level. The differences in the mean residence time (MRT), total clearance (CL) and distribution volume at steady state (Vss) were insignificant between groups A and B. Whereas total and the hepatic clearance of diclofenac were not affected by pentobarbitone, biliary clearance was extensively reduced. It took a relatively long time for diclofenac to move from the sinusoid into the bile and the time was not affected by pentobarbitone.

Inhibitory effect of pentobarbital on biliary excretion of diclofenac in a rat liver perfusion system

The effect of pentobarbital on the biliary excretion of diclofenac was investigated in a rat liver perfusion system following a pulse input of the drug. Without albumin in the perfusate, a trace amount of diclofenac was detected in the outflow from the liver (< 0.1%). The total biliary excretion of diclofenac (intact diclofenac plus its glucuronide) decreased from 23.8% (diclofenac 6.01, glucuronide 17.8%) to 16.3% (diclofenac 5.09, glucuronide 11.2%) with an increase in the perfusate concentration of pentobarbital from 0 to 2.5 micrograms mL-1. At pentobarbital concentrations exceeding 2.5 micrograms mL-1, the biliary excretion of diclofenac and its glucuronide (14% total diclofenac) was not reduced further. The mean local excretion times of both diclofenac and its glucuronide were approximately 17 min and were unchanged at all pentobarbital concentrations tested. The ratios of biliary excreted diclofenac and its glucuronide to total diclofenac were 22 and 78%, respectively, and these values were virtually constant at all concentrations of pentobarbital in the perfusate. These results suggest that the glucuronidation of diclofenac and the biliary excretion of its glucuronide are rapid processes and that pentobarbital blocks a step before glucuronidation.

Effects of hypothermia on the elimination of ethanol, diazepam and oxazepam in rat liver slice incubations

The elimination of ethanol, diazepam and oxazepam which are metabolised by different enzymes, has been studied for 30, 60, 90 and 120 min at 37, 27, 17 and 7 degrees C in rat liver slice incubations. Ethanol elimination followed zero-order kinetics at all temperatures, while the benzodiazepines consistently displayed first-order kinetics. No sign of phase transition was observed in the respective Arrhenius-plots. Ethanol elimination was more temperature dependent than the elimination of diazepam, while the elimination of oxazepam was little influenced by temperature. This is shown by the temperature ratios (Q10) and energies of activation (Ea) of 1.76, 1.56, 1.24 and 40.5, 31.9, 15.2 for ethanol, diazepam and oxazepam, respectively. This means that ethanol, diazepam and oxazepam elimination was reduced by 25, 22 and 14%, respectively, for each 10 degrees C of temperature reduction, which is considerably lower than the commonly observed 50% reduction of enzyme activity. We conclude that observations made for one drug on temperature dependent elimination may not apply to other drugs.

Hepatic Function during General Hypothermia in the Pig: Assessment by Aminopyrine Breath Test

We investigated the influence of pre-harvesting general hypothermia on liver metabolic activity by means of Aminopyrine Breath Test (ABT). This study was conducted in pigs which were anesthetized, curarised and cooled on an ice bed. Each animal received labelled aminopyrine and 14CO2 in expired air was measured between 37.5 and 25.5° C. The liver metabolic activity at 31.5° C represents 53.3% of the normothermic value. At 25.5° C, this activity is reduced by 75.1%. The mean decreasing rate is 6.2%/° C for a fall in temperature of 12° C. A change of slope occurred at 31.5° C. The first decreasing rate is 7.47 ± 1.62%/° C and the second one is 4.48 ± 2.27%/° C. Thus, use of general hypothermia during liver harvesting should improve the quality of organ preservation: the important reduction of metabolism should decrease the oxygen debt resulting from anaerobic cold perfusion.

Comparison of acid/base status in conscious and anaesthetized rats during acute hypothermia

Acute hypothermia was surface-induced in unrestrained conscious rats at two different levels, moderate (30 degrees C TB) and severe (20 degrees C TB). Data reflecting the acid/base status were determined. The values obtained for moderate hypothermia were compared with the acid/base pattern observed during hypothermia induced by two different anaesthetics, sodium pentobarbital and urethane, at room temperature. Conscious, hypothermic animals developed an apparent respiratory alkalosis, with an increase in pHa (from 7.476 to 7.546 in moderate hypothermia and from 7.484 to 7.563 in severe hypothermia) correlated with a decrease in arterial bicarbonate levels (from 22.9 to 16.8 mmol l-1 and from 20.7 to 14.9 mmol l-1 respectively). Lactate increased slightly in conscious, severely hypothermic rats (1.02 mmol l-1). This acid/base pattern was clearly different from that seen in sodium pentobarbital (mild respiratory acidosis) and urethane-induced hypothermia (metabolic acidosis). These results suggest that conscious rats follow a pattern closer to that underlying the relative alkalinity shown by many poikilotherms than to that underlying the constant pH shown in hibernating mammals. This latter pattern, nevertheless, approaches that observed during moderate pentobarbital hypothermia and the acid/base pattern during shallow hypothermia in birds. Anaesthesia may interfere with the development of the processes that lead to the acid/base pattern observed in conscious animals.

Recovery of hepatic clearance and extraction following a release of common bile duct obstruction in the rat

The rate of recovery for hepatic clearance and extraction following release of common-duct obstruction was investigated in the rat. Male Wistar rats underwent ligation of a cannulated common bile duct. Two weeks later, the cannula was opened and implanted into the duodenum, thus re-establishing enterohepatic circulation. Hepatic extraction and indocyanine green clearance were determined in three groups of six rats each, which differed by the time elapsed from the re-establishment of communication between the common bile duct and duodenum, i.e., 1, 48 and 168 h, respectively. A fourth group, in which a sham operation was performed, served as a control. Clearance was reduced from 16.9 +/- 2.5 ml/min per kg in the control group to 2.9 +/- 0.8, 5.4 +/- 2.4, and 8.5 +/- 3.3 ml/min per kg 1, 48, and 168 h, respectively, after release of common-bile-duct obstruction. Extraction rate was reduced from 37.3 +/- 5.9% to 17.5 +/- 2.7% in the 1st hour and recovered completely at 1 week. Thus, in the rat, release of a 2-week common-bile-duct obstruction is associated with complete recovery of the extraction capacity of the liver within a week, but only incomplete recovery of clearance. This decrease in clearance seems to be due to a decrease in effective hepatic blood flow, mostly probably due to the development of porto-systemic shunts.

Differential effects of hypothermia upon blood acid-base state and blood gases in sodium pentobarbital and urethane anaesthetised rats

1. The effects of two anaesthetics, sodium pentobarbital and urethane, and the effects of anaesthesia-associated hypothermia on acid-base status and blood gases were studied in rats without assisted ventilation. 2. Manipulation of conscious rats produces a progressive increase in arterial lactate associated with slight hyperventilation. 3. Sodium pentobarbital anaesthesia produces mild respiratory acidosis accompanied by increase in lactate arterial values. Urethane anaesthesia leads to partially compensated metabolic acidosis. 4. Hypothermia reduces metabolic acidosis and hypercapnia induced by sodium pentobarbital anaesthesia. No difference between hypothermic and normothermic values was observed in urethane anaesthesia.

The effect of diethyl ether, pentobarbitone and urethane anaesthesia on diflunisal conjugation and disposition in rats

1. The disposition of diflunisal (DF) at 10 mg/kg i.v. was investigated over 4 h in bile-exteriorized male rats continuously anaesthetized with (a) diethyl ether inhalation (as required), (b) pentobarbitone sodium i.p. (55 mg/kg initially), (c) urethane i.p. (1500 mg/kg initially) or (d) urethane i.v. (750 mg/kg initially), and compared to that obtained in conscious rats. 2. Diethyl ether decreased the plasma clearance of DF to about 30% of control values, by inhibition of both glucuronidation and sulphation of DF. 3. Pentobarbitone anaesthesia caused only modest inhibition of DF elimination, with plasma clearance decreased to about 80% of control values. 4. Plasma profiles and biliary recovery of DF and its conjugates were little altered by urethane i.p. anaesthesia, but urinary recovery was low and variable because of the nearanuria produced by urethane via this administration route. 5. Urinary recovery of DF and its conjugates was satisfactory in rats given urethane i.v., but tissue distribution of DF was substantially decreased. 6. Pentobarbitone was considered to interfere least with DF disposition at the 10 mg/kg dose, and was selected as the most suitable anaesthetic agent for ongoing studies of disposition of DF and its conjugates in anaesthetized rats.

Liver blood flow measurement in the rat. The electromagnetic versus the microsphere and the clearance methods

This study describes the simultaneous measurement of hepatic arterial and portal venous blood flow in the pentobarbital anesthetized rat by means of electromagnetic flowmeters. Hepatic arterial flow was 0.21 +/- 0.02 mL/min/g liver, and portal venous flow was 1.53 +/- 0.19 mL/min/g liver (n = 20). Flows remained stable for more than 3 hr. A clear advantage of the electromagnetic technique is that it allows the continuous simultaneous separate measurement of hepatic arterial and portal venous blood flow. Simultaneous measurement of hepatic blood flow by the electromagnetic method and the microsphere method yielded almost identical results. Adenosine infusion (100 micrograms/min) did not affect hepatic blood flow measured electromagnetically nor via the microsphere technique. The suitability of indocyanine green (ICG) as an indicator of hepatic blood flow was evaluated by comparing the ICG clearance at steady-state conditions to the values of liver flow obtained by direct electromagnetic measurement. ICG clearance was only 30% of the electromagnetically measured blood flow. These data strongly suggest that ICG clearance does not reflect liver flow in the rat. Intravenous infusion of the vasoactive agents phenylephrine (2, 4, 10 micrograms/min during 5 min) and adenosine (20, 40, 200 micrograms/min) did not affect portal venous nor hepatic arterial flow, measured by the electromagnetic method, although brisk effects on mean arterial blood pressure were observed. This suggests autoregulatory responses of the hepatic vascular bed. The data suggest that electromagnetic flowmeters may be used to measure portal venous and hepatic arterial flow simultaneously in anesthetized rats. The continuous measurement of both flows simultaneously offers a clear advantage over other methods of hepatic flow measurement.

The effect of PGE2 on hepatic blood flow and bile indocyanin green in the rat

The effect of Prostaglandin E2 (PGE2) on hepatic blood flow and bile excretion has been only partially investigated. We studied in the rat the effect of PGE2 on indocyanine green (ICG) clearance, bile flow and ICG recovery in the bile. PGE2 administered to rats at a rate of 2 micrograms/kg/min had no effect on ICG clearance as compared to vehicle-treated rats (14.6 +/- 2.5 Vs, 16.9 +/- 2.5 ml/min/kg) and on bile flow (1.23 +/- 0.15 Vs. 1.24 +/- 0.11 microliters/min/gm liver). However, ICG excretion in bile was significantly increased as compared to a vehicle-treated group, 0.48 +/- 0.09 and 0.25 +/- 0.02 micrograms/microliters bile, respectively (P less than 0.005). This effect of PGE2 on ICG recovery in bile was found to be dose-dependent, and when PGE2 was administered at a rate of 1 microgram/kg/min, bile ICG decreased to 0.33 +/- 0.10 microgram/microliter bile. The effect of PGE2 on bile ICG is not due to an increased extraction of ICG by the liver since using the isolated perfused rat liver, the mean ICG extraction rate over a 20 minute period, with and without PGE2 infusion, was very similar, 33 +/- 3.5 and 37 +/- 6.2 percent, respectively. The lack of effect of PGE2 on ICG clearance and extraction and on bile flow, with an increase in biliary excretion of ICG, suggests a decreased hepatic storage of ICG induced by PGE2 possibly due to an accelerated rate of ICG excretion from the hepatocytes to the bile canaliculi.

Pharmacokinetic evaluation of local drug delivery: the intratesticular and intrarenal administration of acenocoumarol in the rat

According to theory, the regional increase in drug concentration during target organ directed drug delivery as compared with systemic drug delivery is related to the quotient of the clearance of the drug and the blood flow of the target organ. We investigated the steady-state pharmacokinetic disposition of S-acenocoumarol in plasma, liver, testis, and kidney following its administration (constant rate infusion by an osmotic minipump) directly into the testis or the kidney of rats. The effects of clearance induction (phenobarbitone treatment) on the disposition of the drug were also investigated. The results confirm the theory of target-directed drug delivery.

Influence of salicylate and anaesthesia on the rectal absorption of theophylline in rats

The effect of sodium salicylate and anaesthesia on the rectal absorption of theophylline was evaluated in rats. Theophylline proved to be slowly but completely absorbed on rectal infusion in conscious rats, compared with intravenous infusion. Pentobarbital anaesthesia did not influence absorption. In contrast to literature data, the results with salicylate showed that rectal absorption of theophylline was not enhanced.