Effects of diethyl ether anaesthesia on the pharmacokinetics of antipyrine and paracetamol in the rat

Pharmacokinetics of Azalomycin F, a Natural Macrolide Produced by Streptomycete Strains, in Rats

As antimicrobial resistance has been increasing, new antimicrobial agents are desperately needed. Azalomycin F, a natural polyhydroxy macrolide, presents remarkable antimicrobial activities. To investigate its pharmacokinetic characteristics in rats, the concentrations of azalomycin F contained in biological samples, in vitro, were determined using a validated high-performance liquid chromatography–ultraviolet (HPLC-UV) method, and, in vivo, samples were assayed by an ultra-high performance liquid chromatography–tandem mass spectrometric (UPLC–MS/MS) method. Based on these methods, the pharmacokinetics of azalomycin F were first investigated. Its plasma concentration-time courses and pharmacokinetic parameters in rats were obtained by a non-compartment model for oral (26.4 mg/kg) and intravenous (2.2 mg/kg) administrations. The results indicate that the oral absolute bioavailability of azalomycin F is very low (2.39 ± 1.28%). From combinational analyses of these pharmacokinetic parameters, and of the results of the in-vitro absorption and metabolism experiments, we conclude that azalomycin F is absorbed relatively slowly and with difficulty by the intestinal tract, and subsequently can be rapidly distributed into the tissues and/or intracellular f of rats. Azalomycin F is stable in plasma, whole blood, and the liver, and presents plasma protein binding ratios of more than 90%. Moreover, one of the major elimination routes of azalomycin F is its excretion through bile and feces. Together, the above indicate that azalomycin F is suitable for administration by intravenous injection when used for systemic diseases, while, by oral administration, it can be used in the treatment of diseases of the gastrointestinal tract.

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.

Effects of cold stress and ether stress on aminopyrine demethylation kinetics in vivo

The effect of cold stress and ether stress on mixed function oxidase activity in vivo has been studied using the aminopyrine-−14CO2 exhalation rate (CER) method. Cold stress following both acute and chronic exposure resulted in enhanced rate of demethylation of both aminopyrine (CER α half-life) and its monomethyl metabolite (CER β half-life). However the time course of the response to cold stress differs for the two demethylations. The characteristics of the cold-induced enhancement of aminopyrine-CER would appear to differ in several aspects from that reported for phenobarbitone pretreatment. In contrast, ether exposure did not produce any consistent effect on drug metabolizing status of the rat.

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Differences in the effects of model inducers of cytochrome P450 on the biotransformation of scoparone in rat and hamster liver

The hamster is known to display very high rates of monooxygenase-mediated biotransformation. In comparison with other species little knowledge has been gathered with respect to the nature of its cytochrome P450 enzymes and their respective inducibility. We studied the consequences of induction of P450 enzymes in rats and Syrian golden hamsters using the regioselective oxidative O-demethylation of the coumarin derivative scoparone. This metabolic conversion indicates differential effects of P450 inducers in the rat, in which various types of inducers cause different shifts in the isoscopoletin/scopoletin metabolite ratio (I/S-ratio). Liver microsomes from hamster not treated with P450 inducers oxidized scoparone much more efficiently than liver microsomes of untreated rats. In rat liver microsomes total demethylation rates of scoparone increased upon in vivo treatment with phenobarbital or beta-naphthoflavone. Phenobarbital reduced the I/S-ratio whereas beta-naphthoflavone caused an increase in this ratio. In hamster liver microsomes both phenobarbital and beta-naphthoflavone treatments resulted in a decrease in the I/S ratio. In this species the total scoparone demethylation rate was not much affected by phenobarbital, but beta-naphthoflavone caused a huge increase in over-all scoparone biotransformation. In both species, dexamethasone, isoniazid and clofibrate were much less effective. In contrast to the rat, in the hamster the scoparone biotransformation profile cannot be used to differentiate between phenobarbital- or beta-naphthoflavone-treated animals.

Rats with portacaval shunt as a potential experimental pharmacokinetic model for liver cirrhosis: application to carvedilol stereopharmacokinetics

As an experimental model for reduced liver function rats with surgical portacaval shunts (pcs) may be used. Carvedilol, a nonselective beta-adrenoceptor antagonist with vasodilating activity, is extensively metabolised by phase I as well as phase II pathways. In order to study the stereoselective pharmacokinetics of carvedilol in liver disease, pcs and control rats were given rac-carvedilol intravenously and p.o. The carvedilol enantiomers and their conjugates were assayed in plasma, urine, and bile. Carvedilol was highly bound to plasma proteins; binding was reduced by pcs. In all groups, the plasma concentrations of (R)-carvedilol exceeded those of (S)-carvedilol significantly. In comparison to the control group the plasma concentrations of both enantiomers increased after pcs, while the difference between the stereoisomers decreased. The total clearance decreased proportionally to the decrease in liver weight (30%). Both the apparent oral clearance, as well as its stereoselectivity were reduced, by up to 90 and 43%, respectively. The biliary clearance of the parent drug after i.v. dosage increased in rats with pcs due to the reduced hepatic metabolism.

Inhibition of microsomal N-nitrosodimethylamine demethylase by diethyl ether and other anesthetics

The inhibitory actions of diethyl ether and several other anesthetics on the metabolism of N-nitrosodimethylamine (NDMA) and other substrates were studied with rat liver microsomes. Diethyl ether was an effective inhibitor of the low Km NDMA demethylase, showing characteristics of a competitive inhibition. Inhibition of NDMA metabolism was also observed in the liver post-mitochondrial supernatant fraction prepared from ether-anesthetized rats. Selectivity in the inhibitory action of diethyl ether was demonstrated; the ether was most effective against NDMA demethylase, less potent against p-nitroanisole demethylase and N-nitrosomethylbenzylamine demethylase, and not effective against the metabolism of aminopyrine or benzphetamine. Other anesthetics such as chloroform, isoflurane, enflurane, and halothane also effectively inhibited NDMA demethylase. The work demonstrates that diethyl ether is an efficient inhibitor of NDMA metabolism by the microsomal monooxygenase systems.

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.

Biochemical changes associated with the potentiation of acetaminophen hepatotoxicity by brief anesthesia with diethyl ether

Acetaminophen hepatotoxicity in male CD-1 mice was enhanced markedly by brief anesthesia with diethyl ether (ether), and particularly so if acetaminophen was given several hours after ether. The present study was conducted to examine the possible biochemical mechanisms behind this delayed toxicologic synergism. In vitro biochemical studies indicated that ether anesthesia produced a delayed reduction in the activities of glucuronyl transferase and glutathione (GSH) S-transferase, and in the hepatic content of GSH. The hepatic content but not activity of the cytochromes P-450 was initially reduced by ether but recovered by the time of maximal toxicologic enhancement. In vivo studies showed that ether produced a small decrease in the plasma concentrations of glucuronide and sulfate conjugates of acetaminophen, with a concomitant, minor increase in the half-life of acetaminophen, and a major increase in the bioactivation of acetaminophen, as determined by an early, 2-fold increase in the plasma GSH and cysteine conjugates of acetaminophen, and a 3-fold increase in the covalent binding of acetaminophen to hepatocellular protein. Decreases produced by ether in the in vivo production of acetaminophen glucuronide correlated with increasing plasma concentrations of unmetabolised acetaminophen, decreasing hepatic GSH content and increasing covalent binding of acetaminophen to hepatocellular protein when these measurements were performed in the same animals. The biochemical mechanisms underlying the potentiation of acetaminophen hepatoxicity as measured by plasma glutamic pyruvic transaminase concentrations appeared to be due to delayed, complex effects of ether upon multiple enzymatic pathways of acetaminophen elimination and detoxification.

Effect of renal failure or biliary stasis on the pharmacokinetics of amiodarone in the rat

The single dose intravenous pharmacokinetics of amiodarone (50 mg/kg) were examined in rats with 72 h of biliary stasis secondary to bile duct ligation compared with paired control animals; and in rats with uranyl nitrate induced acute renal failure compared with paired control animals. Plasma and tissue levels (liver, kidney, heart, and lung) of amiodarone (1) and its N-deethyl metabolite 2 were obtained at 4 and 24 h following drug administration. Pharmacokinetic parameters were derived from plasma samples obtained over a 24-h period. Compared with controls, biliary stasis caused a decrease in the total clearance of 1 (1.74 versus 0.35 L/h/kg) and in the volume of distribution at steady state (21.1 versus 5.0 L/kg); renal failure caused a decrease in total clearance (1.67 versus 0.9 L/h/kg) and an increase in apparent elimination half-life (13.7 versus 10.1 h). Both disease processes produced significantly higher plasma levels of 1 when compared with control animals at 4 and 24 h. However, only the cholestatic animals had consistently higher tissue levels of 1 in the face of elevated plasma levels. In normal rats, no 1 or 2 was detected in the urine after a 50 mg/kg intravenous dose of 1, and less than 0.5% of the total dose of amiodarone (1) was excreted into bile by 12 h.

Effect of diethyl ether on the bioactivation, detoxification, and hepatotoxicity of acetaminophen in vitro and in vivo

Our working hypothesis for designing this study involved early inhibition by ether of P-450-dependent bioactivation and glucuronyl transferase-dependent "detoxification", with an earlier recovery of bioactivation. The combined in vivo and in vitro results from the same animals indicate that the increased susceptibility to acetaminophen hepatotoxicity may have been due to a combination of delayed decreases induced by ether in the activities of glucuronyl transferase, sulfotransferase and GSH S-transferase, along with a depletion of hepatic GSH. The small decrease in hepatic content of cytochromes P-450 at 2 hr when toxicologic enhancement was minimal, together with repletion at 8 hr when enhancement was maximal, while the above detoxification pathways were inhibited, is compatible with our hypothesis. However, the lack of an accompanying change in the activity of P-450 suggests either that a different P-450 isoenzyme is involved, or that P-450 activity was not toxicologically limiting. The toxicological imbalance in the bioactivation and detoxification of acetaminophen observed after ether pretreatment was evidenced by significant increases both in the plasma concentrations of GSH and cysteine conjugates, and in the covalent binding of acetaminophen to hepatocellular protein.

Effects of anesthetic agents on bile pigment excretion in the rat

Anesthesia-induced alterations in bilirubin conjugation were studied. Rats were fitted with bile duct and jugular vein catheters while anesthetized with diethyl ether, ketamine or pentobarbital. As anesthesia abated, bile was collected for the next 5 hr and analyzed for flow rate, total bilirubin excretion and bilirubin glucuronide composition. The high-performance liquid chromatography method used allowed direct analysis of bile without derivatization or extraction. Ether anesthesia was associated with a reversible suppression of diglucuronide formation and total bilirubin excretion, with reciprocal monoglucuronide changes. Bile flow and pigment excretion were variable with ketamine. Pentobarbital provided the most uniform excretion data, although the ratio of C-8:C-12 monoglucuronide varied with all drugs. These data are consistent with recently reported drug-induced alterations in hepatic uridine diphosphoglucuronic acid concentration and support the hypothesis that alterations in this substrate concentration are capable of influencing rates of hepatic glucuronide formation.

Acute interaction of halothane and enflurane with the metabolism of ethanol in isolated hepatocytes and liver cytosol preparations from the rat

The effects of halothane and enflurane on ethanol (40 mM) oxidation were studied in isolated rat hepatocytes. Anaesthetic (halothane, enflurane and diethyl ether) effect on the activity of alcohol dehydrogenase (ADH) was studied in incubations of cytosol preparations from rat liver. Mean rates of ethanol metabolism ranged from 0.44 to 0.49 mumol ethanol metabolized/mg cell protein/hour in control hepatocytes from fasted and fed animals. These rates were enhanced by 2- and 3-fold in hepatocytes from fed and fasted animals, respectively, when pyruvate (5 mM) was added. Halothane and enflurane both caused dose dependent inhibition of ethanol metabolism (15-40%) in all hepatocytes without exogenous addition of pyruvate. The inhibitory effect was present also after pyruvate stimulation in hepatocytes from fasted animals, but disappeared in hepatocytes from fed animals when pyruvate was added. The rate of ethanol oxidation by cells from fed rats was enhanced by approximately 40% when the concentration of ethanol was increased from 20 mM to 80 mM. The anaesthetic inhibition of ethanol metabolism was about 20% more pronounced at the higher ethanol concentration compared to the lower concentration when no pyruvate was added. In the presence of pyruvate the effect of anaesthetics was again reversed regardless of ethanol concentration. Halothane (2 mM) and enflurane (2 mM) both caused about 25% inhibition of the ADH-activity in cytosol preparations while ether (30 mM) caused more than 50% inhibition. No inhibition of hepatocyte uptake of ethanol was caused by any of the three anaesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of mode of intravenous administration and blood sample collection on rat pharmacokinetic data

The influence of the mode of intravenous dosing and blood sample collection on the pharmacokinetics of 4-[(3-methoxyphenyl)-methyl]-2,2,6,6-tetramethyl-1-oxa-4-aza-2, 6-disilacyclohexane hydrochloride (I) was studied in the rat. Blood samples obtained from the tail and by exsanguination following injection of the 14C-labeled drug into the caudal vein, the jugular vein, and the heart were analyzed for total radioactivity, and the concentration profiles from the different treatments were compared. Dosing and sampling from the tail vein resulted in significantly different blood levels (and related pharmacokinetic parameters) when compared to other methods, probably attributable to a local depot effect. Intracardiac administration tended to cause higher drug levels in the heart than intravenous doses, although no significant differences were found between the respective blood concentrations. The results showed that caudal vein injection is a simple and adequate method of intravenous administration in rats designated for exsanguinated blood and tissue collection. For serial blood sampling in individual animals, the dose may be given via the jugular vein and the blood collected from the cut tail. These methods require little or no surgical preparations and are particularly suitable for prolonged sampling in studies where a relatively large number of animals are involved.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Acute effects of halothane and enflurane on drug metabolism and protein synthesis in isolated rat hepatocytes

The metabolism of sulphanilamide, antipyrine and paracetamol was studied in the absence and presence of the anaesthetics halothane and enflurane at three different concentrations (0.5, 1.0 and 2.0 mM) in isolated hepatocytes from the rat. Cell viability and protein synthesis were monitored to evaluate toxic effects. A strong concentration related inhibition of antipyrine oxidation (40-70%) and paracetamol conjugation (20-40%) was caused by both halothane and enflurane. Acetylation of sulphanilamide was not inhibited, however, as a slight augmentation was noticed. A significant dose related decrease of cell viability (3-13%) was caused by both anaesthetics. Dose dependent inhibition of the synthesis of stationary cell proteins (15-60%) and the synthesis/secretion of medium proteins (35-85%) was caused by halothane. Similar but slightly less pronounced effects were caused by enflurane. The present findings show that volatile anaesthetics may have general effects as well as different degrees of specific effects on both membrane bound enzyme and soluble enzyme activities.

Antipyrine metabolite kinetics in rats: studies on dose and time dependence

Antipyrine metabolite kinetics have been investigated in the rat with respect to dose and time dependence. The metabolic pathways, 4-hydroxylation, benzylic oxidation, and N-demethylation, are of equal quantitative importance (approximately 20 per cent of dose) and show no dose dependence over the range 25-500 mg kg-1. By using [N-methyl-14C]-antipyrine, the single carbon fragment lost by N-demethylation may be monitored as 14CO2. Serial sampling of 14CO2 exhalation rate provides a half-life estimate which, according to theoretical principles, reflects the antipyrine plasma half-life. When both half-lives were measured in the same animals a statistically significant correlation was evident. At doses of 250 mg kg-1 and 500 mg kg-1 there is an increase in CER half-life (218 and 303 min respectively) when compared to a dose below 100 mg kg-1 (152 min). The metabolite formation rate constants are decreased accordingly at the high doses but are invariant over the dose range 25-100 mg kg-1. Although inter-rat variation in antipyrine metabolite kinetics was substantial, intra-rat variability was small. The noninvasive nature of determining antipyrine metabolite kinetics via breath and urine analysis provides a potentially useful animal model system to investigate the factors influencing hepatic mixed function oxidase activity in vivo.

Antipyrine and acetaminophen kinetics in the rat: comparison of data based on blood samples from the cut tail and a cannulated femoral artery

Antipyrine and acetaminophen kinetics were determined from concentration data obtained by simultaneous blood sampling from the cut end of the tail and a cannulated femoral artery in the rat. Significant differences in concentrations and kinetics for both drugs were found by comparison of the two sampling sites. The hypothesis that the differences were due to a low tail blood flow was tested. The tail blood flow was measured with a microsphere technique, and tail antipyrine concentrations were calculated from the relationship between arterial antipyrine concentration, tail flow, and time for comparison with the observed antipyrine concentrations. Mean blood flow of the rat tail was 0.02 ml/min/ml tail tissue at 22 degrees, which was 8.8 and 0.9% of the liver and kidney flow, respectively. Tail flow increased more than twofold by elevation of the tail temperature to 37 degrees. The calculated tail antipyrine concentration versus time curve showed a very close correspondence to the observed antipyrine tail concentration versus time curves. The results show that tail flow is a major determinant of antipyrine tail concentration in the rat. Kinetic data based on blood samples from the cut end of the tail, therefore, should be interpreted with caution.