Barbiturate Metabolism Research Articles

Barbiturate ingestion in three adult captive tigers (Panthera tigris) and concomitant fatal botulism of one : clinical communication

Zoo animals, including tigers, have been reported to suffer from barbiturate intoxication, with pentabarbitone being most commonly recorded. Clinical signs range from mild ataxia to general anaesthesia with recovery over hours to days with several factors affecting hepatic barbiturate metabolism and tissue partitioning. Botulism is an often fatal intoxication in man, animals, birds and certain fish. The occurrence in carnivores is uncommon to rare, with only 2 reports found of botulism in felids. This report relates to 3 adult captive cohabiting tigers that simultaneously developed signs of abdominal discomfort, progressive ataxia, recumbency and comatose sleep resembling stage 2 anaesthesia, alternating with periods of distracted wakefulness and ataxic movements. These signs occurred 4 days after being fed the carcass of a horse that had ostensibly died of colic and not been euthanased. The male tiger that was the dominant animal in the feeding hierarchy was worst affected and had to be given intravenous fluids. The female that was lowest in hierarchy was unaffected. After 48-72 hours of treatment at the Onderstepoort Veterinary Academic Hospital the females could eat and made an uneventful recovery. The male tiger showed partial recovery but died during the night a few hours after drinking water on his return to the owner. Necropsy revealed severe oesophageal dilation and impaction with decaying grass; some of this material and water were present in the pharynx and trachea, and had been aspirated causing acute widespread bronchopneumonia. Colon content tested negative for common pesticides but, together with liver, tested positive for barbiturate. Serum taken on the day of admission had tested negative for barbiturate and the residual serum from the 3 animals later tested negative for botulinum toxin. Colon and oesophageal content from the male at necropsy were positive for Clostridium botulinum toxin type C by the mouse bioassay neutralisation test, confirming that this male had had concomitant barbiturate toxicity and botulism, and had succumbed to aspiration bronchopneumonia secondary to pharyngeal, laryngeal and oesophageal paralysis and oesophageal

Combined gas-liquid chromatography—mass spectrometry in the study of barbiturate metabolism

Abstract Combined gas-liquid chromatography-mass spectrometry is shown to be a useful technique for the identification of barbiturates and their metabolites, when applied to NN’-dimethyl derivatives.

1-(2-Cyclohex-2-enylpropionyl)-3-methylurea, 2-ethyl-5-methylhexanamide and 2-ethylpentanamide: three products of barbiturate decomposition

The three title compounds were obtained by reactions which mimic, with more extreme conditions, the in vivo metabolism of barbiturates. 1-(2-Cyclohex-2-enylpropionyl)-3-methylurea, C(11)H(18)N(2)O(2), (I), and 2-ethylpentanamide, C(8)H(17)NO, (III), both crystallize with two unique molecules in the asymmetric unit; in the case of (III), one unique molecule exhibits whole-molecule disorder. 2-Ethyl-5-methylhexanamide, C(9)H(19)NO, (II), crystallizes as a fully ordered molecule with Z' = 1. In the crystal structures, three different hydrogen-bonding motifs are observed: in (I) a combination of R(2)(2)(4) and R(2)(2)(8) motifs, and in (II) and (III) a combination of R(4)(2)(8) and R(2)(2)(8) motifs. In all three structures, one-dimensional ribbons are formed by N-H···O hydrogen-bonding interactions.

Inhibition of cytochrome P450-dependent monooxygenases by an alkaloid fraction from Helietta apiculata markedly potentiate the hypnotic action of pentobarbital

Crude alkaloid fraction (CAF) isolated from the leaves of Helietta apiculata showed the presence of furoquinolines. The extract was investigated to determine if it can enhance the sensitivity of the central nervous system (CNS) to the hypnotic action of pentobarbital. Administration of CAF to mice in a dose range of 300–500 mg/kg prior to an injection of pentobarbital (40 mg/kg, i.p.) was associated with a statistically significant decrease of sleep latency and prolongation of pentobarbital-induced sleeping time. Pretreatment of rats with the same alkaloid extract (150 mg/kg, i.p. for 4 days) prior to administration of pentobarbital (40 mg/kg, i.p.) caused not only significant reduction of the levels of microsomal proteins, total cytochrome P450 enzymes and a decrease of aminopyrin- N-demethylation and 3,4-benz(a)pyrene hydroxylation but also changed the pattern of cytochrome P450. It is concluded that the CAF isolated from H. apiculata can potentiate the CNS depressant effect of pentobarbital due to alteration of barbiturate metabolism through inhibition, mainly, of cytochrome P450-dependent enzymes.

N-Glucosidation of amobarbital in the cat.

N-Glucosidation is a novel pathway of barbiturate metabolism, so far known to occur only in man. A search for an animal model, conducted through in vitro screening, revealed that amobarbital-N-glucoside was formed in liver preparations from the cat. The presence of amobarbital-N-glucoside was demonstrated in cat urine, following i.p. administration of amobarbital.

Effect of lipid solubility on hepatic first-pass metabolism of barbiturates in rabbits

The disposition of 7 barbiturates (hexobarbital, pentobarbital, cyclobarbital, amobarbital, allobarbital, phenobarbital and barbital) followed first-order kinetics after intravenous or oral administration to rabbits. The area under the plasma concentration-time curves increased in proportion to the dose after the administration of 20–80 mg/kg of barbiturates by either route. In spite of the complete absorption of barbiturates from the gastrointestinal tract as unchanged drug, the systemic availability values after oral administration were lower than unity and decreased in proportion to the in vitro rates of metabolism of the drugs by rabbit liver microsomes. Therefore, the reduction of systemic availability of these barbiturates is considered to be a result of first-pass metabolism through the liver before reaching the systemic circulation. The hepatic intrinsic clearances of unbound drug for these barbiturates estimated from a simple perfusion model based on the clearance concept varied widely in the range of 0.18–132.94 ml/min per kg of body weight and increased with increase in the lipid solubilities. The rates of metabolism of barbiturates by microsomal enzymes have been reported to depend on the lipid solubility of the drugs. As might be expected from these relations, a good linear relationship was found between the in vivo hepatic intrinsic clearance of unbound drug and the in vitro rate of metabolism by rabbit liver microsomes. Therefore, the increased hepatic first-pass metabolism and the diminished systemic availability of orally administered barbiturates are due to the increased hepatic intrinsic clearance, which may be related to the lipid solubility, of the drugs.

PROCEEDINGS OF THE AUSTRALASIAN SOCIETY OF CLINICAL AND EXPERIMENTAL PHARMACOLOGISTS

PROCEEDINGS OF THE AUSTRALASIAN SOCIETY OF CLINICAL AND EXPERIMENTAL PHARMACOLOGISTS

Isolated rat hepatocytes and 9000 g rat liver supernatant as metabolic systems for the study of the pharmacokinetics of barbiturates.

1. A method for metabolic studies with isolated rat hepatocytes has been developed. The rate of metabolism of a number of barbiturates has been studied both in hepatocytes and in 9000 g rat liver supernatant suspension. 2. The half-lives and clearance values, relative to heptabarbital, determined in hepatocytes were similar to the values obtained with isolated perfused rat liver. The values obtained with the 9000 g supernatant preparation were not. 3. In respect of metabolites formed, the metabolism of barbiturates in hepatocytes is comparable to that of the intact rat or liver perfusion. The metabolism in the 9000 g supernatant, however, is not.

Effect of chronic DDT exposure on pentobarbital tolerance and intolerance in the rat

Barbiturate tolerance and intolerance were studied in female albino rats. Three consecutive daily ip doses of pentobarbital (30 mg/kg) were shown to produce a significant tolerance in naive rats, with sleeping times significantly shortened on both day 2 (to 65% of control) and day 3 (to 53%). This tolerance was correlated with a significant increase in the in vitro rate of hepatic microsomal pentobarbital metabolism (235% at day 3). On day 23 (after 20 days of barbiturate abstinence) the animals exhibited an intolerance (sleeping time 126%) not associated with any change in hepatic enzyme activity. All parameters returned to control values by day 43. These results suggest that the intolerance is nonhepatogenic and likely related to enhanced CNS sensitivity. In parallel studies, chronic dietary exposure to DDT ( p,p-DDT 13.1 ppm; o,p-DDT 5.3 ppm) caused increased liver enzyme activity which did not prevent the appearance of tolerance by day 3 (sleeping time 72%; barbiturate metabolism 139%) but did prevent the appearance of day 23 intolerance. The hepatic induction produced by DDT appears to offset any CNS sensitivity associated with delayed barbiturate intolerance.

The Effect of Ethanol Administration on Drug Oxidations and Possible Mechanism of Ethanol-Barbiturate Interactions

Chronic administration of ethanol to rats has no effect on the plasma half life of pentobarbitone, which indicates that ethanol does not induce the microsomal enzyme system and that the latter is therefore not identical with the 'microsomal ethanol oxidizing system'. The microsomal oxidation of ethanol observed in vitro is probably the result of metabolism of ethanol by contaminating alcohol dehydrogenase and by catalase peroxidation. Chronic administration of ethanol to rats leads to stimulation of the activity of the hepatic drug metabolizing enzymes in vitro which is not paralleled by any corresponding increase in the concentration of cytochrome P-450, the terminal enzyme in the microsomal mono oxygenase system. However, no changes were seen in hexobarbital sleeping time or zoxazolamine paralysis time, confirming that ethanol pretreatment does not lead to any significant stimulation of the hepatic microsomal drug metabolizing enzymes in vivo. The observed inhibition of barbiturate metabolism after simultaneous administration of large amounts of ethanol may be attributed to nonspecific interaction with the active site of the microsomal drug metabolizing enzymes. Ethanol has also been shown to potentiate the enzyme induction effect of phenobarbitone in vitro in respect of cytochrome P-450 and cytochrome b5. The effect was even more pronounced in vivo when none of the animals pretreated with both phenobarbitone and ethanol exhibited zoxazolamine paralysis and only one third of these animals exhibited narcosis after hexobarbitone administration. The lipophilic character of ethanol allows it to penetrate the lipid environment of the endoplasmic reticulum and to interact with cytochrome P-450 giving a type II substrate interaction spectrum, although it is not metabolized by the microsomal mono oxygenase system. The molar concentration of ethanol required to give a type II spectrum is much higher than that required for aniline suggesting that the interaction of ethanol with cytochrome P-450 is a non specific interaction with the lipophilic phase of the microsomal membrane adjacent to the haemoprotein. Ethanol may therefore act as an activator of the microsomal mono oxygenase enzyme system either by modification of the membrane environment, by allosteric effects, or by displacement of other endogenous or exogenous substrates already bound to the enzymic sites. The potentiation of phenobarbitone induction of this enzyme system may be the result of stabilization of the substrate enzyme complex, as it has been suggested that induction of the microsomal enzymes is a function of the stability of the enzyme substrate complex.

Induction of hepatic microsomal hydroxylating enzymes by technical piperonyl butoxide and some of its analogs

Although they are usually considered to be inhibitors of hepatic microsomal enzymes, technical piperonyl butoxide (PB) and some of its analogs fed to female adolescent rats for 15 days increased the activity of these enzymes. Responses measured included hexobarbital sleep time, O-demethylation of p-nitroanisole, oxidative cleavage of EPN, microsomal cytochrome P450, and concentration of microsomal protein. PB was fed at 10, 100, 1000, 10,000, and 100,000 ppm with marginally increased activity at 100 ppm, maximum activity at 10,000 ppm, and mortality at 100,000 ppm. The PB analogs safrole, isosafrole, eugenol methyl ether, and isoeugenol methyl ether when fed at 10,000 ppm, all produced marked induction; there was greater response to the methylenedioxyphenyl structures than to the dimethoxyphenyl compounds. Safrole prevented reduction of hexobarbital sleep time, presumably due to an anesthetic effect of safrole itself counteracting the stimulatory effect of enhanced barbiturate metabolism. In 2 × 2 factorial experiments involving dietary levels of 5000 ppm, additivity of induction effects was observed for PB in combination with either phenobarbital or safrole. However, the combination of phenobarbital and safrole produced no greater response than either compound singly, indicating that they probably act by different mechanisms. Conclusions are: (1) inductive ability following repeated exposure is directly proportional to inhibitory ability following acute exposure, (2) these compounds apparently act as alternate substrates of microsomal enzymes, and (3) there is a possibility of induction in animals and man, due to the ubiquity and potential additivity of inductive effects for compounds of this class.

Barbiturate metabolism as affected by neoplastic disease.

(1971). Barbiturate Metabolism as Affected by Neoplastic Disease. Clinical Toxicology: Vol. 4, No. 3, pp. 361-368.

Inhibitory mechanism of imipramine on barbiturate metabolism in rat liver.

Inhibition of barbiturate metabolisms by imipramine (IP) was studied systematically in the in vitro system and intact rats with an emphasis on the contribution of metabolism of the inhibitor itself. Both IP and its major metabolite, desmethylimipramine (DMI) were found to inhibit the in vitro microsomal metabolisms of various barbiturates. The time courses of the inhibitory activities of IP and DMI were also studied in the intact animals. The maximum inhibition was observed at approximately 3 hours after the administration of both IP and DMI and the significant inhibition was observed for further about 10 hours. Hexobarbital hypnosis as a pharmacological action was similarly affected by IP administration. The possible participation of the metabolite DMI was confirmed by studying the effect of preincubation of IP in the in vitro system and determining directly the levels of IP and DMI in the liver tissue at various times after IP administration. The effect of dose and long-term pretreatment of IP on the microsomal metabolism of barbiturates was examined and the dispositions of IP were found to be of linear nature. Species differences in the in vitro metabolic inhibition by IP were also examined. And furthermore, as a measure of the physiological availabilities of IP and DMI, lipid solubility, ionization constant, and absorption from the rat small intestine were discussed in connection with the inhibitory mechanism of IP.

Effects of acute hypoxia on brain-sensitivity and metabolism of barbiturates in mice.

Acute exposure of mice to hypobaric hypoxia, or reduced oxygen at normal pressure, markedly potentiated barbiturate-induced hypnosis. The mice showed lower body concentration of pentobarbital on awakening and reduced rate of pentobarbital disappearance from the body during exposure to the hypoxic environment. These data suggest that acute exposure to hypoxia depresses in vivo metabolism of pentobarbital and enhances CNS sensitivity to the barbiturates.

Alterations in brain sensitivity and barbiturate metabolism unrelated to aggression in socially deprived mice.

Male and female mice deprived of social interactions showed increased resistance to hexobarbital narcosis. However, only the isolated males developed aggressiveness. The more rapid disappearance of hexobarbital from the whole body of isolated mice and the higher drug concentration at awakening in these animals indicate enhanced hepatic degradation with a concomitant increase in CNS excitability. Development of aggressiveness in isolated male mice did not correlate temporally with the reduced response to hexobarbital. These data suggest that alterations in barbiturate sensitivity and development of aggressiveness following social deprivation have different biological bases.

The effects of oestrogens and progestins on the response of mice to barbiturates.

1. Mestranol (oestrogen) prolonged, whilst lynestrenol (progestin) reduced, the duration of pentobarbitone and hexobarbitone sleep in mice, whilst the effects of barbitone were not altered.2. The effects of these steroids on pentobarbitone sleep were dose-related, did not show tachyphylaxis, and produced optimal effects after only 4 days pretreatment.3. The effects of lynestrenol were abolished by SKF 525A, whilst those of mestranol were markedly potentiated, suggesting a different mechanism and/or locus of action for mestranol and SKF 525A.4. Examination of plasma levels of pentobarbitone in mice pretreated with mestranol, lynestrenol or SKF 525A showed that lynestrenol increased whilst mestranol and SKF 525A reduced the rate of clearance of barbiturate from the plasma.5. The effects of lynestrenol disappeared when pentobarbitone was prevented from inducing hypothermia, whilst some significant prolongation of pentobarbitone sleep persisted in mestranol treated mice. This suggested that the ability to potentiate hypothermia was not the sole mechanism by which the effects of pentobarbitone were enhanced by mestranol.6. It is concluded these steroids alter the duration of action of pentobarbitone (and hexobarbitone) by changing the rate of barbiturate metabolism. In the case of mestranol, this might be a combination of an effect upon basal metabolic rate (enhancing hypothermia) and a direct effect on the liver. An effect upon renal clearance cannot be excluded by these results.

Use of radioactive microspheres to assess distribution of cardiac output in rabbits.

Use of radioactive microspheres to assess distribution of cardiac output in rabbits.

Metabolism of barbiturates in man

Barbiturate effeets in man are limited by various proeesses. These include redistribution of unehanged drug within the body, development of toleranee, renal exeretion, and metabolie transformation. Biotransformation of barbiturates is mediated primarily by mierosomal enzyme systems residing in the liver. Yet, beeause of the abundant reserve eapacity of this organ, barbiturates need rwt be withheld from patients with liver disease unless the aeeompanying funetional impairment is extreme. Side ehain oxidation is the major meehanism of inactivation of barbiturates in vivo; 5‐dealkylation has also been deseribed. Ring cleavage, of theoretieal import, has still to be demonstrated in man. The products of other metabolie pathways sueh as N‐demethylation, desulfuration and (if verified) N‐methylation retain pharmacologic activity; these are not mechanisms for termination of barbiturate effect. Barbiturates may conveniently be divided into two groups according to whether their rate of metabolism is measured in days (long‐ and medium‐acting barbiturates) or in hours (anesthetie and short‐acting barbiturates). None is metabolized rapidly. Aeeeleration of mierosomal enzyme activity indueed by drugs and other eompounds, amply documented in animals, may ultimately prove a signifieant factor in barbiturate metabolism in man, whose drug history is often complex.