The distribution of amylobarbitone, butobarbitone, pentobarbitone and quinalbarbitone and the hydroxylated metabolites in man

Fluid and tissue specimens collected from 30 subjects at autopsy have been assayed for their content of common sedative barbiturates and the corresponding hydroxylated metabolites by g.l.c. Where one barbiturate had been ingested an inverse relationship between lipid solubility of the drug and the distribution in fluids and tissues was observed. In most cases the liver, and in the remainder the spleen, contained the highest concentrations of barbiturate. Bile concentrations were often in excess of those in the corresponding liver. The metabolites of the four sedative barbiturates were usually present in lower amounts than the parent drugs in the fluids and tissues of most subjects but urine often contained much higher concentrations of metabolites--sometimes exceeding that of the parent drug in the liver. Administration of two or more barbiturates together did not appear to affect the distribution and metabolism of the individual drugs.

Barbiturate protection in tolerant and nontolerant hypoxic mice: comparison with hypothermic protection

The effects of pentobarbital on survival times of mice exposed to oxygen, 5 per cent, were studied over a large dosage range in normal mice and in mice made tolerant to the effect of barbiturates. Tolerance was induced by pretreatment with phenobarbital, 210 mg/kg, for three days, which increased the median anesthetic dose (AD50) for pentobarbital from 34 to 53 mg/kg. In nontolerant mice there was a dose-related increase in mean survival times for doses between 35 and 60 mg/kg, with a maximum increase to 303 per cent above control. At doses of more than 60 mg/kg survival times progressively decreased toward control. For tolerant mice survival time as a function of pentobarbital dosage was shifted to the right, i.e., protection necessitated higher doses. This shift was not explained by lower brain concentrations of pentobarbital in tolerant animals, but rather parallelled the increased tolerance to the anesthetic effect of the barbiturate. The authors conclude that in this model the protective effect of barbiturate is a function of the anesthetic effect rather than the barbiturate concentration in brain per se. Hypothermia (29 C) resulted in an increase in mean survival time comparable to that in barbiturate-treated animals. This supports the hypothesis that protection is ultimately a function of decreased cerebral metabolism, whether produced by anesthesia or by hypothermia. This model measures only the effect on spontaneous respiration during hypoxia. It is possible that other mechanisms are involved if barbiturates protect in other situations, such as during or after periods of complete ischemia.

Enhancement of pharmacologic responses of pentobarbital by dopram

The effect of Dopram (doxapram hydrochloride, A. H. Robins Co.) on the pharmacologic responses to pentobarbital was evaluated. In naive and pentobarbital-tolerant mice, Dopram was shown to enhance significantly sodium pentobarbital-induced narcosis in a dose-related manner. The effect of the duration of action of Dopram on pentobarbital narcosis also was assessed. It was observed that Dopram (40 mg/kg, i.p.) significantly increased pentobarbital-induced narcosis even when administered 2 hr prior to challenge with sodium pentobarbital (60 mg/kg, i.p.) A significantly increased hypothermic response to sodium pentobarbital was seen in Dopram-treated animals. The half-life of pentobarbital in brain and serum was shown to be increased significantly in animals receiving Dopram, 40 mg/kg, i.p. The waking brain and serum pentobarbital concentrations were not significantly different in either group. These studies show that Dopram potentiates pentobarbital's effects. Further study is necessary to determine the sites of operation and mechanism of this potentiation.

Development of functional tolerance to pentobarbital in goldfish

The acute effects of pentobarbital were measured by the times required for the loss of righting reflex (overturn point) and the pentobarbital brain concentration associated with the overturn. The test consisted of immersing the fish in a 0.3 mg/ml sodium pentobarbital in 0.1 M Tris buffer challenge solution until the overturn point was reached. To examine the development of tolerance the fish were pre-exposed to 0.1 M Tris buffer solutions containing 0.0, 0.010, 0.015 and 0.025 mg/ml of sodium pentobarbital for 6, 24 or 48 hr at which time the overturn times and the pentobarbital brain concentrations at overturn in the challenge solution were determined. The mean pentobarbital content in the brain at overturn of fish pre-exposed to 0.015 or 0.025 mg/ml solution was significantly higher (P less than .01) than in control fish. The loss of tolerance was determined at 3 hr after termination of the pre-exposure of the fish to the various pentobarbital solutions; tolerance was measured only in the group of animals pre-exposed to the 0.025 solution by the significant increase (P less than .01) in the pentobarbital brain levels over control fish. The equilibration curve for fish swimming in 0.025 mg/ml of sodium pentobarbital was determined for 48 hr. A steady state was attained within 6 hr with brain levels that reached approximately 80% the concentration of the external solution.

Drug kinetics and alcohol ingestion

Acute and chronic ethanol ingestion can alter both the pharmacodynamics and pharmacokinetics of other drugs. For psychotherapeutic drugs, modification of drug action by alcohol is much more important than kinetic interaction, such as ethanol induced drug metabolism. In contrast, the importance of the effects of alcohol on the kinetics of other classes of drug is incomplete. The probability and mechanism of alcohol kinetic interactions with other drugs can nevertheless be anticipated, in part, on the basis of the extent of binding of the drug to plasma proteins, the capacity of the liver for extracting the drug from blood passing through the liver and the true distribution space of the drug. Highly bound drugs with low intrinsic hepatic clearance are among the most commonly reported to have their kinetics altered by ethanol (e.g. benzodiazepines, phenytoin, tolbutamide and warfarin). Less highly bound drugs are less consistently affected (e.g. meprobamate, glutethimide, pentobarbitone and phenobarbitone). Acute administration of ethanol to laboratory animals or incubation of microsomal preparations with ethanol inhibits the mixed function oxidase activity. In the human, the elimination half-life of meprobamate, pentobarbitone and tolbutamide is increased by acute ethanol administration. Chronic administration of ethanol to rats and humans causes proliferation of the smooth endoplasmic reticulum, increase in microsomal protein content and cytochrome P450 and results in an augmentation in drug metabolising ability of the microsomes in vitro. Even though the plasma half-life of some drugs is decreased by chronic ethanol ingestion, the clinical determination of the mechanism is incomplete because few studies have measured drug metabolite levels. In addition, alcohol effects on drug distribution have not been studied very extensively. The effects of chronic alcohol ingestion on drugs with low and high hepatic extraction, high and low binding, important tissue localisation and microsomal and non-microsomal metabolism will be quite different. Systematic studies of the mechanism of alcohol kinetic interactions are needed. Such kinetic studies should be combined with pharmacodynamic measures in order to establish the clinical importance of changes in drug kinetics.

The disposition and metabolism of flurbiprofen in several species including man

Flurbiprofen was rapidly absorbed in all species studied. 2. Half-lives of elimination measured 0 to 12 h after a single dose were: mouse 3.4 h, rat 2.5 h, dog 10.1 h, baboon 3.1 h and man 3.9 h. A second phase of elimination was seen in the dog. Flurbiprofen accumulated in the circulation of the dog on repeated dosing. 3. After dosing with [14C]flurbiprofen, tissue levels of radioactivity in dog and baboon were similar to that in plasma. In the rat, levels were slightly elevated in liver, kidney, large intestine and thyroid after repeated dosing. 4. The dog excreted equal amounts of radioactivity in urine and faeces. In other species renal excretion was the more important route. 5. Six metabolites have been detected, the most important being: 2-(2-fluoro-4'-hydroxy-4-biphenylyl)propionic acid (metabolite 1), 2-(i-fluoro-3',4'-dihydroxy-4-biphenylyl)propionic acid (metabolite 2) and 2-(2-fluoro-3'-hydroxy-4'-methoxy-4-biphenylyl)propionic acid (metabolite 3). The proportions of the metabolites and the extents of their conjugation varied among the species. 6. Metabolites were detected in the circulation of rat, mouse and baboon but not in dog and man. 7. Flurbiprofen did not affect the hepatic drug-metabolizing enzyme system of rat. 8. Flurbiprofen was extensively bound to serum protein of rat, dog, baboon and man.

Pentobarbital absorption from capsules and suppositories in humans

Serum pentobarbital levels following administration of the sodium salt as a 100-mg capsule orally and as two 120-mg suppository formulations (A and B) rectally were measured. From these data and previously determined kinetic constants after intravenous administration, the absorption rates and bioavailability of pentobarbital from each dosage form were determined. All three dosage forms were 100% absorbed. Peak serum pentobarbital levels occurred at 1, 4, and 10 hr for the capsule, Suppository A, and Suppository B, respectively. In vitro studies agreed with the serum data in that Suppository A released drug in an in vitro aqueous pH 1.4 system at a much greater rate that Suppository B. The capsule and Suppository A both appeared to be absorbed by simple first-order processes; however, Suppository B had a complex absorption pattern, which was modeled using sequential zero-order and first-order absorption.

Effect of hyperbilirubinemia on the pharmacokinetics of pentobarbital in the rat

The pharmacokinetics of intravenously administered pentobarbitol have been studied in heterozygous (non-jaundiced) and homozygous (jaundiced) Gunn rats following single doses of 30 mg/Kg. Plasma pentobarbital concentration time course data from heterozygous animals were fitted to a biexponential equation while a triexponential equation was required to fit the data from homozygous animals. A slower plasma clearance rate and longer elimination half-life were observed in homozygous animals as well as increased overall volumes of pentobarbital distribution. These results suggest a possible effect of bilirubin on the distribution and elimination of pentobarbital. The volume of pentobarbital plasma distribution, however, was approximately equal in heterozygous and homozygous data, suggesting no competition between bilirubin and pentobarbital for serum protein.

Physical dependence to barbital compared to pentobarbital. IV. Influence of elimination kinetics

The withdrawal characteristics of barbital and pentobarbital after "chronically equivalent" treatment suggested that the longer acting barbital was less liable to produce physical dependence. Therefore, to distinguish this potential pharmacodynamic difference from the known pharmacokinetic differences between the two drugs, the rate of elimination of each was adjusted to mimic that of the other. The rate of barbiturate elimination after chronically equivalent pentobarbital dosing was reduced by barbital substitution or by first-order pentobarbital dose reduction, with the result that withdrawal signs became mild and appeared later (3 days postdrug). The rate of barbiturate elimination after chronically equivalent barbital dosing was increased by pentobarbital substitution or by peritoneal dialysis of barbital, with the result that withdrawal signs became severe and appeared sooner (within 1 day). These findings conclusively support the key role of the rate of barbiturate elimination to expose underlying physical dependence to barbiturates. Furthermore, "physical dependence" and its expression in "withdrawal" must be regarded separately to evaluate and compare critically the dependence capability of different drugs.

Interaction of rubratoxin B and pentobarbital in mice

Rubratoxin B, a metabolite of Penicillium rubrum, is a potent mycotoxin that produces hepatic mid-zonal necrosis. Using propylene glycol as the solvent, the acute LD50 (i.p.) in male mice was estimated at 1.42 mg/kg. Pretreatment with s.c. pellets of pentobarbital reduced this toxicity approximately 42 percent, while castration and SKF 525A pretreatment enhanced toxicity approximately 20 percent and 60 percent, respectively. These studies also demonstrated that rubratoxin B potentiated pentobarbital-induced narcosis and enhanced pentobarbital hypothermia. The effect of SKF-525A and pentobarbital on the LD50 value of rubratoxin B suggests that the toxicity of this mycotoxin in mice does not arise from a toxic metabolite but from the parent compound.

Transport of heterocyclic acids across rat small intestine in vitro

A study has been made of the steady-state fluxes of barbituric acid, six of its substituted derivatives, and 5,5-dimethyloxazolidinedione (DMO) across the wall of rat jejunum in vitro. For each of the compounds tested the mucosal (M) to serosal (S) flux was significantly larger than the S to M flux. Both M to S and S to M fluxes increased linearly with concentration, and the transport of one acid was not influenced by the presence of a tenfold greater concentration of a second heterocyclic acid. The fluxes decreased as the pH of the incubation saline was increased, but neither the M to S, nor the S to M fluxes could be described in terms of simple nonionic diffusion. It was found that the relation between the flux ratios of the transported acids and their pKalpha values could be described by an equation derived from consideration of the transport of a weak acid in a series three compartment system, and it has been concluded that the three compartment system provides a good working hypothesis for the mechanism of heterocyclic acid transport across rat jejunum. It was found that the best fit of the theoretical curve to the experimental data was obtained when the ratio of permeabilities to the ionized and nonionized forms of a weak acid at one of the barriers was assigned the value 5 X 10(-1). It is suggested that this value may be characteristic of a noncellular restriction to diffusion, such as a layer of connective tissue, and substantiates previous suggestions that the intermediate compartment of the intestinal three compartment system is a component of the sub-epithelial extracellular space.

Dose dependent reduction of glucose utilization by pentobarbital in rat brain

A new method of determining the rate of glucose utilization in brain regions of individual rats has been used to measure the dose dependency of the reduction of the metabolic activity of the cerebral cortex by pentobarbital. Cerebral cortical glucose utilization is depressed to a basal level of 44% of the control rate when cerebral pentobarbital levels exceed 50 microgram per g of tissue. The major portion of this effect occurs between the cerebral pentobarbital range of 10--20 microgram per g, which can be achieved by 1/5 to 1/10 the normal anesthetic intraperitoneal dosage. If a depression of brain metabolism is responsible for the previously reported protection of the brain from ischemic damage, these data suggest a substantial reduction of brain metabolic rate is achieved in the rat at a barbiturate dosage which may be therapeutically relevant in the human after acute brain ischemia.

Pentobarbital sleeping time and waking blood levels in spontaneously hypertensive rats

Pentobarbital sleeping times and blood levels on arousal were determined in spontaneously hypertensive rats (SHR) and normotensive controls (WKR). Sleeping time for SHR was significantly less than for WKR, but blood levels of [14C]pentobarbital at awakening were not significantly different. The shorter sleeping time in SHR appears not to result from decreased brain sensitivity to pentobarbital. Instead, SHR appear to differ from WKR in the rate of metabolic clearance of the drug or in the distribution of the drug between blood and brain.