In vitro and in vivo GABAA Receptor Interaction of the Propanidid Metabolite 4-(2-[Diethylamino]-2-Oxoethoxy)-3-Methoxy-Benzeneacetic Acid.

BACKGROUND: Propanidid is a γ-aminobutyric acid type A (GABAA) receptor agonist general anesthetic and its primary metabolite is 4-(2-[diethylamino]-2-oxoethoxy)-3-methoxy-benzeneacetic acid (DOMBA). Despite having a high water solubility at physiologic pH that might predict low-affinity GABAA receptor interactions, DOMBA is reported to have no effect on GABAA receptor currents, possibly because the DOMBA concentrations studied were simply insufficient to modulate GABAA receptors. Our objectives were to measure the propanidid and DOMBA concentration responses on -GABAA receptors and to measure the behavioral responses of DOMBA in mice at concentrations that affect GABAA receptor currents in vitro. METHODS: GABAA receptors were expressed in oocytes using clones for the human GABAA α1, ß2 and γ2s subunits. The effects of DOMBA (0.2-10 mmol/L) and propanidid (0.001-1 mmol/L) on oocyte GABAA currents were studied using standard 2-electrode voltage clamp techniques. Based on in vitro results, 6 mice received -DOMBA 32 mg intraperitoneal and were observed for occurrence of neurologic effects and DOMBA plasma concentration was measured by liquid chromatography tandem mass spectrometry. RESULTS: DOMBA both directly activates GABAA receptors and antagonizes its GABA-mediated opening in a concentration-dependent manner at concentrations between 5-10 and 0.5-10 mmol/L respectively. In vivo, DOMBA produced rapid onset sedation at plasma concentrations that correlate with direct GABAA receptor activation. CONCLUSION: DOMBA modulation of GABAA receptors is associated with sedation in mice. Metabolites of propanidid analogues currently in development may similarly modulate GABAA, and impaired elimination of these metabolites could produce clinically relevant neurophysiologic effects.

Different induction of microsomal carboxylesterases, palmitoyl-CoA hydrolase and acyl-L-carnitine hydrolase in rat liver after treatment with clofibrate.

The levels of hepatic carboxylesterases, including palmitoyl-CoA hydrolase and decanoyl-D,L-carnitine hydrolase, were studied in total homogenates and subcellular fractions prepared from the livers of male rats fed diets containing 0.3% clofibrate. The microsomal carboxylesterase as well as the fatty acyl-thioesterase are differently induced by clofibrate feeding. The specific activities of acetanilide carboxylesterase and decanoyl-D,L-carnitine hydrolase increased more than 3-fold in the microsomal fraction, compared to pellet-fed control animals. The microsomal activities of palmitoyl-CoA hydrolase and propanidid hydrolase were decreased by about 20 to 40% in clofibrate-treated rats. The specific clofibrate hydrolase activity remained unchanged after clofibrate administration, indicating that this microsomal carboxylesterase is not induced by its own substrate. The data suggest a different distribution of the differing carboxylesterase along the endoplasmic reticulum.

Effects of phenobarbital and p,p'-DDT on carboxylesterase activity and hydrolysis of propanidid in rat liver.

Time-dependent responses of microsomal and cytosolic carboxylesterases (EC 3.1.1.1) to administration of phenobarbital or p,p'-DDT were studied in rat liver using p-nitrophenylacetate and propanidid as substrates. In vitro and in vivo studies indicate that the hepatic carboxylesterases are induced by phenobarbital and p,p'-DDT. The induced metabolism of propanidid does not appear to be due to the microsomal polysubstrate monooxygenase system of the liver.

Hydrolysis of ester- and amide-type drugs by the purified isoenzymes of nonspecific carboxylesterase from rat liver.

Five purified carboxylesterases from rat liver microsomes show a differing capacity for the hydrolysis of ester- and amide-type drugs. The two closely related enzymes that are responsible for the microsomal hydrolysis of palmitoyl-CoA and long chain monoacylglycerides exhibit the highest propanidid-and aspirin-cleaving rates. The predominant nonspecific esterase of microsomes is responsible for the hydrolysis of procaine, clofibrate, isoarecaidine esters, butanilicaine, octanoylamide, and possibly butyryl thiocholine. Finally, the palmitoyl carnitine-cleaving esterase splits phenacetin and acetanilide. The purified nonspecific esterase with the lowest isoelectric point is not involved in the metabolism of the drugs mentioned.

The effects of phenobarbital, bis-p-nitrophenyl phosphate and disulfiram on the hydrolysis of propanidid in Wistar rats.

The short-acting anesthetic propanidid was used in vivo as a model substrate in studies of phenobarbital-inducible carboxylesterases in Wistar rats. Phenobarbital shortened the duration of anesthesia produced by intravenous (25 mg/kg) or intraperitoneal (750 mg/kg) administration of propanidid which was in good agreement with the lowered plasma concentration and the shortened half-life of propanidid in phenobarbital-pretreated rats. The treatment of rats with bis-p-nitrophenyl phosphate or disulfiram with or without phenobarbital-pretreatment prolonged the sleeping time and the half-life of propanidid in plasma.

[Pharmacokinetics of intravenous non-steroidal anesthetics]. / Pharmacocinétique des anesthésiques veineux non stéroïdiens.

Even though the anesthetic agents thiopental, ketamine, propanidid and etomidate all belong to very different chemical families they are all characterized by a very large degree of liposolubility. This explains their rapid penetration into the brain. The pharmacokinetic model of thiopental is a three compartment model. There is strong protein binding and only the free fraction is active. The very short action of the product after a single injection is due to the rapid redistribution of the agent into the muscle mass because its hepatic metabolization is very slow. However, when given over prolonged time the adipose compartment plays an important role in the mixture of the product, explaining the prolonged sleep produced. The central depressant actions of thiopental and consequently its action on CMRO2 depend on the initial dose and the route of administration. A single and massive injection produces a small and temporary reduction in the CMRO2 even though the plasmic concentration is high. In contrast prolonged intravenous infusion produces more severe and longer lasting depression of the CMRO2. The pharmacokinetic model of ketamine is tri-compartmental. There is weak protein binding. After IV injection ketamine rapidly enters the brain and the maximum concentration is reached one minute later. After that the cerebral concentration rapidly falls as does the plasma level. Signs of waking are seen at a concentration of 130 micrograms per gram of tissue. An increase in the dose of ketamine does not much influence the duration of analgesia but increase the waking time. This suggests that its indication in ambulatory anesthesia should be looked at with care. It is metabolized by the liver with the formation of several metabolites of which some are active. The kinetics of propanidid can be explained on the basis of a monocompartmental model. The speed of the fall in plasma level of the product is related to the speed of injection. High plasma concentrations mobilize a larger quantity of plasma pseudo-cholinesterases, increasing thus the speed of degradation. The product is rapidly hydrolized (plasma and liver cholinesterases). The duration of action is longer when used at low doses or when it is administered at a constant dose. Propanidid does not have any accumulative effect. The kinetics of etomidate follow a tri-compartmental model. It is very rapidly and largely distributed in the organism, the peak cerebral concentration being reached in less than one minute. There is strong protein binding. Repeated administration of the drug produces an increase in anesthetic sleep but also a delay in recovery. Etomidate is hydrolized by hepatic esterases.

[Animal experiments to study placental passage of Sombrevin and Hexobarbital and their concentrations in various foetal organ systems (author's transl)]. / Tierexperimentelle Studien zur Plazentapassage von Sombrevin und Hexobarbital sowie deren Konzentration in verschiedenen fetalen Organsystemen.

Studies were conducted into 55 rabbits and their 327 foetuses to measure the two intravenously applied anaesthetics, Sombrevin and Hexobarbital, for their passage through the placenta and for the concentrations which eventually settle in various organ systems of the foetuses. Fast placental passage, but in different concentrations, was established for either anaesthetic. Sombrevin was found to reach the foetal organism only in low quantity and for short-time retention, whereas Hexobarbital passed the placental barrier in much higher concentration to stay detectably for a longer period of time in the foetal blood. While Sombrevin was not recordable from foetal organs, its intraplacental buildup reached higher concentrations. Relatively high concentrations of Hexobarbital were recordable from both the placenta and vital foetal organs. The conclusions so far derived from those animal experiments seem to suggest a superiority of Sombrevin over Hexobarbital and regard to applicability to obstetric anaesthesia.

[Comparative studies of placentar diffusion of propanidid (Sombrevin) and barbiturate in premature pregnancies and cesarean sections]. / Vergleichende Untersuchungen der plazentaren Diffusion von Propanidid (Sombrevin) und Barbiturat bei Frühgeburten und Kaiserschnitt

The placental diffusion and fetal elimination of propanidid (Sombrevin) and hexobarbituralnatrium adhibited for the introduction of narcosis in cases of prematures requiring caesarean section is studied by virtue of maternal, umbilical cord, and new-born blood samples. On the basis of the results we come to the conclusion that for the introduction of narcosis in prematures requiring caesarean section, Sombrevin seems to be more suitable than hexobarbituralnatrium, owing to the quicker propanidid elimination.

Prolongation of anesthetic action by BNPP (bis-[p-nitrophenyl] phosphate).

Bis-[p-nitrophenyl] phosphate, BNPP, an enzyme inhibitor of the organophosphate class, has been used to inhibit the enzyme, carboxylic ester hydrolase EC 3.1.1.1. Esterases play a major role in the rapid metabolism of propanidid in vivo; in fact, the short duration of action of this intravenous anesthetic agent is due to this rapid hydrolysis. The duration of anesthesia with propanidid alone in healthy mongrel dogs was 10.1 +/- 2.1 (SEM) minutes. When the dogs were pretreated with BNPP, propanidid anesthesia time was prolonged to 38.2 +/- 7.9 (SEM) minutes. Measurements of serum propanidid concentration demonstrated that prolonged high levels of propanidid were associated with the extended anesthesia time. Therefore, BNPP can significantly alter the anesthetic action of propanidid by inhibition of the enzyme system responsible for the rapid hydrolysis of the agent. The experimental model used in the present study provides a means for investigation of effects of certain drugs when their metabolism is impaired.

Metabolic degradation of propanidid by various tissues. Short communication.

The very short anaesthetic action of 3-methoxy-4-(N,N-diethylcarbamoyl-methoxy)-phenylacetic acid propylester (propanidid, Epantol) is due to its rapid hydrolysis in vivo. In this study, the rates of degradation of this drug under the influence of various isolated tissues have been established. The drug was hydrolysed much more rapidly by liver homogenate than by whole blood or serum. It is concluded that inactivation of propanidid in vivo occurs mainly by the liver, whereas blood esterases play an insignificant part in hydrolyzing the ester. Moreover, it was demonstrated that cardiac tissue is able to hydrolyze the drug as well. It is postulated that the inactivation of propanidid by heart muscle might partly compensate for the severe cardiodepressive action of the anaesthetic agent. A gaschromatographic determination of propanidid has been developed.

The negative inotropic action of propanidid. influence on calcium movements in atrial tissue.

Propanidid (Epontol), a general anaesthetic agent with a particularly short action in vivo significantly depressed the contraction amplitude of guinea pig isolated atria. A steep concentration-response curve could be established. Contractile force of electrically driven atria (180/min) was reduced by approximately 50% at a propanidid concentration of 3.5 x 10-4 M in the medium. The negative inotropic effect developed rapidly (less than 10 min). At concentrations of 4.5 x 10-4 M and less propanidid hardly reduced the frequency of spontaneously beating atria. The uptake of extracellular 45 Ca by spontaneously beating atria occurred significantly more slowly in presence of propanidid (4.5 x 10-4 M ), whereas the exchangeable calcium fraction remained unchanged. Accordingly, propanidid reduced the rate of exchange of calcium so that less ionized calcium was available for excitation-contraction coupling. Propanidid (4.5 x 10-4 M) accelerated the uptake of 45Ca by isolated plasma membranes, obtained from guinea pig ventricular muscle. Moreover, the binding capacity for calcium by isolated membranes was increased in presence of propanidid. These observations imply that less ionized calcium is available for activation of the contractile system. It is concluded that the negative inotropic action of propanidid is probably due to the drug's influence on membrane function, thus bringing about an important change in cellular calcium metabolism.