Enterohepatic circulation of clioquinol in the rat

Clioquinol is sulfated by human jejunum cytosol and SULT1A3, a human-specific dopamine sulfotransferase

Clioquinol, originally marketed as an oral intestinal amebicide, was widely used for multiple intestinal disorders. Its use as an oral agent was, however, discontinued because of its possible association with subacute myelo-optico-neuropathy or SMON. Meanwhile, its use for neurodegenerative diseases has recently been suggested. The metabolic fate of clioquinol, however, is poorly described. Since clioquinol is excreted as a sulfate in animals and humans, we have sought to identify a human sulfotransferase (SULT) responsible for the sulfation. We found that sulfating activities of human jejunal cytosols to clioquinol were well correlated with those to dopamine, a typical SULT1A3 substrate. Consistently, recombinant SULT1A3 showed the highest activity to clioquinol in vitro among the human SULTs examined. The S(50) value for the clioquinol sulfation by SULT1A3 was similar to the K(m) value for that by cytosols from human jejunum, where SULT1A3 is abundantly expressed. Moreover, clioquinol inhibited both human jejunal cytosol- and SULT1A3-mediated sulfations of dopamine in a dose-dependent manner, showing similar IC(50) values. These results suggest that SULT1A3, which is highly expressed in intestine but not in liver, is responsible for the clioquinol sulfation in humans, raising a possibility that orally administered clioquinol might inhibit dopamine sulfation in human intestines.

Evaluation of intestinal absorption into the portal system in enterohepatic circulation by measuring the difference in portal-venous blood concentrations of diclofenac

PURPOSE

We evaluated the first-pass effects in vivo by the intestine and liver during enterophepatic circulation (EHC) by simultaneously measuring the portal and venous plasma concentrations of the rat.

METHODS

The venous and upper portal blood vessels were cannulated through the jugular and the pyloric veins, respectively, to obtain simultaneously blood samples from both sites. After diclofenac was injected as a bolus through the jugular vein, the concentrations of diclofenac in the portal and jugular veins were measured at time intervals. The absorption rate from the intestinal tract into the portal system was determined using the portal-venous difference in plasma concentrations of diclofenac, considering 40% partitioning of diclofenac into erythrocytes.

RESULTS

After one hour, the plasma concentration in the portal vein was always higher than that in the jugular vein in awakening rats with intact EHC (portal-venous blood concentration difference). No portal-venous difference was observed in awakening rats with bile-duct cannulation. Therefore, it was concluded that this portal-venous concentration difference was not due to the hepatic clearance but to diclofenac reabsorption from the intestinal tract.

CONCLUSIONS

Approximately 40% of the dose of diclofenac was reabsorbed over 8 hours from the intestinal tract into the portal system. By comparing the reabsorbed amounts in the portal system and in the systemic circulation, the hepatic extraction ratio in vivo (FH) of diclofenac was estimated to be 63%.

Kinetic analysis of glycyrrhetic acid, an active metabolite of glycyrrhizin, in rats: role of enterohepatic circulation

The role of enterohepatic circulation of glycyrrhetic acid (GA) in rats was determined by kinetic analysis of GA. The concentrations of GA in the plasma of the control rat (without bile duct cannulation) during the first 5 h after intravenous (iv) administration of GA (2, 5, 10, and 20 mg/kg) were similar to those in the bile duct-cannulated rat at each dose. No significant difference was observed in the values of the terminal half-life, the total body clearance, the distribution volume at steady state, the area under the curve of concentration in plasma versus time, and the mean residence time in each dose between both groups. When GA (2, 5, 10, and 20 mg/kg) was administered i.v. to the bile duct-cannulated rat, excretion of unchanged GA in bile was < 1% of each dose, that of the acid-hydrolyzed products was 14-16%, and that of GA-3-O-glucuronide was only 1-2%. In the control rat, a secondary peak of GA concentration was observed 12 h after i.v. administration of GA (20 mg/kg). The enterohepatic circulation of GA was confirmed by the linked-rat method in which bile of the donor rat after i.v. administration of GA (20 mg/kg) was allowed to flow directly into the duodenum of the recipient rat. GA was found in the plasma of the recipient rat after 6 h, and its concentration reached the maximum (approximately 0.5 microgram/mL) 8-12 h after dosing the donor rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Oral activated charcoal in the treatment of intoxications. Role of single and repeated doses

Activated charcoal has an ability to adsorb a wide variety of substances. This property can be applied to prevent the gastrointestinal absorption of various drugs and toxins and to increase their elimination, even after systemic absorption. Single doses of oral activated charcoal effectively prevent the gastrointestinal absorption of most drugs and toxins present in the stomach at the time of charcoal administration. Known exceptions are alcohols, cyanide, and metals such as iron and lithium. In general, activated charcoal is more effective than gastric emptying. However, if the amount of drug or poison ingested is very large or if its affinity to charcoal is poor, the adsorption capacity of activated charcoal can be saturated. In such cases properly performed gastric emptying is likely to be more effective than charcoal alone. Repeated dosing with oral activated charcoal enhances the elimination of many toxicologically significant agents, e.g. aspirin, carbamazepine, dapsone, dextropropoxyphene, cardiac glycosides, meprobamate, phenobarbitone, phenytoin and theophylline. It also accelerates the elimination of many industrial and environmental intoxicants. In acute intoxications 50 to 100g activated charcoal should be administered to adult patients (to children, about 1 g/kg) as soon as possible. The exceptions are patients poisoned with caustic alkalis or acids which will immediately cause local tissue damages. To avoid delays in charcoal administration, activated charcoal should be a part of first-aid kits both at home and at work. The 'blind' administration of charcoal neither prevents later gastric emptying nor does it cause serious adverse effects provided that pulmonary aspiration in obtunded patients is prevented. In severe acute poisonings oral activated charcoal should be administered repeatedly, e.g. 20 to 50g at intervals of 4 to 6 hours, until recovery or until plasma drug concentrations have fallen to non-toxic levels. In addition to increasing the elimination of many drugs and toxins even after their systemic absorption, repeated doses of charcoal also reduce the risk of desorbing from the charcoal-toxin complex as the complex passes through the gastrointestinal tract. Charcoal will not increase the elimination of all substances taken. However, as the drug history in acute intoxications is often unreliable, repeated doses of oral activated charcoal in severe intoxications seem to be justified unless the toxicological laboratory has identified the causative agent as not being prone to adsorption by charcoal. The role of repeated doses of oral activated charcoal in chronic intoxication has not been clearly defined.(ABSTRACT TRUNCATED AT 400 WORDS)