THE INFLUENCE OF ENTEROHEPATIC CIRCULATION ON TOXICITY OF DRUGS

Hepatoenteric recycling is a new disposition mechanism for orally administered phenolic drugs and phytochemicals in rats

Many orally administered phenolic drugs undergo enterohepatic recycling (EHR), presumably mediated by the hepatic phase II enzymes. However, the disposition of extrahepatically generated phase II metabolites is unclear. This paper aims to determine the new roles of liver and intestine in the disposition of oral phenolics. Sixteen representative phenolics were tested using direct portal vein infusion and/or intestinal perfusion. The results showed that certain glucuronides were efficiently recycled by liver. OATP1B1/1B3/2B1 were the responsible uptake transporters. Hepatic uptake is the rate-limiting step in hepatic recycling. Our findings showed that the disposition of many oral phenolics is mediated by intestinal glucuronidation and hepatic recycling. A new disposition mechanism 'Hepatoenteric Recycling (HER)", where intestine is the metabolic organ and liver is the recycling organ, was revealed. Further investigations focusing on HER should help interpret how intestinal aliments or co-administered drugs that alter gut enzymes (e.g. UGTs) expression/activities will impact the disposition of phenolics.

Measuring fecal metabolites of endogenous steroids using ESI-MS/MS spectra in Taiwanese pangolin, (order Pholidota, family Manidae, Genus: Manis): A non-invasive method for endangered species

Pangolins are 'keystone species' driven towards extinction due to a lack of profound awareness and illegal trade. The drivers urge for immediate development in the understanding of demographics and reproductive dynamics of this species. In this study, we developed and validated a quantitative method to measure pangolin fecal extracts using the electrospray (ESI-MS/MS) interface in positive ionization mode. The method aids in the measurement of hormones from the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axis, making it a possibly appropriate technique to understand the cross-talk between the axes. The study aims to measure the relative abundance of adrenal and gonadal hormones such as corticosterone, cortisol, estrone, estradiol-17β, progesterone, testosterone, and a number of its metabolites. From the dried fecal extract, the principal metabolite identified from the estrogen family was estradiol-17β, whereas the gestagen family revealed that the pregnane series is predominated in 5α-configuration. On the other hand, epiandrosterone was seen as the dominant form in the male fecal extracts. Additionally, the glucocorticoids are excreted majorly as corticosterone, but traces of cortisol are also present in both the male and female fecal samples. The physiological validation confirmed that the ESI-MS/MS technique is suitable to determine physiologically caused differences in the fecal steroid concentrations. Physiologically, the age structure in pangolin is not responsible for causing differences within gender. However, the results revealed that glucocorticoids might vary between the sexes, i.e., males have a higher relative abundance of glucocorticoids over females. Therefore, our studies show that some of the main adrenal and gonadal metabolites can be predicted by exploiting MS/MS, which can steer research to potentially assess the reproductive status of captive and free-ranging pangolin species.

The Effect of Sevelamer on Serum Levels of Gut-Derived Uremic Toxins: Results from In Vitro Experiments and A Multicenter, Double-Blind, Placebo-Controlled, Randomized Clinical Trial

High serum levels of gut-derived uremic toxins, especially p-cresyl sulfate (pCS), indoxyl sulfate (IS) and indole acetic acid (IAA), have been linked to adverse outcomes in patients with chronic kidney disease (CKD). Sevelamer carbonate could represent an interesting option to limit the elevation of gut-derived uremic toxins. The aim of the present study was to evaluate the adsorptive effect of sevelamer carbonate on different gut-derived protein-bound uremic toxins or their precursors in vitro, and its impact on the serum levels of pCS, IS and IAA in patients with CKD stage 3b/4. For the in vitro experiments, IAA, p-cresol (precursor of pCS) and indole (precursor of IS), each at a final concentration of 1 or 10 µg/mL, were incubated in centrifugal 30 kDa filter devices with 3 or 15 mg/mL sevelamer carbonate in phosphate-buffered saline at a pH adjusted to 6 or 8. Then, samples were centrifuged and free uremic toxins in the filtrates were analyzed. As a control experiment, the adsorption of phosphate was also evaluated. Additionally, patients with stage 3b/4 CKD (defined as an eGFR between 15 and 45 mL/min per 1.73 m²) were included in a multicenter, double-blind, placebo-controlled, randomized clinical trial. The participants received either placebo or sevelamer carbonate (4.8 g) three times a day for 12 weeks. The concentrations of the toxins and their precursors were measured using a validated high-performance liquid chromatography method with a diode array detector. In vitro, regardless of the pH and concentration tested, sevelamer carbonate did not show adsorption of indole and p-cresol. Conversely, with 10 µg/mL IAA, use of a high concentration of sevelamer carbonate (15 mg/mL) resulted in a significant toxin adsorption both at pH 8 (mean reduction: 26.3 ± 3.4%) and pH 6 (mean reduction: 38.7 ± 1.7%). In patients with CKD stage 3b/4, a 12-week course of treatment with sevelamer carbonate was not associated with significant decreases in serum pCS, IS and IAA levels (median difference to baseline levels: −0.12, 0.26 and −0.06 µg/mL in the sevelamer group vs. 1.97, 0.38 and 0.05 µg/mL in the placebo group, respectively). Finally, in vitro, sevelamer carbonate was capable of chelating a gut-derived uremic toxin IAA but not p-cresol and indole, the precursors of pCS and IS in the gut. In a well-designed clinical study of patients with stage 3b/4 CKD, a 12-week course of treatment with sevelamer carbonate was not associated with significant changes in the serum concentrations of pCS, IS and IAA.

Role of enterohepatic recirculation in drug disposition: cooperation and complications

Enterohepatic recirculation (EHC) concerns many physiological processes and notably affects pharmacokinetic parameters such as plasma half-life and AUC as well as estimates of bioavailability of drugs. Also, EHC plays a detrimental role as the compounds/drugs are allowed to recycle. An in-depth comprehension of this phenomenon and its consequences on the pharmacological effects of affected drugs is important and decisive in the design and development of new candidate drugs. EHC of a compound/drug occurs by biliary excretion and intestinal reabsorption, sometimes with hepatic conjugation and intestinal deconjugation. EHC leads to prolonged elimination half-life of the drugs, altered pharmacokinetics and pharmacodynamics. Study of the EHC of any drug is complicated due to unavailability of the apposite model, sophisticated procedures and ethical concerns. Different in vitro and in vivo methods for studies in experimental animals and humans have been devised, each having its own merits and demerits. Involvement of the different transporters in biliary excretion, intra- and inter-species, pathological and biochemical variabilities obscure the study of the phenomenon. Modeling of drugs undergoing EHC has always been intricate and exigent models have been exploited to interpret the pharmacokinetic profiles of drugs witnessing multiple peaks due to EHC. Here, we critically appraise the mechanisms of bile formation, factors affecting biliary drug elimination, methods to estimate biliary excretion of drugs, EHC, multiple peak phenomenon and its modeling.

Biliary excretion of foreign compounds. Benzene and its derivatives in the rat

1. The extent of the excretion in the bile of the rat of benzene and 21 of its simple derivatives was studied. 2. Some 16 compounds of molecular weight less than 200, and including neutral molecules (benzene and toluene), aromatic acids, aromatic amines and phenols, were injected in solution intraperitoneally into biliary-cannulated rats. Metabolites in the bile were identified and estimated. The extent of biliary excretion of these compounds was low, i.e. 0-10% of the dose in 24hr., and most appeared in the bile mainly as conjugates. 3. The biliary excretion of six conjugates of molecular weight less than 300, including three glycine conjugates, one sulphate conjugate, one glucuronic acid conjugate and two acetyl derivatives, was low (less than 3% of the dose). 4. It is concluded that simple benzene derivatives of molecular weight less than about 300 are poorly excreted in rat bile.

Studies on the disposition, metabolism and hepatotoxicity of coumarin in the rat and Syrian hamster

The hepatotoxicity, metabolism and disposition of coumarin has been compared in male Sprague-Dawley rats and Syrian hamsters. The treatment of rats for 12, 24 and 42 weeks with diets containing 0.2 and 0.5% coumarin resulted in hepatotoxicity and increased relative liver weights. While levels of cytochrome P450 (CYP) and CYP-dependent enzymes were decreased, levels of reduced glutathione (GSH) and activities of UDP glucuronosyltransferase, gamma-glutamyltransferase and GSH S-transferase were increased. In contrast, coumarin produced few hepatic changes in the Syrian hamster. Following a single oral dose of 25 mg/kg [3-14C]coumarin, radioactivity was rapidly excreted by the rat and Syrian hamster with the urine containing 63.5 and 89.9%, respectively, and the faeces 38.0 and 12.4%, respectively, of the administered dose after 96 h. The biliary excretion of radioactivity was greater in the rat than in the Syrian hamster. Analysis of 0-24-h urine samples revealed that both species were poor 7-hydroxylators of coumarin. In the rat, treatment with 0.5% coumarin in the diet for 24 weeks was found to increase the urinary excretion of single oral gavage doses of 25 and 300 mg/kg [3-14C]coumarin. The marked species difference in hepatotoxicity between the rat and Syrian hamster observed in this study may be at least partially attributable to differences in coumarin disposition. However, additional studies are required to elucidate the metabolic pathways of coumarin in both species.

The in vivo dermal absorption and metabolism of [4-14C] coumarin by rats and by human volunteers under simulated conditions of use in fragrances

The disposition and metabolic fate of [4-14C]coumarin in a 70% aqueous ethanol solution was studied in male Lister Hooded rats after occluded dermal application and in three male volunteers after an exposure designed to simulate that which may be encountered when using an alcohol-based perfumed product. In both cases, the 6-h exposure was 0.02 mg/cm(2) (rats 0.023 mg/kg and humans 0.77 mg/kg). In both, coumarin was quickly absorbed, distributed and excreted in urine and feces, although fecal excretion of coumarin in humans was only 1% of the applied dose as opposed to 21% in rats. Total absorption was 72% of the applied dose with rats and 60% with humans. Peak plasma radioactivity in both was at 1 h. The mean plasma half-life of coumarin and metabolites was approximately 1.7 h for humans and 5 h for rats. In humans, coumarin was primarily metabolized to and excreted in urine as 7-hydroxycoumarin glucuronide and 7-hydroxycoumarin sulfate. Small amounts of unconjugated 7-hydroxycoumarin and o-hydroxyphenylacetic acid (o-HPAA) were also excreted. In rats, about twenty metabolites were present, but only o-HPAA was identified. These studies show the rat is a very poor model for humans and toxicity in the rat cannot be extrapolated to humans.

Coumarin metabolism, toxicity and carcinogenicity: relevance for human risk assessment

The metabolism, toxicity and results of tests for carcinogenicity have been reviewed with respect to the safety for humans of coumarin present in foodstuffs and from fragrance use in cosmetic products. Coumarin is a natural product which exhibits marked species differences in both metabolism and toxicity. The majority of tests for mutagenic and genotoxic potential suggest that coumarin is not a genotoxic agent. The target organs for toxicity and carcinogenicity in the rat and mouse are primarily the liver and lung. Moreover, the dose-response relationships for coumarin-induced toxicity and carcinogenicity are non-linear, with tumour formation only being observed at high doses which are associated with hepatic and pulmonary toxicity. Other species, including the Syrian hamster, are seemingly resistant to coumarin-induced toxicity. There are marked differences in coumarin metabolism between susceptible rodent species and other species including humans. It appears that the 7-hydroxylation pathway of coumarin metabolism, the major pathway in most human subjects but only a minor pathway in the rat and mouse, is a detoxification pathway. In contrast, the major route of coumarin metabolism in the rat and mouse is by a 3,4-epoxidation pathway resulting in the formation of toxic metabolites. The maximum daily human exposure to coumarin from dietary sources for a 60-kg consumer has been estimated to be 0.02 mg/kg/day. From fragrance use in cosmetic products, coumarin exposure has been estimated to be 0.04 mg/kg/day. The total daily human exposure from dietary sources together with fragrance use in cosmetic products is thus 0.06 mg/kg/day. No adverse effects of coumarin have been reported in susceptible species in response to doses which are more than 100 times the maximum human daily intake. The mechanism of coumarin-induced tumour formation in rodents is associated with metabolism-mediated, toxicity and it is concluded that exposure to coumarin from food and/or cosmetic products poses no health risk to humans.

Anticarcinogenic, hypocholesterolemic, and antagonistic activities of Lactobacillus acidophilus

Lactobacillus acidophilus is considered to possess health-promoting attributes. These include anticarcinogenic and hypocholesterolemic properties and antagonistic action against intestinal and food-born pathogens. L. acidophilus can also survive the hostile environment and establish in the complex ecosystem of the gastrointestinal tract. Therefore, the beneficial effects of ingesting L. acidophilus accrue over a longer period than those organisms that cannot colonize the gut. However, the exact mechanisms of these attributes are not known. Presumably, the anticarcinogenic activity may be attributed to production of compounds and/or conditions that inhibit the proliferation of tumor cells, suppression of microorganisms that convert procarcinogens to carcinogens, and degradation of carcinogens formed. They hypocholesterolemic effect is probably exerted by inhibition of 3-hydroxy-3-methylglutaryl CoA reductase, which is a rate-limiting enzyme in endogenous cholesterol biosynthesis in the body and by promoting the excretion of dietary cholesterol in feces as a result of coprecipitation in the presence of deconjugated bile acids in the intestine and/or adsorption by the organisms. The antagonistic effect against pathogens and other organisms is possibly mediated by competition for nutrients and adhesion sites, formation of metabolites such as organic acids, hydrogen peroxide, and production of antibiotic-like compounds and bacteriocins.

Comparison of 3,5,3'-triiodo-L-thyronine and L-thyroxine absorption from the intestinal lumen of the fasted rainbow trout, Oncorhynchus mykiss

The absorptions of 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) from the intestinal lumen of the rainbow trout were compared in vivo. Tracer doses of [(125)I]T4 ((+)T4) or [(125)I]T3 ((*)T3) were injected through an anal cannula into the duodenum of trout fasted for 3 days at 12°C, and radioactivity was measured in blood and tissues at 4-48 h. (*)T3 was removed more extensively than (*)T4 from the intestinal lumen and more radioactivity was absorbed into the blood and tissues of u+T3-injected trout than (*)T4-injected trout. HPLC analysis showed that a high proportion of the radioactivity in the plasma, liver, kidney and intestinal lumen of (*)T3-injected trout remained as the parent (*)T3. However, in (*)T4-injected trout most plasma radioactivity was in the form of (125)I(-), and by 24 h a high proportion of luminal radioactivity was (125)I(-). By 48 h, over 4% of the injected (*)T3 and 1% of the injected (*)T4 dose resided in the gall bladder, primarily as derivatives of (*)T3 or (*)T4. We conclude that T3 is absorbed more effectively than T4 from the intestinal lumen of fasted trout, indicating the potential for an enterohepatic T3 cycle.

Absorption, distribution, and elimination of a single oral dose of [14C]tri-o-cresyl phosphate in hens

The absorption, distribution, elimination, and metabolism of a single oral dose of 50 mg (4.6 microCi)/kg of uniformly phenyl-labeled [14C]tri-o-cresyl phosphate (TOCP) was investigated in adult chickens. Three treated hens were killed at each time interval: 0.5, 1, 2, and 5 days. TOCP was absorbed from the gastrointestinal tract and subsequently distributed throughout the body. Generally, the highest concentrations of radioactivity were associated with gastrointestinal tract parts, bile, kidneys, liver, and lungs. Most of the radioactivity (47%) was excreted in the combined fecal-urinary excreta during the first 12 h. Very small fractions of the dose were deposited in egg albumen and egg yolk, 0.12% and 0.24%, respectively during the 5-day study. After 5 days, 99% of the dose was eliminated in excreta. TOCP and its metabolites in bile and the combined fecal-urinary excreta were analyzed by high-performance liquid chromatography and liquid scintillation spectrometry. TOCP and nine of its metabolites were identified. In the bile a TOCP active metabolite, saligenin cyclic-o-cresyl phosphate, was the predominant compound found compared to the parent compound in the excreta. These results suggest that in the hen TOCP is excreted slower than the rat and also undergoes metabolic activation. The absorption, elimination, and metabolic profile of TOCP in the hen may contribute to its sensitivity to delayed neurotoxicity.

Comparison of the metabolism and disposition of [3-14C]coumarin in the rat and marmoset (Callithrix jacchus)

Male Sprague-Dawley rats and marmosets were given a single oral 25 mg/kg dose of [3-14C]coumarin and the excretion of radioactivity in the expired air, urine and faeces monitored up to 96 h. Excretion profiles were similar in both species with the bulk of the dose being excreted in the urine and faeces within 24 h. Chromatographic analysis of 0-48 h urine samples revealed similar metabolic profiles with only small amounts of unchanged coumarin and very little 7-hydroxycoumarin. Coumarin 7-hydroxylase activity was not detectable in hepatic microsomes from either species. These results demonstrate that the disposition of [3-14C]coumarin was similar in the rat and marmoset, a New World primate, and that both species, unlike man, are poor 7-hydroxylators of coumarin.

Biliary excretion of linamarin in the Wistar rat after a single dose

The biliary excretion of linamarin (2[beta-D-glucopyranosyloxy]isobutyronitrile) was studied in male albino Wistar rats injected i.p. with single doses of 300 mg linamarin/kg following cannulation of the bile duct in vivo; 24 hr faeces of uncannulated rats, similarly dosed, was examined for excretory products. Enzymatic and spectrophotometric analyses of the bile exudate showed that glucosidic cyanide (linamarin, and non-glucosidic cyanide were excreted; the elimination of both cyanide forms exhibits biphasic kinetics. Thiocyanate ion was undetectable. T.l.c. of the test bile followed by enzymatic and chemical investigation of the chromatogram confirmed the presence of unchanged linamarin, and four different u.v. fluorescent non-glucosidic cyanide metabolites. Neither linamarin nor cyanide ion was detectable in faeces of the uncannulated rats.

Studies on the metabolism of the neurotoxic tri-o-cresyl phosphate. Distribution, excretion, and metabolism in male cats after a single, dermal application

The metabolism of a single, dermal dose of 50 mg/kg of [14C]tri-o-cresyl phosphate (TOCP) was studied in male cats. TOCP was applied to an unprotected, preclipped area on the back of the neck. Three animals were sacrificed on each of 0.5, 1, 2, 5, and 10 days following application. Radioactivity disappeared biexponentially from the dosing site with a faster initial rate; 73% of the dose disappeared in the first 12 h followed by a slower phase with a half-life of 2.03 days. No radioactivity was detected in the expired air. TOCP was absorbed from the skin and subsequently distributed throughout the body. Generally, the highest concentrations of radioactivity were associated with bile, gall bladder, urinary bladder, kidneys, and liver; the lowest were found in the neural tissues, muscle, and spleen. Within the 10-day experimental period, approximately 28% and 20% of the applied dose were recovered in the urine and feces, respectively. TOCP and its metabolites in the urine, feces, bile, and plasma were analyzed by high performance liquid chromatography and liquid scintillation counting. TOCP was the predominant compound in the feces (26.3% of total fecal radioactivity); it was detected in a smaller percentage in the urine (2.3% of total urinary radioactivity). The major metabolite in the urine was o-cresol followed by di-o-cresyl hydrogen phosphate and o-cresyl dihydrogen phosphate; in the feces di-o-cresyl hydrogen phosphate was the predominant metabolite followed by o-cresyl dihydrogen phosphate. Trace amounts of saligenin cyclic-o-tolyl phosphate, hydroxymethyl, and di(hydroxymethyl) TOCP were also detected in the urine and feces. Other metabolites identified in the urine and feces were the stepwise oxidation products of the methyl group of o-cresol. Unlike the feces, the bile contained mostly metabolites with only trace amounts of TOCP detected at 12 h and 24 h following application. o-Cresyl dihydrogen phosphate and di-o-cresyl hydrogen phosphate were the prevalent metabolites in the bile and plasma. While di(hydroxymethyl) TOCP was present in trace amounts in plasma, an appreciable amount of saligenin cyclic-o-tolyl phosphate, believed to be the active neurotoxic metabolite, was detected. This study shows that skin is an important port of entry for TOCP. Since TOCP represents organophosphorous chemicals capable of producing delayed neurotoxicity in test animals and in humans, it is essential that industrial hygiene control prevents skin contamination of workers handling these chemicals.

The effect of diet on the mammalian gut flora and its metabolic activities

The review will encompass the following points: A brief introduction to the role of the gut flora in the toxicology of ingested food components, contaminants, and additives, including known pathways of activation and detoxication of foreign compounds and the implication of the flora in enterohepatic circulation of xenobiotics. The advantages and disadvantages of the various methods of studying the gut flora (classical bacteriological techniques, metabolic and enzymological methods) will be critically discussed with special reference to their relevance to dietary, toxicological, and biochemical studies. Sources of nutrients available to the gut flora will be described including host products (mucus, sloughed mucosal cells, hormones, proteins) and exogenous nutrients derived from diet. An account of the problems involved in studies of dietary modification with special reference to the use of stock laboratory animal diets, purified diets, and human dietary studies. The influence of dietary modification on the flora will be assessed on the basis of changes in numbers and types of bacteria and their metabolic activity, drawing on data from human and animal studies. The effects of manipulation of the quantity and quality of protein, fat, and indigestible residues (fiber) of the diet will be described together with their possible implications for toxicity of ingested compounds.

Pharmacokinetic analysis of concentration-time data obtained following administration of drugs that are recycled in the bile

A pharmacokinetic model for calculating the pharmacokinetic parameters for a compound that is recycled in the bile is presented and tested using theoretical as well as experimental data. The results indicate that this method is stable and only slightly susceptible to sampling and recycling times. It is apparent from the present study that pharmacokinetic terms that have been used in classical situations are not directly applicable to drugs that enter the enterohepatic circulation. Effective half-life and effective clearance are used to describe the intrinsic ability of the eliminating organs to remove drug from the blood, whereas net half-life and net clearance are used to describe the irreversible elimination of the drug from the body.

Phenobarbital-digitoxin interaction in the guinea pig liver

The influence of phenobarbital pretreatment on liver concentration of digitoxin and its metabolites was studied in guinea-pigs after i.p. administration of the cardiac glycoside. During the first hour an increase in liver uptake was observed in pretreated animals. The differences detected in the hepatic subcellular distribution do not seem to explain the higher concentrations found in the liver of phenobarbital pretreated animals. About 80% of the liver radioactivity was found in the supernatants. Inhibition of digitoxin biotransformation by phenobarbital was demonstrated by chromatographic analysis of the organic soluble compounds present in the supernatants. The possible binding of digitoxin and its metabolites to soluble proteins of liver cytosol was excluded by thin-layer gel filtration. The decrease in digitoxin biotransformation seems to be the reason for the increase in liver uptake and for the decrease in bile concentrations, observed in phenobarbital pretreated animals.

A review of the genotoxicity of food, drug and cosmetic colours and other azo, triphenylmethane and xanthene dyes

The genetic toxicology of the major dyestuffs used in foods, drugs and cosmetics has been reviewed. Published data for azo, triphenylmethane and xanthene dyes from short-term assays for muta-carcinogenicity have been summarized and discussed according to usage, current and previous worldwide legislative status. Certain other synthetic food dyes, commercial mixtures, natural and polymeric colourants as well as a section on aminoazobenzene and its derivatives have been included. Genotoxicity has been discussed with reference to structural chemistry, levels of exposure, absorption and metabolism and to epidemiological information. The extent of agreement between data from different tests and correlations with animal cancer assays have been considered. Synthetic dyes from the 3 major structural classes exhibit genotoxicity, whilst only 2 natural colours have proved active. Activity may be due to the presence of certain functional groups, notably nitro- and amino-substituents which are metabolized to ultimate electrophiles that may be stabilized by electronic interaction with aryl rings. Metabolic processes such as azo-reduction may be activating or detoxifying. the low but significant correlation between animal carcinogenicity and short-term test data may be increased with further screening, especially involving chromosome assays. It is suggested that a human cancer hazard may exist where significant quantities of finished benzidine dye samples are handled. Such risks from exposures to other colours and the possibility of human germ-line mutation induction by dyestuffs cannot be meaningfully assessed.

Enterohepatic recycling of phenolphthalein, morphine, lysergic acid diethylamide (LSD) and diphenylacetic acid in the rat. Hydrolysis of glucuronic acid conjugates in the gut lumen

1. Biliary elimination in female Wistar albino rats 3 h after i.p. injection of [3H]phenolphthalein, [3H]morphine, 14C-LSD and [14C]diphenylacetic acid was 90%, 45%, 75% and 57% respectively, predominantly as glucuronides. 2. Infusion of 3 h bile from the previous experiments into the duodena of bile-duct-cannulated animals demonstrated enterohepatic circulation, amounting in 24 h to 85%, 41%, 28% and 66% of the infused doses of the conjugates of phenolphthalein, morphine, LSD and diphenylacetic acid respectively. 3. Pretreatment with antibiotics to suppress intestinal microflora decreased this enterohepatic recirculation to 22%, 8.6% and 21% in 24 h for phenolphthalein, morphine and diphenylacetic acid glucuronides respectively. Antibiotic pretreatment did not influence the absorption and re-excretion of infused doses of the free aglycones, thus demonstrating the importance of bacterial beta-glucuronidase hydrolysis of the biliary conjugates. 4. The extent of intestinal absorption of the aglycones after bacterial beta-glucuronidase hydrolysis of the conjugates is related to their lipid-solubility as estimated by octan-1-ol:0.1 M phosphate buffer partition ratios (P-values). 5. The persistence of compounds in the enterohepatic circulation is determined by the faecal and urinary elimination of the circulating compounds. Faecal elimination is governed by the extent of intestinal absorption of the circulating compounds, which is influenced by the efficacy of intestinal hydrolysis of the conjugates and the relative lipophilicity of the aglycones released.

Studies of metabolic fate of a new antiallergic agent, azelastine (4-(p-chlorobenzyl)-2-[N-methylperhydroazepinyl-(4)]-1-(2H)-phthalazinone hydrochloride)

The metabolic fate of a new antiallergic agent, azelastine (4-(p-chlorobenzyl)-2-[N-methylperhydroazepinyl-(4)]-1-(2H)-phthalazinone hydrochloride) in rats and guinea pigs was investigated using its 14C-labelled compound. The blood level of radioactivity reached the maximum at 1-1.5 hr after oral administration, indicating the rapid absorption of the drug from gastrointestinal tract. A high concentration of radioactivity was detected in the lung of both species following either oral or intravenous administration. The major pathway of excretion of radioactivity was by way into feces, in both species. The radioactivity excreted in feces was attributable to that which was excreted in bile and exsorbed into gastrointegtinal tract. When the drug was given to pregnant rats, the concentration of radioactivity in the fetus was significantly lower than those in placenta and uterus, indicating the limited placental transfer of the drug. The successive oral administration of the drug in lower doses exerted no effect on the activity of microsomal drug-metabolizing enzymes of rat liver, while in higher doses, had a slight effect.

Steroid absorption and enterohepatic recycling

A short review on steroid absorption and enterohepatic recycling in man with special emphasis on contraceptive and related steroids is presented. Some new experimental data on the intestinal metabolism of steroids is described and includes further observations on the effect of antimicrobial agents on steroid hormone metabolism. Evidence is presented that plasma levels of steroids may be influenced if the intestinal microflora is altered. Some formation of biologically active steroids, like estradiol, may occur in the intestinal tract and this may have both biological and pathological consequences and may be influenced by such factors as diet and sex. It is concluded that our knowledge of the intestinal and especially, the mucosal metabolism of steroids is scanty and further studies are needed to clarify the role of the intestine and enterohepatic circulation in determining the bioavailability of natural and synthetic steroids.

Disposition of ORF 9326, a novel contragestational steroid, in animals

The disposition of ORF 9326 [17BETA-acetoxy-2alpha-chloro-3(p-nitrophenoxy) imino-5-androstane], an O-aryl oxime of 2beta-chlorodihydrotestosterone acetate, was studied in rats, dogs, monkeys and rabbits. Intravenous administration of 3H-ORF 9326 dissolved in PEG-400 to rats, dogs and monkeys resulted in a rapid decline of radioactivity in blood followed by a terminal slope suggesting long retention of radioactivity. Apparent half lives of radioactivity in blood were calculated to be from 50--95 hours for the three species, which peak levels of radioactivity in whole blood occurring within 4--7 hours after administration of the compound. Tissue distribution studies in the rat and dog indicate that body fat is one of the major depot areas for the drug and/or its metabolites. The major route of excretion for ORF 9326 and/or its metabolites in dog and rat is biliary whereas in monkey and rabbit it appears to be renal. Greater than 90% of the radioactive compounds excreted in the urine of dogs and monkeys following intravenous administration of 3H-ORF 9326 appear to be in the form of conjugates.

Disposition and metabolism of benoxaprofen in laboratory animals and man

1. The absorption, distribution, metabolism and excretion of benoxaprofen, a novel anti-inflammatory compound, has been studied in the dog, mouse, rat, rabbit, rhesus monkey and man. 2. Benoxaprofen was well absorbed after oral administration of doses of 1 to 10 mg/kg in all six species. Only unchanged drug was detected in plasma. It was extensively bound to plasma proteins, the highest binding occurring in man (99.8%) and rhesus monkey (99.6%). 3. Species differences were observed in the plasma elimination half-life, the longest being in man (33 h). The rat and mouse also had high values (28 and 24 h respectively) whereas in the other species, values were less than 13 h. 4. After an oral dose of [14C]benoxaprofen (20 mg/kg) to female rats, tissue concn. was highest in liver, kidney, lungs, adrenals and ovaries. Tissue distribution in the pregnant rat was identical to the normal female. The compound was found in the foetus but at a concn. lower than in all maternal organs. 5. There was a marked species difference in the route of excretion. In man, rhesus monkey and rabbit, excretion in the urine was a major route, whilst biliary--faecal excretion was the only effective route in the rat and dog. 6. No major metabolic transformation of benoxaprofen was observed. Man and dog excreted the compound predominantly as the ester glucuronide whereas the rat, mouse, rabbit and rhesus monkey excreted a large proportion of the dose unchanged.

Selectivity of food colours for different organic acid transport systems in rat renal cortex

Seventeen food colours were tested as inhibitors of the simultaneous uptake of labelled o-iodohippurate and iodipamide into slices of rat renal cortex. The results have been interpreted in terms of inhibition of two different aniontransport systems: the hippurate of H-system and the liver-like L-system. No clearcut relation was found between inhibition ability and system specificity on the one hand and moleculr weight or structure on the other. However, most disulphonic azo dyes and quinoline yellow show an L-system affinity while tri-and tetra-sulphonic azo dyes, triarylmethanes and erytrosine show very little predilection for either of the two systems.

Pharmacokinetics of a neurotoxic oral dose of leptophos in hens

A pharmacokinetic profile of [14C] leptophos was determined in laying hens following a single oral dose of 400 mg/kg (0.58 muCi). Most of the radioactivity was excreted into the urine and feces. After 15 days, the total radioactivity recovered in the combined urinary-fecal excretion was 73.5% of the administered dose. A major part of the absorbed leptophos was concentrated in the eggs. The total 14C radioactivity in egg albumen was 7.29% of the administered dose as compared to 4.67% in egg yolk. The half-life for the disappearance of radioactivity from the birds' bodies following the administration of [14C] leptophos was 11.55 days. A correlation between the pharmacokinetics of leptophos and the susceptibility of the hen to delayed neurotoxicity is discusses.

Excretion of phenol red and its glucuronide in the dogfish shark

1. The biliary and urinary excretion of phenol red and its glucuronide was measured in dogfish sharks, Squalus acanthias, in both intact animals and animals with biliary fistula. 2. Phenol red is extensively metabolized to its glucuronide by the dogfish shark and both forms of the compound are actively transported into bile and urine. 3. Both compounds are transferred from plasma to urine and bile against a large concentration gradient; the transfer process is saturable most easily in the renal compartment but also in the hepatic compartment; and both excretion routes for the free drug and the glucuronide are inhibited by probenecid. 4. There were no significant differences in the 48 h biliary excretion of total (free + glucuronide) phenol red, but the urine or intact fish contained two-fold more total drug than did animals with fistulae.

Biliary excretion of xenobiotics

The biliary route is very important for the elimination of some foreign compounds from the body. For many of these compounds, an increase in the rate at which they are excreted into the bile will decrease their toxicity and vice versa. A number of factors which are known to alter the biliary excretion of xenobiotics, as well as the current concepts of the physiological mechanisms responsible for the excretion of foreign compounds, have been enumerated. However, much remains still to be understood; essentially nothing is known at the subcellular level about the biliary excretion of foreign compounds. It has recently been concluded that our knowledge of the biliary excretion of compounds is about 40 years behind that of the renal excretion mechanism.