Biliary excretion of foreign compounds. Biphenyl, stilboestrol and phenolphthalein in the rat: molecular weight, polarity and metabolism as factors in biliary excretion

Metabolism of 4-benzamido-1-[4-(indol-3-yl)-4-oxobutyl]piperidine in rats and monkeys

1. Studies on the absorption, biotransformation and excretion of the potential anti-hypertensive agent 4-benzamido-1-[4-(indol-3-yl)-4-oxobutyl]piperidine (Wy 23699) have been carried out in monkeys and rats. 2. Absorption of the drug in both species was good, as shown by the relative proportions of radioactivity found in the urine after i.v. and oral dosage. 3. Biotransformation was extensive in both species, but the major routes of metabolism were different. In monkey N-dealkylation gave rise to 4-benzamidopiperidine as the principal metabolite; two other minor metabolites were identified as 2-oxo-4-benzamidopiperidine and 4-benzamido-1-[4-(5-hydroxyindol-3-yl)-4-oxo-butyl]piperidine. In the rat, this latter compound (as its conjugate) was the major metabolite; small amounts of benzamidopiperidine but not 2-oxobenzamidopiperidine were found. 4. There was a marked species difference in the route of excretion. Monkeys excreted greater than 60% dose of the drug in the urine, while rats excreted only 19% by this route. This may reflect the species difference in routes of metabolism, metabolic scission of the drug molecule seen in the monkey favouring renal rather than biliary excretion. The toxicological consequences of this are discussed.

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.

Kinetic measurements of the biliary excretion of metabolized compounds

1. Rats received constant infusions of bromosulphophthalein or [carboxy-14C]cholic acid at a range of concentrations. 2. Kinetic parameters describing the biliary excretion of the compounds were determined. 3. The biliary excretion of both compounds could be described by the same kinetic parameters already calculated for phenolphthalein disulphate, which is excreted in the bile unchanged.

Biliary excretion of drugs in man

Biliary excretion is an important route for the elimination of some drugs and drug metabolites in man. The factors which determine elimination via the biliary tract include characteristics of the drug such as chemical structure, polarity and molecular size as well as characteristics of the liver such as specific active transport sites within the liver cell membranes. A drug excreted in bile may be reabsorbed from the gastrointestinal tract or a drug conjugate may be hydrolysed by gut bacteria, liberating original drug which can be returned to the general circulation. Enterohepatic circulation may prolong the pharmacological effect of certain drugs and drug metabolites, but the quantitative importance of this in man appears to be less than in animals. Biliary elimination may play a role in the interindividual differences in drug response observed in healthy subjects and in patients with certain diseases. Cholestatic disease states, in which normal bile flow is reduced, will influence drug elimination by this route resulting in increased risk of drug toxicity. Bile may serve as an alternate route of elimination in renal failure, but this has not been determined in man. Lack of reliable information regarding the biliary excretion of drugs in man is partly due to the relative inaccessibility of the human biliary tract. Most studies of drug excretion in human bile have been performed in post-surgical patients with T-tube drainage. This method of bile collection is not ideal because bile flow and composition are often severely altered during the period of study, not all bile is collected and enterohepatic circulation is partially interrupted. Recent advances in the methods of collection of bile may improve future studies of drug excretion in human bile.

Uptake of glucuronides into isolated hepatocytes and their effects on glucuronide and sulphate conjugation

Uptake studies, using radioactive labelled glucuronides, have demonstrated the ability of 4-nitrophenyl glucuronide and phenolphthalein glucuronide to enter isolated rat hepatocytes. Of these glucuronides 4-nitrophenyl glucuronide was distributed in a similar manner to O-methylglucose, whereas phenolphthalein glucuronide was bound to cellular constituents. Phenolphthalein glucuronide had an effect on theconjugation of harmol in the isolated hepatocytes when glucuronidation was found to be markedly inhibited and sulphation slightly stimulated. The glucuronidation of 4-nitrophenol, 4-methylumbelliferone and harmol in native microsomes was inhibited by phenolphthalein glucuronide. 4-Nitrophenyl glucuronide and also naphthyl glucuronide were without effect both in hepatocytes and microsomes. In control hepatocytes harmine was metabolized to form harmolsulphate mainly. Phenolphthalein glucuronide only affected this metabolic pattern to a minor extent. However, in hepatocytes from phenobarbital treated rats, where the rate of harmine metabolism is increased about five times and the main metabolite is harmol glucuronide, phenolphthalein glucuronide inhibited the formation of the conjugate with a concomitant increase in free harmol.

Metabolism of arylacetic acids. 3. The metabolic fate of diphenylacetic acid and its variation with species and dose

1. [carboxy-14C]Diphenylacetic acid has been administered to seven primate species including man, and four other mammals and the qualitative and quantitative aspects of its elimination determined. 2. In most species, 50-100 percent of the administered 14C was excreted in the urine in 48 h; 2-30 percent of the dose was recovered unchanged in the 24 h urine. 3. In all species the only urinary metabolite detected by radiochromatogram scanning was diphenylacetylglucuronide (10-70 percent of dose). Reverse isotope dilution additionally revealed the formation of trace amounts (less than 1 percent of dose) of the glycine conjugate by four species and of the taurine conjugate by the cat. No evidence was found for the formation of a glutamine conjugate. 4. The influence of dose on the pattern of metabolism and excretion of diphenylacetic acid has been studied in the rat. In this species diphenylacetic acid undergoes extensive elimination and enterohepatic circulation.

Metabolism of arylacetic acids. 1. The fate of 1-naphthylacetic acid and its variation with species and dose

1. [Carboxy-14C]-1-Naphthylacetic acid has been administered to man, 6 primate species and 4 other mammalian species and the urinary metabolites examined by radiochromatogram scanning and reverse isotope dilution. Animals all received a dose of 100 mg/kg and man received 5 mg, orally. 2. Most species excreted at least 60% of the 14C in the urine in 48 h. Unchanged acid was a minor (0-17% dose) excretion product in all species except the cynomolgus monkey (35%). 3. In man, in 24 h 95% of 14C was excreted as 1-naphthylacetyl-glucuronide and 5% as 1-naphthylacetyltaurine. 4. 1-Naphthylacetylglucuronide was the major excretion product in all species except the bushbaby (21% dose) and the cat, which did not form this conjugate. 5. 1-Naphthylacetylglutamine was formed only by the cynomolgus, squirrel and capuchin monkeys and marmoset, and in no case accounted for more than 3% dose. 6. 1-Naphthylacetylglycine was found in the urines of 4 primate and 3 non-primate species, and was the major metabolite in the squirrel monkey, bushbaby and cat. 7. 1-Naphthylacetyltaurine was excreted by all species except the rabbit and the fruit bat. It was a major excretion product in the squirrel and capuchin monkeys, the marmoset and the cat. 8. The influence of dose on the pattern of metabolism and excretion of 1-naphthylacetic acid has been investigated in the rat.

Metabolism of arylacetic acids. 2. The fate of [14C]hydratropic acid and its variation with species

1. (+/-)-[methyl-14C]-Hydratropic acid was administered to man, rhesus monkey, cat, rabbit and fruit bat. 2. All species excreted 60-100% of administered 14C in the urine in 24 h, and unchanged hydratropic acid accounted for 0-17% of the dose. 3. In man, the urinary 14C consisted of a very small quantity (1%) of unchanged hydratropic acid with the remainder as hydratropylglucuronide. 4. Hydratropylglucuronide was the major urinary excretion product in the 4 animal species, while the glycine conjugate was present in the urine of cat and rat. Additionally, cats excreted the taurine conjugate of hydratropic acid. 5. Bile-duct cannulated rats excreted 20-30% of an injected dose of [14C] hydratropic acid in the bile in 3 h mainly as hydratropylglucuronide.

Fate and distribution of the herbicides 2,4-dichlor-phenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in the dogfish shark

1. The urinary and biliary excretion, tissue distribution and metabolism of 14C-labelled 2,4-dichloro- or 2,4,5-trichloro-phenoxyacetic acids (2,4-D or 2,4,5-T) were measured in dogfish sharks, Squalus acanthias. 2. Both herbicides are extensively metabolized (greater than 90%) to the corresponding taurine conjugates, and are excreted predominantly via the urine, where ca. 70% of the administered dose appears within 4-6 days after treatment. 3. The highest tissue levels of 2,4-D or 2,4,5-T were found in liver and kidney. Penetration of both herbicides into the CNS was restricted. 4. Plasma elimination was rapid and the 0.5 for either phenoxyacetic acid was less than 45 min. Similarly, rapid clearance as seen from renal tissue. Final t0.5 values for muscle were about 2-3 days while the major organ showing 2,4-D or 2,4,5-T retention was the liver, where t0.5 values were about 5 days for both the herbicides. 5. The overall pharmacokinetics in the dogfish shark for these herbicides resembled those seen in some mammals.

The metabolism of biphenyl. IV. Phenolic metabolites in the guinea pig and the rabbit

The phenolic metabolites of biphenyl in guinea pigs and rabbits were qualitatively and quantitatively analysed as trimethylsilyl (TMS) ethers by combined gas chromatography/mass spectrometry and gas chromatography, respectively. The parent compound was hydroxylated to monohydroxylated biphenyls and minor amounts of dihydroxylated derivatives, and the main route of body clearance appeared to be by the urine in both species. Thus, in the urine of guinea pigs 32.9% of the dose was detected 96 hrs after dosing, while the major part (29.5%) was eliminated during the first day as conjugates. The main metabolite was 4-hydroxybiphenyl (25.5%). During the first 24 hrs faecal recovery was 20.3% of the dose, and most of this (14.3%) consisted of biphenyl itself. Biliary excretion of the metabolites of biphenyl origin amounted to 3.3% of the dose during the first day, and 4-hydroxybiphenyl was the major metabolite. In the urine of rabbits 49.1% of the dose was recovered 96 hrs after dosing, and most of this (25.4 and 15.9%, respectively) was eliminated during the first two days as conjugates. The major metabolite was 4-hydroxybiphenyl (35.3%). On the first day faecal recovery was 1.6%, of which 1.4% was detected as biphenyl itself. Less than 1% of the dose was found in the 7 hrs rabbit bile, and exclusively as 4-hydroxybiphenyl. The experiments thus show that both qualitative and quantitative differences in the metabolism of biphenyl exist between the guinea pig and the rabbit even though 4-hydroxybiphenyl was the most prominent metabolite of biphenyl in both species.

Streptozotocin: its excretion and metabolism in the rat

The excretion of radioisotope following the administration of three specifically 14C-labelled forms of streptozotocin was investigated in the rat using ureter and bile duct cannulation techniques. The urine collected during the first hour following the administration of the drug contained the highest proportion of injected radioactivity (approximately 34% with (3'-methyl-14C)-streptozotocin and approximately 40% each with (1-14C)-and (2'-14C)-streptozotocin. Over the entire experimental period (6 hours), approximately 70% of the injected radioactivity of (1-14C)- and (2'-14C)-streptozotocin appeared in the urine. With (3'-methyl-14C)-streptozotocin, only 53% of the injected radioactivity appeared in the urine over the same period. In contrast to the high urinary excretion, less than 3% of the injected radioactivity from all three radiolabelled streptozotocin samples appeared in the bile. The in vivo and in vitro metabolism of streptozotocin was also investigated. In addition to substantial amounts of unchanged drug, three radiolabelled metabolites (two major and one minor) were detected in the urine during the 6 hour collection period following the administration of (1-14C)- and (2'-14C)-streptozotocin. In contrast, only unchanged (3'-methyl-14C)-streptozotocin was detected in the urine collected over the same period following the administration of the methyl labelled drug. The two major metabolites were also produced when (1-14C)-and (2'-14C)-streptozotocin were incubated with a rat liver supernatant fraction (100,000 X g). The liver was further demonstrated to be the major site of metabolism in isolated liver perfusion studies in which both (1-14C)- and (2'-14C)-streptozotocin were quantitatively converted to the two major metabolites. The two major metabolites of (1-14C)-streptozotocin, whether produced in vivo or in vitro, were chromatographically homogenous with the two major metabolites formed from (2'-14C)-streptozotocin. Nicotinamide pretreatment had no apparent effect on the urinary excretion of streptozotocin and its metabolites.

The properties of uridine diphosphate glucuronyltransferase(s) which catalyse the synthesis of steroid glucuronides in microsomal fractions from guinea-pig liver

The properties of the UDP-glucuronyltransferase(s) of guinea-pig liver that catalyse the synthesis of steroid glucuronides were examined. There are many similarities between apparently different substrate-specific forms of these enzymes in that all are activated by bivalent metal ions, and all contain at least 2 thiol groups important for enzyme activity. On the other hand, there are significant differences between the enzymes conjugating steroids and those conjugating non-steroids. Only the latter are activated by UDP-N-acetylglucosamine, which enhances their relatively poor affinity for UDP-glucuronic acid. The steroid-conjugating forms of UDP-glucuronyltransferase are not activated by UDP-N-acetylglucosamine and have relatively high apparent affinities for UDP-glucuronic acid. The rate of glucuronidation of testosterone was inhibited by treatment with phospholipase A. Treatment with cholate or Triton X-100 did not enhance the rates of glucuronidation of any steroid tested. The data indicate several similarities between different forms of UDP-glucuronyltransferase, suggesting that there is a large family of related proteins. At the same time there are important differences in the parameters that modulate the rates of different glucuronidation reactions.

Metabolism of nitrosamines in vivo. V Investigation on 14CO2 exhalation, liver RNA labelling and isolation of two metabolites from urine after administration of [2, 5-14C-]dinitrosopiperazine to rats

The synthesis of N, N'-dinitrosopiperazine and N-nitrosopiperazine, both 14C-labelled in the 2-and 5-position is described. After i.p. application of 10 mg/69.5 muCi/kg[2, 5-14C]-N, N'-dinitrosopiperazine to rats no labelled 7-methylguanine was detected in the liver RNA; 1% of the radioactivity was exhaled as 14CO2, 1% excreted via the bile and about 40% excreted in the urine. Two of the urine metabolites were identified as 3-hydroxynitrosopyrrolidine and 1-nitrosopiperazinone-(3).

Bile and urine as complementary pathways for the excretion of foreign organic compounds

1. The urinary and biliary excretion in the rat of 30 aromatic compounds with mol. wt. of 100-850, and largely excreted unchanged, has been studied. 2. These compounds fall into three groups as regards their pattern of elimination, which is related to mol. wt: group 1, with mol. wt. less than 350 and the major route of elimination the urine. When urinary excretion is prevented by ligating the renal pedicles the biliary excretion remains low. group 2, with mol, wt. of 450-850 which are excreted predominantly in bile. Even when the bile duct is obstructed, only small amounts of these compounds are found in urine. group 3, with mol. wt. of 350-450, which are eliminated extensively in both urine and bile. When one of these routes is blocked excretion by the other increases. 3. These studies emphasize the interrelationship of urine and bile as excretory routes for organic compounds. Urine and bile are complementary pathways; the extent of urinary excretion is greatest for the compounds of lowest mol. wt. and tends to decrease as mol. wt. increases and biliary excretion becomes more extensive.

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.

Chemical carcinogenesis and the pancreas

Although carcinoma of the pancreas is an increasingly prevalent form of human cancer, there has been relatively little experimental work on the etiology of this tumor until recently, probably because of the lack of adequate experimental models. However, at least two good experimental systems are now available. Several epidemiologic investigations suggest that chemical carcinogens may induce cancer of the pancreas in humans. If this is so, chemicals must reach the pancreas either through the blood supply or by reflux into the pancreatic duct from bile or duodenal contents. The route of exposure may vary according to the chemical and physical characteristics of different chemical carcinogens. Additional work is needed to determine the ability of different classes of chemicals to reach the pancreatic duct by these routes and the presence of enzymes required for activation of carcinogens in the pancreas. Levels of such enzymes as well as the response of cells of the pancreatic duct to carcinogens may be affected by the physiologic state of the pancreas or pathologic conditions within it. Research is needed to investigate these possibilities.

Choleresis and hepatic transport mechanisms. II. Influence of bile salt choleresis and biliary micelle binding on biliary excretion of various organic anions

To investigate, whether binding to micelles has a function in hepatic transport, biliary excretion of three organic anions, phenolphthalein-beta-D-glucuronide (PG), dibromosulphthalein (DBSP) and indocyanine green (ICG) was studied in rats during saline, taurocholate or dehydrocholate administration. Taurocholate causes a weak choleresis with formation of biliary micelles, dehydrocholate a strong choleresis with little micelle formation. The two bile salts did not uniformly influence biliary excretion of the organic anions: biliary excretion of ICG (12.9 mumoles/kg) and DBSP (75.0 mumoles/kg) was stimulated by both bile salts: ICG excretion most pronounced by taurocholate and DBSP excretion most strongly by dehydrocholate. Biliary output of PG (25.8 and 200 mumoles/kg) was not stimulated by bile salt administration. Binding of PG, DBSP and ICG to biliary micelles was studied in sedimentation experiments by ultracentrifugation. PG, DBSP and ICG in bile showed a similar sedimentation pattern as 3H-taurocholate in bile, which indicates an association of all three anions with biliary micelles. Thus, the influence of bile salts on biliary transport of organic anions varies with the compound studied and the bile salt used, effects which cannot be explained by differences in binding to biliary micelles.

Biliary excretion kinetics of phenolphthalein glucuronide after intravenous and retrograde biliary administration

Several organic compounds of large molecular weight have previously been shown to be rapidly and primarily excreted via the biliary system of the rat after intravenous or retrograde biliary infusion. Poor biliary reabsorption has been suggested to explain these findings. The kinetics of the biliary excretion of Phenolphthalein glucuronide have been examined with concurrent plasma data. A spectrophotometric method, capable of measuring Phenolphthalein glucuronide in amounts as small as 4 nmol per 100 μl of plasma, was developed. The glucuronide (20–40 μmol kg−1) was administered to 14 rats intravenously and to 12 rats by retrograde biliary infusion. There was a significant concentration of Phenolphthalein glucuronide in the systemic blood after glucuronide administration by either route and the kinetics of elimination of the glucuronide were similar. The plasma availability of biliary infused doses was over 45 % of the availability from the intravenous doses based on area under the curve calculations for the average plasma level-time curves. The results demonstrate the need to sample the plasma as well as the bile before any conclusion can be made about reabsorption of a compound from the biliary ducts.

Biliary excretion of some diquaternary ammonium cations in the rat, guinea pig and rabbit

1. The extent of the excretion in the bile and urine of the (14)C-labelled dications, diquat, paraquat, morfamquat, decamethonium and dimethyltubocurarine in bile-duct-cannulated rats, guinea pigs and rabbits was examined. 2. These compounds were excreted unchanged in bile and urine, except diquat, which was metabolized to a significant extent (18% of the dose) in the rabbit only. 3. The extent of the biliary excretion of diquat (mol wt. of ion 184), paraquat (186), decamethonium (258) and morfamquat (469) was less than 10% of the dose in the three species, whereas that of dimethlytubocurarine (653) was greater than 10% in the rat and rabbit but not in the guinea pig. 4. These results together with data from the literature suggest that the molecular weight at which the excretion of dications in the bile exceeds 10% of the dose is in the region of 500-600, which differs from the values for monocations (Hughes et al., 1973) and anions (Millburn et al., 1967; Hirom et al., 1972).

Molecular weight as a factor in the excretion of monoquaternary ammonium cations in the bile of the rat, rabbit and guinea pig

1. The excretion in the bile and urine of intraperitoneally injected (14)C-labelled monoquaternary ammonium or pyridinium cations was measured in bile-duct-cannulated rats (ten compounds) and in guinea pigs and rabbits (six compounds). 2. Seven of these, namely N-methylpyridinium, tetraethylammonium, trimethylphenylammonium, diethylmethylphenylammonium, methylphenyldipropylammonium, dibenzyldimethylammonium and tribenzylmethylammonium, were excreted largely unchanged in the bile and urine. 3. 3-Hydroxyphenyltrimethylammonium, 3-bromo-N-methylpyridinium and cetyltrimethylammonium were metabolized to an appreciable extent in the rat. 4. In intact rats intraperitoneally injected trimethylphenylammonium (mol.wt. 136) was excreted mainly in the urine, dibenzyldimethylammonium (mol.wt. 226) was excreted in roughly equal amounts in the urine and faeces, and tribenzylmethylammonium (mol.wt. 302) was excreted mainly in the faeces. The faecal excretion of these compounds corresponded to their biliary excretion in bile-duct-cannulated rats. About 3-4% of tribenzyl[(14)C]methylammonium was eliminated as (14)CO(2). 5. In rats the extent of biliary excretion of four cations with molecular weights in the range 94-164 was less than 10% of the dose, whereas that of five cations with molecular weights 173-302 was greater than 10%. These results and other data from the literature suggested that the molecular weight needed for the biliary excretion of such cations to an extent of 10% or more of the dose was about 200+/-50. Studies with six cations in guinea pigs and rabbits suggest that this value applies also to these species. 6. The results suggest that the threshold molecular weight for the appreciable (>10%) biliary excretion of monoquaternary cations is different from that for anions (Millburn et al., 1967a; Hirom et al., 1972b). With rats, guinea pigs and rabbits, no significant species difference was noted, whereas with anions there is a marked species difference.