A. G. Renwick

The metabolism of intense sweeteners

Three organic acids (saccharin, acesulfame-K and cyclamate) are used or have been used extensively as intense sweeteners. Once absorbed from the gut they are eliminated, largely in the urine, without undergoing metabolism. Early studies using radiolabelled saccharin indicated the existence of limited metabolism, but this was not confirmed by later more extensive studies using highly purified compound. Metabolism could not be induced by a variety of pretreatments. Following an initial report of the presence of traces of cyclohexylamine in the urines of subjects given cyclamate, it was shown that chronic administration of the sweetener caused the induction of extensive metabolism. The metabolism, which showed wide inter- and intra-individual variability was performed the gut microflora. The peptide sweeteners (aspartame and thaumatin) are metabolized to their constituent amino acids in the gastro intestinal tract, prior to absorption. As such they are incorporated into normal intermediary metabolism and their low-calorie applications derive from their intense sweetness.

The pharmacokinetics of saccharin in man

1. After intravenous administration of the non-nutritive sweetener, saccharin (10 mg/kg), to normal volunteers; the plasma concentration--time curve fitted a two-compartment open model with a terminal half-life of 70 min. Renal clearance was high and the dose was recovered quantitatively in the urine. The elimination rate and clearance were decreased significantly by concurrent probenecid administration. 2. After oral administration (2 g) more complex and variable plasma concentration--time curves were obtained and these were reflected in the urinary excretion of saccharin. The fraction absorbed was about 0.85 as determined by the recovery in urine and the area under the plasma concentration--time curves. 3. No indication of saturation of renal elimination was found after oral or intravenous doses that were many times the average daily intake.

The reduction of nitrobenzoic acids in the rat.

1. 4-Nitrobenzoic acid was excreted in rat urine as 4-aminobenzoic acid and its conjugates after oral (20% dose) and intraperitoneal (17% dose) administration. Suppression of gut microflora by oral administration of antibiotics decreased the reduction in vivo (5% dose was reduced after high dose antibiotics and 13% after low dose antibiotics). The liver, intestinal wall and content of the caecum and colon reduced 4-nitrobenzoic acid extensively in vitro. Pre-treatment with antibiotics decreased the activities of intestinal wall and hind gut contents but not of liver.2. 3-Nitrobenzoic acid was excreted in rat urine as 3-aminobenzoic acid and its conjugates after oral administration to normal (16% dose) and low dose antibiotic-treated animals (7% dose). 2-Nitrobenzoic acid underwent less reduction in vivo in normal animals (10% dose) and this was largely suppressed by low dose antibiotic treatment (1% of dose). The abilities of liver, intestinal wall and caecum and colon contents to reduce 3-nitrobenzoic ac...

The metabolism of 14C-cyclohexylamine in mice and two strains of rat

After administration of 14C-cyclohexylamine (35-500 mg/kg) to male mice and rats, 80% of the dose of 14C was excreted in the urine, mostly within the first 24 h after dosing. In Wistar rats, 7-9% of the 14C in the 0-24 h urine was present as cis-4-aminocyclohexanol, with a similar amount as the corresponding 3-isomers. In the DA rat, only 1-2% of the 14C, and in mouse less than 1% of the 14C was present in the urine as aminocyclohexanols; unchanged cyclohexylamine accounted for about 95% of the activity. The extent of metabolism was not affected by either dose or route of administration. The species differences in metabolism may be implicated in the differences in toxicity during chronic high-dose administration.

The site of reduction of sulphinpyrazone in the rabbit.

Comparison of oral and i.v. administration of sulphinpyrazone (10 mg/kg) to rabbits showed that the oral route was associated with an incomplete bioavailability and a six-fold greater formation of the active sulphide metabolite. The bile was an important route of elimination of unchanged sulphinpyrazone in rabbits (18% of an i.v. dose in four hours). Only small amounts of the sulphide appeared in the bile after i.v. administration. Pretreatment with oral antibiotics decreased the area under the plasma concentration-time curve (AUC) for the sulphide but increased that of the parent drug. Excretion of the p-hydroxysulphide metabolite in urine was decreased 30-fold by antibiotic treatment. The contents of the caecum showed the greatest capacity for sulphinpyrazone reduction in vitro. The liver possessed a slight ability to reduce sulphinpyrazone in vitro under anaerobic, but not aerobic, conditions. The gut bacteria are the main site of reduction of sulphinpyrazone to the active sulphide metabolite in the rabbit. These findings contrast with those obtained for sulindac which was reduced extensively under both aerobic and anaerobic conditions by rabbit-liver soluble fraction in vitro. The sulphide metabolites of both sulphinpyrazone and sulindac were oxidized to the parent drug by rabbit-liver microsomes.

The intestinal metabolism and DNA binding of benzo[a]pyrene in guinea-pigs fed normal, high-fat and high-cholesterol diets

Strains of intestinal bacteria were capable of deconjugating benzo[a]pyrene metabolites in vitro. The hydrolysis products, and other primary oxidative metabolites of benzo[a]pyrene, were stable to further degradation by the strains tested. Cytochromes P-450 and b5 were detectable in the mucosa of the guinea-pig small intestine, but not in the mucosae of the colon or rectum. The concentrations were unaltered by administration of benzo[a]pyrene and/or the feeding of high-fat or high-cholesterol diets. Benzo[a]pyrene hydroxylase was measurable in the mucosa of the upper intestine, but was present in the lower gut only at very low levels in some animals. The activity was inducible, by oral administration of benzo[a]pyrene, in the small intestinal mucosa of guinea-pigs fed normal diet but not in those fed high-fat and high-cholesterol diets. Low levels of covalent binding of 3H to DNA of liver and gut mucosa were obtained in guinea-pigs dosed orally with 3H-benzo[a]pyrene. Comparison with data for animals given 3H2O suggested that approx. one quarter of the binding was probably due to 3H exchange during metabolism. The feeding of high-fat and high-cholesterol diets did not increase this binding. Guinea-pigs fed high-fat and high-cholesterol diets excreted a greater proportion of an oral dose of 3H-benzo[a]pyrene in urine, and less in faeces than animals fed a normal diet. Due to the low, and apparently non-inducible, levels of benzo[a]pyrene hydroxylase activity and of covalent binding in the colonic mucosa, the administration of benzo[a]pyrene to guinea-pigs fed high-fat or high-cholesterol diets appears unlikely to provide a novel animal model for studies on mechanisms of colon carcinogenesis.

The hepatic metabolism and biliary excretion of benzo [a] pyrene in guinea-pigs fed normal, high-fat or high-cholesterol diets

Approx. one-third of an i.v. dose of 14C-benzo[a]pyrene was excreted within four hours in the bile of guinea-pigs fed a normal diet. The extent of excretion was not altered by feeding high-fat or high-cholesterol diets. Hepatic cytochromes P-450 and b5, and benzo[a]pyrene hydroxylase activity were unaltered by the administration of high-fat and high-cholesterol diets. Pretreatment with low oral doses of benzo[a]pyrene (6 X 3 mg/kg) did not induce these parameters in animals given any of the diets. High-fat and high-cholesterol diets altered the pattern of benzo[a]pyrene metabolites in the bile, with significantly increased excretion of dihydrodiol glucuronides in both the high-fat and high-cholesterol groups. Hepatic epoxide hydrolase activity and glutathione content were unaltered by the high-fat or high-cholesterol diets, and therefore cannot explain the alteration in the profile of biliary metabolites of benzo[a]pyrene. The altered pattern of biliary excretion in animals fed high-fat or high-cholesterol diets would lead to an increase in the delivery to the colon of dihydrodiol metabolites of benzo[a]pyrene.

The fate of saccharin impurities: The excretion and metabolism of toluene-2-sulphonamide in man and rat

Oral doses (20 mg/kg) of [Me-14C]toluene-2-sulphonamide were rapidly eliminated by rats (92% of dose in 24 h). Most of the 14C (88%) was recovered in the urine within 7 days with litte (5%) in the faeces. Larger oral doses (125 and 200 mg/kg) were eliminated more slowly (70 and 43% respectively in 24 h) but the overall distribution of 14C between urine and faeces was unchanged. 2. Low oral doses (0.2--0.4 mg/kg) of [Me-14C]toluene-2-sulphonamide were excreted more slowly in man than in the rat, with about 50% recovered in the urine in 24 h and 80% in 48 h. Negligible 14C (less than 1%) appeared in the faeces. 3. The main metabolites of toluene-2-sulphonamide were 2-sulphamoylbenzyl alcohol and its sulphate and glucuronic acid conjugates (80% of the 14C in the urine of rats and 35% in man) and saccharin (35% in man and 3% in the rat). Other metabolities found in the urine were 2-sulphamoylbenzoic acid (2% in the rat and 4% in man) and N-acetyltoluene-2-sulphonamide (6% in rat and 2% in man) together with unchanged compound (5% in rat and 3% in man).

The Fate of Saccharin Impurities: The Excretion and Metabolism of [3-14C]Benz[d]-isothiazoline-1,1-dioxide (BIT) in Man and Rat

1. The 14C label of [3-14C]benz[d]isothiazoline-1,1-dioxide (BIT) (40 mg/kg) was rapidly eliminated (97% dose in 24 h), largely in the urine (92% dose in 24 h), after oral administration to rats. Larger doses (400 mg/kg) were eliminated more slowly after oral or parenteral administration (45--60% within 24 h) mostly in the urine (42--53%). Little 14C (2--3% dose) was present in the faeces after intraperitoneal (400 mg/kg) or low oral (40 mg/kg) doses, but the presence of larger amounts (12% dose) after larger oral doses (400 mg/kg) indicated incomplete absorption. 2. Metabolites identified in the urine of rats were saccharin (about 30% of urinary 14C), 2-sulphamoylbenzoic acid (about 35% urinary 14C) and 2-sulphamoylbenzyl alcohol (15% urinary 14C) in addition to unchanged compound (5--10% urinary 14C). The urine also contained a polar, labile metabolite that gave BIT on acid hydrolysis. The pattern of metabolism was not significantly affected by dose or route of administration. 3. In man, urine was the major route of elimination of 14C (93% dose) after administration of 14C-BIT (0.5 mg/kg). Negligible 14C was recovered in the faeces (less than 1% dose). Excretion was rapid (59% dose in 6 h; 80% dose in 12 h) and little 14C was eliminated on the second (3%) or subsequent days after dosing. 4. Identified metabolites in man included saccharin (about 50% of urinary 14C), 2-sulphamoylbenzoic acid (7% urinary 14C) and 2-sulphamoylbenzyl alcohol (8% urinary 14C unconjugated and 40% conjugated) with negligible unchanged compound. Only traces of the polar labile metabolite were detected. 5. the possible significance of metabolic interrelationships of toluene-2-sulphonamide and BIT to studies on the metabolism of saccharin are discussed.

The Fate of Saccharin Impurities the Excretion and Metabolism of [14C]Toluene-4-sulphonamide and 4-Sulphamoyl[14C]benzoic Acid in the Rat

1. 4-Sulphamoyl[carboxy-14C]benzoic acid was rapidly eliminateda after oral administration to rats (94% dose in 24 h). After 6 days most of the 14C (73-83% dose) was recovered in the urine with significant amounts (18-32% dose) in the faeces due to incomplete absorption. 2. The 14C in the urine and faeces was unchanged 4-sulphamoylbenzoic acid. No 14CO2 was detected in the expired air. 3. After oral administration of [methyl-14C]toluene-4-sulphonamide to rats the label was rapidly eliminated largely in the urine (66-89% dose) with little in the faeces (2-8% dose). The 14C in the faeces was 4-sulphamoylbenzoic acid, which probably originated in the tissues since the gut flora was unable to effect this biotransformation. 4. The urine of rats given [14C]toluene-4-sulphonamide contained 4-sulphamoylbenzoic acid as the major metabolite (93% of the urinary 14C) together with small amounts of unchanged compound (1.5-2.3% of urinary 14C), 4-sulphamoylbenzyl alcohol (2.0-3.9%), 4-sulphamoylbenzaldehyde (0-1.5%) and at higher doses N-acetyltoluene-4-sulphonamide (2.1-2.3%).