Search

Health-based guidance values, such as the ADI, use chemical-specific data to determine the highest intake that would be without significant adverse health effects. A threshold of toxicological concern (TTC) is a level of intake predicted to be without adverse effects based on the toxicity of structurally related compounds. The main advantage of the use of TTCs is that the risk of low exposures can be evaluated without the need for chemical-specific animal toxicity data. TTCs have been used for many years for screening the safety of packaging migrants by the FDA in the USA, and of flavoring substances, by the JECFA. A recent reassessment of the use of TTCs, organized by ILSI Europe, has developed a decision tree which allows a systematic approach to the evaluation of low levels of diverse chemicals in food. The decision tree incorporates a series of increasing TTC values into a step-wise approach. Potentially genotoxic carcinogens are considered first, based on the presence of known structural alerts. Aflatoxin-like, azoxy- and nitroso-compounds are removed from consideration because they are the most potent, and a practical TTC could not be established. Other compounds with structural alerts for genotoxicity are allocated a TTC of 0.15 microg/person per day. Compounds without structural alerts for genotoxicity are evaluated based on chemical structure and intake using a series of TTC values derived by the application of a 100-fold uncertainty factor to the 5th percentile of the distribution of NOAELs from chronic studies on compounds sharing similar structural characteristics.

A group of 14 subjects, who had been identified from 261 volunteers in a 1-week screen as being able to metabolize the sweetener cyclamate to cyclohexylamine (>0.2% of a daily dose), and 31 nonconverters (<0.2% metabolism) were given calcium cyclamate tablets (equivalent to 250 mg cyclamic acid, 3 times daily) for a period of 13 weeks. The metabolism of cyclamate to cyclohexylamine was determined using twice-weekly timed (3 h) urine collections during week 1-3 and 7-13. Urine specimens were collected on all other study days to investigate day-to-day fluctuations in cyclohexylamine excretion. Analyses of the twice weekly timed urine collections showed that subjects recruited as nonconverters essentially remained nonconverters. Of the converters, three showed consistently low metabolism, five showed erratic metabolism, five showed low metabolism initially, which increased during the latter part of the study, and one subject showed consistently high metabolism throughout the study. Analysis of the day-to-day urine specimens showed marked intrasubject variability. The plasma concentrations of cyclohexylamine measured on weeks 1-3 and 7-13 reflected the urine profiles. The highest individual long-term average steady-state excretion values based on the 3-h urine collections and daily samples were 21%, 23%, 25%, 29%, 34%, and 38%. The maximum % metabolism detected in the high converters occasionally reached the value of 60% reported in previous short-term studies, but this high activity was not maintained, and was followed by periods of lower metabolism. The results of this metabolism study support an acceptable daily intake (ADI) of 0-11 mg/kg body weight per day.

The teratogenicity of all-trans-retinoic acid, 13-cis-retinoic acid, and retinol was investigated in pregnant Wistar rats given a single oral dose on Day 10 of gestation. External malformations showed dose-dependent increases and the order of potency was all-trans-retinoic acid > retinol > 13-cis-retinoic acid. The metabolites in maternal plasma were determined following a single oral dose on Day 10 of gestation. Equipotent teratogenic doses of all-trans-retinoic acid and 13-cis-retinoic acid had similar plasma levels of all-trans-retinoic acid; however, retinol teratogenicity could not be accounted for by circulating all-trans-retinoic acid or its metabolites. The teratogenicity and maternal pharmacokinetics of all-trans-retinoic were compared in pregnant Wistar rats when given as a single dose (50 mg/kg) and as three equal doses (16.66 mg/kg) over 6 hr. Divided doses were of slightly greater potency than the single dose but the maximum observed concentration (Cmax) and area under the plasma concentration-time curve (AUC) values for the second and third doses were greatly attenuated compared with the first dose; in consequence, both the total AUC and Cmax were reduced compared with the single dose. The altered profile could not be explained by increased formation of all-trans-retinoic acid glucuronide or increased isomerisation to 13-cis-retinoic acid. The maternal plasma levels of all-trans-retinoic acid in pregnant rabbits were reduced by a dose given 24 hr earlier. These data show that all-trans-retinoic acid in maternal plasma is a poor indicator of fetal exposure to teratogenic risk.

Intravenous infusion of cyclohexylamine (30-120 mg/kg) caused a dose-dependent increase in blood pressure in urethane-anesthetized rats. The increase in blood pressure was inversely related to the duration of the infusion. The blood pressure returned to baseline values (+/- 5 mm Hg) within 60 min of the end of the infusion after doses of 30 and 60 mg/kg were administered over 20 and 40 min. The plasma concentrations of cyclohexylamine were related linearly to the administered dose and decreased only 27% between the end of the infusion and 60 min later. The concentration-effect relationship showed clockwise hysteresis, indicative of tachyphylaxis, as has been reported in humans (Eichelbaum et al., 1974). Administration of bolus doses of tyramine at the end of the infusion or 60 min later demonstrated the presence of an indirect sympathomimetic response, although this was attenuated to a greater extent by high doses and more rapid infusions of cyclohexylamine. An almost complete loss of response to tyramine was found only in animals given 120 mg/kg over 40 min. The presence of a tyramine response 60 min after the infusion of 60 mg/kg occurred when there was an essentially complete reversal of the hypertensive effect of cyclohexylamine. These data indicate that the hypertensive effect of the indirectly acting sympathomimetic amine cyclohexylamine occurs primarily during rapid increases in plasma concentrations. Tachyphylaxis develops rapidly after the cessation of the infusion which is probably due largely to reuptake of released noradrenaline at low doses and depletion of releasable noradrenaline at high doses.

A group of 194 diabetic patients were given calcium cyclamate (1 g/day as cyclamic acid equivalents) for a period of 7 days. Blood and urine samples were collected to determine the formation of cyclohexylamine, which is an indirectly acting sympathomimetic amine. Blood pressure and heart rate were recorded before and after treatment. Urine samples were collected each day and analyzed for cyclamate (to check compliance) and cyclohexylamine (to monitor the development of metabolizing activity). After 7 days intake most individuals (78%) did not excrete significant amounts of cyclohexylamine (less than 0.1% of the daily dose of cyclamate) but a small number (8; 4% of the group) excreted more than 20% of the daily dose as cyclohexylamine in the urine. Similar interindividual variations were found in the plasma concentrations of cyclohexylamine after 7 days intake of cyclamate, with 8 individuals having concentrations of 300-1942 ng/ml. The changes in cardiovascular parameters in these 8 subjects between pre- and postdosing were similar to those found in 150 subjects with plasma cyclohexylamine concentrations less than 10 ng/ml. Twenty of the subjects were restudied after receiving calcium cyclamate for 2 weeks at a daily dose equivalent to 2 g of cyclamic acid (0.66 g tds). Plasma concentrations of cyclohexylamine, heart rate, and blood pressure were measured every 30 min for a period of 8 hr (one dose interval) after the final dose. Twelve patients had plasma concentrations of cyclohexylamine greater than 10 ng/ml (89-2043 ng/ml) at the start of the dose-interval investigations. There were no transient increases or decreases in plasma concentrations of cyclohexylamine which might have resulted in a transient change in blood pressure or heart rate. These data indicate that the metabolism of cyclamate (2 g/day) to cyclohexylamine would not affect blood pressure or heart rate even in individuals with high metabolizing ability.

Adult male rats were fed diets containing 0 to 10% saccharin, 2% tryptophan, or 2% tryptophan plus 5% saccharin ad libitum for 1 to 2 months. Saccharin produced a dose-related increase in the urinary excretion of indican which is the main metabolite of indole. The renal clearances of both indican and saccharin were reduced at high plasma concentrations (200 to 300 micrograms/ml) of saccharin, suggesting saturation of renal tubular secretion. The increased amounts of indican in the urine, and of indole in the cecum arose from accumulation of protein and tryptophan in the cecum rather than an increase in the enzyme tryptophanase which metabolizes tryptophan to indole. The high levels of protein in the ceca of rats fed saccharin-containing diets were associated with a dose-related increase in the weight of the contents and wall of the cecum. Administration of 2% tryptophan in the diet increased significantly the amounts of tryptophan in the cecum and in plasma, but produced only small increases in the size of the cecum, the amount of indole present, and the excretion of indican. The effects of saccharin and tryptophan were additive. These effects are consistent with saccharin having a major effect on protein digestion in the intestine such that increased amounts of tryptophan are available in the cecum for microbial metabolism to indole. The greater formation of indole and excretion of indican in the urine suggest that saccharin increases the catabolism of dietary tryptophan to metabolites with known cocarcinogenic activity toward the rat bladder.

Cyclohexylamine showed dose-dependent kinetics after administration of single oral doses up to 500 mg/kg in rats, with a reduction in plasma clearance, an increase in apparent half-life, and an increased area under the testicular concentration-time curve. Cyclohexylamine was absorbed and eliminated more rapidly by mice. Saturation of cyclohexylamine uptake by rat renal cortical slices in vitro and of renal tubular secretion in vivo occurred at concentrations and doses comparable to the oral dose studies. During chronic dietary administration the concentrations of cyclohexylamine in the plasma and testes showed a pronounced diurnal variation in rats which was not detected in mice. The steady-state plasma clearance in rats was approximately one-half that in mice. The concentrations of cyclohexylamine in the plasma and testes of rats, but not mice, showed a nonlinear relationship to dietary intake. Elevated concentrations were found at intakes greater than 200 mg/kg/day. The pharmacokinetics of cyclohexylamine make an important contribution to the difference in sensitivity to testicular atrophy in rats and mice and the dose-response relationship for this toxicity in rats.

Low dietary levels of sodium saccharin (0-2%) fed to male rats for 6 weeks produced a dose-related increase in the urinary excretion of p-cresol, a major microbial metabolite of tyrosine. Some animals fed higher levels of saccharin (5-7.5%) for 6 weeks excreted increased amounts of p-cresol, but many excreted negligible amounts so that the overall dose-response relationship was bell shaped. After 20 weeks of exposure, all rats in the higher dose groups showed increased p-cresol excretion and by 26 weeks the 7.5% saccharin group showed a 36-fold increase over animals fed the 0% saccharin diet. The urinary excretion of phenol, another microbial amino acid metabolite, was constant in animals fed dietary levels of saccharin below 2% for 6 weeks, but was virtually abolished at higher levels. The excretion of indican (formed from indole, a microbial metabolite of tryptophan) was increased by saccharin in a dose-related fashion at all time points, but showed only a 3-fold increase at 7.5% compared with the 0% group. p-Cresol may therefore prove more sensitive than indican as an indicator of altered microbial metabolism due to saccharin. In a separate study the effect of 7.5% saccharin on p-cresol and indican excretion was shown to be largely reversible and the excretion of phenol increased rapidly when saccharin was withdrawn from the diet. Chronic saccharin administration to man at high doses (1 g/day for 4 weeks) had no perceptible effect on the excretion of these three metabolites.

Cyclohexylamine hydrochloride has been given in the diet to mice and to Wistar and DA rats for 13 weeks, to provide a constant intake of 400 mg of the base/kg/day. Significantly decreased food intake and body weight gain were found in both strains of rats but not mice. The metabolism of [14C]cyclohexylamine was widely different in Wistar and DA rats and in rats and mice, and these differences were not altered appreciably by chronic intake for 13 weeks. The differences in metabolism resulted in marked and persistent differences in the concentrations of the hydroxylated metabolites in the plasma and testes of treated animals with Wistar much greater than DA much greater than mice. After 7 and 13 weeks testicular atrophy was demonstrated in both strains of rats given cyclohexylamine diet by a decrease in organ weight and by histological changes. DA rats appeared more sensitive to testicular toxicity from cyclohexylamine than Wistar rats, while mice showed no evidence of testicular damage. These data show that the development of testicular toxicity is not related to the extent of hydroxylation. The concentrations of cyclohexylamine in the plasma and testes of the treated animals were lower in mice than in either strain of rats despite a similar daily intake. This suggests that species differences in pharmacokinetics may contribute to the apparent difference in sensitivity to testicular toxicity.