PROPYLENETHIOUREA (PTU) First draft prepared by M. Watson Pesticide Safety Directorate, Ministry of Agriculture, Fisheries and Food Harpenden, Hertfordshire, United Kingdom EXPLANATION Propylenethiourea (PTU) is a plant and animal metabolite and a degradation product of propineb. Results of previous toxicological evaluation of propineb (Annex I, references 28, 34, 40, and 44) indicate that the effects of propineb on the rat thyroid may be caused primarily by PTU. PTU is also of interest because it forms part of the terminal residue to which consumers of produce treated with propineb are exposed and because the levels of PTU in treated produce generally increase during food processing as the level of propineb decreases. A toxicological monograph has not been prepared previously on PTU. This monograph contains summaries of relevant studies contained in previous toxicological monographs on propineb (Annex I, references 29, 35, and 46), as well as studies on PTU that have become available since the last previous review of propineb. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOLOGICAL DATA Biochemical aspects Absorption, distribution and excretion Radiolabelled 14C-PTU was orally administered to groups of 5 male Sprague-Dawley rats at dosage levels of 0.5 to 50 mg/kg bw. Additional groups of rats were given intravenous doses of 5 or 10 mg/kg bw. Intraduodenal doses of 5 mg/kg bw were also administered to rats with bile duct fistulas. Radioactivity in urine, faeces, exhaled air, bile and body tissues/organs was measured for up to 10 days. Whole body autoradiograms were prepared. Total recoveries of radioactivity in experiments ranged from 86% to 98%. More than 95% of orally administered doses was absorbed. Absorption and excretion of radioactivity were rapid. At 48 hours, 85-92% of orally or intravenously administered doses was recovered in the urine, 7-14% in the faeces, < 0.2% in exhaled air, and only 1-2% remained in the body. The elimination half-life for the first 48 hours was about 6 hours. After 5 days, the elimination of half-life was 2-2.5 days, which indicated a biphasic elimination pattern. Biliary excretion was also observed, as was evidence for enterohepatic circulation. Two hours after oral doses of 5 mg/kg bw, radioactivity was essentially uniformly distributed in body tissues/organs, with the exception of the thyroid, which had a 12-fold higher level of radioactivity. Thyroid levels of radioactivity reached a peak at 8 hours, which was about double that observed at 2 hours, and radioactivity remained considerably higher than levels in other tissues/organs for the remainder of the 10-day study. By 48 hours, radioactivity levels in body tissues/organs, with the exception of the thyroid, declined about 50-fold. Somewhat higher levels of radioactivity were observed in the kidney and liver. Selective uptake and accumulation of radioactivity in the thyroid was apparent throughout the 10-day study (Weber et al., 1978). Biotransformation No data were available to the Meeting on the metabolic fate of PTU. Toxicological studies Acute toxicity No data were available to the Meeting on the acute toxicity of PTU. Short-term toxicity studies (including special studies on thyroid function) Rats In a 21-day comparative study of the effects of PTU and various other thyro-suppressive agents, Wistar rats (50/sex/group) were treated orally with 0 or 50 mg/kg bw/day of the following compounds: pure propineb, two technical samples of propineb, PTU, ETU, technical zineb and methyl thiouracil as a positive control. Rats (10/sex/group) were killed after 7, 14 or 21 days treatment and after 14 or 28 days recovery. Investigation of thyroid activity was limited to recording of thyroid weight, no biochemical analyses or histopathology were included in this study. PTU, ETU and methyl thiouracil caused significant increases in absolute and relative thyroid weight in rats of both sexes, while the three propineb samples caused an increase in relative weight in females only. Propineb had only a moderate effect compared to methyl thiouracil, whereas PTU had an equivalent effect to that of methyl thiouracil in its goitrogenic activity and was somewhat more potent than ETU. The thyroid enlargement induced by propineb was partially reversible during the 28-day withdrawal period (Kimmerle, 1972). PTU of about 99% purity was administered in the diet of rats (Wistar: 40/sex/dose) for 24 months at concentrations of 0, 1, 10, 100, or 1000 ppm. The homogeneity, stability and accuracy of admixture of the test material into the test diet was not checked during the study. At 3, 7, and 14 days, and at 1, 3, 6, 12, and 24 months, 5 rats/sex/group were subjected to thyroid function tests. Twenty-four hours following oral administration of 2 µCi of 125I-NaI tracer, the rats were sacrificed. Accumulation of iodine by the thyroid was determined by measuring radioactivity in excised and weighed thyroid tissue. PBI was also assayed by measuring radioactivity in plasma protein precipitated by trichloroacetic acid. PTU inhibited thyroid function in a dose-related manner. Both sexes were affected to about the same degree. Initial effects were observed at all dose levels at 3 days, as evidenced by dose-related decreases in iodine uptake by the thyroid and by decreased PBI levels. By 7 days, compensatory reactions were observed in the 1000 ppm and 100 ppm groups. In the 1000 ppm group, thyroid weights increased approximately 6-fold over control weights, reaching peak levels at 6 months in females and at 12 months in males. Iodine uptake by the thyroid, expressed as the amount of radioactive iodine per mg of tissue, decreased to about 10% of the control levels at these times. PBI, after an initial decrease to approximately 20% of the control levels, later rose to control levels by 1-3 months. Mean body weights in this group were significantly decreased from 7 days to the end of the study. In the 100 ppm group, thyroid weights increased 2-3-fold over control weights, reaching peak levels at 6 months. Iodine concentration in the thyroid, however, remained similar to control levels throughout the study. PBI levels that were initially decreased, later increased to 2-3 times control levels at 1 month. At 10 ppm, initially reduced iodine uptake and PBI levels recovered to control levels at 7 days and remained similar to control levels for the remainder of the study. However, this is considered to be a normal physiological response, without lasting adverse effect, and the NOAEL in this study was thus 10 ppm, equivalent to 0.5 mg/kg bw/day (Weber & Patzschke, 1979). The effect of PTU on thyroid function was investigated in a 63- day study in Wistar rats. Groups of 36 males received PTU (> 97% purity) in the drinking water at concentrations of 0, 0.1, 0.3, 1 or 10 ppm. Stability and accuracy of admixture in the drinking water was checked during the study and found to be adequate. On days 7, 21 and 63, thyroid function was investigated in groups of 12 rats by determination of thyroid weight, measurement of 131I incorporation into the thyroid and by radioimmunoassay determination of T3, T4 and TSH in the serum. Food and water intake, as well as weight gain, were not affected by treatment. Thyroid weights were unaffected by treatment at any of the examinations. Although minor intergroup differences in results of other investigations attained a level of statistical significance when compared with controls, there were no consistent, dose-related effects. Histopathological examination of thyroids revealed no treatment-related effects. The NOAEL in this study was 10 ppm, the highest dose tested, equal to 1 mg/kg bw/day (Weber et al., 1991). Long-term toxicity/carcinogenicity studies Mice PTU of 99.3% purity was administered in the diet to groups of CF1/W74 mice (60/sex/dose) at levels of 0, 1, 10, or 100 ppm for 24 months. An additional group of mice received 1000 ppm in the diet, which was regularly alternated on a weekly basis with control diet. The stability of the admixture of the test material into the test diet was reported checked and found to be adequate before the start of the study. General examinations did not indicate any notable differences between test and control animals. Food consumption of 1000 ppm mice was increased compared to control mice. Mean body weights for 1000 ppm male mice were consistently lower than those of control male mice from week 16 to termination of the study. No other meaningful body-weight differences were observed for any of the groups. Mortality was equivalent in all test and control groups. Haematological examinations did not reveal any meaningful differences between control and test group male or female mice. Plasma ALP activity at 1000 ppm was increased at 12 and 24 months. ALP levels in other dose groups were generally similar to control levels. Cholesterol levels were increased at 1000 ppm at 12 and 24 months. Gross necropsies on mice sacrificed at 12 months revealed clearly enlarged thyroids in 1000 ppm male mice. For mice dying or sacrificed during the study, increased incidences of enlarged or swollen livers, sometimes with nodules, were observed in 10, 100, and 1000 ppm groups. For mice sacrificed at 24 months, dose-related increased incidences of enlarged or swollen livers and/or nodular alterations were observed in all treated male and female groups. These livers also tended to be partly hardened and brittle. Thyroid weights were significantly increased in 1000 ppm male mice, but not in female mice at 12 months. Liver weights of 1000 ppm male mice were significantly increased at 12 months and at 24 months, and of female mice at 24 months. Liver weights of 100 ppm male and female mice were also significantly increased at 24 months. Kidney weights from female mice were significantly increased at 24 months at 1000 ppm, at 100 ppm, and possibly also at 10 ppm. Histopathological examination revealed increased incidences of thyroid hypercellularity in the high-dose male mice at the interim 12-month sacrifice and at the terminal sacrifice. Clearly increased incidences of hepatocellular tumours were observed in treated male and female groups. For male mice, percentages of animals with adenomas 0, 11, 10, 7, and 26%, and of animals with carcinomas 0, 13, 29, 31, and 21% for the control, 1, 10, 100, and 1000 ppm groups, respectively. For female mice, comparable percentages of adenomas were 0, 10, 13, 32, and 18%, and of carcinomas 2, 2, 5, 26, and 38%, respectively. Historical control data were supplied, but the Meeting concluded that the figures were not helpful in considering the results of this study. Hepatic tumour incidence was variable from one study to another and the slides had been read by different pathologists from a number of different institutions. In the thyroid, only 2 adenomas (both in the one ppm male group) and 2 adenocarcinomas (one in the 100 ppm female group and one in the 1000 ppm female group) were observed. The Meeting concluded that it was not possible to establish a NOAEL in this study since liver tumour incidence in all treated groups was higher than in controls. However, there was evidence of a dose-response relationship and the lowest dose level (equal to 0.2 mg/kg bw/day) was considered to be a marginal effect level (Bomhard & Loeser, 1981). Rats PTU of approximately 99% purity was fed to Wistar TNO/W74 rats for 24 months at dosage levels of 0, 1, 10, 100, or 1000 ppm. The homogeneity, stability and accuracy of admixture of the test material into the test diet was not checked during the study. The control group consisted of 100 rats/sex/group and each of the test groups of 50 rats/sex/group. Additional satellite groups of 25 rats/sex/group were used for clinical laboratory studies. From the third week onward, animals in the 1000 ppm group gained little or no body weight and were cachectic. Food consumption was substantially below that of control animals. Most rats had a ruffled hair coat and/or heavy loss of hair. The skin of some rats was covered with a brownish scale. Mortalities commenced at 3 months. For male rats, 31/50 died by 6 months and 34/50 by 12 months. For female rats, 38/50 died by 6 months and 40/50 by 12 months. At 65 weeks the last survivors in this group, 12 male and 3 female rats, were sacrificed. Haematological examinations on male and female rats in this dose group revealed a marked and widespread decrease in haematopoiesis, and differential white blood cell counts revealed a relative increase in mature polymorphonuclear neutrophils and a relative decrease in lymphocytes. Clinical chemistry revealed highly increased plasma ALP activities and urea, creatinine and cholesterol levels were increased. Gross necropsies revealed enlarged thyroids in some animals. Histopathological examination of organs/tissues revealed nodular and generalized hyperplasia of the thyroid, calculi and vacuolation of tubular cells in kidneys, aplasia and decreased haematopoiesis in marrow (particularly in males), vacuolation of hepatocytes in liver (particularly in males), and atrophy and reduced spermatogenesis in testes. Nine male rats (of 42 examined) had thyroid adenomas. Two female rats (of 25 examined) also had thyroid adenomas and one additional female rat had a thyroid cystadenoma. Daily observations of animals receiving lower dietary levels indicated no differences from the control animals. Food consumption was also similar in these test and control groups. Body weights of 100 ppm male rats were consistently lower than those of control male rats from week 20 to termination of the study. Body weights of all other test groups were similar to those of the respective control groups. Slightly increased mortality in the 100 ppm male group was observed at 18 and 24 months. Mortalities were equivalent in all other test and control groups. The number of male rats surviving to termination of the study were 85/100, 41/50, 44/50, and 38/50 for the control, 1, 10, and 100 ppm groups, respectively. The respective numbers for female rats were 83/100, 40/50, 42/50, and 43/50. Haematological examinations and clinical chemistry, including determination of PBI in plasma, did not reveal any indication of reaction to treatment at 1, 10 or 100 ppm. Gross necropsies revealed enlarged thyroids in numerous rats treated with 100 ppm. Thyroid weights from rats treated with 100 ppm were significantly increased. Histopathological examination of the thyroids revealed nodular hyperplasia in 3 males treated with 1 ppm and 5 males and 2 females treated with 100 ppm (in addition to 6 males and 4 females treated with 1000 ppm). Historical control data were not presented, but the report states that "this change may be expected in normal control rats". The presence of large follicles and colloid cysts in the thyroids of rats treated with 100 ppm was probably treatment- related. Inter-group differences in incidence of other findings in the thyroid were probably not treatment-related: vacuolated cells lining the follicles, often seen in active thyroids, were present in 2 control males and 4 treated with 100 ppm, and in no control females, compared with 1 at 10 ppm and 3 at 100 ppm. Small follicles with little colloid were seen in 1% of control females, 22% at 1 ppm, 46% at 10 ppm, and 2% at 100 ppm. In males the corresponding incidences were 7%, 18%, 20% and 16%. In summary, at 1000 ppm, a clearly excessive dosage level, thyroid tumours were definitely related to the test material. At 100 ppm non-neoplastic thyroid changes and reduced body-weight gain were seen. The NOAEL in this study was 10 ppm, equal to 0.56 mg/kg bw/day (Bomhard & Loeser, 1980). Reproduction studies No data were available to the Meeting regarding multigeneration reproduction studies with PTU. Special studies on embryo/fetotoxicity Rats A teratology study of propineb and PTU is described in the published literature in which groups of 15 Sprague-Dawley rats received PTU by gavage on days 6 to 16 of pregnancy at levels of 0, 11, 23, 45 or 90 mg/kg bw/day. PTU showed teratogenic effects at doses which did not show any maternal toxicity (45 and 90 mg/kg bw/day). However, slightly reduced body-weight gain and increased thyroid weights were reported at 45 and 90 mg/kg bw/day and the Meeting considered these findings to be evidence of slight maternal toxicity. Results were not reported in sufficient detail to fully evaluate this study (Vicari et al., 1985). Earlier studies showed that PTU had teratogenic potential in rats following one or several doses (> 20 mg/kg bw/day) on days 12 or 13 of gestation (Ruddick et al., 1976; Bleyel & Lewerenz, 1978). No data were available to the Meeting regarding special studies with PTU on embryo/fetotoxicity in species other than rats. Special studies on genotoxicity The results of genotoxicity studies are summarized in Table 1. Observations in humans No information available. Table 1. Results of genotoxicity assays on PTU Test system Test Object Concentration Purity Results Reference Ames test S. typhimurium up to 12.5 mg/plate 99.5% Negative Herbold, 1980 TA98, TA100, TA1535, TA1537 Ames test S. typhimurium up to 12.5 mg/plate 99.5% Negative Herbold, 1981a TA98, TA100, TA1535, TA1537 Pol A1 E. coli p3478, W3110 up to 1 mg/plate 99.5% Negative Herbold, 1981b Micronucleus test Mouse ? ? Negative Rolandi et al., 1984 DNA metabolism and CF1 mice 100 mg/kg bw 99.5% Negative Klein, 1981 binding COMMENTS Following oral administration to rats, PTU was rapidly absorbed and eliminated in the urine and faeces. Less than 0.2% of the administered dose was detected in the exhaled air and after 10 days only 1-2% remained in the body. Biliary excretion was also observed, as was evidence for enterohepatic recirculation. Distribution in body tissues was essentially uniform, with the exception of the thyroid which had a 12-fold higher level than other tissues. The results of toxicity studies clearly indicate that PTU has a goitrogenic effect in rats. In a 21-day comparative study of the effects of propineb, PTU, ETU, zineb and methyl thiouracil on thyroid weight, PTU had an equivalent effect to that of methyl thiouracil, and was somewhat more potent than ETU. The thyroid enlargement was partially reversible during a 28-day withdrawal period and the results suggested that the effects of propineb on the thyroid in rats may be caused primarily by the metabolite, PTU. In a 63-day study in male rats, in which PTU was administered in the drinking water at levels of 0, 0.1, 0.3, 1 or 10 ppm, no consistent effects on thyroid function were seen at doses up to 10 ppm, the highest dose tested, which was equal to 1 mg/kg bw/day. In a 24- month study on thyroid function, using dietary levels of 0, 1, 10, 100 or 1000 ppm, the NOAEL (based on increased thyroid weight at higher doses) was 10 ppm in the diet, equivalent to 0.5 mg/kg bw/day. In long-term studies, treatment-related alterations in tumour incidence were seen in rats and mice. In a 2-year study in mice, using dietary levels of 0, 1, 10, 100 and 1000 ppm, it was considered not possible to establish a NOAEL since liver tumour incidence in all treated groups was higher than in controls. However, there was evidence of a dose-response relationship and the lowest dose level (equal to 0.2 mg/kg bw/day) was considered to be a marginal effect level. Thyroid tumour incidence was not affected by treatment of mice with PTU. In a 2-year study in rats, also using dietary levels of 0, 1, 10, 100 or 1000 ppm, the NOAEL was 10 ppm in the diet, equal to 0.56 mg/kg bw/day. Treatment-related thyroid tumours were seen at 1000 ppm. This dietary level was accompanied by increased mortality, while non-neoplastic thyroid changes and reduced body-weight gain were seen at 100 ppm. A published article indicated that PTU showed teratogenic effects in rats at 45 and 90 mg/kg bw/day, doses which showed slight maternal toxicity. Results were not, however, reported in sufficient detail to fully evaluate this study. No information was available to the Meeting regarding special studies with PTU on embryo/fetotoxicity in species other than rats. PTU is not mutagenic in bacteria and does not cause damage to mouse DNA in vivo. The Meeting could not reach any conclusion regarding the genotoxicity of PTU because of the limited database. A temporary ADI was allocated to PTU, which was based on the marginal effect level in the 2-year study in mice (1 ppm in the diet, equal to 0.2 mg/kg bw/day). The Meeting felt reassured that, in view of the metabolic conversion of propineb to PTU, the long- term study in mice with PTU identified the same target organ as the long-term study in mice with propineb. However, in view of the overall inadequacy of the toxicological data for PTU, the Meeting concluded that a 1000-fold safety factor was necessary. TOXICOLOGICAL EVALUATION Level causing no toxicological effect Mouse: 1 ppm in the diet, equal to 0.2 mg/kg bw/day (marginal effect level in 2-year study) Rat: 10 ppm in the diet, equivalent to 0.5 mg/kg bw/day (2-year thyroid function study) 10 ppm in the diet, equal to 0.56 mg/kg bw/day (2-year study) Estimate of temporary acceptable daily intake for humans 0-0.0002 mg/kg bw Studies without which the determination of a full ADI is impractical Results to be submitted to WHO by 1998: 1. Long-term carcinogenicity study in mice, identifying a NOAEL. 2. Clarification of the genotoxic potential of PTU. 3. Clarification of embryo/fetotoxicity and teratogenicity potential of PTU in rodents. REFERENCES Bleyl, D.W. & Lewerenz, H.J. (1978) Zur teratogenen Wirkung von Propylenthioharnstoff bei Ratten. Monatshefte fur Veterinarmedizin, 33: 139-197, Jena. Bomhard, E. & Loeser, E. (1980) Propylenethiourea, chronic toxicity study on rats (two-year feeding experiment). Bayer AG Institute of Toxicology, Report No. 9345. Unpublished report submitted to WHO by Bayer AG. Bomhard, E. & Loeser, E. (1981) Propylenethiourea, chronic toxicity study with mice (feeding study over two years). Bayer AG Institute of Toxicology, Report No. 10102. Unpublished report submitted to WHO by Bayer AG. Herbold, B. (1980) Propylenethiourea, Salmonella/microsome test for detection of point mutagenic effects. Bayer AG Institute of Toxicology, Report No. 9563. Unpublished report submitted to WHO by Bayer AG. Herbold, B. (1981a) Propylenethiourea, Salmonella/microsome test to evaluate for point mutations employing liver homogenates from strain CFWI male mice. Bayer AG Institute of Toxicology, Report No. 10116. Unpublished report submitted to WHO by Bayer AG. Herbold, B. (1981b) Propylenethiourea, Pol Al-test on E. coli to evaluate for DNA damage. Bayer AG Institute of Toxicology, Report No. 10146. Unpublished report submitted to WHO by Bayer AG. Kimmerle, G. (1972) Comparative test of antracol, propylenethiourea, zineb and ethylene thiourea for thyroid action in rats. Report No 3551, Bayer AG, Institute of Toxicology. Unpublished report submitted to WHO by Bayer AG. Klein, W. (1981) Study for the effect of PTU on DNA metabolism. Seibersdorf Research Centre, Institute of Biology, Project No. PS/2277, Bayer Study No. T 4003 443 (PTU). Unpublished report submitted to WHO by Bayer AG. Rolandi, A., De Marinis, E. & De Caterina, M. (1984) Dithiocarbamate pesticides: activity of propineb in the micronucleus test in mice. Mut. Res. 135: 193-197. Ruddick, J.A., Newsome, W.H. & Nash, L. (1976) Correlation of teratogenicity and molecular structure: Ethylene thiourea and related compounds. Teratology, 13: 263-266. Vicari, L., de Dominicis, G., Vito, M., Placida, C., De Marinis, E. (1985) Teratogenic and goitrogenic activity of propineb and propylenethiourea in the rat. Boll. Soc. It. Biol. 61: 271-278. Weber, H., Patzschke, K. & Wegner, L.A. (1978) Propylenethiourea- 14C, biokinetic study on rats. Bayer AG Institute of Pharmacokinetics, Report No. 7397. Unpublished report submitted to WHO by Bayer AG. Weber, H. & Patzchke, K. (1979) Effect of long-term administration of propylenethiourea (PTU) on thyroid function of male and female rats. Bayer AG Institute of Pharmacokinetics, Report No. 8494. Unpublished report submitted to WHO by Bayer AG. Weber, H., Mager, H., Ditgens, K. & Ohs, P. (1991) PTU feeding study: Effect of the compound on thyroid function in male Wistar rats after uptake with the drinking water in the low dose range up to 10 ppm over a time interval up to 63 days. Bayer AG Institute of Toxicology. Unpublished report submitted to WHO by Bayer AG.
See Also: Toxicological Abbreviations