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Carcinogenicity

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In summary, the authors of the rat NTP study (1988) concluded that the there was some evidence of carcinogenic activity of MBT for male and female F344/N rats. However, the poor survival rate in male rats, indicating that the MTD (maximum tolerated dose) was presumable exceeded. The overall rates of individual tumors in treated animals do not exceed the historical range and in general, a dose-response relationship is missing. The increase incidences of adrenal gland pheochromocytomas in rats seem to be associated with nephropathy and endocrine disturbance. The relevance of this finding for human is questionable. The results of the mice carcinogenicity NTP study (1988) was discussed as negative for male mice and equivocal for female mice. In a chronic feeding study no carcinogenic effects were indicated (Ogawa 1989). In addition, in an early carcinogenicity study (NCI 1968) no carcinogenic potential of MBT was noted. Taking all available epidemiological information into consideration the totality of available evidence is inadequate to support a conclusion that MBT may be a human carcinogen and, more specifically, a bladder carcinogen. 
In conclusion, the animal data are conclusive but not sufficient for classification. For the human data, the totality of available evidence is inadequate to support a conclusion that MBT may be a human carcinogen and, more specifically, a bladder carcinogen.

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Justification for classification or non-classification

No classification is required according to the classification criteria 67/548/EWG and regulation no. 1272/2008 (GHS).

Additional information

Non-human information

Carcinogenicity: oral

The carcinogenic potential of MBT was evaluated in an early carcinogenicity study (NCI 1968, Innes 1969). Even through the study was not performed according to present standards, it should be mentioned that MBT was not found to be tumorigenic when given orally to mice for 18 months at the maximal tolerated dose, which was fixed at 100 mg/kg MBT by gavage (dissolved in 0.5 % gelatine) from day 7 to 28 of age and at 323 ppm in the diet from day 28 for additional 17 months.

A 20 months feeding study (Ogawa 1989) done with mice (strain Slc:ddY) did not reveal a carcinogenic potential of MBT up to 1920 ppm (ca 248 mg/kg bw) in the diet, a dose that produced evident signs of toxicity (cell infiltration in the interstitium of the kidney) (for more details see chapter repeated dose toxicity).

Carcinogenicity studies with MBT in rats and mice were performed by the National Toxicology Program (NTP 1988).

Male and female Fischer 344 rats were administered with MBT for 103 weeks. Groups of 50 male rats were administered 0, 375, or 750 mg/kg 2-mercaptobenzothiazole in corn oil by gavage, 5 days per week for 103 weeks. Groups of 50 female rats were administered 0, 188, or 375 mg/kg 2-mercaptobenzothiazole in corn oil by gavage on the same schedule.

All animals were observed two times per day, and clinical signs were recorded once per week. Body weights by cage were recorded once per week for the first 12 week and once per month thereafter. A necropsy was performed on all animals including those found dead. During necropsy, all organs and tissues were examined for grossly visible lesions. Necropsy and histological exams were performed on all animals and all relevant tissues. No haematology and clinical biochemistry were performed. Tumour incidences were calculated with different statistic methods (Life table test, incidental tumour test, Cochran Armitage Trend test and Fisher exact test).

Survival of the low dose group of male rats was statistically significant lower (p < 0.001) than that of the vehicle controls after week 85; survival of the high dose group was statistically significant lower (p< 0.001) than that of the vehicle controls after week 83. Mortality of treated females were not significant different from control. Mean body weights of dosed male rats were similar to or greater than those of the vehicle controls. Mean body weights of dosed female rats were generally greater (up to 11 %) than those of the vehicle controls. Rats were lethargic and showed prostration after they were dosed.

Nephropathy, characterized by tubular degeneration and regeneration, was present in all male rats and in more than 75 % of the female rats; a severity grade from minimal to severe (1- 4) was recorded for each animal. The mean severity of nephropathy was increased in dosed male rats (vehicle control: mild-moderate; low dose and high dose: moderate-severe). However, these changes were later discussed as questionable (TSCA 1991-1993 cited in BG Chemie 2000).

Ulcers and inflammation were observed at increased incidences in dosed rats, and epithelial hyperplasia and hyperkeratosis were observed at increased incidences in dosed male and low dose female rats. These effects were later discussed as treatment related effect (bolus-effect) but not as substance-related effect.

The number of animals of the high dose group which developed tumours was comparable to control, whereas the number of animals with tumours was increased in the low dose group (males: control 49, low dose 50, high dose 48, females: control 37, low dose 46, and high dose 40). The total number of primary tumours was similar in control and high dose animals (males: control 123, high dose 125, females: control 73, high dose 79); the number of animals with primary tumours were elevated in the low dose group (males: 169, females 104). The tumour types were roughly similar between the two groups. The time to tumour was increased in treated females (control 62 wk, low dose 66 wk, high dose 63 wk) and decreased in treated males (control 90 wk, low dose 77 wk, high dose 56 wk).

The authors of the NTP study conclude that the incidences of mononuclear cell leukemia, pituitary gland adenomas and pancreatic acinar cell adenomas in male rats were increased only in the low dose groups and thus not dose-dependent. In males of the low and high dose groups the incidences of adrenal gland benign pheochromocytoma and the overall rate of benign and malignant pheochromocytoma was significant increased; the incidences of preputial gland adenomas or carcinomas (combined) in dosed groups were significant greater than those in the vehicle controls. However the increases were more pronounced in the low dose group than in the high dose group and thus not dose-dependent. In high dose females incidences of adenomas of the pituitary gland and adrenal gland pheochromocytomas were significant greater (p< 0.05) than those in the vehicle control and occurred with a significant positive trends (p < 0.05).

All tumour incidences were within the range of the historical control data. In addition, there was no linear trend in dose, except for the adrenal gland pheochromocytomas in males and females and adenomas of the pituitary gland in females. Moreover, it is also known that gavage application of corn oil itself (vehicle control) increases significant the incidence of pancreatic acinar cell adenomas (Haseman 1990).

The authors of the NTP study (1988) suggested that examples of neoplasms with dose-related trends included pituitary gland adenomas in female rats and adrenal gland pheochromocytomas in each sex of rats. These responses suggested that 2-mercaptobenzothiazole expressed some carcinogenic activity in rats at doses sufficient to accelerate mortality. In the meantime more insight is available on the mechanism and relevance of pheochromocytomas and MBT exposure. Recent data analysis revealed that nephrotoxicity in rats, is associated with occurrence of pheochromocytomas. It is hypothesized that nephropathy and calcifications produce a disturbance in renal elimination of calcium and phosphate. Impairment of calcium homeostasis results in proliferation of chromafin cells in the adrenal gland with an increased secretion of catecholamines. The nephropathy and endocrine disturbance seems to induce and/or enhances the development of pheochromocytomas (Greim 2009). This mode of action seems to be rat specific and thus the relevance for humans is questionable.

In the mice NTP carcinogenicity study (NTP 1988) male and female B6C3F1 mice were administered with MBT for 103 weeks (NTP 1988). Groups of 50 male and 50 female mice were administered 0, 375, or 750 mg/kg 2-mercaptobenzothiazole in corn oil by gavage, 5 days per week for 103 weeks. The study design and schedule were similar to the Fischer 344 rat carcinogenicity study discussed.

Survival of the high dose group of female mice was significantly lower than that of the vehicle controls after week 27. The survival rat of the female low dose group and the male treatment groups were comparable to control. Six high dose male and four high dose female mice died on the same day during week 13. Since they were mistakenly dosed twice within a 16-hour period, these mice were censored from the statistical incidence of survival after week 12.

Mean body weights of high dose male mice were 6 %-14 % lower than those of the vehicle controls from week 3 to week 64; the mean body weights of low dose males were 4 to 8 % lower compared to control from week 6 to week 64. In females the mean body weights of the high dose group were within 6 % of the vehicle control, whereas the mean body weights from the low dose group were generally greater than those of the vehicle controls throughout the studies.

Similar to the rat study, mice were lethargic and showed prostration after they were dosed.

No substance-related non-neoplastic lesions were indicated in any of the treated mice.

The incidence of hepatocellular adenomas or carcinomas (combined) in low dose female mice was significantly greater (P = 0.028) than that in the vehicle controls (control: 4/50, low dose: 12/49, high dose: 4/50). Adenomas and adenomas or carcinomas (combined) of the pituitary gland occurred with significant negative trends in female mice. Lymphomas in female mice occurred with a significant negative trend, and the incidence in the low dose group was significantly lower than that in the vehicle controls. The tumour incidences of treated male mice were not significant different from controls. The authors conclude that there was no evidence of carcinogenic activity of 2-mercaptobenzothiazole for male B6C3F1 mice dosed with 375 or 750 mg/kg. There was equivocal evidence of carcinogenic activity for female B6C3F1 mice, indicated by increased incidences of hepatocellular adenomas or carcinomas (combined). However there was no dose response relationship indicated and the observed incidences of hepatocellular adenomas or carcinomas were within the bounds of historical control range.

Carcinogenicity: other routes

Parallel to the oral carcinogenicity study (NCI 1968, Innes 1969) mice were treated once subcutaneous with MBT suspended in DMSO. Groups of 18 B6C3F1 and 18 B6AKF1 mice of each sex received a single subcutaneous injection in 0.05 ml suspension (1000 mg/kg) MBT in the nape of the neck with sacrifice at ca. 17 months after injection (no more data available). An equal number of mice of each strain served as vehicle control. Survival of the MBT-treated mice was comparable to the control group in both strains. No adverse effects were reported and no evidence of a carcinogenic effect in treated mice (NCI 1968, Innes 1969).

In an additional approach a single subcutaneous administration (215 mg/kg Captax in 0.5% gelatine) was given in the nape of the neck to weanling mice on approximately the 28th day of age ((NCI 1968, Innes 1969). A dose-independent increase in incidences in reticulum cell sarcoma was noted in treated male B6CF31 mice (5 vs. 2 in control); the total number of males with tumour and the incidences of hepatoma and pulmonary adenomas and carcinomas were not increased compared to control. The tumour incidences of female B6CF31 mice and male and female B6AKF1 mice were comparable to control.

Human information

Mortality trends for 1059 production workers at a rubber chemical plant in Nitro, West Virginia (Nitro cohort) were examined to find whether they had increased mortality from cancer associated with exposure to 2-mercaptobenzothiazole (MBT) and/or its derivates (sodium mercaptobenzothiazole, n-cyclohexyl-2-benzothiazole sulphenamide, benzothiazyl disulphide, n-tertiary-butyl-2-benzothiazole sulphenamide, 2-(morpholinothio) benzothiazole, 2-(2,6-dimethylmorpholinothio) benzothiazole, 2-(hexamethyleneiminothio) benzothiazole, and 1,3 bis(2-benzothiazolylmercaptomethyl) urea) (Strauss 1993). This chemicals and its derivates have been manufactured at the plant since 1935. Analyses were conducted on MBT exposed employees by cumulative exposure and time since first exposure. In addition to MBT and its derivates, p-aminobiphenyl (PAB), a potent bladder carcinogen, was used in this plant between 1935 and 1955. The study population included 1059 full time, white, male production workers who were active at the Nitro plant between 1955 and 1977 for one day or more. 600 of the 1059 plant workers had a history of exposure to MBT. Annual air borne exposure estimates throughout the study period were developed for all hourly production jobs with sampling data available from 1977 to1989, historical company documents, and interviews with knowledgeable retirees for the period before 1977. All jobs with potential exposure to MBT were assigned to one of four exposure categories. The highest exposure to MBT and/or its derivates occurred between 1943 and 1954 (>2->3 mg/m3). For the remaining years an exposure of = 1mg/m3 was estimated and since 1970 the exposure was reduced to less than 0.25 mg/m3 (Collins 1999). Because of concern about the potential confounding effect of exposure to PAB all workers exposed to PAB were identified. Those workers whose work covered all parts of the plant, such as yard labour, maintenance or general production jobs were not considered exposed to PAB, although it was recognised that some of these jobs may have involved such exposure. A cohort study design was used to compare death rates of workers with rates for the general population. Standardised mortality ratios (SMSs) were calculated by taking the ratio of observed deaths to expected deaths multiplied by 100. Person-years of follow up were counted from 1955 to the date of death or the study end date, 1987. Mortality of the production workers was significant lower than local mortality, whereas SMR for all cancers was slightly raised. This was due to the raised rates seen for lung cancer, prostate cancer and bladder cancer. Standardised mortality ratios for other site specific cancers and for non-neoplastic diseases were either near or considerably lower. Mortality for the MBT subcohort was slightly lower than local mortality; whereas the mortality for all cancers was slightly raised. Increased death rates were seen for lung cancer, prostate cancer and bladder cancer. Standardized mortality ratios for MBT workers with PAB exposed job assignments were significantly increased for all cancers, lung cancer (observed10 to 3.4 expected), and bladder cancer (observed 7 to 0.22 expected); there were no deaths due to prostate cancer. The standardized mortality ratios for the subcohort of MBT exposed, but not PAB exposed workers (511 workers) were slightly lower than expected for all cancers, but were increased for bladder cancer (observed 3 to 0.66 expected) and prostate cancer (observed 4 to 1.99 expected); whereas the mortality for lung cancer was lower than expected. The total number of deaths from MBT exposed workers hired in 1956 or later, after termination of PAB production, was significant lower than expected, and there were no deaths due to cancer, or bladder cancer. The authors concluded that there were no associations between exposure to MBT and its derivatives and increased rates of most malignant neoplasms in workers without an exposure to PAB. They discussed that the association between exposure to MBT and bladder cancer based on only three deaths was very imprecise; and there was no strong trend of increasing rates with increasing exposure, although there were too few deaths for meaningful trend analysis. All three deaths from bladder cancer in the MBT cohort occurred in the two highest cumulative exposure categories, and with at least 20 years since first exposure. Evaluation of bladder cancer results for MBT may also have been confounded by exposure to PAB, even through these findings related to workers never assigned to PAB departments. The processes involving PAB and MBT overlapped for a 25 year period and workers exposed to MBT may have had undocumented exposure to PAB if they had a job working in many parts of the plant between 1935 and 1955.

In a follow up study, the mortalities of workers at the Nitro plant (West Virginia) with exposure to 2-mercaptobenzothiazole were evaluated (Collins 1999). In this study the mortalities of 1059 full time white male production workers employed at the plant from 1955 to 1977 were analyses. A detailed exposure assessment was done on the 600 workers with exposure to MBT. Nine years of additional follow up to the original study (Strauss 1993) were added. Although the number of all cancers in the original study was greater than expected in workers (cohort: 1059 white male production workers) at the plant, the number of cancers in the combined follow up was at expected levels. For the entire follow up period (1988-1996) lung cancer, prostate cancer, and the other cancer sites examined were at expected levels. Among the cancers, only rates of bladder cancer (3/0.9) were greater than expected for the entire follow up period. The standardised mortality ratios for total cancer of workers exposed to MBT (subcohort 600 production workers) were similar to the entire study group and to the local population. Rates for lung cancer and prostate cancer were at expected levels. The standardised mortality ratio for bladder cancer was greater than expected when compared with the external reference group (13/ 1.5). The number of total cancers was greater than expected (23/11.5) for MBT workers with one or more jobs with exposure to PAB (subcohort 89 production workers). The number of lung cancers (11/4.6) and bladder cancers (8/0.3) for this group of workers were greater than expected. The MBT workers without a job definite exposure to PAB during their careers (subcohort 511 production workers) had an observed total cancer rate which was lower than expected (40/52.3). Observed deaths from bladder cancer (5/1.2), however, were greater than expected in this group based on five deaths. The five workers who died of bladder cancer held jobs with plant-wide responsibilities. These five workers had job titles of maintenance worker, yard labourer, or general production worker and thus the authors suggested a possible risk of exposure to PAB. Workers with no potential exposure to PAB, or those workers hired after 1955 (subcohort 270 production worker), have a lower observed than expected risk of total cancer (3/9.7) and there were no deaths from bladder cancer. The authors concluded that MBT workers had bladder cancer rates which were greater than expected, but an unknown portion of these workers had exposure to PAB. The potential confounding exposure to PAB, a potent bladder carcinogen, in an unknown portion of the MBT workers makes it impossible to evaluate risk of bladder cancer in this population. Although the lack of bladder cancers among workers hired after PAB manufacture ended would argue against a relation with exposure to MBT, the number of expected deaths in this group of workers is too small to be conclusive.

In another epidemiological study mortality of workers employed at a chemical production plant in north Wales (Ruabon cohort) (Sorahan 1993) was investigated. MBT was used and manufactured along with vulcanisation inhibitors and accelerators, antioxidants, and many other proprietary products for the rubber industry. It was first manufactured on the site in 1932. Operations also involved sodium MBT and zinc MBT. Production of MBT derivatives such as dibenzothiazyl disulphide (MBTS), n-oxydiethyene-2-benzothiazolesulphenamide (MBS), and n-cyclohexyl-2-benzothiazolesulphenamide (CBS) began in 1939. The occupational exposure to the following compounds were also selected for investigation 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), n-cyclohexylthiophthalimide (CTP), phenyl-ß-napthylamine (PBN), aniline and ortho-toluidine. The study population included 2410 workers (2160 men, 250 women) who were active at the plant between 1955 and 1986 for six months minimum. Job histories were available in terms of some 300 job and department titles. Assessments of eight hour time weighted average exposure to both MBT and MBT derivatives were available for each job and department title and where used for assign exposure categories to MBT and/or its derivates (zero exposure, very low exposure 0-1 mg/m3, low 1-2.5 mg/m3, medium 2.5-6 mg/m3, or high exposure 6-20 mg/m3). These estimates based on monitoring data for the period 1977 and onward, review of process manuals and other company records for earlier years and discussions with long serving employees. Expectations were calculated from serial mortalities for England, Wales and local population. In general, the overall mortality experience of the cohort, both for all cancers, and all non-cancers, was close to (or below) expectation for all periods of follow up (including 30 or more years from first employment). The bladder cancer mortality rate of MBT exposed workers were not significant different from control population. There was considerable overlap in the membership of the subcohorts because of exposure to more than one chemical at the plant. Of the 360 men in the MBT subcohort, 45 were also exposed to PBN, 191 to aniline and 90 to TMQ. From the 360 worker of the MBT subcohort 119 died during the study period, three deaths from bladder cancer. Two of the three deaths from bladder cancer in the MBT subcohort were among the three deaths from bladder cancer in the aniline or ortho-toluidine cohort.

In a follow up study, the mortalities of workers at the chemical production plant in north Wales with exposure to 2-mercaptobenzothiazole were evaluated (Sorahan 2000). The mortality (1955-1996) and cancer morbidity experience (1971 to 1992) of a cohort of 2160 male production workers from chemical factory in north Wales were investigated. All subjects had at least 6 months employment at the factory and some employment in the period 1955 to 1984. Detailed job histories were abstracted from company computerised records and estimates of individual cumulative exposure to 2-mercaptobenzothiazole (MBT) and its derivates obtained, with a job exposure matrix derived by former factory hygienist. Duration of employment in the aniline, PBN and o-toluidine departments was also calculated. Based on serial rates for the general population of England and Wales, observed mortality for the total cohort was close to expectations for all causes. Significant excess were shown for cancer of the large intestine among worker exposed to MBT (observed: 7, expected: 2.73). However, the authors suggested that there is no convincing evidence that occupational chemical exposure can influence the risks of cancer of the large intestine, and the excess mortality for this disease in the subcohort exposed to MBT may well be a change finding or represent the effects of non-occupational factors. Significant excess were also shown for cancer of the bladder among workers exposed to MBT (observed: 7, expected 1.72). There was a significant (p<0.05) excess mortality from cancer of the bladder in the 605 study subjects potentially exposed to one or more of the four chemicals being investigated. This excess was dependent primarily on deaths occurring > 20 years after first exposure in those who started employment before 1955. There were 30 subjects in the total study cohort who, on basis of death certificates or cancer registration particulars, had had malignant bladder cancer. In separate analyses of the four exposure history variables, Poisson regression showed significant positive trends for risk of notification of bladder cancer increasing with cumulative duration of employment in the PBN, and o-toluidine departments; similar findings were not obtained for cumulative exposure to MBT or for duration of employment in the aniline department. The authors concluded that it seems likely that some members of this cohort have had occupational bladder cancer. Confident interpretation is difficult because of small numbers in the exposed subcohorts, relatively crude measures of exposure assessment for the four chemicals under study, and presence of unconsidered potential chemical confounders.

In a more recent follow up study, the mortalities and morbidity of workers at the chemical production plant in north Wales with exposure to 2-mercaptobenzothiazole were evaluated (Sorahan 2008, 2009). The mortality (1955–2005) and cancer morbidity experience (1971–2005) of a cohort of 2160 male production workers were investigated. Different subcohorts with MBT, PBN, ortho-toluidine and aniline exposure were analysed. There was considerable overlap in the membership of more than one of the four subcohorts. Two analytical approaches were used, indirect standardization and Poisson regression. Mortality from all causes combined in the overall cohort was close to expectation (observed 1334, expected 1302.3), except mortality from lung cancer, which was lower (observed 120, expected 131.9). Mortality from bladder cancer in the overall cohort was non-significantly elevated (observed 22, expected 14.43). The combined subcohorts (MBT, PBN, ortho-toluidine, aniline n = 611) showed an increase in bladder cancer mortality (observed 11, expected 3.96). There was also an increase in bladder cancer incidence in the combined subcohorts (observed 18, expected 8.43). In the combined subcohorts there was a significant positive trend in SMRs for bladder cancer (p<0.05) with period from commencing exposure but not indication of risks decreasing with period from ceasing exposure. In the MBT subcohort (n = 363) mortality from all causes was close to expectation; whereas a significant excess mortality was found for all neoplasm, cancers of the large intestine (observed: 8, expected: 3.45) and bladder (observed: 8, expected: 2.14). Non-significant excesses were shown for lung cancer and multiple myeloma. Non-significant excesses were also shown for cancer of the kidney and other cancers of the urinary tract. Based on national cancer incidence rates, significant excess morbidity was found for all malignant neoplasms, for cancer of the bladder (observed: 12, expected: 4.75) and multiple myeloma (observed: 4, expected: 0.86). Non-significant excesses were shown for cancers of the large intestine and lung. Non-significant excesses were also shown for cancer of the kidney and for other cancers of the urinary tract. Significant positive trends are shown for cancer of the large intestine and multiple myeloma in relation to MBT exposure; however the significant trend shown for multiple myeloma is based on very small numbers of deaths (MBT: none: 2, 0.01 to 21.24 mg/m3 y: 2, 21.25 to 63.74 mg/m3 y: 2, >63.75 mg/m3 y: 0). Moreover, the dose-response relationship is questionable, no cases in the highest category. Referring to the findings on the large intestine the author suggested in an earlier publication (Sorahan 2000) “that there is no convincing evidence that occupational chemical exposure can influence the risks of cancer of the large intestine, and the excess mortality for this disease in the subcohort exposed to MBT may well be a change finding or represent the effects of non-occupational factors.” A significant positive trend was seen in the MBT subcohort to relative risk of bladder cancer when analysis was performed adjusted for age and calendar period. However no significant positive trend was noted when single analysis was performed in which all four variables are considered simultaneously and simultaneous analysis was limited to three exposures. However, the reliability of these findings are limited because of the relatively small number of cases in the exposed subcohorts, the crude measures of exposure assessment for the four chemicals under study and the possible of unconsidered potential chemical confounding.

A re-evaluation of above discussed human data has been finished (Environ 2010); with special emphasis of the published data from the Ruabon cohort (Sorahan 2000, 2008, 2009). The re-evaluation of the study data of the Ruabon cohort did not reveal new or compelling evidence supporting a conclusion that MBT may be a human bladder carcinogen. Limitations of this study include difficulties in assigning employees to exclusive exposure specific sub-cohorts, due to the broad cases hampering substance specific evaluation of a possible cancer risk; and combining non-cancer (i.e. benign bladder tumors) with true bladder cancers. The authors of the re-evaluation agreed that some of the members of the Ruabon cohort likely suffered occupational bladder cancer, the evidence remains inadequate to clearly delineate whether any may have been caused by exposure to MBT.

In summary, the re-evaluation of all available epidemiological information cannot identify any specific new evidence that would substantiate the notion that MBT is a bladder carcinogen in humans.

In conclusion:

Oral route:

The authors of the 2-year gavage study (NTP 1988) concluded that under the conditions of this study, there was some evidence of carcinogenic activity of MBT for male F344/N rats, indicated by increased incidences of mononuclear cell leukemia, pancreatic acinar cell adenomas, adrenal gland pheochromocytomas, and preputial gland adenomas or carcinomas (combined). They also stated that there was some evidence of carcinogenic activity for female F344/N rats, indicated by increased incidences of adrenal gland pheochromocytomas and pituitary gland adenomas. However, the increase in mononuclear cell leukemia, pituitary gland adenomas and pancreatic acinar cell adenomas in male rats were increased only in the low dose groups and thus not dose-dependent. The incidences of adrenal gland pheochromocytomas, and preputial gland adenomas or carcinomas (combined) were significant increased in low and high dose males. The increase noted was more pronounced in the low dose group than in the high dose group; thus no clear dose-effect relationship was revealed. In high dose females incidences of adenomas of the pituitary gland and adrenal gland pheochromocytomas were significant increased and occurred with a significant positive trend. The observed incidences were within the bounds of historical control range. However, the relevance of adrenal gland pheochromocytomas in rats is questionable. Recent data analysis revealed that occurrence of pheochromocytomas in MBT treated rats, is associated with nephrotoxicity associated with endocrine disturbance (Greim 2009). The relevance of this finding for humans is questionable. In summary it should be noted that the study is severely compromised by the poor survival rate in male rats, indicating that the MTD (maximum tolerated dose) was presumable exceeded, and by the fact of the higher tumour rates in the low dose than in the high dose groups in general (absence of a dose-response relationship). The observed differences in the tumour incidences of dosed animals compared to the concurrent controls do not appear to be significant when historical controls are taken into account. The overall rates of individual tumours in treated animals do not exceed the historical range.

In the other two 2-year gavage study (NTP 1988), the author concluded that there was no evidence of carcinogenic activity of 2-mercaptobenzothiazole for male B6C3F1 mice dosed with 375 or 750 mg/kg; there was equivocal evidence of carcinogenic activity for female B6C3F1 mice, indicated by increased incidences of hepatocellular adenomas or carcinomas (combined). However, this increase was not dose-dependent and the incidences were within the historical control data range.

The carcinogenic potential of MBT was evaluated in an early carcinogenicity study (NCI 1968) and was not found to be tumorigenic when given orally to mice for 18 months at the maximal tolerated dose. Moreover, in a chronic feeding study (Ogawa 1989) done with mice (strain Slc: ddY) no carcinogenic potential of MBT was revealed up to 1920 ppm (ca 248 mg/kg bw) in the diet, a dose that produced evident signs of toxicity.

Other routes:

No carcinogenic potential of MBT was observed in an early carcinogenicity study (NCI 1968) with mice, which were treated once subcutaneous with MBT suspended in DMSO. In another subcutaneous early carcinogenicity study (NCI 1968) 215 mg/kg MBT in 0.5% gelatine was given in the nape of the neck to weanling mice on approximately the 28th day of age. A dose-independent increase in incidences in reticulum cell sarcoma was noted in treated male B6CF31 mice (5 vs. 2 in control); the total number of males with tumour and the incidences of hepatoma and pulmonary adenomas and carcinomas were not increased compared to control. However, the biological relevance of these findings is questionable. The tumour incidences of female B6CF31 mice and male and female B6AKF1 mice were comparable to control.

Human data:

Epidemiologic evaluations were done to investigate the cancer risk of MBT exposed workers.

Mortality trends for production workers at a rubber chemical plant in Nitro, West Virginia were examined to find whether they had increased mortality from cancer associated with exposure to 2-mercaptobenzothiazole (MBT) and/or its derivates (Strauss 1993). Because of concern about the potential confounding effect of exposure to PAB, a potent bladder carcinogen, all worker exposed to PAB were identified. Mortality for the MBT subcohort was slightly lower than local mortality; whereas the mortality for all cancers was slightly raised. Increased death rates were seen for lung cancer, prostate cancer and bladder cancer. An increase for all cancers, lung cancer and bladder cancer were noted in MBT workers with PAB exposed job assignments. For the subcohort of MBT exposed, but not PAB exposed workers standardized mortality ratios were increased for bladder cancer and prostate cancer. Whereas the ratios of MBT exposed worker, which were hired after termination of PAB production was significant lower than expected, and there were no deaths due to cancer or bladder cancer. However, the increase in bladder cancer observed in MBT workers, not exposed to PAB according to job assignment, based only on three deaths. All three cases occurred in the two highest cumulative exposure categories, and with at least 20 years since first exposure. The authors did not exclude a compounding effect by PAB exposure, a potent bladder carcinogen. Processes involving PAB and MBT overlapped for 25 year period and workers exposed to MBT may have had undocumented exposure to PAB. This suggestion is supported by the finding, that workers hired after termination of PAB production did not show increase ratios of cancer or bladder cancer.

In the follow up study (Collins 1999) an increase in bladder cancer was also noted in the follow up in workers exposed to MBT and PAB and workers exposed to MBT without a job definite exposure to PAB; whereas workers with no potential exposure to PAB or those workers hired after 1955, had no death from bladder cancer. The authors concluded that MBT workers had bladder cancer rates which were greater than expected, but an unknown portion of these workers had exposure to PAB. The potential confounding exposure to PAB, a potent bladder carcinogen, in an unknown portion of the MBT workers makes it impossible to evaluate risk of bladder cancer in this population. Although the lack of bladder cancers among workers hired after PAB manufacture ended would argue against a relation with exposure to MBT.

The mortality of workers employed at a chemical production plant in north Wales was evaluated in a parallel epidemiological study (Sorahan 1993). The study population included 2410 workers who were active between 1955 and 1986. Workers were exposed to MBT and its derivates. An occupational exposure to TMQ, CTP, phenyl-ß-napthylamine (PBN), aniline and orto-toluidine was assumed and was also investigated. The overall mortality experience of the cohort, both for all cancers, and non-cancers, was close to (or below) expectation for all periods of follow up. In general, the bladder cancer mortality rate of MBT exposed workers was not significant different from control. There was considerable overlap in the membership of the subcohorts because of exposure to more than one chemical at the plant. Two of the three bladder cancer deaths in the MBT subcohort were among the three deaths from bladder cancer in the aniline or ortho-toluidine cohort.

In a follow up study the mortality (1955 – 1996) and cancer morbidity experience (1971 to 1992) of a cohort of 2160 male production workers from chemical factory in north Wales were investigated (Sorahan 2000). A significant increase was shown for cancer of the large intestine among worker exposed to MBT (7/2.73). However the relevance of the finding is questionable, because there is no convincing evidence that occupational chemical exposure can influence the risks of cancer of the large intestine, and the excess mortality for this disease in the subcohort exposed to MBT may well be a change finding or represent the effects of non-occupational factors. A significant increase was also shown for cancer of the bladder (7/ 1.72) among worker exposed to MBT (subcohort 357 men). However, separate analysis of relative risk of bladder cancer with cumulative exposure to MBT did not reveal a significant positive trend. A chemical confounding by the three other chemicals could not be excluded.

In a more recent follow up study the mortality and cancer morbidity from worker at the chemical production plant in north Wales were evaluated (Sorahan 2008, 2009). An increase in bladder cancer mortality and morbidity was seen in MBT exposed workers and in combined subcohorts exposed to the four chemicals MBT, PBN, ortho-toluidine, and aniline. In the combined subcohort there was a significant positive trend in SMRs for bladder cancer. A significant positive trend was seen in the MBT subcohort to relative risk of bladder cancer when analysis was performed adjusted for age and calendar period; whereas no significant positive trend was noted when single analysis was performed in which all four variables are considered simultaneously or simultaneous analysis was limited to three exposures. However, the reliability of these findings are limited because of the relatively small number of cases in the exposed subcohorts, the crude measures of exposure assessment for the four chemicals under study and the possible of unconsidered potential chemical confounding. The relevance of cancer of large intestine in MBT exposed workers is questionable, because there is no convincing evidence that occupational chemical exposure can influence the risks of cancer of the large intestine. The relevance of multiple myeloma is also questionable because of the low case numbers and the lack of a dose-response relationship.

The re-evaluation (Environ 2010) of all available epidemiological information with specially emphasis of data from the Ruabon cohort (Sorahan 2000, 2008, 2009) cannot identify any specific new evidence that would substantiate the notion that MBT is a bladder carcinogen in humans.