Sulfur mustard carcinogenicity

J. Ashby, H. Tinwell, R. D. Callander and N. Clare, Genetic-Activity of the Human Carcinogen Sulfur Mustard Towards Salmonella and the Mouse Bone-Marrow, Mutat. Res., 1991, 257, 307-311. 

Because of the correlation between mutagenicity and carcinogenicity, one would expect sulfur mustard to be carcinogenic on the basis of mutagenicity data alone. This expectation is borne out by carcinogenicity tests in experimental animals and by data from human exposures. The International Agency for Research on Cancer classifies sulfur mustard as one of relatively few chemical agents on which the data are adequate to show an association with the induction of cancer in humans.7... 

Sulfur mustards (designated H [mustard], HD [distilled mustard], and HT [HD and T mixture]) do not present acute lethal hazards. Their principal effect is severe blistering of the skin and mucous membranes. Epidemiological evidence indicates a causal relationship between exposure to mustard agent at high concentrations and the development of chronic nonreversible respiratory disorders, such as chronic bronchitis and asthma, and ocular diseases, such as delayed recurrent keratitis and prolonged, intractable conjunctivitis (IOM, 1993). Sulfur mustard has been classified as a known human carcinogen based on evidence of in-... 

Sulfur mustard is a known human carcinogen, and some of its degradation products may also be carcinogenic (IOM, 1993). Sulfur mustard acts as a vesicant or blister agent and shows acute systemic toxicity in addition to its effects on skin, eyes, and the respiratory tract. 

Caution The intermediate 2-chloroethyl sulfides are very toxic (sulfur mustards) and are very powerful vesicants and potent carcinogens. This protocol must be undertaken in an efficient hood and disposable vinyl and/or latex gloves and chemical-resistant safety goggles must be worn at all times. 

SF. Because of that deficiency, the RfD for sulfur mustard is estimated by extrapolating from a subchronic study in animals, and the SF is established by applying comparative carcinogenic potency methods. The absence of chronic oral toxicity data can be addressed by conducting a chronic oral animal bioassay. It is important that sulfur mustard be delivered to animals at a slow rate (i.e., in the diet) rather than by stomach tube, because it is corrosive at the point of entry. 

Because sulfur mustard is the only agent identified in this report as a carcinogen, a description of the derivation of the carcinogenic slope factor is presented in the chapter on sulfur mustard (see Chapter 7). 

The subcommittee agrees with ORNL that calculating an SF for sulfur mustard using the relative potency approach was more appropriate than using estimates from inhalation unit risk. The subcommittee notes, however, that a recent study by Culp et al. (1998) reported a lower carcinogenic potency value for B[a]P. That chronic exposure study of B[a]P in feed was conducted under Good Laboratory Practice conditions in B6C3Fi female mice (Culp et al. 1998). The incidence of forestomach tumors was found to be 1 of 48, 3 of 47, and 36 of 46 at concentrations... 

In the absence of a chronic bioassay for sulfur mustard, the two approaches described above for estimating an upper limit on the carcinogenic potency give remarkably similar results—1.6 and 5.3 per mg/kg per day for lifetime exposure. Those potency values are less than an order of magnitude lower than the 9.5 per mg/kg per day derived by ORNL (see Table 7-2). That would indicate that the potency estimate of 132 per mg/kg per day relative to BCME (described earlier) is too high... 

TABLE 7-2 Estimates of the Upper Limit for Carcinogenic Potency of Sulfur Mustard... 

The strength of evidence for the SF of sulfur mustard is poor because no epidemiological or animal carcinogenicity studies have been conducted... 

The approach selected by ORNL to calculate the SF for sulfur mustard was scientifically valid given the absence of epidemiological or animal carcinogenicity studies of sulfur mustard. Another approach using the MTD for estimating the potency of sulfur mustard yielded a similar result. Using an updated estimate for B[a]P potency, the subcommittee believes that the Army s interim SF of 9.5 per mg/kg per day should be lowered to 1.6 per mg/kg per day. 

Watson, A.P., T.D. Jones, and G.D. Griffin. 1989. Sulfur mustard as a carcinogen Application of relative potency analysis to the chemical warfare agents H, HD, and HT. Regul. Toxicol. Pharmacol. 10 1-25. 

Information on the potential carcinogenicity of sulfur mustard is available primarily from studies on rats and mice. McNamara et al. (1975) exposed SDW rats, ICR Swiss albino and A/J mice, rabbits, guinea pigs, and dogs to sulfur mustard vapors for varying exposure durations up to one year. The test animals were exposed to 0.001 mg HD/m continuously or to 0.1 mg HD/m for 6.5 hr followed by... 

Inhalation unit risks have been derived for sulfur mustard directly from experimental animal data as weU as from an analysis of the relative carcinogenic potency of sulfur mustard in comparison with that of known carcinogens for which there are both long-and short-term data. Epidemiological and long-term animal data are not available to directly derive an oral slope factor for sulfur mustard. Estimates of the oral slope factor are made from the inhalation unit risk and from the relative potency method. 

U.S. EPA (1991) derived a cancer inhalation unit risk for sulfur mustard based on the results of inhalation animal studies conducted by McNamara et al. (1975, see Section 3.7.2) however, it was emphasized in the EPA report that the studies of McNamara et al. (1975) contained deficiencies which made a quantitative analysis difficult. Conducted in 1970, the studies do not conform to the modem norms of acceptable experimental protocol, and it is likely that there was bias in the assignment of the animals to the test categories (U.S. EPA, 1991). In addition, many of the exposures were very brief, included only a few animals, and many of the animals were sacrificed (and some were replaced) before their capacity to develop late-appearing tumors was fully developed (U.S. EPA, 1991). Despite these shortcomings, it was noted by EPA that the McNamara et al. data are the best available for estimating the carcinogenic potency of sulfur mustard. The authors of the EPA report analyzed two sets of McNamara s data one from a toxicity study and one from a carcinogenicity study (see Section 3.7.2). 

Considering all the above data, the U.S. EPA (1991) selected the unit risk of 8.5 x 10 per pg/m, derived from the Weibull time-to-tumor model, as the recommended upper bound estimate of the carcinogenic potency of sulfur mustard for a lifetime exposure to HD vapors. However, U.S. EPA (1991) stated that "depending on the unknown true shape of the dose-response curve at low doses, actual risks may be anywhere from this upper bound down to zero". The Weibull model was considered to be the most suitable because the exposures used were long-term, the effect of killing the test animals before a full lifetime was adjusted for, and the sample size was the largest obtainable from the McNamara et al. (1975) data. 

The inhalation carcinogenicity of sulfur mustard has also been evaluated using relative potency methods. Using the results of studies by Heston (1950, see Section 3.7.2) and Shimkin and McClelland (1949), U.S. EPA (1991) determined that the potency of sulfur mustard to induce pulmonary tumors in strain A mice was equivalent to that of 20-methylcholanthrene (MC). EPA then used the results of studies conducted by Stoner et al. (1984) to determine that MC was 10-13 times more potent than benzo(a)pyrene (BaP) in inducing lung tumors in this same strain of mice. Since the potency of sulfur mustard was... 

Long-term oral carcinogenicity studies have not been conducted on sulfur mustard. In oral subchronic studies in which the agent was administered by gavage to rats (see section 3..3), epithehal hyperplasia of the forestomach occurred in 5/12 males and 5/12 females dosed with 0.3 mg HD/kg/day (in sesame oil), 5 days/week for 13 weeks. In hght of the known carcinogenicity of sulfur mustard, the epithehal hyperplasia is suggestive of a pre-neoplastic condition. 

The estimated comparative carcinogenic potency can be used to derive a slope factor (SF) or qi for sulfur mustard. The slope factor converts the estimated daily intake averaged over a lifetime exposure to incremental risk of an individual developing cancer. Because the slope factor is an upper 95th percentile confidence limit on the probability of response based on experimental animal data, the carcinogenic risk will generally be an upper-bound estimate. 

Because sulfur mustard is known to be a strong and direct DNA alkylating agent, the likelihood is very high that it functions as a non-threshold carcinogen. Conseqnently, the risks associated with exposures to low concentrations require evaluation, and the McNamara et al. (1975) stndy provides the only data set that allows for a quantification of carcinogenic potency following exposnre to snlfnr mnstard vapors. 

In the absence of human or animal oral dose-response data, the relative potency approaches developed by Watson et al. (1989) and U.S. EPA (1991) are considered to be appropriate methods for estimating the tnmorigenic potency of sulfur mustard by the oral route of exposure. The oral slope factor derived by Watson et al. is approximately one order of magnitude less than the one derived from the relative potency estimated by U.S. EPA (1991). In the emerging area of relative potency analysis, a factor of 10 difference represents a good fit. There is no significant difference between the estimates of sulfur mustard carcinogenic potency relative to B(a)P pnblished by Watson et al. (1989) and U.S. EPA (1991). 

In the review by Papirmeister et al. (1991), it was noted that sulfur mustard-induced cytotoxicity is dose dependent and that DNA appeared to be more sensitive to mustard-induced alkylation than are other cellular constituents. The low-dose effects of sulfur mustard are characterized by gen-otoxicity and inhibition of mitosis. The loss of cellular reproduction may be due to bifunctional alkylation that ultimately prevents normal DNA replication. It was hypothesized that monofunctional DNA damage might be responsible for low-dose mutagenic and possibly carcinogenic effects. 

The carcinogenicity of sulfur mustard in animals has been reviewed in lARC (1975), Watson et al. (1989), lOM (1993),... 

---