NSRL for MX

The cancer potency of MX ("mutagen X"; 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)furanone) was estimated from dose-response data of multiple MX-responding tumor sites observed among male and female rats exposed orally (Komulainen et al., 1997; 2000). These sites were liver, adrenal and thyroid in both sexes and mammary gland in the female. Other sites were associated with treatment, but were judged likely to contribute only minimally to the overall potency estimate, and thus were not included in the analyses. For each of the tumor sites listed above, a probability distribution of cancer potency estimates was derived using likelihood theory. The linear term (q1) of the multistage model fit to dose response data for a given site represents the cancer potency for that site. A combined distribution representing cancer potency for all selected sites affected by MX was derived through Monte Carlo analysis. The upper 95 percent confidence bound indicated by the combined distribution for these MX-related tumor sites was taken as the cancer potency for MX. The potency derivation takes into account body size differences between humans and experimental animals. The Proposition 65 “no significant risk level” (NSRL) is defined in regulation as the daily intake level posing a 10 lifetime risk of cancer. The cancer potency estimate and corresponding NSRL are given in Table 1.