The induction of urothelial hyperplasia by methyl methanesulphonate and ethyl methanesulphonate.

The early and late morphological changes induced in rat bladder urothelium by intravesicular administration of the alkylating agents methyl methanesulphonate (MMS) and ethyl methanesulphonate (EMS) are described. In the short-term, both compounds produced dose-related toxic damage followed by a regenerative hyperplasia of the urothelium. At any given dose-level, the effects of MMS were more severe than those of EMS. Two years after administration of multiple doses of 2.5 mg MMS or 7.5 mg EMS the majority of animals had dose-related simple urothelial hyperplasias with occasional mild dysplasia. However, in three MMS-treated animals the hyperplasias had progressed to well-differentiated transitional-cell carcinomas. No bladder neoplasms were seen in EMS-treated animals. The urothelial response of the rat to MMS and EMS is discussed with reference to the known chemical reactivity of these compounds. It is concluded that EMS is a mitogen for the urothelium and that the few carcinomas which develop following topical exposure of the bladder to MMS do not necessarily reflect any initiating potential in this compound. Rather it is argued that the results are consistent with MMS acting as a promoter in cells which have either been previously initiated or which carry a latent oncogene.