Rapid induction of thymic lymphomas by isopropyl methanesulfonate: a preliminary report.

The direct-acting SN1 alkylating agent isopropyl methanesulfonate (IMS) was carcinogenic by subcutaneous injection in female Hsd:(ICR)BR mice, causing thymic lymphoid neoplasms within 7 months in at least 20 of 32 treated mice. No such neoplasms were observed in mice treated with the direct-acting SN2 methyl homolog, methyl methanesulfonate (MMS). Both the IMS-treated mice and the MMS-treated mice initially received 20 mumole of the respective compounds by sc injection once weekly; however, because of toxic effects the dose of IMS was reduced to 10 mumole per injection on the 63rd day and further reduced to 5 mumole per injection on the 120th day, after which this dose was maintained until the 202nd day when the last surviving IMS-treated mouse became moribund and was sacrificed. In 2 of the MMS-treated mice, 93% of which were alive at 288 days, tumors were observed at the site of injection, one being a papilloma and the other a subcutaneous sarcoma. IMS has not previously been implicated as a carcinogen, to our knowledge. Its induction of thymic lymphomas may conceivably be related to its ability to alkylate exocyclic oxygen atoms in the DNA of hemopoietic cells.