Spontaneous mononuclear cell leukemia is a confounding factor in evaluating chemical leukemogenicity in the NTP 2-year carcinogenicity studies. A short-term assay for F344 rat leukemia was developed to better discriminate between age-induced and chemically-enhanced leukemia. The accuracy and sensitivity of the transplant model for predicting the long-term leukemogenic potency of chemicals was confirmed in short-term assays with 7 chemicals that had increased or decreased the prevalence of leukemia in previous 2-year carcinogenicity studies. Additional studies with the short-term assay revealed structure-activity relationships for chemicals that were either negative or positive for leukemic trends. Nine different glycol ethers were evaluated in the short-term assay for anti-leukemic activity. Of these, only ethylene glycol monomethyl ether and ethylene glycol monoethyl ether exhibited chemotherapeutic potential. Ethylene glycol monomethyl ether was a more potent antileukemic agent than the monoethyl ether, and at non-toxic doses in drinking water completely eliminated the early manifestations of leukemia, prevented early mortality, and doubled the tumor latency period. These data were confirmed by in vitro tests with suspended leukemic cell cultures. Presently, the chemotherapeutic potential for the acid and aldehyde metabolites of ethylene glycol monomethyl ether, as well as chemicals in the propylene glycol monomethyl glycol alkyl ether series, are being evaluated. By contrast, two chemicals containing dimethyl esters of phosphoric acid (dichlorvos and trichlorfon) enhanced the expression of leukemia in the short-term assay and in 2-year carcinogenicity tests. Three other chemicals with the same structural relationship: dimethyl hydrogen phosphite, dimethyl methylphosphonate, and dimethylmorpholinophosphoramidate also increased the incidence of leukemia in recently completed 2-year studies. These observations suggest that the dimethyl phosphoric acid ester moiety should be considered a structural alert for leukemogenicity. Presently, the leukemogenic potential of acetaminophen is being evaluated in the short-term leukemia transplant model to confirm and extend the observations obtained from 2-year studies.
