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Dose-response studies with a pure tumor-promoting agent, phorbol myristate acetate.

The dose-response relationship of the mouse skin tumor-promoting agent, phorbol myristate acetate (PMA), was determined over a dose range of 0.02 to 25 µg/application. The effect of frequency of application was also examined by three times weekly or once weekly treatments. Administration of PMA three times weekly gave a maximal tumor response with all of the mice at risk in each of the three highest dosage groups, 25, 5, and 2.5 µg, developing papillomas. At the lower dose, 0.5 µg, the tumor incidence was 72% of the population at risk, and no tumors were observed at the two lowest doses, 0.1 and 0.02 µg. The time to 50% incidence of animals with tumors was 7.5 to 8 weeks for each of the three maximal response groups, indicating that the rate of appearance of tumor-bearing mice was similar in each of these groups. However, if the total number of papillomas is compared, a dose-response relationship is clearly apparent. It was found that 2.5 µg of PMA given three times weekly is an optimal dose to obtain 100% of the mice with papillomas, although the total numbers of papillomas and cancers are significantly fewer than at the two higher doses of tumor promoter. While tumor promotion is accompanied by epidermal hyperplasia and inflammation, these manifestations are in all likelihood not related to the mode of action of PMA. A new abbreviated method for the preparation of pure PMA is described.