Inhibition of mouse skin tumor promotion by adriamycin and daunomycin in combination with verapamil or palmitoylcarnitine

The anti-cancer drugs Adriamycin (ADR) and Daunomycin (DAU) alone were unable to inhibit the promotion of skin papillomas by repeated applications of 8.5 nmol of 12-O-tetradecanoylphorbol-13-acetate (TPA) in 7,12-dimethylbenz(a)anthracene (DMBA)-initiated mice. Pretreatments with 50 μg of ADR also failed to alter the tumor-promoting activities of smaller doses of TPA. Therefore, the effects of the anthracycline antibiotics on skin tumor promotion were evaluated in combination with the Ca²⁺ antagonist verapamil (VRP) and the protein kinase C (PKC) inhibitor palmitoylcarnitine (PC), compounds known to circumvent drug resistance. When applied simultaneously with each promotion treatment with 8.5 nmol of TPA, 2.5 mg of VRP inhibited the number of papillomas/mouse by 26%. But the combination of VRP + 50 μg of ADR or DAU inhibited the yields of papillomas by 50 or 47%, respectively, suggesting that VRP was required to reveal the antitumorpromoting activities of otherwise ineffective drugs. Similarly, the promotion of skin tumors by TPA was inhibited synergistically by the combinations of 2 μmol of PC + 50 μg of ADR or DAU. For instance, ADR and DAU had no effects alone but inhibited the incidence of skin papillomas by 78 and 86%, respectively, in the presence of PC, a compound which alone inhibited the tumor incidence by only 44%. The results indicate that ADR and DAU are effective against the promoting component of skin carcinogenesis only if they are applied in combination with Ca²⁺ antagonists or PKC inhibitors at a time when they can inhibit the early biochemical effects induced by TPA.