Synergistic and additive combinations of several antitumor drugs and other agents with the potent alkylating agent adozelesin.

Adozelesin is a highly potent alkylating agent that undergoes binding in the minor groove of double-stranded DNA (ds-DNA) at A-T-rich sequences followed by covalent bonding with N-3 of adenine in preferred sequences. On the basis of its high-potency, broad-spectrum in vivo antitumor activity and its unique mechanism of action, adozelesin has entered clinical trial. We report herein the cytotoxicity for Chinese hamster ovary (CHO) cells of several agents, including antitumor drugs, combined with adozelesin. The additive, synergistic, or antagonistic nature of the combined drug effect was determined for most combinations using the median-effect principle. The results show that in experiments using DNA- and RNA-synthesis inhibitors, prior treatment with the DNA inhibitor aphidicolin did not affect the lethality of adozelesin. Therefore, ongoing DNA synthesis is not needed for adozelesin cytotoxicity. Combination with the RNA inhibitor cordycepin also did not affect adozelesin cytotoxicity. In experiments with alkylating agents, combinations of adozelesin with melphalan or cisplatin were usually additive or slightly synergistic. Adozelesin-tetraplatin combinations were synergistic at several different ratios of the two drugs, and depending on the schedule of exposure to drug. In experiments using methylxanthines, adozelesin combined synergistically with noncytotoxic doses of caffeine or pentoxifylline and resulted in several logs of increase in adozelesin cytotoxicity. In experiments with hypomethylating agents, adozelesin combined synergistically with 5-azacytidine (5-aza-CR) and 5-aza-2'-deoxycytidine (5-aza-2'-CdR). Combinations of adozelesin with tetraplatin or 5-aza-2'-CdR were also tested against B16 melanoma cells in vitro and were found to be additive and synergistic, respectively. The synergistic cytotoxicity to CHO cells of adozelesin combinations with tetraplatin, 5-aza-CR, or pentoxifylline was not due to increased adozelesin uptake or increased alkylation of DNA by adozelesin.