[Combination effects of etoposide with other antitumor drugs in vitro and in vivo].

Combination effects of etoposide (ET) with each of 10 antitumor drugs were examined with P 388 leukemia cells in vitro and in vivo. Median effect analysis was applied for the evaluation of in vitro effect by the growth inhibition, and the in vivo effect by comparison of the increase of life span (ILS) in a combined group with the sum of ILS's in 2 single agent groups. Among 10 drugs combined with ET, cyclophosphamide (melphalan was used for in vitro study), cisplatin and 6-mercaptopurine exhibited a strong synergism both in vitro and in vivo. The combination of ET with mitomycin C, vincristine, vindesine or cytarabine produced additive or slightly synergistic effect in the both systems. However, methotrexate + ET combination showed an antagonistic effect. Although the combination of ET with doxorubicin or fluorouracil showed a slight synergism in vitro, it was antagonistic in vivo. Thus, the in vitro and in vivo combined effects were consistent in 8 of 10 drugs. The employed methods in the present studies could distinguish high efficacy of ET + cyclophosphamide and ET + cisplatin, which are clinically approved as effective combinations against lung cancer. The methods seem to be useful to assess the drug efficacy in experimental combination.