Chemotherapy with [SP-4-3-(R)]-[1,1-cyclobutanedicarboxylato(2-)](2- methyl-1,4-butanediamine-N,N')platinum (CI-973, NK121) in combination with standard agents against murine tumors in vivo.

[SP-4-3-(R)]-[1,1-cyclobutanedicarboxylato(2-)](2-methyl-1,4- butane-diamine-N,N')platinum (CI-973) is a cisplatin analogue that is currently in clinical trial. Preclinically, CI-973 retained activity against L1210, P388, K562, and human ovarian carcinoma sublines resistant to cisplatin in vitro. CI-973 also retained substantial activity [ratio of median life span of treated to control groups x 100% (%T/C) > 190] against cisplatin resistant L1210 and P388 sublines in vivo. Good activity (stasis or tumor burden reduction) was also obtained against five murine solid tumors including breast, colon, and sarcoma. Binary combination therapy with CI-973 and seven clinically utilized anticancer agents was evaluated against murine tumors in vivo for the ability of each combination to produce a superior therapeutic response compared to optimal single agent therapy alone at tolerated doses. The seven agents combined with CI-973 were mitomycin C, cyclophosphamide, doxorubicin, vinblastine, etoposide, ifosfamide, and methotrexate. Of the combination regimens evaluated, only the combination of CI-973 and methotrexate was therapeutically superior to single agent therapy. Against i.v. inoculated P388 leukemia, combination therapy with CI-973 at 32 mg/kg/injection and methotrexate at 43 mg/kg/injection produced 5.3 logs greater cell kill (%T/C = 284, with one cell surviving therapy) than either methotrexate therapy (%T/C = 208, with 2.5 x 10(5) cells surviving therapy) or CI-973 therapy (%T/C = 193 with 2.5 x 10(6) cells surviving therapy) alone. The combination toxicity index of 1.0 indicated additive normal host tissue toxicity with the same target organs for dose limiting toxicity. To better understand the enhanced cell kill in vivo, the combination of CI-973 and methotrexate was evaluated against P388 leukemia in vitro. Clonogenic survival studies showed that the combination of CI-973 and methotrexate was additive rather than synergistic with respect to cell killing in vitro. The lack of greater than additive cell kill in vitro suggested that the mechanism responsible for synergistic kill in vivo was not operative in vitro. The in vivo results suggest that the combination of CI-973 and methotrexate may be useful in the clinic.