Studies on drug resistance in a human melanoma xenograft system

Alkylating agents and their functional analogues belong to the most useful antineoplastic drugs in the treatment of disseminated malignant melanoma. In conjunction with an open clinical phase II trial evaluating the combination of cisplatin and ifosfamide, 17 melanoma xenograft lines were established from patients often refractory to dacarbazine (DTIC). These xenograft lines were exposed to cisplatin, dacarbazine, dibromodulcitol, ifosfamide, methyl-CCNU, mitomycin C, and malonato-diaminocyclohexane-platinum II (PHM) at the respective LD 10/30 doses. Growth delay values less than 2 corresponded in 26/27 instances with progressive disease, whereas values greater than 2 corresponded in only 10/13 instances with achievement of a no-change status or a partial remission of the donor patient's disease. Among the panel of DNA-damaging agents tested, cross-resistance was incomplete. Some xenograft lines revealed unique chemosensitivity patterns in contrast to a uniform pattern of drug resistance in others (pleiotropic or multidrug resistance). The data confirm independently of results obtained in the phase II study that the combination of cisplatin and ifosfamide is effective against malignant melanoma refractory to dacarbazine. Suboptimal drug exposure, repeated up to 21 transplant generations, was employed to induce secondary resistance to either dacarbazine, melphalan or methyl-CCNU in a melanoma xenograft line originally quite sensitive to drug treatment. When the resistant sublines were exposed to the other agents, only partial cross-resistance was observed. Tumour volume responses to treatment with dacarbazine correlated with persisting DNA damage assayed 24 h after in vivo drug exposure in a sensitive line and the absence of such lesions in a resistant line.