Manipulation of oxygenation in a human tumour xenograft with BW12C or hydralazine: effects on responses to radiation and to the bioreductive cytotoxicity of misonidazole or RSU-1069.

The influence of altered tumour oxygenation on the responses to radiation and/or bioreductive 2-nitroimidazole compounds was studied in a well differentiated, human, colon adenocarcinoma (MAWI), grown as a subcutaneous xenograft in nude mice. Tumour growth delays were measured after local, single 5-18 Gy doses of X-rays. BW12C, which inhibits dissociation of oxyhaemoglobin, produced radioprotection similar to that resulting from clamping off the tumour blood supply during irradiation. Hydralazine, a vasoactive agent, also appeared to give radioprotection. BW12C or misonidazole (MISO) alone had no measurable inhibitory effect on xenograft growth. Hydralazine or RSU-1069 slightly increased the time for tumours to reach 6 times their original volumes. When hydralazine was given 40 min after a dose of 800 mg/kg of MISO, without X-rays, growth delays in excess of 5 tumour volume doubling times resulted and fewer tumour cells were present in histological sections. Lower doses of MISO combined with hydralazine were ineffective. Other combinations of bioreductive cytotoxic agents and methods of manipulation of tumour blood flow/oxygenation induced slight and inconsistent growth delays. Hydralazine was injected after irradiation of tumours in mice previously treated with various doses of MISO in an attempt to exploit the bioreductive cytotoxic potential of MISO in conjunction with its radiosensitizing properties; however, tumour growth delays were similar with or without hydralazine after irradiation. Thus, post-irradiation restriction of tumour blood flow appears to be an ineffective therapeutic strategy in this human xenograft tumour model.