[Biological basis for combined radio-chemotherapy in radioresistant tumors]

Human tumor cells such as melanoma or glioblastoma are intrinsically radioresistant on an average than cells of more common tumors in radiotherapy such as squamous cell carcinoma or adenocarcinoma. Mean inactivation dose (D) for glioblastoma A-7 cells was 3.1 Gy for cells growing exponentially, but was 4.3 Gy for cells grown in large spheroids with hypoxic cells and PLD recovery. The D for cells of squamous cell carcinoma was about 2.1 Gy. This indicates that local control of the radioresistant tumors may be achieved if a drug showing an enhancement ratio of about 2.0. Data on our experiments with others in the literature indicate that drugs which selectively sensitize hypoxic cells and inhibit PLD recovery may be useful to increase the therapeutic ratio. Experimental evidence on a nitrosourea, ACNU has been presented for such mechanisms of the action. Multivariate analysis with Cox's model on malignant gliomas of 209 patients indicated that a significant increase in the survival time was obtained in the radiotherapy combined with ACNU.