Predictability of response to clinical thermochemotherapy by the clonogenic assay

In order to assess the value of the clonogenic assay for predicting clinical response to dimethyl-triazeno-imidazole-carboxamide (DTIC) plus hyperthermia (42 degrees C), the responses of patients with measurable disease, who received combined therapy, were compared with assay results. The clonogenic assay was used independently to determine in vitro sensitivities of 53 melanomas to DTIC, with and without hyperthermia. Separate cell suspensions were incubated for 1 hour with DTIC at 37 degrees C and at 42 degrees C. In vitro sensitivity was determined by inhibition of colony formation in a double-layer agar system. Three of the 53 (6%) melanomas were sensitive to DTIC at 37 degrees C, 13 of the 53 (25%) were sensitive to 42 degrees C hyperthermia alone, and 22 of the 53 (42%) were sensitive to DTIC at 42 degrees C. Nine patients were treated with DTIC, plus hyperthermia, to the areas of their melanoma metastases (one pulmonary, four hepatic, and four subcutaneous). In five patients, the clonogenic assay results predicted positive tumor sensitivity to combined therapy, and 4 of the 5 had objective tumor regression. Tumors were resistant in vitro for four patients, and all had disease progression during treatment. Statistical analysis suggested that some responses were due to synergism of the combination of heat and drug, whereas others were due to an additive effect. The apparent direct correlation between in vitro tumor cell sensitivity to DTIC at 42 degrees C and actual clinical response to chemotherapy, plus hyperthermia, in this limited trial, has been encouraging. The clonogenic assay and in vitro evaluation of drug-heat interaction may prove helpful for selecting those patients in whom hyperthermia should be used as an adjunct to chemotherapy, and may help determine the most effective drug/heat scheduling. Further trials with other malignancies and other chemotherapeutic agents are warranted.