Optimizing the factors which modify thermal enhancement of melphalan in a spontaneous murine tumor

Background: Hyperthermia enhances the cytotoxicity of some chemotherapeutic agents. Both clinical and laboratory studies suggest melphalan may be an important drug when hyperthermia is added to chemotherapy treatments. Factors that may modify the thermal enhancement of melphalan were studied to optimize its clinical use with hyperthermia. Methods: The tumor studied was an early-generation isotransplant of a spontaneous C3Hf/Sed mouse fibrosarcoma, Fsa-II. All studies were performed under supervision of the Animal Care and Use Committee. Hyperthermia was administered by immersing the tumor-bearing foot into a constant temperature water bath. Four factors were studied: duration of hyperthermia, sequencing of hyperthermia and melphalan, intensity of hyperthermia, and tumor size. To study duration of hyperthermia tumors were treated at 41.5°C for 30 or 90 min immediately after intraperitoneal administration of melphalan. For sequencing of hyperthermia and melphalan, animals received hyperthermia treatment of tumors for 30 min at 41.5°C immediately after drug administration, both immediately and 3 h after administration of drug or only at 3 h after administration of drug. Intensity of hyperthermia was studied using heat treatment of tumors for 30 min at 41.5 or 43.5°C immediately following drug administration. Effect of tumor size was studied by delaying experiments until three times the tumor volume (113 mm³) was observed. Treatment of tumors was for 30 min at 41.5°C immediately following drug administration. Tumor response was studied by the mean tumor growth time. Results: Hyperthermia in the absence of melphalan had a small but significant effect on tumor growth time at 43.5°C but not at 41.5°C. Hyperthermia at 41.5°C immediately after melphalan administration doubled mean tumor growth time at 30 min and caused a threefold increase at 90 min (P=0.0002) when compared to tumors treated with melphalan alone at room temperature. Application of hyperthermia for one-half hour immediately following drug administration was the most effective in delaying tumor growth. No significant difference in mean tumor growth time was observed with an increase in temperature from 41.5 to 43.5°C. For large tumors heat alone and melphalan alone caused a moderate increase in tumor growth delay. These effects in large tumors were greatly increased by a combination of chemotherapy and hyperthermia. Conclusions: From our data it would appear that the administration of intraperitoneal melphalan immediately prior to 90 min of heat at 41.5°C may optimize anti-neoplastic activity. These data may be useful in formulating clinical protocols in which melphalan and heat are combined.