Interleukin-1 modification of the effects of cyclophosphamide and fractionated irradiation

Studies were performed to determine whether recombinant human interleukin-1 (IL-1) modifies the tumor cytotoxicity of cyclophosphamide (CY) combined with fractionated X-irradiation. RIF-1 tumors were implanted intradermally in C3H Km mice and therapeutic effect was evaluated by the regrowth delay method, that is, the time for treated tumors to grow to 3 times their volume at the start of treatment relative to that for untreated tumors. A single intraperitoneal treatment of 15 μ/kg IL-1 given 24 hr after 100 or 200 mg/kg CY and immediately before the first of 5 daily fractionated treatments of 1–4 Gy increased tumor growth delay beyond that produced by CY and irradiation without the IL-1. However, the IL-1 given with either CY or fractionated irradiation did not extend the time for tumor regrowth beyond that produced by the agents themselves. Thus, while CY and fractionated irradiation together produce a greater than additive effect, IL-1 seems to extend this phenomenon. From these findings, it appears that IL-1 enhances the cytotoxic effects of CY and X ray against tumors, an effect that would have considerable practical significance in the light of the protective effects shown elsewhere for the same lymphokine on normal tissues.