Preclinical evaluation of the orally active camptothecin analog, RFS-2000 (9-nitro-20(S)-camptothecin) as a radiation enhancer

Purpose: To test for enhancement of radiation effects in vitro and in vivo by the orally administered camptothecin derivative, 9-nitrocamptothecin (RF S-2000); to study whether the mechanism of this enhancement involves inhibi tion of sublethal damage recovery. Methods and Materials: In vitro: H460 human lung carcinoma cells were incub ated with RFS-2000 for various times at 37 degrees C, irradiated, immediate ly rinsed, and assessed for colony-forming ability. Sublethal damage recove ry (SLDR) was also assessed using two split doses of radiation. In vivo: H4 60 cell xenografts were used in nude mice. Tumors were grown subcutaneously on the flank, then treated with RFS-2000 (1 mg/kg) and/or radiation (2 Gy) for 5 consecutive days. Tumor growth delay was then measured for each trea tment group. Results: Radiation enhancement was observed in vitro for incubation times b etween 4 and 24 hr with 10 nM RFS-2000. Using a 24-hr treatment, the radiat ion dose enhancement ratio values (DER) for 5, 10, and 15 nM were 1.22, 1.5 4, and 2.0, respectively. Incubation with 10 nM RFS-2000 inhibited SLDR by a factor of 2. The results of three independent in vivo experiments showed that RFS-2000 can enhance the effects of fractionated radiotherapy, with an enhancement factor (EF) of 1.64. Conclusion: Our results show that RFS-2000 can enhance the effects of radia tion in human lung cancer cells both in vitro and in vivo, and that the mec hanism of this effect may involve the inhibition of SLDR. (C) 2000 Elsevier Science Inc.