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 (RFS-2000); to study whether the mechanism of this enhancement involves inhibition of sublethal damage recovery. Methods and Materials: In vitro: H460 human lung carcinoma cells were incubated with RFS-2000 for various times at 37°C, irradiated, immediately rinsed, and assessed for colony- forming ability. Sublethal damage recovery (SLDR) was also assessed using two split doses of radiation. In vivo: H460 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 treatment group. Results: Radiation enhancement was observed in vitro for incubation times between 4 and 24 hr with 10 nM RFS-2000. Using a 24-hr treatment, the radiation dose enhancement ratio values (DER) for 5, 10, and 15 nM were 1.22, 1.54, 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 radiation in human lung cancer cells both in vitro and in vivo, and that the mechanism of this effect may involve the inhibition of SLDR.