Proceed ings of the 40th Annua l ASTRO Meet ing

133 RADIATION-INDUCED HYPERSENSITIVITY FROM LOW DOSE EXPOSURES

Smith, Lewis G., Miller, Richard C., Richards, Marcia and Hall, Eric J. Department of Radiation Ontology, Center for Radiological Research, College of Physicians & Surgeons of Columbia University, New York, NY

Purpose/Objective: Hypersensitivity of exponentially growing cells to X and gamma rays has been reported for doses below about 0.5 Gy, with measured resistance to higher doses. The reported results have been interpreted to mean that a dose of 0.5 Gy or less is not sufficient to trigger an inducible repair mechanism. The purpose of this study was to examine whether hypersensitivity is maintained for multiple sequential small doses &gamma rays.

Materials & Methods: V79 hamster lung and C3H 10TI/2 mouse embryo cells were grown to confluence in culture vessels. While in plateau phase of growth, cells were exposed to 6 Gy of Cs-137 gamma rays. Cells were treated to 1 x 6 Gy, 2 x 3 Gy, 3 x 2 Gy, 6 x 1 Gy or 20 x 0.3 Cry. Either immediately or 6 hr after the final dose fraction, cells were replated at low density and assayed for clonogenicity.

Results: Compared to a 6 Gy single dose exposure, either 3 fractions of 2 Gy/fraction or 2 fractions of 3 Gy/fraction doses resulted in sparing of radiation-induced cytotoxicity. However, after 20 fractions of 0.3 Gy/fraction or 6 fractions of 1 Gy/fraction, survival was reduced significantly in both V79 or C3H 10T1/2 cells when compared to cells exposed to a single dose of 6 C,-y. Examination of potentially lethal damage repair (PLDR) in platean-phase C3H 10T1/2 cells by delayed plating demonstrated similar results of the hypersensitivity of cells to low dose fractions.

Conclusion: These results support the notion that byperfraetionation with many very small doses (less than 0.5 Gy) may be particularly effective for resistant tumors. This approach, whereby low dose exposures are given below the dose that would trigger repair may be advantageous in the treatment of tumors currently considered to be radioresistant, such as glioblastoma multiforme and sott tissue sarcomas. Several years ago and more recently, in vitro studies have shown that hypoxic cells are more sensitive than oxic cells when given low doses of X rays. In slow growing tumors with chronic and acutely hypoxic cells, all cells, but especially hypoxic cells may be more sensitive to low dose exposures and thus contribute to therapeutic gain when small dose fractions are used in therapy.

191

134 PRE-CLINICAL TOXICITY AND EFFICACY STUDY OF ORAL IPDR (QD X 14D) AS A PRODRUG FOR IUDR-RADIOSENSITIZATION IN U25~ HUMAN GLIOBLASTOMA XENOGRAFTS

Timothy J. Kinsella, Kathleen A. Vielhuber, and Keith A. Kunugi

Dept. of Radiation Oncology, Case Western Reserve University, School of Medicine, University Hospitals of Cleveland, Cleveland, OH 44106

PURPOSE/OBJECTIVE: Oral (po) IPdR has been shown to be efficiently converted to IUdR in liver and results in a greater therapeutic index for radiosensitization than po or IV IUdR alone with short exposures (QD x 4-6D) (Can. Res. 54:2695-2700, 1994; Clin. Can. Res. 4:99-109, 1998). We now investigate the host (athymic mice) systemic toxicity of longer (QD x 14 D) IPdR exposures and the radiosensitizing effect of this longer IPdR exposure in subcutaneous (sc) human glioblastoma (U251) xenografts compared to po IUdR.

MATERIALS AND METHODS: 2 x 10 6 U251 human tumor cells were injected sc into the dorsal flank of 10 wk old female athymic mice to establish measurable xenograft tumors. Groups of 6 mice (with or without xenografts) received 0, 750 or 1500 mg/kg/D x 14 D oflPdR and were observed for 28 D. Animals were weighted QD prior to (x 4 D), during (x 14 D) and following (x 28 D) IPdR. To evaluate radiosensitization in SC tumor xenografts, a standard regrowth delay assay was used. Groups of 6 mice with SC xenografts measuring 0,25 - 0.3 c m 2 w e r e

randomized to one of seven treatment arms including: 2 XRT alone treatments (2 Gy/D x 4 D; 4 Gy/D x 4 D); po IPdR alone (1500 mg/kg/D x 14 D); po IUdR alone (MTD; 100 mg/kg/D x 14 D); 2 drug-XRT combined treatments (po IPdR x 14 D + 2 Gy/D x 4 D; po IUdR x 14 D + 2 Gy/D x 4 D) and control (no drug nor XRT). A linear interpolation of the tumor growth data with time to 300% growth was used to compare treatment arms and to calculate a sensitizer enhancement ratio (SER). % IUdR-DNA incorporation in xenograft tumors, normal bone marrow, normal intestine, and normal liver were also measured immediately following po IPdR (1500 mg/kg/D x 14 D) using a HPLC nuclcoside analysis.

RESULTS: No body weight loss was noted during the 14 D IPdR treatment period and the 28 D observation period. No tumor growth delay was found with either po IPdR or po IUdR alone. A significant SER was found for the combination of po IPdR + XRT (SER - 1.31; p = 0.05) but not for the combination ofpo IUdR + XRT (SER = 1.07, p = 0.57) in the U251 xenografts. The % IUdR - DNA incorporation in the tumor cells was higher following po IPdR QD x 14 D (3.7 + 0.3%) than po IUdR QD x 14 D (1.4 + 0.1%). The % IUdR - DNA incorporation following 14 D of IPdR at 1500 mg/kg/D in normal bone marrow, normal small intestine and normal liver were 1.2 _+ 0.2%, 3.8 + 0.2% and 0.2 _+ 0.1%, respectively.

CONCLUSIONS: A 14 D po schedule oflPdR using up to 1500 mg/kg/D results in no significant systemic toxicity and is associated with significant radiosensitization using this human glioblastoma multiforme xenograft model. Based on these data and the previously published data, which also demonstrate an improve therapeutic index oflPdR to IUdR a Phase I study ofpo IPdR QDX 14 D as a radiosensitizer will be designed. (Supported in part by NIH Grant CA 50595 and Sparta Pharmaceutical's, Inc., Horsham, PA 19044).