The early supra-additive apoptotic response of R3327-G prostate tumors to androgen ablation and radiation is not sustained with multiple fractions

Purpose: The treatment of R3327-G tumor-bearing rats with androgen ablation (AA) via castration results in a supra-additive increase in apoptosis when 2-8 Gy gamma-irradiation (RT) is given as a single dose 3-14 days afterwar ds. We report here the dose response and effect of multiple fractions on th is supra-additive apoptotic response. Materials and Methods: Dunning R3327-G tumors were grown in the flanks of C openhagen rats and the experiments were initiated at a tumor volume of 1.0- 1.5 cc. Androgen ablation was achieved by castration 3 days prior to gamma- irradiation. Apoptosis was measured with a terminal deoxynucleotidyl transf erase dUTP-biotin nick end-labeling assay 6-h after RT, unless otherwise sp ecified, Results: The dose response of the supra-additive apoptotic response was ass essed by irradiating castrated animals with single doses of 2, 4, 8, or 16 Gy (n = 5 per group); tumor cell apoptosis at 6-h following irradiation was 2.4% +/- 0.7% (+/- SEM), 4.2% +/- 0.8%, 6.5% +/- 1.4%, and 1.6% +/- 0.3%, respectively. The RT only and AA only controls had < 1% apoptosis, The effe ct of fractionated RT on apoptosis was investigated to determine if the sup ra-additive apoptotic response was sustained with repeated 2-8 Gy fractions . When tumor-bearing animals were treated with repeated daily 2-Gy fraction s, there was a reduction in the level of the supra-additive apoptotic respo nse. After five 2-Gy fractions at 24-h intervals, apoptosis in the combined treated tumors was at levels seen in the AA controls. This raised the poss ibility that more than 24 h are required for recovery of the high supra-add itive apoptotic levels seen after one fraction. When the interfraction inte rval was extended to 96 h, there was no significant increase in apoptosis o ver the additive effect of AA and RT, Although there was a decline in supra -additive apoptosis with repeated fractions, a dose response for tumor grow th delay was evident for RT alone using 2.5-Gy fractions. Moreover, the com bination of AA + fractionated RT resulted in a supra-additive enhancement i n tumor growth delay to 5 cc Conclusion: The early supra-additive apoptotic response from AA and single fraction radiation is not seen at high single fraction doses and is not sus tained with repeated fractions. Therefore, the classical apoptotic response that occurs within 24 h of irradiation is not Likely to be the main mechan ism responsible for any clinical benefit seen with this combination. (C) 20 00 Elsevier Science Inc.