Lack of prostate cancer radiosensitization by androgen deprivation

Purpose: The majority of clinical trials have shown that high-grade prostat e cancer patients treated with androgen deprivation (AD) plus radiation (RT ) have a survival advantage over those treated with RT alone. One possible mechanism for such a favorable interaction is that AD sensitizes cells to r adiation. Animal model studies have provided suggestive evidence that AD se nsitizes cells to radiation, but this mechanism is difficult to confirm con clusively in vivo. This question was investigated in LNCaP cells grown in v itro. Methods and Materials: LNCaP cells were cultured in vitro in Dulbecco's mod ified Eagle's medium (DMEM)-F12 medium, containing 10% fetal bovine serum ( complete medium [CM]). AD was achieved by culture in charcoal-stripped seru m (SS)-containing medium. Replacement of androgen was done by adding the sy nthetic androgen R1881 at 1 X 10(-10) M to SS. Apoptosis was measured with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end label ing (TUNEL) assay. Clonogenic survival was used to determine overall cell d eath, and the results were corrected for differences in plating efficiency from the various growth conditions. Results: LNCaP cells were grown in CM, SS, or SS + R1881 medium, and cell c ounts obtained at 3,4, and 5 days. Cell number increased exponentially in C M, whereas no increase in cell number was observed in SS medium. Cell count s from growth in SS + R1881 were intermediate between these extremes. Apopt osis was measured to determine if the combination of AD + RT in vitro resul ted in supra-additive cell death, as has been previously described in an in vivo model system. The cells were cultured for 3 days before RT and apopto sis quantified 24 h after RT. There was a consistent supra-additive increas e in apoptosis in cells exposed to AD + RT (2 or 8 Gy), as compared to eith er treatment given individually. In contrast, significant radiosensitizatio n by AD was not observed by clonogenic survival even when the conditions of AD were varied. No radiosensitization was observed upon incubation in SS m edium for 3, 4, or 5 days before RT, or extending AD after RT for 6 h befor e plating or 24 h after plating. Conclusion: The results show that in LNCaP prostate tumor cells supra-addit ive apoptosis does not translate into radiosensitization by clonogenic surv ival. Because clonogenic survival is a measure of overall cell death, eithe r the level of apoptosis is too small a component of overall cell death or the increases in apoptosis occurred in a subpopulation that would have been killed by other mechanisms. Although the findings indicate that AD does no t act by sensitizing prostate cancer cells to RT, the additive cell death a nd growth inhibitory effects of AD + RT are clinically meaningful. (C) 2001 Elsevier Science Inc.