Effect of the vitamin D3 analog ILX 23-7553 on apoptosis and sensitivity to fractionated radiation in breast tumor cells and normal human fibroblasts

Purpose: Previous work from this laboratory has demonstrated that the vitamin D-3 analogs EB 1089 and ILX 23-7553 enhance the response of breast tumor cells to ionizing radiation and promote radiation-induced apoptotic cell death. The current studies were designed to more closely simulate clinical radiotherapy in the treatment of breast cancer by examining the utility of ILX 23-7553 as an adjunct to fractionated ionizing radiation. The potential toxicity to normal tissue of the combination of ILX 23-7553 and fractionated radiation was assessed in a model of BJ human fibroblasts in culture. Methods: MCF-7 cells and human fibroblasts were treated with fractionated radiation alone (5x2 Gy over 3 days), ILX 23-7553 alone (50 nM) or ILX 23-7553 followed by 5x2 Gy. Viable cell numbers were determined by trypan blue exclusion and apoptosis by the TUNEL assay. A statistical model of additivity was utilized to assess the nature of the interaction between ILX 23-7553 and fractionated radiation. Results: Radiation and ILX 23-7553 each alone reduced viable cell numbers by 72 +/- 3.1% and 62 +/- 4.8%, respectively. Pretreatment with ILX 23-7553 followed by 5x2 Gy reduced viable cell numbers by 93.2 +/- 0.7%. The interaction between ILX 23-7553 and fractionated radiation appeared to be additive despite the fact that the combination of ILX 23-7553 and fractionated radiation also promoted a twofold increase in apoptotic cell death. ILX 23-7553 failed to enhance the response to radiation or to promote apoptosis in BJ human foreskin fibroblasts. Conclusions: ILX 23-7553 enhanced the antiproliferative and apoptotic effects of fractionated ionizing radiation in MCF-7 breast cancer cells. These effects appeared to be selective in that similar responses were not observed in a model of normal human fibroblasts. Vitamin D-3 analogs such as ILX 23-7553 may prove to have utility in combination with conventional radiotherapy of breast cancer as well as other malignancies which are sensitive to vitamin D-3.