We have obtained two procaspase 3 activators (PAC-1 and sPAC-1) from our collaborators at the University of Illinois, and have obtained a panel of cell lines on which to test the compounds with and without X-irradiation. Experiments with a number of carcinoma cell lines have shown that S-PAC-1 is a potent radiation sensitizer in all cell lines tested. The most potent radiation sensitization was in breast cancer cell lines, where a 30% enhancement of radiation dose was achieved when S-PAC-1 was added to cells. We have demonstrated that these effects correlate with increased presence of double stranded DNA breaks in treated cells, and with slower repair of these breaks. Experiments on the mechanism of death suggest that cells in fact surprisingly do not die predominantly by apoptosis after treatment with S-PAC-1 and ionizing radition. In fact, cells do die at higher numbers by mitotic catastrophe, as is typical for radiation-induced cell death. Experiments to determine how S-PAC-1 is effecting greater cell killing by radiation suggest that S-PAC-1 has a previously unknown effect inhibiting the DNA repair machinery. Confirmatory experiments are ongoing. The effects of S-PAC-1 with radiation are also being tested in a mouse xenograft tumor model. We have begun preparing these results for publication.
