Abstract 5716: Interference with repair of sub-lethal radiation damage by ABT-888, an inhibitor of poly(ADP-ribose) polymerase (PARP), and theobromine

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL BACKGROUND: Upon DNA damage, PARP-1 and -2 use NAD to build a polymer of ADP-ribose on themselves, histones, and other proteins, altering chromatin and recruiting repair complexes to the damage. Inhibition of PARP activity sensitizes cancer cells or tumor xenografts to radiation therapy (RT) and inhibits DNA repair. RT is usually delivered as fractionated daily doses, allowing sub-lethal damage (SLD) to be repaired between doses. We asked if ABT-888, a specific PARP inhibitor, or theobromine (TB), which radiosensitizes via a PARP-independent mechanism, could inhibit SLD repair. METHODS: Human lung cancer cells A549 and H460 were plated at a range of cell densities, exposed to Cs-137 γ-radiation (0-8 Gy), as single doses or as multiples of 2 Gy/day, with or without drugs, then allowed to form colonies. DNA double-strand breaks (DSB) were estimated as γH2AX foci. RESULTS: In agreement with literature data, 2.5 μM ABT-888 produced a dose enhancement factor (DEF) of 1.4 at 10% survival for single radiation doses, and 10 μM was only modestly better (DEF 1.5). A549 cells showed marked repair of SLD (Table), such that the surviving fraction (SF) after 6 Gy was lower (0.13) than for 3 x 2 Gy (0.52). When ABT-888 or TB was added 1 h before the first irradiation, cells were sensitized to 6 Gy single dose or 3 x 2 Gy. The combination was more effective. ABT-888-sensitized cells responded to 3 x 2 Gy with similar killing as unsensitized cells to a single 6 Gy dose (p = 0.93). The combined effects are reflected in a decrease in mean inactivation dose (MID). Higher levels of γH2AX foci in drug-treated cells indicate inhibition of damage processing. CONCLUSIONS: Radiosensitization of human lung cancer cells by ABT-888 is at least partially via inhibition of SLD repair. Although more ABT-888 produces little additional benefit, addition of TB, which acts through a complementary mechanism, further improves the response. ![Figure][1] Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5716. doi:1538-7445.AM2012-5716 [1]: pending:yes