Inhibition of poly(ADP-ribose) polymerase (PARP) increases the therapeutic efficacy of the G-quadruplex ligand RHPS4/irinotecan combination in colon cancer xenografts

Background and Aim: The G-quadruplex ligand RHPS4 is known to induce a rapid activation of DNA damage response at telomeres and poly(ADP-ribosyl)ation is involved in the regulation of many cellular processes such as DNA damage repair.We recently showed that the combination of RHPS4 with camptothecins has a strong synergistic interaction in vitro on colon cancer lines and produced a marked antitumor activity on xenografts. Unfortunately, this combination failed to cure animals as no complete remissions were reported. So, based on the observation that PARP inhibitors sensitize tumor cells to camptothecins and in the attempt to search for a more effective anticancer therapy, we evaluated a multicomponent strategy based on the addition of the PARP inhibitor GPI 15427 to the previously established camptothecins/ RHPS4 combination. Material and Methods: PARP activity and poly(ADP-ribose) formation have been evaluated in cells upon RHPS4 treatment. Moreover, the presence of poly(ADP-ribose) at the telomeres has been investigated by coimmunofluorescence experiments using an anti-TRF1 antibody to mark telomeres. The involvement of PARP activity in the cellular response to RHPS4 has been studied by treatment with the PARP inhibitor GPI 15427 followed by analysis of the formation of TIFs (Telomeres Disfunction Induced Foci) and evaluation of the clonogenic ability of the human colon carcinoma cells line HT29. In vivo experiments were performed on HT29 xenografts to evaluate the therapeutic efficacy of GPI 15427/Irinotecan/RHPS4 combination. Results and Conclusions: RHPS4 produced a rapid induction of PARP activity and, interestingly, most of the ADP-ribose polymers induced by RHPS4 were localized at telomeres. Consistently with these results, the PARP- 1 protein was found to be localized at telomeres upon drug treatment, thus providing a rational for RHPS4 and PARP inhibitors combination. Indeed, RHPS4/GPI 15427 combination strongly increased the number of TIFs/cell and reduced the survival of HT29 cells compared to the treatment with RHPS4 alone. The in vivo experiments confirmed these results and demonstrated the high efficacy of GPI 15427/Irinotecan/RHPS4 combination as this multicomponent strategy produced an impressive inhibition of tumor weight and a marked tumor growth delay. Notably, after the triple combination a complete regression of tumors was observed in most of the mice treated accompanied by a significant increase of overall survival. Finally, a complete cure of about 50% of mice was observed. Interestingly, this treatment remained effective even when started at a very late stage of tumor growth. In conclusion, these data suggest that the integration of a PARP inhibitor with Irinotecan/RHPS4 combination could be a promising strategy to improve the response of solid tumors to antitumoral agents.