New Insights into PARP Inhibitors' Effect on Cell Cycle and Homology-Directed DNA Damage Repair

In preclinical and clinical studies, olaparib and veliparib are the most represented PARP inhibitors (PARPi), which mainly target homologous DNA damage repair pathway-deficient cancer cells. Their off-target effects are not fully understood, especially with regard to cell cycle and homology-directed DNA damage repair. Our objective was to comparatively evaluate olaparib and veliparib in this context and correlate our findings with their therapeutic potential. We used a well-established direct repeat GFP (DR-GFP) reporter assay in U2OS(DR-GFP) and H1299(DR-GFP) cells and measured DNA damage repair activity upon drug treatment. Olaparib-treated U2OS(DR-GFP) cells showed a dramatic decrease in DNA damage repair versus veliparib irrespective of inhibitory potency. We demonstrate that this effect was a result of olaparib's strong effect on the cell cycle. Unlike in veliparib-treated U2OS(DR-GFP) cells, in olaparib-treated cells S-phase decreased and G(2)-phase increased sharply, indicating a G(2)-phase arrest-like state and replicative stress. This was further confirmed by upregulation of p53 and p21 and accumulation of cyclin A. Lack of the same effect in p53-null H1299(DR-GFP) cells suggested that olaparib's effect is p53 related, which was confirmed in p53-depleted U2OS(DR-GFP) and p53-null HCT116 cells. Importantly, we also demonstrate that olaparib, but not veliparib, induced a robust phosphorylation of Chk1, a crucial component of the replicative stress response pathway. Our data show olaparib and veliparib differ in their off-target effects; olaparib, unlike veliparib, mitigates DNA damage repair activity via G(2) cell-cycle arrest-like effect in a p53-dependent manner. These off-target effects may add to PARPis' anticancer properties.