Combinatorial study of a novel poly (ADP-ribose) polymerase inhibitor and an HDAC inhibitor, SAHA, in leukemic cell lines

Abstract

Background: Cancer is a multifactorial disease, which makes it difficult to cure. Since more than one defective cellular component is often involved during oncogenesis, combination therapy is gaining prominence in the field of cancer therapeutics. Objective: The purpose of this study was to investigate the combinatorial effects of a novel PARP inhibitor, P10 and HDAC inhibitor, SAHA, in leukemic cells. Methods: Combinatorial effects of P10 and SAHA were tested using propidium iodide staining in different leukemic cells. Further, flowcytometry-based assays such as calcein-AM/ethidium homodimer staining, annexin-FITC/PI staining and JC-1 staining were carried out to elucidate the mechanism of cell death. In addition, cell-cycle analysis, immunocytochemistry studies and western blotting analysis were conducted to check the combinatorial effect in Nalm6 cells. Results: Propidium iodide staining showed that P10 in combination with SAHA induced cell death in Nalm6 cells, in which PARP expression and activity is high with a combination index of <0.2. Annexin-FITC/PI staining, JC-1 staining and other biochemical assays revealed that P10 in combination with SAHA induced apoptosis by causing a change in mitochondrial membrane potential in >65% cells. Importantly, combinatorial treatment induced S phase arrest in 40-45 % cells due to DNA damage and plausible replicative stress. Finally, we demonstrated that treatment with P10 led to DNA strand breaks, which were further potentiated by SAHA (p < 0.01), leading to activation of apoptosis and increased cell death in PARP-positive leukemic cells. Conclusions: Our study reveals that coadministration of PARP inhibitor with SAHA could be used as a combination therapy against leukemic cells that possess high levels of intrinsic PARP activity.