Abstract
Arsenic trioxide (ATO) is an anticancer agent used for the treatment of acute promyelocytic leukemia (APL). However, 5-10% of patients fail to respond or experience disease relapse. Based on poly(ADP-ribose) polymerase 1 (PARP1) involvement in the processing of DNA demethylation, here we have tested the in vitro susceptibility of ATO-resistant clones, derived from the human APL cell line NB4, to PARP inhibitors (PARPi) in combination with hypomethylating agents (azacitidine and decitabine) or high-dose vitamin C (ascorbate), which induces 5-hydroxymethylcytosine (5hmC)-mediated DNA demethylation. ATO-sensitive and -resistant APL cell clones were generated and initially analyzed for their susceptibility to five clinically used PARPi (olaparib, niraparib, rucaparib, veliparib and talazoparib). The obtained PARPi IC50 values were far below (olaparib and niraparib), within the range (talazoparib) or above (rucaparib and veliparib) the Cmax reported in patients, likely due to differences in the mechanisms of their cytotoxic activity. ATO-resistant APL cells were also susceptible to clinically relevant concentrations of azacitidine and decitabine and to high-dose ascorbate. Interestingly, the combination of these agents with olaparib, niraparib or talazoparib resulted in synergistic antitumor activity. In combination with ascorbate, PARPi increased the ascorbate-mediated induction of 5hmC, which likely resulted in stalled DNA repair and cytotoxicity. Talazoparib was the most effective PARPi in synergizing with ascorbate, in accordance with its marked ability to trap PARP1 at damaged DNA. These findings suggest that ATO and PARPi have non-overlapping resistance mechanisms and support further investigation on PARPi combination with hypomethylating agents or high-dose ascorbate for relapsed/ATO-refractory APL especially in frail patients.
Significance Statement In this study we found that poly(ADP-ribose) inhibitors (PARPi) show activity as single agents against human acute promyelocytic leukemia cells resistant to arsenic trioxide at clinical relevant concentrations. Furthermore, PARPi enhance the in vitro efficacy of high-dose vitamin C, azacitidine and decitabine, all agents that alter DNA methylation. In combination with vitamin C, PARPi increase the levels of 5-hydroxymethylcytosine, likely because of altered processing of the oxidized intermediates associated with DNA demethylation.
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