PARP1 Inhibitors Eliminated Imatinib-Refractory Chronic Myeloid Leukemia Cells in Bone Marrow Microenvironment Conditions

Bone marrow microenvironment (BMM) also defined as a stem cell niche provides a major obstacle for current anti-leukemia treatment modalities including tyrosine kinase inhibitors (TKIs) such as imatinib. We recently reported that chronic myeloid leukemia (CML) stem cells are sensitive to PARP inhibitor (PARPi)-triggered synthetic lethality in conditions mimicking peripheral blood (normoxia, no stromal cells). We reported that PARPi olaparib and talazoparib eliminate imatinib-refractory CML cells in the conditions mimicking BMM (hypoxia, stromal cells present). The effect, however, is not as robust as that observed in normoxia implicating BMM-specific protective impact. Therefore, more efficient targeting of PARP is required to enhance synthetic lethal effect against CML in BMM.

Most PARPi's have been designed to compete with NAD for a binding site on the PARP1 molecule. This strategy resulted in the discovery of nucleotide-like PARPi's that not only target PARP, but, unfortunately, other enzymatic pathways involving NAD and other nucleotides as co-factors. Using such inhibitors affects multiple NAD/nucleotide-dependent enzymatic pathways, which results in secondary toxic effects proceeding from the inactivation of other pathways, while the efficiency against the PARP pathway is diminished. Thus, the challenge is to design inhibitors based on other activities of PARP1. We reported that DSBs-dependent interaction of PARP1 with histone 4 (H4) resulted in activation of PARP1 enzymatic activity and stimulation of Alt-NHEJ.

Using high-throughput screen we identified non-NAD-like inhibitors, which interfered with H4-mediated activation of PARP1 and were effective against several types of tumors including BRCA1-deficient CML cells. Here we show that combination of two structurally different PARPi's, NAD-like olaparib or talazoparib and non-NAD-like 5F02 resulted in more abundant elimination of CML cells and reduced toxicity to normal counterparts. Finally, the combination exerted synergistic effect in humanized immunodeficient mice bearing primary CML xenografts. Altogether, non-NAD-like PARP1i synergistically enhanced synthetic lethal effect of NAD-like PARPi in tumor cells without increasing the cytotoxic effect in normal cells.

While synthetic lethality mediated by NAD-like PARPi is usually associated with enhanced accumulation of potentially lethal DSBs, non-NAD-like PARP1i 5F02 did not induce DSBs when used as a single agent and also in combination with olaparib. Thus, the mechanistic aspect of the synergistic effect of NAD-like and non-NAD-like PARPis needs to be elucidated. Nevertheless, our data suggest that combining NAD-like and allosteric non-NAD-like PARPi's may represent a viable therapeutic strategy for enhancing CML response to PARP inhibition and reducing the resistance that inevitably results from treatment with NAD-like PARPi's alone.