Pb1719: niraparib in b-cell lymphoid malignancies: exploring synthetic lethality as target for therapy

Topic: 1. Acute lymphoblastic leukemia - Biology & Translational Research Background: DNA damage, if left unrepaired, may induce cell death or carcinogenesis through cell mutations, chromosomal aberrations,, and genomic instability, one of the hallmarks of cancer. DNA damage agents may induce base loss, DNA single-strand breaks (SSBs), double-strand breaks (DSBs) and formation of adducts that impair base pairing and/or blocking DNA replication and transcription. Cells have developed a response mechanism to these lesions consisting in a network of proteins that sense, signal and/or repair DNA lesions, with PARP1/2 being involved in these mechanisms. Dysregulations related to DNA repair are seen in B-cell lymphoid neoplasms such as acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and multiple myeloma (MM). New treatments are being developed, many of which are based on synthetic lethality, targeting tumor cells with altered DNA repair pathways while preserving normal cells with efficient repair. Aims: This work aims to explore the therapeutic potential of niraparib, a PARP1/2 inhibitor, in lymphoid neoplasms, namely ALL, CLL and MM in in vitro models and correlate DNA damage and repair in these models with niraparib sensitivity. Methods: For this purpose, three cell lines were used: U-266 (MM cell line), HG-3 (CLL cell line), and 697 (ALL cell line). DNA damage and repair activity characterization was assessed by cytokinesis-blocked micronucleus assay, without and with exposure to H2O2. Further, the cells were incubated in the absence and presence of increasing concentrations of niraparib. The antiproliferation and cytotoxic effects of this DDR inhibitor were assessed using trypan blue assay for 72h. Cell cycle distribution was assessed by flow cytometry (FC) using propidium iodide/RNAse assay and cell death by annexin-V/7-AAD by FC as well as by optic microscopy (May-Grünwald-Giemsa staining). The results were statistically analyzed and were considered a significance level of 95%. Results: HG-3 and U-266 cells showed higher baseline chromosome damage compared to 697 cells, with micronuclei as the predominant damage. However, only the HG-3 line has efficient repair. Niraparib reduced cell proliferation and viability depending on dose, time and cell line. U-266 was the most resistant cell line (IC50 57 M), followed by HG-3 (IC50 55 μM) and 697 was the most sensitive (IC50 2 μM). Cell death evaluated by FC revealed that niraparib sparked a significant decrease in viable cells and increase in apoptotic cells, mainly with the administration of the higher doses. These results were confirmed by optical microscopy, observing the morphological effects compatible with apoptosis in niraparib conditions. Regarding cell cycle distribution, niraparib induces an arrest in the G0/G1 phase in HG-3 cells and a G2/M phase arrest in 697 cells, the most sensitive cells to niraparib. Additionally, 697 was the only cell line that present BCRA1 and PARP1 mutations. Summary/Conclusion: In conclusion, these lymphoid cell lines have different lesion profiles and sensitivity to Niraparib. In the future, a better characterization of compensatory repair pathways (including BRCA1 mutations and homologous repair efficiency) may help identify patients who will benefit from therapy with PARP1/2 inhibitors, as niraparib. Keywords: Targeted therapy, Apoptosis, DNA damage, Lymphoid malignancy