Abstract 268: PARP3 inhibitors in cancer therapy
Authors: Sharif-Askari Bahram , Davidson David , Amrein Lilian , Panasci Lawrence 
Published: July 2016

Abstract
Poly-ADP-ribose-polymerase (PARP1) is an important regulator of DNA damage response (DDR). After induction of certain types of DNA damage, including nicks and double strand breaks (DSB)s, PARP1 is rapidly recruited to altered DNA sites were its catalytic activity produces protein-conjugated long-branched-poly-ADP-ribose (PAR) chains. These protein modifications result in the recruitment and activation of multiple proteins involved in DNA repair. Although there is considerable research describing the characteristics and activity of PARP1/2, there are 15 other PARP family members that are less well characterized. Of these PARP3 has been shown to have a distinct role in DNA repair. This protein has been shown to have a critical role in DSB resolution and to interact with partner proteins know to function in classical nonhomologous end joining (NHEJ) such as DNA-PKcs, Ku70, and Ku80.
In order to examine the role of PARP3 vis-à-vis chemotherapy, we utilized the specific PARP3 inhibitor, ME-0328 in combination with activated cyclophosphamide (4HC) in the chronic lymphocytic leukaemia (CLL) cell lines, MEC1 and MEC2.
Results from MTT cytotoxicity assays showed that the 50% inhibitory concentration (IC50) (50% of control) of 4HC in MEC-1 cells was 12.9μM. When used in combination with a nontoxic concentration (2μM) of the specific PARP3 inhibitor ME-0328 the IC50 of 4HC was reduced to 3.3μM. In MEC-2 cells the IC50 of 4HC was 10.8μM and was reduced to 2.1μM in the presence of ME-0328 at 2μM. ABT-888, a specific PARP1/2 inhibitor in clinical trials had a minimal effect on 4HC cytotoxicity in these cell lines. 4HC results in interstrand crosslinks (ICLs) which are believed to be repaired by homologous recombination. Intriguingly, PARP3 negatively regulates class switch recombination via activation-induced cytidine deaminase in B-lymphocytes. This may be involved in ME-0328 sensitization of 4HC in B-lymphocytic malignancies.
Given these preliminary results we hypothesize that the PARP3 inhibitor, ME-0328 will sensitize B-lymphocytic cancers to chemotherapies. To explore this hypothesis this project will pursue the following objectives:
1. Characterize the selective sensitization of ME-0328 with 4HC and/or bendamustine (an ICL-inducing drug utilized in the treatment of B-lymphocyte malignancies) in B-lymphoma and CLL cell lines plus CLL clinical samples.
2. Determine the mechanisms by which ME-0328 sensitizes CLL cells to 4HC and/or bendamustine (i.e. reduction of PARP1 or 3 activity and/or altered activity of known DNA repair proteins (Rad51, DNA-PK, H2AX) and/or altered activation-induced cytidine deaminase).
3. Determine the in-vivo activity of 4HC and/or bendamustine with or without ME-0328 in a Rag2 /-γcc-/- xenograft model using the MEC1-CLL cell line.
The inhibition of PARP3 may increase the sensitivity of tumor cells to DNA damaging chemotherapies. Our results should stimulate development of specific PARP3 inhibitors for clinical use.
Citation Format: Bahram Sharif-Askari, David Davidson, Lilian Amrein, Lawrence Panasci. PARP3 inhibitors in cancer therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 268.