Abstract 2609: AZD9574 is a novel, brain penetrant PARP-1 selective inhibitor with activity in an orthotopic, intracranial xenograft model with aberrant DNA repair

Abstract The Poly (ADP-ribose) polymerase (PARP) family has numerous essential functions in cellular processes such as transcription, chromatin remodelling, DNA damage response and repair as well as apoptosis. PARP inhibition blocks base excision repair and results in conversion of SSBs to DNA double-strand break (DSBs). DSBs are the most deleterious form of DNA damage that can be generated by exogenous DNA damaging agents or endogenous replication stress. DSBs can be repaired by homologous recombination repair (HRR) or non-homologous end joining (NHEJ). The physiological importance of HRR is underscored by the observation of genomic instability in HRR-deficient (HRD+) cells and, importantly, the association of cancer predisposition and developmental defects with mutations in HRR genes. PARP1 and PARP2 are required for SSB repair, while PARP1 is also involved in the repair of DNA double-strand breaks (DSBs) and replication fork damage. Recent reports suggest the PARP1 inhibition is sufficient to elicit an anti-proliferative effect and that PARP2 is essential for the survival of hematopoietic stem and progenitor cells in animal models.AZD9574 is a novel brain penetrant PARP1 inhibitor that acts by selectively inhibiting and trapping PARP1 at the sites of SSBs. AZD9574 exhibited >8000-fold selectivity for PARP1 compared to PARP2 and other members of the PARP family (PARP2, PARP3, PARP5a and PARP6) in biochemical assays. While AZD9574 inhibited PARP1 enzymatic activity in all tested cell lines irrespective of the HRR status (IC50 range between 0.3 - 2 nM), colony formation assay in isogenic cell lines pairs confirmed higher potency and selectivity towards HRD+ models (DLD1 BRCA2-/-; SKOV-3 BRCA2-/- and SKOV-3 PALB2-/-). For example, AZD9574 IC50 in the BRCA2-/- DLD1 cells was 1.38 nM compared to IC50 > 40 µM BRCA2wt cells, which corresponds to a ~20,000-fold greater efficacy in the BRCA2-/-cells (ratio of AZD9574 IC50 in BRCA2wt divided by BRCA2-/-) compared to the wild type parental line. In vivo, AZD9574 demonstrated dose-dependent efficacy in a BRCA1 mutant MDA-MB-436 subcutaneous xenograft model. Anti-tumour effects of AZD9574 were manifested by pronounced growth regressions that were durable after treatment withdrawal. An intracranial xenograft model of breast cancer brain metastases was developed to assess the efficacy of AZD9574 in the context of blood-brain barrier penetrance. Treatment of animals with established intracranial lesions using a dose of 3 mg/kg AZD9574 showed sustained tumour growth suppression resulting in a significantly extended survival of tumour-bearing mice. Collectively, we believe that our data support the development of AZD9574 as a potential therapy for patients with HRD+ breast cancer whose disease has spread to the brain. Citation Format: Kunzah Jamal, Anna Staniszewska, Jacob Gordon, Shenghua Wen, Frank McGrath, Gregory Dowdell, Dominic Kabbabe, Giuditta illuzzi, Matthew Griffin, Barry R. Davies, Petra Hamerlik. AZD9574 is a novel, brain penetrant PARP-1 selective inhibitor with activity in an orthotopic, intracranial xenograft model with aberrant DNA repair [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2609.