Synergize a novel homologous recombination inhibitor with DNA damagingagents in TNBC

15-20% of all breast cancers are triple negative, being devoid of the three receptors that classify and define treatment strategies for most mammary cancers: estrogen receptor (ER), progesterone receptor (PR) and ERB2 (also known as HER2). Because of this, no targeted therapies are available for TNBC patients. This poses a destitute situation, as TNBC is an aggressive cancer that disproportionally affects younger women and those having African origins. Currently, standard care for TNBC patients includes surgery, radiation treatment and chemotherapy, where the two latter have toxic side effects. Poly-ADP-ribose polymerase inhibitor (PARPi) monotherapy is FDA approved in BRCA1/2-deficient TNBC patients as they present with deficiencies in DNA repair which are exacerbated by PARPi and thus lead to killing of cancer cells. However, about 80% of all TNBC patients are wild type for BRCA1/2 genes and thus not eligible for PARPi therapies or stop responding to PARPi as restoration of DNA repair is one of the frequent PARPi resistance mechanism. We tested a library of novel compounds (nitro fatty acids) that inhibit homologous recombination (HR)-mediated DNA DSB repair through specifically and reversibly adducting to the recombinase RAD51 thus inhibiting its activity. A lead candidate (CP- 8) was identified that induces chemically an HR-defect (HRD) DNA repair in TNBC cells proficient in HR- mediated DNA repair and synergizes with DNA damaging treatments such PARPi and ionizing radiation (IR). Preliminary toxicity studies in mice showed no adverse effects by CP-8 which not only diminishes TNBC tumor growth when combined with PARP inhibitors in vitro and in vivo, but also alleviates toxic side effects of PARP inhibitors on bone marrow cells. Our overarching working hypothesis is that CP-8 is a safe, reversible and potent sensitizer of HR-proficient (naïve or PARPi resistant) primary and brain metastatic TNBC tissues to DNA damaging therapies by reducing RAD51 functionality. Moreover, we expect that co-treatment of CP-8 would allow dose reduction of DNA damaging agent thus reducing toxic side effects in patients. Aim 1 will examine a TNBC cell line panel, TNBC PDX explants and in vivo mouse models for drug synergism of CP-8 with DNA damaging agents varying in mechanism. Three PARPi (ola, tala and nir) resistant TNBC cell lines have been already generated in the laboratory to assess re-sensitization to PARPi by CP-8. PDX models were chosen from a recent study considering their relative low response to DNA damaging agents. Target engagement of RAD51 by CP-8 as well as DNA DSB will be assessed in cell lines, TNBC PDX explants and in vivo tissues and correlated with drug efficacies. Cell lines and tissues will be analyzed by already established click chemistry-assisted affinity capture and HPLC-MS/MS for possible additional CP-8 targets and metabolite content. Aim 2 will perform IND enabling studies of the in vitro and in vivo CP-8 pharmacology in collaboration with PharmaDirections with the short-term goal to prepare CP-8 for a Phase 2 (R42) evaluation.