STING agonism enhances anti-tumor immune responses and therapeutic efficacy of PARP inhibition in BRCA-associated breast cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors exert their efficacy by inducing synthetic lethal effects, as well as cGAS/STING-mediated immune responses in BRCA- and other homologous recombination repair-deficient cancer cells. Here we investigated whether the immunologic and therapeutic effects of PARP inhibition in BRCA-deficient breast cancer models could be augmented by synthetic cyclic dinucleotide agonists of STING. Combined PARP inhibition and STING agonism induced a greater degree of STING pathway activation and proinflammatory cytokine production compared to monotherapies in BRCA1-deficient human and mouse triplenegative breast cancer cell lines. In a mouse model of BRCA1-deficient TNBC, the combination also induced an improved immune response compared to either monotherapy alone, evidenced by a greater degree of cytotoxic T cell recruitment and activation, and enhanced dendritic cell activation and antigen presentation. Nanostring mRNA analysis indicated that combinatorial effects were the result of augmented interferon signaling and antigen processing, as well as of heightened leukocyte and dendritic cell functions. Finally, the combination markedly improved anti-tumor efficacy in vivo compared to monotherapy treatment, with evidence of complete tumor clearance and prolongation of survival. These results support the development of combined PARP inhibition and STING agonism in BRCA-associated breast cancer.