MEK Inhibition Remodels the Immune Landscape of Mutant Tumors to Overcome Resistance to PARP and Immune Checkpoint Inhibitors.

Mutant tumors are associated with poor outcomes, at least in part, due to decreased therapeutic sensitivity. Here, we show that mutations are associated with resistance to monotherapy and combination therapy with PARP inhibitors (PARPi) and immune checkpoint blockade with anti-PD-L1 antibodies. In mutant tumors, inhibition of KRAS signaling with MEK inhibitors (MEKi) triggered and amplified PARPi-induced DNA damage, cytosolic double-stranded DNA accumulation, STING pathway activation, and CD8 T-cell recruitment. Moreover, MEKi decreased myeloid-derived suppressor cell infiltration, in part, by inhibiting IL6 and GMCSF production. Importantly, addition of MEKi to PARPi and anti-PD-L1 resulted in marked tumor inhibition in immunocompetent mutant tumor models. This study provides the underlying mechanistic data to support evaluation of PARPi, MEKi, and anti-PD-L1 combination in clinical trials of mutant tumors. SIGNIFICANCE: This study provides key insights into the potential for using MEKi combined with PARPi and anti-PD-L1 for the treatment of all mutant tumors. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/10/2714/F1.large.jpg.