Rational combination therapy with PARP and MEK inhibitors capitalizes on therapeutic liabilities in <i>RAS</i> mutant cancers.

Abstract

Mutant <i>RAS</i> has remained recalcitrant to targeted therapy efforts. We demonstrate that combined treatment with poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitors and mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitors evokes unanticipated, synergistic cytotoxic effects in vitro and in vivo in multiple <i>RAS</i> mutant tumor models across tumor lineages where <i>RAS</i> mutations are prevalent. The effects of PARP and MEK inhibitor combinations are independent of <i>BRCA1/2</i> and <i>p53</i> mutation status, suggesting that the synergistic activity is likely to be generalizable. Synergistic activity of PARP and MEK inhibitor combinations in <i>RAS</i> mutant tumors is associated with (i) induction of BIM-mediated apoptosis, (ii) decrease in expression of components of the homologous recombination DNA repair pathway, (iii) decrease in homologous recombination DNA damage repair capacity, (iv) decrease in DNA damage checkpoint activity, (v) increase in PARP inhibitor-induced DNA damage, (vi) decrease in vascularity that could increase PARP inhibitor efficacy by inducing hypoxia, and (vii) elevated PARP1 protein, which increases trapping activity of PARP inhibitors. Mechanistically, enforced expression of FOXO3a, which is a target of the RAS/MAPK pathway, was sufficient to recapitulate the functional consequences of MEK inhibitors including synergy with PARP inhibitors. Thus, the ability of mutant <i>RAS</i> to suppress FOXO3a and its reversal by MEK inhibitors accounts, at least in part, for the synergy of PARP and MEK inhibitors in <i>RAS</i> mutant tumors. The rational combination of PARP and MEK inhibitors warrants clinical investigation in patients with <i>RAS</i> mutant tumors where there are few effective therapeutic options.