Novel Strategies for Combatting Acquired Resistance to Combined BRAF and MEK Inhibition in Mutant BRAF Melanomas

The melanoma treatment landscape has been revolutionized by the rational design of small molecules targeting the genetic drivers of the disease and by harnessing the immune system to attack aberrantly growing cells. Despite dramatic advancements, both treatment modalities have limitations that mitigate clinical success. In the case of targeted therapies, the durability of the dramatic initial response is hampered by the acquisition of one or more resistance mechanisms which allow tumors to grow on while on treatment. Mutant BRAF melanoma patients treated with combined BRAF and MEK inhibitors can expect regression for about one year before relapse, and effective salvage treatment options are lacking. Acquired mechanisms of resistance to BRAF monotherapy and combined BRAF plus MEK inhibitors most often implicate re-activation of MEK-ERK1/2 signaling, although resistance to the combination is less studied. Using an in vivo model of ERK1/2 activity and resistance to combined BRAF and MEK inhibition, we provide pre-clinical proof-of-concept that temporal monitoring of ERK1/2 activity models tumor response to clinically used intermittent dosing schedules. Proteomic pathway analysis of changes in expression during treatment with BRAF and MEK inhibitors indicated expression of cell mitotic regulator polo-like kinase 1 (Plk1) is maintained in treated resistant melanomas. Plk1 inhibition selectively induced apoptosis in combination with resistant melanomas. Using a mutant BRAF syngeneic immune-competent mouse model, we also identified intrinsic tumor alterations that drove immune-dependent changes in response and relapse to targeted therapy, and demonstrate induction of pyroptotic death as a novel salvage therapy. These data provide insight on strategic and therapeutic opportunities that may be employed in melanoma during treatment or after progression on BRAF plus MEK inhibitors