RAF-independent MEK mutations drive refractory histiocytic neoplasms but respond to ERK inhibition

Histiocytic neoplasms are clonal disorders of the monocyte/macrophage lineage defined by mutations activating mitogen-activated protein kinase (MAPK) signaling. Recently, the MEK1/2 inhibitor cobimetinib was FDA-approved for patients with adult histiocytoses. Here, aided by a prospective registry of patients with histiocytoses (NCT03329274), we identify that MEK1/2 mutations which constitutively activate MEK independently of RAF are associated with worse progression-free survival with MEK1/2 inhibition as compared to patients with other MEK1/2 mutational classes. The most common RAF-independent MEK1 mutation (MEK1^E102_I103del) drove a lethal histiocytic-like neoplasm in mice, which was sensitive to the ERK1/2 inhibitor ulixertinib. We subsequently treated five MEK1^E102_I103del-mutant patients with ulixertinib on prospective protocols, four of whom were refractory to MEK inhibition. Four of five patients experienced objective responses to ulixertinib. These data reveal the impact of oncogenic MEK mutations in vivo, identify patients with likelihood of resistance to MEK inhibition, and nominate ERK inhibition to overcome resistance to MEK inhibition in histiocytoses.