PURPOSE: Dual MAPK pathway inhibition (dMAPKi) with BRAF and MEK inhibitors 
improves survival in BRAF V600E/K mutant melanoma, but the efficacy of dMAPKi in 
non-V600 BRAF mutant tumors is poorly understood. We sought to characterize the 
responsiveness of class II (enhanced kinase activity, dimerization dependent) 
BRAF mutant melanoma to dMAPKi.
EXPERIMENTAL DESIGN: Tumors from patients with BRAF wild-type (WT), V600E (class 
I), and L597S (class II) metastatic melanoma were used to generate 
patient-derived xenografts (PDX). We assembled a panel of melanoma cell lines 
with class IIa (activation segment) or IIb (p-loop) mutations and compared these 
with WT or V600E/K BRAF mutant cells. Cell lines and PDXs were treated with 
BRAFi (vemurafenib, dabrafenib, encorafenib, and LY3009120), MEKi (cobimetinib, 
trametinib, and binimetinib), or the combination. We identified 2 patients with 
BRAF L597S metastatic melanoma who were treated with dMAPKi.
RESULTS: BRAFi impaired MAPK signaling and cell growth in class I and II BRAF 
mutant cells. dMAPKi was more effective than either single MAPKi at inhibiting 
cell growth in all class II BRAF mutant cells tested. dMAPKi caused tumor 
regression in two melanoma PDXs with class II BRAF mutations and prolonged 
survival of mice with class II BRAF mutant melanoma brain metastases. Two 
patients with BRAF L597S mutant melanoma clinically responded to dMAPKi.
CONCLUSIONS: Class II BRAF mutant melanoma is growth inhibited by dMAPKi. 
Responses to dMAPKi have been observed in 2 patients with class II BRAF mutant 
melanoma. These data provide rationale for clinical investigation of dMAPKi in 
patients with class II BRAF mutant metastatic melanoma.See related commentary by 
Johnson and Dahlman, p. 6107.