Background: Dacarbazine (D) and paclitaxel (P) have been used to treat MM pts with limited effect. The MM treatment landscape has recently changed with the approval of vemurafenib and ipilimumab in 2011, but secondary malignancies or other toxicities are of concerns. T is a reversible, highly selective allosteric inhibitor of MEK1/2 activation and kinase activity. In a PhII trial (NCT01037127), pts with BRAFV600E mutation MM had median PFS of 5.3 mos. This PhIII trial (NCT01245062) was conducted in pts with BRAFV600E/K mutant advanced or MM. Methods: Pts were randomized 2:1 to T (2mg QD) or C (D or P). Pts were stratified by baseline LDH level and prior C; pts in the C arm were allowed to crossover to receive T after confirmation of PD. Primary endpoint was PFS in pts with BRAFV600E mutation-positive MM and no prior brain mets; secondary endpoints were OS, ORR and safety in primary and ITT. PFS and OS were compared using a stratified log-rank test. The study was designed with ≥99% power and one-sided α = 0.025 to detect 57% reduction in the risk of PD or death in pts treated with T vs C. Results: Between Dec 2010 and Jul 2011, 322 pts were randomized to T (n=214) or C (n=108); 273 pts were BRAFV600E mutation-positive with no prior brain mets. HR for primary population for PFS by investigator was 0.44 (95% CI 0.31–0.64; p<0.0001) in favor of T with a median PFS of 4.8 mo vs 1.4 mo with C. PFS benefit in favor of T was observed in ITT; this was confirmed by an independent review. The confirmed ORR was 24% with T and 7% with C. HR for interim OS was 0.53 (95% CI 0.30–0.94; p=0.0181), in favor of T in primary population. OS benefit was consistent in ITT pop despite 51 pts crossover from C to T. The most frequent AEs with T were skin rash, diarrhea, edema, hypertension, fatigue. Known MEKi class effects were observed, e.g. chorioretinopathy (<1%) and decreased ejection fraction (7%). Grade 3 AEs in T arm were hypertenstion (12%) and rash (7%). Conclusions: T is the first in class MEKi associated with a significant improvement of PFS and OS compared to C in pts with BRAF_V600E/K mutant MM.