Background: This first-in-human study evaluated the safety, pharmacokinetics (PK), pharmacodynamics (PD), MTD, and efficacy of TAK-733 – an oral, selective, allosteric inhibitor of MEK1/2 – in pts with advanced solid tumors (NCT00948467; completed study). Methods: Eligibility: age ≥18 y; ECOG PS 0–2; evaluable tumors. Pts received escalating doses of TAK-733 QD in a modified 3+3 design for 21 d in a 28-d cycle to determine the MTD based on DLTs in cycle 1. Plasma (PK) and peripheral blood samples (PD: pERK reduction in PBMCs) were obtained pre-dose (d 1, 8, 15, 21) and post-dose (d 1, 21) in cycle1. Results: 51 pts (median age 58 y; 51% M) received escalating doses of TAK-733 (0.2–22 mg; median 2 cycles, range 1–11 [5 pts ≥6 cycles]). 4 pts had DLTs: grade 3 acneiform dermatitis, 1 each at 11.8 and 16mg; grade 3 pustular rash and grade 2 rash/stomatitis (qualifying as a DLT) at 22mg, leading to the 16 mg dose being selected as MTD. 45 pts (88%) had a drug-related AE; most frequent was acneiform dermatitis (47%). 10 pts (20%) had a grade ≥3 drug-related AE; most frequent were creatine phosphokinase increase and acneiform dermatitis (each n=3, 6%). 7 pts discontinued due to AEs. TAK-733 exhibited a moderately fast absorption with a median T_max of 3 hr. Steady-state exposure of TAK-733 (0.2–22mg) did not increase in a dose proportional manner based on the power model analysis. The mean terminal t_1/2 (11.8, 16, and 22 mg) was 43 hr. Overall mean accumulation ratio was 3.5 following QD dosing for 21 d. On d 21, E_max of blood pERK modulation was 56–99%, and time-averaged modulation over the dosing interval at steady-state was 76–98% at MTD. This range correlates well to the 76–89% for pERK modulation associated with maximal efficacy in xenograft models. 1 pt (16 mg) with melanoma (BRAF L597R) had a confirmed partial response after 4 cycles (treated for 9 cycles). 15 pts had a best response of stable disease (4–11.7 months in 6 pts). Conclusions: From preliminary data, TAK-733 appears generally well tolerated, pharmacodynamically active and shows signs of anti-tumor activity in pts with advanced solid tumors. MTD was associated with significant pERK inhibition in peripheral blood. Clinical trial information: NCT00948467.