9000 Background: PLX4032 is an oral, selective inhibitor of the oncogenic V600E mutant BRAF kinase with preclinical activity. V600E BRAF is the most common kinase mutation in melanoma (60%), also found in colorectal carcinomas (10%), most anaplastic and papillary thyroid carcinomas, and low-grade serous ovarian carcinomas. Methods: Phase I, dose-escalation study designed to determine maximum tolerated dose (MTD), safety, pharmacokinetic (PK) / pharmacodynamic (PD), and efficacy (RECIST evaluation every 8 wks) of PLX4032 in sequential cohorts of 3 to 6 patients (pts). Plasma PK samples were collected on days 1, 8 and 15. Results: 54 pts have been enrolled: metastatic melanoma (n=49), thyroid (n=3), rectal (n=1), or ovarian carcinoma (n=1). 26 pts received a crystalline formulation (CF) continuously at doses from 100 mg BID to 1600 mg BID with associated exposures below target plasma levels. 28 pts received an optimized formulation with increased bioavailability, predicted to have 10-fold greater bioavailability, at doses from 160 mg BID to 1120 mg BID. AUC was dose-proportional and above target levels at 240 mg BID and higher. There was 1 DLT at 720 mg BID (G4 pancytopenia); treatment was restarted at 360 mg BID without myelosuppression. At 1120 mg BID, 3 of 5 pts had DLT (rash and fatigue). One pt had grade 3 increased ALT at 360 mg BID. 13 melanoma pts (77 %M1C) treated at doses of 240 mg BID or higher of the increased bioavailability formulation have a minimum follow-up of 8 weeks. 5 of the 7 BRAF V600E+ pts treated at ≥ 240 mg BID had tumor regression, up to 83%, with 1 confirmed partial response (PR) and 1 unconfirmed PR (too early); 2 of 4 pts with unknown V600E status had tumor regression, up to 50%, with 1 confirmed PR; 2 BRAF wild-type pts had progressive disease. All 7 pts with tumor regression remain progression-free, ranging from 4 to 14 months. 3 thyroid cancer pts with V600E mutations have tumor regression (range 9–16%) and are progression-free (4–7 months). Conclusions: Dose escalation of PLX4032 reached DLTs at 1120 mg BID. 720 mg BID is the current MTD, but 960 mg BID may be explored. PLX4032 exhibits antitumor activity in V600E BRAF mutant tumors. These observations confirm that V600E BRAF is a valid therapeutic target in human cancer. [Table: see text]