Background: LGX818, a potent and selective BRAF inhibitor (BRAFi) being investigated in BRAF V600 mutant melanoma, has unique biochemical properties with a dissociation half-time > 10 times longer than other BRAF inhibitors. Methods: A phase I trial of LGX818 administered orally once (qd) or twice (bid) daily in BRAF V600 tumors was initiated to define the maximum tolerated dose (MTD)/recommended phase II dose (RP2D) and to assess pharmacokinetics and clinical activity in BRAFi–naive or pretreated patients with BRAF V600 mutant advanced melanoma. Baseline assessment of biomarkers from MAPK/PI3K pathways and pharmacodynamics were also evaluated. Results: Fifty-four patients have been enrolled in the dose-escalation phase (dose levels [DLs], 50-700 mg qd [n=42] and 75-150 mg bid [n=12]). LGX818 plasma concentrations increased proportionally by dose with a mean t_1/2 of 4 hours and steady state in ≈ 15 days. The MTD/RP2D (450 mg qd) was well tolerated. Seven patients had a dose limiting toxicity (DLT): 5 at qd (1 each with hand-foot skin reaction [HFSR], foot pain, fatigue, diarrhea/rash, insomnia/asthenia) and 2 at bid (1 facial paresis/confusion, 1 musculoskeletal pain/neuralgia). All DLTs were grade 3 and reversible. The most common adverse events (≥ 20%) suspected to be treatment related were cutaneous (rash, dry skin, HFSR, pruritus, keratosis pilaris, alopecia), pain in extremity, arthralgia, and fatigue. Squamous cell carcinoma was observed in 2 patients (1 naive and 1 pretreated). As of 30 Sept 2012, the preliminary efficacy (all DLs) in patients with at least 1 postbaseline tumor assessment was 16 partial responses [PRs] (67%; 12 confirmed) out of 24 BRAFi–naive patients and 2 PRs (8.3%; 1 confirmed) among 24 BRAFi–pretreated patients. Responses were seen at all DLs from 50 to 550 mg qd. Updated safety and efficacy including time to event endpoints will be reported. Conclusions: Initial results from this study identified the MTD/RP2D as 450 mg/day and provided an early sign of promising activity in advanced melanoma. Expansion cohorts are ongoing in BRAFi–naive and BRAFi–pretreated melanoma and colorectal cancer. Clinical trial information: NCT01436656.