Background: Belvarafenib (HM95573/GDC-5573) is an oral type II pan-RAF kinase inhibitor which demonstrates selective anti-tumor activity in several non-clinical cancer models and in cancer patients with RAS- or RAF- mutation. Here we present overall safety and efficacy findings of two phase 1 studies, consisting of dose escalation and dose expansion stages. Methods: Patients with advanced solid cancers harboring documented RAS- or RAF- mutation were enrolled. In the dose escalation study, patients were treated with Belvarafenib at a starting dose of 50 mg once daily (QD) to 800 mg BID to assess safety and tolerability and identify the recommended dose (RD). Dose escalation was guided based on pharmacokinetic data and used a traditional 3+3 design. The dose expansion study was comprised of 6 cohorts (according to the type of tumor and RAS- or RAF gene mutation) and patients received the RD of Belvarafenib. The primary objective was to explore anti-tumor activity (per RECIST 1.1) and pharmacodynamic effects. Results: The dose escalation study included 72 patients in 9 dose cohorts (cut-off date of 18 Jan 2017). Dose dependent increase in exposures observed up to 650 mg BID. The most common treatment-emergent adverse events that occurred in more than 20% of patients were rash, dermatitis acneiform and pyrexia. A total of 4 DLTs (different kinds of rashes) were reported and included 2 DLTs at the 800 mg BID level. Therefore, 650 mg BID was considered the MTD and 450 mg BID was identified as the RD for Belvarafenib. There were 7 partial responses (3 confirmed PRs) from 200 mg QD to 800 mg BID in NRAS-mutant melanoma, BRAF-mutant melanoma, KRAS-mutant sarcoma, and BRAF-mutant GIST. Four of nine patients with NRAS-mutant melanoma had a PR (ORR 44%). The dose expansion study included 63 patients in 5 indication-specific and basket cohorts administered with 450 mg BID Belvarafenib (cut-off date of 6 Oct 2018). No new safety signal was detected. There were two PRs each in patients with NRAS-mutant melanoma (2/9), BRAF-mutant melanoma (2/6) and BRAF-mutant CRC (2/7), respectively. Conclusions: Belvarafenib was well tolerated and exhibited anti-tumor activity in patients with advanced solid tumors harboring RAS or RAF mutations. Belvarafenib is being further investigated in combination with the MEK inhibitor cobimetinib. Clinical trial information: NCT02405065, NCT03118817