Cell experiment:

For MTT assays, 2×103 cells are seeded in triplicate in 96 wells in their regular culture medium (containing PLX4720 for PRT lines). Next day, cells are washed twice with PBS and then the medium is replenished containing the indicated RAF inhibitor. Medium is changed 48 hours later and after a further 48 hours, 10 μL of 5 mg/mL MTT reagent is added to wells, and incubated for three hours. Formazan crystals are then solubilized overnight with a 1:10 dilution of 0.1 M glycine (pH 10.5) in DMSO. Wells are then analyzed at 450 nM in a Multiskan(R) Spectrum spectrophotometer. Results depicted are normalized to DMSO conditions and are a composite of three independent experiments. Error bars shown are representative of the standard error of mean (SEM).

Animal experiment:

H1755 tumor xenografts are generated by injection of 5×106 cells in a 50/50 mixture for matrigel and PBS into 6- to 8-wk-old female NOD/SCID mice. Mice are randomized to treatment groups once tumors reach an average size of 150 mm3. H1755 cells are s.c. implanted and allowed to grow to appr 200 mm3 (4 wk after implantation). Mice are then treated with vehicle, PLX4032, or PLX8394 for 15 d. The vehicle for daily oral gavage is PEG 400 [20% (vol/vol)], tocopheryl polyethylene glycol succinate (TPGS) [5% (vol/vol)], water [75% (vol/vol)]. PLX8394 is dissolved in PEG 400 [20% (vol/vol)], TPGS [5% (vol/vol)], and water [75% (vol/vol)] and vortexed continuously throughout the dosing period. PLX8394 is given daily by oral gavage at a dose of 150 mg/kg/d.

PLX8394 is a potent and selective Raf inhibitor, with an IC50 of appr 5 nM for BRAFV600E.

PLX8394 potently inhibits phosphorylation of ERK1/2 in PRT #3 and PRT #4 cells at >25 nM and in addition to parental cells at 10 nM. PLX8394 effectively reduces cyclin D3 and cyclin D1, phosphorylation of retinoblastoma protein, and expression of cyclin A2 in parental cells and PRT #3 and PRT #4 cells. PLX8394 inhibits ERK1/2 phosphorylation and the growth of vemurafenib-resistant cells harboring either a BRAF V600K/L505H double mutation or an transposon-induced, N-terminal truncated form of BRAF[1]. PLX8394 significantly impairs tumor cell growth and suppresses MAPK signaling in LA cell lines expressing either endogenous V600E or non-V600 mutant forms of BRAF[2].

PLX8394 (150 mg/kg/d) substantially suppresses tumor growth, MAPK pathway signaling and tumor cell proliferation in these H1755 xenograft tumors without overt toxicity in mice. PLX8394 combines with erlotinib yields plasma erlotinib concentrations of >1 μM[2].

[1]. Basile KJ, et al. Inhibition of mutant BRAF splice variant signaling by next-generation, selective RAF inhibitors. Pigment Cell Melanoma Res. 2014 May;27(3):479-484. [2]. Okimoto RA, et al. Preclinical efficacy of a RAF inhibitor that evades paradoxical MAPK pathway activation in protein kinase BRAF-mutant lung cancer. Proc Natl Acad Sci U S A. 2016 Nov 22;113(47):13456-13461