Antitumor Efficacy of the Novel RAF Inhibitor GDC-0879 Is Predicted by BRAFV600E Mutational Status and Sustained Extracellular Signal-Regulated Kinase/Mitogen-Activated Protein Kinase Pathway Suppression

Abstract

Oncogenic activation of the BRAF serine/threonine kinase has been associated with initiation and maintenance of melanoma tumors. As such, development of pharmacologic agents to target RAF proteins or their effector kinases is an area of intense investigation. Here we report the biological properties of GDC-0879, a highly selective, potent, and orally bioavailable RAF small-molecule inhibitor. We used extracellular signal-regulated kinase (ERK)-1/2 and mitogen-activated protein kinase/ERK kinase (MEK)-1/2 phosphorylation as biomarkers to explore the relationship between tumor outcome and pharmacodynamic inhibition of the RAF-MEK-ERK pathway. In GDC-0879-treated mice, both cell line-and patient-derived BRAF V600E tumors exhibited stronger and more sustained pharmacodynamic inhibition (>90% for 8 hours) and improved survival compared with mutant KRAS-expressing tumors. Despite the involvement of activated RAF signaling in RAS-induced tumorigenesis, decreased time to progression was observed for some KRAS-mutant tumors following GDC-0879 administration. Moreover, striking differences were noted for RAF and MEK inhibition across a panel of 130 tumor cell lines. Whereas GDC-0879-mediated efficacy was associated strictly with BRAF V600E status, MEK inhibition also attenuated proliferation and tumor growth of cell lines expressing wild-type BRAF (81% KRAS mutant, 38% KRAS wild type). The responsiveness of BRAF V600E melanoma cells to GDC-0879 could be dramatically altered by pharmacologic and genetic modulation of phosphatidylinositol 3-kinase pathway activity. These data suggest that GDC-0879-induced signaling changes are dependent on the point of oncogenic activation within the RAS network. Taken together, these studies increase our understanding of the molecular determinants for antitumor efficacy resulting from RAF pathway inhibition and have implications for therapeutic intervention in the clinic.

The RAF inhibitor PLX4032 inhibits ERK signaling and tumor cell proliferation in a V600E BRAF-selective manner

“…Whether this development results from the induction of ERK signaling by PLX4032 is unknown. Others also have noted ERK activation in tumors with BRAF WT that have been exposed to RAF inhibitors, and it has been suggested that this may lead to the acceleration of the growth of such tumors (7,33). We found that induction of ERK phosphorylation in mutant RAS tumor cells induced only transient expression of some ERKdependent genes.…”

“…In contrast, although PLX4032 inhibited MEK and ERK phosphorylation in tumors with BRAF V600E , it rapidly induced MEK and ERK phosphorylation in all other tumor cells tested and in normal keratinocytes. Induction of ERK phosphorylation by other putative RAF inhibitors has been described recently (7,25,32) and suggests that paradoxical induction of ERK phosphorylation is common property of ATP-competitive RAF-kinase inhibitors in cells with BRAF WT , but not BRAF V600E (26,33). The mechanism of ERK induction in cells with BRAF WT has been attributed to negative feedback (23) or to selective inhibition of BRAF causing activation of CRAF (32).…”

“…T he prevalence of deregulation of ERK signaling in human tumors (1)(2)(3) suggests that drugs that inhibit the pathway might have significant therapeutic activity (4)(5)(6)(7). Selective, allosteric MAPK/ERK kinase (MEK) inhibitors (MEKi) have been useful for determining the utility and feasibility of therapeutic targeting of the ERK pathway.…”

An Integrated Model of RAF Inhibitor Action Predicts Inhibitor Activity against Oncogenic BRAF Signaling

“…To identify potent inhibitors of dimeric BRAF, we compared eight structurally diverse RAF inhibitors: AZ-628 (AZ) (McDermott et al, 2007), TAK-632 (TAK) , LY3009120 (LY) (Peng et al, 2015), GDC-0879 (GDC) (Hoeflich et al, 2009), SB-590885 (SB) (King et al, 2006), PLX7904/Paradox Breaker (PB), which does not to induce paradoxical activation in wild-type BRAF cells , Vemurafenib (VEM) and Dabrafenib (DAB) (Rheault et al, 2013) (Figure S1A, Table S1). To compare potencies of inhibitors in cells endogenously expressing monomeric or dimeric BRAF V600E , we treated parental (PAR) SKMEL239 cells expressing full length BRAF V600E and the SKMEL239 derivative (clone C3) that is resistant to VEM due to enhanced dimerization of endogenous splice variants of BRAF V600E that lack the RAS-binding domain (Poulikakos et al, 2011).…”

BRAF mutations in non-small cell lung cancer: has finally Janus opened the door?

“…Moreover, administration of GDC0879 significantly improved survival rates of mice harboring B-Raf mutant tumors in comparison with a control group of mice harboring Ras mutations (Hoeflich et al, 2009). GDC0879 inhibitory effect was strictly associated with V600E B-Raf mutations, which may limit its use for NSCLC, where V600E comprise only a minority of the B-Raf mutations.…”