Drug resistance in targeted cancer therapies with RAF inhibitors

Abstract

Hyperactive RAS/RAF/MEK/ERK signaling has a well-defined role in cancer biology. Targeting this pathway results in complete or partial regression of most cancers. In recent years, cancer genomic studies have revealed that genetic alterations that aberrantly activate the RAS/RAF/MEK/ERK signaling mainly occur on RAF or upstream, which motivated the extensive development of RAF inhibitors for cancer therapy. Currently, the first-generation RAF inhibitors have been approved for treating late-stage cancers with BRAF(V600E) mutations. Although these inhibitors have achieved promising outcomes in clinical treatments, their efficacy is abolished by quick-rising drug resistance. Moreover, cancers with hyperactive RAS exhibit intrinsic resistance to these drugs. To resolve these problems, the second-generation RAF inhibitors have been designed and are undergoing clinical evaluations. Here, we summarize the recent findings from mechanistic studies on RAF inhibitor resistance and discuss the critical issues in the development of next-generation RAF inhibitors with better therapeutic index, which may provide insights for improving targeted cancer therapy with RAF inhibitors.

Keywords: RAF inhibitors; RAF/KSR family kinase; RAS/RAF/MEK/ERK signaling; drug resistance; oncogenic mutation; regulatory spine; targeted therapy.

Figure 1

The regulatory mechanism of RAS/RAF/MEK/ERK signaling pathway. RAF and MEK exist as heterodimers in cytosol of quiescent cells (A). Upon Ras activation, these heterodimers are recruited to the plasma membrane where they form transient tetramers through side-to-side dimerization of RAF. Further, this side-to-side RAF dimerization activates RAF themselves and also loosens RAF/MEK heterodimers, which facilitate the MEK dimerization on the surface of RAF dimer. Subsequently, active RAF dimer phosphorylates MEK dimer docking on its surface or promotes intra-dimer MEK transphosphorylation. Thereafter, active MEK dimer docking on RAF dimer or released from RAF dimer phosphorylates and activates ERK. Since, unlike BRAF, CRAF and ARAF do not heterodimerize with MEK in inactive state, how MEK is delivered to CRAF and ARAF for activation remains unknown in current studies (B). EGF: Epidermal growth factor; RBD: RAS binding domain.

Figure 2

Genetic mutations that hyperactivate the RAS/RAF/MEK/ERK signaling pathway in human cancers. The RAS/RAF/MEK/ERK signaling is initiated on the plasma membrane where engagement of EGFR by its ligand activates Sos. In turn, Sos functions as a RAS-GEF to catalyze the GTP loading on RAS. Subsequently, RAS-GTP triggers the RAF/MEK/ERK kinase cascade and ultimately activates ERK. As the terminal kinase of this signaling cascade, active ERK phosphorylates numerous substrates and induces diverse cell responses. This signaling cascade is tightly regulated under physiological conditions through complicated mechanisms that include MEK-mediated positive feedback (red arrows) as well as NF1-driven GTP hydrolysis and ERK-mediated negative feedback (blue arrows). In cancer cells, this signaling cascade is mainly hyperactivated by genetic mutations on EGFR, NF1, RAS, and RAF. The hotspots for oncogenic mutations in these genes are labeled as yellow stars. The mutational prevalence of these genes in different type of cancers (calculated from TCGA, cBio, and COSMIC databases) are shown in boxes. TCGA data are shown in small circles forming square, while COSMIC data and cBio data are shown as a donut plot and a box graph, respectively. Numbers on the legends indicate the percentages of mutations in the particular cancer types. EGF: Epidermal growth factor.