A Combination of Conformation-Specific RAF Inhibitors Overcome Drug Resistance Brought about by RAF Overexpression (original) (raw)
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Cell systems, 2018
Clinically used RAF inhibitors are ineffective in RAS mutant tumors because they enhance homo- and heterodimerization of RAF kinases, leading to paradoxical activation of ERK signaling. Overcoming enhanced RAF dimerization and the resulting resistance is a challenge for drug design. Combining multiple inhibitors could be more effective, but it is unclear how the best combinations can be chosen. We built a next-generation mechanistic dynamic model to analyze combinations of structurally different RAF inhibitors, which can efficiently suppress MEK/ERK signaling. This rule-based model of the RAS/ERK pathway integrates thermodynamics and kinetics of drug-protein interactions, structural elements, posttranslational modifications, and cell mutational status as model rules to predict RAF inhibitor combinations for inhibiting ERK activity in oncogenic RAS and/or BRAFV600E backgrounds. Predicted synergistic inhibition of ERK signaling was corroborated by experiments in mutant NRAS, HRAS, and...
Drug resistance in targeted cancer therapies with RAF inhibitors
Cancer Drug Resistance, 2021
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 firstgeneration 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.
Targeting Aberrant RAS/RAF/MEK/ERK Signaling for Cancer Therapy
Cells
The RAS/RAF/MEK/ERK (MAPK) signaling cascade is essential for cell inter- and intra-cellular communication, which regulates fundamental cell functions such as growth, survival, and differentiation. The MAPK pathway also integrates signals from complex intracellular networks in performing cellular functions. Despite the initial discovery of the core elements of the MAPK pathways nearly four decades ago, additional findings continue to make a thorough understanding of the molecular mechanisms involved in the regulation of this pathway challenging. Considerable effort has been focused on the regulation of RAF, especially after the discovery of drug resistance and paradoxical activation upon inhibitor binding to the kinase. RAF activity is regulated by phosphorylation and conformation-dependent regulation, including auto-inhibition and dimerization. In this review, we summarize the recent major findings in the study of the RAS/RAF/MEK/ERK signaling cascade, particularly with respect to ...
Combined MEK and ERK inhibition overcomes therapy-mediated pathway reactivation in RAS mutant tumors
PLoS ONE, 2017
Mitogen-activated protein kinase (MAPK) pathway dysregulation is implicated in >30% of all cancers, rationalizing the development of RAF, MEK and ERK inhibitors. While BRAF and MEK inhibitors improve BRAF mutant melanoma patient outcomes, these inhibitors had limited success in other MAPK dysregulated tumors, with insufficient pathway suppression and likely pathway reactivation. In this study we show that inhibition of either MEK or ERK alone only transiently inhibits the MAPK pathway due to feedback reactivation. Simultaneous targeting of both MEK and ERK nodes results in deeper and more durable suppression of MAPK signaling that is not achievable with any dose of single agent, in tumors where feedback reactivation occurs. Strikingly, combined MEK and ERK inhibition is synergistic in RAS mutant models but only additive in BRAF mutant models where the RAF complex is dissociated from RAS and thus feedback productivity is disabled. We discovered that pathway reactivation in RAS mutant models occurs at the level of CRAF with combination treatment resulting in a markedly more active pool of CRAF. However, distinct from single node targeting, combining MEK and ERK inhibitor treatment effectively blocks the downstream signaling as assessed by transcriptional signatures and phospho-p90RSK. Importantly, these data reveal that MAPK pathway inhibitors whose activity is attenuated due to feedback reactivation can be rescued with sufficient inhibition by using a combination of MEK and ERK inhibitors. The MEK and ERK combination significantly suppresses MAPK pathway output
Mechanism and consequences of RAF kinase activation by small-molecule inhibitors
British Journal of Cancer, 2014
Despite the clinical success of RAF inhibitors in BRAF-mutated melanomas, attempts to target RAF kinases in the context of RAS-driven or otherwise RAF wild-type tumours have not only been ineffective, but RAF inhibitors appear to aggravate tumorigenesis in these settings. Subsequent preclinical investigation has revealed several regulatory mechanisms, feedback pathways and unexpected enzymatic quirks in the MAPK pathway, which may explain this paradox. In this review, we cover the various proposed molecular mechanisms for the RAF paradox, the clinical consequences and strategies to overcome it.
Cancer Research, 2013
Tumors with mutant RAS are often dependent on extracellular signal-regulated kinase (ERK) signaling for growth; however, MEK inhibitors have only marginal antitumor activity in these tumors. MEK inhibitors relieve ERK-dependent feedback inhibition of RAF and cause induction of MEK phosphorylation. We have now identified a MEK inhibitor, CH5126766 (RO5126766), that has the unique property of inhibiting RAF kinase as well. CH5126766 binding causes MEK to adopt a conformation in which it cannot be phosphorylated by and released from RAF. This results in formation of a stable MEK/RAF complex and inhibition of RAF kinase. Consistent with this mechanism, this drug does not induce MEK phosphorylation. CH5126766 inhibits ERK signaling output more effectively than a standard MEK inhibitor that induces MEK phosphorylation and has potent antitumor activity as well. These results suggest that relief of RAF feedback limits pathway inhibition by standard MEK inhibitors. CH5126766 represents a new type of MEK inhibitor that causes MEK to become a dominant-negative inhibitor of RAF and that, in doing so, may have enhanced therapeutic activity in ERK-dependent tumors with mutant RAS. Cancer Res; 73(13); 4050-60. Ó2013 AACR.
Inhibitors that stabilize a closed RAF kinase domain conformation induce dimerization
Nature Chemical Biology, 2013
D ysregulation of the RAS-RAF-MEK-ERK pathway is conducive to tumor formation 1,2 . Although activating mutations in the RAS genes (HRAS, KRAS and NRAS) are the most recurrent lesions driving oncogenic RAS/ERK signaling, gain-of-function mutations in BRAF are arguably among the leading causes 3,4 . Under normal conditions, RAF activation is initiated at the plasma membrane by binding growth factor-stimulated RAS GTPases. This triggers the sequential phosphorylation and activation of MEK and ERK. Active ERK then phosphorylates a diverse set of substrates, eliciting various cell-specific responses including proliferation and survival.
The Therapeutic Promise of Anti-Cancer Drugs Against the Ras/Raf/MEK/ERK Pathway
Topics in Anti-Cancer Research, 2013
The Ras/Raf/MEK/ERK mitogen-activated protein kinase (MAPK) pathway mediates cellular responses to different growth signals and is frequently deregulated in cancer. There are three Raf kinases-A-Raf, B-Raf, and C-Raf; however, only B-Raf is frequently mutated in various cancers. The most common B-Raf mutation involves a substitution of a glutamic acid residue to a valine moiety at codon 600. Subsequently, the MAPK pathway is constitutively activated, even in the absence of any growth signals. Although early attempts to target Ras have not yielded any viable drug candidates, many novel compounds inhibiting the activities of B-Raf and MEK have been developed and investigated in clinical trials in recent years and have shown promising result. The first MEK inhibitor (CI-1040) lacked efficacy in clinical trials, but its low toxicity encouraged the search for novel compounds-now there are over a hundred open clinical trials employing various B-Raf and MEK inhibitors. Several of these trials are now in Phase III. In this chapter, we will discuss new patents and patent applications related to inhibitors of the Ras/Raf/MEK/ERK pathway and some recent clinical trial results.