KRAS No Longer Undruggable

KRAS is one of the most highly mutated proteins in cancer accounting for nearly 30% of cancer related mutations, and 86% of all RAS related mutations (reviewed in (1, 2)).  Common mutations include G12C, G12D, G12V which account for nearly 80% of all KRAS mutations.  KRAS functions as a molecular switch in growth factor pathways, regulating proliferation by alternating between its active (GTP-bound) vs inactive (GDP-bound) state.  Mutations to codon 12 in KRAS maintains kRAS in an active, GTP-bound state through preventing GTPase-activating proteins (GAP) association. 
Although the RAS family of proteins was discovered nearly four decades ago there has not been a viable drug therapy developed to effectively blocks mutant RAS dysfunction. Targeting RAS directly is difficult because of the absence of known allosteric regulatory sites, as well as the fact that it has picomolar affinity to GTP/GDP (3).  Alternative drugs have been developed to target Farnesyl transferase and downstream MEK targets, but these have failed for various reasons (reviewed in (1, 2)). Recently, a drug that targets mutant RAS G12C specifically has shown promising clinical results and is now the first FDA approved RAS-targeting drug for the treatment of NSCLC, read on for a summary of this drugs journey from discovery to approval, and roadblocks that still lie ahead.

Figure Legend: A) Chemical structure for AMG510. B) AMG510 bound to KRASG12C (10.2210/pdb6OIM/pdb)

Early Discovery
RASG12C occurs in 13% of non-small-cell lung cancers (NSCLC) thus therapies that target RASG12C is paramount.  Seminal work by Ostrem et al published in 2013 identified a specific covalent inhibitor that could bind Cys-12 of KRASG12C (4).  Interestingly, Dr. Shokat’s group identified small molecules that specifically capitalize on a new binding pocket that forms when the cysteine mutation is present; thus, these drugs have no effect on wild type RAS at the tested doses (4).  This binding pocket is beneath the effector binding switch-II region and results in disruption of the switch regions leading to a preference for GDP versus GTP.  The drug underwent significant rounds of modifications to produce a drug suitable for in vivo applications, these efforts resulted in ARS-1620 which has been used by the scientific community to further define the utility of this class of drugs (5).  ARS-1620 showed benefits in vivo, and structural studies confirmed that the molecule covalently binds to Cys-12 while the quinazolone core of the molecule occupied the new binding pocket beneath the switch II loop. 
Clinical Success
Additional optimization efforts were needed to improve cellular potency for therapeutic viability. Amgen, in collaboration with Carmot therapeutics, was developing a Cys-12 covalent small molecule, and their initial compound showed multi-fold enhancement in potency compared to ARS-1620 in part through binding a cryptic pocket comprised of Y96/H95/Q99 in KRASG12C (6).  Unfortunately, the biopharmaceutical properties of this drug were unsuitable.  The group went through many rounds of modifications to identify a drug that could capitalize on interacting with the cryptic pocket to maintain potency while enhancing its bioavailability, which also required overcoming a configurational stability issue. The result was the identification of AMG510 (Sotorasib), which was the first Cys-12 covalent KRASG12C inhibitor tested in clinical trials (7).  It showed great promise in phase I clinical trails with demonstrated anti-tumor activity (8).  Importantly, of the 129 patients treated with Sotorasib no dose-limiting toxic effects or treatment-related deaths were observed.  Of the patients with NSCLC 32.2% had an objective response and 88.1% had disease control.  In the Codebreak 1000 phase II clnical trials similar positive results were reported with an objective response rate of 36% and a objective response rate of 81% (reviewed in (9)).  These results provided the groundwork for the FDA’s accelerated approval to use Sotorasib as a therapy for NSCLC.
Acquired Resistance and Summary
Sotorasib has been a monumental success by any measure; still, there is evidence in preclinical models that acquired resistance will be problematic(10).  Recently, Tanaka et al. sought to identify mechanisms by which acquired resistance occurred to KRASG12C targeting drugs, and they determined that 10 heterogenous alterations occurred in a patient with acquired resistance(11).  These alterations targeted four genes, KRAS, NRAS, BRAF, and MAP2k1, all of which converged to reactive the RAS-MAPK signaling. Looking closer at KRAS led to the identification of a KRAS Y96D mutation which altered the switch-II pocket and suppressed drug interaction. This escape mechanism was validated in KRASG12C cancer models.  New investigation into next generation drugs will be necessary to overcome this acquired resistance, and Cytoskeleton is proud to offer a panel of RAS tools to aide in these studies. 


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