RAS family genes, including HRAS, KRAS and NRAS, are the most common oncogenes in human cancer, and encode extremely similar proteins made up of chains of 188 to 189 amino acids. The sequences and structural features of these three proteins are highly conserved, except for their carboxyl-terminal domains and post-translational lipid modifications [
1,
2]. HRAS, KRAS and NRAS are regulated in a similar manner within the cell. The RAS genes encode monomeric GTPases that function as molecular switches in signal transduction pathways regulating cell proliferation, differentiation and survival in mammalian cells [
1]. Mutations that can constitutively activate RAS have been found in 20% ~ 25% of all human cancers [
3]. KRAS has the highest mutation rate compared to HRAS and NRAS in various types of cancers [
4,
5]. KRAS is a proto-oncogene and its gene product was first found as a p21 GTPase. KRAS binds to GTP in its active state and possesses an intrinsic enzymatic activity which cleaves the terminal phosphate of the nucleotide, converting it to GDP. Upon conversion of GTP to GDP, KRAS is deactivated. The rate of conversion is usually slow, but can be increased dramatically by an accessory GTPase-activating protein (GAP) [
4]. In turn, KRAS can bind to guanine nucleotide exchange factors (GEFs) (such as SOS), which force the release of bound nucleotide (GDP). GTP binding enables several residues, primarily in the switch I region (residues 30–40) and switch II region (residues 60–70), to adopt a conformation that permits KRAS effector proteins to bind; these switches are regulated by GAPs and GEFs [
6,
7]. In mammalian cells, endogenous KRAS proteins are predominantly in the GDP state and activation is transient [
8]. However, the common oncogenic mutations in KRAS proteins interfere with GTP hydrolysis, resulting in proteins that remain in the active GTP state and continue to transmit signals to effector pathways [
1]. Thus, KRAS acts as a molecular on/off switch. Once it is turned on, it recruits and activates proteins necessary for the propagation of signaling of growth factors and other receptors, such as c-Raf and PI3K [
9‐
12].
Somatic KRAS mutations are found at high rates in leukemia [
13,
14], colorectal cancer [
15], pancreatic cancer [
16] and non-small cell lung cancer (NSCLC) [
17]. In NSCLC, KRAS mutation is observed in up to 30–40% of cases [
11,
12,
18]. The most common KRAS mutations include G12C, G12D, G12R, G12S, G12 V, G13D and Q61H [
19,
20]. Beyond the most common hotspot alleles in exons 2 and 3, mutations in exon 4 of KRAS, including K117 N and A146T, have also been found in patients with colorectal cancer [
21,
22]. KRAS mutations constitutively activate KRAS and subsequently its downstream Raf/MEK/ERK1/2 and PI3K/PIP3/AKT survival pathways in various cancers, including lung cancer [
9‐
12]. However, over the past two decades, evidence has gradually accumulated to support a paradoxical role for RAS proteins in the initiation of cell death pathways [
1,
23‐
25]. Hyperactive RAS forces cells into the pathway of programmed cell death [
26]. Vitamin C treatment selectively kills mutant KRAS expressing tumor cells, but not wild-type KRAS containing cells [
27]. Interestingly, either glucose withdrawal or glucose-mediated hyperactivation of RAS is able to trigger apoptosis [
26,
28]. RAS oncogenes trigger apoptosis only under specific conditions [
26]. Thus, manipulation of the opposing functions of KRAS in cell proliferation/survival versus cell death should be an attractive approach to develop new strategies for the treatment of various types of cancers, especially those with mutant KRAS. Currently, there are no effective targeted therapies for patients with KRAS mutant cancers [
29,
30]. KRAS has been considered an “undruggable” target and it is difficult to inhibit its intracellular activity [
5,
31] for the following reasons [
31]: first, whether KRAS adopts an active or inactive form depends on its GTP or GDP binding status rather than it being a substrate of catalytic reactions; second, there is a picomolar affinity between KRAS and GTP while micromolar concentrations of GTP exist in cancer cells; third, KRAS lacks a sufficiently large and deep hydrophobic pocket for small molecule binding, aside from the challenging nucleotide-binding site [
4‐
6,
32]. Therefore, numerous efforts made by industry and academic laboratories have failed to design a drug to inhibit KRAS activity in cancer cells by directly targeting KRAS. Since intracellular KRAS activity is difficult to disrupt, and activated KRAS has been demonstrated to trigger death pathways [
1], changing the nature of KRAS signaling from pro-survival to pro-death by directly targeting KRAS in cancer cells may represent an entirely new strategy for cancer therapy. Here we identified KRA-533 as a novel KRAS agonist that binds to the GTP/GDP binding pocket in the KRAS protein to prevent GTP cleavage, resulting in the accumulation of constitutively active GTP-bound KRAS that triggers both apoptotic and autophagic cell death pathways in cancer cells, leading to potent suppression of mutant KRAS lung cancer in vitro and in animal models.