Oncogenic KRAS blockade therapy: renewed enthusiasm and persistent challenges

Abstract

Across a broad range of human cancers, gain-of-function mutations in RAS genes (HRAS, NRAS, and KRAS) lead to constitutive activity of oncoproteins responsible for tumorigenesis and cancer progression. The targeting of RAS with drugs is challenging because RAS lacks classic and tractable drug binding sites. Over the past 30 years, this perception has led to the pursuit of indirect routes for targeting RAS expression, processing, upstream regulators, or downstream effectors. After the discovery that the KRAS-G12C variant contains a druggable pocket below the switch-II loop region, it has become possible to design irreversible covalent inhibitors for the variant with improved potency, selectivity and bioavailability. Two such inhibitors, sotorasib (AMG 510) and adagrasib (MRTX849), were recently evaluated in phase I-III trials for the treatment of non-small cell lung cancer with KRAS-G12C mutations, heralding a new era of precision oncology. In this review, we outline the mutations and functions of KRAS in human tumors and then analyze indirect and direct approaches to shut down the oncogenic KRAS network. Specifically, we discuss the mechanistic principles, clinical features, and strategies for overcoming primary or secondary resistance to KRAS-G12C blockade.

Keywords: Covalent inhibitor; Drug resistance; Gene mutation; KRAS; Targeted therapy.

Fig. 1

Type and frequency of RAS mutations in human cancers. a. Somatic mutations of RAS oncogene in the top 10 human cancers. b. The frequency and location of G12, G13, and Q61 mutations in the exons of RAS oncogenes. c. The frequency and type of KRAS mutations in codon 12 in pancreatic cancer, colorectal cancer, and lung adenocarcinoma. The data were derived from recent studies using the COSMIC or cBioPortal database [2, 14, 19]

Fig. 2

Principle of inhibiting oncogenic KRAS activation. a. The wild-type (WT) KRAS protein maintains a balance between the inactive state of guanosine diphosphate (GDP) binding and the active state of guanosine triphosphate (GTP) binding. This process is mediated by GTPase activating protein (GAP) and guanine nucleotide exchange factor (GEF). b. The KRAS oncoprotein (e.g., KRAS-G12C) disrupts GAP-mediated GTP hydrolysis, allowing these mutants to accumulate in a continuous GTP-binding active state, which is responsible for oncogenic activity. c. The covalent inhibitor of KRAS-G12C protein (G12Ci) achieves allosteric inhibition of mutant cysteine 12 (12C) to prevent GEF-catalyzed nucleotide exchange and block subsequent effector pathways

Fig. 3

Indirect KRAS suppression strategy. The activation of receptor tyrosine kinases, such as members of the epidermal growth factor receptor (EGFR) family, activate KRAS through the growth factor receptor-bound protein 2 (GRB2)-SH2–containing protein tyrosine phosphatase 2 (SHP2)-SOS Ras/Rac guanine nucleotide exchange factor 1 (SOS1) pathway. The mutant KRAS protein accumulates in the guanosine triphosphate (GTP)-bound state, leading to the activation of downstream effector pathways, especially the RAF-MEK-extracellular signal regulated kinase (ERK) and the phosphatidylinositol 3-kinase (PI3K)-AKT-mechanistic target of rapamycin (mTOR) pathways. The localization of KRAS on the cell membrane is the first step in subsequent KRAS activation, which is mediated by enzymes, including but not limited to farnesyltransferase (FT), geranylgeranyltransferase 1 (GGT1), and isoprenylcysteine carboxyl methyltransferase (ICMT). In addition to directly inhibiting KRAS (exemplified by covalent allele-specific inhibitors that bind to KRAS-G12C), multiple approaches can indirectly inhibit the oncogenic pathway of KRAS by targeting upstream regulators, downstream effectors, and KRAS expression and processing. The main drugs or reagents used for indirect KRAS inhibition are shown in red (for clinical trials or approved for use in patients) or green (for preclinical research)

Fig. 4

The immunosuppressive function of extracellular KRAS-G12D protein in the tumor microenvironment. KRAS-G12D protein can be released during ferroptosis, which is a regulated cell death caused by reactive oxygen species (ROS) and subsequent lipid peroxidation. The release of KRAS-G12D protein is mediated by exosomes, which are cargo extracellular vesicles produced by multivesicular bodies derived from endosomes. The small GTPase RAB27A regulates exocytosis of multivesicular endosomes, which leads to exosome secretion. This process is further enhanced by autophagy-related 5 (ATG5)-dependent autophagosome formation and autophagy-meditated secretion. Once released, the extracellular KRAS-G12D protein from exosomes is taken up by advanced glycosylation end product-specific receptor (AGER) on macrophages, leading to phosphorylation and activation of signal transducer and activator of transcription 3 (STAT3). Nuclear STAT3 acts as a transcription factor to produce cytokines, such as transforming growth factor beta 1 (TGFB1), interleukin 10 (IL10), and arginase 1 (ARG1), for polarization of M2 macrophages, which limits antitumor immunity

Fig. 5

Immunostimulation by sotorasib acting on the tumor microenvironment. Sotorasib is a highly selective inhibitor of KRAS-G12C that reacts with mutant cysteine at position 12 by connecting to a structural feature called the switch II pocket. Sotorasib can induce the production of chemokines, such as C-X-C motif chemokine ligand 10 (CXCL10) and CXCL11, as well as the release of damage-associated molecular patterns (DAMPs), leading to dendritic cell (DC) maturation and activation. The priming of naive T cells to generate cytotoxic T lymphocytes (CTLs) requires mature DC-mediated antigen presentation. The number and function of tumor-targeted CTLs is a prerequisite for the immune system to attack cancer cells. However, the expression of immune checkpoint substances (such as programmed cell death protein 1 [PD-1]) limit the anticancer activity of CTLs, and the administration of anti–PD-1 antibodies reverses this process

Fig. 6

Mechanisms of adaptation or resistance to KRAS-G12C inhibitors. a. Production of new KRAS-G12C protein. Activation of the pathway involving epidermal growth factor receptor (EGFR)–SH2-containing protein tyrosine phosphatase 2 (SHP2)–SOS Ras/Rac guanine nucleotide exchange factor 1 (SOS1) is necessary to maintain the newly produced KRAS-G12C protein in an active GTP-bound form, which leads to the adaptation of ARS-1620 through the RAF-MEK-extracellular signal regulated kinase (ERK) pathway. The cell cycle regulator aurora kinase A (AURKA) can further enhance KRAS-G12C–mediated activation of mitogen-activated protein kinase (MAPK) effector pathways. b. Activating wild-type NRAS and HRAS. Multiple receptor tyrosine kinases (RTKs), rather than a single RTK, activate wild-type NRAS and HRAS, leading to acquired resistance to ARS-1620 and sotorasib by the RAF-MEK-ERK and the phosphatidylinositol 3-kinase (PI3K)-AKT-mechanistic target of rapamycin (mTOR) pathways. c. Inducing epithelial-to-mesenchymal transition (EMT). The insulin-like growth factor receptor (IGFR)-insulin receptor substrate 1 (IRS1) pathway mediates PI3K activation in a SHP2-independent manner, leading to acquired resistance to sotorasib or ARS-1620 through snail family transcriptional repressor 1 (SNAI1)-mediated EMT. d. Inducting secondary genetic alterations. An analysis of the genetic alterations of patients with acquired adagrasib resistance showed that 45% of the cases had a putative genetic mechanism of drug resistance. In short, acquired KRAS mutations in drug binding sites or oncogenic hotspots, gain-of-function mutations in the MAPK pathway, and loss-of-function mutations in tumor suppressor genes favor the acquisition of resistance to KRAS-G12C inhibitors