The KRASG12C Inhibitor MRTX849 Provides Insight toward Therapeutic Susceptibility of KRAS-Mutant Cancers in Mouse Models and Patients

Adagrasib has favorable pharmacokinetic properties, including a 23-hour half-life, dose-dependent pharmacokinetics, and CNS penetration.

“Adagrasib has favorable pharmacokinetic properties, including a long half-life (23 hours), dose-dependent pharmacokinetics, and central nervous system (CNS) penetration.8,21,22”

KRAS G12C occurs in 3 to 4% of colorectal cancer patients and impairs GTP hydrolysis, shifting KRAS to its active GTP-binding state.

“The KRAS glycine-to-cysteine mutation at codon 12 (KRAS G12C) occurs in 3 to 4% of patients with colorectal cancer and impairs GTP hydrolysis, which shifts KRAS to the active GTP-binding state to drive protumorigenic effector signaling.8,10–12”

Keeping adagrasib exposure continuously above a target threshold inhibits KRAS-dependent signaling throughout the dosing interval and is predicted to maximize antitumor activity.

“Maintaining continuous adagrasib exposure above a target threshold allows for the inhibition of KRAS-dependent signaling for the duration of the dosing interval and is predicted to maximize antitumor activity.8,20”

Despite clinical benefit, acquired resistance to single-agent KRASG12C inhibitor therapy eventually occurred in most patients.

“Despite the clinical benefit that was observed for many patients treated with KRASG12C inhibitors, acquired resistance to single-agent therapy eventually occurred in most patients.”

In early-phase trials, adagrasib and sotorasib showed promising results in NSCLC and more modest efficacy in colorectal cancer.

“In early-phase clinical trials, two potent, selective, and irreversible small-molecule KRASG12C inhibitors have shown promising results in non–small-cell lung cancer (NSCLC) and more modest efficacy in colorectal cancer: adagrasib (MRTX849) and sotorasib (AMG510).6,7,10”

One major route leading to recovery of RAS activity is activation of RTKs through genetic or non-genetic mechanisms.

“One major route leading to such recovery of RAS activity is the activation of RTKs through either genetic (e.g., gene amplification or fusion) or non-genetic mechanisms.2,4,5,6,7,8,22,23,24,25”

Sotorasib and adagrasib were effective at halting the growth of G12C-mutant cancer cells in 2-dimensional, 3-dimensional, and xenograft models.

“Sotorasib and adagrasib, two prominent members of this class, were effective at halting the growth of G12C-mutant cancer cells in 2-dimensional, 3-dimensional, and xenograft models, clearing the path for clinical trials [48,49,50,51].”

Treatment-emergent mutations were observed in genes from other signaling pathways, including ALK, APC, CDKN2A/B, CTNNB1, IDH1/2, KEAP1, MET, MYC, NRAS, PTCH1, PTEN, RB1, RET, RICTOR, SMARCA4, and TP53.

“These included ALK, APC, CDKN2A/B, CTNNB1, IDH1/2, KEAP1, MET, MYC, NRAS, PTCH1, PTEN, RB1, RET, RICTOR, SMARCA4, and TP53 [50,97,102].”

KRAS inhibition leads to short-term decreases in expression of genes related to glycolysis, mitotic spindle formation, G2M checkpoint, and downstream targets of E2F, MYC, and mTOR signaling.

“KRAS inhibition leads to short-term decreases in the expression of genes related to glycolysis, mitotic spindle formation, G2M checkpoint, and the downstream targets of E2F, MYC, and mTOR signaling [50].”

CRISPR screens using adagrasib found deletion of tumor suppressors KEAP1 and RB1 as mechanisms of resistance to KRAS inhibition in both in vitro and in vivo models.

“CRISPR screens using the KRAS G12C inhibitor adagrasib found the deletion of tumor suppressors KEAP1 and RB1 as mechanisms of resistance to KRAS inhibition in both in vitro and in vivo models [50].”

Inhibition of mutant KRAS rapidly decreases phosphorylation of MEK and ERK, leading to decreased expression of nuclear ERK-regulated genes including DUSP proteins, RSK, MYC, SPRY1, and SPRED1.

“The inhibition of mutant KRAS rapidly decreases the phosphorylation of MEK and ERK, leading to a decrease in the expression of nuclear ERK-regulated genes including DUSP proteins, RSK, MYC, SPRY1, and SPRED1 (Figure 2) [50,62].”

Prior studies with KRAS G12C, RAF, and MEK inhibitors indicated that blockade of KRAS-dependent signaling activated a transcriptional program suppressing DUSP, SPRY, and ETV family genes which cooperate to mediate increased signaling flux though RTK, SOS, and MEK/ERK family members resulting in conditional bypass dependence on KRAS.

“Prior studies with KRAS G12C , RAF, and MEK inhibitors indicated that blockade of KRAS-dependent signaling activated a transcriptional program suppressing DUSP , SPRY , and ETV family genes which cooperate to mediate increased signaling flux though RTK, SOS, and MEK/ERK family members resulting in conditional bypass dependence on KRAS [4, 32-34].”

Studies evaluating the anti-tumor activity of KRAS G12C inhibitors indicated that the dependence of cells on KRAS mutations for growth and survival as oncogenic drivers was distinct in in vitro cell lines compared with in vivo model systems.

“However, studies evaluating the anti-tumor activity of KRAS G12C inhibitors indicated that the dependence of cells on KRAS mutations for growth and survival as oncogenic drivers was distinct in in vitro cell lines compared with in vivo model systems ([4, 22]).”

Clinical trials of multiple KRASG12C inhibitors have shown promising efficacy in advanced, previously treated NSCLC, leading to FDA accelerated approval of adagrasib and sotorasib.

“Clinical trials of multiple KRASG12C inhibitors have shown promising efficacy in advanced, previously treated non–small cell lung cancer (NSCLC), leading to United States Food and Drug Administration (FDA) accelerated approval of two of these therapies (adagrasib and sotorasib) and many others being investigated in this setting (9–17).”

Acquired resistance to KRASG12C inhibition may be mediated by activation of multiple RTK regulators, including EGFR, FGFR, IGF1R, HER2, SHP2, and downstream RAS effectors such as MYC and mTOR.

“Recent studies using preclinical models have indicated that acquired resistance to KRASG12C inhibition may be mediated by the activation of multiple receptor tyrosine kinase (RTK) regulators, including epidermal growth factor receptor (EGFR), fibroblast growth factor receptor (FGFR), insulin-like growth factor 1 receptor (IGF1R), human epidermal growth factor 2 (HER2), and Src homology 2 domain-”

Most KRAS G12C mutant NSCLC patients experience clinical benefit from selective KRAS G12C inhibition, while CRC patients bearing the same mutation rarely respond.

“Most KRAS G12C mutant non-small cell lung cancer (NSCLC) patients experience clinical benefit from selective KRAS G12C inhibition, while patients with colorectal cancer (CRC) bearing the same mutation rarely respond.”

Clinical trials results have shown limited efficacy of KRAS G12C covalent inhibitors in CRC; only one CRC patient treated with MRTX849 had a 47% tumor reduction by RECIST criteria.

“Clinical trials results have shown limited efficacy of KRAS G12C covalent inhibitors in CRC; only one CRC patient treated with MRTX849 had a 47% tumor reduction by RECIST criteria(15).”

RTK/kinase feedback activation is among the main mechanisms of adaptive resistance to KRASG12C inhibitors.

“Previous studies evidenced that RTK/kinase feedback activation is among the main mechanisms of adaptive resistance to KRASG12C inhibitors”

Sensitivity to AZ'1569 was not predicted by KRAS allele zygosity status, presence of concomitant mutations (including TP53 mutations), or baseline levels of kinases within the EGFR/KRAS axis.

“we found that sensitivity to AZ’1569 was not predicted by the KRAS allele zygosity status, the presence of concomitant mutations (including TP53 mutations), or baseline levels of kinases within the EGFR/KRAS axis”

Adagrasib has demonstrated clinical activity in previously treated patients with multiple tumor types, including colorectal cancer.

“Adagrasib, an oral small-molecule inhibitor of mutant KRAS G12C protein, has shown clinical activity in pretreated patients with several tumor types, including colorectal cancer.”

Adagrasib is an oral small-molecule covalent inhibitor of KRAS G12C that irreversibly and selectively binds the mutant protein in its inactive GDP-bound state.

“Adagrasib (MRTX849) is an orally available, small-molecule covalent inhibitor of KRAS G12C that irreversibly and selectively binds the mutant protein in its inactive, GDP-bound state.8,20”

Compounds bind covalently to cysteine 12 in the switch II pocket of GDP-bound KRASG12C, preventing oncogenic signaling and causing tumor regressions in preclinical models.

“compounds have been identified that bind covalently to cysteine 12 within the switch II pocket of the GDP-bound form of the KRASG12C protein, preventing oncogenic signaling and causing potent tumor regressions in preclinical models.3-7”

Combining mTOR inhibitor vistusertib with MRTX849 showed strong synergy against a panel of sensitive cell lines and xenograft models.

“Combining the mTOR inhibitor vistusertib with MRTX849 treatment showed strong synergy against a panel of sensitive cell lines and xenograft models [50].”

SHP2 inhibitors synergize with KRAS G12C inhibitors against sensitive mouse tumors.

“SHP2 inhibitors synergize with KRAS G12C inhibitors against sensitive mouse tumors, possibly because they prevent feedback reactivation driven by the loss of ERK-mediated EGFR repression when KRAS activity is abrogated and/or because SHP2 inhibitors drive KRAS to be in the G12C inhibitor-sensitive GDP-bound state of KRAS [50,133].”

Covalent inhibitors targeting mutated cysteine residues in KRASG12C exploited a binding pocket in the switch II domain which enabled the development of clinically active small molecule inhibitors for patients with this mutation.

“However, covalent inhibitors targeting the mutated cysteine residues in KRAS G12C exploited a binding pocket in the switch II domain which enabled the development of clinically active small molecule inhibitors for patients with this mutation [3-5].”

Direct small-molecule inhibitors of KRAS, including covalent inhibitors of KRASG12C, impair oncogenic signaling and cause tumor regressions in preclinical models.

“Recent advances in medicinal chemistry have enabled the development of direct small-molecule inhibitors of KRAS, including covalent inhibitors of the KRASG12C protein that impair oncogenic signaling and cause tumor regressions in preclinical models (8–10).”

The mechanisms of acquired resistance to adagrasib and sotorasib are currently unknown.

“The mechanisms of acquired resistance to these therapies are currently unknown.”

In 2021, the FDA approved sotorasib for the treatment of adult patients with KRAS G12C-mutated NSCLC, while adagrasib received similar approval the following year.

“In 2021, the FDA approved sotorasib for the treatment of adult patients with KRAS G12C-mutated NSCLC, while adagrasib received similar approval the following year [48,50,51,52,53].”

Studies conducted with MRTX849 in KRAS G12C mutated nonclinical tumor cell lines and in vivo models generally demonstrated a lack of effectiveness for the combination with BYL719.

“Studies conducted with MRTX849 in KRAS G12C mutated nonclinical tumor cell lines and in vivo models generally demonstrated a lack of effectiveness for the combination with BYL719 [4].”

AMG510 and MRTX849, which target KRAS G12C, produced less profound activity in CRC than in non-small cell lung cancer patients.

“Although two small molecules, AMG510 and MRTX849, which target KRAS G12C have entered clinical trials, these inhibitors produced less profound activity in CRC than non-small cell lung cancer patients [6–10].”

Adagrasib has a long half-life of 23 hours.

“Adagrasib is a selective, irreversible inhibitor of KRASG12C and has favorable properties, including long half-life (23 hours), dose-dependent pharmacokinetics, and central nervous system penetration (3–5).”

Adagrasib is a selective, irreversible inhibitor of KRASG12C.

“Adagrasib is a selective, irreversible inhibitor of KRASG12C and has favorable properties, including long half-life (23 hours), dose-dependent pharmacokinetics, and central nervous system penetration (3–5).”

Adagrasib has dose-dependent pharmacokinetics.

“Adagrasib is a selective, irreversible inhibitor of KRASG12C and has favorable properties, including long half-life (23 hours), dose-dependent pharmacokinetics, and central nervous system penetration (3–5).”

Adagrasib has central nervous system penetration.

“Adagrasib is a selective, irreversible inhibitor of KRASG12C and has favorable properties, including long half-life (23 hours), dose-dependent pharmacokinetics, and central nervous system penetration (3–5).”

AMG-510 and MRTX849 are structural derivatives of ARS-1620 with improved sensitivity of K-RasG12C recognition.

“AMG-510 and MRTX849 are structural derivatives of ARS-1620 with improved sensitivity of K-RasG12C recognition.”

Another major determinant of KRASG12C inhibitor resistance is the reactivation of upstream tyrosine kinase signaling that promotes SOS1-dependent GTP loading on RAS family members.

“Another major determinant of KRASG12C inhibitor resistance is the reactivation of upstream tyrosine kinase signaling that promotes SOS1-dependent GTP loading on RAS family members (19, 38, 44, 48–50).”

Results from early-stage clinical trials indicate that many cancer patients in the KRASG12C subgroup could significantly benefit from novel K-RasG12C-targeted therapeutics.

“Results from early-stage clinical trials indicate that many cancer patients in this subgroup could significantly benefit from these novel therapeutics.6,7”

In vivo responses to MRTX849 in 25 KRASG12C mutant human cell line-derived xenografts were individually represented as sensitive, partially sensitive, or refractory.

“In addition, the in vivo responses to MRTX849 in 25 KRASG12C mutant human cell line-derived xenografts were individually represented as sensitive, partially sensitive, or refractory.7”

K-RasG12C inhibitors act primarily through targeting the MAPK/ERK pathway for growth inhibition.

“Instead, K-RasG12C inhibitors act primarily through targeting MAPK/ERK pathway for growth inhibition.4, 5, 6, 7,49”

Preclinical findings strongly imply that intrinsic resistance might be accountable for heterogeneous responses to K-RasG12C inhibition in the clinic.

“These preclinical findings strongly imply that intrinsic resistance might be accountable for the heterogeneous responses to K-RasG12C inhibition in the clinic.4, 5, 6, 7,49”

Adaptive resistance to both AMG-510 and MRTX849 has been observed in preclinical animal models.

“Although long-term efficacy of K-RasG12C-targeted therapies in patients is still unknown,6,50 adaptive resistance to both AMG-510 and MRTX849 has been observed in preclinical animal models.7,11”

In most KRASG12C mutant models, phosphorylation of AKT and mTORC1-effector ribosomal protein S6 is barely affected by K-RasG12C inhibition.

“In most KRASG12C mutant models, the phosphorylation status of AKT and mTORC1-effector ribosomal protein S6 is barely affected by K-RasG12C inhibition.6,7,12,49”

Combination of MRTX849 and RMC-4550 further inhibited K-Ras signaling and exhibited increased antitumor responses in MRTX849-sensitive and -refractory models.

“In another study, the combination of MRTX849 and RMC-4550, a more clinically relevant SHP2 inhibitor, was found to further inhibit K-Ras signaling and exhibit increased antitumor responses in MRTX849-sensitive and -refractory models.7”

Individual genetic alterations including TP53, CDKN2A, STK11, and KRAS mutant allele frequency failed to predict adaptive resistance.

“although individual genetic alterations, including TP53 (tumor protein p53), CDKN2A (cyclin-dependent kinase inhibitor 2A), STK11 (serine/threonine kinase 11), and KRAS mutant allele frequency failed to predict adaptive resistance.6,7,11”

Expression of select HER family RTKs and genes controlling early cell-cycle transition correlated with the degree of therapeutic response to MRTX849.

“For example, the expression of select HER (human epidermal growth factor receptor) family members of RTKs and genes controlling early cell-cycle transition exhibited a correlation with the degree of therapeutic response to MRTX849,7”

Co-targeting K-RasG12C with PI3K effectors such as AKT and mTOR is effective in preclinical studies.

“Furthermore, co-targeting K-RasG12C with PI3K effectors, such as ATK and mTOR has also been shown to be effective in a series of preclinical studies.6,7,10,15,49”

Pre-existing clustered genetic variations, including overexpression of selected HER family genes, could provide clues on the therapeutic response to MRTX849.

“Although no individual genetic alteration at baseline seems to correlate with the therapeutic efficacy of K-RasG12C inhibitors, pre-existing clustered genetic variations, including the overexpression of selected HER family genes could provide some clues on the therapeutic response to MRTX849.7”

Co-treatment of MRTX849 with palbociclib induced more comprehensive tumor regression in several MRTX849-refractory models by profoundly suppressing the Rb/E2F pathway.

“Similarly, co-treatment of MRTX849 with palbociclib, a recently FDA-approved CDK4/6 inhibitor, induced more comprehensive tumor regression in several MRTX849-refractory models by profoundly suppressing the retinoblastoma protein (Rb)/E2F transcription factor (E2F) pathway.7”

No individual genetic alteration at baseline seems to correlate with the therapeutic efficacy of K-RasG12C inhibitors.

“Although no individual genetic alteration at baseline seems to correlate with the therapeutic efficacy of K-RasG12C inhibitors, pre-existing clustered genetic variations, including the overexpression of selected HER family genes could provide some clues on the therapeutic response to MRTX849.7”

Improved drug design has produced inhibitors selective for mutant KRAS in either its active or inactive state.

“improvements in drug design have culminated in the development of inhibitors that are selective for mutant KRAS in its active or inactive state.”

In xenografts and preclinical models, the precursor tool compound MRTX1257 showed promising results, leading to development of MRTX849.

“In xenografts and preclinical models, a precursor tool compound MRTX1257 showed promising results, leading to development of MRTX84918,19.”

Possible escape mechanisms for the tumor were identified through in vitro and in vivo models and CRISPR screens and included HER family receptor tyrosine kinases, EGFR, SHP2, and mTOR/S6 pathways.

“Possible escape mechanisms for the tumor were identified through in vitro and in vivo models and CRISPR screens and included HER family receptor tyrosine kinases, EGFR, SHP2, and mTOR/S6 pathways19.”

Each tested combination of afatinib, RMC-4550, vistusertib, everolimus, or palbociclib with adagrasib significantly enhanced anti-tumor activity compared to any individual therapy alone.

“Each tested combination proved to significantly enhance anti-tumor activity of adagrasib compared to any individual therapy alone19.”

Maximal response to adagrasib is associated with ERK activity inhibition.

“When achieving maximal response from adagrasib, association was seen with ERK activity inhibition.”

AMG-510 and MRTX849 were the first KRAS G12C inhibitors introduced in clinic.

“Using the His-95 groove as an additional binding site, AMG-510 and MRTX849 were the first KRAS G12C inhibitors that were introduced in clinic [71, 91, 92].”

KRASG12C mutant cancer cell lines treated with G12C inhibitors develop adaptive resistance.

“Recent reports (and our unpublished observations; see Results) show that KRASG12C mutant cancer cell lines treated with G12C-Is also develop adaptive resistance (Hallin et al., 2020; Lou et al., 2019; Misale et al., 2019; Ryan et al., 2020; Xue et al., 2020).”

G12C inhibitors have significant but limited efficacy in KRAS-mutant malignancies, mostly due to adaptive resistance.

“G12C inhibitors (G12C-Is) have significant but limited efficacy in KRAS-mutant malignancies, mostly due to “adaptive resistance.””

Covalent cross-linking to codon 12 cysteine induces a conformational fit and traps KRASG12C in its inactive GDP-bound state.

“Covalent cross-linking to codon 12 cysteine leads to an induced fit of the inhibitor in KRASG12C and trap the protein in its inactive, GDP-bound state (20–23).”

Adaptive response to G12C inhibitors can be minimized by combining G12C inhibitor with RTK or SHP2 inhibitors.

“These studies reported that adaptive response to G12C-Is could be minimized by combining G12C-I with RTK or SHP2-Is (Hallin et al., 2020; Lou et al., 2019; Ryan et al., 2020; Xue et al., 2020).”

In early clinical studies, KRASG12C inhibitors show promising activity in patients whose tumors harbor KRASG12C mutation.

“In early clinical studies, they have demonstrated promising activity in patients whose tumors harbor KRASG12C mutation (23, 24).”

In preclinical studies, KRASG12C inhibitors show significant selective in vitro toxicity in KRASG12C mutant cancer cells and inhibit growth of KRASG12C mutant xenograft and PDX tumors in mice.

“In preclinical studies, KRASG12C inhibitors have demonstrated significant and selective in vitro toxicity in KRASG12C mutant cancer cells; and these inhibitors are effective at inhibiting the growth of KRASG12C mutant xenograft and PDX tumors in mice (21–23).”

Genetic and pharmacological screens identified RTK signaling to PI3K and mTOR, and autophagy, as collateral dependencies with KRAS and MAPK pathway inhibitors.

“These experiments have identified receptor tyrosine kinase (RTK) signaling to PI3K and mTOR (23, 37–40), as well as the autophagy pathway (30) as collateral dependencies with KRAS and MAPK pathway inhibitors.”

Sotorasib and adagrasib, both KRASG12C inhibitors, have shown encouraging activity in KRASG12C-mutant solid tumors, especially non-small cell lung cancer with KRASG12C.

“Two KRASG12C inhibitors, sotorasib (AMG510) and adagrasib (MRTX849) have entered clinical development and they have demonstrated promising activities in solid tumors with KRASG12C mutation, particularly among patients with KRASG12C non-small cell lung cancer (23, 24).”

Clinically active, covalent KRASG12C inhibitors have provided the first opportunity to directly target this key oncoprotein.

“Clinically active, covalent KRASG12C inhibitors have provided the first opportunity to directly target this key oncoprotein (Canon et al., 2019; Hallin et al., 2020; Stephen et al., 2014).”

Initial reports from phase I trials show tumor responses in KRASG12C tumors are partial and restricted to a subset of patients.

“initial reports from phase I trials show tumor responses in KRASG12C tumors are partial and restricted to a subset of patients (Canon et al., 2019; Hallin et al., 2020).”

ARS is less potent than other G12C-Is, such as AMG-510 and MRTX1257.

“ARS is less potent than other G12C-Is, such as AMG-510 and MRTX1257 (Canon et al., 2019; Hallin et al., 2020).”

MRTX849 (adagrasib) is a highly selective covalent inhibitor of KRAS G12C and is currently in phase I/II clinical studies.

“MRTX849 (adagrasib) developed by Mirati Therapeutics, has been identified as a highly selective covalent inhibitor of KRAS (G12C) and is currently in phase I/II clinical studies [44, 45].”

In KRAS G12C positive cell lines and patient-derived xenograft models from multiple tumor types, 65% of the models showed significant tumor regression.

“In KRAS (G12C) positive cell lines and patient-derived xenograft models from multiple tumor types, 65% of the models showed significant tumor regression (Table 1) [60].”

Hallin et al. illustrated that genetic alterations in cell cycle regulators resulted in cell cycle dysregulation and changed KRAS mutant allele frequency, identifying additional factors that could attenuate the therapeutic response to MRTX849.

“Hallin et al. illustrated that genetic alterations in cell cycle regulators resulted in cell cycle dysregulation and changed KRAS mutant allele frequency, identifying additional factors that could attenuate the therapeutic response to MRTX849 [60].”

Inhibition of cell cycle genes in vivo in MRTX849-treated xenografts intensified further tumor growth inhibition over the effects of KRAS inhibition alone.

“Inhibition of cell cycle genes in vivo in MRTX849-treated xenografts intensified further tumor growth inhibition over the effects of KRAS inhibition alone [60].”

Up to 20% of KRAS-mutant NSCLC has concurrent CDKN2A mutations.

“It was found that up to 20% of KRAS-mutant NSCLC has concurrent CDKN2A mutations.”