Targeting KRAS(G12C): From Inhibitory Mechanism to Modulation of Antitumor Effects in Patients.

KRAS mutations are among the most common genetic alterations in lung, colorectal, and pancreatic cancers. Direct inhibition of KRAS oncoproteins has been a long-standing pursuit in precision oncology, one established shortly after the discovery of RAS mutations in human cancer cells nearly 40 years ago. Recent advances in medicinal chemistry have established inhibitors targeting KRAS(G12C), a mutation found in ∼13% of lung adenocarcinomas and, at a lower frequency, in other cancers. Preclinical studies describing their discovery and mechanism of action, coupled with emerging clinical data from patients treated with these drugs, have sparked a renewed enthusiasm in the study of KRAS and its therapeutic potential. Here, we discuss how these advances are reshaping the fundamental aspects of KRAS oncoprotein biology and the strides being made toward improving patient outcomes in the clinic.

A Small Molecule Targeting Mutagenic Translesion Synthesis Improves Chemotherapy.

Intrinsic and acquired drug resistance and induction of secondary malignancies limit successful chemotherapy. Because mutagenic translesion synthesis (TLS) contributes to chemoresistance as well as treatment-induced mutations, targeting TLS is an attractive avenue for improving chemotherapeutics. However, development of small molecules with high specificity and in vivo efficacy for mutagenic TLS has been challenging. Here, we report the discovery of a small-molecule inhibitor, JH-RE-06, that disrupts mutagenic TLS by preventing recruitment of mutagenic POL ζ. Remarkably, JH-RE-06 targets a nearly featureless surface of REV1 that interacts with the REV7 subunit of POL ζ. Binding of JH-RE-06 induces REV1 dimerization, which blocks the REV1-REV7 interaction and POL ζ recruitment. JH-RE-06 inhibits mutagenic TLS and enhances cisplatin-induced toxicity in cultured human and mouse cell lines. Co-administration of JH-RE-06 with cisplatin suppresses the growth of xenograft human melanomas in mice, establishing a framework for developing TLS inhibitors as a novel class of chemotherapy adjuvants.

Pan-KRAS inhibitor disables oncogenic signalling and tumour growth.

KRAS is one of the most commonly mutated proteins in cancer, and efforts to directly inhibit its function have been continuing for decades. The most successful of these has been the development of covalent allele-specific inhibitors that trap KRAS G12C in its inactive conformation and suppress tumour growth in patients1-7. Whether inactive-state selective inhibition can be used to therapeutically target non-G12C KRAS mutants remains under investigation. Here we report the discovery and characterization of a non-covalent inhibitor that binds preferentially and with high affinity to the inactive state of KRAS while sparing NRAS and HRAS. Although limited to only a few amino acids, the evolutionary divergence in the GTPase domain of RAS isoforms was sufficient to impart orthosteric and allosteric constraints for KRAS selectivity. The inhibitor blocked nucleotide exchange to prevent the activation of wild-type KRAS and a broad range of KRAS mutants, including G12A/C/D/F/V/S, G13C/D, V14I, L19F, Q22K, D33E, Q61H, K117N and A146V/T. Inhibition of downstream signalling and proliferation was restricted to cancer cells harbouring mutant KRAS, and drug treatment suppressed KRAS mutant tumour growth in mice, without having a detrimental effect on animal weight. Our study suggests that most KRAS oncoproteins cycle between an active state and an inactive state in cancer cells and are dependent on nucleotide exchange for activation. Pan-KRAS inhibitors, such as the one described here, have broad therapeutic implications and merit clinical investigation in patients with KRAS-driven cancers.

Diverse alterations associated with resistance to KRAS(G12C) inhibition.

Inactive state-selective KRAS(G12C) inhibitors1-8 demonstrate a 30-40% response rate and result in approximately 6-month median progression-free survival in patients with lung cancer9. The genetic basis for resistance to these first-in-class mutant GTPase inhibitors remains under investigation. Here we evaluated matched pre-treatment and post-treatment specimens from 43 patients treated with the KRAS(G12C) inhibitor sotorasib. Multiple treatment-emergent alterations were observed across 27 patients, including alterations in KRAS, NRAS, BRAF, EGFR, FGFR2, MYC and other genes. In preclinical patient-derived xenograft and cell line models, resistance to KRAS(G12C) inhibition was associated with low allele frequency hotspot mutations in KRAS(G12V or G13D), NRAS(Q61K or G13R), MRAS(Q71R) and/or BRAF(G596R), mirroring observations in patients. Single-cell sequencing in an isogenic lineage identified secondary RAS and/or BRAF mutations in the same cells as KRAS(G12C), where they bypassed inhibition without affecting target inactivation. Genetic or pharmacological targeting of ERK signalling intermediates enhanced the antiproliferative effect of G12C inhibitor treatment in models with acquired RAS or BRAF mutations. Our study thus suggests a heterogenous pattern of resistance with multiple subclonal events emerging during G12C inhibitor treatment. A subset of patients in our cohort acquired oncogenic KRAS, NRAS or BRAF mutations, and resistance in this setting may be delayed by co-targeting of ERK signalling intermediates. These findings merit broader evaluation in prospective clinical trials.

Rapid non-uniform adaptation to conformation-specific KRAS(G12C) inhibition.

KRAS GTPases are activated in one-third of cancers, and KRAS(G12C) is one of the most common activating alterations in lung adenocarcinoma1,2. KRAS(G12C) inhibitors3,4 are in phase-I clinical trials and early data show partial responses in nearly half of patients with lung cancer. How cancer cells bypass inhibition to prevent maximal response to therapy is not understood. Because KRAS(G12C) cycles between an active and inactive conformation4-6, and the inhibitors bind only to the latter, we tested whether isogenic cell populations respond in a non-uniform manner by studying the effect of treatment at a single-cell resolution. Here we report that, shortly after treatment, some cancer cells are sequestered in a quiescent state with low KRAS activity, whereas others bypass this effect to resume proliferation. This rapid divergent response occurs because some quiescent cells produce new KRAS(G12C) in response to suppressed mitogen-activated protein kinase output. New KRAS(G12C) is maintained in its active, drug-insensitive state by epidermal growth factor receptor and aurora kinase signalling. Cells without these adaptive changes-or cells in which these changes are pharmacologically inhibited-remain sensitive to drug treatment, because new KRAS(G12C) is either not available or exists in its inactive, drug-sensitive state. The direct targeting of KRAS oncoproteins has been a longstanding objective in precision oncology. Our study uncovers a flexible non-uniform fitness mechanism that enables groups of cells within a population to rapidly bypass the effect of treatment. This adaptive process must be overcome if we are to achieve complete and durable responses in the clinic.

Myocardial Hypertrophic Remodeling and Impaired Left Ventricular Function in Mice with a Cardiac-Specific Deletion of Janus Kinase 2.

The Janus kinase (JAK) system is involved in numerous cell signaling processes and is highly expressed in cardiac tissue. The JAK isoform JAK2 is activated by numerous factors known to influence cardiac function and pathologic conditions. However, although abundant, the role of JAK2 in the regulation or maintenance of cardiac homeostasis remains poorly understood. Using the Cre-loxP system, we generated a cardiac-specific deletion of Jak2 in the mouse to assess the effect on cardiac function with animals followed up for a 4-month period after birth. These animals had marked mortality during this period, although at 4 months mortality in male mice (47%) was substantially higher compared with female mice (30%). Both male and female cardiac Jak2-deleted mice had hypertrophy, dilated cardiomyopathy, and severe left ventricular dysfunction, including a marked reduction in ejection fractions as assessed by serial echocardiography, although the responses in females were somewhat less severe. Defective cardiac function was associated with altered protein levels of sarcoplasmic reticulum calcium-regulatory proteins particularly in hearts from male mice that had depressed levels of SERCA2 and phosphorylated phospholamban. In contrast, SERCA2 was unchanged in hearts of female mice, whereas phosphorylated phospholamban was increased. Our findings suggest that cardiac JAK2 is critical for maintaining normal heart function, and its ablation produces a severe pathologic phenotype composed of myocardial remodeling, heart failure, and pronounced mortality.

North American ginseng (Panax quinquefolius) suppresses ß-adrenergic-dependent signalling, hypertrophy, and cardiac dysfunction.

There is increasing evidence for a beneficial effect of ginseng on cardiac pathology. Here, we determined whether North American ginseng can modulate the deleterious effects of the ß-adrenoceptor agonist isoproterenol on cardiac hypertrophy and function using in vitro and in vivo approaches. Isoproterenol was administered for 2 weeks at either 25 mg/kg per day or 50 mg/kg per day (ISO25 or ISO50) via a subcutaneously implanted osmotic mini-pump to either control rats or those receiving ginseng (0.9 g/L in the drinking water ad libitum). Isoproterenol produced time- and dose-dependent left ventricular dysfunction, although these effects were attenuated by ginseng. Improved cardiac functions were associated with reduced heart masses, as well as prevention in the upregulation of the hypertrophy-related fetal gene expression. Lung masses were similarly attenuated, suggesting reduced pulmonary congestion. In in vitro studies, ginseng (10 µg/mL) completely suppressed the hypertrophic response to 1 µmol/L isoproterenol in terms of myocyte surface area, as well as reduction in the upregulation of fetal gene expression. These effects were associated with attenuation in both protein kinase A and cAMP response element-binding protein phosphorylation. Ginseng attenuates adverse cardiac adrenergic responses and, therefore, may be an effective therapy to reduce hypertrophy and heart failure associated with excessive catecholamine production.

CD73-TNAP crosstalk regulates the hypertrophic response and cardiomyocyte calcification due to α1 adrenoceptor activation.

Cluster of differentiation 73 (CD73) is an ecto-5' nucleotidase which catalyzes the conversion of AMP to adenosine. One of the many functions of adenosine is to suppress the activity of tissue nonspecific alkaline phosphatase (TNAP), an enzyme important in regulating intracellular calcification. Since myocardial calcification is associated with various cardiac disease states, we studied the individual roles and crosstalk between CD73 and TNAP in regulating myocyte responses to the α1 adrenoceptor agonist phenylephrine in terms of calcification and hypertrophy. Cultured neonatal rat cardiomyocytes were treated with 10 µM phenylephrine for 24 h in the absence or presence of the stable adenosine analog 2-chloro-adenosine, the TNAP inhibitor tetramisole or the CD73 inhibitor α,ß-methylene ADP. Phenylephrine produced marked hypertrophy as evidenced by significant increases in myocyte surface area and ANP gene expression, as well as calcification determined by Alizarin Red S staining. These responses were associated with reduced CD73 gene and protein expression and CD73 activity. Conversely, TNAP expression and activity were significantly increased although both were suppressed by 2-chloro-adenosine. CD73 inhibition alone significantly reduced myocyte-derived adenosine levels by >50 %, and directly induced hypertrophy and calcification in the absence of phenylephrine. These responses and those to phenylephrine were abrogated by TNAP inhibition. We conclude that TNAP contributes to the hypertrophic effect of phenylephrine, as well as its ability to produce cardiomyocyte calcification. These responses are minimized by CD73-dependent endogenously produced adenosine.

In vivo vulnerabilities to GPX4 and HDAC inhibitors in drug-persistent versus drug-resistant BRAFV600E lung adenocarcinoma.

The current targeted therapy for BRAFV600E-mutant lung cancer consists of a dual blockade of RAF/MEK kinases often combining dabrafenib/trametinib (D/T). This regimen extends survival when compared to single-agent treatments, but disease progression is unavoidable. By using whole-genome CRISPR screening and RNA sequencing, we characterize the vulnerabilities of both persister and D/T-resistant cellular models. Oxidative stress together with concomitant induction of antioxidant responses is boosted by D/T treatment. However, the nature of the oxidative damage, the choice of redox detoxification systems, and the resulting therapeutic vulnerabilities display stage-specific differences. Persister cells suffer from lipid peroxidation and are sensitive to ferroptosis upon GPX4 inhibition in vivo. Biomarkers of lipid peroxidation are detected in clinical samples following D/T treatment. Acquired alterations leading to mitogen-activated protein kinase (MAPK) reactivation enhance cystine transport to boost GPX4-independent antioxidant responses. Similarly to BRAFV600E-mutant melanoma, histone deacetylase (HDAC) inhibitors decrease D/T-resistant cell viability and extend therapeutic response in vivo.

Ouabain increases iNOS-dependent nitric oxide generation which contributes to the hypertrophic effect of the glycoside: possible role of peroxynitrite formation.

In addition to inotropic effects, cardiac glycosides exert deleterious effects on the heart which limit their use for cardiac therapeutics. In this study, we determined the possible contribution of ouabain-induced iNOS stimulation to the resultant hypertrophic as well as cytotoxic effects of the glycoside on cultured adult rat ventricular myocytes. Myocytes were treated with ouabain (50 µM) for up to 24 h. Ouabain significantly increased gene and protein levels of inducible nitric oxide synthase (iNOS) which was associated with significantly increased release of NO from myocytes as well as increased total release of reactive oxygen species (ROS), superoxide anion (O(2) (-)), and increased peroxynitrite formation as assessed by protein tyrosine nitration. Administration of ouabain was also associated with increased levels of myocyte toxicity as determined by myocyte morphology, trypan blue staining and lactate dehydrogenase (LDH) efflux. The nonspecific NOS inhibitor Nω-nitro-L: -arginine methyl ester and the more selective iNOS inhibitor 1400W both abrogated the increase in LDH release but had no significant effect on either morphology or trypan blue staining. Ouabain also significantly increased both myocyte surface area and expression of atrial natriuretic peptide indicating a hypertrophic response with both parameters being completely prevented by NOS inhibition. The effects of iNOS inhibitors were associated with diminished ouabain tyrosine nitration as well as abrogation of ouabain-induced p38 and ERK phosphorylation. Our study shows that ouabain is a potent inducer of NO formation, iNOS upregulation, and increased production of ROS. Inhibition of ouabain-dependent peroxynitrite formation may contribute to the antihypertrophic effect of iNOS inhibition possibly by preventing downstream MAPK activation.

Suppressing Nucleotide Exchange to Inhibit KRAS-Mutant Tumors.

Guanine nucleotide exchange factors (GEF) control the rate-limiting step of physiologic RAS activation. In this issue of Cancer Discovery, Hofmann and colleagues describe the discovery of a selective inhibitor targeting the GEF, SOS1, along with its preclinical effects in suppressing KRAS-mutant tumor growth.See related article by Hofmann et al., p. 142.

The G protein signaling regulator RGS3 enhances the GTPase activity of KRAS.

Recently reported to be effective in patients with lung cancer, KRASG12C inhibitors bind to the inactive, or guanosine diphosphate (GDP)­bound, state of the oncoprotein and require guanosine triphosphate (GTP) hydrolysis for inhibition. However, KRAS mutations prevent the catalytic arginine of GTPase-activating proteins (GAPs) from enhancing an otherwise slow hydrolysis rate. If KRAS mutants are indeed insensitive to GAPs, it is unclear how KRASG12C hydrolyzes sufficient GTP to allow inactive state­selective inhibition. Here, we show that RGS3, a GAP previously known for regulating G protein­coupled receptors, can also enhance the GTPase activity of mutant and wild-type KRAS proteins. Our study reveals an unexpected mechanism that inactivates KRAS and explains the vulnerability to emerging clinically effective therapeutics.

Adaptive Resistance to Dual BRAF/MEK Inhibition in BRAF-Driven Tumors through Autocrine FGFR Pathway Activation.

PURPOSE: Combined MAPK pathway inhibition using dual BRAF and MEK inhibitors has prolonged the duration of clinical response in patients with BRAFV600E-driven tumors compared with either agent alone. However, resistance frequently arises. EXPERIMENTAL DESIGN: We generated cell lines resistant to dual BRAF/MEK inhibition and utilized a pharmacologic synthetic lethal approach to identify a novel, adaptive resistance mechanism mediated through the fibroblast growth factor receptor (FGFR) pathway. RESULTS: In response to drug treatment, transcriptional upregulation of FGF1 results in autocrine activation of FGFR, which potentiates extracellular signal-regulated kinases (ERK) activation. FGFR inhibition overcomes resistance to dual BRAF/MEK inhibitors in both cell lines and patient-derived xenograft (PDX) models. Abrogation of this bypass mechanism in the first-line setting enhances tumor killing and prevents the emergence of drug-resistant cells. Moreover, clinical data implicate serum FGF1 levels in disease prognosis. CONCLUSIONS: Taken together, these results describe a new, adaptive resistance mechanism that is more commonly observed in the context of dual BRAF/MEK blockade as opposed to single-agent treatment and reveal the potential clinical utility of FGFR-targeting agents in combination with BRAF and MEK inhibitors as a promising strategy to forestall resistance in a subset of BRAF-driven cancers.

Sodium-hydrogen exchange inhibition attenuates glycoside-induced hypertrophy in rat ventricular myocytes.

AIMS: Cardiac glycosides induce cardiomyocyte hypertrophy via yet to be defined mechanisms. These hypertrophic effects are likely related to changes in intracellular signalling secondary to Na(+)-K(+) ATPase (NKA) inhibition which would produce elevations in intracellular sodium concentrations. Sodium-hydrogen exchanger isoform 1 (NHE-1) also contributes to intracellular sodium regulation. Accordingly, we determined the contribution of NHE-1 to cardiac glycoside-induced hypertrophy. METHODS AND RESULTS: The majority of the experiments were performed on cultured neonatal rat ventricular myocytes exposed to either ouabain (100 microM) or digoxin (40 microM) for 24 h, although additional experiments were also done using adult left ventricular myocytes with 30 microM of either glycoside. Both glycosides increased cell surface area by 30% and atrial natriuretic peptide gene expression by two- to three-fold (P < 0.05 for both). These effects were associated with a significant reduction in the expression of two NKA isoforms, alpha(2) and alpha(3), whereas the alpha(1) isoform was unaffected. Conversely, both glycosides increased NHE-1 expression in cardiomyocytes by approximately two-fold and significantly increased intracellular sodium concentrations by more than 60% (P < 0.05). Both ouabain and digoxin were also found to significantly increase phosphorylation of mitogen-activated protein kinases. All these effect were prevented when identical experiments were carried out in the presence of the NHE-1 inhibitors EMD 87580 or AVE 4890. Identical results were obtained using adult myocytes, although this was associated with downregulation of all three NKA isoforms. Glycoside-induced increase in cell shortening or intracellular Ca(2+) transients was not significantly affected by NHE-1 inhibition. CONCLUSION: When taken together, these studies show that NHE-1 inhibition attenuates the hypertrophic effect of cardiac glycosides without affecting inotropic parameters and suggest a possible approach to limiting glycoside-induced hypertrophic responses while preserving therapeutic, i.e. inotropic, actions.