Concurrent inhibition of oncogenic and wild-type RAS-GTP for cancer therapy

RAS oncogenes (collectively NRAS, HRAS and especially KRAS) are among the most frequently mutated genes in cancer, with common driver mutations occurring at codons 12, 13 and 611. Small molecule inhibitors of the KRAS(G12C) oncoprotein have demonstrated clinical efficacy in patients with multiple cancer types and have led to regulatory approvals for the treatment of non-small cell lung cancer2,3. Nevertheless, KRASG12C mutations account for only around 15% of KRAS-mutated cancers4,5, and there are no approved KRAS inhibitors for the majority of patients with tumours containing other common KRAS mutations. Here we describe RMC-7977, a reversible, tri-complex RAS inhibitor with broad-spectrum activity for the active state of both mutant and wild-type KRAS, NRAS and HRAS variants (a RAS(ON) multi-selective inhibitor). Preclinically, RMC-7977 demonstrated potent activity against RAS-addicted tumours carrying various RAS genotypes, particularly against cancer models with KRAS codon 12 mutations (KRASG12X). Treatment with RMC-7977 led to tumour regression and was well tolerated in diverse RAS-addicted preclinical cancer models. Additionally, RMC-7977 inhibited the growth of KRASG12C cancer models that are resistant to KRAS(G12C) inhibitors owing to restoration of RAS pathway signalling. Thus, RAS(ON) multi-selective inhibitors can target multiple oncogenic and wild-type RAS isoforms and have the potential to treat a wide range of RAS-addicted cancers with high unmet clinical need. A related RAS(ON) multi-selective inhibitor, RMC-6236, is currently under clinical evaluation in patients with KRAS-mutant solid tumours (ClinicalTrials.gov identifier: NCT05379985).

Insights into how adeno-squamous transition drives KRAS inhibitor resistance

The histologic transformation of adenocarcinoma (ADC) to squamous cell carcinoma (SCC), known as adeno-squamous transition or AST, is frequently observed in patients with lung cancer undergoing cancer therapy. In this issue, Tong and colleagues investigate genetic and epigenetic mechanisms that drive AST to confer resistance to KRAS inhibitors in preclinical models and patients.

Long-term benefit of sotorasib in patients with KRAS G12C-mutated non-small-cell lung cancer: plain language summary

WHAT IS THIS SUMMARY ABOUT?

This is a plain language summary of a study called CodeBreaK 100. The CodeBreaK 100 study included patients with non-small-cell lung cancer that had spread outside the lung (advanced). Lung cancer is one of the most common forms of cancer. CodeBreaK 100 specifically looked at patients with a particular change(mutation) in the KRAS gene resulting in the mutated protein called KRAS G12C. The KRAS G12C mutation can lead to development and growth of lung cancer. Patients received a treatment called sotorasib, which has accelerated approval or full approval in over 50 countries for patients with non-small-cell lung cancer with the KRAS G12C mutation. The CodeBreaK 100 study looked at whether sotorasib is a safe and effective treatment for advanced non-small-cell lung cancer. Sotorasib is designed to specifically target and lock the mutated KRAS protein in the inactive state to treat non-small-cell lung cancer.

WHAT WERE THE RESULTS?

In total, 174 adults were treated with sotorasib. Treatment-related side effects were seen in 70% of patients and were severe in 21% of patients. The most common side effects included diarrhea, increased liver enzymes, nausea and tiredness. 70 (41%) patients responded to sotorasib and 144 (84%) patients had tumors that either remained stable or shrunk in size. 29 (41%) patients who responded to sotorasib responded for over 12 months. After 2 years, 9 patients with a response remained on sotorasib; there were no notable increases in tumor size or development of new tumors over this time. There were 5patients who received sotorasib for more than 2 years and continued to respond. Long-term benefit was seen for some patients. Patients also benefitted from treatment when the tumor expressed different amounts of a protein called PD-L1.In total, 33% of patients were still alive after 2 years.

WHAT DO THE RESULTS MEAN?

Results show the long-term benefit of sotorasib therapy for people with advanced KRAS G12C-mutated non-small-cell lung cancer. Clinical Trial Registration: NCT03600883 (CodeBreaK 100) (ClinicalTrials.gov).

Outcomes of Combination Platinum-Doublet Chemotherapy and Anti-PD(L)-1 Blockade in KRASG12C-Mutant Non-Small Cell Lung Cancer

BACKGROUND

Direct KRASG12C inhibitors are approved for patients with non-small cell lung cancers (NSCLC) in the second-line setting. The standard-of-care for initial treatment remains immune checkpoint inhibitors, commonly in combination with platinum-doublet chemotherapy (chemo-immunotherapy). Outcomes to chemo-immunotherapy in this subgroup have not been well described. Our goal was to define the clinical outcomes to chemo-immunotherapy in patients with NSCLC with KRASG12C mutations.

PATIENTS AND METHODS

Through next-generation sequencing, we identified patients with advanced NSCLC with KRAS mutations treated with chemo-immunotherapy at 2 institutions. The primary objective was to determine outcomes and determinants of response to first-line chemo-immunotherapy among patients with KRASG12C by evaluating objective response rate (ORR), progression-free survival (PFS), and overall survival (OS). We assessed the impact of coalterations in STK11/KEAP1 on outcomes. As an exploratory objective, we compared the outcomes to chemo-immunotherapy in KRASG12C versus non-G12C groups.

RESULTS

One hundred and thirty eight patients with KRASG12C treated with first-line chemo-immunotherapy were included. ORR was 41% (95% confidence interval (CI), 32-41), median PFS was 6.8 months (95%CI, 5.5-10), and median OS was 15 months (95%CI, 11-28). In a multivariable model for PFS, older age (P = .042), squamous cell histology (P = .008), poor ECOG performance status (PS) (P < .001), and comutations in KEAP1 and STK11 (KEAP1MUT/STK11MUT) (P = .015) were associated with worse PFS. In a multivariable model for OS, poor ECOG PS (P = .004) and KEAP1MUT/STK11MUT (P = .009) were associated with worse OS. Patients with KRASG12C (N = 138) experienced similar outcomes to chemo-immunotherapy compared to patients with non-KRASG12C (N = 185) for both PFS (P = .2) and OS (P = .053).

CONCLUSIONS

We define the outcomes to first-line chemo-immunotherapy in patients with KRASG12C, which provides a real-world benchmark for clinical trial design involving patients with KRASG12C mutations. Outcomes are poor in patients with specific molecular coalterations, highlighting the need to develop more effective frontline therapies.

Chemical remodeling of a cellular chaperone to target the active state of mutant KRAS

The discovery of small-molecule inhibitors requires suitable binding pockets on protein surfaces. Proteins that lack this feature are considered undruggable and require innovative strategies for therapeutic targeting. KRAS is the most frequently activated oncogene in cancer, and the active state of mutant KRAS is such a recalcitrant target. We designed a natural product-inspired small molecule that remodels the surface of cyclophilin A (CYPA) to create a neomorphic interface with high affinity and selectivity for the active state of KRASG12C (in which glycine-12 is mutated to cysteine). The resulting CYPA:drug:KRASG12C tricomplex inactivated oncogenic signaling and led to tumor regressions in multiple human cancer models. This inhibitory strategy can be used to target additional KRAS mutants and other undruggable cancer drivers. Tricomplex inhibitors that selectively target active KRASG12C or multiple RAS mutants are in clinical trials now (NCT05462717 and NCT05379985).

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.

Long-Term Outcomes and Molecular Correlates of Sotorasib Efficacy in Patients With Pretreated KRAS G12C-Mutated Non-Small-Cell Lung Cancer: 2-Year Analysis of CodeBreaK 100

Clinical trials frequently include multiple end points that mature at different times. The initial report, typically based on the primary end point, may be published when key planned co-primary or secondary analyses are not yet available. Clinical Trial Updates provide an opportunity to disseminate additional results from studies, published in JCO or elsewhere, for which the primary end point has already been reported.In the longest follow-up, to our knowledge, for a KRASG12C inhibitor, we assessed the long-term efficacy, safety, and biomarkers of sotorasib in patients with KRAS G12C-mutated advanced non-small-cell lung cancer (NSCLC) from the CodeBreaK 100 clinical trial (ClinicalTrials.gov identifier: NCT03600883). This multicenter, single-group, open-label phase I/phase II trial enrolled 174 patients with KRAS G12C-mutated, locally advanced or metastatic NSCLC after progression on prior therapies. Patients (N = 174) received sotorasib 960 mg once daily with the primary end points for phase I of safety and tolerability and for phase II of objective response rate (ORR). Sotorasib produced an ORR of 41%, median duration of response of 12.3 months, progression-free survival (PFS) of 6.3 months, overall survival (OS) of 12.5 months, and 2-year OS rate of 33%. Long-term clinical benefit (PFS ≥ 12 months) was observed in 40 (23%) patients across PD-L1 expression levels, in a proportion of patients with somatic STK11 and/or KEAP1 alterations, and was associated with lower baseline circulating tumor DNA levels. Sotorasib was well tolerated, with few late-onset treatment-related toxicities, none of which led to treatment discontinuation. These results demonstrate the long-term benefit of sotorasib, including in subgroups with poor prognosis.

Clinical and Genomic Features of Response and Toxicity to Sotorasib in a Real-World Cohort of Patients With Advanced KRAS G12C-Mutant Non-Small Cell Lung Cancer

PURPOSE

With the recent approval of the KRAS G12C inhibitor sotorasib for patients with advanced KRAS G12C-mutant non-small cell lung cancer (NSCLC), there is a new need to identify factors associated with activity and toxicity among patients treated in routine practice.

MATERIALS AND METHODS

We conducted a multicenter retrospective study of patients treated with sotorasib outside of clinical trials to identify factors associated with real-world progression free survival (rwPFS), overall survival (OS), and toxicity.

RESULTS

Among 105 patients with advanced KRAS G12C-mutant NSCLC treated with sotorasib, treatment led to a 5.3-month median rwPFS, 12.6-month median OS, and 28% real-world response rate. KEAP1 comutations were associated with shorter rwPFS and OS (rwPFS hazard ratio [HR], 3.19; P = .004; OS HR, 4.10; P = .003); no significant differences in rwPFS or OS were observed across TP53 (rwPFS HR, 1.10; P = .731; OS HR, 1.19; P = .631) or STK11 (rwPFS HR, 1.66; P = .098; OS HR, 1.73; P = .168) comutation status. Notably, almost all patients who developed grade 3 or higher treatment-related adverse events (G3+ TRAEs) had previously been treated with anti-PD-(L)1 therapy. Among these patients, anti-PD-(L)1 therapy exposure within 12 weeks of sotorasib was strongly associated with G3+ TRAEs (P < .001) and TRAE-related sotorasib discontinuation (P = .014). Twenty-eight percent of patients with recent anti-PD-(L)1 therapy exposure experienced G3+ TRAEs, most commonly hepatotoxicity.

CONCLUSION

Among patients treated with sotorasib in routine practice, KEAP1 comutations were associated with resistance and recent anti-PD-(L)1 therapy exposure was associated with toxicity. These observations may help guide use of sotorasib in the clinic and may help inform the next generation of KRAS G12C-targeted clinical trials.

Overall survival with circulating tumor DNA-guided therapy in advanced non-small-cell lung cancer

Circulating tumor DNA (ctDNA) sequencing guides therapy decisions but has been studied mostly in small cohorts without sufficient follow-up to determine its influence on overall survival. We prospectively followed an international cohort of 1,127 patients with non-small-cell lung cancer and ctDNA-guided therapy. ctDNA detection was associated with shorter survival (hazard ratio (HR), 2.05; 95% confidence interval (CI), 1.74-2.42; P < 0.001) independently of clinicopathologic features and metabolic tumor volume. Among the 722 (64%) patients with detectable ctDNA, 255 (23%) matched to targeted therapy by ctDNA sequencing had longer survival than those not treated with targeted therapy (HR, 0.63; 95% CI, 0.52-0.76; P < 0.001). Genomic alterations in ctDNA not detected by time-matched tissue sequencing were found in 25% of the patients. These ctDNA-only alterations disproportionately featured subclonal drivers of resistance, including RICTOR and PIK3CA alterations, and were associated with short survival. Minimally invasive ctDNA profiling can identify heterogeneous drivers not captured in tissue sequencing and expand community access to life-prolonging therapy.

Clinicopathologic characterization of ERK2 E322K mutation in solid tumors: Implications for treatment and drug development

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Background: MAPK1 encodes ERK2, a kinase component of the mitogen activated signaling (MAPK) pathway. ERK2 E322K is a known activating mutation that leads to increased phosphorylation and ERK signaling. In vitro studies found this mutation to be associated with resistance to dabrafenib, trametinib, but potential sensitivity to ERK inhibitors. Despite its potential as a drug target, little is known about the clinicopathologic characteristics of this hotspot mutation across solid tumors. Methods: Patients with solid tumors underwent tumor next-generation sequencing at Memorial Sloan Kettering Cancer Center between Jan 2015 and Sep 2020 using the MSK-IMPACT assay. Using the cBioPortal database and clinical charts, we analyzed tumors harboring MAPK1/ERK2 E322K mutations, assessed their clinicopathologic characteristics, co-mutational status and overall survival (OS). OS was measured from time of tumor sequencing to date of death or last follow-up. Results: A total of 37 tumor samples from 35 patients were identified in 59,822 tumors sequenced (0.06%) to harbor an ERK2 E322K mutation. The distribution across tumor types was as follows: head and neck squamous cell carcinoma (29%), bladder cancer (20%), lymphomas (9%), colorectal cancers (9%), gastric cancers (9%), cholangiocarcinoma (6%), cervical cancers (6%), lung cancers (6%), germ cell tumor (3%), Merkel cell carcinoma (3%), and breast cancers (3%). The OS in patients with metastatic disease and ERK2 E322K was 22.29 months (95%CI: 7.56-NA) months. Other mutations in RAS pathway frequently co-occurred with ERK2 E322K mutation (17/37, 46%). Concurrent mutations are also involved in pathways of cell cycle (71%), PI3K (71%), TP53 (66%), NOTCH (57%), RTK (51%), HIPPO (29%), TGF-beta (29%), WNT (26%), NRF2 (20%), MYC (14%). The median TMB score of samples from solid malignancies was 12.3 (range:0-101, quartiles: 6.9-33.0) mutation/Mb. Two patients (2/35, 6%) had microsatellite-instability high (MSI-H) tumors. The most frequent concurrent activating mutations include ARID1A (29%), FBXW7 (26%), PI3KCA (22%), PI3KR1/2/3 (20%), CDKN2A (11%), PTEN (8%), BRCA1/2(8%), FGFR3 (8%), BRAF (6%), Only one of these 35 patients received treatment targeting BRAF/MEK/ERK pathway and achieved partial response. One patient with NSCLC harboring a concurrent EGFR L858R mutation did not respond to erlotinib. One patient with PI3KCA mutated head and neck cancer did not respond to PI3K inhibitor. Two patients had TMB score of 100.9 and 12.9 mutation/Mb had partial response to pembrolizumab. Conclusions: ERK2 E322K mutation is a rare oncogenic mutation across diverse solid tumor types, associated with a high co-occurrence of other activating mutations and a high TMB. The lack of response to other targeted therapies suggests ERK2 E322K is a potential driver mutation. These findings may inform treatment and further development of ERK inhibitors.

Immune biomarkers and response to checkpoint inhibition of BRAFV600 and BRAF non-V600 altered lung cancers

BACKGROUND

While 2-4% of lung cancers possess alterations in BRAF, little is known about the immune responsiveness of these tumours.

METHODS

Clinical and genomic data were collected from 5945 patients with lung cancers whose tumours underwent next-generation sequencing between 2015 and 2018. Patients were followed through 2020.

RESULTS

In total, 127 patients with metastatic BRAF-altered lung cancers were identified: 29 tumours had Class I mutations, 59 had Class II/III alterations, and 39 had variants of unknown significance (VUS). Tumour mutation burden was higher in Class II/III than Class I-altered tumours (8.8 mutations/Mb versus 4.9, P < 0.001), but this difference was diminished when stratified by smoking status. The overall response rate to immune checkpoint inhibitors (ICI) was 9% in Class I-altered tumours and 26% in Class II/III (P = 0.25), with median time on treatment of 1.9 months in both groups. Among patients with Class I-III-altered tumours, 36-month HR for death in those who ever versus never received ICI was 1.82 (1.17-6.11). Nine patients were on ICI for >2 years (two with Class I mutations, two with Class II/III alterations, and five with VUS).

CONCLUSIONS

A subset of patients with BRAF-altered lung cancers achieved durable disease control on ICI. However, collectively no significant clinical benefit was seen.

With one eye on the future

In the past 20 years we have seen the rise of a new era of cancer research that moved its focus away from the cancer cell itself and revealed a complexity of interactions, both within the tumor and with the host, that ultimately dictate the evolution and progression of the disease. We have witnessed the development of immunotherapies that changed the fate of many patients and new diagnostic strategies with the potential of changing clinical practice. In this article, several experts discuss what lies ahead.

Phase 1 Clinical Trial of Trametinib and Ponatinib in Patients With NSCLC Harboring KRAS Mutations

Introduction

Somatic KRAS mutations occur in 25% of patients with NSCLC. Treatment with MEK inhibitor monotherapy has not been successful in clinical trials to date. Compensatory activation of FGFR1 was identified as a mechanism of trametinib resistance in KRAS-mutant NSCLC, and combination therapy with trametinib and ponatinib was synergistic in in vitro and in vivo models. This study sought to evaluate this drug combination in patients with KRAS-mutant NSCLC.

Methods

A phase 1 dose escalation study of trametinib and ponatinib was conducted in patients with advanced NSCLC with KRAS mutations. A standard 3-plus-3 dose escalation was done. Patients were treated with the study therapy until intolerable toxicity or disease progression.

Results

A total of 12 patients with KRAS-mutant NSCLC were treated (seven at trametinib 2 mg and ponatinib 15 mg, five at trametinib 2 mg and ponatinib 30 mg). Common toxicities observed were rash, diarrhea, and fever. Serious adverse events potentially related to therapy were reported in five patients, including one death in the study and four cardiovascular events. Serious events were observed at both dose levels. Of note, 75% (9 of 12) were assessable for radiographic response and no confirmed partial responses were observed. The median time on study was 43 days.

Conclusions

In this phase 1 study, in patients with KRAS-mutant advanced NSCLC, combined treatment with trametinib and ponatinib was associated with cardiovascular and bleeding toxicities. Exploring the combination of MEK and FGFR1 inhibition in future studies is potentially warranted but alternative agents should be considered to improve safety and tolerability.

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.

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 state–selective inhibition. Here, we show that RGS3, a GAP previously known for regulating G protein–coupled 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.

Acquired Resistance to KRASG12C Inhibition in Cancer

BACKGROUND

Clinical trials of the KRAS inhibitors adagrasib and sotorasib have shown promising activity in cancers harboring KRAS glycine-to-cysteine amino acid substitutions at codon 12 (KRASG12C). The mechanisms of acquired resistance to these therapies are currently unknown.

METHODS

Among patients with KRASG12C -mutant cancers treated with adagrasib monotherapy, we performed genomic and histologic analyses that compared pretreatment samples with those obtained after the development of resistance. Cell-based experiments were conducted to study mutations that confer resistance to KRASG12C inhibitors.

RESULTS

A total of 38 patients were included in this study: 27 with non-small-cell lung cancer, 10 with colorectal cancer, and 1 with appendiceal cancer. Putative mechanisms of resistance to adagrasib were detected in 17 patients (45% of the cohort), of whom 7 (18% of the cohort) had multiple coincident mechanisms. Acquired KRAS alterations included G12D/R/V/W, G13D, Q61H, R68S, H95D/Q/R, Y96C, and high-level amplification of the KRASG12C allele. Acquired bypass mechanisms of resistance included MET amplification; activating mutations in NRAS, BRAF, MAP2K1, and RET; oncogenic fusions involving ALK, RET, BRAF, RAF1, and FGFR3; and loss-of-function mutations in NF1 and PTEN. In two of nine patients with lung adenocarcinoma for whom paired tissue-biopsy samples were available, histologic transformation to squamous-cell carcinoma was observed without identification of any other resistance mechanisms. Using an in vitro deep mutational scanning screen, we systematically defined the landscape of KRAS mutations that confer resistance to KRASG12C inhibitors.

CONCLUSIONS

Diverse genomic and histologic mechanisms impart resistance to covalent KRASG12C inhibitors, and new therapeutic strategies are required to delay and overcome this drug resistance in patients with cancer. (Funded by Mirati Therapeutics and others; ClinicalTrials.gov number, NCT03785249.).

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.

Chemo-immunotherapy outcomes of KRAS-G12C mutant lung cancer compared to other molecular subtypes of KRAS-mutant lung cancer

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Background: KRAS mutations are identified in approximately 30% of NSCLC, with G12C mutations being the most common subtype and representing 12% of all non-small cell lung cancer cases. Novel direct inhibitors are in clinical development and have shown promising activity, although the efficacy of these agents compared to other standard therapies for lung cancer is not yet known. We hypothesized that patients with KRAS-G12C mutations may have distinct responses to chemo-immunotherapy regimens both with respect to STK11 and KEAP1 co-mutation status and compared to patients with non-G12C subtypes. Methods: Patients with KRAS-mutant lung cancers at Memorial Sloan Kettering Cancer Center and Dana Farber Cancer Institute treated with chemo-immunotherapy regimens as first line therapy for advanced/metastatic disease were identified. Subset with KRAS G12C mutations non-G12C subtypes were compared and response to therapy was assessed by investigator. Baseline characteristics were compared with the Chi-square and Fisher’s exact test for categorical data and Wilcoxon rank-rum test for continuous data. Response evaluations where performed by investigators and compared between groups with the Fisher’s exact test. Progression free survival and overall survival was calculated from start of therapy to date of progression or death/last follow up, respectively and compared between groups using the Cox proportional-hazards model. Results: We identified 137 patients with KRAS -mutant NSCLC treated with chemo-immunotherapy: 45% (62/137) had mutations in KRAS-G12C and 55% harbored non-G12C mutations (17% G12V, 15% G12D, 4% G12A, 4% G12S, 3% G13D). The median OS was 21 and 14 months for G12C and non-G12C patients, respectively (p = 0.24). ORR to chemo-immunotherapy for patients harboring a KRAS-G12C mutation was 40% (25/62) compared to 31% (23/75) in non-G12C subtypes (p = 0.3). Median PFS was similar for both G12C and non-G12C subtypes (7.3 vs 6.1 months, respectively, p = 0.12). Concurrent STK11 mutation was identified in 40% of patients with KRAS-G12C and KEAP1 alterations were observed in 32% of patients. In patients with KRAS-G12C, co-mutation in STK11 and/or KEAP1 was associated with shorter PFS (15.8 vs 5.1 months, p = 0.01). Conclusions: KRAS-G12C mutations are present in 12% of patients with NSCLC and represent a relevant subtype of NSCLC given KRAS G12C inhibitors now in clinical development. Treatment outcomes to chemo-immunotherapy are similar in patients with G12C and non-G12C subtypes. Outcomes are poor for patients with concurrent STK11 and/or KEAP1 mutations representing a significant unmet need.

KRAS G12C Mutation Is Associated with Increased Risk of Recurrence in Surgically Resected Lung Adenocarcinoma

PURPOSE

KRAS ^G12C is the most common KRAS mutation in primary lung adenocarcinoma. Phase I clinical trials have demonstrated encouraging clinical activity of KRAS ^G12C inhibitors in the metastatic setting. We investigated disease-free survival (DFS) and tumor genomic features in patients with surgically resected KRAS ^G12C-mutant lung adenocarcinoma.

EXPERIMENTAL DESIGN

Patients who underwent resection of stage I-III lung adenocarcinoma and next-generation sequencing (NGS) were evaluated. Exclusion criteria were receipt of induction therapy, incomplete resection, and low-quality NGS. Mutations were classified as KRAS wild-type (KRAS ^wt), G12C (KRAS ^G12C), or non-G12C (KRAS ^other). DFS was compared between groups using the log-rank test; factors associated with DFS were assessed using Cox regression. Mutual exclusivity and cooccurrence, tumor clonality, and mutational signatures were assessed.

RESULTS

In total, 604 patients were included: 374 KRAS ^wt (62%), 95 KRAS ^G12C (16%), and 135 KRAS ^other (22%). Three-year DFS was not different between KRAS-mutant and KRAS ^wt tumors. However, 3-year DFS was worse in patients with KRAS ^G12C than KRAS ^other tumors (log-rank P = 0.029). KRAS ^G12C tumors had more lymphovascular invasion (51% vs. 37%; P = 0.032) and higher tumor mutation burden [median (interquartile range), 7.0 (5.3-10.8) vs. 6.1 (3.5-9.7); P = 0.021], compared with KRAS ^other tumors. KRAS ^G12C mutation was independently associated with worse DFS on multivariable analysis. Our DFS findings were externally validated in an independent The Cancer Genome Atlas cohort.

CONCLUSIONS

KRAS ^G12C mutations are associated with worse DFS after complete resection of stage I-III lung adenocarcinoma. These tumors harbor more aggressive clinicopathologic and genomic features than other KRAS-mutant tumors. We identified a high-risk group for whom KRAS ^G12C inhibitors may be investigated to improve survival.

Treatment Outcomes and Clinical Characteristics of Patients with KRAS-G12C-Mutant Non-Small Cell Lung Cancer

PURPOSE

KRAS mutations are identified in approximately 30% of patients with non-small cell lung cancer (NSCLC). Novel direct inhibitors of KRAS G12C have shown activity in early-phase clinical trials. We hypothesized that patients with KRAS G12C mutations may have distinct clinical characteristics and responses to therapies.

EXPERIMENTAL DESIGN

Through routine next-generation sequencing, we identified patients with KRAS-mutant NSCLC treated at Memorial Sloan Kettering Cancer Center (New York, NY) from 2014 to 2018 and reviewed tumor characteristics, overall survival, and treatment outcomes.

RESULTS

We identified 1,194 patients with KRAS-mutant NSCLC, including 770 with recurrent or metastatic disease. KRAS G12C mutations were present in 46% and KRAS non-G12C mutations in 54%. Patients with KRAS G12C had a higher tumor mutation burden (median, 8.8 vs. 7 mut/Mb; P = 0.006) and higher median PD-L1 expression (5% vs. 1%). The comutation patterns of STK11 (28% vs. 29%) and KEAP1 (23% vs. 24%) were similar. The median overall survivals from diagnosis were similar for KRAS G12C (13.4 months) and KRAS non-G12C mutations (13.1 months; P = 0.96). In patients with PD-L1 ≥50%, there was not a significant difference in response rate with single-agent immune checkpoint inhibitor for patients with KRAS G12C mutations (40% vs. 58%; P = 0.07).

CONCLUSIONS

We provide outcome data for a large series of patients with KRAS G12C-mutant NSCLC with available therapies, demonstrating that responses and duration of benefit with available therapies are similar to those seen in patients with KRAS non-G12C mutations. Strategies to incorporate new targeted therapies into the current treatment paradigm will need to consider outcomes specific to patients harboring KRAS G12C mutations.

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.

KRASG12C Inhibition with Sotorasib in Advanced Solid Tumors

BACKGROUND

No therapies for targeting KRAS mutations in cancer have been approved. The KRAS p.G12C mutation occurs in 13% of non-small-cell lung cancers (NSCLCs) and in 1 to 3% of colorectal cancers and other cancers. Sotorasib is a small molecule that selectively and irreversibly targets KRASG12C.

METHODS

We conducted a phase 1 trial of sotorasib in patients with advanced solid tumors harboring the KRAS p.G12C mutation. Patients received sotorasib orally once daily. The primary end point was safety. Key secondary end points were pharmacokinetics and objective response, as assessed according to Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1.

RESULTS

A total of 129 patients (59 with NSCLC, 42 with colorectal cancer, and 28 with other tumors) were included in dose escalation and expansion cohorts. Patients had received a median of 3 (range, 0 to 11) previous lines of anticancer therapies for metastatic disease. No dose-limiting toxic effects or treatment-related deaths were observed. A total of 73 patients (56.6%) had treatment-related adverse events; 15 patients (11.6%) had grade 3 or 4 events. In the subgroup with NSCLC, 32.2% (19 patients) had a confirmed objective response (complete or partial response) and 88.1% (52 patients) had disease control (objective response or stable disease); the median progression-free survival was 6.3 months (range, 0.0+ to 14.9 [with + indicating that the value includes patient data that were censored at data cutoff]). In the subgroup with colorectal cancer, 7.1% (3 patients) had a confirmed response, and 73.8% (31 patients) had disease control; the median progression-free survival was 4.0 months (range, 0.0+ to 11.1+). Responses were also observed in patients with pancreatic, endometrial, and appendiceal cancers and melanoma.

CONCLUSIONS

Sotorasib showed encouraging anticancer activity in patients with heavily pretreated advanced solid tumors harboring the KRAS p.G12C mutation. Grade 3 or 4 treatment-related toxic effects occurred in 11.6% of the patients. (Funded by Amgen and others; CodeBreaK100 ClinicalTrials.gov number, NCT03600883.).

Abstract 622: Rapid non-uniform adaptation to conformation-specific KRAS G12Cinhibition

KRAS GTPases are activated in one-third of cancers and KRAS G12C is the most common activating alteration in lung adenocarcinoma. KRAS G12C-specific inhibitors (G12Ci) are in Phase-I clinical trials and early data show only partial responses in lung cancer patients. How cancer cells bypass inhibition, to prevent maximal responses to therapy, is not understood. Because KRAS G12C cycles between an active and inactive conformation, and the covalent G12Ci only bind to the latter, we tested whether isogenic cell populations respond non-uniformly by studying the effect of treatment at a single-cell resolution. Using single-cell RNA sequencing and a fluorescent quiescence biosensor, we show that shortly after treatment, most cancer cells are sequestered in a quiescent state with low KRAS activity, while a small population reactivates KRAS to resume proliferation. This rapid divergent response is due to synthesis of new, drug-free KRAS protein, resulting from increased KRAS transcription in response to suppressed MAPK signaling. Combining cell fate-specific gene expressions and results from a CRISPR-Cas9 screen, we identified that adaptive signals such as epidermal growth-factor receptor and aurora kinase A signaling modulate the heterogeneous response to treatment with G12Ci. These upstream signals help to maintain new KRAS G12C protein in its active, drug-insensitive state, which restores KRAS signaling and transcriptional output in a subset of cells to allow escape from G12Ci-induced quiescence. Cells without these adaptive changes (or cells where they are pharmacologically inhibited) remain sensitive to G12Ci treatment, because new KRAS G12C is either not available, or it exists in its inactive, drug-sensitive state. Combined inhibition of these adaptive signals along with KRAS G12C produced more potent antitumor effects in xenograft models. 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 to maximize the therapeutic potential of conformation-specific KRAS G12C inhibitors in the clinic.

Citation Format: Jenny Y. Xue, Yulei Zhao, Jordan Aronowitz, Trang T. Mai, Alberto Vides, Besnik Qeriqi, Dongsung Kim, Chuanchuan Li, Elisa de Stanchina, Linas Mazutis, Davide Risso, Piro Lito. Rapid non-uniform adaptation to conformation-specific KRAS G12Cinhibition [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 622.

Abstract 709: The utility of plasma ctDNA for detection of KRAS G12C and other mutations in lung cancers

Background: KRAS is the most common oncogene in lung cancer, but has historically been considered undruggable. The recent development of mutant-selective KRAS G12C inhibitors has for the first time created potential therapeutic opportunities for this unmet need. Simultaneously, circulating tumor DNA (ctDNA) is increasingly being used to detect targetable oncogenes in patients with metastatic lung cancers. There is limited data regarding the utility of plasma ctDNA in specifically identifying KRAS G12C mutations.

Methods: Plasma was collected from 599 patients with lung cancer seen at Memorial Sloan Kettering Cancer Center between 10/2016 and 1/2019. ctDNA sequencing was performed using the ctDX-Lung Assay (Resolution Bioscience; Kirkland, WA). Tissue DNA sequencing was carried out using the MSK-IMPACT assay.

Results: Mutations in KRAS (KRAS+) were detected in 129 patients (21.5%). Of patients with KRAS+ lung cancers, 116 had metastatic disease at the time of plasma testing. Plasma testing was carried out within 90 days of metastatic diagnosis in 92 of the 116 metastatic KRAS+ patients (79.3%), of whom 66 patients had both plasma and tissue sequencing available for comparison. 59 of the 66 patients (89.4%) had not had systemic treatment at the time of plasma testing. Average turn-around-time (TAT) for ctDNA testing in this cohort of 66 patients was 10 days, while average TAT for tissue sequencing was 22 days. ctDNA detected a KRAS mutation in 48 of the 66 patients (72.7%). A G12C mutation was found in the tissue and/or blood from 29 patients (43.9%), while 37 patients (56.1%) had other KRAS mutations. In one patient with a G12C mutation detected on plasma testing, no KRAS mutation or other known oncogenic drivers were found on tissue testing, though with limited tumor cells in the tissue sample. 75.6% of the patients with G12C mutations had KRAS detectable in the plasma, as compared to 70.3% of patients with other KRAS mutations (p=0.6). Within the cohort of patients with paired tissue and plasma testing, at a median follow-up of 202 days post-metastatic diagnosis, survival was longer in those patients without detectable KRAS in the plasma as compared to those patients with detectable plasma KRAS (log rank (Mantel-Cox), p&lt;0.001, log rank HR 7.9, 95% CI: 3.7-16.8).

Conclusion: Plasma testing was able to rapidly detect KRAS G12C mutations in the majority of patients with this alteration, which may guide G12C inhibitor therapy. In our cohort, patients that shed KRAS mutant DNA into the plasma had shorter survival.

Citation Format: Yonina R. Murciano-Goroff, Kathryn C. Arbour, Michael D. Offin, Hai-Yan Y. Tu, Emily S. Lebow, Tristan S. Shaffer, Caterina Bertucci, Syed A. Hosseini, Kavita Garg, Lee P. Lim, Mark Li, Jason C. Chang, Jorge S. Reis-Filho, Pedram Razavi, James M. Isbell, Gregory J. Riely, David M. Hyman, Piro Lito, Bob T. Li. The utility of plasma ctDNA for detection of KRAS G12C and other mutations in lung cancers [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 709.

COVID-19 in patients with lung cancer

BACKGROUND

Patients with lung cancers may have disproportionately severe coronavirus disease 2019 (COVID-19) outcomes. Understanding the patient-specific and cancer-specific features that impact the severity of COVID-19 may inform optimal cancer care during this pandemic.

PATIENTS AND METHODS

We examined consecutive patients with lung cancer and confirmed diagnosis of COVID-19 (n = 102) at a single center from 12 March 2020 to 6 May 2020. Thresholds of severity were defined a priori as hospitalization, intensive care unit/intubation/do not intubate ([ICU/intubation/DNI] a composite metric of severe disease), or death. Recovery was defined as >14 days from COVID-19 test and >3 days since symptom resolution. Human leukocyte antigen (HLA) alleles were inferred from MSK-IMPACT (n = 46) and compared with controls with lung cancer and no known non-COVID-19 (n = 5166).

RESULTS

COVID-19 was severe in patients with lung cancer (62% hospitalized, 25% died). Although severe, COVID-19 accounted for a minority of overall lung cancer deaths during the pandemic (11% overall). Determinants of COVID-19 severity were largely patient-specific features, including smoking status and chronic obstructive pulmonary disease [odds ratio for severe COVID-19 2.9, 95% confidence interval 1.07-9.44 comparing the median (23.5 pack-years) to never-smoker and 3.87, 95% confidence interval 1.35-9.68, respectively]. Cancer-specific features, including prior thoracic surgery/radiation and recent systemic therapies did not impact severity. Human leukocyte antigen supertypes were generally similar in mild or severe cases of COVID-19 compared with non-COVID-19 controls. Most patients recovered from COVID-19, including 25% patients initially requiring intubation. Among hospitalized patients, hydroxychloroquine did not improve COVID-19 outcomes.

CONCLUSION

COVID-19 is associated with high burden of severity in patients with lung cancer. Patient-specific features, rather than cancer-specific features or treatments, are the greatest determinants of severity.

Abstract 12: Characterizing KRAS G12C and other mutations in plasma ctDNA from patients with lung cancer

Background: KRAS is the most common oncogene in lung cancers, but despite decades of intense research, there are no FDA-approved drugs targeting these cancers. Recent recognition that KRAS G12C harbors a binding pocket near the mutant cysteine residue has enabled the development of a new class of allele-specific inhibitors that are currently in early-phase trials. However, the mutational landscape and clinical characteristics of KRAS G12C mutant lung cancers are not well understood. The clinical use of plasma circulating tumor DNA (ctDNA) provides an opportunity to characterize this disease.

Methods: Plasma was collected from 636 patients seen at Memorial Sloan Kettering Cancer Center between 11/2016 and 5/2019. ctDNA was extracted and analyzed using the validated 23-gene ctDx-Lung assay (Resolution Bioscience; Kirkland, WA). Categorical comparisons were carried out using Fisher’s exact test.

Results: 95 NSCLC patients were identified as having alterations in KRAS based on ctDNA analysis, of whom 93.7% were metastatic at the time of plasma testing. 81.8% and 80.6% of G12C and non-G12C patients were systemic treatment naive at the time of testing, respectively. 33 patients had KRAS G12C mutations (34.7%), and 62 had other KRAS mutations (65.3%), including mutations in G12D in 24 patients, G12V in 9 patients, Q61H in 5, G12A in 8, G12S in 7, G13C in 3, G13D in 2, G13V and G12F in one patient each, as well as both Q61L and G13C in 2 patients. 60.6% of KRAS G12C patients and 53.2% of KRAS non-G12C were female. 97% of patients with G12C mutations were former smokers as compared to 77.4% in the non-G12C group (p=0.02). Of patients with stage IV adenocarcinoma with plasma KRAS detection, 9 of 25 patients with G12C were alive at the time of data analysis (36%), while 28 of 53 (52.8%) patients with KRAS non-G12C disease were alive (p =0.23; days from metastatic diagnosis to data analysis: G12C patients: 113-608, median: 396.0; non-G12C patients: 79-2248, median: 427.5). For patients with KRAS G12C mutations, co-alterations were found in TP53 (n=12, 36.4%), as well as SNVs in ROS1 (n=2), and in ALK (n=1). In patients with non-G12C KRAS detected on plasma, co-alterations were found in TP53 (n=26, 41.9%), and PIK3CA (n=3), with additional concurrent alterations in RTKs, including one patient with EGFR K714N, one with EGFR L858R, one patient with an FGFR1 amplification, and one each with an FGFR1 and FGFR3 mutation. Alterations were also detected in MET R988C (n=1) and a MET exon 14 splice variation, with SNVs in RET (n=2), as well as in ALK, ROS1, and AKT1, with a MET as well as a RICTOR amplification in one patient each.

Conclusion: Patients with KRAS G12C mutations detected on plasma analysis were more likely to be smokers. Our data support the need for further analysis of co-alterations both in plasma and on tissue sequencing to aid the development of therapies for patients with KRAS mutations as well as to advance research on combinations of G12C inhibitors with other pathway inhibitors.

Citation Format: Yonina R. Murciano-Goroff, Emily S. Lebow, Hai-Yan Tu, Mark Li, Lee P. Lim, Kathryn C. Arbour, William Travis, David B. Solit, Marc Ladanyi, David R. Jones, Charles M. Rudin, Andres Martinez, Mackenzie L. Myers, Alexander Makhnin, Pedram Razavi, Michael D. Offin, James W. Isbell, Gregory J. Riely, David M. Hyman, Piro Lito, Bob T. Li. Characterizing KRAS G12C and other mutations in plasma ctDNA from patients with lung cancer [abstract]. In: Proceedings of the AACR Special Conference on Advancing Precision Medicine Drug Development: Incorporation of Real-World Data and Other Novel Strategies; Jan 9-12, 2020; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_1):Abstract nr 12.

Clinical characteristics and anti-PD-(L)1 treatment outcomes of KRAS-G12C mutant lung cancer compared to other molecular subtypes of KRAS-mutant lung cancer

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Background: KRAS mutations are identified in approximately 30% of NSCLC. There are no FDA approved targeted therapies for patients with KRAS-mutant non-small cell lung cancer (NSCLC) but novel direct inhibitors of KRAS G12C have shown some activity in early phase clinical trials. We hypothesized that patients with KRAS-G12C mutations may have distinct clinical characteristics and responses to systemic therapies compared to patients with non-G12C subtypes. Methods: We identified patients with KRAS-mutant lung cancers who underwent next-generation sequencing with MSK-IMPACT, between January 2014 and December 2018. Baseline characteristics were compared with the Chi-square and Fisher’s exact test for categorical data and Wilcoxon rank-rum test for continuous data. Overall survival was calculated from time of diagnosis of metastatic/recurrent disease to date of death or last follow up, with left truncation to account for time of MSK-IMPACT. Overall survival was compared between groups using the Cox proportional-hazards model. Response evaluations where performed by independent thoracic radiologists according to RECIST 1. and compared between group with the Fisher’s exact test. Results: We identified 1194 patients with KRAS -mutant NSCLC, 772 with recurrent or metastatic disease. Of patients with advanced disease, 46% (352/772) had mutations in KRAS-G12C and 54% harbored non-G12C mutations (15% G12D, 16% G12V, 8% G12A, 4% G13D). Co-mutation patterns were similar with respect to KEAP1 (p=0.9) and STK11 (p=1.0). Patients with non-G12C mutations had a higher proportion of never smokers (10% vs 1.4% p<0.001). The median OS from diagnosis was 13 months for G12C and non-G12C patients (p=0.99). 45% (347/772) received 1L or 2L line treatment with PD-(L)1 inhibitor. RECIST measurements were available for 290/347 cases (84%). ORR with anti-PD-(L)1 treatment was 24% vs 28% in G12C vs non-G12C patients (p=0.5). In patients with PD-L1 50% (n=103), ORR was 39% for G12C vs 58% non-G12C patients (p=0.06). Conclusions: KRAS G12C mutations are present in 12% of patients with NSCLC and represent a relevant subtype of NSCLC given KRAS G12C inhibitors now in clinical development. Baseline characteristics including co-mutation patterns are similar between patients with G12C and non-G12C, except for smoking history. The efficacy of KRAS G12C direct inhibitors will need to be compared to other available therapies for KRAS mutant NSCLC (chemotherapy and PD-(L)1 inhibitors) to identify most effective therapeutic strategy.

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.

Abstract LB-A04: Rapid non-uniform adaptation to conformation-specific KRAS G12C inhibition

KRAS GTPases are activated in one-third of cancers and KRAS G12C is the most common activating alteration in lung adenocarcinoma. KRAS G12C inhibitors are in Phase-I clinical trials and early data show only partial responses in lung cancer patients. How cancer cells bypass inhibition, to prevent maximal responses to therapy, is not understood. Because KRAS G12C cycles between an active and inactive conformation, and the inhibitors only bind to the latter, we hypothesized that isogenic cell populations respond non-uniformly. Here we studied the effect of treatment at the single cell level and showed that shortly after treatment, some cancer cells were sequestered in a quiescent state with low KRAS activity, while others reactivated KRAS to resume proliferation. By combining cell fate-specific gene expressions and results from a CRISPR-Cas9 screen, we identified that this rapid divergent response is due to new KRAS G12C produced in response to suppressed MAPK output. Upstream-acting adaptive signals, such as epidermal growth-factor receptor and aurora kinase signaling, maintain new KRAS G12C protein in its active/drug-insensitive state to restore KRAS output. Cells without these adaptive changes (or cells where they are pharmacologically inhibited) remain sensitive to drug treatment, because new KRAS G12C is either not available, or it exists in its inactive/drug-sensitive state. Combined inhibition of these adaptive signals along with KRAS G12C produced more potent antitumor effects in vivo. 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 to maximize the therapeutic potential of conformation-specific KRAS G12C inhibitors in the clinic.

Citation Format: Jenny Xue, Yulei Zhao, Jordan Aronowitz, Trang T Mai, Alberto Vides, Besnik Qeriqi, Dongsung Kim, Chuanchuan Li, Elisa de Stanchina, Linas Mazutis, Davide Risso, Piro Lito. Rapid non-uniform adaptation to conformation-specific KRAS G12C inhibition [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr LB-A04. doi:10.1158/1535-7163.TARG-19-LB-A04

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

Despite decades of research, efforts to directly target KRAS have been challenging. MRTX849 was identified as a potent, selective, and covalent KRAS^G12C inhibitor that exhibits favorable drug-like properties, selectively modifies mutant cysteine 12 in GDP-bound KRAS^G12C, and inhibits KRAS-dependent signaling. MRTX849 demonstrated pronounced tumor regression in 17 of 26 (65%) KRAS^G12C-positive cell line- and patient-derived xenograft models from multiple tumor types, and objective responses have been observed in patients with KRAS^G12C-positive lung and colon adenocarcinomas. Comprehensive pharmacodynamic and pharmacogenomic profiling in sensitive and partially resistant nonclinical models identified mechanisms implicated in limiting antitumor activity including KRAS nucleotide cycling and pathways that induce feedback reactivation and/or bypass KRAS dependence. These factors included activation of receptor tyrosine kinases (RTK), bypass of KRAS dependence, and genetic dysregulation of cell cycle. Combinations of MRTX849 with agents that target RTKs, mTOR, or cell cycle demonstrated enhanced response and marked tumor regression in several tumor models, including MRTX849-refractory models. SIGNIFICANCE: The discovery of MRTX849 provides a long-awaited opportunity to selectively target KRAS^G12C in patients. The in-depth characterization of MRTX849 activity, elucidation of response and resistance mechanisms, and identification of effective combinations provide new insight toward KRAS dependence and the rational development of this class of agents.See related commentary by Klempner and Hata, p. 20.This article is highlighted in the In This Issue feature, p. 1.

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 BRAF^V600E-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.

Abstract B015: An approach to suppress the evolution of resistance in BRAFV600E-mutant cancer

Tumors evolve as they adapt to environmental cues. The principles governing evolution of tumors under the selective pressure of targeted therapy are not well understood. We aimed to evaluate the evolution of resistance and to identify therapeutic modalities that prevent this process in BRAFV600E-mutant tumors. We modeled the selection and propagation of BRAFV600E amplification (BRAFamp) in patient-derived tumor xenografts (PDX) treated with a direct ERK inhibitor. Single-cell sequencing and multiplex-fluorescence in situ hybridization mapped the emergence of extra-chromosomal amplification in multiple subclones of the same tumor shortly after treatment. The evolutionary selection of BRAFamp is determined by the fitness threshold, the barrier subclonal populations need to overcome to regain fitness in the presence of therapy. This differed for ERK signaling inhibitors, and single-cell sequencing of a melanoma PDX model showed that drugs of the same pathway do not necessarily select for the same subclones. These data suggest that sequential monotherapy is not optimal, but concurrent targeting of RAF, MEK, and ERK, however, imposes a sufficiently high fitness threshold to prevent the propagation of subclones with high-level amplification. Administered on an intermittent schedule, this treatment inhibited tumor growth without apparent toxicity in 11/11-lung cancer and melanoma PDX models with various additional alterations. Thus, gene amplification can be acquired and expanded through parallel evolution, enabling tumors to adapt while maintaining their intratumoral heterogeneity. Treatments that impose a high fitness threshold, such as our intermittent triple therapy, will likely prevent the evolution of resistance-causing alterations and merit testing in patients.

Citation Format: Yaohua Xue, Luciano Martelotto, Timour Baslan, Alberto Vides, Martha Solomon, Kalyani Chadalavada, Elisa DeStanchina, Gouri Nanjangud, Michael Berger, Scott Lowe, Jorge S. Reis-Filho, Neal Rosen, Piro Lito. An approach to suppress the evolution of resistance in BRAFV600E-mutant cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B015.

Effects of Co-occurring Genomic Alterations on Outcomes in Patients with KRAS-Mutant Non-Small Cell Lung Cancer

Purpose:KRAS mutations occur in approximately 25% of patients with non-small cell lung cancer (NSCLC). Despite the uniform presence of KRAS mutations, patients with KRAS-mutant NSCLC can have a heterogeneous clinical course. As the pattern of co-occurring mutations may describe different biological subsets of patients with KRAS-mutant lung adenocarcinoma, we explored the effects of co-occurring mutations on patient outcomes and response to therapy.Experimental Design: We identified patients with advanced KRAS-mutant NSCLC and evaluated the most common co-occurring genomic alterations. Multivariate analyses were performed incorporating the most frequent co-mutations and clinical characteristics to evaluate association with overall survival as well as response to platinum-pemetrexed chemotherapy and immune checkpoint inhibitors.Results: Among 330 patients with advanced KRAS-mutant lung cancers, the most frequent co-mutations were found in TP53 (42%), STK11 (29%), and KEAP1/NFE2L2 (27%). In a multivariate analysis, there was a significantly shorter survival in patients with co-mutations in KEAP1/NFE2L2 [HR, 1.96; 95% confidence interval (CI), 1.33-2.92; P ≤ 0.001]. STK11 (HR, 1.3; P = 0.22) and TP53 (HR 1.11, P = 0.58) co-mutation statuses were not associated with survival. Co-mutation in KEAP1/NFE2L2 was also associated with shorter duration of initial chemotherapy (HR, 1.64; 95% CI, 1.04-2.59; P = 0.03) and shorter overall survival from initiation of immune therapy (HR, 3.54; 95% CI, 1.55-8.11; P = 0.003).Conclusions: Among people with KRAS-mutant advanced NSCLC, TP53, STK11, and KEAP1/NFE2L2 are the most commonly co-occurring somatic genomic alterations. Co-mutation of KRAS and KEAP1/ NFE2L2 is an independent prognostic factor, predicting shorter survival, duration of response to initial platinum-based chemotherapy, and survival from the start of immune therapy. Clin Cancer Res; 24(2); 334-40. ©2017 AACR.

An approach to suppress the evolution of resistance in BRAFV600E-mutant cancer

The principles governing evolution of tumors exposed to targeted therapy are poorly understood. Here we modeled the selection and propagation of BRAF amplification (BRAF^amp) in patient-derived tumor xenografts (PDX) treated with a direct ERK inhibitor, alone or in combination with other pathway inhibitors. Single cell sequencing and multiplex-fluorescence in situ hybridization mapped the emergence of extra-chromosomal amplification in parallel evolutionary tracts, arising in the same tumor shortly after treatment. The evolutionary selection of BRAF^amp is determined by the fitness threshold, the barrier subclonal populations need to overcome to regain fitness in the presence of therapy. This differed for ERK signaling inhibitors, suggesting that sequential monotherapy is ineffective and selects for a progressively higher BRAF copy number. Concurrent targeting of RAF, MEK and ERK, however, imposes a sufficiently high fitness threshold to prevent the propagation of subclones with high-level amplification. Administered on an intermittent schedule, this treatment inhibited tumor growth in 11/11-lung cancer and melanoma PDX without apparent toxicity in mice. Thus, gene amplification can be acquired and expanded through parallel evolution, enabling tumors to adapt while maintaining their intratumoral heterogeneity. Treatments that impose the highest fitness threshold will likely prevent the evolution of resistance-causing alterations and merit testing in patients.

Predicting MEK Inhibitor Response in Lung Cancer: A Proper Signature Is Required

The ERK signaling pathway is one of the most commonly deregulated pathways in cancer. Assays that accurately measure ERK signaling output in clinical specimens would be extremely helpful not only in determining the pharmacodynamic effects of drug treatment but also in selecting those patients most likely to respond to therapy. Clin Cancer Res; 23(6); 1365-7. ©2016 AACRSee related article by Brant et al., p. 1471.

Large Cell Neuroendocrine Carcinoma of the Lung: Clinico-Pathologic Features, Treatment, and Outcomes

BACKGROUND

Large cell neuroendocrine carcinoma (LCNEC) accounts for approximately 3% of lung cancers. Pathologic classification and optimal therapies are debated. We report the clinicopathologic features, treatment and survival of a series of patients with stage IV LCNEC.

MATERIALS AND METHODS

Cases of pathologically-confirmed stage IV LCNEC evaluated at Memorial Sloan Kettering Cancer Center from 2006 to 2013 were identified. We collected demographic, treatment, and survival data. Available radiology was evaluated by Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 criteria.

RESULTS

Forty-nine patients with stage IV LCNEC were identified. The median age was 64 years, 63% of patients were male, and 88% were smokers. Twenty-three patients (n = 23/49; 47%) had brain metastases, 17 at diagnosis and 6 during the disease course. Seventeen LCNEC patients (35%) had molecular testing, of which 24% had KRAS mutations (n = 4/17). Treatment data for first-line metastatic disease was available on 37 patients: 70% (n = 26) received platinum/etoposide and 30% (n = 11) received other regimens. RECIST was completed on 23 patients with available imaging; objective response rate was 37% (95% confidence interval, 16%-62%) with platinum/etoposide, while those treated with other first-line regimens did not achieve a response. Median overall survival was 10.2 months (95% confidence interval, 8.6-16.4 months) for the entire cohort.

CONCLUSION

Patients with stage IV LCNEC have a high incidence of brain metastases. KRAS mutations are common. Patients with stage IV LCNEC do not respond as well to platinum/etoposide compared with historic data for extensive stage small-cell lung cancer; however, the prognosis is similar. Prospective studies are needed to define optimum therapy for stage IV LCNEC.

Abstract LB-071: Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism

Oncogenic mutations impair the GTPase activity of RAS, causing it to accumulate in the activated, GTP-bound conformation. Here we describe the mechanism of action of a novel compound that binds to KRASG12C to selectively inhibit proliferation and induce apoptosis in cancer cells with this mutation. The inhibitor interacted preferentially with the GDP-bound form of the protein, yet it reduced the level of GTP-bound KRASG12C. This suggested that GTPase activity is required for inhibition, which turned out to be the case. The inhibitor traps GDP-bound KRASG12C and prevents nucleotide exchange to the GTP-bound form. Genetic experiments revealed that mutants completely lacking GTPase activity, and those promoting exchange, reduced the potency of the drug. In contrast, mutants with diminished exchange were more susceptible to inhibition. In agreement with these findings, targeting receptor signaling potentiated KRASG12C inhibition, whereas its activation had the opposite effect. This mechanism reveals that KRASG12C undergoes nucleotide cycle in cancer cells and provides a basis for the development of effective combination therapies to treat KRASG12C-driven cancers.

Citation Format: Piro Lito, Martha Solomon, Rasmus Hansen, Lian-Sheng Li, Neal Rosen. Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-071.

Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism

It is thought that KRAS oncoproteins are constitutively active because their guanosine triphosphatase (GTPase) activity is disabled. Consequently, drugs targeting the inactive or guanosine 5'-diphosphate-bound conformation are not expected to be effective. We describe a mechanism that enables such drugs to inhibit KRAS(G12C) signaling and cancer cell growth. Inhibition requires intact GTPase activity and occurs because drug-bound KRAS(G12C) is insusceptible to nucleotide exchange factors and thus trapped in its inactive state. Indeed, mutants completely lacking GTPase activity and those promoting exchange reduced the potency of the drug. Suppressing nucleotide exchange activity downstream of various tyrosine kinases enhanced KRAS(G12C) inhibition, whereas its potentiation had the opposite effect. These findings reveal that KRAS(G12C) undergoes nucleotide cycling in cancer cells and provide a basis for developing effective therapies to treat KRAS(G12C)-driven cancers.

Hematopoietic stem cell origin of BRAFV600E mutations in hairy cell leukemia

Hairy cell leukemia (HCL) is a chronic lymphoproliferative disorder characterized by somatic BRAFV600E mutations. The malignant cell in HCL has immunophenotypic features of a mature B cell, but no normal counterpart along the continuum of developing B lymphocytes has been delineated as the cell of origin. We find that the BRAFV600E mutation is present in hematopoietic stem cells (HSCs) in HCL patients, and that these patients exhibit marked alterations in hematopoietic stem/progenitor cell (HSPC) frequencies. Quantitative sequencing analysis revealed a mean BRAFV600E-mutant allele frequency of 4.97% in HSCs from HCL patients. Moreover, transplantation of BRAFV600E-mutant HSCs from an HCL patient into immunodeficient mice resulted in stable engraftment of BRAFV600E-mutant human hematopoietic cells, revealing the functional self-renewal capacity of HCL HSCs. Consistent with the human genetic data, expression of BRafV600E in murine HSPCs resulted in a lethal hematopoietic disorder characterized by splenomegaly, anemia, thrombocytopenia, increased circulating soluble CD25, and increased clonogenic capacity of B lineage cells-all classic features of human HCL. In contrast, restricting expression of BRafV600E to the mature B cell compartment did not result in disease. Treatment of HCL patients with vemurafenib, an inhibitor of mutated BRAF, resulted in normalization of HSPC frequencies and increased myeloid and erythroid output from HSPCs. These findings link the pathogenesis of HCL to somatic mutations that arise in HSPCs and further suggest that chronic lymphoid malignancies may be initiated by aberrant HSCs.

Abstract 4758: Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant tumors

MEK inhibitors are clinically active in BRAF V600E melanomas, but only marginally so in KRAS-mutant tumors. While MEK inhibitor treatment resulted in sustained inhibition of ERK phopshorylation in BRAF V600E tumors, it was associated with a rebound in ERK phosphorylation after prolonged exposure in KRAS mutant tumors. To understand this phenomenon, we performed an RNAi screen in a KRAS-mutant model and found that CRAF knockdown enhanced the antiproliferative effects of MEK inhibition both in vivo and in vitro. In agreement with previous findings, MEK inhibitor treatment in KRAS mutant cells resulted in CRAF reactivation and induction of MEK phosphorylation. However, MEK activated by CRAF was less susceptible to traditional MEK inhibitors, such as PD0325901, compared to when activated by BRAF V600E. Consistent with this observation, the rebound in ERK phosphorylation following MEK inhibitor treatment in KRAS mutant tumors was dependent on intact CRAF expression. Furthermore, traditional MEK inhibitors induced RAF-MEK complexes in KRAS mutant models, and disrupting these complexes enhanced the inhibition of CRAF-dependent ERK signaling. In an effort to identify more effective compounds we found that newer MEK inhibitors, such as trametinib and CH5126766, do not only target the catalytic activity of MEK, but in addition, they also impair its reactivation by CRAF. This occurs through two distinct mechanisms: trametinib disrupts RAF-MEK complex formation, as evidenced by co-immunoprecipitation of endogenous proteins and surface plasmon resonance assays with purified enzymes. Instead, CH512676 induces the interaction of MEK with RAF, yet it prevents the phosphorylation of MEK in this complex. To elucidate the mechanism responsible for the latter property, we determined the ternary structure of CH512676-bound MEK1. CH5126766 interacts with Asn 221 and Ser 222 residues in MEK1 to displace the activation segment and disrupt RAF-mediated phosphorylation. These data provide a blueprint for developing better MEK inhibitors that effectively inhibit ERK signaling in KRAS mutant tumors.

Citation Format: Piro Lito, Anna Saborowski, Jingyin Yue, Martha Solomon, Eric Joseph, Christof Fellman, Kazuhiro Ohara, Kenji Morikami, Takaaki Miura, Christine Lucas, Nobuya Ishii, Scott Lowe, Neal Rosen. Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4758. doi:10.1158/1538-7445.AM2014-4758

Abstract 3140: Context specific effects of the BRAFV600E mutation on hematopoiesis identifies novel models of BRAF mutant hematopoietic disorders

BRAFV600E mutations have recently been identified in nearly 100% of patients with the chronic lymphoproliferative disorder hairy cell leukemia (HCL), as well as a small percentage of patients with the plasma cell malignancy multiple myeloma. Despite extensive knowledge regarding the functional effects of BRAFV600E expression in epithelial tissues, very little is understood about the role of the BRAFV600E mutation in hematopoietic transformation. We therefore utilized a conditional BRafV600E murine model crossed with Mx1-cre, Vav-cre, Cd19-cre, and Cγ1-cre transgenic mice to delineate the effects of mutant BRaf expression in pre-natal and post-natal hematopoietic stem and progenitor cells (HSPCs), B-lineage cells, and germinal center B cells respectively. We also investigated the origin of the BRAFV600E mutation in HCL patient bone marrow samples using prospective isolation of sorted HSPC populations followed by quantitative sequencing for the BRAFV600E mutation.

Surprisingly, we identified the presence of the BRAFV600E mutation in long-term hematopoietic stem cells of HCL patients, and we also observed marked alterations in HSPC frequencies. Consistent with the human genetic data, expression of BRafV600E in HSPCs of mice resulted in a lethal transplantable hematopoietic disorder characterized by splenomegaly, anemia, thrombocytopenia, increased circulating soluble CD25, and increased clonogenic capacity of B-lineage cells- all classic features of human HCL. In contrast, restricting expression of BRafV600E to B-lineage cells did not result in disease even up to 1.5 years of age.

We next assessed the effects of the BRafV600E mutation on HSPC self-renewal and lineage specification. We plated whole BM cells from Mx1-cre BRafV600E mice in methylcellulose containing myeloid/erythroid cytokines or lymphopoietic cytokes (IL-7). BRafV600E cells demonstrated impaired colony formation in myeloid/erythroid conditions. However, BRafV600E HSPCs exhibited limitless replating capacity when plated in the presence of IL-7, indicating that the BRAF mutation induces aberrant B lineage cell self-renewal. A clear competitive advantage was also seen with competitive transplantation of BRafV600E BM cells, identifying an increase in HSPC self-renewal associated with the BRAF mutation.

Data from the murine models studied here and characterization of the BM compartment in HCL patients suggest that the cytopenias seen in HCL patients are due in part to HSPC-intrinsic effects of the BRAFV600E mutation on erythropoiesis, megakarypoiesis, and myelopoiesis. Moreover, these data suggest that the use of therapies targeting MAP-kinase signaling in HCL may lead to durable remissions not only through effects on mature leukemic cells, but also through targeted inhibition of signaling and survival in mutant HSPCs.

Citation Format: Eunhee Kim, Stephen S. Chung, Jae H. Park, Young Rock Chung, Piro Lito, Julie Feldstein, Wenhuo Hu, Wendy Beguilin, Sebastien Monette, Cihangir Duy, Raajit Rampal, Leon Telis, Minal Patel, Min Kyung Kim, Ari M. Melnick, Neal Rosen, Martin S. Tallman, Christopher Y. Park, Omar Abdel-Wahab. Context specific effects of the BRAFV600E mutation on hematopoiesis identifies novel models of BRAF mutant hematopoietic disorders. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3140. doi:10.1158/1538-7445.AM2014-3140

Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant tumors

MEK inhibitors are clinically active in BRAF(V600E) melanomas but only marginally so in KRAS mutant tumors. Here, we found that MEK inhibitors suppress ERK signaling more potently in BRAF(V600E), than in KRAS mutant tumors. To understand this, we performed an RNAi screen in a KRAS mutant model and found that CRAF knockdown enhanced MEK inhibition. MEK activated by CRAF was less susceptible to MEK inhibitors than when activated by BRAF(V600E). MEK inhibitors induced RAF-MEK complexes in KRAS mutant models, and disrupting such complexes enhanced inhibition of CRAF-dependent ERK signaling. Newer MEK inhibitors target MEK catalytic activity and also impair its reactivation by CRAF, either by disrupting RAF-MEK complexes or by interacting with Ser 222 to prevent MEK phosphorylation by RAF.

Abstract 4630: Feedback dependent suppression of mitogenic signaling and its effect on RAF inhibition in BRAFV600E melanomas

RAF inhibitors have remarkable activity in melanomas harboring BRAFV600E mutations. We previously found that active Ras induces RAF dimers in tumors with wild-type RAF, and, that RAF inhibitors transactivate these dimers. Because BRAFV600E melanomas have enhanced ERK signaling, they are hypothesized to have feedback-mediated suppression of Ras in order to maintain monomeric and inhibitor-sensitive BRAFV600E. To test this hypothesis we evaluated the response of these tumors to RAF inhibitors over time. We found that in the treatment-naive state, BRAFV600E melanomas had suppressed RTK signaling and maintained a low level of Ras-GTP. This occurred through a multifactorial negative feedback process driven by active ERK. In this baseline state, BRAFV600E was sensitive to RAF inhibitors. After treatment with RAF inhibitors, however, inhibition of ERK signaling lead to loss of feedback, a process that restored signaling from RTKs and resulted in Ras activation. In turn, this diminished the effect of RAF inhibitors and lead to a partial, but sustained, reactivation of ERK signaling. In this post-treatment state, ERK signaling was insensitive to RAF inhibitors but sensitive to MEK inhibitors. Targeting the regulatory components of this new steady state, either through co-administration of a MEK inhibitor or a RTK inhibitor along with a RAF inhibitor resulted in improved antitumor activity. These results highlight the need to understand the adaptive response to ERK pathway inhibition in order to design better combinatorial therapies and achieve maximal antitumor effects.

Citation Format: Piro Lito, Christine Pratilas, Eric Joseph, Madhavi Tadi, Ensar Halilovic, Matthew Zubrowski, Alan Huang, Elisa de Stanchina, Sarat Chandarlapaty, Poulikos Poulikakos, Neal Rosen. Feedback dependent suppression of mitogenic signaling and its effect on RAF inhibition in BRAFV600E melanomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4630. doi:10.1158/1538-7445.AM2013-4630

Relief of profound feedback inhibition of mitogenic signaling by RAF inhibitors attenuates their activity in BRAFV600E melanomas

BRAF(V600E) drives tumors by dysregulating ERK signaling. In these tumors, we show that high levels of ERK-dependent negative feedback potently suppress ligand-dependent mitogenic signaling and Ras function. BRAF(V600E) activation is Ras independent and it signals as a RAF-inhibitor-sensitive monomer. RAF inhibitors potently inhibit RAF monomers and ERK signaling, causing relief of ERK-dependent feedback, reactivation of ligand-dependent signal transduction, increased Ras-GTP, and generation of RAF-inhibitor-resistant RAF dimers. This results in a rebound in ERK activity and culminates in a new steady state, wherein ERK signaling is elevated compared to its initial nadir after RAF inhibition. In this state, ERK signaling is RAF inhibitor resistant, and MEK inhibitor sensitive, and combined inhibition results in enhancement of ERK pathway inhibition and antitumor activity.

Abstract 1223: Relief of feedback inhibition of RTK and RAS signaling in V600E BRAF melanomas exposed to RAF inhibitors buffers their effects on signaling

Activation of ERK signaling causes feedback inhibition of receptor tyrosine kinase and RAS signaling by multiple mechanisms and limits the steady-state output of the pathway. The V600E BRAF mutant does not depend on RAS-dependent dimerization and is not susceptible to feedback inhibition of RAS. In V600E BRAF melanomas, ERK pathway output and the expression of ERK genes such as DUSP6 and members of the SPROUTY family are markedly elevated. In these tumors, RTK signaling and RAS activation are actively suppressed. In the context of elevated DUSP6, RAF inhibitors rapidly inhibit ERK phosphorylation and output. This relieves the feedback inhibition of various RTKs and induces RAS activity and the formation of activated wild type RAF dimers that are insensitive to RAF-inhibitors. This results in a rebound in ERK phosphorylation and output that is sensitive to MEK, but not RAF inhibitors and suggests that the latter cannot maximally inhibit ERK signaling in these tumors. In support of this idea, combined treatment with MEK and RAF inhibitors reduces ERK output and the growth of V600E BRAF melanoma xenografts to a greater degree than either alone.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1223. doi:1538-7445.AM2012-1223

Sprouty 2 regulates DNA damage-induced apoptosis in Ras-transformed human fibroblasts

We have reported that expression of Sprouty 2 (Spry2) is necessary for tumor formation by HRas(V12)-transformed fibroblasts. We now report on the role of Spry2 in the inhibition of UV(254 nm) radiation-induced apoptosis in HRas(V12)-transformed human fibroblasts. Silencing Spry2 in this context resulted in increased apoptosis, associated with decreased Akt activation and decreased phosphorylation of HDM2 at Ser-166, which has been shown to stabilize HDM2. As a consequence, when cells with silenced Spry2 were UV-irradiated, they exhibited diminished levels of HDM2 and elevated levels of p53. In agreement with these findings, overexpression of Spry2 in the parental non-transformed fibroblasts led to increased Akt activation and to the stabilization of HDM2. It also led to diminished expression of p53 and decreased apoptosis following UV irradiation. Silencing Spry2 in HRas-transformed cells decreased Rac1 activation, but independent expression of Spry2 in the non-transformed parental cells had no effect on Rac1, suggesting a specific involvement in the activation of Rac1 by Ras. Silencing Spry2 in HRas(V12)-transformed cells resulted in diminished interaction between HRas and Tiam1, a Rac1-specific nucleotide exchange factor. Expression of constitutively active Rac1 in cells with silenced Spry2 partly reversed the effect of Spry2 down-regulation. Furthermore, loss of Spry2 expression in HRas(V12)-transformed cells augmented the cytotoxicity of the DNA-damaging, chemotherapeutic agent cisplatin, a process that was also reversed by active Rac1. Together, these data show that Spry2 inhibits apoptosis in response to DNA damage by regulating Akt, HDM2, and p53, by a process mediated partly by Rac1.