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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. [ABSTRACT FROM AUTHOR]

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 KRASG12C signaling and cancer cell growth. Inhibition requires intact GTPase activity and occurs because drug-bound KRASG12C 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 KRASG12C inhibition, whereas its potentiation had the opposite effect. These findings reveal that KRASG12C undergoes nucleotide cycling in cancer cells and provide a basis for developing effective therapies to treat KRASG12C-driven cancers. [ABSTRACT FROM AUTHOR]

Summary: MEK inhibitors are clinically active in BRAFV600E melanomas but only marginally so in KRAS mutant tumors. Here, we found that MEK inhibitors suppress ERK signaling more potently in BRAFV600E, 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 BRAFV600E. 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 FROM AUTHOR]

RAF kinase inhibitors have substantial therapeutic effects in patients with BRAF-mutant melanoma. However, only rarely do tumors regress completely, and the therapeutic effects are often temporary. Several mechanisms of resistance to RAF inhibitors have been proposed. The majority of these cause ERK signaling to become insensitive to treatment with RAF inhibitors by increasing the amount of RAF dimers in cells, whereas others bypass the dependence of the tumor on mutant RAF. One motivation for studying mechanisms of drug resistance is that such efforts may suggest new therapeutic targets or rational combination strategies that delay or prevent the emergence of drug-resistant clones. Here, we review the current model of RAF inhibitor resistance with a focus on the implications of this model on ongoing laboratory and clinical efforts to develop more effective therapeutic strategies for patients with BRAF-mutant tumors. [ABSTRACT FROM AUTHOR]

Summary: BRAFV600E 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. BRAFV600E 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 FROM AUTHOR]

We have reported that expression of Sprouty 2 (Spry2) is necessary for tumor formation by HRas[supV12]-transformed fibroblasts.We now report on the role of Spry2 in the inhibition of UV[sub254 nm] radiation-induced apoptosis in HRas[supV12]-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[supV12]-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[supV12]-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. [ABSTRACT FROM AUTHOR]

Sprouty-2 (Spry2) acts as an inhibitor of receptor tyrosine kinase signaling. Spry2 also prevents the c-Cbl-induced degradation of epidermal growth factor receptor (EGFR). We compared human fibroblasts malignantly transformed by overexpression of HRasV12 oncogene to their non-transformed parental cells and found that the malignant cells express a high level of Spry2. These cells also exhibited an increase in the level of EGFR compared to their precursor cells. We found that intact EGFR was required if HRas-transformed cells were to grow in the absence of exogenous growth factors or form large colonies in agarose. When we decreased expression of Spry2, using a Spry2-specific shRNA, the HRasV12-transformed fibroblasts could no longer form large colonies in agarose, grow in reduced levels of serum, or form tumors in athymic mice. The level of active HRas in these cells remained unaltered. A similar, but less pronounced, effect in tumor formation was observed when Spry2 was down-regulated in human patient-derived fibrosarcoma cell lines. In HRas-transformed cells Spry2 sustained the level and the downstream signaling activity of EGFR. In the parental, non-HRas-transformed, fibroblasts expression of Spry2 resulted in the inhibition of HRas and ERK activation, suggesting that the positive effect of Spry2 in tumor formation is specific to HRas-transformation. Co-immunoprecipitation studies with HRas-transformed cells revealed that Spry2 and HRas interact, and that HRas interacts with Spry2-binding partners, c-Cbl and CIN85, in a Spry2-dependent manner. These data show that Spry2 plays a critical role in the ability of HRas-transformed cells to form sarcomas in athymic mice. [ABSTRACT FROM AUTHOR]

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. Kim et al. review recent advances in the effort to therapeutically target KRAS(G12C), one of the most common oncoproteins in lung cancer and one previously thought to be undruggable. Mechanistic insights with implications for the emergence of resistance and optimal combination therapy are discussed. [ABSTRACT FROM AUTHOR]

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. Real-world analysis by @rohit_thum @RielyMD @KCArbourMD identifies KEAP1 mutations associated with resistance and recent anti–PD-(L)1 therapy exposure associated with toxicity among patients receiving sotorasib for advanced KRAS G12C+ NSCLC. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]