The Improbable Targeted Therapy: KRAS as an Emerging Target in Non-Small Cell Lung Cancer

KRAS is a frequent oncogenic driver in solid tumors, including non-small cell lung cancer (NSCLC). It was previously thought to be an “undruggable” target due to the lack of deep binding pockets for specific small-molecule inhibitors. A better understanding of the mechanisms that drive KRAS transformation, improved KRAS-targeted drugs, and immunological approaches that aim at yielding immune responses against KRAS neoantigens have sparked a race for approved therapies. Few treatments are available for KRAS mutant NSCLC patients, and several approaches are being tested in clinicals trials to fill this void. Here, we review promising therapeutics tested for KRAS mutant NSCLC.


Introduction

Mutations in Kirsten rat sarcoma viral oncogene homolog (KRAS) are one of the most common genomic alterations identified in solid tumors, especially lung cancer, colorectal cancer, and pancreatic ductal adenocarcinoma (Figure 1). These cancers rely on continued activation and signaling of KRAS, therefore making it an ideal therapeutic target. Unfortunately, therapeutic approaches specifically targeting KRAS, such as attempts to inhibit KRAS membrane localization with farnesyltransferase inhibitors, failed in clinical trials and have thus far not been successful. In non-small lung cancer (NSCLC), the current treatment for KRAS mutant patients relies on chemotherapy, with an average overall survival (OS) of 22 months, which is less than desired.1 Since previous efforts failed to specifically target KRAS, researchers began to indirectly target KRAS through its role in various signaling pathways. Unfortunately, targeting KRAS dependencies in NSCLC has highlighted the difficulty in the effective inhibition of indirect targeting of KRAS.2,3 In addition, downstream pathway blockade of rapidly accelerated fibrosarcoma kinase/mitogen-activated protein kinase/phosphatidylinositol 3-kinase (RAF/MEK/PI3K) with inhibitors has not yet been proven to be effective as shown by the negative results of the Selumetinib Evaluation as Combination Therapy-1 (SELECT-1) trial of selumetinib combined with docetaxel.4, 5, 6 Small-molecule inhibitors have been challenging to develop since mutant KRAS has a very high affinity for guanosine triphosphate (GTP) and the catalytic sites are small and tough to target. Analyses of the KRAS subpopulation in landmark clinical trials in NSCLC have provided information on response (overall response rate of 17%–18%) and survival (median OS of 3 months) of this patient cohort for approved therapeutics,7, 8, 9 but there is still a lack of effective treatments for mutant KRAS. Consequently, the search for therapies that successfully target frequent KRAS mutations has continued due to its large incidence in various cancers.


In < 5 years, the availability of novel therapeutics has transformed the landscape of KRAS treatments, and a target that was once considered “undruggable” now has several therapeutic inhibitors undergoing clinical trials.10, 11, 12 There are essentially four novel approaches to target KRAS that can be divided into targeted therapies and immunotherapies, potentially in combination with one another:1 a novel class of direct KRAS inhibitors that specifically inhibit the G12C mutations through direct interaction with the cysteine residue, as well as a somewhat broader approach using a pan-KRAS inhibitor that does not attempt to inhibit specific mutations;2 novel signaling inhibitors that can inhibit chronic activation of KRAS-dependent pathways;3 immune checkpoint inhibitors (ICIs) that can be used in combination with other approaches;4 and approaches that take advantage of the neoantigen feature of mutated KRAS protein and attempt to improve the T cell response. Therefore, we present in this review recent advances in therapeutic options from a clinical standpoint, to understand the mechanisms of these inhibitors and potential future considerations regarding KRAS and therapeutic efficacy in NSCLC.


Mutated KRAS is a prominent oncogene in lung cancer

The intracellular guanine nucleotide-binding protein (G protein) KRAS is a member of the small GTPases RAS gene family that also includes NRAS and HRAS. KRAS encodes for six exons, resulting in two major splice variants, KRAS4A and KRAS4B, which differ in the C-terminal sequence. This region is required for membrane localization and contains a dual membrane-targeting motif in KRAS4A, in contrast to KRAS4B, which only contains one farnelysation site (Figure 2A). Since KRAS4B is the dominant form and KRAS4A is only marginally expressed in normal cells, it was long thought that the splicing of KRAS was of little consequence. In tumors, the overall expression of KRAS4A can be greatly elevated.13 This is of interest since KRAS4A has been reported to be required for the formation of chemically induced lung cancer in mice.14 Also, active KRAS4A functions as a regulator of hexokinase 1, which depends on its palmitoylation and colocalization with the metabolic enzyme on the outer mitochondrial membrane, thus effectively modulating glucose metabolism.15 Most mutations in KRAS affect codons 12, 13, 61, and 146; however, codon 146 is rarely altered in some cancers, such as NSCLC. Oncogenic KRAS missense mutations result in the high-affinity binding of GTP and loss of GTPase activity, thereby leaving KRAS in an “on” state and deregulating various signaling pathways that rely on active RAS (Figure 2B).16,17 More recent work suggests that another potential mechanism of oncogenic KRAS activation (A146T mutant and possibly also V14I and G12D) involves the acceleration of intrinsic and guanine nucleotide exchange factor-induced nucleotide exchange.18 The most frequent KRAS mutations in NSCLC are G12C, with almost half of all cases, followed by G12V and G12D (Figure 2C).19,20 Thus, lung cancer cells express mutations in both KRAS4A and KRAS4B.


Lung cancer is the number one cause of cancer-related death worldwide and accounts for ~228,820 new cases and 140,730 deaths in 2020.21 NSCLC is the most frequent lung cancer subtype, and patients who are diagnosed with metastatic disease have reported 5-year OS rates of only 6%.22 With ~25% to 30% of cases, KRAS mutations are represented in a large portion of NSCLC patients,23 but effective therapies against KRAS have yet to be approved by the US Food and Drug Administration (FDA). The oncogenic mutations that drive NSCLC include not only KRAS but also epidermal growth factor receptor (EGFR) (17% in NSCLC), anaplastic lymphoma kinase (ALK) (7% in NSCLC), mesenchymal epithelial transition kinase (MET) (3% in NSCLC), and others, which are generally mutually exclusive.24, 25, 26, 27 It should be noted that there are ethnic differences in the frequency of KRAS mutations in NSCLC, with a higher incidence rate of KRAS mutations in White NSCLC patients as compared to Black or east Asian patients.28, 29, 30, 31 However, these differences are not known to affect the potential efficacy of novel KRAS therapeutics. Numerous targeted therapies against these actionable mutations have been developed and tyrosine kinase inhibitors (TKIs) targeting EGFR, ALK, or MET are already standard of care. In fact, a poor prognosis and a shorter median survival are reported for patients with KRAS mutant NSCLC versus patients with actionable mutations coupled with approved personalized therapies (2.41 versus 3.5 years, p < 0.001).32 These data demonstrate the impact of targeted therapies on response and OS in NSCLC cases caused by a specific driver mutation. Fortunately, several KRAS-targeted therapies are under investigation in clinical trials. Interestingly, many of the oncogenes in NSCLC signal through KRAS, and some of the approaches developed against active KRAS may also be applicable in these patients.

KRAS mutations and clinical characteristics

In past studies, KRAS mutations have been reported as a prognostic indicator of poor OS in NSCLC,33, 34, 35 while other studies have demonstrated no difference in response to chemotherapy/targeted therapy or OS.36, 37, 38 Although its true prognostication is somewhat unclear due to the complexity of KRAS-driven tumors, several analyses have noted a predictive value of KRAS, indicating poor prognosis.39,40 For example, a retrospective analysis of 482 lung adenocarcinoma cases at a single institution characterized the incidence of KRAS and its association with age, gender, race, and smoking history.41 The study determined that NSCLC patients who are positive for KRAS mutations are typically White and have a history