Role of KRAS in NSCLC: present and future

Abstract

Lung cancer is the leading cause of cancer-related deaths worldwide. Despite therapeutic advances over the last several decades, the overall 5-year survival remains only 16%. (Siegel R, 2013). Molecular studies have indicated that Adenocarcinomas have distinct genomic alterations allowing classification into clinically relevant molecular subsets. Such specific molecular level alterations are sometimes important for initiation and maintenance of the tumour serving as “drivers” in lung cancer tumorigenesis. Not only is mutant KRAS difficult to restrain, its presence in some types of cancers predicts that there will be no response to other targeted drugs. For example, patients with colorectal cancer may benefit from additional treatment with an antibody drug that targets the EGFR protein (cetuximab or panitumumab). However, this benefit is seen only in patients who have a “wild-type” (not mutated) KRAS gene. The reason for this is that EGFR and other related receptor proteins rely on KRAS to transmit proliferation signals.

Short Commentary

Lung cancer is the leading cause of cancer-related deaths worldwide. Despite therapeutic advances over the last several decades, the overall 5-year survival remains only 16%. (Siegel R, 2013). Molecular studies have indicated that Adenocarcinomas have distinct genomic alterations allowing classification into clinically relevant molecular subsets. Such specific molecular level alterations are sometimes important for initiation and maintenance of the tumour serving as “drivers” in lung cancer tumorigenesis. Not only is mutant KRAS difficult to restrain, its presence in some types of cancers predicts that there will be no response to other targeted drugs. For example, patients with colorectal cancer may benefit from additional treatment with an antibody drug that targets the EGFR protein (cetuximab or panitumumab). However, this benefit is seen only in patients who have a “wild-type” (not mutated) KRAS gene. The reason for this is that EGFR and other related receptor proteins rely on KRAS to transmit proliferation signals.

RAS is a member of the ras gene family which encodes small G proteins with intrinsic GTPase activity that leads to protein inactivation and activates downstream effectors involved in multiple pathways including proliferation, differentiation, and apoptosis. Mutations in the KRAS gene occur frequently in non-small cell lung cancer (NSCLC), especially in Adenocarcinoma (~30%) though less common in squamous cell carcinoma (about 7%) (Pao W, 2011, Weir BA, 2007). Although mutationally activated KRAS tumours were originally identified in 1982 (Cox AD, 2010) to date there are no successful treatment strategies that target these mutations. However, key pathways activated by KRAS and mutation-associated vulnerabilities may be therapeutically targetable including the MEK, phosphoinositide 3-kinase (PI3K), GSK-3α and RAL/TBK1 pathways (Mitin N 2005, Sos ML 2009) Mutations in tumor suppressor genes TP53 and STK11 are also common in lung Adenocarcinoma and often co-occur with KRAS mutations. (Gibbons DL, 2014). While much is known about tumor promotion mechanisms of TP53, less is known about STK11 function and impact on disease progression and patient survival. The STK11 gene encodes a serine/threonine protein kinase known as liver kinase β1 (LKB1). (Shackelford DB, 2009), The most common STK11 mutations are deletion or inactivating mutations (Sanchez-Cespedes M, et al. 2002), which, along with murine studies provide strong evidence for a tumor suppressor function for this gene (Ji H, 2007).

Recent studies have defined gene expression changes triggered by RAS. For example, a RAS (Bild AH, et al. 2006). signature associated with MEK pathway activation is also associated with sensitivity to MEK inhibitors (MEKi). (Loboda A, 2010) Gene expression studies have also defined signatures associated with enhanced tumor progression and reduced patient survival. Examples of such signatures include the malignancy risk signature reported by our group, which is rich in cell cycle regulating genes and therefore associated with highly proliferative tumours. (Chen DT, et al. 2011) It is now well established that a functional immune system is crucial in controlling tumor growth. (Dunn GP, 2006) Consequently, T cell presence in tumours is associated with immune surveillance and improved patient survival (Schreiber RD, 2011). It is important to note that benefit from immunotherapy, including T cell checkpoint blockade, is also commonly associated with high tumor expression of immuno-stimulatory genes and T cell infiltration. (Ji RR, et al. 2012). Thus, activation of key immune regulatory pathways such as JAK-STAT and NF-κB pathways (Gajewski TF, 2013) in tumor cells or tumor infiltrating non-malignant cells likely enhances the response to immunotherapy. We recently identified a gene expression signature of NF-κB regulated genes that is associated with an immune-active tumor microenvironment. (Hopewell EL, et al. 2013) The role and potential association between common lung cancer mutations and the immune surveillance response has however not been investigated. The goal of studies described here was to better define molecular heterogeneity in KRAS mutant tumours, especially as it relates to effects of co-occurring mutations in STK11 and TP53 tumor suppressors in shaping KRAS mutant tumor biology, proliferative and immune surveillance responses in tumours.

Unlike mutations in proteins known as receptor tyrosine kinases, like EGFR or ALK, mutated KRAS is a very difficult protein to target with cancer drugs. (So much so that the National Institutes of Health (NIH) has undertaken a special effort to intensify the effort towards successful targeting of mutant KRAS, known as the RAS Initiative.) There have been numerous attempts to inhibit the proteins that are activated by mutant KRAS (i.e., those that are “downstream” of KRAS), most notably MEK1 and MEK2. These two proteins are part of an important pathway named MAPK, which is governed by KRAS. MEK1/2 are activated by KRAS and by another protein, BRAF, which is frequently mutated in melanoma. The U.S. Food and Drug Administration (FDA) has already approved two inhibitors of MEK proteins, trametinib and cobimetinib, as part of treatment for melanoma with mutated BRAF. MEK inhibitors, however, have shown little to no activity in clinical trials for cancers with mutant KRAS. Just recently, a large trial that compared the MEK inhibitor selumetinib plus the chemotherapy drug docetaxel versus docetaxel in NSCLC produced disappointing results. Nevertheless, there are still trials ongoing with the MEK inhibitors trametinib and binimetinb (also known as MEK162) combined with other drugs (chemotherapy or targeted drugs) in KRAS-mutant cancers. Some success was reported from a small trial that combined an investigational MEK inhibitor known as PD-0325901 with palbociclib. The latter is an inhibitor of CDK4/6, proteins that drive cell proliferation, and are ultimately activated by KRAS. Palbociclib is already FDA-approved for breast cancer in combination with hormonal therapy (KRAS is rarely mutated in breast cancer). In this trial, NCT02022982, patients with KRAS-mutant cancers (lung, ovarian, or thyroid) experienced clinical benefit that lasted for over 6 months in 7 out of 25 patients.

Another possibility is that KRAS-mutant cancers, in particular NSCLC, may have a better response to immunotherapy-treatment that helps the immune system fight cancer. KRAS mutations are found much more frequently in NSCLC that develops in smokers. These cancers are associated with a high total burden of mutations, and data indicate that this is a good predictive marker of response to immunotherapy drugs known as immune checkpoint drugs (nivolumab, pembrolizumab, etc). Given these results, trials that combine inhibitors of proteins downstream from KRAS with immunotherapy are of interest. For instance, one ongoing study (NCT02779751) combines a newer CDK4/6 inhibitor, abemaciclib, with the immune checkpoint inhibitor pembrolizumab. More trials like this are currently being designed.

NSCLC remains a very heterogeneous disease and KRAS represents one of the most commonly mutated genes in this disease, hence KRAS mutation might represent an intriguing and elusive clinical targets for ongoing drug development and KRAS mutational status should be routinely incorporated into clinical trial design for both RAS and non-RAS targeted approaches Giuve the rapid expansion of the New generation sequencing technique, KRAS mutational status may eventually become an important biomarker in the treatment of patients with advanced NSCLC even outside the context of clinical trials.