By Jackie Man
With non-small-cell lung cancers (NSCLCs) accounting for more than 85% of all cases of lung cancer, lung cancer is among one of the most common and lethal cancers worldwide (Martin, Leighl, Tsao and Shepherd, 2013). The most famous gain-of-function alternation in NSCLC is the carcinogenic Kirsten Rat Sarcoma Viral Oncogene Homolog (KRAS), which accounts for ~30% of lung adenocarcinomas (LADC) in Western countries and 10% of Asian LADCs (Yang, Liang, Schmid and Peng, 2019). Through the advancement of molecular sequencing, selective therapeutic agents and precision medicine, NSCLC therapies have improved significantly over the past 15 years. Yet, KRAS-mutant NSCLCs remain an “undruggable target” and no effective anti-RAS inhibitors are currently used in routine clinical practice (Adderley, Blackhall and Lindsay, 2019).
To understand the reason behind the unresponsiveness of KRAS-mutant NSCLCs to existing treatments and future possibilities for therapeutic strategies, the biology behind KRAS-mutant NSCLCs must first be understood. KRAS is one of the important members of the RAS subfamily. RAS proteins are small GTPases that function as a binary switch between active GTP-bound and inactive GDP-bound state, regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) (Yang, Liang, Schmid and Peng, 2019). In a normal resting cell, RAS protein encoded by the RAS proto-oncogene is bound to GDP, and is inactive. However, upon arrival of extracellular stimuli, RAS proteins would be activated by GEF through the exchange of GDP for GTP, resulting in transient formation of active GTP-bound form of RAS. This in turn activates different downstream signalling pathways that regulate fundamental cell processes such as proliferation, growth, motility and survival (Adderley, Blackhall and Lindsay, 2019).
In normal cells, activated GTP-bound RAS would be deactivated via GAPs, resulting in rapid conversation from active to inactive GDP-bound RAS. However, carcinogenic mutations in RAS may impair the ability of KRAS to hydrolyse and breakdown GTP. Such mutation would lock the KRAS oncoprotein in a constitutively active state, resulting in constitutive activation of KRAS downstream signalling pathway cascades, thus leading to uncontrolled cell proliferation and survival via signal transduction for most receptor tyrosine kinases including ERFG, MET or ALK and the progression of cancer development (Ghimessy et al., 2020).
As a consequence of this mechanism, the development of KRAS-targeting cancer therapy has been challenging due to KRS amino acid substitution, associated with poor prognosis in patients with lung cancer (Yang, Liang, Schmid and Peng, 2019). Previous oncologists have tried to characterise the predictive role of KRAS mutation in patients receiving chemo-, targeted, or immunotherapy – as well as combinations of these treatments. For instance, studies conducted by S. Rodenhuis et al. and J. H. Schiller et al. both investigated the associations of RAS mutations in response to chemotherapy, yet none of the findings show KRAS to be a predictive factor for response rate (Schiller, J H et al., 2001). The unresponsiveness of such KRAS-targeted cancer therapies was further confirmed in a study in 2014, which determined KRAS mutation to be a prognostic factor due to the underscores of KRAS-mutant lung cancer on disease-free survival (DFS) and overall survival (OS) (Nadal et al., 2014). However, with technological advancements and continued characterization of KRAS, recent novel research has led to new theories regarding methods by which to target KRAS mutations directly, which have been demonstrated in multiple early-phase clinical trials.
This breakthrough of direct RAS targeting was first made by Ostrem et al., who developed a strategy to target mutant KRAS G12C without affecting wild-type protein through tethering (novel screening technology). Lim et al. bought this discovery further by reporting a GDP analogue – guanine nucleotide-binding pocket (SML-8-73-1) – with the ability to be remodified to compete with GTP and GDP for active binding sites even at high concentrations of GTP. As the effects of SML-8-73-1 on mutant KRAS G12C is irreversible, competing for the sites would therefore lock KRAS in its inactive GDP-bound state irreversibly, thus blocking the proliferative activity of the KRAS-mutant cells. However, despite SML-8-73-1’s preclinical effects on KRAS G12C, follow-up studies showed that specificity of the inhibitor is rather low resulting in a variety of off-target effects when used in clinics (Ghimessy et al., 2020).
This research led to the discovery of a more recent and promising KRS G12C-specific inhibitor: AMG 510 (sotorasib). Similar to SML-8-73-1, AMG 510 is a novel small molecule that covalently binds to the cysteine amino acid and again locks KARS in its inactive GDP-bound state irreversibly. Treatments with AMG510 in preclinical trials caused regression in KRAS G12C tumors and increased the effectiveness of chemotherapy and selective chemotherapeutic agents. Encouraging phase I clinical trial data in 32 patients with KRAS G12C mutation were just released in ASCO 2019 where a partial response was achieved in 54% and stable disease in 46% of the patients, in total achieving a disease control rate of 100% (Ghimessy et al., 2020).
Another inhibitor which also shows promising development as a potential KRAS-mutant treatment is the MRTX 849, a mutation-selective and orally available irreversible small-molecule inhibitor of KRAS G12C. The mechanisms of this inhibitor show great similarity with the AMG 510 and results also show anti-tumor activity and complete tumor regression when delivered to patients and cell-derived in vivo models (Yang, Liang, Schmid and Peng, 2019).
Other than focusing on direct targeting of KRAS, other conventional approaches in treatment for patients harboring KRAS mutations include revitalizing chemotherapy. As previously stated, efficacy of chemotherapy is very limited in patients with KRAS-mutant lung cancer due to its short durable response. However, new approaches that aim to combine chemotherapy and mTOR inhibitors prove a way out of this challenge. It has been found that the mTOR signaling mediates a key resistant mechanism to chemotherapy in KRAS-mutant lung cancer, shown by hyperactivation of the pathway in lung cancer patients samples compared to wild-type KRAS. Therefore, it is thought that combining clinically approved mTOR inhibitor and chemotherapy will lead to an inhibition of proliferation of cancer cells, specifically those cancer cells harboring the KRAS-mutation. That said, more evidence in the form of clinical trials is required of this therapy in cancer patients with KRAS mutations in order to reach a conclusion as to the efficacy of this treatment (Ghimessy et al., 2020).
In conclusion, although new advancements provide us with a better understanding of the KRAS-mutant oncogene, the precise role of KRAS and the effects of the downstream cascade pathways are yet to be fully understood. However, the emergence of new promising agents based on a deeper understanding of the pathobiology of KRAS – such as covalent KRAS G12 inhibitors and the recently proposed combinatorial approaches – point to a potential revolutionization of KRAS-mutant NSCLS therapies. Thus, the success of future KRAS-mutant NSCLCs therapy awaits further clinical studies following the recently proposed approaches of tailed treatments for KRAS-mutant NSCLCs.
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