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Page 4 of 19 Cordover et al. J Cancer Metastasis Treat 2020;6:45 I http://dx.doi.org/10.20517/2394-4722.2020.101

such as transcription factors and cytoplasmic signaling proteins which have key roles in the regulation of
[8]
cell growth and function . Due to the alteration of the Ras/ERK pathway in many cancers, components of
the pathway are considered to be promising drug targets for cancer therapies.

Ras inhibitors
The RAS genes are among the most common oncogenes in human cancer and are of particular interest
for targeting the ERK signaling cascade in cancer. The Ras family of proteins consists of three different
isoforms - K-Ras, N-Ras, and H-Ras. Point mutations in all three of the RAS genes are associated with
different types of cancer. For instance, K-RAS mutations are seen in over half of pancreatic tumors and
[12]
N-RAS mutations are present in almost 20% of skin cancers . As Ras is inactive in quiescent cells, its
constitutive activation drives tumor formation. Previous efforts to target Ras in cancer therapies have
proven to be unsuccessful. However, the development of new direct Ras inhibitors is a promising area of
research.

One way to inhibit Ras is by targeting Ras membrane localization. In order to exert its oncogenic effects,
Ras must localize to the plasma membrane. The farnesyltransferase inhibitors (FTIs) were created to block
Ras membrane localization. The C-terminal CaaX box is a motif that is conserved in all Ras proteins and is
essential for the addition of a farnesyl lipid group. The addition of a farnesyl group to a targeted cysteine in
[13]
the CaaX motif is catalyzed by farnesyltransferase (FTase) and allows Ras to interact with the membrane .
FTIs have been shown to be ineffective in the treatment of solid tumors with K-RAS and N-RAS mutations,
but are promising in treating certain malignancies with H-RAS mutations, such as thyroid and bladder
[14]
cancers . The failure of FTIs in blocking K-Ras membrane localization is attributed to K-Ras having
a higher affinity for FTase than H-Ras. Additionally, K-Ras can interact with geranylgeranyltransferase
(GGTase) which allows for the covalent attachment of a geranylgeranyl lipid group to the CaaX domain,
thus allowing K-Ras to localize to the membrane without farnesylation by FTase. Nonetheless, researchers
have designed a neo-substrate that binds to the farnesylated cysteine in the CaaX domain to prevent
[13]
geranylgeranylation by GTTase and subsequently, K-Ras membrane localization .

Another advancement is to exploit the G12C mutation in K-Ras to suppress oncogenic activity. Targeting
tumors with the K-Ras G12C mutant is of clinical importance because this mutation is frequently expressed in
[15]
lung and colon adenocarcinomas . In addition, there were few known mechanisms for inhibiting K-Ras
activity in cancer cells until the recent discovery of a druggable pocket in K-Ras G12C . This target, called
the switch-II pocket, has initiated the innovation of several irreversible inhibitors to lock K-Ras G12C in its
inactive GDP state. There are currently several K-RAS G12C inhibitors that are in Phase I/II of clinical trials.
For instance, AMG510, developed by Amgen, interacts with the His95 small groove within the switch-II
pocket. Preclinical studies have shown regression of K-RAS G12C tumors and elevated T cell infiltration at
[16]
tumor sites in mice treated with AMG510 . Likewise, MRTX849 (Mirati Therapeutics) also binds K-Ras
in the switch II pocket. MRTX849 has proved to be as effective as a K-Ras G12C inhibitor by inhibiting GTP-
[16]
loading . AMG10 and MRTX849 have both been shown to lead to inhibition of ERK phosphorylation and
signaling, thus contributing to anti-tumor activity in humans. Further work is being done to determine the
[17]
efficacy of K-Ras G12C inhibitors in combination with other anticancer drugs .

Another area of interest is to investigate Ras dimerization as a potential therapeutic target for cancer.
Previous studies show that monomeric Ras can bind Raf, but numerous studies indicate that dimerization
of Ras is necessary for the activation of Raf. Under normal cellular conditions, Ras-GTP is recruited to the
plasma membrane where it activates the Raf kinase domain. Raf in turn activates the MAP kinase pathway.
[18]
As such, all three isoforms of Ras activate Raf, and K-Ras is the most powerful activating agent of Raf .
Multiple studies have found evidence for the role of Ras homodimerization in Raf activation. For instance,
by using a K-Ras mutant that impairs dimerization, K-Ras dimerization was shown to be critical for the