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Mutations in the oncoprotein KRAS occur in about 25% of cancer cases and are associated with poor prognosis of the disease. Therefore, scientists believe that blocking KRAS signaling is a potential anti-cancer pathway. However, previous decades of research have shown that this protein may be a “non-medicinal” target because its surface is very smooth except for a GTP binding pocket, so it is difficult for drug developers to The surface finds the “foothold” of small molecule drug candidates (binding pockets).

Knowledge Card: In cells, KRAS is inactivated when bound to guanosine diphosphate (GDP) and is activated when bound to guanosine triphosphate (GTP), and abnormal activation of KRAS is closely related to various cancers.
However, about 10 years ago, researchers found that KRAS is actually more targeted than imagined, which has made a breakthrough in the development of drugs targeting KRAS. As shown in Table 1, at least five KRAS modulators are in clinical development. Among them, AMG 510, MRTX849 and JNJ-74699157/ARS-3248 are only targeting the G12C mutant of KRAS (KRAS-G12C), while BI 1701963 (inhibiting KRAS by binding to SOS1) and mRNA-5671 ( A cancer vaccine that targets KRAS proteins has a different mechanism of action and may play a role in a wider population.


Knowledge Card: The inactivation and activation of KRAS are regulated by two types of factors: 1) guanine nucleotide exchange factor (GEF, such as SOS protein), which catalyzes the binding of KRAS to GTP, thereby promoting KRAS activation; GTPase-activating proteins (GAPs), which promote the hydrolysis of GTP that binds to KRAS to GDP, thereby inhibiting KRAS activation.

Source: Nature Reviews Drug Discovery


1. KRAS-G12C inhibitor


Overall, advances in KRAS inhibitors are closely related to the industry’s increased interest in “small molecule drugs that bind to targets through covalent interactions.” For a long time, drug developers have avoided using covalent binders because they fear that this mixed and irreversible combination may cause off-target toxicity. However, with the EGFR inhibitor afatinib, BTK inhibitor ibrutinib and other drug candidates confirm that this toxicity is controllable, many companies have begun to explore whether this covalent binding agent can also be applied to other targets.


Kevan Shoka, a chemical biologist at the University of California, San Francisco, discovered the possibility of developing a KRAS covalent binder early on. Because, they found that the KRAS-G12C mutant is characterized by the introduction of a new cysteine, and this nucleophilic amino acid is easy to form a covalent bond, if a small molecule can be covalently bound to the KRAS-G12C mutant. Cysteine, then, they are expected to be used to treat patients carrying such KRAS mutations.


Knowledge Card: The G12C mutation is one of the most common KRAS mutations, specifically the glycine mutation at the 12th position of KRAS to cysteine ​​(cysteine). This mutation is present in approximately 13% of lung adenocarcinomas, 3% of colorectal cancers, and 2% of other solid tumors. Other common KRAS mutations include G12D, G12V, which are highly expressed in colorectal cancer and pancreatic cancer.
In 2013, in a Nature paper, Shokat first reported the feasibility of using small molecules to covalently bind KRAS-G12C mutants (the team adhered to a six-year screening in order to obtain such molecules). Through crystallographic analysis, Shokat et al. found that covalent compounds can bind to previously undiscovered “switch-II pockets” on KRAS.


Three major drug candidates


This discovery immediately caused a sensation. Later, Wellspring Biosciences, a subsidiary of Araxes Pharma, co-founded by Shokat, confirmed that a modified version of this compound identified by Shokat could reduce tumors in mice transplanted with human tumors carrying the KRAS-G12C mutation. Soon after, Araxes teamed up with Johnson’s Janssen to advance the drug candidate (JNJ-74699157/ARS-3248 mentioned earlier) into clinical trials.


While Araxes is making progress, the pharmaceutical giant Amgen has also made important gains. In collaboration with Carmot Therapeutics, they have long discovered that some covalently bound compounds can even open a “mystery pocket” on the “lid” of the “switch-II pocket.”


“This is one of our key findings. Opening this mysterious pocket may bring more potential binding sites to candidate molecules,” said Margaret Chu-Moyer, vice president of research at Amgen. (In 2017, Shokat and colleagues reported the same “mystery pocket” they found in Cell Chemical Biology)


In August 2018, Amgen’s first KRAS-G12C inhibitor (AMG510) entered clinical studies. A few months later (January 2019), competitor Mirati Therapeutics’ KRAS-G12C inhibitor MRTX849 also began human trials, Araxes and Janssen JNJ-74699157 followed closely in July this year.


Amgen recently published preliminary data on the AMG 510 I/II trial at ESMO (September). Of the 13 NSCLC patients treated with AMG 510, 7 (54%) had partial remission and the tumor volume became smaller; the other 6 patients were stable after treatment. In addition, patients appear to be well tolerated by AMG 510, and dose-limiting toxicity has not been shown to date. Currently, many patients have been treated with AMG 510 for more than 6 months.


Greg Friberg, head of tumor development at Amgen, believes that because the patients with NSCLC who participated in the trial have deteriorated and AMG 510 is on average for their four-line treatment, these preliminary data suggest that this KRAS-G12C inhibitor has a good long-lasting response. .


However, Amgen’s early results for trials of colorectal cancer patients are not “bright”. Only 12 patients (8%) underwent partial remission, and the remaining 10 patients were stable. Friberg said that differences in the efficacy of AMG 510 in different cancers may reflect a slightly different role in the KRAS-G12C pathway in different diseases.


Mirati announced data on MRTX849 at the AACR-NCI-EORTC conference and Cancer Discovery in October. Of the 6 evaluable NSCLC patients, 3 (50%) achieved partial remission; of the 4 colorectal cancer patients, 1 (25%) achieved partial remission.


Combination therapy


In addition to monotherapy, developers have also studied combination therapy based on KRAS-G12C inhibitors. For example, Amgen is studying the association of AMG 510 with the PD-1 antibody pembrolizumab. In a paper published in Nature on October 30, the researchers confirmed that 9 of the 10 mice treated with this combination strategy disappeared permanently. In addition, mice cured by “AMG 510+pembrolizumab” also possessed the ability to reject KRAS-G12D tumors, suggesting that this combination therapy may drive an adaptive immune response.


Currently, Amgen has tested this combination therapy strategy in NSCLC patients and plans to test it in patients with colorectal cancer by the end of this year. Mirati also plans to use MRTX849 in combination with pembrolizumab.


In addition to PD-1 antibodies, SHP2 inhibitors are another “joint partner” that KRAS-G12C inhibitor developers are looking for. SHP2 is a phosphatase that is involved in the activation-inactivation mechanism of KRAS. Earlier this month, Amgen had just partnered with a company called Revolution Medicines to obtain its experimental SHP2 inhibitor, RMC-4630. However, when the trial of the “AMG 510+ RMC-4630” was not disclosed. Mirati has previously partnered with Novartis to develop the “SHP2 inhibitor TNO155+ MRTX849” combination therapy, and the relevant Phase I/II trial is planned for 2020.


Other combined strategies under investigation include “KRAS-G12C inhibitor + CDK4 inhibitor”. Using genome-wide CRISPR interference screening, the Shokat team recently discovered that CDK4 is one of the genes critical for cell survival when KRAS is inhibited. He hopes that the trial to investigate this combined strategy will begin as soon as possible.


2. Targeting other KRAS mutants


In addition to KRAS-G12C inhibitors, the industry has made significant progress in developing drugs for other KRAS mutations. Among them, a company called Moderna Therapeutics is working with Merck to develop an mRNA cancer vaccine (Table 1 and the aforementioned mRNA-5671), which enables it to make antigens in vivo to initiate T cell search and destruction of expression. Cells of key KRAS mutants (G12C, G12D, G12V and G13C).


In a phase I trial, Moderna and Merck are testing the potential of mRNA-5671 monotherapy and “mRNA-5671 + pembrolizumab combination therapy” in patients with KRAS mutations or metastatic NSCLC, colorectal cancer or pancreatic cancer.


In addition, as shown in Table 1, Mirati plans to conduct the research required to submit the G12D inhibitor IND application to the FDA in 2020.


3. Indirect inhibition of KRAS


The drugs for pan-KRAS (pan-KRAS) have also made exciting progress in the near future. At the end of October, Boehringer Ingelheim (BI) announced that its first pan-KRAS inhibitor, BI1701963, entered clinical development. BI1701963 targets the SOS1 protein, which promotes the activation of KRAS. Preclinical studies have shown that this highly specific SOS1 inhibitor reduces KRAS-GTP levels and MAPK signaling in cellular and animal models. All early indications indicate that BI1701963 has a broad inhibitory effect on KRAS activating mutations.

KRAS is in an “on” state when combined with GTP, and is in a “closed” state when combined with GDP. Its conformation differs in different states. This difference regulates its ability to combine “partners” such as SOS1, PI3K, RAF, and GAPs.

“Unlike KRAS-G12C inhibitors, SOS1 inhibitors block the interaction of SOS1-KRAS and thus are not related to mutations in KRAS,” explains Norbert Kraut, head of BI cancer research.


However, BI has found that the effect of SOS1 inhibitor monotherapy is not outstanding, only to make the tumor “stagnant”, but not to kill the tumor. In order to obtain tumor cell killing activity, they must use BI1701963 in combination with a MEK inhibitor (trametinib). The company expects that this combination strategy will improve efficacy and potentially delay the emergence of adaptive resistance.


In addition to BI, in February of this year, in a PNAS paper, Bayer also announced that it has discovered a SOS1 conjugate that selectively inhibits KRAS-SOS1 interaction. The company is said to be developing SOS1 inhibitors, but has not yet announced any research and development schedule.


4. Other possibilities in the future


Finally, it is worth mentioning that in addition to the indirect inhibition of KRAS’s SOS1 inhibitor BI1701963, BI has another combination with the newly identified pocket on the KRAS, the switch I/II pocket. Small molecule BI-2852. Studies have shown that this newly discovered pocket is expected to block the interaction of KRAS with all its “activating partners”, effector proteins and inactive proteins. BI is actively promoting the development of compounds that combine this pocket.


Given the strong desire of the pharmaceutical industry to target KRAS over the past few decades, it is believed that the discovery of these new pockets will further fuel the R&D of KRAS drugs.

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