Getting Closer to Targeting an ‘Untargetable’ Cancer Causing Gene

Scientists may have uncovered a potent inhibitor of one of the most common KRAS mutations that drives cancers

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Cancer is a devastating disease and currently the second leading cause of death in America surpassed only by heart disease. Progress in the last few decades has identified new treatments and preventative measures against cancer that has led to a decrease in the cancer mortality rate since 1991, especially for the four most common types – lung, colorectal, breast, and prostate. However, scientists continue diligently working to understand cancer and develop therapeutics with the hope that one day there may be a cure.

Cancer is a disease where genetic mutations cause a group of cells to no longer function normally and instead grow uncontrollably and metastasize (or spread) to other parts of the body. This uncontrolled growth occurs for several reasons. Normal cells respond to chemical signals (like hormones) that tell them to start or stop growing. When mutations occur in genes involved in interpreting these signals, it can lead to cancerous cells no longer responding to stop signals and no longer needing start signals to grow. Normal cells are also limited for how many times they are able to divide, whereas cancer cells develop replicative immortality. Cancer cells also avoid destruction. When normal cells age or become faulty they induce a programmed cell death (or self destruction) called apoptosis. When this process is disrupted by mutations it allows cancer cells to avoid self-destruction and continue growing. Cancer cells also avoid destruction by evading the immune system or tricking the immune system into protecting it.

Genes that have the potential to promote cancer development when mutated are called oncogenes. Mutated forms of an oncogene can sometimes be inherited – or develop due to encountering carcinogens (chemicals that increase the risk of cancer developing by damaging DNA) – or just develop due to the natural mutations that occur over one’s lifetime as cells make errors in DNA replication during growth. One of the most frequently mutated oncogenes is the KRAS gene, which can act as a major driver of cancer, especially in pancreatic, colorectal, and lung cancers. The KRAS gene encodes the KRAS protein that acts like a switch that can be turned ‘on’ to trigger downstream signaling to promote cell growth among other roles. Typically a cell receives a signal that activates the KRAS protein; however, when KRAS becomes mutated, it gets stuck in an ‘on’ state and continuously promotes cell growth and the development of cancer.

Because KRAS mutations (like KRAS^G12C and KRAS^G12D) are common drivers of cancer, there has been a longstanding effort since the 1980s to identify a way to directly target this protein and inhibit it. However, this has proven to be quite challenging for at least two reasons: (1) an inhibitor must specifically target the mutated protein within cancer cells but not target the normal protein within healthy cells; and (2) a protein needs to contain a site where a drug can bind to it to be targetable, such as a deep binding pocket, which is not always present.

For years, the KRAS protein was considered ‘untargetable’ because it was perceived to contain no deep pockets in its structure for drugs to bind to. Then, a major breakthrough was published in November 2013 of the discovery of a cryptic binding site on KRAS that can be used for mutant specific targeting. Since then, there has been some success in identifying potential inhibitors for the KRAS^G12C mutation. One is called Adagrasib, which acts by covalently binding to the mutated site of the protein and another is called Sotorasib, which also covalently binds to mutated KRAS^G12C and is FDA approved as a treatment.

Finding these inhibitors has ignited immense interest to find inhibitors for other mutant versions of KRAS as well, like the KRAS^G12D mutation, which is the most prevalent mutation among KRAS driven cancers. Scientists have been getting closer to achieving this goal, and in a study from December 2021, used a structure-based-drug-design strategy and identified a compound as a potential inhibitor of this KRAS mutation called MRTX1133. Now, in a recent study, the scientists have gone further and characterized mechanisms by which MRTX1133 works and found ways to increase its efficacy.

In this study, the researchers first tested the specificity of MRTX1133 to bind to the mutated KRAS^G12D protein versus healthy KRAS. They tested specificity using multiple biochemical binding experiments and found that the inhibitor binds to the mutated form about 700 times more strongly than the healthy form – showing its strongly specific. Because binding was specific, the scientists next wanted to test if MRTX1133 turns down KRAS signaling in actual human cell cultures. They did this by performing a biochemical experiment in both pancreatic and colorectal cancer cell lines to measure the activation of proteins downstream of KRAS signaling (as in proteins KRAS activates). They found that the inhibitor decreases downstream signaling from the KRAS^G12D protein in a concentration-dependent manner for both cancer cell lines, however, it was less effective in the colorectal cancer cell lines at later time points. This told the scientists that MRTX1133 can inhibit KRAS^G12D function in cells – but would this have an overall effect on cancer growth?

To test this question, the scientists used a cell viability assay to measure if the inhibitor reduces the survival of several KRAS^G12D driven cancer cell lines (pancreatic, colorectal, lung, and gastric). They found in most cell lines it reduced viability. To address the question further, the scientists tested MRTX1133 on the different cancer types in xenograft mouse models, where a human tumor is transplanted into a mouse. In these models, the inhibitor showed it was able to cause >30% tumor regression (or tumor shrinking) in 73% [8 out of 11] of the pancreatic tumors tested, 25% [2 out of 8] of the colorectal tumors tested, and in the one gastric tumor tested. It was unable to cause tumor regression in the lung cancer tumors tested and only inhibited growth to some degree. So it seems the inhibition of KRAS^G12D by MRTX1133 has tumor-type-dependent efficacy.

To identify ways to improve the inhibitor’s efficacy, the scientists performed a type of genetic screen to identify proteins that impact MRTX1133 inhibition. Multiple proteins were identified as candidates (including EGFR and PI3K𝛼) that may be beneficial to co-target with other drugs as a therapeutic strategy to enhance the inhibition of MRTX1133. The scientists tested MRTX1133 in combination with other drugs and found they could increase the efficacy of the inhibitor, notably for both colon and pancreatic cancer xenografts, when MRTX1133 was paired with the drug cetuximab – an EGFR inhibitor or Alpelisib – a PI3K𝛼 inhibitor.

Overall, this study provides a proof-of-principle that the KRAS^G12D mutation is not ‘untargetable’. MRTX1133 can target it and has proven to be a potential candidate to be used as a therapeutic agent against KRAS^G12D driven cancers when combined with other drugs. Additionally, other scientists are working on ways to inhibit KRAS^G12D, such as in a study from January 2022 where scientists identified a possible strategy to develop a binding inhibitor via salt bridges.

It will be highly impactful if any KRAS^G12D inhibitors translate into human treatments and enter clinical trials, however, more work needs to be done first. This includes follow up studies looking at effective combinations of drugs with MRTX1133 to maximally increase its effect. It will also be beneficial to learn why the inhibitor has tumor-type-dependent efficacy.

These studies should give us all hope that a KRAS^G12D mutation specific drug is possible and can be strived for. Although there is still much more to do before we see a cure for KRAS driven cancers, we should remain hopeful that the collective effort from numerous labs studying KRAS mutations will eventually get us there.