Explore the ‘undruggable’ oncology target RAS, inhibitory techniques used on downstream pathways, and the preclinical models available to test up-and-coming RAS targeted agents.
RAS Family Mutations and Cancer
One-third of cancers diagnosed each year are driven by mutations in RAS family genes, including 95% of pancreatic cancers and 45% of colon cancers. This, in theory, presents an attractive family of targets for treating many cancer types.
The RAS family is represented by three members:
- KRAS, the most frequently mutated (85% of all RAS-driven cancers).
- Followed by NRAS (12%).
- And HRAS (3%).
All three RAS proteins share over 80 percent of their amino acid sequence, with mutations occurring predominantly in three genes, in codons 12, 13, and 61.
RAS Proteins and Cell Proliferation
RAS proteins are membrane-associated G proteins meaning they can bind GTP and GDP. When RAS is bound to GTP, the protein is switched to an active state inducing cell growth, proliferation. and migration. GDP binding switches the protein to an inactive state.
When RAS family members are mutated they are permanently bound to GTP, driving cell proliferation continuously.
RAS: An Undruggable Oncology Target
One solution to stop oncogenic RAS would be to block its activation. However, attempts to develop drugs that target mutant RAS proteins have been unsuccessful.
This is due to the relative cellular abundance of GTP, and the binding affinity of RAS for GTP being extremely high. There’s also an apparent lack of suitable surfaces in critical regions of RAS proteins necessary for small-molecules to bind, making tumors bearing these mutations among the most difficult to treat. RAS-related treatment strategies instead use an indirect approach, targeting pathways downstream of RAS.
Downstream RAS Pathway Drug Targets
RAS-RAF-MEK-ERK
One important signaling route is the RAS-RAF-MEK-ERK (MAPK) cascade initiated by ligand binding to receptor tyrosine kinases (RTK). Once RAF activation takes place, RAF phosphorylates and activates mitogen-activated protein kinases 1 and 2 (MEK1 and MEK2). These proteins can then both activate the downstream extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2).
ERK 1 and 2 regulate transcription factors in the nucleus (i.e. c-Myc, c-Fos, and CREB) via phosphorylation, acting on gene expression involved in cellular growth, cell differentiation, and division.
PI3K-Akt
RAS also interacts with and stimulates the PI3K-Akt pathway. Once PI3K is activated by RAS, AKT is in turn phosphorylated and activated, with a number of downstream effects. These include activation of mTOR, which regulates the protein synthesis necessary for cell growth, proliferation, and angiogenesis.
Current Inhibitory Routes: BRAF and MEK
Targeting MAPK signaling using BRAF inhibitors (e.g. vemurafenib, dabrafenib) and/or MEK inhibitors (e.g. trametinib, cobimetinib) has proven to be an effective treatment for a variety of different cancers. Resistance remains a major challenge, however -- 30% of tumors don’t respond to the inhibitors, and progression is often observed within the 70% that do respond.
Preclinical Models for RAS Targeted Therapies
A variety of preclinical models are available for assessing agents targeting RAS and downstream pathways, including genetically engineered mouse models (GEMM) and patient-derived xenografts (PDX).
Genetically Engineered Mouse Models
Preclinical GEMM recapitulate disease characteristics, where genes that are strongly associated with tumor progression and development are deleted, overexpressed, or mutated. This results in spontaneous tumor formation.
There’s a large collection of GEMM available covering many common cancer indications, such as lung, prostate, breast, colon, and pancreatic cancers.
KRAS mutant GEMM are of particular interest for testing therapies targeting the RAS oncogene family. Indeed, conditional expression of KRAS mutants in the lung, pancreas, and gastrointestinal tract induces preneoplastic epithelial hyperplasias, adenomas, pancreatic intraepithelial neoplasia, and adenocarcinomas. These models have been successfully used to demonstrate the efficacy of BRAF, MEK and mTOR inhibitors both as as single agents and in combinations.
Patient-Derived Xenografts
PDX are also valuable and translational preclinical models to evaluate RAS targeting therapies. PDX are animal models derived directly from patient tissue samples, which maintain genotypic and phenotypic fidelity to the patient from whom they were derived. This provides more predictive models than traditional xenografts.
Because of their temporal proximity to the patient, and having never been subjected to the selection pressures of cell culture, PDX models closely recapitulate patient disease. These models show high fidelity in histological presentation of the patient’s cancer, but also in response to chemotherapy, radiotherapy, and targeted therapies.
PDX models harboring relevant mutation have been successfully used to evaluate the efficacy of inhibitors of RAS downstream targets and recently to identify small-molecule compound binding to KRAS.
Summary
RAS proteins are essential components of the signaling networks controlling cell growth, proliferation, and migration. RAS mutations are frequently found in human tumors and are largely recalcitrant to targeted therapies.
GEMM and PDX RAS mutant models recapitulating critical aspects of human disease provide important models for oncology research, and are widely used for preclinical validation of targets and therapies.
