Drugging the Undruggable: The Ras Protein

Photo Credits: Scientific American

Understanding the RAS Protein

The RAS proteins—KRAS, NRAS, and HRAS—are small GTPases involved in transmitting signals inside cells that regulate growth, survival, and differentiation. They play a pivotal role in the MAPK and PI3K-AKT signaling pathways, both of which control cellular processes like division and metabolism. Under normal conditions, RAS proteins switch between an active state (when bound to GTP) and an inactive state (when bound to GDP). This cycling process allows them to regulate various cellular functions in a highly controlled manner.

However, mutations in RAS genes can lock the protein in its active form, leading to continuous signaling that drives cancerous cell growth. In fact, RAS mutations are found in around 30% of all human cancers, with particular prevalence in lung, colorectal, and pancreatic cancers. The most common and well-studied mutations occur in the KRAS gene, particularly in codon 12, 13, or 61.

These mutations make RAS proteins difficult to target with traditional therapies, as they render the proteins resistant to the normal regulatory mechanisms of the cell. Despite this challenge, the medical community has made significant progress in developing drugs that can inhibit mutant RAS-driven cancers.

RAS Mutations and Their Impact on Cancer

RAS mutations have a profound impact on cancer biology. By constantly signaling for cell division and survival, mutant RAS proteins drive tumor growth and make cancer cells more resistant to apoptosis (programmed cell death). These mutations also promote resistance to many conventional cancer therapies, which rely on targeting rapidly dividing cells.

For example, in pancreatic cancer, which harbors one of the highest rates of KRAS mutations, the prognosis is often poor. KRAS mutations in colorectal cancer and lung cancer have also been linked to more aggressive disease and poorer outcomes, making these cancers particularly difficult to treat.

Breakthrough in RAS Inhibition: FDA-Approved Drugs

For decades, researchers struggled with finding an effective way to directly target RAS proteins. Due to their structure and mechanism of action, RAS proteins were deemed "undruggable" for a long time. However, recent innovations have changed this narrative. Two FDA-approved drugs have now opened the door to treating cancers driven by mutant RAS proteins.

1. Sotorasib (Lumakras)

Approved by the FDA in May 2021, Sotorasib (brand name: Lumakras) was the first drug to directly target the KRAS G12C mutation, one of the most common mutations found in cancers such as non-small cell lung cancer (NSCLC). This drug is a small molecule inhibitor that binds to the KRAS G12C mutant protein, locking it in an inactive state and preventing it from initiating the signaling pathways that promote tumor growth.

Sotorasib has shown promising results in clinical trials, particularly for patients with advanced or metastatic NSCLC who have the KRAS G12C mutation. In one pivotal trial, Sotorasib led to an objective response rate (ORR) of 37.1% in patients with previously treated KRAS G12C-mutant NSCLC. While its efficacy in other cancers with KRAS mutations is still being explored, Sotorasib represents a breakthrough in directly targeting a mutant form of RAS.

2. Adagrasib (Kraso)

Approved by the FDA in December 2022, Adagrasib (brand name: Kraso) is another small-molecule inhibitor of KRAS G12C. It works similarly to Sotorasib, by irreversibly binding to the mutant protein and preventing it from remaining active. Adagrasib has demonstrated strong efficacy in clinical trials for patients with advanced or metastatic NSCLC harboring the KRAS G12C mutation, showing an objective response rate comparable to that of Sotorasib.

In addition to NSCLC, Adagrasib is also being tested in other cancers with KRAS G12C mutations, including colorectal cancer and pancreatic cancer, though these indications are still being studied. The approval of Adagrasib, following closely on the heels of Sotorasib, underscores the growing importance of targeting KRAS mutations in precision oncology.