Drugging more enzymes in KRas’s family

Although there are more than 150 guanosine triphosphate (GTP)–binding proteins, or GTPases, only the GTPase KRas G12C has proved susceptible to small-molecule drugs. That’s because it’s tough to find a spot for small molecules to bind to GTPases. Now, researchers have used what they know about the weak spot in KRas G12C to locate a toehold for small molecules on other GTPases. The finding could lead to new drugs and biological probes.

GTPases are involved in many cellular processes. They bind GTP and hydrolyze it to guanosine diphosphate. In doing so, they function as molecular switches in myriad processes, including protein biosynthesis and cell differentiation, proliferation, division, and movement. GTPases have been implicated in cancer and neurodegeneration. “You could find them in almost any disease area,” says Kevan M. Shokat, a chemical biologist at the University of California, San Francisco, who led the research.

GTPases were considered undruggable until 2013. That’s when scientists in Shokat’s lab showed that the GTPase mutant KRas G12C—in which a glycine is replaced by a cysteine at position 12—has a cryptic binding pocket. This area, dubbed switch II, is invisible if nothing is bound to the enzyme. But Shokat’s team showed that a small molecule can slip into this pocket and shut the enzyme down. Since then, the US Food and Drug Administration has approved two drugs, Amgen’s Lumakras (sotorasib) and Bristol Myers Squibb’s Krazati (adagrasib), as treatments for non-small cell lung cancer that target KRas G12C.

Shokat says that even though scientists have been studying the biology of other GTPases, there has been no pharmacology for these enzymes. “There have been no chemicals developed to perturb them, so it’s sort of like a wealth of biology but pent-up chemistry,” he says.

To see if cryptic pockets were also present on other GTPases, Shokat and coworkers overlaid the sequence of these enzymes onto KRas G12C. They then made mutant versions of these enzymes, inserting a cysteine at the same position the mutant cysteine appears in KRas G12C. This cysteine was able to latch on to molecules designed to make covalent bonds with sulfur, thereby revealing the cryptic pocket in several GTPases (Cell 2024, DOI: 10.1016/j.cell.2024.08.017).

The cryptic pockets in these GTPases are surprisingly similar. Johannes Morstein, a postdoctoral researcher in Shokat’s lab and the paper’s first author, says it wasn’t obvious that this would be the case. “We did sequence alignments, and some of the residues are quite different,” he says. What’s more, molecules can bind to some of the pockets and not others. This suggests that drugs could be designed to specifically target certain GTPases.

Victor Cee, who was involved with the development of Lumakras and is now senior vice president of drug discovery at Hexagon Bio, says in an email that the study reaches “the tantalizing conclusion that many members of the GTPase superfamily contain potentially druggable switch II pockets.” Like Shokat’s earlier work with KRas G12C, Cee says, “it is not a stretch to suggest that this new understanding will spur screening campaigns and medicinal chemistry efforts directed toward generating therapeutic inhibitors of the less well-studied, but no less important, members of the GTPase superfamily.”