The small GTPase RAS protein is a critical molecular switch that cycles between GTP-bound (active) and GDP-bound (inactive) states. Over the last 40 years, it has garnered significant interest because 1) It regulates several critical growth and proliferation signaling pathways, 2) has been identified as a key oncogenic driver, and 3) is one of the most highly mutated cancer genes, with nearly 19% of all cancers patients having a mutation in one of the 3 RAS isoforms (H-RAS, N-RAS, and K-RAS) (reviewed in 1). Yet, for much of the last 40 years, RAS was deemed undruggable due to its “smooth” surface with no obvious drug-binding features.
In 2021, that changed when work pioneered by the Shokat group2 led to the first approved KRAS targeting drugs, sotorasib and adagrasib. These drugs function by covalently binding to GDP-bound KRAS-G12C mutants. The impact of this scientific breakthrough has been twofold – 1) in the clinic where it has had a tremendous impact on KRAS-G12C-dependent cancers and 2) it has muted the idea That KRAS is an undruggable target. Unfortunately, these drugs can’t target wild-type KRAS and other KRAS mutants, such as KRAS G12D which occurs at a higher rate than KRAS-G12C, for example (reviewed in 3). Fortunately, several new RAS targeting approaches are in development, and this newsletter will discuss some of these technologies that may lead to the next generation of novel RAS therapeutics.
K-RAS Small Inhibitor Advancements
While KRAS-G12C targeting drugs have been successful, emerging resistance mechanisms have already popped up that limit the overall benefit4,5. Therefore, novel binding regions on KRAS needed to be identified and exploited. Several approaches are being used to make second-generation KRAS inhibitors such as small molecules, nanobodies, and peptides(reviewed in 3). Additionally, identifying targeted therapies for KRAS G12D-dependent cancers that occur at a higher frequency in highly lethal cancers like pancreatic cancer is an ongoing goal. One challenge for targeting KRAS G12D is that the aspartic acid mutation (G12D) is far less reactive than the cysteine in (G12C), thus, identifying a small molecule inhibitor that could covalently bind like the 1st generation KRAS G12C inhibitors seemed daunting. The Mirati group that developed adagrasib did not let that deter them, and recently published work identifying the small molecule, MRTX1133, that bound non-covalently to KRAS G12D, but with extremely high affinity6. Another benefit of this drug is that it may be able to target both the GTP- and GDP-bound form of KRAS G12D, and their recent work showed that the drug has an IC50 of < 2nM to GDP bound KRAS G12D, and has a greater than 700 fold affinity for this mutant form of KRAS versus wild type7. Importantly, it appeared to reduce tumor growth in xenograft models. This could be a significant leap forward if RAS mutants can be effectively targeted by non-covalent drugs.
Figure Legend. Chemical structure of MRTX1133 the noncovalent but selective and potent inhibitor of KRAS G12D. Structure showing the MRTX1133 drug molecule interacting with the G12D amino acid of KRAS. Figure adapted from J. Med. Chem. 2022, 65, 4, 3123-3133
Molecular Glues – Forming Inhibitory Complexes
Molecular glue in its simplest terms is a molecule that can make two proteins come together and interact which results in a desired function. In the case of drug therapy, the goal is to inhibit the dysfunctional protein through interaction with the other recruited protein. That is exactly what the group at Revolution Medicines did when they created a molecular glue that forms a tricomplex with cyclophilin A and KRAS G12C8. The forced interaction between these two proteins prevents KRAS G12C from binding with its effectors, essentially inhibiting its activity. The molecular glue compound, RMC-4996, is a highly modified version of a fusion between the cyclophilin A binding motif of Sanglifehrin A and a promiscuous cysteine-reactive warhead that targets KRAS G12C, and is described in detail in the Schulze C. et al. publication8. Unlike the 1st generation small molecule inhibitors, this molecular glue has been shown to bind the active form of KRAS G12C and was shown to promote tumor regression in several human cancer models. Interestingly, the group has since modified the KRAS binding end of their molecular glue and have developed two promising alternatives, RMC-6236 which functions as a pan-RAS inhibitor, and RMC-9805 which functions as a “cool covalent” KRAS-G12D inhibitor(reviewed in 9,10). Another novel molecular glue, developed by the Shokat lab, combines a medium chain lipid tail with the covalent KRAS G12C binder (MRTX849-warhead) to promote forced, protein-membrane interaction11. This additional membrane interaction resulted in disrupted KRAS lateral mobility and prevented nano-cluster formation which was shown to be important for downstream signaling.
PROTACs – Degraders
Targeted protein degradation using proteolysis-targeting chimeras (PROTACs) has garnered significant interest from the pharmaceutical industry. Modern PROTACs are heterobifunctional small molecules that are comprised of two ligands and a linker region(reviewed in 12). One ligand targets the protein of interest while the other ligand targets an E3 ligase. Once the proteins interact the protein of interest will be ubiquitinated and targeted for degradation. Importantly, the PROTAC itself may not be consumed in the reaction; thus, can target multiple copies of the protein of interest, a significant advantage over most small molecule inhibitors. There are also molecular glues that can promote targeted protein degradation. A drug known as thalidomide, was shown to enhance the protein-protein interaction between the E3 ligase Cereblon and specific target proteins. In the realm of RAS, this degrader technology was utilized by fusing a monobody, 12VC1, that targets KRAS G12C with an E3 ligase targeting ligand13. The results were promising as KRAS signaling was blunted, and the benefits of this fusion was more sustained than 12VC1monobody treatment alone13. Due to size limitations resulting in low cell permeability, this degrader had to be expressed in the cell and is currently not a viable therapeutic option. A PROTAC was developed by linking the KRAS G12C-warhead to thalidomide, but while it bound to endogenous KRAS G12C it did not promote degradation of the target14. The Crews lab created a PROTAC that combined the MRTX849 warhead with a molecule that recruits the E3 ligase VHL15. This molecular glue promoted the degradation of KRAS G12C and suppressed downstream MAPK signaling. Most promisngly, a group at Astellas developed a PROTAC, ASP3082, that specifically depletes KRAS G12D, and this drug has shown promising pre-clinical results, and is now in phase I clinical trials(reviewed in 9).
These studies highlight the rapid advancements in new therapies being developed to treat cancers that harbor KRAS mutations. It will be interesting to see how these technologies are mixed and matched to create very potent, yet precise and targeted therapies. Furthermore, it seems feasible that other undruggable targets such as other small GTPases that have traditionally been hard to target may benefit from the technologies.