A team from the National Cancer Institute RAS Initiative and collaborators at Lawrence Livermore National Laboratory and BridgeBio TheRas Pharma have made a dual discovery involving the much-studied protein KRAS, identifying a better model and a highly potent anticancer compound. 

Their findings, the product of a long-term partnership, appear in the Journal of Biological Chemistry and focus on targeting mutated versions of KRAS in human cancer cell lines. 

cartoon structure shows binding site of BBO inhibitor (L) to G12C KRAS-GppNHp
Illustrated structure shows binding site of BBO inhibitor (L) to G12C KRAS-GppNHp

The anticancer compound, BBO-8956, is one of the first dual inhibitors of the cancer-causing KRAS G12C mutant, said Anna Maciag, Ph.D., senior principal scientist at Frederick National Laboratory’s Cancer Research Technology Program, where the RAS Initiative is headquartered. In the team’s laboratory models, it potently blocked cancer cells from proliferating. 

BBO-8956 is an experimental compound used for laboratory studies but is closely related to a compound that’s being evaluated as a potential treatment. As with all new therapeutics, studies in humans will be needed to assess effectiveness in people.  

“It’s basically a very close analogue of a compound we codeveloped that just has been approved by FDA as an investigational new drug,” Maciag said, referring to the FDA’s authorization that the compound may be administered to humans in experimental or investigational contexts. 

Disabling cancer cell division 

A dual inhibitor is noteworthy because of KRAS’ biological role. The KRAS protein is present in many types of cells and acts as a molecular on/off switch. When it’s bound to a nucleotide cofactor—a type of molecule—called GTP, its shape changes to one that allows other key proteins to bind it and establish an “on” signal that results in the cell dividing. In normal cells, GTP eventually undergoes a chemical reaction that converts it to a cofactor called GDP, which switches KRAS “off.” A dual inhibitor can bind to KRAS in its “on” and “off” forms alike. 

This is especially useful because the cancer-causing mutated versions of KRAS remain in the “on” form, which allows the cells to divide unchecked and establish a growing tumor. Blocking that is crucial, but existing KRAS inhibitors so far have only been able to target the “off,” GDP-bound KRAS. Often, they stunt the cancer’s progression but aren’t enough to eradicate the tumor. 

The team found that BBO-8956 latches onto KRAS G12C in both forms, essentially blocking its activity. The “off” form remains off and is unable to be switched on, while the “on” form becomes disabled, its molecular structure pushed so far out of its usual configuration that the proteins that would usually bind to trigger cell division can’t. The cancer cells lose the signal to propagate and can’t carry out critical functions they need to survive.  

‘We have to be very careful’ 

As part of the study, the team also examined models of normal KRAS and its mutant versions in their “on” form via nuclear magnetic resonance spectroscopy (NMR), a method for obtaining high-resolution 3D structures of proteins. These studies used a stable analogue of GTP called GppNHp, which promotes KRAS activity like GTP does and is broadly used in KRAS structural studies since it’s easier to work with. 

However, they noticed that while GppNHp acts as GTP does, it causes a large population (40–60%) of KRAS’ “on” form to assume a previously identified structural configuration known as “state one.” This was a surprising finding, since state one’s structure prevents other proteins from binding to KRAS and activating cell division. 

“That was very difficult to understand for us. Why would a cancer cell have this high population of an inactive state in an oncogenic protein? It doesn’t make sense,” Maciag said. 

Alok Sharma, Ph.D., a scientist on the team, then used NMR to compare the effects of GppNHp and GTP, leading to another surprise. In both normal KRAS and its mutant versions, state one was almost nonexistent in the GTP-bound KRAS proteins, nowhere close to the high percentages seen in the GppNHp-bound proteins. The verdict was clear, said Maciag: GppNHp, despite being a close stand-in for GTP, creates an excess of an artificial state that rarely occurs in nature. It isn’t always a suitable model. 

It shows “we have to be very careful when we use anything that is analogous, not the natural substrate,” Maciag said. 

Both discoveries would have been more challenging were it not for the partnerships surrounding the study, Maciag added. While her team specializes in drug screening, some of the analyses and the chemistry required capabilities and staffing only available via their collaborators at Lawrence Livermore National Laboratory and TheRas, respectively. 

“It’s a very nice collaboration. We’ve been working together since—I think—2017,” Maciag said. “It’s really an amazing team.” 

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