Mutant forms of the protein KRAS are found in approximately 30 percent of all cancers. They are responsible for many of the hallmarks of these cancers, and KRAS is, therefore, considered an important therapeutic target. However, attempts to develop clinically useful KRAS-targeted drugs have been unsuccessful.
"KRAS is a molecular switch," said Michael Burns, a doctor of medicine and doctor of philosophy candidate at Vanderbilt School of Medicine in Nashville, Tenn. "In the 'on' state it transmits signals that drive cell growth and survival. In many cancers, KRAS is permanently in the on state, and it is a highly validated therapeutic target.
"KRAS switches from off to on most efficiently when it is attached to a protein called SOS," explained Burns. "Each SOS protein attaches to two KRAS proteins, and we have identified a number of small molecules that bind to a particular part of SOS when it is in a complex with two KRAS proteins. These small molecules disrupt the function of the complex, ultimately causing inhibition of the signaling pathways downstream of KRAS that drive cell growth and survival. Although our data were generated in biochemical assays and cell lines, they suggest a potential way to therapeutically target KRAS, which has not been possible to date."
KRAS switches from off to on during a process called guanine nucleotide exchange, and SOS increases the rate at which this process occurs. Burns and colleagues hypothesized that small molecules that blocked SOS-mediated guanine nucleotide exchange would prevent KRAS switching on and, therefore, inhibit the signaling pathways downstream of KRAS that drive cell growth and survival.
Instead, they found that a number of small molecules that attached to a special pocket in a region of SOS called the CDC25 domain and increased SOS-mediated guanine nucleotide exchange actually inhibited two of the major signaling pathways downstream of KRAS: the MAPK and PI3K signaling pathways.
The researchers are actively investigating why small molecules that increased SOS-mediated guanine nucleotide exchange in biochemical assays blocked signaling downstream of KRAS in cell lines. They are also working to optimize the small molecules before they conduct studies in preclinical models of cancer.