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Switching off a lung cancer oncogene using small molecules

Institution: University of California, San Francisco
Investigator(s): Ulf Peters, Ph.D.
Award Cycle: 2010 (Cycle 19) Grant #: 19FT-0069 Award: $135,000
Subject Area: Cancer
Award Type: Postdoctoral Fellowship Awards

Initial Award Abstract
One of the most commonly mutated proteins in cancer is Ras. Oncogenic mutations turn Ras into its active state and uncontrolled growth and cancer occur very frequently. Since many signaling pathways go through Ras, this protein does not need to be activated itself by a mutation, but turning on upstream proteins and pathways can have the same effect. Targeting Ras for intervention in cancer by turning it into its inactive form makes sense on two levels: Preventing Ras-mutants from causing constant activation and/or preventing normal Ras protein from relaying signals for activating cell growth and proliferation.

Why have no drugs been developed that bind to Ras and turn it inactive? The natural activating ligand of Ras, GTP, has a very high concentration in cells and binds extremely tightly, so it is very difficult for any small molecule to displace GTP. However, there is a potential solution. Formation of a chemical bond between a small molecule drug and Ras could lock-in the drug irreversibly and therefore prevent GTP from binding and activating Ras. We would like to test whether Ras can be targeted in such a manner, using irreversible chemical bond formation, and whether such irreversible drugs can turn off Ras and associated pathways involved in cancer.

Formation of a chemical bond requires complementary reactive groups on both the small molecule drug and the protein. While wild-type Ras does not possess very reactive groups, one of the common oncogenic mutants in lung cancer (G12C) contains a well-positioned cysteine in the binding site, an amino acid that is poised to react with the right partner. Chemical groups that can react with such cysteines are known and can be installed on small molecule drugs. That leaves the problem of finding a small molecule that can bind Ras and contain the reactive group to fuse irreversibly with the cysteine. We envision several starting points for making an irreversible drug: modify natural ligands, synthesize novel chemicals, or develop drugs from reported low-affinity binders of Ras. Thus, we will synthesize new compounds following these approaches, test them against Ras and Ras mutants, and improve our leads using the structure of Ras as a guide. To understand how to switch Ras off we will engineer additional reactive cysteines into the drug binding site. This will allow us to obtain crucial insights into how the inactive form of Ras can be induced. If introduction of a non-naturally occurring cysteines makes the protein susceptible to specially designed small molecules, these molecules could then also be used as basic science probes for the biological roles of Ras.

The developed small molecules will be tested in several lung cancer cell lines with various mutations in Ras. In addition, cell lines will be chosen that have common mutations in lung cancer relevant proteins that signal upstream or downstream of Ras. These results will give an indication if active Ras and Ras-mutants can be turned off in cells with our compounds. They will also tell us how targeting Ras may affect the overall growth and proliferation pathways.

Finding compounds that target Ras and testing their effects in cells will be a first step to validate Ras as a direct lung cancer target. With our approach, using irreversible chemical bond formation, we hope to overcome previous problems and will gain a “toe-hold” into the chemistry of the small molecule binding site of Ras. How will lung cancer patients benefit from our research? Direct inhibition of Ras represents a novel, unexplored target in chemotherapy. Small molecule drugs against Ras may significantly expand our arsenal of weapons for the cure of lung cancer. By severing Ras as central signaling hub we hope to overcome two common problems in lung cancer: activating mutations in other parts of the signaling pathway and resistance mutations that arise during treatment. Ras is one member of a large class of proteins called “small GTPases” that have eluded targeting by small molecule drugs. The development of Ras inhibitors could serve as a blueprint to target other members of this class of proteins, several of which have significant roles in cancer.