Cancer Metastasis Hobbled by New Compound

Fighting cancer means killing cancer cells. However, oncologists know that it’s also important to halt the movement of cancer cells before they spread throughout the body.

Researchers at Northwestern University, Chicago, and Oregon Health & Science University (OHSU) in Portland, have developed a small-molecule drug that can stop the movement of cancer cells and so prevent their spread to other areas of the body. Early tests in mouse models of different human cancers showed that the orally administered compound KBU2046 inhibited metastasis, reduced bone destruction, and prolonged survival, without any evident toxicity.

“Cancers are lethal because they move,” says senior researcher Raymond Bergan, M.D., division chief of hematology and medical oncology and professor of medicine at OHSU. “This drug is designed to stop that movement.  We started off with a chemical that stopped cells from moving, then we increasingly refined that chemical until it did a perfect job of stopping the cells with no side effects. All drugs have side effects, so you look for the drug that is the most specific as possible. This drug does that.”

Dr. Bergan and Karl Scheidt, Ph.D., professor of chemistry and professor of pharmacology, director of the Center for Molecular Innovation and Drug Discovery, and executive director of the NewCures accelerator at Northwestern University, have set up a company, Third Coast Therapeutics, to further develop anticancer drugs that block cell motility and metastasis. Dr. Scheidt’s laboratory designed and created the candidate molecules, which were then evaluated by Dr. Bergan’s team, which has been studying cancer motility for decades.

The researchers report on the development of KBU2046 and their in vitro and in vivo studies in a paper entitled “Precision Therapeutic Targeting of Human Cancer Cell Motility,” which is published in Nature Communications.

Increased cell mobility is a hallmark of cancer and represents the first step in a cancer migrating from its primary organ to distant organ sites. This metastasis to other sites reduces the likelihood of survival. “For the vast majority of cancer – breast, prostate, lung, colon, and others – if it is detected early when it is a little lump in that organ and it has not spread, you will live,” says Bergan, who is also the associate director of medical oncology in the OHSU Knight Cancer Institute and director of the OHSU Bergan Basic Research. “And generally, if you find it late, after it has spread throughout your body, you will die….Movement is the key: the difference is black and white, night and day. If cancer cells spread throughout your body, they will take your life. We can treat it, but it will take your life.”

Cellular processes that drive this increased mobility thus represent “high-value therapeutic targets,” the authors state. “However, comprehensive endeavors aimed at selectively inhibiting cancer cell motility and resultant metastasis have met with failure.” Part of the problem is identifying druggable pathways that regulate cell motility selectively.

The researchers’ development of KBU2046 started with a chemical scaffold, 4′,5,7-trihydroxyisoflavone (genistein), which was known to have antimotility properties, and which their previous studies had shown inhibit human prostate cancer cell invasion in vitro and metastasis in a mouse model of cancer. Genistein itself wouldn’t be suitable as a potential anticancer drug because it also exhibits other biological effects, but it did represent a promising starting point for further refinement and modification. “While its diverse spectrum of biological effects render it unusable as a selective and potent biological probe, these same properties maximize its potential to selectively probe a wide spectrum of bioactive sites upon chemical diversification,” the researchers note.

They developed a series of genistein structure modifications and tested them for their ability to inhibit human prostate cancer cell invasion in vitro. Compounds that displayed off-target effects and estrogen receptor binding – genistein is also known to have estrogenic effects – were discarded. This process led to the identification of KBU2046, which was found to inhibit prostate cancer cell invasion at least as effectively as genistein, and also inhibited the migration of human prostate, breast, colon, and lung cancer cells.

The new compound also demonstrated high selectivity in cellular assays, did not activate estrogen-responsive genes in relevant human breast cancer cells, and demonstrated no toxicity to a range of cell types, including human prostate cells and human bone marrow stem cells. Encouragingly, tests in mouse models of human prostate cancer confirmed that at nanomolar blood concentrations, KBU2046 inhibited metastasis by up to 92%, including bone metastases, and also prolonged survival.

The collaboration, involving researchers from Xiamen University in China, the University of Washington, and the University of Chicago, provided key multidisciplinary areas of expertise.”We used chemistry to probe biology to give us a perfect drug that would only inhibit the movement of cancer cells and wouldn’t do anything else,” Bergan says. “That basic change in logic led us to do everything we did.”

As we identified areas we were lacking, we looked at new cutting-edge technologies, and if there was something that didn’t meet our needs, we developed new assays to address our needs,” adds Ryan Gordon, Ph.D., research assistant professor in the OHSU School of Medicine and co-director of the Bergan lab.

Initial molecular studies designed to identify the mechanism of action of KBU2046 indicated that the compound decreased phosphorylation of the heat shock protein HSP90β, and that this was linked with the selective inhibition of cancer cell motility. Classical HSP90 inhibitors are cytotoxic and bind directly to HSP90, which impacts on a number of cellular kinases and other client proteins. In contrast, KBU2046 demonstrates no cytotoxicity, “and its effects on protein phosphorylation were highly specific, demonstrating a lack of effects on kinase function.”

HSP90β is part of a large multiprotein chaperone complex that binds a large, but specific, set of regulatory proteins, and it seemed reasonable to assume that KBU2046 might somehow selectively affect the binding of client proteins that regulate cell motility. Further assays demonstrated that KBU2046 in fact binds to a CDC37/HSP90β heterocomplex – “CDC37 is a co-chaperone that mediates the binding of over 350 client proteins to HSP90β, including over 190 kinases” – but not to either the isolated HSP90β or CDC37 proteins.

Also unlike other known HSP90 inhibitors that act broadly on kinase function, KBU2046 binding to the CDC37/HSP90β heterocomplex had highly selective effects on kinase binding. Of 420 kinases screened, KBU2046 led to increases in the binding of just ten, and decreased the binding of another seven. “These findings are in contrast to classical inhibitors of HSP90 function, which have been shown, through this same assay, to affect the binding of the majority client kinase proteins.”

Three kinases in particular were significantly affected by KBU2046: RAF1 and RIPKI (both decreased binding) and SGK3 (increased binding). “All three proteins have been shown by others to regulate cell motility,” the authors note. They then separately showed that knocking down any one of these three kinases decreased cell mobility. “However, only knockdown of RAF1 or of RIPK1 (i.e., the two kinases whose binding to the heterocomplex was decreased by KBU2046) mitigated KBU2046 efficacy, while KBU2046 still retained efficacy in the face of SGK3 knockdown.”

“From our findings, we propose an integrated structural and functional model. KBU2046 binds to a cleft that is formed when HSP90β and CDC37 bind to form a heterocomplex,” the authors comment. “This in turn affects the ability of the hetercomplex to bind client kinase proteins in a very precise manner, selectively affecting those that regulate cell motility.”

They claim that their findings provide a “rational platform” to progress into human studies, and are currently raising the funds needed to carry out Investigational New Drug (IND)-enabling studies. “We’ve taken a clue provided by nature and through the power of chemistry created an entirely new way to potentially control the spread of cancer,” Scheidt says.

“Our eventual goal is to be able to say to a woman with breast cancer: Here, take this pill and your cancer won’t spread throughout your body,” Bergan adds. “The same thing for patients with prostate, lung, and colon cancer.…This drug is highly effective against four cancer types (breast, colon, lung, prostate) in the in vitro model so far. Our goal is to move this forward as a therapy to test in humans.”