AACR: Developmental Signaling Takes the Front Stage

Hans Clevers, MD, PhD

Timothy Hoey, PhD

Washington, DC-The Wnt signaling pathway was demonstrated as a promising target in cancer stem cell therapy through a series of talks highlighting the promise and efficacy of a new generation of inhibitors that are making their way into the clinic.

Three talks were given in separate sessions by Hans Clevers, Shifeng Pan, and Timothy Hoey, focusing on the role of Wnt signaling in normal and cancer stem cell biology, at the annual meeting of the American Association for Cancer Research (AACR), held April 6–10 in Washington, DC. In addition, preclinical data for two compounds currently in phase I clinical trials were presented to the public for the first time.

Cancer as a Disease of Stem Cells

In the first plenary session of the meeting, Hans Clevers, MD, PhD, from the Hubrecht Institute, described lineage tracing experiments that have proven critical in our understanding of intestinal biology. Crypt structures in the colon and small intestine are formed by invaginations in the epithelial tissue. At the base of these crypts reside the intestinal stem cells, which require Wnt ligands in order to maintain their pluripotency. Wnt ligands are given to the intestinal stem cells by adjacent Paneth cells. His work showed how the stem cells are able to asymmetrically divide, producing one copy of a daughter cell that is forced upwards in the crypt and leaving the stem cell behind. This process is essential for the turnover of colon epithelial tissue, which occurs approximately every 4 days.

Dr. Clevers also showed how the normal intestinal stem cells can become dysregulated. Adenoma formation, like that which occurs with the disease familial adenomatous polyposis, was described by Dr. Clevers as “a caricature of the true stem cell niche,” where cells that have aberrant Wnt signaling fail to differentiate as they are forced upward in the crypt structure. Instead of reaching the top and undergoing programmed cell death, these cells grow outward and form a polyp. This mechanism for adenoma formation in colon disease may present a common means by which cancers can form. It gives evidence that cancer itself begins with disruption of the normal stem cells and provides a rationale for the targeting signaling pathways that are important in maintaining stemness, such as Wnt.

Targeting Wnt With Biologics

At this time, four drugs targeting Wnt signaling have reached clinical trials. Two of these compounds are biologics developed by OncoMed, and for the first time, the preclinical data for OMP-54F28 were shown in a presentation by Timothy Hoey, PhD, senior vice president, cancer biology at OncoMed. OMP-54F28 was developed as a fusion protein of the ligand-binding domain of frizzled-related protein 8, one of the endogenous receptors for the Wnt ligand, and an antibody Fc region. This drug is capable of scavenging Wnt ligands with high potency, and it was shown to act on a broad spectrum of different Wnt ligands.

In the mouse breast cancer model MMTV-Wnt1, in which spontaneous tumors form as a result of overexpression of the Wnt1 ligand, OMP-54F28 was shown to block tumor growth. In a patient-derived xenograft pancreatic cancer model, it sharply reduced the rate of relapse in tumors when given in combination with gemcitabine. This experiment was especially important, because relapse of pancreatic cancer and subsequent metastasis are key mortality events.

Next, Dr. Hoey described results of experiments to directly test the ability for OMP-54F28 to suppress cancer stem cells and metastasis. First, patient-derived pancreatic xenografts were surgically removed, and extracted cells were sorted for their expression of CD44, a marker of cancer stem cells. In vehicle-treated mice, 12.7% of the tumor cells showed expression of CD44. This phenomenon was sharply decreased when mice were treated with OMP-54F28, where 1.9% of the tumor cells stained for CD44. This was not necessarily due to bulk tumor killing either, since tumors from gemcitabine-treated mice showed 13.9% of extracted cells staining for CD44, suggesting traditional therapies do little to impact the population of cancer stem cells. Serial transplantation experiments confirmed that tumor cells from mice treated with OMP-54F28, when transplanted into new mice, were less tumorigenic. In fact, mice treated with the combination of gemcitabine and OMP-54F28 produced tumors that were completely incapable of making a new tumor. “Our interpretation of this is that blockade of Wnt signaling blocks cell survival pathways and sensitizes these tumorigenic cells to the cytotoxic effects of chemotherapy,” Dr. Hoey said.

Moving Down to Small Molecules

Data for one of the other Wnt signaling inhibitors currently in trials was presented by Shifeng Pan, PhD, from Novartis. This compound, LGK-974, is an orally bioavailable small molecule inhibitor targeting the enzyme porcupine. Porcupine is a critical acyltransferase in Wnt-secreting cells that facilitates the modification of the Wnt ligand. Acylation of Wnt allows for its subsequent secretion from the cell, and without porcupine, Wnt production is blocked. Dr. Pan’s data showed the development of this compound from the initial hit to the drug, and he showed its dramatic efficacy in both cell and mouse models of Wnt signaling-driven cancer. He mentioned that Novartis had screened over 300 cell lines for efficacy of this compound, and he showed data from two: head and neck squamous cell carcinoma and pancreatic cancer. LGK-974 was shown to inhibit anchorage-independent growth of these cell lines, suggesting it could be an effective off-switch for pathways that promote cancer cell survival in adverse conditions.

Conclusion

Each of the Wnt signaling presentations give tantalizing clues to the efficacy of targeting developmental pathways in cancer. The clinical trials are both shrouded in secrecy, and it is anybody’s guess as to whether efficacy and safety will be demonstrated in patients. In particular, there are serious concerns about severe gastrointestinal side effects, as highlighted in the question and answer section of Dr. Hoey’s presentation. Time will tell whether targeting the Wnt signaling pathway will fulfill the promise of all the preclinical work.