University of California San Francisco researchers may have discovered why the experimental anti-cancer drug Onyx-015 works more broadly than had been expected, a finding that could solidify and expand use of the drug and which reveals a potential new target for therapy that could prompt the development of other cancer drugs.
Onyx-015 is a genetically modified version of a cold virus that was designed to home in on cancer cells, specifically those in which the p53 tumor-suppressor gene - a key sentinel of cell health -- does not function. P53 is mutated and does not function in 60 percent of cancers. The theory has been that, without the p53 defense, Onyx-015 could replicate in cancer cells.
However, researchers have observed in clinical trials and cell culture studies that Onyx-015 also works against tumors in which the p53 tumor-suppressor gene appears to remain intact. This has created heated debate in the scientific community about the drug's efficacy and mechanism of action.
Now, the UCSF researchers believe they have solved the conundrum: In the October issue of Nature Medicine, they report that while some tumors responding to Onyx-015 have mutations in the p53 gene itself, others have a defect in another gene, known as p14 ARF, which is located "upstream" in the p53 pathway and which indirectly regulates p53 function. They showed that loss of p14 ARF leads to the deregulation of another protein, Mdm2, which in turn inhibits p53.
As a result, p53 cannot shut down the engineered virus's attempt to take over the cellular machinery, replicate incessantly and eventually kill the cell -- action that, counterintuitive as it may be, is just what researchers want to happen when it comes to cancer cells.
The finding "solves a big puzzle about the drug's mechanism of action," says a co-author of the study and the drug's creator, Frank McCormick, PhD, director of the UCSF Comprehensive Cancer Center, and co-founder of Onyx Pharmaceutical Inc., which develops the drug. "Inactivating p14ARF is just another way of turning off p53."
The finding also suggests the possibility of new applications for Onyx-015, says the senior author of the study, Michael Korn, MD, UCSF assistant adjunct professor of medicine. "Our work supports the idea of exploring the use of Onyx-015 in tumors that we presumed would be immune to the drug because they were thought to frequently have p53 genes, such as such as melanoma and glioblastoma [a form of brain cancer]. Clinical trials have to show if the p14 ARF and p53 status could serve as a reliable predictor of response to treatment with Onyx-015."
The finding could also lead researchers to consider how to design other genetically engineered viruses that take the p14 ARF mechanism into consideration, says Korn.
Moreover, he says, the discovery suggests that p14 ARF could potentially become a therapeutic target in itself. A drug that could reactivate p14 ARF would reactivate p53, thereby reinvigorating the cell's defenses.
To date, researchers have focused clinical trials of Onyx-015 on cancer types that previous studies have shown often have inactivated p53, though the drug has been administered randomly. The new findings, says Korn, will encourage the researchers to clarify the status of 53 and p14ARF in the individual tumors targeted.
In a recent multi-institutional phase II trial of Onyx-015 led by investigators at University of Texas M.D. Anderson Cancer Center, 30 patients with advanced head and neck cancer, most of whom had failed surgery or radiation therapy, were treated. The trial was designed to determine the appropriate dosage for the drug. In 63 percent of the patients, the tumors shrank by 50 percent or more, and in eight of these 19 responding patients the tumors disappeared altogether. (Nature Medicine, August 2000). The patients were treated with a combination of Onyx-015 and chemotherapy.
Because researchers later determined that the response to the combination therapy occurred both in tumors in which p53 was mutated and in which it was in tact, the finding left open the possibility that chemotherapy was the critical factor in the shrinking of tumors. However, says Korn, the new finding raises the possibility that the tumors in which p53 remained intact might have been defective in p14 ARF, and were, therefore, responding to Onyx-015. Based on this hypothesis, he says, "It will be important to assess the effect of Onyx-015 without chemotherapy, to see if the drug has the same potent effect."
According to Onyx Pharmaceuticals, researchers participating in ongoing phase II trials of Onyx-015 in different cancer types, including colorectal and pancreatic cancer, have observed a response to therapy in some patients. As is the case with all experimental cancer drugs, Onyx-015 still must be evaluated in a phase III trial, a much larger trial designed to test the efficacy of a drug in a large population. A phase III, multi-institutional clinical trial of Onyx-015 for head and neck cancer has begun.
In another study, Onyx Pharmaceuticals will evaluate the p53 status of tumors in patients who are participating in a phase II clinical trial in which Onyx-015 will be administered intravenously. To date, the drug has been administered in different ways: by direct injection into the tumor by installation into the peritoneal cavity in patients with ovarian cancer, and by injection into the artery that supports the liver in patients with gastrointestinal cancer metastatic to the liver. Onyx-015 is an example of the new wave of cancer therapies emerging based on researchers' growing understanding of the molecular mechanisms of cancer and viruses. Researchers know that p53 plays the crucial role of halting cell division when it detects that a cell's DNA has been damaged, as occurs in cancer or when a virus enters a cell. In many cases, loss of p53 function is one of the first of several steps leading to the disruption of a cell's regulatory processes.
McCormick conceived the drug based on this knowledge of p53 - and the fact that when the adenovirus cold virus, a relatively harmless irritant, enters a cell, its first trick is to dismantle the p53 tumor suppressor gene, preventing it from signaling the cell to self-destruct in the face of an invader. Once the normal cold virus has taken over the cell's molecular machinery, it begins replicating, ultimately killing the cell. The adenovirus's victory leads to nothing more than sneezing and congestion.
But McCormick realized that if he disabled the protein within the cold virus that sabotages p53, the virus would not be able to replicate in normal cells, because p53 would remain intact and thwart the virus's efforts. The virus would, however, be able to grow in cancer cells in which the p53 gene was already disabled. In this way, the crippled adenovirus would discriminate between cancer cells and normal cells - a Holy Grail of cancer therapy.
In their effort to elucidate how Onyx-015 works in cells that appear to have normal p53 genes, the researchers examined, in culture, colorectal cancer cells that respond to Onyx-015 even though their p53 gene appears to remain in tact. They observed that when p14 ARF was mutated, p53 did not function, even though the p53 gene itself was not damaged. But when14 ARF was reintroduced in cells, p53 began functioning again and Onyx-015 was no longer able to replicate in the tumor cells. The explanation, they demonstrated, is that the loss of p14 ARF leads to the deregulation of another protein, Mdm2, which in turn inhibits p53. In this way, p53 is prevented from inducing the protective effects that would otherwise shut down the engineered virus.
To confirm their findings, the researchers will examine tumor samples from cancer patients who have intact p53 genes but respond to Onyx-015, to see whether the p14 ARF pathway is, in fact, defective in these cells, says the lead author of the study, Stefan Ries, PhD, a postdoctoral fellow in McCormick's lab.
In addition, researchers in McCormick's lab are attempting to create a mouse version of the Onyx-015 virus. If they succeed, they will then "knock out" either the p14ARF gene or the p53 gene in mouse tumors to see how the drug fares.
"We're definitely making progress in understanding Onyx-015," says Ries. "This was a very nice discovery."
Co-authors of the study are Christian H. Brandts, MD, a postdoctoral fellow, Alicia Chung, a staff research associate, Carola H. Biederer, PhD, a postdoctoral fellow, and Ettie M. Lipner, staff research associate, all in the McCormick laboratory, and Bryon C. Hann, MD, PhD, a specialist in the laboratory of Alan Balmain, PhD, UCSF professor of biochemistry.
The ongoing research is funded by Onyx Pharmaceuticals, Inc. (NASDAQ: Onxx). McCormick remains a paid scientific advisor to the company, and is a stockholder in the company. Korn is a paid clinical advisor to the company.
Materials provided by University Of California, San Francisco. Note: Content may be edited for style and length.
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