Reoviruses as Cancer Therapeutics

Traditional cytotoxic cancer therapies can in some instances cause serious side effects that include nausea, hair loss, and damage to major organ systems. Similarly, therapeutic radiation can cause systemic malaise, low blood counts, and irritation when mucus membranes fall within the radiation field.  The trade off has been that the benefits that patients receive from therapy exceeds the drawbacks of the side effects of treatment. 

The most significant advances in cancer therapy over the last 30 years have been the so-called targeted therapies. Herceptin, a monoclonal antibody, is directed at breast cells expressing the Her2-neu receptor, and the anti-angiogenesis agent Avastin targets blood vessels that feed tumors. Herceptin is also part of another exciting trend in cancer treatment, combination therapies, which combine an anti-cancer agent with a biomarker test for identifying patients most likely to respond to treatment.

Dozens of development-stage cancer drugs target extremely narrow, well-defined cellular pathways thought to be unique or essential to cancer, for example phosphorylation through tumor-specific kinases. Because they act primarily on cancer-promoting mechanisms, these agents are expected to show favorable therapeutic results.

Similarly, genomics and proteomics research labs have identified hundreds of genes that confer susceptibility to specific types of cancer treatment (existing or yet-to-be discovered), or resistance to the most serious side effects. The challenge will be to discover and promote molecules that work safely within these patient populations, while providing concrete survival benefit. There is every reason to believe that within 20 years targeted cancer therapies will be the norm, rather than the exception, for most types of cancer.

So far, the model for targeted therapies is to use them along with traditional cytotoxic agents and/or radiation, to improve the likelihood of an objective response. This strategy combines broad-based and targeted treatments, the old and the new models for treating cancer, to maximum benefit.

Viruses As Anti-Cancer Agents – A Brief History
Oncolytic viruses bridge that gap between established and emerging treatment regimes. Oncolytic viruses are targeted agents whose replication within tumors causes cancer cells to die. The promise of oncolytic viral therapy is the development of agents that act broadly against a range of cancer types, while simultaneously sparing patients from the most serious side effects of traditional therapy.

Medical case studies over the past century have described spontaneous cancer remissions in late-stage disease following contraction of a known, or in some cases unknown, viral infection. In these cases, viruses are believed to have acted as agents that somehow preferentially select tumor cells for their growth and replication, and as a consequence kill their host cell. Understanding the mechanisms through which viruses achieve their cancer-killing abilities has been largely unknown until the last decade or so. The recent accumulation of the results of basic and applied research on cancer and viruses provides the scientific rationale for expecting certain viruses to infect cancer cells preferentially over healthy cells. At this juncture, extensive evidence supports the idea that the virus-cancer connection might be exploited to treat tumors.

The groundbreaking product in this category was ONYX-015, a modified adenovirus that normally infects the upper respiratory tract. This agent selectively replicates within, and kills cancer cells that lack a functioning p53 tumor suppressor gene. More than half of all cancers express this maladaptive alteration. ONYX-015 carries a “loss-of-function” mutation, meaning that unlike wild-type adenoviruses, it lacks a protein that binds to and inactivates p53 in cancer cells. Wild-type adenoviruses disable this gene before viral replication can occur. Thus, the ONYX-015 adenovirus infects only cancer cells expressing the p53 abnormality, and leaves normal cells unaffected. These cells are susceptible to the virus, which infects them and kills the cells through viral replication.

ONYX-015 was ultimately not pursued for approval in North America but it is now licensed for sale in China. Nevertheless, other adenovirus constructs are in various stages of clinical testing, and experience suggests that researchers are on the right track. In 2000, Canadian researchers reported some success in preclinical testing of vesicular stomatitis virus injected directly into mouse tumors. And last year, a group at Memorial Sloan Kettering Cancer Center in New York demonstrated the oncolytic potential of weakened herpes viruses, which in unmodified form cause cold sores and a host of other human ailments.

The Case For Reoviruses
Reoviruses are double-stranded RNA viruses that replicate exclusively in the cytoplasm, which means they do not integrate into a cell’s DNA. Among the reoviruses are two pathogenic virus families: rotavirus, which infects the intestinal tracts of children, and “orphan” viruses that infect the respiratory tract but cause no apparent disease.

Oncolytics’ oncolytic virus product, REOLYSIN®, was based on research conducted in the 1990s at the Department of Microbiology and Infectious Diseases at the University of Calgary. REOLYSIN falls into the “orphan” category of non-pathogenic viruses. REOLYSIN replicates only in tumor cells possessing an activated pathway for the Ras oncogenes, which play a role in more than two thirds of human cancers.

REOLYSIN represents a departure from most previously studied oncolytic viruses. First, the virus formulated as REOLYSIN is not associated with any human disease, a fact that greatly expands opportunities to test and use it in humans. Second, the virus is unmodified in any way, naturally infecting cancer cells in its native form. The reovirus particles also enter normal cells but cannot replicate in them.

We have demonstrated in cell culture that REOLYSIN infects and replicates within about two-thirds of known cancer cell types, reflecting the tumors’ expression of the Ras protein. In this respect, it behaves very much like broad-spectrum chemotherapy. However, its specificity for cancer cells suggests that it is unlikely to cause serious side effects, even at very high doses.

In a clinical setting, patients receiving 100 billion virus particles intravenously usually experience mild side effects. The worst cases develop moderate flu-like symptoms which are generally relieved with over-the-counter pain medications. In our experience, patients with terminal cancer tolerate the therapy quite well.

REOLYSIN is effective because tumors with an activated Ras pathway are unable to mount an anti-viral response to reovirus infection. Ras can be thought of as a switch that focuses a cell’s energy and destiny on reproduction, at the expense of other housekeeping functions such as recognizing and repelling viral infection through interferon and other pathways. In normal cells, this pathway is mediated by an RNA-activated protein kinase (PKR), which prevents reovirus replication. Cancer cells lacking PKR activity are susceptible to reovirus infection, and subsequent cell death. Since normal cells do not possess Ras activation, they are immune from reovirus infection. Within Ras-active tumor cells, the reovirus freely replicates and kills the host tumor cell, sending progeny virus particles off to infect and kill neighboring cancer cells. This cycle of infection, replication and cell death repeats until no cells carrying the activated Ras pathway are available.

Reoviruses use cancer cells as a type of bioreactor, producing thousands of copies of themselves within the cytoplasm. Electron microscopy of cancer cells infected with reovirus shows the cytoplasm to be full of virus particles, at which point the cell bursts and dies.

Production And Administration
The type 3 reovirus we use to produce REOLYSIN is manufactured in a similar fashion to viral vaccines. Cells susceptible to infection are inoculated with virus and cultured in suspension. The reovirus rapidly expands, kills the cells, and moves on to infect other cells. Viruses are harvested and isolated by removing the cellular debris followed by chromatography and terminal filtration to yield a preparation of pure virus particles. Frozen reovirus maintains potency in excess of four years. A lyophilized preparation under development is expected to remain stable and active for even longer periods. Manufacture of clinical batches follows current Good Manufacturing Practices.

Development And Clinical Trial Status
As a result of REOLYSIN’s potential broad-based activity, Oncolytics is pursuing a varied approach to its development, including local administration in combination with radiation, and systemic administration with or without chemotherapy. After completing ongoing studies, the company will select appropriate cancer targets for registration studies

Oncolytics has completed a number of clinical trials in Canada and the U.K., and is currently conducting additonal Phase I,  and Phase II trials in the U.S. and the U.K..  We expect REOLYSIN to enter additional Phase II trials over the next few months.

In one Canadian Phase I study of 18 terminal cancer patients who had failed to respond to conventional treatments, none of the patients receiving REOLYSIN experienced product-related adverse events or dose-limiting toxicities. In this trial, REOLYSIN was injected directly into subcutaneous tumors. Tumor regression was observed in 11 of 18 patients (61%) with response ranging from partial to complete (32% to 100%).

In a small, T2 prostate cancer trial, we observed evidence of tumor cell death in four of six patients, with a fifth showing prostate shrinkage of 67%, and a drop in prostate specific antigen levels of 53%.

Similar results were obtained through systemic administration. Phase I study data has demonstrated that intravenously administered reovirus is capable of anti-tumor activity in colorectal, prostate, non-small cell lung, and other tumors in terminal patients. Of 33 subjects receiving REOLYSIN , anti-tumor activity was demonstrated in seven.

Interim results of the company’s first radiation/ REOLYSIN cotherapy trial has demonstrated partial responses and/or stable disease in a majority of the target lesions, and that the product was well tolerated by the patients.

In June 2006, Oncolytics announced final results of a Canadian Phase I trial of REOLYSIN in recurrent malignant glioma. Twelve patients were treated in the study at varying dosages (a maximum tolerated dose was not reached). Three patients lived longer than one year, and one was still alive approximately five years post-treatment.

The Future   
In oncology, it is tempting to think in terms of an “ideal” therapeutic – one with broad spectrum activity but few if any side effects. Despite encouraging results for oncolytic viruses, these agents are still early in clinical development. Thus far, the clinical results suggest that REOLYSIN is well tolerated by terminally ill patients and may provide meaningful, relevant tumor regression. The eventual impact of oncolytic viruses on cancer treatment is a story that waits to be told, which is precisely the position in which monoclonal antibodies found themselves 15 years ago. Thanks to a number of pioneering companies and academic researchers, oncologists have bought into the concept of monoclonals, which have been validated numerous times commercially.

Oncolytic viruses have not yet reached that point, but we expect they will following larger Phase II and Phase III trials. Oncolytics is currently treating patients in seven clinical trials in the U.S. and the U.K.: a Phase II combination REOLYSIN and radiation trial, a Phase II sarcoma trial, three Phase I/II combination REOLYSIN and chemotherapy trials, a Phase I/II recurrent malignant glioma (brain cancer) trial, and a Phase Ib combination REOLYSIN and radiation trial.  In addition, the Company recently received approval to begin a combination REOLYSIN and cyclophosphamide trial in the U.K. The Company is also collaborating with the National Cancer Institute to conduct two human clinical trials using REOLYSIN; a Phase II study administered systemically in patients with melanoma and a Phase I/II study of REOLYSIN co-administered both systemically and intraperitoneally in patients with ovarian cancer.

New agents – and oncolytic viruses are no exception – will most likely enter clinical practice as adjunctive agents to conventional chemotherapy, radiation, or one or more targeted therapies. Although much remains to be learned about REOLYSIN and other oncolytic viruses, our clinical experience with these agents suggests that they are readily combined with standard cancer therapies to produce additive efficacy.  But we won’t have long to wait for those answers.  With a wide variety of clinical trials currently enrolling patients, we expect results to begin to emerge from Phase II trials with REOLYSIN as early as next year.

Contact:
Matt Coffey, Ph.D.
Chief Scientific Officer
Oncolytics Biotech
Calgary, Alberta
403-283-0711
Matt.Coffey@oncolytics.ca