Gene therapy to treat melanoma in dogs

Since the year 1996 CONICET researchers of the Gene Transfer Unit (UTG) at the ‘Ángel H. Roffo’ Institute of Oncology of the University of Buenos Aires work on the development of a combined therapy to combat a special type of cancer in dogs.

“Canine mucosal melanoma is the most aggressive melanoma. Currently, the only available treatment is surgery and yet half of the patients die after three months”, Gerardo Glikin, CONICET independent researcher and co-director of the Unit, explains.

In general, at the time of the diagnosis, the cancer has already spread to other parts of the body and the probabilities of survival are low because this tumor is infiltrating- that is to say it penetrates the surrounding tissue – and it quickly progresses, even tough most of it was removed after the surgery.

The treatment designed by the UTG team includes three steps: first, the surgical procedure to reduce the size of the tumor; then, a local injection with a ‘suicidal’ gene; and finally, vaccines with tumor extracts designed to stimulate the immune response against this cancer in the long term.

“In the first instance, the surgery allows reducing the number of malignant cells because this type of cancer tends to regrowth”, Liliana Finocchiaro, CONICET independent researcher and co-director of the Unit, says. “After this procedure, the survival rate of 50% of the treated patients is over 500 days rather than 90 days, which is expected for untreated patients” she adds.

This local treatment consists of an injection that has a ‘suicidal’ gene that incorporates the tumor cell nuclei. This gene allows the insertion into de DNA of antiviral drugs such as ganciclovir or acyclovir and act as a chain terminator.

“When the DNA polymerase enzyme –in charge of DNA duplication – encounters a molecule of an antiviral, it cannot go on copying the genetic material. This triggers the cell death process with release of free radicals and death factors to the environment and thus the effect is transmitted to the surrounding tumor cells”, Finocchiaro illustrates.

The researcher explains that, in broad terms, the cell recognizes the complex as if it was an invading virus, and activates its immune response. Furthermore, this effect is transmitted to the surrounding cells. To summon, to treat a few cells with the suicidal gene triggers a cascade effect that amplifies the impact.

“With the genetic modification of only one per cent of the cells – depending on the case – it is possible to eliminate up to 90% of the tumor”, Glikin adds.

The third stage was designed to activate the antitumor immunity in the medium and long term. For this reason, a vaccine that contains extracts of the tumor was developed, and it has to be applied in an area distant from the place where the cancer primary located.

“This vaccine also contains two proteins that mediate the inflammatory response, the Interleukin-2 and the Granulocyte-Macrophage colony-stimulating Factor . This allows stimulating the immune response in distant places where there is no tolerance to the tumor. Then, the lymphocytes – that through this vaccine learnt to identify tumor cells- go to the affected area and remain there preventing the proliferation of these cells”, Finocchiaro comments.

As part of the treatment the patient has to receive this vaccine every six months during his lifetime. The results surprise: survival rates increase between five to six times, from three months to a year and a half, up to eight years if we consider death from melanoma. Nevertheless, long term effects last and more than 50 per cent of old dogs die from diseases not related to the tumor.

“The rate of treated patients free of metastases at the moment of their death is of 84%, while this happens only in the case of 51% of the not treated patients. This combined treatment allows chronifying the disease, thus patients end up dying from other factors, in general associated with age”, Glikin concludes.