Cancer drugs can be tested on pet dogs with tumours

Pet dogs may be humans' best friends in a new arena - cancer treatment, according scientists who say that the canines can serve as useful models for studying new cancer drugs.
Pet dogs with naturally occurring (rather than laboratory-induced) tumours can be used in early cancer drug trials, said Timothy Fan, veterinary clinical medicine professor University of Illinois at Urbana-Champaign.
Physiological similarities between dogs and humans, and conserved genetics between some dog and human cancers, can allow pet dogs to serve as useful models for studying new cancer drugs, he said.
"Dogs tend to develop cancer as a geriatric population, just like people. Because the tumours develop spontaneously, there is heterogeneity in that tumour population, as a human being would have," Fan said.

"The size of the tumours and the speed of growth of those tumours are comparable in dogs and human beings. So there are many attributes of a dog that develops cancer spontaneously that recapitulate the biology that we see in people," Fan said.
Some studies have already begun using dogs to test new cancer therapies. Starting in 2007, for example, Fan began testing an anti-cancer drug called PAC-1 in pet dogs with naturally occurring lymphomas and osteosarcomas.

The results in dogs allowed the scientists to advance PAC-1 as a potential therapy against human cancers. The drug is now in phase I human clinical trials in Chicago.

Other investigational therapeutics historically piloted in pet dogs with cancer include muramyl tripeptide, an immune-stimulating agent that could not be tested in immune-deficient mice or rats with induced cancers, Fan said.
"Because you're taking a human cancer tissue and implanting it in a mouse, that's a foreign tissue, and the mouse's immune system will reject it," he said.
"So you have to transplant those tissues into an immunocompromised mouse. Dogs are immunocompetent, and so were an ideal study subject for testing immunomodulatory cancer therapies.
"Another example in which dogs have been important in demonstrating drug activity was an anti-cancer compound produced by the pharmaceutical company Gilead Sciences," he said.

"The company produced a pro-drug, which must be activated by a naturally occurring enzyme in human leukocytes before it can become effective. Mice and rats lack this enzyme, but dogs have it, so the compound was tested in dogs," he said.
There also are limitations to the use of pet dogs in cancer drug trials.
"There are some tumours that will not be that relevant," Fan said.
"Colon cancer, for example, is heavily driven by diet, and we don't see much colon cancer in dogs. So pet dogs might not be a suitable model for colon cancer in humans," he said.