Veterinarians always welcome new cancer drugs to fight the No. 1 killer of dogs and other companion animals.

Soon, they might have a new weapon in their armamentarium. Clinical trials are underway in the United States for Paccal Vet, a paclitaxel (Taxol) analogue that could become the nation’s first government-approved drug for treating canine mast cell tumors.

The research is creating a buzz among oncologists in veterinary medicine.

“I want this drug," said Lisa Barber, an assistant professor of oncology at Tufts University’s Cummings School of Veterinary Medicine. "We were frustrated with the drugs that we had to treat cancers in dogs. All the stuff we use is for humans. It is important to have medications for dogs.”

Roughly a year ago, a Swedish pharmaceutical company contacted Barber to evaluate the potentially groundbreaking chemotherapeutic treatment in dogs in the United States. By November, a large-scale clinical trial for Paccal Vet was launched in multiple veterinary centers to treat dogs with high-grade mast cell tumors, a severe form of the most common type of skin cancer in dogs.

Two clinical trials completed in Europe for Sweden’s Oasmia Pharmaceutical’s product have shown encouraging results, insiders say. The most significant preliminary finding was the absence of severe allergic reactions, which have previously hampered the use of paclitaxel for this condition.

The U.S. Food and Drug Administration (FDA) reportedly has taken note, recognizing the possible medical breakthrough in treating cancers in animals. Last month, the agency's Center for Veterinary Medicine agreed to expedite its review of Paccal Vet, a move that could cut the government review time in half and speed up registration of the drug for market.

The FDA’s decision “will benefit our ongoing phase III study in the U.S. and EU a great deal and an earlier launch on the very important U.S. market,” Oasmia Chief Executive Officer Julian Aleksov said in a statement.

If there are no hitches, Paccal Vet could hit the market within two years, one expert said.

In the meantime, oncologists like Dr. Robert Rosenthal caution against the initial optimism and what appear to be stunning results.

“The results of the proposed study in the United States will be eagerly anticipated, but such anticipation should be tempered until results of that and the European work are available for more general critical review,” said Rosenthal, a Veterinary Information Network consultant. “That the adverse effects of the conventional carrier have been eliminated is good news, but it certainly remains to be seen to what extent any new formulation of paclitaxel is effective in the clinic.

“It is incumbent on all oncologists not to be too optimistic too early in a situation like this one.”

Still, researchers and veterinarians are encouraged by the international scope of a Paccal Vet review and the investment Oasmia is making.

“We have a company looking to put money into the veterinary oncology market and trying to come up with newer drugs that will help us,” said Dr. Craig Clifford, a veterinary oncologist and director of clinical research at Red Bank Veterinary Hospital in Tinton Falls, N.J. “It takes a lot of money to get something approved for a specific species. A lot of companies don’t want to go through with that.”

The U.S. study will tap a diversity of dogs from coast-to-coast. There are 12 private veterinary clinics in 10 states and Washington, D.C. and six universities participating in the trial. As of January, 40 dogs had been treated and the goal is to enroll up to 220 during the next six months, according to Animal Clinical Investigation, a Washington, D.C. group overseeing the study's implementation.

Study subjects can vary in age, breed and gender but must have higher-grade tumors and no prior radiation or chemotherapy treatment. Dog owners pay only for the initial consultation fee. The remaining expenses are covered by the drug company.

Results should determine the effectiveness and safety of Paccal Vet compared to lomustine (CeeNu), a standard chemotherapy treatment for high-grade mast cell tumors. Lomustine treatment has gained popularity during recent years because it is fairly inexpensive, easy to administer in pill form and the side effects are relatively small.

Dr. Tony Rusk, vice president of clinical affairs for Animal Clinical Investigation, said the veterinary oncology community is interested in having approved veterinary products in the oncology market. “We are extremely excited and motivated in studies like this. It may result in a veterinary product for cancer.”

That is good news for animals. Roughly half of all dogs 10 years or older die of cancer-related diseases. It is estimated that one in four dogs will develop some form of tumor during its lifetime. Certain breeds such as Beagles, Bulldogs and Labrador Retrievers have a higher risk of developing mast cell tumors.

Increasingly, dog owners are becoming more aware about pets with cancer and opting for treatment, Clifford said.

“People’s perceptions of pets have certainly changed in the last 20 years," he said. "Before the dog was the family dog. If the dog was sick you put the dog down and got another dog. Now the dog is a family member. We have more and more owners that are electing to go through a chemotherapy for their pets."

Surgery usually is a first-line treatment for mast cell tumors, especially low-grade forms, but it's generally considered ineffective at stopping more serious, high-grade tumors. Combining surgery and radiation therapy improves outcomes with high-grade mast cell tumors, but radiotherapy requires the dog to be anesthetized and can be expensive. Chemotherapeutic options for such tumors include human medications like lomustine.

Veterinary oncologists also have considered the anticancer drug paclitaxel (Taxol), a treatment for human breast and ovarian cancer, as a potential chemotherapeutic for mast cell tumors, but there are drawbacks. The drug is not water soluble and must be dissolved by the castor oil-type elixir, Cremophor, before it can be administered intravenously to patients.

However, Cremophor is known to cause severe side effects. To lessen the impact, cancer patients need to take medications in advance and go through a lengthy infusion period.

“Everybody wanted paclitaxel, but they didn’t want the Cremophor,” Barber said. “The drug is highly toxic to dogs. I don’t think paclitaxel is a fun drug for people, too.”

A 2004 study published in the Journal of Veterinary Internal Medicine found that 64 percent of dogs receiving paclitaxel experienced allergic reactions post treatment. Nearly a quarter required hospitalization, and more than one in 10 died of sepsis.

For Paccal Vet, Oasmia scientists developed a new water-soluble formulation that eliminates the need for Cremophor. The new treatment requires no pre-medication, dramatically shortens infusion time and eliminates serious side effects, according to company research.

Last month, Oasmia publicly detailed highlights from two clinical studies in Europe. The infusion times were cut to about 30 minutes in both trials.

In the first study dealing with dosage amounts and different types of cancers, 74 percent of the dogs responded to the drug. The phase III study, focusing on dogs with serious mast cell tumors, showed a 70-percent response rate and a median of 235 days in which the disease remained stable after treatment. There were no unexpected, severe side effects, the company claims.

“A response rate of 74 percent is absolutely stunning,” Barber said. “We may very well find this drug may have other implications in treating other kinds of cancers.”