Any pet owner who has been told their animal has cancer knows the two emotions: anxiety for the beloved pet’s life, and hope for an effective treatment.

“Many people consider their dogs and cats members of the family,” says Food and Drug Administration veterinarian Lisa Troutman. “Just as the FDA reviews drugs for humans for safety and effectiveness before they can go on the market, the agency does the same for treatments for animals.”

Take, for instance, cancer, which accounts for almost half of the deaths of pets over 10 years of age. Although pets of any age can have cancer, the longer they live, the greater the likelihood of developing it. Dogs get cancer at roughly the same rate as humans.

“Pets are living longer because of preventative health care. And we’re able to diagnose cancers earlier. As a result there is an increased need for better cancer treatments,” Troutman notes.

Until very recently, the only drugs available to treat cancer in animals were those approved for use in humans. But in the last few years, veterinary drug sponsors (the pharmaceutical companies developing the drugs) have brought to market treatments meant specifically for animals.

Troutman explains that “The FDA works closely with these companies to discuss how they can demonstrate that their innovative veterinary drugs are safe and effective, and to address questions that arise while working toward approval of their drug.”

The warning signs of cancer in dogs are similar to those in people, Troutman says: a lump or bump, a wound that doesn’t heal, any kind of swelling, abnormal bleeding. But generally, a pet owner should keep an eye out for what Troutman calls “the basics —changes in the normal functions of eating, drinking, peeing, pooping and sleeping —and contact their veterinarian if they have concerns.

“Emotional state, such as being withdrawn and irritable, can be another sign,” she says.

Both general veterinary practitioners and veterinary oncologists, as well as other specialists, treat cancer in cats and dogs. In general, veterinary practitioners work with veterinary oncologists to provide the diagnosis and the follow-up care for the pet during treatment, which may include blood work and imaging (such as x-rays or ultrasound examinations) to monitor the animal’s progress.

There’s a fundamental difference between treating cancer in pets versus people. “Side effects from cancer treatment are usually fewer than those seen in people, and veterinarians work very hard to manage those side effects and maintain quality of life,” Troutman says. “There are even drugs that have been brought to market with the intent of managing common side effects, like vomiting or lack of appetite.”

Troutman says that sponsors are continuing to develop innovative treatments for different types of cancer in dogs.

“We’re looking at therapies that are more targeted now,” she says. Scientists are identifying proteins or other substances unique to cancer cells and developing treatments that target those substances without harming healthy cells.

Currently, three drugs are approved and another drug is conditionally approved to treat cancer in dogs:

• Palladia (toceranib phosphate), to treat mast cell tumors, was approved in 2009;
• Stelfonta (tigilanol tiglate injection), to treat mast cell tumors, was approved in 2020;
• Tanovea-CA1 (rabacfosadine for injection), to treat lymphoma, conditionally approved in 2016 and fully approved in 2021; and
• Laverdia-CA1 (verdinexor tablets), to treat lymphoma, conditionally approved in 2021

To date, there are no FDA-approved treatments for cancer in cats. Most cancer treatments for dogs and cats use drugs that the FDA has approved for use in humans.

Questions that pet owners may want to ask their veterinarian and veterinary oncologist when their pet has been diagnosed with cancer include:

• What treatments are available?
• What is the prognosis with each treatment?
• What are the side effects of each treatment and how will they affect my pet’s quality of life?
• How long will I need to treat my pet?
• Will this treatment cure my pet's cancer? Will it make him more comfortable?
• What is the cost of each treatment?
• How many visits back to the veterinarian are needed?

To evaluate the safety of any new veterinary drug, companies typically conduct a study in a small number of healthy animals in the same species that the drug is intended for (for example, if the drug is for dogs, it will be tried first in healthy dogs). The findings help the veterinarian anticipate potential side effects when the drug is used to treat a patient and help minimize adverse events that might affect the pet’s quality of life.

Companies also must show in controlled studies that the drug works—that it is effective when used according to the label. For example, for a drug intended for a particular kind of cancer, companies typically run a clinical trial at multiple animal hospitals where pets are being treated for that cancer. In these studies, the patients may receive either the drug being studied or a control. Although the owners and veterinarians are aware that their pets and patients could receive either the experimental drug or the control — a placebo — they don’t know which treatment they actually get. In either case, owners have the option to drop out of a study at any time.

When the goal is to treat a form of cancer that affects smaller numbers of animals, drug companies can use a pathway called conditional approval to bring drug treatments to market more quickly. Conditional approval allows a company to make its drug available to patients after proving the drug fully meets the FDA standard for safety, and showing that there is a reasonable expectation that the drug is effective for the intended purpose.

“Often small exploratory studies are conducted to support a reasonable expectation of effectiveness,” Troutman says.

Conditional approvals have both pros and cons. On the plus side, they allow veterinary drug sponsors to provide patients quicker access to innovative treatments without waiting for the development of "substantial evidence" of effectiveness that would satisfy the requirement for a full approval.

“On the other hand, because the studies used to support a reasonable expectation of effectiveness are small, the drugs may not turn out to be effective when they are used in greater numbers of animals,” Troutman says.

The FDA may allow, through annual renewals, the conditionally-approved products to stay on the market for up to five years while the company collects the remaining effectiveness data required to support a new animal drug application for full approval. Conditional approval automatically expires at the end of five years and the drug is removed from the market if the company has not fully demonstrated that the drug is effective.