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U.S. Department of Health and Human Services

Animal & Veterinary

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FDA Approves Drug To Treat Cancer in Dogs

by Melanie McLean, D.V.M., Communications Staff
FDA Veterinarian Newsletter 2008 Volume XXIII, No VI

Being told your dog has skin cancer is always scary. However, if it’s a type of skin cancer called a cutaneous mast cell tumor (MCT), it may not be so scary anymore.

In May 2009, the Food and Drug Administration approved PALLADIA (toceranib phosphate) to treat canine cutaneous MCTs. PALLADIA, manufactured by Pharmacia & Upjohn Company, a Division of Pfizer, Inc., New York, NY, is the first drug approved specifically for the treatment of cancer in dogs.

MCTs are the most common skin cancer in dogs, accounting for about 20 percent of the cases of canine skin tumors. MCTs can occur anywhere on the dog’s body, and they have no “typical” appearance.

Many cancers, including MCTs, are classified by a histologic grade based on the degree of differentiation of the cancer cells. The term differentiation means how much the cancer cells resemble or differ from the normal cells of the same tissue type. Cancer cells that closely resemble the normal cells are called well differentiated. Poorly differentiated or undifferentiated cancer cells may have a primitive or bizarre appearance. A well differentiated cancer typically behaves less aggressively than a poorly differentiated one; that is, it grows more slowly and is less likely to be invasive or metastasize (spread to other parts of the body).

The three-tiered Patnaik system is used to grade MCTs. Grade I (low grade) MCTs are well differentiated, Grade II (intermediate grade) are moderately differentiated, and Grade III (high grade) are poorly differentiated or undifferentiated. It is important to know the grade of the MCT when discussing a dog’s prognosis. A dog with a Grade I or II MCT has a better prognosis for long-term survival than a dog with a Grade III MCT. The best way to determine the grade of a MCT is with a biopsy.

Many cancers, including MCTs, are also classified by a clinical stage based on the degree of metastasis. There are three components to staging: the size of the primary tumor, the spread to regional lymph nodes, and the presence or absence of metastases (secondary tumors in other parts of the body). There are five stages (Stage 0 to Stage IV) of the generally recognized clinical staging system for MCTs. In most cases, the higher the stage, the poorer the prognosis. MCTs can spread to the liver, spleen, bone marrow, and blood, as well as other sites in the body. Usually, a MCT spreads first to the draining (regional) lymph node. Evaluation of the regional lymph nodes is the most important part in determining the clinical stage of a MCT.

Surgery, radiation therapy, and chemotherapy are available treatment options for MCTs in dogs. Surgery is the treatment of choice. Radiation therapy and chemotherapy are commonly performed after surgery, if necessary. Until the recent approval of PALLADIA, all chemotherapeutic drugs used to treat MCTs in dogs were developed for use in humans and prescribed by veterinarians in an extralabel (“off label”) manner.

PALLADIA is approved for the treatment of Patnaik Grade II or III, recurrent, cutaneous MCTs with or without regional lymph node involvement in dogs. It is a tyrosine kinase inhibitor and works by killing the cancer cells and cutting off their blood supply.