Masitinib Mesylate

What is masitinib mesylate?

Masitinib mesylate (Masivet®) is a prescription drug used in dogs to treat inoperable mast cell tumors that are classified as grade 2 or 3. It is a targeted treatment designed to shrink the size of the tumor and slow the progression of the cancer.

It may be prescribed for off-label use in cats for the treatment of mast cell tumors and other cancerous masses. It has also been used off-label in dogs to treat other tumors and conditions such as melanoma, soft-tissue sarcoma, osteosarcoma, lymphoma, inflammatory bowel disease, and atopic dermatitis.

Masivet® is available as a veterinary prescription drug in the United Kingdom and Europe. The Masivet® label states that the mast cell tumor being treated must express the mutated c-KIT tyrosine kinase receptor, and your veterinarian must confirm its presence before treatment.

Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s direction and cautions carefully, as they may be significantly different from those on the label.

How do I give my pet masitinib mesylate?

• Masitinib mesylate is given orally (by mouth).
• This drug comes in tablet form.
• Tablets must be administered whole and not divided, broken, or ground.
• Give this drug with food.
• If vomiting occurs after dosing your pet, and less than 10 minutes have passed since the tablet was administered, repeat the treatment. If more than 10 minutes have passed, do not repeat the treatment.
• Follow the instructions on the product label, in conjunction with your veterinarian’s directions.

This medication should take effect within 2 to 3 hours; however, effects may not be noted outwardly at the start of treatment and it may take 2 to 3 weeks until improvement in clinical signs is noted.

Masitinib mesylate is not classified as a hazardous drug, but you should use caution when giving this drug to your pet.

• People with known hypersensitivity to masitinib mesylate should not handle the product.
• Children should not come into contact with the drug and should be kept away from feces, urine, litter, and vomit of treated dogs.
• Do not allow treated animals to lick humans for a few days after the last dose.
• To avoid exposure to this drug, wash your hands with soap and water after administering and wear protective gloves to prevent contact with feces, urine, vomit, and broken or crushed tablets.
• Place all waste material in a plastic bag and seal before disposing in the regular trash.
• If you accidentally expose your eyes to the drug, immediately rinse eyes with water.
• If you accidentally ingest this drug, immediately seek medical advice and show the package insert or label to the physician.
• If you are nursing, pregnant, or may become pregnant, pay special attention to these handling precautions because masitinib mesylate may harm a nursing baby or an unborn baby (cause birth defects).

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects from masitinib mesylate?

Masitinib mesylate may cause the following side effects:

• Mild diarrhea
• Mild vomiting
• Loss of appetite
• Mild lethargy
• Hair loss

Serious adverse reactions may occur. If any of the following signs develop, stop giving masitinib mesylate and contact your veterinarian immediately:

• Prolonged vomiting and/or diarrhea
• Bloody stools
• Prolonged inappetence and weight loss
• Changes in thirst or urination
• Extreme lethargy
• Development of a cough
• Enlargement of lymph nodes
• Generalized tissue swelling
• Fever

Drug sensitivities can develop over time, with repeated exposure. Your pet may have no reaction after the first few doses but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of the treatment and notify your veterinarian if you have any concerns.

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. This medication is short acting and should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE masitinib mesylate in:

• Pets that are allergic to it
• Pets that are pregnant
• Nursing puppies
• Pets that are less than 6 months old or that weigh less than 4 kg
• Pets that have inadequate liver or kidney function
• Pets with anemia (low red cell count) or low white blood cell count
• Pets with low protein levels in the bloodstream
• Pets with protein loss in the urine

USE WITH CAUTION in:

• Pets with moderate liver or kidney dysfunction
• Pets using nonsteroidal anti-inflammatory drugs (NSAIDs)

Are there any drug interactions I should be aware of?

The following drugs should be used WITH CAUTION when given with masitinib mesylate:

• Azole antifungals
• Benzodiazepines,
• Beta blockers
• Calcium channel blockers
• Cisapride
• Cyclosporine
• Doxorubicin
• Macrolide antibiotics
• NSAIDs
• SSRIs
• Tricyclic antidepressants
• Warfarin

Masitinib mesylate may have reduced effectiveness in dogs previously treated with chemotherapy and/or radiation therapy. Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment. 

Is any monitoring needed with this medication?

• Pets on long-term treatment should be examined by their veterinarian at least once monthly to assess general body condition, response to treatment, and the development of any side effects.
• Blood tests should be performed regularly to assess red blood cell and white blood cell numbers, as well as kidney and liver blood marker levels.
• Urine tests should be performed regularly to assess protein loss in the urine.
• Follow your veterinarian’s recommendations regarding the frequency of monitoring your pet’s health and laboratory testing.
• At home, monitor for any side effects such as those noted above. If any side effects occur, stop using the medication and contact your veterinarian.

How do I store masitinib mesylate?

• Ensure masitinib mesylate is tightly stored in its original container.
• Store this medication at room temperature of 68°F–77°F (20°C–25°C) in a cool, dry place, protected from light.
• Keep out of reach from pets and children.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet’s weight and symptoms.