On Monday, the US Food and Drug Administration (FDA) approved Stelfonta (tigilanol tiglate injection) to treat nonmetastatic, skin-based mast cell tumors (MCTs) in dogs. The FDA also approved Stelfonta to treat nonmetastatic subcutaneous MCTs in particular areas of dogs’ legs.

Stelfonta is delivered via an intratumoral injection (directly into the MCT) and works by activating a protein that spreads throughout the tumor and disintegrates the cells.

“This is the first approval for an intratumoral injection to treat nonmetastatic mast cell tumors in dogs,” said Steven M. Solomon, DVM, MPH, director of the FDA’s Center for Veterinary Medicine. “This approval provides an additional treatment option to help treat local mast cell tumors on or under the skin in dogs.”

MCTs, the most common malignant skin tumor in dogs, account for approximately 20% of all canine skin tumors, and usually present as a lump on or under the skin.

A study of 118 dogs with a measurable cutaneous or a subcutaneous MCT on the lower leg established the safety and efficacy of Stelfonta: 80 dogs were treated with Stelfonta and 38 were assigned to an untreated control group. Tumor response was assessed nearly a month after the first injection. For dogs treated with Stelfonta, the complete remission rate was 75%.

Eighteen dogs in the Stelfonta group whose treated tumor didn’t completely disappear were re-treated with Stelfonta a second time about four weeks after the first treatment. Approximately one month after receiving their second treatment, 44% of those dogs’ tumors completely disappeared. A month after the study began, dogs in the untreated control group were treated with Stelfonta for the first time and 62% had their tumor vanish.

Some of the reported side effects: “Stelfonta may cause extensive wound formation at the tumor site, including cellulitis and severe tissue sloughing. Most wounds gradually reduced in size or completely resolved.” Other adverse reactions included pain, swelling, bruising, scarring, and lameness in the treated leg.

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