EXPRESS: Suspected Chronic Eosinophilic Leukemia in a Cat Treated with Toceranib

Case summary A 16-year-old spayed female Domestic Shorthair cat was referred to the xxx for evaluation of chronic, progressive eosinophilia accompanied by vomiting and weight loss despite normal appetite. Blood smear evaluation revealed immature eosinophils with dysplastic features and occasional giant forms. Abdominal ultrasound identified small-intestinal thickening, a hepatopathy, and splenomegaly. Screening for FeLV, FIV and heartworm infection were negative. Chronic eosinophilic leukemia was suspected, although hypereosinophilic syndrome could not be ruled out. At the time of referral, the cat was receiving prednisolone and chlorambucil but showed persistent clinical signs and progressive eosinophilia for suspected small cell gastrointestinal lymphoma. Chlorambucil was discontinued and toceranib was initiated alongside prednisolone. No adverse effects attributable to toceranib were observed. A partial response, defined as resolution of clinical signs and a 30% reduction in circulating eosinophils, was observed by week four. A subsequent progression free survival interval of four weeks was achieved. Progressive disease developed at week nine, and given declining quality of life, euthanasia was thereby elected at week ten. Relevance and Novel Information This case report is the first to describe the use of toceranib to treat suspected chronic eosinophilic leukemia, a rare and poorly documented disease process in cats. This novel therapeutic approach is the first publication of a partial response and stable disease in the treatment of suspected chronic eosinophilic leukemia in a cat.