Antitumor activity of protein A administered intravenously to pet cats with leukemia or lymphosarcoma.

Extracorporeal immunoadsorption with protein A (SpA) containing Staphylococcus aureus Cowan I (SAC) has previously been shown to induce an antitumor and antiviral response in some feline leukemia virus (FeLV)-infected, lymphosarcoma (LSA) cats. However, the mechanism by which this response is induced is unknown. Since it is possible that SpA dissociates from the SAC column during treatment, and that intravenous infusion of SpA could be a more efficacious form of treatment than extracorporeal immunoadsorption therapy, 6 normal cats and 15 FeLV-infected pet cats with naturally occurring leukemia or LSA were infused with SpA. No toxic effects resulting from SpA infusion were observed in the normal cats. Antitumor effects were observed in one of the eight cats with LSA and in three of the six cats with leukemia, but there were no antiviral responses. The antitumor responses were objective regressions; however, all tumors recurred. These results demonstrate that infusion of SpA into FeLV-infected LSA or leukemic cats can result in an antitumor but apparently not in an antiviral response.