Expression of erythropoietin in cats treated with a recombinant adeno-associated viral vector

Objective - To characterize the biological effects of IM administration of a recombinant adeno-associated virus serotype 2 (rAAV2) vector containing feline erythropoietin (fEPO) cDNA and determine whether readministration of the vector or removal of muscle tissue at the injection sites alters those effects. Animals - 10 healthy 7-week-old specific pathogen-free cats. Procedure - Cats received 1 × 10⁷ infective units (iU; n = 3), 1 × 10⁸ iU (3), or 1 × 10⁹ iU (2) of rAAV2-fEPO vector IM (day 0). Two control cats received an rAAV2 vector containing the LacZ gene (1 × 10⁹ iU, IM). In all cats, hematologic variables and serum fEPO concentration were measured at intervals; anti-rAAV2 antibody titer was measured on day 227. In cats that did not respond to treatment, the rAAV2-fEPO vector was readministered. Injection sites were subsequently surgically removed. Results - Compared with control cats, cats treated with 1 × 10⁹ iU of rAAV2-fEPO vector had increased Hct and serum fEPO concentrations. One of these cats developed pure BBC aplasia; its Hct normalized following injection site excision. Cats receiving lower doses of vector had no response; on retreatment, 1 of those cats developed sustained erythrocytosis that persisted despite injection site removal and the others did not respond or responded transiently. Antibodies against rAAV2 were detected in all vector-treated cats. Conclusions and clinical relevance - Gene therapy may be an effective treatment for cats with hypoproliferative anemia. However, rAAV2-fEPO vector administration may result in pure BBC aplasia or pathologic erythrocytosis, and injection site removal does not consistently abolish the biological response.