Stimulation of the immune system is a current treatment method used to recognize and target tumor cells. One key cytokine, interleukin-2 (IL-2), is known for these positive effects in treating human carcinoma and melanoma. The growth factor effects of IL-2 elicit proliferation of cytotoxic T cells, T-helper cells and other killer cells. To avoid adverse systemic reactions, viral vectors like canarypox virus are being used as a delivery system. Local application via injection of canarypox virus (Oncept IL-2 from Merial in Europe) into a tumor has been used for successful resolution of feline fibrosarcoma, which has similarities to equine sarcoids.
A German prospective study has evaluated the use of Oncept IL-2 canarypox virus in healthy horses [Loschelder-Ostrowski, J.; Winter, J.C.; Merle, R., et al. Treatment of equine sarcoids using recombinant poxviruses expressing feline intrleukin-2. Veterinary Dermatology 2021; doi:10.1111/vde.12941].
Initially, the researchers performed a safety study on four horses aged 5–25 years with no clinical or dermatological abnormalities. Each horse was injected with Oncept IL-2 twice, at a one-week interval. As a control area, the manufacturer’s solvent was injected at two separate sites subcutaneously on the neck. Biopsies were taken prior to treatment and five days after the second injections. Complete blood counts and fibrinogen testing was done on seven occasions over 12 days.
Once these trials ensured safety, a clinical trial was then performed on 20 client-owned horses with sarcoids smaller than 10 x 10 cm. Up to three sarcoids were treated per horse with intra-tumoral injection under sedation on Day 1 and Day 7. A total of 38 sarcoids received treatment in the study. (It was noted that there were no control horses included in the study.) Complete blood counts and fibrinogen were assessed as in the safety study. Lesions were measured before and the day after the second treatment. Owners continued to perform weekly measurements and photographs through eight weeks after the last treatment, and then monthly for 22-40 months.
The location of a sarcoid was not a significant factor in results, although legs had the greatest reduction in size compared to the head. Nodular lesions responded far less than verrucous lesions, which reduced at least 50% in size. Horses treated with intra-tumoral injection into sarcoids experienced no significant local adverse effects, demonstrating the safety of this product in horses. In addition, Oncept IL-2 is a drug that is safe for human handling. Currently it is only licensed as a veterinary product in Europe.
The authors report the results: “A reduction in tumor size or complete resolution of the tumor after treatment with Oncept IL-2 occurred in 50% of the injected and 68% of all sarcoids in the present clinical study.” Full regression occurred within 30 +/- 5 weeks. It is possible that more than two treatments could result in improved efficacy— more studies are necessary to establish the appropriate treatment regimen.