NOVEL CANCER IMMUNOTHERAPY FOR TREATMENT OF EQUINE GENITAL SQUAMOUS CELL CARCINOMA

Abstract

Squamous cell carcinoma (SCC) is the most common genital malignancy in horses, with most thought to be induced by infection with the equine papillomavirus (EPV), particularly type 2 EPV. The tumor is typically locally invasive in an anatomically challenging location, and is therefore often refractory to treatment, including topical 5 fluorouracil or mitomycin-C, laser ablation, cryosurgery, or surgical procedures. Thus, there is a large need for effective new treatments for this cancer in horses. We have previously shown that local injections of lipid nanoparticles delivering Toll-like receptor agonists (LTNs) were highly effective for local immunotherapy of corneolimbal SCC in horses. Therefore, in the present study, we used in vitro assays to assess leukocyte and tumor cell immune responses to LTNs and conducted an open label clinical trial to evaluate the efficacy and safety of local LTN injections in n = 17 male horses with genital SCC. We found that LTN induced strong innate immune activation, dominated by interferon pathways, in both leukocytes and SCC cell lines. In the 90-day clinical trial, horses received a series of three intratumoral LTN injections, at monthly intervals. A high overall tumor response rate (65%) was observed, with two complete responses and nine partial tumor responses. Injections were well-tolerated, with few adverse events other than transient redness at the injection site. Immunohistochemical analysis of tumor biopsies revealed a consistent increase in infiltrating CD3 T cells 30 days after the first injection, and cytokine and transcriptome analysis showed upregulated interferon pathways. We conclude therefore that LTN immunotherapy has considerable promise as a new immunotherapy option for management of genital SCC in horses.