Exploratory high-throughput screening of repurposed drugs for canine lymphoid malignancies.

BACKGROUND: Lymphoid malignancies are common in dogs. However, the limitations of existing chemotherapy highlight the need for alternative therapies. Drug repositioning is a promising approach for discovering new therapies using existing drugs. In this study, we conducted high-throughput screening (HTS) of clinically used drugs to identify candidates with antiproliferative activity against canine lymphoid tumor cells in vitro. METHODS: A total of 1,824 compounds were screened at 5 µM through HTS using a water-soluble tetrazolium assay in three canine lymphoid tumor cell lines (GL-1, UL-1, CLBL-1) and one non-tumorigenic epithelial cell line (MDCK). Compounds that selectively inhibited tumor cells while sparing the MDCK cells were retained as primary screening candidates. Compounds unsuitable for drug repositioning for cancer treatment, such as anticancer agents or topical formulations, were excluded from the study. The remaining compounds were reviewed based on literature-derived pharmacodynamic or clinical evidence and pharmacokinetic data in dogs. Selected candidates were subjected to secondary screening in which dose-dependent antiproliferative effects were evaluated. Half-maximal inhibitory concentration (IC₅₀) values were determined and compared with reported maximum plasma concentrations (Cmax) in dogs to assess the potential for achieving pharmacologically active concentrations in vivo. RESULTS: Forty-five compounds were identified in a primary screening that showed tumor-selective inhibitory activity against lymphoid tumor cells. Based on the literature, five compounds (artesunate, niclosamide, pentamidine, itraconazole, and dronedarone) were selected for secondary screening. All the five compounds exhibited dose-dependent antiproliferative effects, and their IC₅₀ values were comparable to or below the reported Cmax in dogs. CONCLUSIONS: This exploratory screening study identified clinically approved drugs with available pharmacokinetic data as candidate therapeutic agents for the treatment of canine lymphoid malignancies. Based on this study, further studies are warranted to validate the in vivo efficacy and elucidate the underlying mechanisms of candidate drugs in canine lymphoid malignancies.