The antitumor mechanism of oclacitinib in canine lymphoma.

Janus kinase (JAK) is a tyrosine kinase that binds to cytokine receptors and regulates transcription through signal transducer and activator of transcription (STAT) phosphorylation. Oclacitinib is a JAK1 inhibitor that is approved for the treatment of canine atopic dermatitis because it inhibits IL-31 signaling. Recent studies suggest clinical remission in dogs with cutaneous lymphoma treated with oclacitinib; however, the underlying mechanisms remain unclear. Its effects on high-grade canine lymphomas beyond cutaneous lymphoma are unknown. In this study, we examined the antitumor mechanism of oclacitinib in canine cutaneous lymphoma and determined its effects on high-grade canine lymphoma cell lines in vitro. EO-1 (cutaneous lymphoma) and eight high-grade lymphoma cell lines were treated with oclacitinib. Growth inhibition, cell death, G0/G1 phase cell cycle arrest, and apoptosis based on caspase-3 activation were observed in five sensitive cell lines. JAK1 and STAT5 phosphorylation was detected in these cells and decreased upon treatment, suggesting a dependence on JAK1/STAT5 signaling. In canine cutaneous lymphoma tissues, JAK1/STAT5 phosphorylation correlated with clinical sensitivity to oclacitinib. Similar phosphorylation profiles were observed in some multicentric and gastrointestinal lymphomas, suggesting broader applicability. Taken together, the results indicate that oclacitinib exerts antitumor effects by inhibiting JAK1/STAT5 signaling. The phosphorylation of these proteins may serve as a biomarker for predicting response to this inhibitor in canine lymphoma.