When drug repurposing backfires: antagonistic effects of ivermectin on cisplatin therapy in oral cancer

Cisplatin (CIS) remains the gold standard chemotherapeutic agent for the management of oral cancer. However, its clinical efficacy is frequently limited by chemoresistance and dose-dependent toxicities. Ivermectin (IVM), a well-established antiparasitic agent with emerging anticancer properties, has been proposed as a potential adjuvant capable of enhancing the antitumor efficacy of CIS. In vitro experiments were conducted on HN9 oral squamous cell carcinoma cells, while OEC normal epithelial cells served as control. The experimental evaluation comprised the MTT assay for cell viability, Annexin V/propidium iodide (PI) staining for apoptosis detection, cell cycle distribution analysis, quantitative real-time PCR (RT-qPCR) for the assessment of apoptotic, anti-apoptotic, and inflammatory gene expression, and oxidative stress assay. Contrary to expectations, the CIS–IVM combination demonstrated an antagonistic interaction in HN9 cells. This was evidenced by reduced cytotoxicity of the combination treatment compared to CIS monotherapy, as determined by the MTT assay. Additionally, the combination attenuated CIS-induced S-phase arrest and was associated with decreased expression of pro-apoptotic markers (BAX and Caspase-3). Differential modulation of TNF-α expression and glutathione levels further suggested complex interactions affecting inflammatory and oxidative stress pathways. These findings indicate that IVM may interfere with CIS-induced cytotoxicity, potentially through its antioxidant and anti-inflammatory properties. The present study demonstrated an unexpected antagonistic interaction between CIS and IVM in HN9 oral cancer cells. Further investigations are warranted to elucidate the underlying molecular mechanisms and to determine whether similar effects occur in other oral cancer models or in vivo settings. Reduced cytotoxicity of the cisplatin-ivermectin combination compared to cisplatin alone in HN9 oral cancer cells. Attenuation of S-phase cell cycle arrest when ivermectin is combined with cisplatin. The combination treatment showed diminished expression of apoptotic markers (BAX and Caspase-3) compared to cisplatin alone. Differential effects on TNF-α expression and glutathione levels between single-drug and combination treatments. Ivermectin appears to attenuate the cytotoxic, pro-apoptotic, and cell cycle arresting effects of cisplatin in the oral cancer cell line.