8-O-acetylharpagide, an active compound isolated from A. taiwanensis selectively induced G2/M phase arrest and radiosensitivity in hypopharyngeal cancer cells

Hypopharyngeal cancer (HPC) is a rare but most malignant subtype of head and neck squamous cell carcinoma (HNSCC). Conventional chemo-radiotherapy is the primary approach for the treatment of HPC, while the efficacy is limited and complications. Previously, we have isolated an active compound (AT-1) identified as 8-O-acetylharpagide from <i>Ajuga taiwanensis</i>. Because Ajuga extracts are both edible and known for their anti-cancer properties, it is worthwhile to investigate whether AT-1 can inhibit cancers of otolaryngological origin. In this study, we compared the cell killing effects of AT-1 on FaDu HPC cells and periodontal ligament <i>(</i>PDL) cells. The results showed that AT-1 was more toxic on FaDu cells (IC₅₀ = 0.88 mM) than on PDL cells (IC₅₀ = 1.65 mM), yielding a selectivity index (SI) ≈ 1.85. AT-1 caused significant G2/M phase arrest in FaDu cells with a dose-dependent manner. Concomitantly, high concentration of AT-1 could induce necrosis-like fraction and late apoptosis in FaDu cells. Notably, AT-1 did not influence the cell cycle redistribution and cell death in PDL cells under the same condition of treatment. Furthermore, AT-1 enhanced the radiosensitivity of FaDu cells rather than PDL cells, suggesting that AT-1 induced G2/M phase arrest remains sensitive to ionizing radiation known as a principle of radiobiology. Taken together, current data indicate that AT-1 raises selectively efficacy on HPC cells by increasing G2/M phase arrest, apoptosis, as well as radiosensitivity.