Lung cancer is the main cause of cancer-related death worldwide. The high mortality is probably attributable to early metastasis; however, the mechanism underlying metastasis to regional lymph nodes is still unknown. Cyclooxygenase (COX)-derived prostaglandin E₂ (PGE₂) induces tumor growth and metastasis and is associated with a poor prognosis. The present study investigated the effect of an authentic COX inhibitor, aspirin, on regional lymph node metastasis during the development of lung cancer in mice.

An orthotopic intrapulmonary implantation model based on male C57BL/6 (6–8-weeks-old) mice was used. The lungs were injected with a solution containing Lewis lung carcinoma (LLC) cells overexpressing green fluorescent protein (GFP) and BD Matrigel^®. The effect of aspirin on mediastinal lymph node metastasis of LCC cells from the primary injection sites was then examined.

The implantation process took approximately 30 s per mouse and operative mortality was 10%. Single pulmonary nodules developed at the implanted site in 95% of animals, and regional mediastinal lymph node metastasis was observed at 14 days post-LLC-GFP cell injection in all mice that formed a primary lung tumor. The mean survival time of mice injected with LLC-GFP cells was 15±3 days (range, 12–22 days). Histopathological analysis revealed that no metastatic tumors developed in the regional mediastinal lymph nodes by Day 10–12 post-LLC-GFP cell injection and no metastasis to distant organs or distant lymph nodes was observed by Day 21 post-injection. Oral administration of aspirin (100mg/kg, twice a day) after LLC-GFP cell injection inhibited metastasis to the regional lymph nodes, with no significant suppression of primary tumor growth in the lungs. Aspirin treatment led to a significant reduction in mortality (P<0.0001).

The present lymph node metastasis model is useful for evaluating the efficacy of agents that inhibit tumor metastasis to the regional lymph nodes. Aspirin reduced the metastasis of LLC-GFP cells injection to the regional lymph nodes, with a significant reduction in mortality. These findings suggested that COX inhibitors have potential for preventing lymph node metastasis.