Intratumoral COX‐2 inhibition enhances GM‐CSF immunotherapy against established mouse GL261 brain tumors

Immunotherapy has shown effectiveness against experimental malignant brain tumors, but the clinical results have been less convincing most likely due to immunosuppression. Prostaglandin E 2 (PGE 2 ) is the key immunosuppressive product of cyclooxygenase‐2 (COX‐2) and increased levels of PGE 2 and COX‐2 have been shown in several tumor types, including brain tumors. In the current study, we report enhanced cure rate of mice with established mouse GL261 brain tumors when immunized with granulocyte macrophage‐colony stimulating factor (GM‐CSF) secreting tumor cells and simultaneously treated with the selective COX‐2 inhibitors parecoxib systemically (5 mg/kg/day; 69% cure rate) or valdecoxib intratumorally (5.3 µg/kg/day; 63% cure rate). Both combined therapies induced a systemic antitumor response of proliferating CD4 + and CD8 + T cells, and further analysis revealed T helper 1 (T h 1) cell supremacy. The GL261 tumor cell line produced low levels of PGE 2 in vitro, and co‐staining at the tumor site demonstrated that a large fraction of the COX‐2 + cells were derived from CD45 + immune cells and more specifically macrophages (F4/80 + ), indicating that tumor‐infiltrating immune cells constitute the primary source of COX‐2 and PGE 2 in this model. We conclude that intratumoral COX‐2 inhibition potentiates GM‐CSF immunotherapy against established brain tumors at substantially lower doses than systemic administration. These findings underscore the central role of targeting COX‐2 during immunotherapy and implicate intratumoral COX‐2 as the primary target.