In vitro activity: Naproxen is approximately equipotent inhibitor of COX-1 and COX-2 in intact cells with IC50 of 2.2 μg/mL and 1.3 μg/mL, respectively. Naproxen decreases the in vitro LPS-induced PGE2 and TXB2 production in rats and humans with IC50 of 30.7 μM and 79.5 μM for PGE2 inhibition, 72.4 μM and 48.3 μM for TXB2 inhibition, respectively. Naproxen produces concentration-related inhibition of TXB2 production from human platelets and LPS-induced TXB2 production from human mononuclear cells with plC50 values (-log concentration inhibiting TXB2 by 50%) of 5.7 and 6.4, respectively, and exhibits slightly inhibitory selectivity for constitutive and induced COX-2 with IC50 COX-1/IC50 COX-2 of 6.3. Only high concentration of Naproxen can significantly induce apoptosis at 48 hours in HCA-7 colon cancer cells with IC50 of 1.45 mM.

Kinase Assay: For the determination of COX-1 and COX-2 inhibition, bovine aortic endothelial cells (BAEC) are incubated for 30 minutes with Naproxen (0.1 ng/mL to 1 mg/mL), and cultured J774.2 macrophages are treated with endotoxin at 1 μg/mL for 12 hours to induce COX-2 followed by incubated for 30 minutes with Naproxen (0.1 ng/mL to 1 mg/mL), respectively. Arachidonic acid (30 μM) is then added, and the cells are incubated for a further 15 minutes at 37 °C. The medium is then removed, and radioimmunoassay is used to measure the formation of 6-keto-PGF1α, PGE2, thromboxane B2, or PGF2α for the assessment of IC50 for COX-1 and COX-2. 

Cell Assay: Cells (Human colon cancer HCA-7 cell lines) are exposed to Naproxen for 24 and 48 hours, respectively. At the end of incubation, cells are harvested by trypsinization, stained with trypan blue solution (0.04% wt/vol) and counted in a Neubauer haemocytometer chamber for the determination of cell viability.