Tolfenamic acid- 13 C 6 is the 13 C 6 labeled Tolfenamic acid. Tolfenamic Acid (GEA 6414) is a non-steroidal anti-inflammatory and anti-cancer agent, selectively inhibits COX-2 , with an IC50 of 13.49 μM (3.53 μg/mL) in LPS-treated ( COX-2 ) canine DH82 monocyte/macrophage cells, but shows no effect on COX-1.