PTGS2 and neoplasm: In fact, its IC50 towards the tumour-associated CAIX and CAXII enzymes was determined to be 16 and 18 nM, respectively (Di Fiore et al, 2006), and thus was more than double as potent than its inhibition of COX-2, where the IC50 is 40 nM (Penning et al, 1997).