Cyclopentenones containing a 4-(methylsulfonyl) phenyl group in the 3-position and a phenyl ring in the 2-position are selective inhibitors of cyclooxygenase-2 (COX-2). The selectivity for COX-2 over COX-1 is dramatically improved by substituting the 2-phenyl group with halogens in the meta position or by replacing the phenyl ring with a 2-or 3-pyridyl ring. Thus the 3, 5-difluorophenyl derivative 7 (L-776,967) and the 3-pyridyl derivative 13 (L- ...