PTGS2 and rheumatoid arthritis: The specificity of [11C]MC1 binding to COX-2 was further confirmed by a blocking study with celecoxib, a preferential COX-2 inhibitor; it should be noted, however, that the blocking effect was minimal in one participant with rheumatoid arthritis, perhaps due to limited bioavailability of the drug in response to a single dose or to the variable gastric absorption rate [28].