inhibition.) On the other hand, aspirin forms a covalent bond to a serine residue in the enzyme, and this bond cannot be broken. (This is called irreversible inhibition.) Acetaminophen does not interact with either COX-1 or COX-2, but it is now understood that it may interact with a newly identified cyclooxygenase, COX-3. Selective targeting of the COX enzymes is an exciting area of pharmacology, in which the challenge continues to be the development of drugs that interact with specific COX enzymes.