New insights into the mechanism of action of acetaminophen: Its clinical pharmacologic characteristics reflect its inhibition of the two prostaglandin H2 synthases

Abstract

Acetaminophen (INN, paracetamol) possesses highly selective analgesic and antipyretic effects that result from its inhibitory actions on the synthesis of prostaglandins (PGs). PGs are lipid mediators derived from arachidonic acid that play central roles in the pathogenesis of inflammation, fever, and pain.1 However, acetaminophen differs from the majority of nonsteroidal anti-inflammatory drugs (NSAIDs) and selective inhibitors of prostaglandin H2 synthase (PGHS) 2 because it lacks significant antiinflammatory activity.2 Moreover, as opposed to aspirin, acetaminophen is a poor inhibitor of platelet function at doses that are antipyretic.3,4 PGs are generated by the oxygenation of arachidonic acid to the unstable intermediate prostaglandin H2 (PGH2) by PGHS, of which there are 2 major isoforms—the constitutive PGHS-1 and the (generally) inducible PGHS-2 (discussed later). These enzymes are also commonly referred to as cyclooxygenase (COX) 1 and 2, respectively, in reference to the specific enzymatic active site that catalyzes arachidonic acid oxygenation and provides the target for the majority of pharmacologic inhibitors of these enzymes (discussed later). However, in this commentary we use the term PGHS to describe the enzymes and limit the use of COX to refer specifically to the PGHS cyclooxygenase active site and POX to refer to the PGHS peroxidase active site (as discussed in the "Catalytic mechanisms of PGHS" section). Acetaminophen is an inhibitor of both PGHS isoforms in purified enzyme preparations.6 However, it exhibits a highly variable capacity to inhibit PG synthesis by different cell and tissue types. For example, the analgesic and antipyretic effects of acetaminophen follow its inhibition of prostaglandin E2 (PGE2) generation within the central nervous system (CNS),7,8 whereas the failure of acetaminophen to inhibit platelet-derived thromboxane A2 synthesis9 and inflammatory PGE2 synthesis accords with its weak antiplatelet and anti-inflammatory effects. Recent investigations of the mechanism of action of acetaminophen on the PGHSs have elucidated the basis for this cellular selectivity. This commentary brings together past investigations and recent insights regarding the nature of acetaminophen-PGHS interactions and provides a model that best explains the selective clinical behavior of acetaminophen. In the latter half of the 19th century, phenacetin and acetaminophen, structurally related aniline dye derivatives, were developed as antipyretic agents.11,12 The safety profile of phenacetin was mistakenly favored over acetaminophen, leading to the popular clinical use of the former drug for nearly 80 years.13 However, in the late 1940s Brodie and Axelrod14 elegantly demonstrated that phenacetin's therapeutic efficacy was a result of its major hepatic metabolite, acetaminophen. These seminal experiments published in the Journal of Pharmacology and Experimental Therapeutics were among the first to demonstrate the pharmacologic activity of a drug metabolite formed in vivo and resulted in the rapid popularization of acetaminophen for clinical use. Within 2 decades, acetaminophen was shown to reduce prostaglandin E production in vivo,15 inciting researchers to investigate the molecular basis for such effects. PGHSs catalyze the oxygenation of arachidonic acid to yield PGH2. This reaction occurs in 2 steps catalyzed by distinct active sites within the same enzyme.16 The hydrogen atom on carbon 13 of arachidonic acid is abstracted by the tyrosine 385 radical present in the COX active site. The resulting arachidonyl radical incorporates 2 molecules of oxygen to yield the hydroperoxyendoperoxide intermediate prostaglandin G2 (PGG2). PGG2 then undergoes a 2-electron reduction to the equivalent alcohol, PGH2, in the POX catalytic site. The reduction of hydroperoxides in the POX site yields an oxidized heme radical, which generates the tyrosine 385 radical in the COX site via intramolecular electron transfer. As stated previously, initiation of COX-catalyzed arachidonic acid oxygenation requires that the tyrosine 385 residue within the active site must first be oxidized to a radical (Tyr•) (Fig 1). This priming event is made possible by the initial reduction of an available hydroperoxide substrate by the POX site, generating a ferryl protoporphyrin IX radical cation [Fe(IV)PP•+] (Fig 1, step 1). The POX site can reduce a broad range of hydroperoxides with variable efficacy: hydroperoxides of fatty acids are preferential substrates, whereas hydrogen peroxide is a very weak substrate.16 The Fe(IV)PP•+ radical can then be transferred to the neighboring Tyr 385 in the COX active site, yielding the requisite Tyr• and a partially reduced form of the heme [Fe(IV)] (Fig 1, step 2), which must be further reduced by 1 electron to regenerate the resting enzyme [Fe(III)] (Fig 1, step 6). This heme reduction is performed by any number of reducing cosubstrates within cells that have not been characterized. Using phenol,