Nonsteroidal antiinflammatory drugs inhibiting prostanoid efflux: As easy as ABC? Timothy D. Warner*† and Jane A. Mitchell‡ *The William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, United Kingdom; and ‡National Heart and Lung Institute, Imperial College School of Medicine, Dovehouse Street, London SW3 6LY, United Kingdom onsteroidal antiinflammatory may explain its antinociceptive proper- resistance proteins (MRP) form a sub- drugs (NSAIDs) and prosta- ties (7). Others have found therapeutic family within the ABC transporters. noids continue to be fascinat- effects of NSAID metabolites that are MRP1 is a high-affinity leukotriene C4 N ing research targets. Humans inactive on COX; sulindac sulfone is a transporter, and mice that harbor dele- have been using NSAIDs in one form or clear example. The NSAID sulindac is tions of this gene have an altered re- another, from folk remedies through to an inactive drug metabolized to a phar- sponse to inflammatory stimuli but are the products of modern pharmaceutical macologically active sulfide derivative, otherwise healthy and fertile. MRP2 is research, for thousands of years. How- which potently inhibits COX. Sulindac is the major transporter responsible for ever, we still have not characterized all also metabolized to a sulfone derivative the secretion of bilirubin glucuronides of the systems through which these that is inactive against COX. However, into bile (11). MRP1 and -4 may also be drugs produce their beneficial and like sulindac sulfide, sulindac sulfone involved in the transport of dehydroepi- harmful effects (1, 2). We are certain, both promotes cellular apoptosis and androsterone 3-sulfate (the most abun- from seminal work carried out in the dant circulating steroid in humans), and 1960s and early 1970s, that NSAIDs MRP4 transports conjugated steroids have the common property of inhibiting Some NSAIDs and bile acids (12). Through the use of the activity of cyclooxygenase (COX), model systems such as inside-out vesi- the enzyme that initiates the formation could inhibit the active cles, overexpressing cells, and small in- of prostanoids (1). An explosion of re- terfering RNA knock-down, Reid et al. search in the last decade or so has led transport of (9) show that MRP4 has particular af- us to understand that COX exists in two finity for the transport of prostaglandin forms, COX-1 and -2, and that drugs prostanoids from their (PG)E1 and PGE2 and also displays a targeted against COX-2 still retain the high affinity for PGF1␣, PGF2␣, PGA1, antiinflammatory properties of tradi- generating cells. and thromboxane B2. tional NSAIDs (1, 2). Because the tradi- So might it be that MRP4 is generally tional NSAIDs generally inhibit both the transporter for prostanoids? Because isoforms of COX, it is the ability of inhibits carcinogenesis. This activity sug- prostanoids are produced by almost all NSAIDs to inhibit COX-1 that has be- gests that the chemopreventive proper- tissues, we should expect to see MRP4 come associated with their deleterious ties of sulindac are not necessarily asso- expressed widely. Fortunately, this ap- side effects, particularly within the gas- ciated with its ability to inhibit COX pears to be the case, for, in addition trointestinal tract. However, this dualist (8). So it appears clear that, although to its appearance in cancerous tissues approach, COX-1 ϭ good prostanoids they share the ability to inhibit COX (11), MRP4 has been detected in intesti- and COX-2 ϭ bad prostanoids, does not enzymes, the NSAIDs, as befits a chem- nal (13), neuronal (14), prostate (15), appear to supply a full understanding of ically disparate group of drugs, also in- and renal (16) tissues and within blood the effects of NSAIDs in vivo. For that fluence other systems. vessels (17). Of course, it may well be reason, researchers have continued to In this issue of PNAS, Reid et al. (9) significant that MRP4 is highly ex- look for other targets at which NSAIDs present evidence that some NSAIDs pressed in kidney and prostate, because may exert effects. For example, NSAIDs could inhibit the active transport of pro- they are associated with the production have been shown to inhibit factors stanoids from their generating cells. and clearance of high levels of prosta- within inflammatory pathways, such as This is an interesting idea. After synthe- noids. [Prostanoids, of course, are nuclear factor ␬-B (3), and to elevate sis, how do prostanoids leave cells? It named after the prostate gland, being the production of antiinflammatory would appear unlikely to be via simple first characterized as smooth muscle adenosine (4). Other researchers have diffusion, because prostanoids are contracting activity within seminal suggested that there could be yet more largely water-soluble and cross mem- fluid (18).] COX isoforms on which the NSAIDs branes and tight cell barriers poorly, So what do these findings suggest? act (see ref. 2). Experiments with chiral clearly suggesting an active process to Clearly that there are particular pro- NSAIDs have also shown that enanti- export the prostaglandins. In recent cesses regulating the release of prosta- omers inactive on COX duplicate some years, the ATP-binding cassette (ABC) noids from cells, and that these pro- of the therapeutic effects of NSAIDs. transporter family of proteins has at- cesses could be targets for the NSAIDs. For example, whereas for ketoprofen tracted considerable interest. The hu- Maybe that is not too surprising. We and flurbiprofen only the S- and not the man genome appears to contain 48 know that NSAIDs have a common R-enantiomers inhibit COX (5) at in- ABC genes, 16 of which have known ability to bind to the active site of COX flammatory sites and promote gastroin- functions and 14 of which are associated (19), a site for which arachidonic acid is testinal damage, both enantiomers are with a defined human disease (10). the natural substrate. NSAIDs may also antinociceptive (6). Interestingly, al- ABC transporters move a variety of sub- though R-flurbiprofen is not a COX stances, including lipids, bile salts, and See companion article on page 9244. inhibitor, it does reduce prostaglandin toxic compounds, through membranes †To whom correspondence should be addressed. E-mail: production within the spinal cord, which while consuming ATP. The multidrug [email protected]. 9108–9110 ͉ PNAS ͉ August 5, 2003 ͉ vol. 100 ͉ no. 16 www.pnas.org͞cgi͞doi͞10.1073͞pnas.1733826100 Downloaded by guest on September 29, 2021 COMMENTARY Table 1. Some proposed actions of NSAIDs NSAID Action Target Chemical class Example Inhibit prostanoid production COX All All Inhibit prostanoid release MRP4 Arylpropionic acids Flurbiprofen, ibuprofen, indoprofen, ketoprofen Indole͞indene acetic acids Indomethacin Inhibit COX expression Nuclear factor ␬-B and Diarylheterocycles Rofecoxib related factors; activator Indole͞indene acetic acids Sulindac protein 1 Salicylic acid derivatives Aspirin, sodium salicylate, sulfasalazine Stimulate nuclear receptors PPAR-␥ Anthranilic acids Flufenamic acid Arylpropionic acids Ibuprofen, fenoprofen Indole͞indene acetic acids Indomethacin Promote apoptosis Mitogen-activated Indole͞indene acetic acids Indomethacin protein kinase cMYC, nuclear factor ␬-B, p38 Indole͞indene acetic acids Sulindac Elevate adenosine Increased intracellular Salicylic acid derivatives Aspirin, sodium salicylate breakdown of ATP Inhibit neutrophil adhesiveness Erk Salicylic acid derivatives Aspirin, sodium salicylate Inhibit drug transport Renal organic anion Arylpropionic acids Ibuprofen, flurbiprofen, ketoprofen transporter Indole͞indene acetic acids Indomethacin, naproxen Salicylic acid derivatives Diflunisal Promote neuroprotection Blockade of N-methyl-D- Salicylic acid derivatives Sulphasalazine aspartate receptors NSAIDs are chemically disparate, and it is clear that certain functions are possessed only by certain chemical groups. However, all NSAIDs have the common ability to inhibit COX. This information is not exhaustive and does not discriminate among activities found only at high and supratherapeutic drug concentrations. bind to other similar sites (20) and so the heteroaryl acetic acid family, of MRP4, they might also increase the could well associate with MRP4. How- whereas celecoxib and rofecoxib are dia- effectiveness of drugs transported out ever, because we know that different rylheterocycles. So these data may point of cells by this protein. These drugs NSAIDs bind to the active sites of toward structure–activity relationships might include anticancer agents, pro- COX-1 and -2 to differing extents and that could be exploited to find even viding us with another potential mech- indeed have little effect against lipoxy- more potent inhibitors of MRP4. anism through which NSAIDs could genase, which also uses arachidonic acid The data of Reid et al. (9) may also inhibit the development of cancers. In as a substrate (21), we should expect offer some explanations of the other addition, NSAIDs could increase the that different NSAIDs will bind differ- puzzling properties and actions of effectiveness of anti-HIV drugs, be- ently to MRP4. This is indeed what NSAIDs, including those touched on cause MRP4 is associated with resis- Reid et al. (9) report. Their data show above. For example, it has been recog- tance to nucleoside analogues such as that diclofenac is rather inactive, failing nized for some time that studies with azidothymidine (24). Interestingly, ke-