Diclofenac: an update on its mechanism of action and safety profile

Abstract

Tong J. Gan
Duke University Medical Center, Durham,
Background:
North Carolina
Diclofenac is a proven, commonly prescribed nonsteroidal anti-inflammatory drug (NSAID) that has analgesic, anti-inflammatory, and antipyretic properties, and has been shown to be effective in treating a
Address for correspondence:
variety of acute and chronic pain and inflammatory conditions. As with all NSAIDs, diclofenac exerts its
Tong J. Gan, MD, Department of Anesthesiology,
action via inhibition of prostaglandin synthesis by inhibiting cyclooxygenase-1 (COX-1) and cyclooxygenase-
Duke University, Erwin Rd, Suite 5670B, Durham,
2 (COX-2) with relative equipotency. However, extensive research shows the pharmacologic activity of
NC 27710, USA.
diclofenac goes beyond COX inhibition, and includes multimodal and, in some instances, novel mechanisms
Tel.: þ1 919.681.4660; Fax: þ1 919.681.4698;
[email protected] of action (MOA).

Data sources:
Key words:
Literature retrieval was performed through PubMed/MEDLINE (through May 2009) using combinations of the
COX 1 and 2 – Diclofenac – Enzyme inhibition –
terms diclofenac, NSAID, mechanism of action, COX-1, COX-2, and pharmacology. Reference citations
Equipotency – Mechanism of action – NSAID –
Receptor inhibition resulting from publications identified in the literature search were reviewed when appropriate.

Accepted: 14 April 2010; published online: 17 May 2010
Citation: Curr Med Res Opin 2010; 26:1715–31 Methods:
This article reviews the established, putative, and emerging MOAs of diclofenac; compares the drug’s pharmacologic and pharmacodynamic properties with other NSAIDs to delineate its potentially unique qualities; hypothesizes why it has been chosen for further recent formulation enhancement; and evaluates the potential effect of its MOA characteristics on safety.

Discussion:
Research suggests diclofenac can inhibit the thromboxane-prostanoid receptor, affect arachidonic acid release and uptake, inhibit lipoxygenase enzymes, and activate the nitric oxide–cGMP antinociceptive pathway. Other novel MOAs may include the inhibition of substrate P, inhibition of peroxisome proliferator activated receptor gamma (PPARg), blockage of acid-sensing ion channels, alteration of interleukin-6 production, and inhibition of N-methyl-D-aspartate (NMDA) receptor hyperalgesia. The review was not designed to compare MOAs of diclofenac with other NSAIDs. Additionally, as the highlighted putative and emerging MOAs do not have clinical data to demonstrate that these models are correct, further research is necessary to ascertain if the proposed pathways will translate into clinical benefits. The diversity in diclofenac’s MOA may suggest the potential for a relatively more favorable profile compared with other NSAIDs.
Introduction
Pain is a universally experienced physiologic response to environmental stimuli. The sensation of pain is highly subjective and can vary from person to person and within the same person. Factors such as gender, ethnicity, genetics, and widely variable physiologic profiles contribute to this interindividual and intraindivi- dual variability1,2. The societal and economic effects of pain are substantial and have been linked to increased healthcare costs and lost income3,4.

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For example, pain is the most common reason for health care office visits, resulting in more than 425 million prescriptions for pain medications in the United States, a number that may increase as the average life expectancy rises5.
Despite intense research and an ever-growing knowl- edge of physiologic pain pathways, medications for treat- ing acute pain have evolved little in recent years, with perhaps the most recent notable addition being the cyclooxygenase-2 (COX-2) inhibitor class in 1999. Because new, chemically distinct analgesic entities are lacking, recent trends in the development of new anal- gesics have included (1) improving the delivery of estab- lished medications, (2) combining analgesics from different classes, (3) formulating analgesics with other medications that minimize the analgesic’s side effects (e.g., proton pump inhibitors with nonsteroidal anti- inflammatory drugs [NSAIDs]), or (4) capitalizing on the analgesic characteristics of medications originally developed for other uses (e.g., anticonvulsants and muscle relaxants). The net result has been an increase in the analgesic armamentarium available to physicians and patients established on a relatively small collection of available, traditional candidate analgesics. Despite the lack of new chemical entities, modifying existing analge- sics has provided valuable tools for supporting multi- modal approaches to pain management.
One analgesic that has experienced a recent resurgence of interest is the NSAID diclofenac. Like most NSAIDs, diclofenac possesses analgesic, anti-inflammatory, and antipyretic properties. Globally, diclofenac is the most pre- scribed NSAID5, and since its introduction to the US market in the 1990s, various diclofenac formulations have either been made commercially available or have undergone clinical investigation. Since its initial commer- cial introduction, diclofenac has been used by more than 1 billion patients and ranks as the eighth largest selling drug in the world6,7. Worldwide, diclofenac is commer- cially available in oral, intravenous, suppository, transder- mal patch, or gel formulations.
Several basic science, preclinical, and clinical studies suggest that diclofenac possesses pharmacologic and phar- macodynamic traits that differ from other NSAIDs; this growing amount of data suggests a wider range of physio- logic effects than previously thought. This review focuses on the established, putative, and emerging mechanisms of action (MOAs) for diclofenac. Comparisons with other NSAIDs were also made in an attempt to delineate those qualities of diclofenac that are potentially unique and may help explain why this NSAID is the most commonly used analgesic worldwide and the one chosen for further recent formulation enhancements. In addition, the potential effect of diclofenac’s MOA characteristics on safety was evaluated.

 

Methods
The literature review began with a search of the electronic database PubMed using various forms and combinations of the key words: diclofenac, mechanism of action, NSAID, COX-1, COX-2, and pharmacology. The literature search located more than 200 publications indexed through May 2009 including basic science, preclinical, and clinical stud- ies as well as several articles of historical interest. When appropriate, reference citations from publications identi- fied in the literature search were reviewed. Publications highlighted in this article were extracted based on rele- vancy to established, putative, and emerging MOAs for diclofenac.
Diclofenac
Diclofenac is a phenylacetic acid derivative (2-[2,6- dichloranilino]phenylacetic acid) that is available in oral formulations in sodium, potassium, or sodium/misoprostol forms. The sodium salt of diclofenac (Voltaren*) is an enteric-coated, slow-release tablet that was first intro- duced in Japan in 1974 and is indicated for osteoarthritis (OA), rheumatoid arthritis (RA), ankylosing spondylitis, and mild to moderate pain. This slow-release formulation of Voltaren resists dissolution in the low pH of the gastric fluid, but allows rapid release in the higher pH of the duo- denum. Voltaren is presently available in approximately 120 countries6. An extended-release formulation of the diclofenac sodium salt (Voltaren-XRy) was designed to release diclofenac over a prolonged period of time for chronic pain associated with OA and RA. In contrast, an immediate-release diclofenac potassium sugar-coated tablet (Cataflamz) was introduced in the early 1980s. That formulation was designed to release active drug in the stomach for rapid uptake. Cataflam is also approved for the treatment of acute and chronic signs and symptoms of OA, RA, and ankylosing spondylitis, and the management of pain and dysmenorrhea, when prompt relief is desired. Introduced in the early 1990s, Arthrotecx is a unique for- mulation of sodium diclofenac that contains misoprostol, a synthetic prostaglandin analog with gastric antisecretory and mucosal protective properties. Arthrotec is indicated for the relief of arthritis pain in those at high risk for devel- oping NSAID-induced ulcers and related complications. Arthrotec is a sequentially released combination tablet that contains an enteric-coated diclofenac core with a misoprostol outer shell.
Unlike many other NSAIDs, both sodium and potas- sium diclofenac formulations exhibit signs of variable

*Voltaren is a registered trade name of Novartis, East Hanover, NJ, USA. yVoltaren-XR is a registered trade name of Novartis, East Hanover, NJ, USA.
zCataflam is a registered trade name of Novartis, East Hanover, NJ, USA. xArthrotec is a registered trade name of Pfizer, New York, NY, USA.
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Table 1. Diclofenac formulations: characteristics and pharmacokinetic parameters.

Drug* Formulation Dose, mg Mean (% CV)
AUC, ng ti hr/mL Cmax, ng/mL Tmax, hr
Oral

Voltaren ti Sodium salt
ti Delayed-release (enteric-coated) tablets
50 1429 (38.4) 1417 (22.4) 2.22 (49.8)

Voltaren-XR
ti Sodium salt
ti Extended-release tablets
100
2079 (33.7)
417 (40.7)
5.25 (28.3)

Cataflam
ti Potassium salt
ti Immediate-release tablets
50
1309 (21.7)
1312 (44.1)
1.00 (74.6)

Arthrotec
ti Sodium/misoprostol salt
ti Delayed-release (enteric-coated) tablets
50
1380 (272)
1207 (364)
2.4 (1.0)

Zipsor Topical
ti Potassium salt
ti Liquid-filled soft gelatin capsule
25
597 (25.4)
1087 (38.6)
0.47 (35.4)

Voltaren Solaraze
ti Sodium salt gel ti Sodium salt gel
4000
2000
233 (128)
9 (19)
15 (7.3)
4 (5)
14 (0–24)y 4.5 (8)

Flector ti Epolamine salt patch 180 119.3 (75.7) 17.4 (13.5) 5.4 (3.7)
Pennsaid ti Sodium salt solution 80 drops 177.5 (72.6) 8.1 (5.9) 11.0 (6.4)
*Arthrotec, is a registered trade name of Pfizer, New York, NY, USA; Voltaren, Voltaren-XR, Cataflam, and Voltaren Gel are registered trade names of Novartis, East Hanover, NJ, USA; Zipsor is a registered trade name of Xanodyne Pharmaceuticals, Inc., Newport, KY, USA; Solaraze is a registered trade name of PharmaDerm, Melville, NY, USA; Flector is a registered trade name of King Pharmaceuticals, Bristol, TN, USA; and Pennsaid is a registered trade name of Nuvo Research, Varennes, Quebec, Canada.
yMedian (range).
CV, coefficient of variation; AUC, area under the curve; Cmax, maximum plasma concentration; tmax, time to peak concentration (Cmax).

 
interindividual and intraindividual absorption character- istics8,9. Diclofenac absorption may be influenced by gas- tric emptying rate and mechanical agitation in the
complications. Accordingly, there has been much interest in using topical NSAIDs as it is thought these formulations deliver drug directly to the site of action and may be asso-

stomach
10–13
. However, the small number of head-
ciated with reduced risk of systemic side effects.

to-head clinical trials that have been performed suggest that diclofenac is as efficacious as other NSAIDs14. Nonetheless, the variability in absorption and differences in pharmacokinetic characteristics among the diclofenac formulations suggest that there may be room for improve- ment (Table 1). Recently, a novel formulation of diclofe- nac potassium (Zipsor*) has been approved by the US Food and Drug Administration for the treatment of mild to moderate acute pain in adults15. Zipsor employs ProSorby dispersion technology in a liquid filled, soft gel- atin capsule. ProSorb technology is a proprietary blend of solubilizing and dispersing agents designed to maximize the available diclofenac on contact with stomach acid. Controlled clinical trials demonstrate that Zipsor pro- vides more rapid and consistent absorption of diclofenac potassium than Cataflam16 and provides rapid pain relief17.
Treatment with oral diclofenac and other NSAIDs has been associated with important side effects includ- ing cardiovascular, gastrointestinal (GI), and hepatic
Nonetheless, as with all NSAIDs, topical formulations of diclofenac carry a warning for cardiovascular and GI side effects. Several types of diclofenac topicals are currently
available; diclofenac sodium gel (Voltaren Gelz and Solarazex), diclofenac epolamine transdermal patch (Flectorti), and diclofenac sodium solution (Pennsaid?). Indicated for pain relief of OA in hands and knees, Voltaren gel was introduced in the mid-2000s18. Another diclofenac sodium gel, Solaraze, is approved for the treatment of lesions as the result of actinic keratoses19. Flector, a diclofenac epolamine patch, provides relief from acute pain due to minor strains, sprains, or bruises20. In late 2009, Pennsaid, a diclofenac sodium solution, received approval from the US Food and Drug Administration for the treatment of pain and stiffness associated with OA21.
While the various formulations provide alternative methods of delivering diclofenac potassium, consistent across all these formulations is the delivery of diclofenac, which will presumably have the same MOA regardless of its delivery method or dosage formulation.

 

zVoltaren Gel is a registered trade name of Novartis, East Hanover, NJ, USA.

*Zipsor is a registered trade name of Xanodyne Pharmaceuticals, Inc., Newport, KY, USA.
yProSorb dispersion technology licensed to Xanodyne Pharmaceuticals, Inc., Newport, KY, USA, from AAIPharma Inc., Wilmington, NC, USA.
xSolaraze is a registered trade name of PharmaDerm, Melville, NY, USA.
ti Flector is a registered trade name of King Pharmaceuticals, Bristol, TN, USA. ?Pennsaid is a registered trade name of Nuvo Research, Varennes, Quebec, Canada.

 

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Table 2. Established, putative, and emerging mechanisms of action of diclofenac.

Established
Inhibition of COX-1 and COX-2 activity Inhibition of the synthesis of PGE2 and TXA2
Putative
Inhibition of leukotriene synthesis Inhibition of PLA2
Modulation of free AA levels (e.g., increase uptake into
triglyceride pool)
Stimulation of ATP-sensitive potassium channels via the
L-arginine-NO-cGMP pathway
Central effects: increase plasma ti -endorphin levels and inhibition of
NMDA pathway Emerging
Inhibition of PPARg
Reduction in plasma and synovial substance P and IL-6 levels Inhibition of the thromboxane-prostanoid receptor
Inhibition of acid-sensing ion channels

COX-1, cyclooxygenase-1; COX-2, cyclooxygenase-2; PGE2, prostaglandin E2; TXA2, thromboxane A2; PLA2, phospholipase A2; AA ¼ arachidonic acid; ATP, adenosine triphopshate; NO, nitric oxide; cGMP, cyclic guanosine monophosphate; NMDA, N-methyl-D-aspartate; PPARg, peroxisome prolif- erator activated receptor-g; IL-6, interleukin-6.
Diclofenac mechanisms of action
All NSAIDS appear to have anti-inflammatory, anti- pyretic, and analgesic properties and although these char- acteristics can largely be explained by the inhibition of prostaglandins22, differences in potency, efficacy, and safety profiles among NSAIDs suggest that other MOAs may be employed by this class of drugs. As for all NSAIDs, the exact MOA of diclofenac remains unknown; however, several lines of evidence have led to established, putative, and emerging MOAs for this NSAID (Table 2). These MOAs are depicted in schematic form in Figure 1.
Established mechanism of action
Inhibition of cyclooxygenase and prostaglandin synthesis
The most well recognized and widely accepted MOA of all NSAIDs is the inhibition of cyclooxygenase (COX). Vane23 was the first to propose the inhibition of prosta- glandin production via the inhibition of COX as the mechanism of NSAIDs. The inhibition of prostaglandin and thromboxane synthesis by NSAIDs has been demon- strated in a variety of in vitro experimental models and a wide range of tissue types in vivo24. Typical of most NSAIDs, diclofenac inhibits the synthesis of proinflamma- tory and nociceptive prostaglandins in blood and synovial tissue (Figure 2)25–27. Diclofenac is among the most effec- tive inhibitors of prostaglandin E2 (PGE2) production and has been reported to be 3 to 1000 times more potent on a molar basis compared with other NSAIDs in its ability to inhibit COX activity28,29. PGE2 inhibition by diclofenac is correlated with drug concentration in the plasma30.

 

The elucidation of distinct isoforms of COX has fur- thered the understanding of this class of enzymes and its ability to inhibit prostaglandin synthesis. COX-1 is con- sidered to be the ‘housekeeping’ isoform that is constitu- tively expressed in most tissue types. COX-1 concentration remains relatively stable and is involved with mediating normal platelet function, regulating renal blood flow, and providing cytoprotection of the gastric mucosa via prostaglandin I2 (prostacyclin)24. In contrast, the expression of COX-2 can dramatically increase in response to tissue damage and proinflammatory mediators and is responsible for increased production of prostaglan- din, thromboxane, and leukotriene mediators of inflamma-
24,31
tion and pain . Importantly, there is a distinct difference among NSAIDs in terms of their relative inhi- bition of these two isoforms. Relative 80% inhibitory con- centration (IC80) ratios from an in vitro assay for a variety of NSAIDS are shown in Figure 3. As expected, the so-called COX-2 inhibitor rofecoxib has a 20-fold higher selectivity for COX-2 than for COX-1 (IC80: 5 mM vs 100 mM) compared with diclofenac that has a four-fold higher selectivity for COX-2 (IC80: 0.23 mM vs
32
1.0 mM) . Warner and colleagues32 noted that even though diclofenac has a four-fold higher selectivity for COX-2, at a relevant therapeutic level (i.e., IC80) of diclo- fenac, 70% of COX-1 would also be inhibited (Figure 4). So although the IC80 ratio of COX-2/COX-1 activity for diclofenac shows a ‘selectivity’ toward COX-2, the relative nonselectivity of diclofenac may provide the drug with a safety advantage over COX-2-specific inhibitors that have been linked to adverse cardiovascular effects. Yet, some evidence suggests that dual suppression of COX-1 and COX-2 activity is responsible for the loss of GI protection associated with traditional NSAIDs33. Therefore, the non- selectivity of diclofenac could be used instead to theorize that it may be associated with higher GI side effects than the traditional NSAIDs with greater COX-1 selectivity, but this does not appear to be the case (see ‘Diclofenac Safety Considerations’).
Putative mechanisms of action
Inhibition of leukotriene synthesis, inhibition of phospholipase A2, and modulation of arachidonic acid levels
Studies in the mid-1980s revealed an effect of diclofenac on leukotriene production. Ku et al.29 first described the diclofenac-induced reduction of 5-hydroxyeicosatetrae- noic acid (5-HETE) and leukotriene C4 formation in rat leukocytes, macrophages, and whole blood by employing cellular and in vivo approaches. These effects were achieved using diclofenac concentrations above normal therapeutic levels, and similar effects were observed with indomethacin, naproxen, and ibuprofen, but at much
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ATP-sensitive K Channel
K Phospholipids
TP
NMDA ASIC
Guanylate +

–

Phospholipase A2 –

–

–

cyclase

NO

cGMP
Lipoxygenase
Arachidonic acid
+

Trigylceride pool
–
PPARY
Cox-2

Cyclooxygenase

5-Lipoxygense
COX-1 Constitutive
COX-2 Induced

– – –
PGH2
–
Leukotrienes
B4, C4

5-HETE

 

–

Endothelium
Kidney

Platelets
–

 

–

Substance P
PGI2
GI mucosa protection
TXA2
PGE2
PGF
2a
PGD2

Inhibition of platelet aggregation Inflammation Inflammation PGJ2
Platelet Aggregation –
IL-6
Anti-inflammation Anti-proliferation
Differentiation

Apoptosis

Figure 1. Schematic of established, putative, and emerging mechanisms of action of diclofenac. Sites of established or proposed inhibitory (ti) or stimulatory (þ) action are indicated in circles. ASIC, acid-sensing ion channel; cGMP, cyclic guanosine monophosphate; COX-1, cyclooxygenase-1; COX-2, cyclooxygenase-2; GI, gastrointestinal; 5-HETE, 5-hydroxyeicosatetraenoic acid; IL-6, interleukin-6; K, ATP-sensitive potassium channel; NMDA, N-methyl- D-aspartate receptor; NO, nitric oxide; PGD2, prostaglandin D2; PGE2, prostaglandin E2; PGF2a, prostaglandin F2 alpha; PGH2, prostaglandin H2; PGI2, prostaglandin I2; PGJ2, prostaglandin J2; PPARg, peroxisome proliferator activated receptor gamma; TP, thromboxane-prostanoid receptor; TXA2, thromboxane A2.
higher concentrations. The authors suggested that the reduced arachidonic acid availability was the product of enhanced reincorporation into triglycerides and supported their argument with data showing incorporation of radio- labeled arachidonic acid into phospholipids from rat peri- toneal polymorphonuclear (PMN) leukocytes29. They found no effect of diclofenac on phospholipase A2 (PLA2), an enzyme that hydrolyzes the ester bond at the sn-2 position of phospholipids to liberate arachidonic
arachidonic acid turnover, but rather increased the uptake of arachidonic acid in monocytes and macrophages, where its incorporation into triacylglycerols was enhanced36. A subsequent study investigating interleukin-1-induced PGE2 release in human synovial cells that had been labeled with radioactive arachidonic acid showed that diclofenac inhibited radioactivity associated with free ara- chidonic acid and increased the amount of radioactivity associated with phosphatidylethanolamine and triglycer-

acid
34,35
.
ides37. However, in a recent report using human colon

Those studies were followed by a report by Kothari et al.36 describing reduced 5-HETE, leukotriene B4, and leukotriene C4 production in rat peritoneal neutrophils and macrophages treated with diclofenac after calcium ionophore stimulation. In agreement with the work of Ku, Kothari and coworkers36 found that diclofenac did not inhibit PLA2 activity or interfere with phospholipid
cancer cells, indomethacin decreased the uptake of radio- labeled arachidonic acid, and diclofenac did not, even at concentrations that inhibited COX-2 activity38.
In contrast to the early findings that suggested diclofe- nac does not have a direct inhibitory affect on PLA2, a recent study in patients experiencing acute pancreatitis showed that PLA2, which is prevalent in these patients
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(A)

100
90
80
70
60
50
40
30
20
10
0
–10

 

 

 

 

 

 

 

 

 

25 mg QD 15 mg QD 50 mg TID 800 mg TID 500 mg BID
(B)

100
80
*
*

 

60
40
20
*
*

*

0

Placebo Rofecoxib 25 mg QD
Meloxicam 15 mg QD
Diclofenac 50 mg TID
Ibuprofen 800 mg TID
Naproxen 500 mg BID

–20

Figure 2. Inhibition of (A) prostaglandin E2 (PGE2) and (B) thromboxane B2 (TXB2) by several NSAIDs in whole blood assays. Mean ti SE (back-transformed from the log percent scale) of time-weighted average inhibition vs baseline over 8 hours postdose for each treatment on day 6. PGE2 generated in lipopolysacharide-stimulated whole blood is representative of COX-2 activity and Serum TXB2 generated in clotting whole blood is representative of COX-1 activity (*p50.001 vs baseline and placebo). However, time-weighted average inhibition was contained within the prespecified bounds for similarity to placebo. QD, every day; TID, three times a day; BID, twice a day. Reproduced with permission from Van Hecken et al.25.
yet derived from extrapancreatic origins39, is inhibited over 93% by relatively high concentrations of diclofe- nac40. In that study it was also reported that indomethacin was a more effective PLA2 inhibitor and ketorolac exhib- ited inhibition similar to diclofenac. It is notable that indomethacin has been proven effective in treating exper- imental acute pancreatitis41; its effectiveness in this con- dition may therefore be due in part to its capacity to inhibit PLA2. This contradictory data could be due to the pres- ence of different PLA2 isoforms in extracellular spaces (secreted or group II PLA2s) and intracellular spaces (group IV PLA2s; for a review, see Schaloske and Dennis42).
In a recent study by Singh and coworkers43, diclofenac was shown to produce 90% inhibition of PLA2 purified from snake venom. The authors presented detailed crystal- lography data to 2.7 A˚ resolution depicting the binding of diclofenac to residues crucial to the PLA2 substrate bind- ing site. The affinity of PLA2 for diclofenac (equilibrium

44
and the conformation of diclofenac in the two complexes were observed to be similar43, suggesting appreciable inhi- bition of PLA2 under in vivo conditions.
Diclofenac-mediated inhibition of group II PLA2s may have an important pharmacologic role because secreted
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COX-2 Selectivity COX-1 Selectivity
Rofecoxib
Meloxicam
Celecoxib
Diclofenac
Piroxicam

Ibuprofen

Naproxen

Aspirin

Indomethacin

Flubiprofen

Ketorolac
–3 –2 –1 0 1 2 3
Log IC80 Ratio (COX-2/COX-1)

Figure 3. Cyclooxygenase-1 (COX-1) and COX-2 selectivity profiles of several NSAIDs. IC80, 80% inhibitory concentration. Adapted with permission from Warner et al.32.

metabolized by COX enzymes, the resulting accumulation

100

80

60

40

20
0
RofecoxibMeloxicamCelecoxibDiclofenacPiroxicamIbuprofenNaproxenAspirinIndomethacineKetorolac

 

Figure 4. Cyclooxygenase (COX) selectivity of NSAIDs indicates lower gastrointestinal toxicity is related to COX-2 inhibitory potential. Reproduced with permission from Mitchell and Warner120.

PLA2s can promote inflammation both via contributions to eicosanoid production and through direct activation of proinflammatory cells45. Additional PLA2s have been identified but are yet to be tested as targets for diclofenac inhibition. Further research is necessary to delineate the role of diclofenac and related NSAIDs in influencing path- ways requiring PLA2 activity.
It has been suggested that in an in vivo environment, the effects of diclofenac-induced COX inhibition can have localized negative effects related to leukotriene produc-
promotes shunting of arachidonic acid toward reactions with existing lipoxygenases, promoting formation of alter- nate proinflammatory entities. This appears to occur in the stomach, where elevated leukotriene levels may have a role in promoting irritability of the stomach lining46–48. Inhibitors of leukotriene synthesis and leukotriene recep- tors have been shown to protect against NSAID-associated GI adverse effects49,50. However, this shunting effect may not occur in some target tissues, such as in synovial fluid51. Further, diclofenac had no effect on leukotriene B4 pro- duction by blood mononuclear cells from patients with OA who were treated for 6 months52.
These findings suggest alternative MOAs for diclofenac in arachidonic acid release and leukotriene synthesis path- ways compared with other NSAIDs. Further evidence for diclofenac involvement in related pathways is accumulat- ing, including inhibition of dehydrogenase and hydroxy- dehydrogenase enzymes that themselves inactivate anti-inflammatory eicosanoid-related mediators (e.g., lipoxins)53. These alternative MOAs may potentially enhance efficacy and improve the side effects profile.

Stimulation of peripheral nitric oxide–cyclic guanosine monophosphate–potassium channel pathways
In addition to its role in modulating vascular and GI smooth muscle responsiveness, the nitric oxide–cyclic gua- nosine monophosphate (cGMP) pathway appears to have an important role in peripheral and central analgesia54. As early as 1979, Ferreira and Nakamura55 reported an anal-

tion
46,47
. By sparing arachidonic acid from being
gesic effect of cGMP in a rat paw hyperalgesia model.
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Subsequently, they proposed that the mode of action of some peripheral analgesics may be via the L-arginine– nitric oxide–cGMP pathway given that analgesic effects were blocked by inhibitors of guanylate cyclase and the formation of nitric oxide from L-arginine, and analgesic effects were potentiated by an inhibitor of cGMP phospho- diesterase56–58. Using two different animal pain models, Tonussi and Ferreira59 showed that in addition to inhibit- ing COX, diclofenac’s analgesic action also appears to be due to the functional down-regulation of sensitized, peripheral pain receptors that is the result of stimulation of the L-arginine–nitric oxide–cGMP pathway. They pro- posed that a clearer understanding of the MOA of diclo- fenac may help to explain why some analgesics are more effective under different conditions than others. In addi- tion, diclofenac may have a safety advantage if the same level of analgesia can be obtained with a lower dose because of this apparent dual MOA. Interestingly, in rats nontherapeutic doses of sildenafil (25–100 mg), a selective inhibitor of phosphodiesterase-5 that blocks cGMP degra- dation, markedly increased the antinociceptive effect of a
60
subtherapeutic dose of diclofenac (25 mg) .
A nitric oxide induced increase in cGMP is known to
61–63
facilitate the opening of a variety of ion channels . Using a rat PGE2-induced hyperalgesic paw model, Soares and Duarte64 showed that a cGMP analog induced
peripheral antinociception via activation of the ATP-sensitive potassium channel. Concurrently, and based on the previous finding that suggested ketorolac- induced antinociception involved the L-arginine–nitric oxide–cGMP pathway65, La´zaro-Iba´n˜ez et al.66 demon- strated that local administration of ketorolac produced dose-dependent antinociception by activation of the ATP-sensitive potassium channel via the L-arginine– nitric oxide–cGMP pathway. While not the case for the NSAID indomethacin, diclofenac and rofecoxib both appear to use the L-arginine–nitric oxide–cGMP pathway to activate ATP-sensitive potassium channels and provide an antinociceptive effect67,68, which further illustrates dif- ferences in the MOA of some NSAIDs. Diclofenac acti- vation of ATP-sensitive potassium channels has been shown to be inhibited by a nitric oxide synthase inhibitor, a guanylate cyclase inhibitor, and an ATP-sensitive potas- sium channel opener69. The diclofenac-induced peripheral antinociceptive effect does not appear to also include the activation of calcium-activated or voltage-dependent potassium channels69.
Centrally mediated and neuropathic mechanisms
The COX-dependent antinociceptive activity of NSAIDs in the central nervous system (CNS) was reviewed by Burian and Geisslinger70. They concluded that antinoci- ceptive activity of NSAIDs in the CNS depends on loca- tion of drug targets, site of drug delivery, and the uptake

 

and distribution to the action site. Studies in animals71,72 and humans73 suggest that, at least in part, diclofenac may act directly or indirectly through CNS processes. Bjorkman and colleagues74 reported a dose-dependent reduction in ethacrynic acid induced writhing in mice injected with diclofenac into different areas of the brain with a threshold dose range of 1 to 10 ng. Administration of aspirin75 and sodium salicylate76 into the CNS has also been reported to attenuate the writhing response in rats, although these drugs do not completely inhibit the response. Central activity associated with diclofenac, therefore, is not necessarily shared with other NSAIDs.
Further results from the Bjorkman et al.74 study showed that the central antinociceptive effects of diclofenac in rat was partially reversed by the opioid receptor antagonist naloxone, suggesting a possible role of central opioid path- ways. Diclofenac was shown to decrease the concentration of pituitary ti-endorphin (an endogenous opioid) in rats within 30 minutes of diclofenac given intraperitoneally72.
Adecrease in pituitary ti -endorphin levels is theoretically consistent with an increase in plasma concentrations of the peptide, which is considered an index of nociceptive input into the CNS. Indeed, an increase in plasma con- centrations of ti -endorphin has been reported with diclo- fenac in humans that were pain free73. Plasma levels of ti -endorphin increase in patients experiencing pain. In a study of patients undergoing surgery, ibuprofen-induced analgesia increased as plasma levels of ti -endorphin decreased77. However, it is unknown whether this effect was due to the general analgesic COX-dependent effect of ibuprofen or a direct effect on ti -endorphin secretion.
N-methyl-D-aspartate (NMDA) receptors are involved in synaptic nociceptive transmission in the spinal cord. Several NSAIDs have been shown to have antinociceptive actions in the spinal cord78. Diclofenac has been shown to attenuate NMDA receptor-mediated hyperalgesia in rats via the L-arginine–nitric oxide–cGMP pathway discussed previously79. Diclofenac has also been demonstrated as a selective, competitive inhibitor of NMDA receptors in rat jaw muscles80. Additionally, diclofenac has been shown to markedly increase kynurenic acid concentrations in the spinal cord and diencephalon in rats81. Kynurenic acid is an antagonist of the NMDA receptor and has been asso- ciated with antinociceptive effects82. That observation was confirmed in a recent study that compared the effects of various NSAIDs on kynurenic acid levels. Schwieler et al.83 reported that while diclofenac and indomethacin, two nonselective COX inhibitors, increased kynurate levels in rats, the COX-2-selective inhibitors parecoxib and meloxicam decreased kynurenic acid levels. Those results suggest that any proposed antinociceptive activity of an NSAID via the inhibition of the NMDA receptor pathways is dependent on the COX selectivity profile of that NSAID.
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Emerging mechanisms of action
Inhibition of peroxisome proliferator activated receptor-gamma
Given that the COX-2 is an immediate early gene that is rapidly upregulated in response to injury, it is not surprising that results in some studies suggest that inhibition of COX-2 may be responsible for the anticancer effect of NSAIDs. Indeed, increases in COX-2 expression have been reported in a variety of cancers and experimental cancer models (see Warner and Mitchell84 for review) and celecoxib reduces the number of adenomatous colo- rectal polyps in patients with familial adenomatous polyposis85. The production of PGE2 via the activation of COX-2 is thought to inhibit apoptosis and stimulate
86,87
angiogenesis, leading to tumor growth . Thus, some of the antitumor activity of NSAIDs is presumably due to the drug classes’ inhibition of COX and PGE2 produc- tion. However, several mechanisms of NSAID antitumor activity have been proposed that are independent of COX
31
and PGE2 (see Tegeder et al. for review). For many nonspecific and COX-2-selective NSAIDS, COX- independent antitumor effect is via nuclear factor kappa
B(NF-kB), activator protein 1 (AP-1), mitogen-activated kinase, and cell cycle regulatory pathways31. However, not all NSAIDs exploit these pathways, suggesting there are other routes for COX-independent NSAID antitumor

 

patients with RA a correlation between the ability of diclofenac to increase the activity of PPARg and to reduce cell proliferation by decreasing cell viability and inducing apoptosis. In contrast, diclofenac has also been reported to be an antagonist for PPARg signaling, leading to 60% decrease in PPARg-mediated adipose cell differ- entiation. Diclofenac has also been shown to inhibit the agonistic activity of rosiglitazone94. These latter results suggest diclofenac may be inhibiting PPARg signaling by acting as a competitive antagonist. This paradoxically sug- gests that diclofenac, under certain conditions, might pro- mote inflammation by blocking PPARg anti-inflammatory activity. However, diclofenac, by way of inhibiting COX, may indirectly inhibit PPARg-mediated signaling by decreasing prostanoid levels that act as endogenous ago- nists for the PPARg receptor97. Further, it has been shown in a macrophage model that diclofenac induces COX-2 protein expression via a PPARg pathway, resulting in the release of several anti-inflammatory cytokines includ- ing interleukin-10 (IL-10) and transforming growth factor-ti98. Therefore, diclofenac may have multiple MOAs for inducing anti-inflammatory activity via the PPARg pathway.
Reduction of inflammatory substances
Substance P is a proinflammatory neuropeptide that plays

activity, which again supports the notion that individual
a role in several inflammatory diseases
99,100
. Along with a

NSAIDs can have unique MOA profiles. variety of cytokines, substance P is found in the plasma and

One such proposed alternative route for COX-
synovial fluid of patients with arthritis
101–103
. Because

independent antitumor activity by NSAIDs is the modulation of nuclear receptor activity, in particular the peroxisome proliferator activated receptor gamma (PPARg). PPARg is involved in fatty acid metabolism,
NSAIDs are commonly used to treat pain associated with arthritis, it was hypothesized that NSAIDs may inter- fere with the chemotactic effect of substance P on mono- cyte and PMN cells, a crucial step in arthritic disease.

controls differentiation of adipocytes and macrophages, Experiments by Locatelli and coworkers104,105 showed

is involved in inflammatory processes, and has been shown to play a role in suppressing tumor cell prolifera- tion31. The PPARg pathway has also been shown to be involved in the induction of COX-2 protein in a variety of cell types88–90. In addition, a variety of NSAIDs have been shown to bind and activate PPARg, including indometh- acin at a concentration that induces adipocyte differenti-
ation91, albeit at a relatively low affinity (IC50 ¼ 100 mM) compared with the PPARg agonist rosiglitazone (Kd ¼ 40 nM)92. Indomethacin and sulindac sulfide have been shown, at concentrations higher than required for inhibition of prostaglandin production, to be effective inhibitors of transformed cell growth of non-small cell
that at a clinically relevant concentration, diclofenac (10ti8 M) inhibited the chemotactic response of human PMN cells by approximately 40%. In contrast, at this clin- ically relevant concentration (10ti8 M), indomethacin105 and ibuprofen104 did not appear to inhibit the chemotactic effect induced by substance P. In a substance-P-induced itch-associated response murine model, an inhibitor of 5-lipoxygenase and a leukotriene B4 antagonist inhibited the substance P effect, but neither diclofenac nor indomethacin at 10 mg/kg 30 minutes prior to injection of substance P suppressed the response106. However, in a direct measurement of substance P levels in the murine snout, a longer exposure to diclofenac with daily intrave-

lung cancer cells via a proposed PPARg pathway
93
.
nous doses of 0.5 mg/kg markedly depleted substance P107.

Diclofenac has been reported to have an affinity for PPARg 50 times greater than that reported for other
94
NSAIDs (IC50 ¼ 700 nM) . In addition, indomethacin and diclofenac have been shown to be selective agonists for PPARg rather than the other PPAR isoforms95. Yamazaki et al.96 demonstrated in synovial cells from
More importantly, in an open-label placebo-controlled study in patients with RA who had elevated synovial fluid cytokine and substance P levels at baseline, diclofe- nac sodium 50 mg and naproxen 250 mg three times a day for 7 days significantly ( p50.05) decreased synovial fluid levels of substance P compared with placebo, whereas no
! 2010 Informa UK Ltd www.cmrojournal.com Diclofenac MOA and safety Gan 1723

 
significant difference was seen with indomethacin 25 mg. Therefore, it appears that one MOA of diclofenac may be to decrease substance P, and although not suggested by the results from the substance-P-induced itch-associated response murine model, it is tempting to speculate that diclofenac may provide an analgesic effect by inhibiting the leukotriene pathway (as previously discussed) by the depletion of substance P. However, this hypothesis needs to be specifically investigated in humans.
A myriad of cytokines are involved in initial inflamma- tion. Interleukin-6 (IL-6) is both a pro-inflammatory and anti-inflammatory cytokine that is held in balance by the acute inflammatory mediator IL-10. Prostaglandins regu- late the release of IL-6, and IL-10 down-regulates the expression of IL-6 and other proinflammatory cytokines. There are several lines of evidence that suggest that NSAIDs may play a role in reducing proinflammatory cytokines by a prostaglandin-independent mechanism. Indomethacin, aspirin, and ibuprofen inhibit the expres- sion and synthesis of IL-6 in human peripheral blood mononuclear cells with no direct relationship with PGE2 production108. Likewise, ketoprofen, indomethacin, and diclofenac down-regulate the expression and production of IL-6 independently, in part, of PGE2 production in
109
human T cells . Further, diclofenac significantly decreases IL-6 production and fully blocks PGE2 synthesis in human chondrocytes110. IL-6 concentrations have been shown to be significantly lower and IL-10 levels signifi- cantly higher in patients undergoing major surgery and receiving diclofenac for 12 hours compared with pla- cebo111. Plasma and synovial fluid IL-6 levels were signif- icantly reduced in patients with RA who underwent

 

decrease of IL-6 by PMN cells was seen with prolonged (180 days) diclofenac treatment in patients with OA52.

Inhibition of the thromboxane receptor
Thromboxane A2, via the thromboxane–prostanoid (TP) G-protein coupled receptor, promotes platelet activation and aggregation112. Diclofenac appears to be more potent than meloxicam (COX-2-specific inhibitor) at halting the generation of serum thromboxane B2 by platelets in clot- ting whole blood26; thromboxane B2 is an inactive product of, and therefore a marker for, thromboxane A2 production via COX-1 activity. In addition to inhibiting thrombox- ane A2 production, it appears that at therapeutic levels diclofenac binds to and can act as a competitive antagonist of the TP receptor113. In contrast, the COX-2-specific inhibitors celecoxib, rofecoxib, lumiracoxib (a diclofenac derivative), and the COX-1-specific inhibitor flubiprofen did not display antagonistic activity against the TP recep- tor. Further, Van Hecken et al.25 showed that diclofenac significantly inhibited platelet aggregation compared with placebo, whereas meloxicam and rofecoxib did not (Figure 5). The greater inhibition of platelet aggrega- tion when compared with COX-2 inhibitors and the differentiation in TP receptor inhibition between diclofe- nac and other NSAIDs (irrespective of their COX-speci- ficity) may suggest a potential cardiovascular safety advantage for diclofenac (see ‘Diclofenac Safety Considerations’).

Inhibition of acid-sensing ion channels
Tissue acidosis is a component of inflammation that

diclofenac treatment for 7 days
102
. Lastly, a similar
contributes to the sensation of pain by direct excitation
100
*

80
60

*

 

40

*
20
0

 

–20
Placebo Rofecoxib 25 mg QD
Meloxicam 15 mg QD
Diclofenac 50 mg TID
Ibuprofen 800 mg TID
Naproxen 500 mg BID
Figure 5. Diclofenac inhibits platelet aggregation to a greater extent than COX-2-specific inhibitors. Mean ti SE (back-transformed from the log percent scale) of time-weighted average inhibition vs baseline over 8 hours postdose for each treatment on day 6. Arachidonic acid 1 mM was the agonist used in these assays (*p50.001 vs baseline and placebo). QD, every day; TID, three times a day; BID, twice a day. Reproduced with permission from Van Hecken et al.25.
1724 Diclofenac MOA and safety Gan www.cmrojournal.com ! 2010 Informa UK Ltd

 
of nociceptive sensory neurons via acid-sensing ion chan- nels (ASIC). Voilley et al.114 demonstrated that diclofenac selectively inhibits ASIC3 and ibuprofen selectively inhi- bits ASIC1a in a simian virus 40 transformed simian cell line (COS cells). In addition, the authors showed that these NSAIDs prevented the inflammation-induced expression of ASIC in sensory neurons in these cells. However, the COX-2-selective NSAIDs rofecoxib and piroxicam did not inhibit ASIC activity or expression. Diclofenac (half maximal inhibitory concentration
50 have also been shown to inhibit proton-induced currents in rat hippocampal interneurons115. Both of these agents decreased the amplitude and rate of decay of acid-evoked currents; however, only diclofenac shifted the steady state desensitization curve to more alkaline pH, presumably slowing down the recovery from desensitization of ASICs. Topical diclofenac has been shown to decrease acid-evoked pain in humans likely through the attenua- tion of ASIC activity116. Indomethacin and acetylsalicylic acid have also been shown to have a direct inhibitory effect on acid-induced pain in human skin117.

 

Diclofenac safety considerations
NSAIDs are among the most commonly used and widely prescribed medications worldwide. Because the drug class is first choice in the treatment of pain associated with chronic conditions, such as RA and OA, the use of NSAIDs is likely to increase within the aging population. Further, the development of COX-2-selective inhibitors was associated with a large increase in the total number of NSAID prescriptions in the United States118, a trend that is expected to continue as more NSAID formulations

 

Gastrointestinal concerns
From their first description, traditional NSAIDs have been associated with GI side effects including life-threatening GI hemorrhage119. GI adverse effects are the most frequent patient complaint with the use of NSAIDs. Shortly after the discovery of two different COX isoforms, it was theo- rized that the extent of GI toxicity associated with a given NSAID was related to its relative activity against COX-1119. A comparison of the relative COX inhibition profiles of a variety of NSAIDs to their relative GI toxicity is shown in Figure 6120. Diclofenac, as discussed earlier, has more specificity to COX-2 than COX-1, and is associated with a relatively low level of GI toxicity compared with the other NSAIDs. Nonetheless, in an analysis of case– control and cohort studies, the authors concluded that, in general, ibuprofen has the lowest GI risk among the NSAIDs, and diclofenac and naproxen were associated with a slightly higher GI risk compared with ibuprofen121.
Because of purported causal relationship between COX-1 inhibition and GI adverse effects, the COX-2-spe- cific NSAIDs were developed to lower the GI risk. In a long-term double-blind, randomized safety trial (VIGOR Study) in patients with RA, rofecoxib was shown to be associated with a lower incidence of perforation, hemor- rhage, or symptomatic peptic ulcer in the upper GI tract compared with naproxen122. Similar results were reported in a 6-month combined analysis of patients from two clin- ical trials (CLASS Study) that compared the safety of celecoxib with diclofenac or ibuprofen123. However, a careful analysis of the prespecified endpoints at 12 months showed no significant difference in clinically sig- nificant upper GI events (i.e., bleeding, perforation, gastric outlet obstruction) between celecoxib 400 mg and diclo-
124
fenac 75 mg (0.43% vs 0.5%; p ¼ 0.640) or celecoxib 400 mg and ibuprofen 800 mg (0.43% vs 0.55%;

become available. It is not surprising then given the number of patients exposed to this drug class that safety concerns can become an important public health issue. Although as a class NSAIDs are generally well tolerated, several safety considerations should be measured against the clinical benefits associated with each compound. Of particular note are the US Food and Drug Administration’s label requirements for all NSAIDs, regardless of COX spe-
11
10
9
8
7
6
5
4
Ketoprofen
Indomethacin
Aspirin
Naproxen

Ibuprofen
Fenoprofen
Diflunisal

 

 

Tolmetin

cificity, that emphasize the increased risk of serious GI
3 Piroxicam

adverse events including bleeding, ulceration, and perfo- ration of the stomach or intestines and the increased risk of
2
1
0

Diclofenac
Sulindac

serious cardiovascular thrombotic events, myocardial infarction, and stroke. Moreover, as the inhibition of COX-2 can affect prostaglandin formation and transami- nases levels, renal failure and hepatotoxicity are potential complications of treatment with diclofenac and, therefore, should be included as an integral part when evaluating safety considerations.
0 1 2 3 4 5 6 7 8 9 10 11
Relative GI Toxicity (increasing toxicity )
Figure 6. Relative cyclooxygenase (COX-1/COX-2) selectivity compared with relative gastrointestinal toxicity of several traditional nonsteroidal anti-inflammatory drugs. Reproduced with permission from Mitchell and Warner120.
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p ¼ 0.414). However, in a large meta-analysis of 69 ran- domized controlled trials, COX-2-selective inhibitors produced significantly fewer gastroduodenal ulcers and clinically important ulcer complications compared with nonselective NSAIDs125, which supports the prevailing opinion that COX-2 inhibitors offer a superior GI safety profile.
Although the mode of action is not clear, there is evi- dence to suggest that NSAIDs can initiate damage in the lower GI tract126. It has been suggested that the magnitude of this injury may be greater than that of NSAID- associated gastropathy127–129. Gut inflammation, increased gut permeability, and malabsorption are typi- cally reported as adverse effects in the lower GI tract during NSAID treatment130. However, events of GI bleed- ing and perforation are considered to be most clinically relevant as they contribute to an increased risk of morbid- ity and mortality associated with NSAIDs.
A post hoc analysis of a randomized, controlled trial designed to evaluate the incidence of recurrent upper GI bleeding in high-risk patients receiving either celecoxib or diclofenac demonstrated that one-third of all clinically relevant bleeding events were related to the lower GI tract (perforation, anemia, colitis, and angiodysplasic bleeding)130,131. In a double-blind trial designed to evalu- ate the incidence of cardiovascular events in patients receiving diclofenac and etoricoxib, greater than 50% of all GI complications reported were the result of lower GI events (bleeding, perforation, and obstruction). Moreover, the incidence of clinically relevant lower GI events was similar to that observed in the upper GI tract132. Patients with a prior lower GI event and patients aged ti 65 years were determined to be at a higher relative risk for devel- oping a lower GI clinical event.
Cardiovascular considerations
The COX-2-selective inhibitors were designed to limit the known GI adverse events associated with the drug class. However, with the increased use of COX-2 inhibitors, fur- ther analysis of the data, and additional reports, it became clear that COX-2-selective inhibitors may be associated with an increase in cardiovascular risk, which led to the eventual withdrawal of rofecoxib and valdecoxib from the market. Large randomized controlled trials and epidemio- logic studies have confirmed that these drugs are associated with an elevated risk of myocardial infarction and stroke (see Ong et al. for review121). In addition, in a meta- analysis of 29 trials, patients who received rofecoxib had a 2.3-fold increased risk for myocardial infarction com- pared with those receiving placebo or other traditional NSAIDs133.
The increased cardiovascular risk associated with COX- 2-selective inhibitors has been suggested to be due to a

 

disruption in the normal balance between the prothrom- botic activity of thromboxane A2 (TXA2) (derived from COX-1 activity) and the inhibition of platelet aggregation by PGI2 (derived from COX-2 activity). In other words, bleeding or excessive platelet aggregation is hypothesized to occur if this balance is tipped toward relatively higher activity of PGI2 or TXA2, respectively. For example, it is thought that excessive bleeding associated with traditional NSAIDs is due to inhibition of constitutively expressed COX-1 activity (and subsequent decrease in platelet aggregation), via a decrease in TXA2, the major product of platelet COX-1134. Notably, this decrease in TXA2 activity is also used to explain the cardioprotective effect of traditional NSAIDs, such as aspirin135. In contrast to TXA2, on activation of the pathway, PGI2 is the major product of COX-2136. Indeed, the biosynthesis of PGI2 is coupled with the expression of COX-2137. While the COX-2-specific activity of celecoxib blocks a major source of PGI2 biosynthesis, it does not inhibit TXA2- mediated platelet aggregation136. The absence of PGI2 production to oppose constitutive TXA2 activity could therefore contribute to the cardiovascular adverse effects of COX-2 inhibitors138. Endogenous PGI2 has been shown to modulate platelet-vascular actions of TXA2 in mice138. Diclofenac, often referred to as an equipotent inhibitor of both COX isoforms despite an IC80 ratio closer to celecoxib than ibuprofen (Figure 4), has activity against COX-1, and would theoretically inhibit constitutive TXA2 production while inhibiting PGI2 production. Antagonistic activity against the TP receptor may further protect against a potential imbalance toward TXA2-medi- ated platelet aggregation seen with COX-2-specific inhib- itors. Indeed, van der Hoorn and colleagues139 showed in their study that a TP antagonist counteracted the adverse cardiovascular effects of unbalanced TXA2 activity in mice treated with the COX-2-specific inhibitor rofecoxib. However, such an effect has not been shown in humans. Further, in a recent double-blind study (MEDAL Study) in patients with OA, diclofenac had a similar thrombotic cardiovascular risk to that associated with the COX-2-spe- cific inhibitor etoricoxib, suggesting no cardiovascular safety advantage140. In a population-based analysis of over 1 million healthy individuals and compared with no NSAID use, diclofenac had a hazard ratio for death/myo- cardial infarction of 1.63 (95% CI: 1.52–1.76) compared with 2.13 for rofecoxib (95% CI: 1.89–2.41) and 2.01 for celecoxib (95% CI: 1.78–2.27)141.
Diclofenac also appears to have a protective effect on leukocyte–endothelium interactions. Adherence of leuko- cytes to the vascular endothelium is an early step in plaque formation that contributes significantly to cardiovascular disease (for a recent review, see Rao et al.142). Diclofenac appears to reduce chemotaxis and protease production in PMN leukocytes, and to reduce expression of key contrib- utors to the leukocyte–endothelial cell adhesion process,
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including L-selectin, E-selectin, intercellular adhe- sion molecule-1, and vascular cell adhesion mole-
143–146
cule-1 . In addition, unlike indomethacin and

 

Adverse events associated with topical formulations

naproxen
147,148
, diclofenac does not appear to counteract
Topical NSAIDs have been postulated to be as efficacious

the effects of antihypertensive agents149. Therefore, diclo- fenac seems to be a viable NSAID option for patients receiving treatment for hypertension.
Renal complications
In addition to GI and cardiovascular concerns, the pro- longed use of NSAIDs has been linked to renal injury and toxicity150. Although reports have demonstrated that selective COX-2 inhibitors have effects on renal function similar to those observed with nonselective
as oral treatment without GI, hepatic, and renal side effect complications. As the risk of these events increases with age161, topical NSAIDs are an attractive option for the treatment of pain relief. Overall, reports in the literature regarding adverse events for topical diclofenac are limited and details regarding safety are often generalized. Typically reported adverse events for topical diclofenac include localized skin reactions including rash, itching, or burn-
162–164
ing . In comparison to oral treatment, topical diclo- fenac is generally well tolerated with a lower incidence of
165,166
systemic adverse events including GI complaints .

NSAIDs
151,152
, acute treatment with therapeutic doses of
However, with a limited number of reports summarizing

diclofenac has not been associated with significantly impaired renal function in healthy adults153 or postsurgical patients154. However, the results from a long-term study (46 months) have indicated that prolonged treatment with diclofenac can be associated with decreases in creat- inine clearance, a predictor in renal dysfunction. Importantly, a time course evaluation of decreased clear- ance revealed that the highest frequency of this change occurred in early visits during the treatment period155.
Hepatic adverse effects
As with antihypertensives, antimicrobials, and immuno- suppressants, NSAIDs can cause drug-induced liver
safety of topical diclofenac, additional studies are neces- sary to ascertain any long-term safety risk of topical diclo- fenac specifically or for any safety comparisons with oral formulations. Currently, topical formulations of diclofenac carry the same warnings for an increased risk of cardiovas- cular and GI events, as well as renal and hepatic adverse effects that are carried by oral formulations of diclofenac and other NSAIDs.

 
Discussion
Lack of correlation between antinociceptive and anti-

injury
156,157
. It has been reported that diclofenac causes
inflammatory activity suggests that the analgesic proper-

elevations of transaminase levels more commonly than other NSAIDs158. Yet, a review of in vitro and in vivo animal studies did not indicate that diclofenac is hepato- toxic159. While anecdotal reports rarely associate diclofe- nac with hospitalizations due to liver toxicity and acute liver failure, a systemic review of over 60 randomized, con- trolled clinical trials did not associate an increase in clin- ical liver events with diclofenac compared with other NSAIDs158. Likewise, a prospective assessment of 17,289 patients with RA or OA who received diclofenac for a mean duration of 18 months during randomized, con- trolled trials reported that a markedly higher rate of ami- notransferase elevations with diclofenac as compared with other NSAIDs may not be paralleled by a proportional marked increase in the rate of serious clinical liver injury160. Nonetheless, it is recommended that physicians educate their patients on the warning signs of hepatotox- icity, and to minimize potential risk by treating patients with NSAIDs with the lowest effective dose for the short- est duration. Further, as with other NSAIDs, transaminase levels should be measured periodically in patients receiv- ing diclofenac as severe hepatic reactions can occur at any time during treatment.
ties of NSAIDs cannot be attributable entirely to anti-
167,168
inflammatory effects . In addition, because some COX inhibitors significantly reduce pain only when administered at doses 100-fold greater than necessary to inhibit COX-derived prostaglandin synthesis169, not all NSAID analgesic activity can be explained by COX inhi- bition59. Diclofenac, one of the most widely used NSAIDs worldwide, possesses some unique and beneficial MOA characteristics. In addition, as a relatively equipotent inhibitor of both COX-1 and COX-2170, diclofenac may have an advantage over other NSAIDs due to its relatively low GI toxicity, and potentially lower cardiovascular tox- icity than COX-2 inhibitors, and minimal effects on renal and hepatic activity.
This review was not designed to be a comparison of the MOAs of diclofenac to other NSAIDs; therefore, it is not possible to comment on the similarities and differences of the mechanistic models of these compounds. None of the proposed putative and emerging MOAs are supported by clinical data. Therefore, in addition to the need for more preclinical studies to confirm the putative and emerging MOAs of diclofenac, clinical trials will be required to dem- onstrate the translation of these MOAs to clinical benefit.
! 2010 Informa UK Ltd www.cmrojournal.com Diclofenac MOA and safety Gan 1727

 
Transparency
Declaration of funding
This research was supported by Xanodyne Pharmaceuticals, Inc. Declaration of financial/other relationships
T.J.G. has disclosed that he has received grant/research support from Baxter, Eisai, Schering-Plough, Acacia, and Aspect, and that he has participated in speakers bureaus for Baxter, Edwards Lifesciences, Fresenius, Xanodyne Pharmaceuticals, Inc., and GlaxoSmithKline.
Peer reviewers may receive honoraria from CMRO for their review work. The peer reviewers have disclosed no relevant financial relationships.

Acknowledgment
The author would like to thank Xanodyne Pharmaceuticals, Inc., for support of this manuscript. He also thanks Lamara D. Shrode, PhD, of The JB Ashtin Group, Inc., who, on the behalf of Xanodyne Pharmaceuticals, Inc., provided editorial assistance. This editorial assistance was funded by Xanodyne Pharmaceuticals, Inc.
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