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Mechanism Of Action

ARTHROTEC (diclofenac/misoprostol) is a combination product containing diclofenac, a nonsteroidal anti-inflammatory drug (NSAID) with analgesic, anti-inflammatory and antipyretic properties, and misoprostol, a GI mucosal protective prostaglandin E1 analog.


The mechanism of action of diclofenac, like that of other NSAIDs, is not completely understood but involves inhibition of cyclooxygenase (COX-1 and COX-2).

Diclofenac is a potent inhibitor of prostaglandin (PG) synthesis in vitro. Diclofenac concentrations reached during therapy have produced in vivo effects. Prostaglandins sensitize afferent nerves and potentiate the action of bradykinin in inducing pain in animal models. Prostaglandins are mediators of inflammation. Because diclofenac is an inhibitor of prostaglandin synthesis, its mode of action may be due to a decrease of prostaglandins in peripheral tissues.


Misoprostol is a synthetic prostaglandin E1 analog with gastric antisecretory and mucosal protective properties. NSAIDs inhibit prostaglandin synthesis. A deficiency of prostaglandins within the gastric and duodenal mucosa may lead to diminishing bicarbonate and mucus secretion and may contribute to the mucosal damage caused by NSAIDs.

Misoprostol can increase bicarbonate and mucus production, but it has been shown at doses 200 mcg and above that are also antisecretory. It is therefore not possible to differentiate whether the ability of misoprostol to reduce the risk of gastric and duodenal ulcers is the result of its antisecretory effect, its mucosal protective effect, or both.

In vitro studies on canine parietal cells using titrated misoprostol acid as the ligand have led to the identification and characterization of specific prostaglandin receptors. Receptor binding is saturable, reversible, and stereo-specific. The sites have a high affinity for misoprostol, for its acid metabolite, and for other E type prostaglandins, but not for F or I prostaglandins and other unrelated compounds, such as histamine or cimetidine. Receptor-site affinity for misoprostol correlates well with an indirect index of antisecretory activity. It is likely that these specific receptors allow misoprostol taken with food to be effective topically, despite the lower serum concentrations attained.

Misoprostol, over the range of 50–200 mcg, inhibits basal and nocturnal gastric acid secretion, and acid secretion in response to a variety of stimuli, including meals, histamine, pentagastrin, and coffee. Activity is apparent 30 minutes after oral administration and persists for at least 3 hours. In general, the effects of 50 mcg were modest and shorter-lived, and only the 200 mcg dose had substantial effects on nocturnal secretion or on histamine-and meal-stimulated secretion.

Misoprostol also produces a moderate decrease in pepsin concentration during basal conditions, but not during histamine stimulation. It has no significant effect on fasting or postprandial gastrin nor intrinsic factor output.


General Pharmacokinetic Characteristics

The pharmacokinetic profiles of diclofenac and misoprostol administered as the fixed combination (ARTHROTEC 50 or 75) are similar to the profiles when the two drugs are administered as separate tablets (see Table 2). No pharmacokinetic interaction between the two drugs has been observed following multiple dosing. The diclofenac total exposure [area under the curve (AUC)] is dose-proportional within the range of 25 mg to 150 mg. Approximately dose-proportional increase in misoprostol exposure was also observed within the range of 200 to 400 mcg. Neither diclofenac nor misoprostol accumulated in plasma following repeated doses of ARTHROTEC given every 12 hours under fasted conditions.

Table 2: Pharmacokinetic Parameters of Diclofenac and Misoprostol Acid Following Single Oral Doses of ARTHROTEC or Separate Products in Healthy Subjects

Treatment (n=36) Cmax (pg/mL) Tmax (hr) AUC(0–4h) (pg·hr/mL)
ARTHROTEC 50 441 (137) 0.30 (0.13) 266 (95)
Misoprostol 478 (201) 0.30 (0.10) 295 (143)
ARTHROTEC 75 304 (110) 0.26 (0.09) 177 (49)
Misoprostol 290 (130) 0.35 (0.12) 176 (58)
Treatment (n=36) Cmax (ng/mL) Tmax (hr) AUC(0–12h) (ng·hr/mL)
ARTHROTEC 50 1207 (364) 2.4 (1.0) 1380 (272)
Diclofenac 1298 (441) 2.4 (1.0) 1357 (290)
ARTHROTEC 75 2025 (2005) 2.0 (1.4) 2773 (1347)
Diclofenac 2367 (1318) 1.9 (0.7) 2609 (1185)
SD: Standard deviation of the mean; AUC: Area under the curve; Cmax: Peak concentration; Tmax: Time to peak concentration


Diclofenac: Diclofenac is completely absorbed from the GI tract after oral administration under fasted condition, and peak plasma levels are achieved in 2 hours (range 1–4 hours), and the area under the plasma concentration curve (AUC) is dose-proportional within the range of 25 mg to 150 mg. Peak plasma levels are less than dose-proportional and are approximately 1.5 and 2.0 mcg/mL for 50 mg and 75 mg doses, respectively. The diclofenac in ARTHROTEC is in a pharmaceutical formulation that resists dissolution in the low pH of gastric fluid but allows a rapid release of drug in the higher pH environment of the duodenum. Only 50% of the absorbed dose is systemically available due to first pass metabolism (i.e., oral bioavailability is 50%).

Misoprostol: Misoprostol is rapidly absorbed following oral administration of ARTHROTEC, and misoprostol acid (active metabolite) reaches a maximum plasma concentration in approximately 20 minutes. Maximum plasma concentrations of misoprostol acid are diminished when the dose is taken with food, and total availability of misoprostol acid is reduced by use of concomitant antacid. Clinical trials were conducted with concomitant antacid; this effect does not appear to be clinically important.

Food decreases the multiple-dose bioavailability profile of ARTHROTEC 50 and ARTHROTEC 75.


Diclofenac: The volume of distribution of diclofenac is approximately 0.55 L/kg. More than 99% of diclofenac is bound to plasma albumin.

Misoprostol: The plasma protein binding of misoprostol acid is less than 90% and is concentration-independent in the therapeutic range.

After a single oral dose of misoprostol to nursing mothers, misoprostol acid was excreted in breast milk. The maximum concentration of misoprostol acid in expressed breast milk was achieved within 1 hour after dosing and was 7.6 pg/mL (CV 37%) and 20.9 pg/mL (CV 77%) after single 200 mcg and 600 mcg misoprostol administration, respectively. The misoprostol acid concentrations in breast milk declined to <1 pg/mL at 5 hours post-dose. These data may not reflect drug level in mature milk and in a daily dosing regimen for osteoarthritis or rheumatoid arthritis.


Diclofenac: Diclofenac metabolism is predominantly mediated via CYP2C9 in the liver. Five metabolites (4'hydroxy-, 5-hydroxy-, 3'-hydroxy-, 4',5-dihydroxy-and 3'-hydroxy-4'-methoxy diclofenac) have been identified. The major metabolite (4'-hydroxy-diclofenac) has very weak pharmacologic activity.

Both diclofenac and its oxidative metabolites undergo glucuronidation or sulfation followed by biliary excretion. Acylglucuronidation mediated by UGT2B7 and oxidation mediated by CYP2C8 may also play a role in diclofenac metabolism. CYP3A4 is responsible for the formation of minor metabolites, 5-hydroxy and 3'-hydroxy-diclofenac.

Misoprostol: It undergoes rapid and extensive metabolism to its biologically active metabolite, misoprostol acid.


Diclofenac: Diclofenac is eliminated through metabolism and subsequent urinary and biliary excretion of the glucuronide and the sulfate conjugates of the metabolites. Approximately 65% of the dose is excreted in the urine and 35% in the bile. The elimination half-life of diclofenac is approximately 2 hours. The clearance of diclofenac is approximately 350 mL/min (equivalent to 21 L/h).

Conjugates of unchanged diclofenac account for 5–10% of the dose excreted in the urine and for less than 5% excreted in the bile. Little or no unchanged unconjugated drug is excreted. Conjugates of the principal metabolite account for 20–30% of the dose excreted in the urine and for 10–20% of the dose excreted in the bile.

Conjugates of three other metabolites together account for 10–20% of the dose excreted in the urine and for small amounts excreted in the bile. The elimination half-life values for these metabolites are shorter than those for the parent drug. Urinary excretion of an additional metabolite (half-life = 80 hours) accounts for only 1.4% of the oral dose. The degree of accumulation of diclofenac metabolites is unknown. Some of the metabolites may have activity.

Misoprostol: After oral administration of radio-labeled misoprostol, approximately 70% of detected radioactivity appears in the urine. The elimination half-life is approximately 30 minutes.

Specific Populations

Age: Geriatric Population

A 4-week study, comparing plasma level profiles of diclofenac (50 mg two times a day) in younger adults (26–46 years, N=10) versus elderly subjects (66–81 years, N=10), did not show differences between age groups. In subjects over 64 years of age, the AUC for misoprostol acid was increased.

In a multiple-dose crossover study of 24 elderly subjects aged 65 years or older, the misoprostol contained in ARTHROTEC (two times a day) did not affect the pharmacokinetics of diclofenac.

Age: Pediatric Population

Diclofenac and misoprostol have not been investigated in pediatric patients.

Race: Pharmacokinetic differences due to race have not been identified.

Renal Impairment

In patients with renal impairment (N=5, creatinine clearance 3 to 42 mL/min) following intravenous administration of 50 mg diclofenac, AUC values and elimination rates were comparable to those in healthy subjects.

Pharmacokinetic studies with misoprostol in patients with varying degrees of renal impairment showed an approximate doubling of elimination half-life, Cmax, and AUC compared to healthy subjects.

Hepatic Impairment

In patients with biopsy-confirmed cirrhosis or chronic active hepatitis (variably elevated transaminases and mildly elevated bilirubin, N=10), diclofenac concentrations and urinary elimination values following administration of 100 mg oral solution were comparable to those in healthy subjects.

In a study of subjects with mild to moderate hepatic impairment, mean misoprostol acid AUC and Cmax showed approximately twice high as the mean values obtained in healthy subjects. Three subjects who had the lowest antipyrine and lowest indocyanine green clearance values had the highest misoprostol acid AUC and Cmax values.

Drug Interaction Studies


Aspirin: When ARTHROTEC was administered with aspirin, the protein binding of diclofenac was reduced, although the clearance of the free diclofenac was not altered. The clinical significance of this interaction is not known. See table 1 for clinically significant drug interactions of NSAIDs with asprin [see DRUG INTERACTIONS].

Voriconazole: When a single dose diclofenac (50 mg) was coadministered with the last dose of voriconazole (400 mg every 12 hours on Day 1, followed by 200 mg every 12 hours on Day 2), the mean Cmax and AUC of diclofenac were increased by 114% and 78%, respectively, when compared to diclofenac alone [see DRUG INTERACTIONS].

In vitro, diclofenac interferes minimally with the protein binding of prednisolone (10% decrease in binding). Benzylpenicillin, ampicillin, oxacillin, chlortetracycline, doxycycline, cephalothin, erythromycin, and sulfamethoxazole have no influence, in vitro, on the protein binding of diclofenac in human serum.

Other drugs: In small groups of patients (7-10 patients/interaction study), the concomitant administration of azathioprine, gold, chloroquine, D-penicillamine, prednisolone, doxycycline or digitoxin did not significantly affect Cmax and AUC of diclofenac.


Diazepam: Misoprostol given for 1 week had no effect on the steady state pharmacokinetics of diazepam when the two drugs were administered 2 hours apart.

Other drugs: Pharmacokinetic studies also showed a lack of drug interaction with antipyrine or propranolol given with misoprostol.

Animal Toxicology

A reversible increase in the number of normal surface gastric epithelial cells occurred in the dog, rat, and mouse during long-term toxicology studies with misoprostol. No such increase has been observed in humans administered misoprostol for up to 1 year. An apparent response of the female mouse to misoprostol in long-term studies at 100 to 1000 times the human dose was hyperostosis, mainly of the medulla of sternebrae. Hyperostosis did not occur in long-term studies in the dog and rat and has not been seen in humans treated with misoprostol.

Clinical Studies


Diclofenac sodium, as a single ingredient or in combination with misoprostol, has been shown to be effective in the management of the signs and symptoms of osteoarthritis.

Rheumatoid Arthritis

Diclofenac sodium, as a single ingredient or in combination with misoprostol, has been shown to be effective in the management of the signs and symptoms of rheumatoid arthritis.

Upper Gastrointestinal Safety

Diclofenac, and other NSAIDs, have caused serious gastrointestinal toxicity, such as bleeding, ulceration, and perforation of the stomach, small intestine or large intestine. Misoprostol has been shown to reduce the incidence of endoscopically diagnosed NSAID-induced gastric and duodenal ulcers. In a 12-week, randomized, double-blind, dose-response study, misoprostol 200 mcg administered four, three or two times a day, was significantly more effective than placebo in reducing the incidence of gastric ulcer in OA and RA patients using a variety of NSAIDs. The three times a day regimen was therapeutically equivalent to misoprostol 200 mcg four times a day with respect to the prevention of gastric ulcers. Misoprostol 200 mcg given two times a day was less effective than 200 mcg given three or four times a day. The incidence of NSAID-induced duodenal ulcer was also significantly reduced with all three regimens of misoprostol compared to placebo (see Table 3).

Table 3

Misoprostol 200 mcg Dosage Regimen
  Placebo two times a day three times a day four times a day
Gastric ulcer 11% 6%* 3%* 3%*
Duodenal ulcer 6% 2%* 3%* 1%*
N=1623; 12 weeks
*Misoprostol significantly different from placebo (p<0.05)

Results of a study in 572 patients with osteoarthritis demonstrate that patients receiving ARTHROTEC have a lower incidence of endoscopically defined gastric ulcers compared to patients receiving diclofenac sodium (see Table 4).

Table 4

Osteoarthritis patients with history of ulcer or erosive disease (N=572), 6 weeks Incidence of ulcers
Gastric Duodenal
ARTHROTEC 50 three times a day 3%* 6%
ARTHROTEC 75 two times a day 4%* 3%
diclofenac sodium 75 mg two times a day 11% 7%
Placebo 3% 1%
*Statistically significantly different from diclofenac (p<0.05)

Last reviewed on RxList: 6/20/2016
This monograph has been modified to include the generic and brand name in many instances.

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