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Beginning in 1829, with the isolation of salicylic acid from the folk remedy willow bark, NSAIDs have become an important part of the pharmaceutical treatment of pain (at low doses) and inflammation (at higher doses). Part of the popularity of NSAIDs is that, unlike opioids, they do not produce sedation or respiratory depression and have a very low addiction rate. NSAIDs, however, are not without their own problems (see below). Certain NSAIDs, including ibuprofen and aspirin, have become accepted as relatively safe and are available over-the-counter without prescription.
Contents |
Mode of action
Most NSAIDs act as non-selective inhibitors of the enzyme cyclooxygenase, inhibiting both the cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) isoenzymes. Cyclooxygenase catalyses the formation of prostaglandins and thromboxane from arachidonic acid (itself derived from the cellular phospholipid bilayer by phospholipase A2). Prostaglandins act (among other things) as messenger molecules in the process of inflammation. This mechanism of action was elucidated by John Vane, who later received a Nobel Prize for his work.
Examples
NSAIDs can be broadly classified based on their chemical structure. NSAIDs within a group will tend to have similar characteristics and tolerability. There is little difference in clinical efficacy between the NSAIDs when used at equivalent doses. Rather, differences between compounds tended to be with regards to dosing regimens (related to the compound's elimination half-life), route of administration, and tolerability profile. Some more common examples are given below.
Paracetamol (acetaminophen), owing to its inhibitory action on cyclooxygenase, is sometimes grouped together with the NSAIDs. Paracetamol, however, does not have any significant anti-inflammatory properties and is not a true NSAID. Though it has not been clearly elucidated, it is suspected that this lack of anti-inflammatory action may be due to the paracetamol inhibiting cyclooxygenase predominantly in the central nervous system. There is also some speculation that paracetamol acts through the inhibition of the recently discovered COX-3 isoform (see below).
Salicylates
- Amoxiprin
Benorilate
Choline magnesium salicylate
Diflunisal
Faislamine
Methyl salicylate
Salicyl salicylate (salsalate)
Arylalkanoic acids
- Aceclofenac
Acemetacin
Bromfenac
Etodolac - Indometacin
- Ketorolac
Nabumetone
Sulindac
Tolmetin
2-Arylpropionic acids (profens)
- Carprofen
- Fenbufen
Fenoprofen
Flurbiprofen - Ketoprofen
- Loxoprofen
- Naproxen
- Tiaprofenic acid
N-Arylanthranilic acids (fenamic acids)
- Mefenamic acid
- Meclofenamic acid
Tolfenamic acid
Pyrazolidine derivatives
- Phenylbutazone
- Azapropazone
Metamizole
Oxyphenbutazone
Oxicams
- Lornoxicam
Meloxicam
Tenoxicam
Coxibs
- Celecoxib
- Etoricoxib
Lumiracoxib
Parecoxib - Rofecoxib (withdrawn from market)
- Valdecoxib (withdrawn from market)
Sulphonanilides
- Nimesulide
Others
- Licofelone
Omega-3 Fatty Acids
Uses
NSAIDs are usually indicated for the treatment of acute or chronic conditions where pain and inflammation are present. Research continues into their potential for prevention of colorectal cancer, and treatment of other conditions, such as cancer and cardiovascular disease.
NSAIDs are generally indicated for the symptomatic relief of the following conditions: (Rossi, 2006)
- Rheumatoid arthritis
Osteoarthritis
Inflammatory arthropathies (e.g. ankylosing spondylitis, psoriatic arthritis, Reiter's syndrome)
Acute gout
Dysmenorrhoea
Metastatic bone pain
Headache and migraine
Postoperative pain
Mild-to-moderate pain due to inflammation and tissue injury
Pyrexia
Renal colic
Aspirin, the only NSAID able to irreversibly inhibit COX-1, is also indicated for inhibition of platelet aggregation; an indication useful in the management of arterial thrombosis and prevention of adverse cardiovascular events.
In 2001, NSAIDs accounted for 70,000,000 prescriptions and 30 billion over-the-counter doses sold annually in the United States. (Green, 2001). With the aging of the Baby Boomer generation and the associated rise in the incidence of osteoarthritis and other such conditions for which NSAIDs are indicated, the use of NSAIDs may increase further still.
Pharmacokinetics
Most NSAIDs are weak acids, with a pKa of 3-5. They are absorbed well from the stomach and intestinal mucosa. They are highly protein-bound in plasma (typically >95%), usually to albumin, so that their volume of distribution typically approximates to plasma volume. Most NSAIDs are metabolised in the liver by oxidation and conjugation to inactive metabolites which are typically excreted in the urine, although some drugs are partially excreted in bile. Metabolism may be abnormal in certain disease states, and accumulation may occur even with normal dosage.
Ibuprofen and diclofenac have short half-lives (2-3 hours). Some NSAIDs (typically oxicams) have very long half-lives (e.g. 20-60 hours).
Adverse effects
The widespread use of NSAIDs has meant that the adverse effects of these relatively safe drugs have become increasingly prevalent. The two main adverse drug reactions (ADRs), associated with NSAIDs relate to gastrointestinal (GI) effects and renal effects of the agents.
These effects are dose-dependent, and in many cases severe enough to pose the risk of ulcer perforation, upper gastrointestinal bleeding, and death, limiting the use of NSAID therapy. An estimated 10-20% of NSAID patients experience dyspe gastrointestinal (GI) effects and renal epsia, and NSAID-associated upper gastrointestinal adverse events are estimated to result in 103,000 hospitalizations and 16,500 deaths per year in the United States, and represent 43% of drug-related emergency visits. Many of these events are avoidable; a review of physician visits and prescriptions estimated that unnecessary prescriptions for NSAIDs were written in 42% of visits. (Green, 2001)
Cardiovascular risk
A recent meta-analysis of all trials comparing non-steroidal anti-inflammatory drugs found a 80% increase in the risk of myocardial infarction with both newer Cox-2 antagonists and high dose traditional anti-inflammatories compared with placebo, with the important exception of naproxen, which was not associated with an increased cardiovascular risk (Kearney et al, BMJ 2006;332:1302-1308).
Gastrointestinal ADRs
The main ADRs associated with use of NSAIDs relate to direct and indirect irritation of the gastrointestinal tract (GIT). NSAIDs cause a dual insult on the GIT - the acidic molecules directly irritate the gastric mucosa; and inhibition of COX-1 reduces the levels of protective prostaglandins.
Common gastrointestinal ADRs include: (Rossi, 2006)
- Nausea
- Dyspepsia
- Gastric ulceration/bleeding
- Diarrhea
Risk of ulceration increases with duration of therapy, and with higher doses. In attempting to minimise GI ADRs, it is prudent to use the lowest effective dose for the shortest period of time, a practice which studies show is not often followed.
There are also some differences in the propensity of individual agents to cause gastrointestinal ADRs. Indomethacin, ketoprofen and piroxicam appear to have the highest prevalence of gastric ADRs, while ibuprofen (lower doses) and diclofenac appear to have lower rates. (Rossi, 2006)
Certain NSAIDs, such as aspirin, have been marketed in enteric-coated formulations which are claimed to reduce the incidence of gastrointestinal ADRs. Similarly, there is a belief that rectal formulations may reduce gastrointestinal ADRs. However, in consideration of the mechanism of such ADRs and indeed in clinical practice, these formulations have not been shown to have a reduced risk of GI ulceration. (Rossi, 2006)
Commonly, gastrointestinal adverse effects can be reduced through suppressing acid production, by concomitant use of a proton pump inhibitor, e.g. omeprazole; or the prostaglandin analogue misoprostol. Misoprostol is itself associated with a high incidence of gastrointestinal ADRs (diarrhoea). While these techniques may be effective, they prove to be expensive for maintenance therapy.
Renal ADRs
NSAIDs are also associated with a relatively high incidence of renal ADRs. The mechanism of these renal ADRs is probably due to changes in renal haemodynamics (bloodflow), ordinarily mediated by prostaglandins, which are affected by NSAIDs. Horses are particularly prone to these adverse affects compared to other domestic animal species.
Common ADRs associated with altered renal function include: (Rossi, 2006)
- Salt and fluid retention
- Hypertension
These agents may also cause renal impairment, especially in combination with other nephrotoxic agents. Renal failure is especially a risk if the patient is also concomitantly taking an ACE inhibitor and a diuretic - the so-called "triple whammy" effect. (Thomas, 2000)
In rarer instances NSAIDs may also cause more severe renal conditions: (Rossi, 2006)
- Interstitial nephritis
Nephrotic syndrome
Acute renal failure
Acute tubular necrosis
Photosensitivity
Photosensitivity is a commonly overlooked adverse effect of many of the NSAIDs. (Moore, 2002) It is somewhat ironic that these anti-inflammatory agents may themselves produce inflammation in combination with exposure to sunlight. The 2-arylpropionic acids have proven to be the most likely to produce photosensitivity reactions, but other NSAIDs have also been implicated including piroxicam, diclofenac and benzydamine.
Benoxaprofen, since withdrawn due to its hepatotoxicity, was the most photoactive NSAID observed. The mechanism of photosensitivity, responsible for the high photoactivity of the 2-arylpropionic acids, is the ready decarboxylation of the carboxylic acid moiety. The specific absorbance characteristics of the different chromophoric 2-aryl substituents, affects the decarboxylation mechanism. While ibuprofen is somewhat of an exception, having weak absorption, it has been reported to be a weak photosensitising agent.
During pregnancy
NSAIDs are not recommended during pregnancy, particularly during the third trimester. While NSAIDs as a class are not direct teratogens, they may cause premature closure of the fetal ductus arteriosus and renal ADRs in the fetus. Additionally, they are linked with premature birth (Ostensen & Skomsvoll, 2004). Aspirin, however, is used together with heparin in pregnant women with antiphospholipid antibodies (Cervera & Balasch, 2004).
In contrast, paracetamol (acetaminophen) is regarded as being safe and well-tolerated during pregnancy (Graham et al., 2005). Doses should be taken as prescribed, due to risk of hepatotoxicity with overdoses (Wilkes et al, 2005).
Other ADRs
Common ADRs, other than listed above, include: raised liver enzymes, headache, dizziness (Rossi, 2006).
Uncommon ADRs include: heart failure, hyperkalaemia, confusion, bronchospasm, rash (Rossi, 2006). Ibuprofen may also rarely cause irritable bowel syndrome symptoms.
Most NSAIDs penetrate poorly into the central nervous system (CNS). However, the COX enzymes are expressed constitutively in some areas of the CNS, meaning that even limited penetration may cause adverse effects such as somnolence and dizziness.
Chirality
Most NSAIDs are chiral molecules (diclofenac is a notable exception). However, the majority are prepared in a racemic mixture. Typically, only a single enantiomer is pharmacologically active. For some drugs, (typically profens) an isomerase enzyme exists in vivo which converts the inactive enantiomer into the active form, although its activity varies widely in individuals. This phenomenon is likely to be responsible for the poor correlation between NSAID efficacy and plasma concentration observed in older studies, when specific analysis of the active enantiomer was not performed.
Ibuprofen and ketoprofen are now available in single, active enantiomer preparations (dexibuprofen and dexketoprofen), which purport to offer quicker onset and an improved side-effect profile. Naproxen has always been marketed as the single active enantiomer.
Newer NSAIDs: selective COX inhibitors
COX-2 inhibitors
The discovery of COX-2 in 1991 by Daniel L. Simmons at Brigham Young University raised the hope of developing an effective NSAID without the gastric problems characteristic of these agents. It was thought that selective inhibition of COX-2 would result in anti-inflammatory action without disrupting gastroprotective prostaglandins.
COX-1 is a constitutively expressed enzyme with a "house-keeping" role in regulating many normal physiological processes. One of these is in the stomach lining, where prostaglandins serve a protective role, preventing the stomach mucosa from being eroded by its own acid. When non-selective COX-1/COX-2 inhibitors (such as aspirin, ibuprofen, and naproxen) lower stomach prostaglandin levels, these protective effects are lost and ulcers of the stomach or duodenum and potentially internal bleeding can result. COX-2 is an enzyme facultatively expressed in inflammation, and it is inhibition of COX-2 that produces the desirable effects of NSAIDs.
The relatively selective COX-2 inhibiting oxicam, meloxicam, was the first step towards developing a true COX-2 selective inhibitor. Coxibs, the newest class of NSAIDs, can be considered as true COX-2 selective inhibitors, and include celecoxib, rofecoxib, valdecoxib, parecoxib and etoricoxib.
Controversies with COX-2 inhibitors
While it was hoped that this COX-2 selectivity would reduce gastrointestinal adverse drug reactions (ADRs), there is little conclusive evidence that this is true. The original study touted by Searle (now part of Pfizer), showing a reduced rate of ADRs for celecoxib, was later revealed to be based on preliminary data - the final data showed no significant difference in ADRs when compared with diclofenac.
Rofecoxib however, which has since been withdrawn, had been shown to produce significantly fewer gastrointestinal ADRs compared to naproxen. (Bombardier et al, 2000). This study, the VIGOR trial, raised the issue of the cardiovascular safety of the coxibs - a statistically insignificant increase in the incidence of myocardial infarctions was observed in patients on rofecoxib. Further data, from the APPROVe trial, showed a relative risk of cardiovascular events of 1.97 versus placebo - a result which resulted in the worldwide withdrawal of rofecoxib in October 2004.
COX-3 inhibitors
Simmons also co-discovered COX-3 in 2002 and analyzed this new isozyme's relation to paracetamol (acetaminophen), arguably the most widely used analgesic drug in the world. (Chandrasekharan et al, 2002). The authors postulated that inhibition of COX-3 could represent a primary central mechanism by which these drugs decrease pain and possibly fever.
The clinical ramifications and knowledge of COX isozymes are rapidly expanding and may offer significant hope for future treatments of pain, inflammation, and fever.
References
- Bombardier C, Laine L, Reicin A, Shapiro D, Burgos-Vargas R, Davis B, et al. Comparison of upper gastrointestinal toxicity of rofecoxib and naproxen in patients with rheumatoid arthritis. N Engl J Med 2000;343(21):1520-8. PMID 11087881
- Cervera R, Balasch J. The management of pregnant patients with antiphospholipid syndrome. Lupus 2004;13(9):683-7. PMID 15485103
- Chandrasekharan NV, Dai H, Roos KL, Evanson NK, Tomsik J, Elton TS, Simmons DL. COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression. Proc Natl Acad Sci U S A 2002;99:13926-31. PMID 12242329
- Graham GG, Scott KF, Day RO. Tolerability of paracetamol. Drug Saf 2005;28(3):227-40. PMID 15733027
- Green GA. Understanding NSAIDS: from aspirin to COX-2. Clin Cornerstone 2002;3:50-59. PMID 11464731
- Moore DE. Drug-induced cutaneous photosensitivity. Drug Safety 2002;25:345-72. PMID 12020173
- Ostensen ME, Skomsvoll JF. Anti-inflammatory pharmacotherapy during pregnancy. Expert Opin Pharmacother 2004;5(3):571-80. PMID 15013926
- Rossi S, editor. Australian Medicines Handbook 2006. Adelaide: Australian Medicines Handbook; 2006. ISBN 0-9757919-2-3
- Thomas MC. Diuretics, ACE inhibitors and NSAIDs – the triple whammy. Med J Aust 2000;172:184-185. PMID 10772593
- Wilkes JM, Clark LE, Herrera JL. Acetaminophen overdose in pregnancy. South Med J 2005;98(11):1118-22. PMID 16351032
