Non-Steroidal Anti-Inflammatory Drugs

Nikita Shokur

25 April 2014

Medicinal Chemistry

Introduction:

Non-steroidal anti-inflammatory drugs (NSAIDs) are drugs

from diverse structural classes that show analgesic, anti-

inflammatory, and antipyretic activities. Ever since the

development of NSAIDs, the need for these drugs has been

exponentially growing due to the broad range of applications for

these medications and the recent development of selective COX-2

inhibitors. The use of NSAIDs is seen primarily in patients with

musculoskeletal illnesses. Their use relieves temporary

conditions such as sprains, strains, back pain, headaches,

menstrual cycle pains, colon polyps and long term conditions such

as rheumatoid arthritis, osteoarthritis, ankylosing spondylitis,

lupus, and clotting of blood.1 However, NSAIDs have also shown

extensive adverse effects including gastrointestinal bleeding,

peptic ulcer disease, hypertension, edema, and renal disease.2 In

2012, the use of NSAIDs has been associated with the risk of

acute myocardial infarction.3

NSAIDs act by blocking cyclooxygenase enzyme systems. In

1990, an important discovery was made differentiating between the

two forms of cyclooxygenase enzyme: the COX-1 that produces

prostaglandins and thromboxanes that regulate gastrointestinal,

renal, and vascular functions and COX-2 that regulates the

produces prostaglandins involved inflammation, pain, and fever.4

In the past two decades, an enormous commercial development of

selective COX-2 enzyme inhibitors, known as coxibs, has been

started. The idea was to reduce the stomach bleeding caused by

the inhibition of COX-1 enzyme, which protected the stomach

lining with mucous otherwise destroyed by acid.

Today, the use of NSAIDs continues to grow at an incredible

rate. Between 1984 and 1988, the number of patients receiving

prescriptions for NSAIDs jumped from 44 million to 70 million; by

1997 the worldwide

market for these medications was

over six billion dollars.2

Recent development of COX-2

selective inhibitor Celebrex has

brought in four billion

dollars since its debut in

1999 and became the sixth best-selling drug.5 Other classes of

NSAIDs include Fenamates, Indole Acetic Acids, Oxicams, Propionic

Acids, and Salicylates (Figure 1).6

Figure 1. Major Classes of NSAIDs.

Discovery:

The first signs of NSAIDs use dates back about 3500 years

when Greek physician Hippocrates prescribed willow bark and

leaves to reduce fever and inflammation.7 In the 17th century,

salicin was identified as an active component of willow bark; in

the midst of the following century, Germany started producing

salicylic acid. It wasn’t until the 1899 that salicylic acid was

converted to acetylsalicylic acid, a more palatable form

currently known as Aspirin, and marketed by Bayer (Figure 2).7

Figure 2. Acetylation of

salicylic acid to

acetylsalicylic acid.

Pharmocology:

The pharmacology of salicylates will be described. Other

classes of drugs have similar pharmacological properties and

follow a comparably similar absorption, distribution, metabolism,

and excretion (ADME).

Salicylates act as antipyretic, analgesic, anti-

inflammatory, and uricosuric drugs. The inhibit blood clotting,

which contributes to the prevention of strokes and heart attacks,

due to inhibition of thromboxane A2. Salicylates are given orally

and have around a 55% bioavailability.2 Majority is absorbed in

the small intestine and some are absorbed in the stomach due to

passive diffusion. Absorption takes between 20-30 minutes after

oral intake as it is readily hydrolyzed in blood and liver .8

They are distributed throughout most of the body’s tissues.

Salicylates are initially converted to salicylic acid. Average

elimination time is about 24 hours; inactivation occurs in the

hepatic endoplasmic reticulum and mitochondria. Majority of it is

excreted after undergoing conjugation with glycine or with

glucuronic acid to form ether or ester, while around 10% of it is

excreted as a free acid, and trace amounts salicylates are

hydroxylated before excretion.2,8

SAR:

General Structure Activity Relationship (SAR) for all anti-

inflammatory non-steroidal drug has been identified.8 All agents

must possess a center of acidity represented by groups such as

carboxylic acids, enols, sulfanomide, or tetrazole groups. Amide

or ester derivatives of carboxylic acids are usually attributed

to the metabolic hydrolysis products. The center of acidity for

NSAIDs is generally located a distance of one carbon away from a

flat surface represented by an aromatic or hetero-aromatic ring.

This distance plays a critical role because as the distance

increases the activity of the drug diminishes. Aryl and hetero-

aryl derivatives are common since they correlate with the double

bond at the 5- and 8- position of the arachidonic acid (Figure

X).

Substitution of a methyl group on the alpha carbon has shown to

increase anti-inflammatory activity. The resulting derivatives,

alpha-methyl acetic, have been given a class name profen. Groups

larger than methyl show a decline in activity. The importance of

the methyl group comes from a creation of a new chiral center;

the anti-inflammatory activity is associated with the S (+)

enantiomer. The activity is further enhanced by an addition of a

second area of lipophilicity that is non-coplanar with the

original aromatic or hetero-aromatic ring. This second

lipophilic area corresponds to the double bond region at the

positon 11- of arachidonic acid.

Figure X. Arachidonic Acid.

Mechanism of Action:

The discovery of the mechanism of aspirin by John Vane in

the seventies has spiked our ability to develop NSAIDs. In the

early 90’s, Needleman, Simmons, and Herschman’s group reported a

presence of an isoform of cyclooxygenase, later named COX-2.7

While traditional NSAIDs inhibited both COX-1 and COX-2, the

adverse effects of gastrointestinal toxicity were attributed to

gastro protective prostaglandins and prostocyclins produced via

COX-1 pathway. Since then scientists and pharmaceutical companies

have shifted their focus on developing selective COX-2

inhibitors. Their efforts led to the first selective COX-2

inhibitor celecoxib and followed by rofexocib (Figure X).

Figure X. COX-2 with bound Celecoxib.

Arachidonic acid is produced from fatty acids by

Phosopholipadase A. Cyclooxygenase then catalyzes the oxidation

of arachidonic acid to prostoglandin G2, which is then reduced to

prostoglandin H2 (Figure X).7 COX-1 is compromised of 602 amino

acids whereas COX-2 has 604 amino acids. The two isoforms share

60-65% homology.7 The major difference is the substitution of

Isoleucine-523 in COX-1 for Valine-523 in COX-2.9 This opens a

new pocket for binding and allows for selectivity.

There are two types of inhibition of cyclooxygenase: the

irreversible ihibition and competitive inhibition of the

substrate. The irreversible inhibition is unique to Aspirin, the

drug permanently acetylates the key Serine-530 within the active

site.8 This inhibition is irreversible since plateles do not

carry out protein synthesis, thus inhibition lasts the cells

lifetime. The competitive inhibition of the substrate

(arachidonic acid) is reversible when the drug washes out or is

competing for the bidning site. The carboxylate part of the drug

bonds to Arginine-120 and Tyrosine-355.10 Methyl and the ring

structures fit into the hydrophobic pocket.

Cyclooxygenase 1 enzymes are thought to be constitutive,

being expressed in most of our bodily tissues. While

cyclooxygenase 2 enzymes is induced by cell-signaling proteins

called cytokines and is partially constitutive.Both are

responsible for the physiological production of prostaglandins.

Prostaglandins have a wide range of functions.8 PGI2 and PGE2 are

responsible for pain by sensitizing nerve endings to bradykinin,

histamine, and substance P. PGI2, PGD2, and PGE2 are vasodilators

responsible for inflammation. PGI2 protects the gastric mucosa.

PGE2 maintains renal blood flow and is responsible for fever. TXA2

stimulates platelet aggregation. PGD2 contracts uterus.

Figure X. Biosynthesis of prostaglandin from arachidonic acid via

COX isoform pathway.

NSAIDs can be classified into four groups.7 Group 1

includes NSAIDs that inhibit both COX-1 and COX-2 with very

little selectivity. Group 2 includes NSAIDs that inhibit COX-2

with 5-50 fold selectivity. Group 3 includes NSAIDs that inhibit

COX-2 with over 50 fold selectivity. And Group 4 shows weak

inhibition of both cyclooxygenase isoforms.

Figure X. Classification of NSAIDs according to their COX-1/2

activities.

Future Developments:

Despite the promise of therapeutic effectiveness of the

selective COX-2 inhibitors, there

have been cases reporting the arousal of severe cardiovascular

effects with the use of these inhibitors.8 Long-term clinical

trials of rofecoxib, a Group 3 coxib, that indicated an increased

risk of myocardial infarction. A later analysis of a vast

population has shown that the risk of heart problems grew many

folds while on rofecoxib compared to some older NSAIDs. These

effects may be related to the degree of selectivity of COX-1 and

COX-2 inhibitors. COX-1 mediates the production of prostaglandins

and thromboxanes, which are responsible for platelet aggregation,

and COX-2 produces prostacyclin, which regulates platelet

aggregation. The inhibition of only COX-2 significantly reduces

the production of prostacyclins, whereas the production of

thrombaxanes is unaffected. Thus, future developments of NSAIDs

may be aimed at preferentially inhibiting COX-2 while also having

an inhibition on COX-1 to a lesser extent.

Recent research has also shown a link between COX-2 and

cancer. The study revealed an over-expression of COX-2 mRNA in

colorectal cancer.12 Levels of COX-2 mRNA were found

overexpressed in almost 80% of the colorectal tumors compared to

normal colorectal mucosa. This suggests that COX-2 could be used

as a possible biomarker for the risk of cancer and that it could

be of value in chemoprevention of colon cancer.

References:

1. American College of Rheumatology Members. ‘‘NSAIDs:

Nonsteroidal Anti-inflammatory

Drugs’’ American College of Rheumatology, 2012, Web.

2. Saraf, S. Non-Steroidal Anti-Inflammatory Drugs, PharmaMed Press, 2008,

12-13,

168-169.

3. Krotz, F.; Schiele, T. M.; Klauss, V.; Sohn, H. Y.;

‘‘Selective COX-2 inhibitors and risk of

myocardial infarction’’ Journal of Vascular Research, 2005, 42(4),

312-24.

4. Rainsford, K. D. ‘‘Anti-Inflammatory Drugs In the 21st

century’’ Subcellular Biochemistry,

2007, 42, 3-27.

5. Halpern, G.M. ‘‘COX-2 Inhibitors: a story of greed, deception,

and death’’

Inflammopharmacology, 2005, 13(4), 419-25.

6. Monteleone, G. ‘‘Classes of NSAIDs’’ SportsMedicine. Web.

7. Pravee Rao, P.N.; Knaus, E.E. ‘‘Evolution of Nonsteroidal

Anti-Inflammatory Drugs

NSAIDs):Cyclooxygenase (COX) Inhibition and Beyond’’ Journal

of Pharmaceutical

Sciences, 2008, 11(2), 81-110.

8. Bkhaitan, M. ‘‘Non-Steroidal Anti-inflammatory Drugs’’

Department of Pharmaceutical

Chemistry, Web.

9. Gierse, J., McDonald, J., Hauser, S.; ‘‘A Single Amino Acid

Difference between

Cyclooxygenase-1 (COX-1) and −2 (COX-2) Reverses the Selectivity of COX-2 Specific

Inhibitors’’ Journal of Biological Chemistry, 1996, 271, 15810-15814.

10. Dunlap, N. ‘‘Enzyme Inhibitors’’ Medicinal Chemistry, Custom

Publishing, 2013, 88-93.

11. Billones, J.; Buenaobra, S. ‘‘Quantitative Structure-ActivityRelationship (QSAR)Study of

Cyclooxygenase-2 (COX-2) Inhibitors’’ Philippine Journal of Science,2008, 140(2),

125-132.

12. Roelofs, H. ‘‘Over-expression of COX-2 mRNA in colorectal

cancer’’ Biomedical Central

Gastroenterology, 2014, 14(1).