NSAIDs, Prostaglandins, COX1 and COX2

• NSAIDs are being discussed

Lecture Outline

• The lecture will cover: Prostaglandins, COX1 and COX2, and the classification of NSAIDs.
• Specific NSAIDs to be discussed include:
  • Paracetamol
  • Aspirin
  • Indomethacin
  • Diclofenac
  • Ibuprofen
  • Naproxen
  • Fenamic acid
  • Piroxicam
  • Meloxicam
  • Celecoxib

Prostaglandins

• Prostaglandins (PGs) are naturally occurring 20-carbon cyclopentano fatty acids.
• PGs belong to a class of eicosanoids that are derived from membrane phospholipids.
• All naturally occurring PGs have a 15α OH group and a trans double bond at C13.
• Different PGs are classified according to the nature and stereochemistry of oxygen substituents at C9 and C11.
• The number of double bonds in the chain is indicated with a subscript. (examples: C13,14; C5,6; C17,18)
• PGs are most commonly derived from arachidonic acid, resulting in PGs with 2 double bonds.
• Corticosteroids inhibit phospholipases, which are essential for PG synthesis.
• Arachidonic acid is acted upon by COX enzymes to form PGs or thromboxanes (TX), or by lipoxygenases to form leukotrienes.

COX1 & COX2

• Two main COX enzymes catalyze dioxygenation of arachidonic acid to PGG2 at the cyclooxygenase active site, and subsequent reduction to PGH2 at the peroxidase site.
• Acetaminophen (paracetamol) has analgesic and antipyretic activity but little anti-inflammatory activity, implying a different COX enzyme (COX3).
• COX1 and COX2 enzymes have similar structures.
• COX2 contains 18 amino acids inserted at the C-terminal, not found in COX1.
• COX2 contains valine (VAL) residues at positions 434 and 523.
• COX 1 has isoleucine (ILE) residues at these positions.
• COX2 inhibitors can access a side pocket not available in COX1.
• Differences in amino acid sequences allow for substrate selectivity:
  • COX1 is more specific for fatty acid substrates.
  • COX2 accepts a wider range of fatty acid substrates.
  • COX1 primarily metabolizes arachidonic acid.
  • COX2 metabolizes C18 and C20 substrates.
• There is little COX2 in resting cells, but expression can be induced in inflammatory sites.
• COX1 functions to produce PGs in normal cellular activity.
• NSAIDs can be non-selective for both COX1 and COX2, or selective for COX2 only.
• Renal and gastric side effects are associated with COX1 inhibition.

Classification of NSAIDs

• NSAIDs are classified into:
  • Carboxylic acids
    • Salicylic acids (e.g., Aspirin)
    • Acetic acids (e.g., Diclofenac, Indomethacin)
    • Propionic acids (e.g., Ibuprofen, Naproxen)
    • Fenamic acids (e.g., Mefenamic acid)
  • Enolic acids
    • Pyrazolones (e.g., Phenylbutazone)
    • Oxicams (e.g., Piroxicam, Meloxicam)
  • Non-acidic compounds
    • Nabumetone

Paracetamol

• Paracetamol (Acetaminophen) is an antipyretic analgesic.
• It is classified as a p-aminophenol.
• Paracetamol has analgesic and antipyretic properties.
• It lacks anti-inflammatory properties.
• It acts by inhibiting prostaglandin synthesis by inhibiting COX3.
• Paracetamol is not water soluble.

Paracetamol Metabolism and Toxicity

• Paracetamol undergoes hydrolysis to yield aniline, which can cause hemolytic anemia.
• CYP450-mediated metabolism is responsible for hepatotoxicity.
• It undergoes rapid first-pass metabolism in the GI tract, yielding O-glucuronides via conjugation reactions.
• N-hydroxamide can be produced, and is toxic in high concentrations.
• This is converted to a toxic metabolite (N-acetyl-p-benzoquinone-imine), which produces nephrotoxicity, but can be detoxified by conjugation with hepatic glutathione (small doses).
• Large doses of paracetamol can lead to liver failure.
• The antidote for paracetamol overdose is N-acetylcysteine, which serves as a glutathione substitute if liver stores are depleted.

Aspirin

• Aspirin is an anti-inflammatory drug and a salicylate.
• It is more active against COX1 than COX2.
• Aspirin prevents endoperoxide and 15-peroxidation of arachidonic acid.
• The active moiety of aspirin is the salicylate anion.
• GI side effects of aspirin seem to be due to the carboxylic acid function.
• Removing the carboxylic acid group reduces anti-inflammatory action.
• Activity is eliminated when the phenolic OH is moved to the ortho or para position.
• Addition of halogen to the aromatic ring enhances potency and toxicity.
• Substitution on the 5-position of the aromatic ring increases anti-inflammatory activity.
• Aspirin is acetyl salicylic acid.

Indomethacin

• Indomethacin is an aryl and heteroarylacetic acid.
• It is one of the most potent NSAIDs in use, more potent than aspirin.
• Indomethacin has many side effects.
• It undergoes rapid oral absorption and is converted to inactive metabolites (Metabolism).
• 50% of the dose is converted to inactive metabolites.

Diclofenac

• Diclofenac is synthesized from N-phenyl-2,6-dichloroaniline.
• It is marketed as Voltaren®.
• Diclofenac is a potent analgesic, antipyretic, and anti-inflammatory agent.
• Diclofenac acts in 3 ways: inhibits COX, inhibits lipoxygenase, and stimulates arachidonic acid reuptake.
• Diclofenac has rapid and almost complete oral absorption.
• It forms 4 major hydroxylated metabolites.
• The remainder of the dose is excreted as sulfate conjugates.
• Can also form toxic benzoquinone imine intermediates that are inactivated by hepatic glutathione.

Ibuprofen

• The S isomer is the active form of ibuprofen.
• Addition of an alpha methyl group on the alkanoic acid portion enhances anti-inflammatory properties and reduces side effects, while acetyl groups increase toxicity and reduce potency.
• All metabolites are inactive.

Naproxen

• Naproxen is marketed as Anaprox®.
• It is 55 times more potent than aspirin for anti-inflammatory properties and 0.7 times as strong as indomethacin.
• For analgesia, it is 7 times more potent than aspirin and only 10% as strong as indomethacin.
• Substitution at position 6 leads to maximum anti-inflammatory activity.
• Small lipophilic substituents result in more active analogs.
• Marketed as optically active isomers.

Fenamic Acids

• Fenamic acids are an anthranillic acid class of NSAIDs.
• They are structural analogs of diclofenac.
• Fenamic acids are potent inhibitors of PG synthesis.
• The compounds possess an aromatic ring, and an additional lipophilic chain leads to enhanced anti-inflammatory activity.
• Substitution on the anthranillic acid ring reduces activity.
• Substituents on the N-aryl ring that force the rings to be coplanar enhance binding and lead to enhanced anti-inflammatory activity.
• The NH moiety is essential for activity.
• Replacement of the CO₂H group has little effect on activity.

Mefenamic Acid

• Mefenamic acid marketed as Ponstel.
• It produces analgesia centrally and peripherally.
• It is absorbed rapidly after oral administration.
• Metabolism occurs through regioselective oxidation of the 3'-methyl group.
• Urinary excretion accounts for 50-55% of the administered dose.
• Metabolites are inactive.

Piroxicam

• Piroxicam has a long duration of action, but many side effects.
• It is extensively metabolized.
• Major metabolites arise from aromatic hydroxylation.

Meloxicam

• Meloxicam is a selective COX2 inhibitor.
• It is metabolized by oxidation of the thiazole methyl group, followed by oxidation to carboxylic acid.
• Its metabolites also have anti-inflammatory activity.

Celecoxib

• Celecoxib is a COX2 selective inhibitor.
• It is synthesized by condensing 4-methyl-acetophenone and ethyltrifluoroacetate with sodium methoxide.
• The resulting derivative is cyclized with 4-hydrazinophenylsulfonamide.
• It is excreted in feces and urine as inactive metabolites.
• Metabolism occurs in the liver.
• Involves hydroxylation of the 4-methyl group to a primary alcohol, which is then oxidized to the corresponding inactive carboxylic acid.
• The acid can also be conjugated.