Lecture 9: Anti-Inflammatories I

Classification of Anti-inflammatory and Immunosuppressant Drugs

• Anti-inflammatory and immunosuppressant medications are categorized into three primary groups:

  • Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) and Coxibs.

  • Antirheumatoid drugs.

  • Biologicals.

The Evolution and Chemistry of Aspirin

• Aspirin (acetylsalicylic acid) was developed as an improvement over salicylic acid. Salicylic acid was known to produce significant gut disturbances. The addition of an acetyl group ($CH_3-C-O$) to create acetylsalicylic acid resolved these gastrointestinal issues, as salicylic acid is no longer present in its original form in the body after administration.

• There are approximately 450 different versions of antipyretic analgesics that act in a manner similar to aspirin.

Arachidonic Acid Metabolism and Lipid Mediators

• The production of lipid inflammatory mediators begins with phospholipids in the cell membrane.

• Phospholipase $A_2$ Pathway: Phospholipase $A_2$ acts on phospholipids to produce Arachidonate and Lyso-glyceryl-phosphorylcholine. This step is inhibited by glucocorticoids, which induce the production of lipocortin.

• Cyclo-oxygenase (COX) Pathway:

  • Arachidonate is converted into cyclic endoperoxides by the enzyme Cyclo-oxygenase.

  • NSAIDs act as inhibitors of the COX enzyme.

  • Cyclic endoperoxides are converted into various prostaglandins ($PG$) and Thromboxanes ($TX$):

    • $PGE_{2}$: Acts as a vasodilator and hyperalgesic. It sensitizes nociceptors to painful stimuli (mediated by substances like bradykinin and histamine), leading to increased excitation. It is also involved in the induction of fever in the hypothalamus.

    • $PGI_{2}$ (Prostacyclin): A vasodilator and hyperalgesic that stops platelet aggregation.

    • $PGD_{2}$: Acts as a vasodilator and inhibits platelet aggregation.

    • $PGF_{2\alpha}$: Causes bronchoconstriction and myometrial contraction; its production is required during labor.

    • $TXA_{2}$ (Thromboxane $A_{2}$): A thrombotic agent and vasoconstrictor. Its synthesis involves $TXA_{2}$ synthase. Antagonists exist for both the synthase and the receptor.

• Lipoxygenase Pathways: Arachidonate is also processed by various lipoxygenases:

  • 5-Lipoxygenase: Produces 5-HPETE, which leads to Leukotrienes ($LT$) and Lipoxins A and B. Inhibitors include drugs like zileuton.

  • Leukotrienes:

    • $LTB_{4}$: Functions as a chemotaxin.

    • $LTC_{4}$, $LTD_{4}$, and $LTE_{4}$: Act as bronchoconstrictors and increase vascular permeability.

    • Leukotriene receptor antagonists include zafirlukast and montelukast.

  • 12-Lipoxygenase: Produces 12-HETE, which acts as a chemotaxin.

  • 15-Lipoxygenase: Also involved in the production of lipid mediators.

• Platelet-Activating Factor (PAF): Acts as a vasodilator, increases vascular permeability, and serves as a bronchoconstrictor and chemotaxin. Antagonists can inhibit its induction.

Mechanisms of Action and Physiological Effects of NSAIDs

• Anti-inflammatory Action: NSAIDs modify the inflammatory reaction by decreasing vasodilation and, consequently, reducing oedema. However, they are ineffective against mediators that contribute to tissue damage in chronic inflammatory conditions.

• Analgesic Action: They reduce specific types of pain by decreasing the production of prostaglandins in damaged and inflamed tissue. This prevents the sensitization of nociceptors to other inflammatory mediators like bradykinin and 5-HT (serotonin). Headaches are often linked to vasodilation, leading to swelling that affects nociceptors in the brain.

• Antipyretic Action: NSAIDs lower raised body temperature. The thermostat in the hypothalamus is activated by Interleukin-1 ($IL-1$), which induces COX-2 production of $PGE_{2}$. By inhibiting this, NSAIDs regulate temperature.

Cyclooxygenase (COX) Isoforms and Structure

• The COX enzyme is a dimer made of two identical subunits, each featuring two catalytic sites: a Cyclooxygenase site and a Peroxidase site. It is located in the Endoplasmic Reticulum (ER) membrane.

• COX-1 (Constitutive): Expressed in most tissues, including platelets, the stomach, kidney, and colon. It plays a homeostatic role, and its inhibition leads to side effects (e.g., gastric damage).

• COX-2 (Inducible): Found in most cells but specifically induced in inflammatory cells after stimulation by cytokines, growth factors, or tumor promoters. It is an immediate-early response gene.

• COX-3: Identified in the Central Nervous System (CNS) of dogs. In humans, it is considered a splice variant of COX-1 and is a potential target for paracetamol.

Selectivity of NSAIDs

• Selectivity is determined by the structural differences in the hydrophobic channels of the enzymes.

• Both COX-1 and COX-2 channels allow the entry of Arachidonic acid from the ER membrane to be converted into Prostaglandin $E_{2}$.

• Structural Difference: COX-1 has a larger Isoleucine residue at a key location, narrowing the channel. COX-2 has a smaller Valine residue, which creates a wider channel and an additional binding site in a side pocket.

• Traditional NSAIDs: These are non-selective and block both channels.

• COX-2 Specific Drugs: These are bulkier molecules that fit into the wider COX-2 channel and its side pocket but are too large to fit into the narrower COX-1 channel.

Classification by COX Selectivity

• Selectivity is often measured by the $log(IC_{50} \text{ ratio } COX-2/COX-1)$.

• COX-1 Selective: Ketorolac, Flurbiprofen, Suprofen, Ketoprofen, Aspirin, Naproxen, Tolmetin, Indometacin (Indomethacin), Ibuprofen.

• Low COX-2 Selectivity (<5-fold): Fenoprofen, Zomepirac, Meclofenamate, Diclofenac, Sodium salicylate, Tomoxiprol, Sulindac, Niflumic acid, Piroxicam, Diflunisal.

• Moderate COX-2 Selectivity (5-50-fold): Etodolac, Meloxicam, Celecoxib, Nimesulide.

• High COX-2 Selectivity (>50-fold): Rofecoxib.

Adverse Effects and Clinical Risks

• Gastrointestinal (GUT): Prostaglandins normally inhibit acid secretion and protect the mucosa. NSAIDs cause dyspepsia, diarrhoea, nausea, vomiting, gastric bleeding, and ulceration. Co-administration of misoprostol (a PG analogue) can be protective.

• Renal Function: Prostaglandins maintain renal blood flow. NSAID use can lead to renal failure. COX-2 inhibitors can affect salt retention in the kidneys.

• Cardiovascular (CV) Risk: High COX-2 selective drugs (e.g., Rofecoxib) can cause serious CV side effects, such as heart failure. Rofecoxib was initially seen as a "wonder drug" until patients died from heart failure related to salt retention and effects on vasodilation/vasoconstriction balance.

• Other Side Effects:

  • Liver damage (specifically associated with paracetamol toxicity).

  • Bronchospasm and asthma attacks.

  • Skin rashes.

Aspirin vs. Paracetamol

• Aspirin:

  • Acts as a "suicide inhibitor" (irreversible inhibitor) by binding covalently to a Serine (Ser) residue in the COX enzyme, blocking arachidonic acid from the cyclooxygenase site.

  • Provides anti-platelet action.

  • Associated with reduced risk of colonic and rectal cancer.

  • Potential reduced risk of Alzheimer’s disease.

  • It is a weak acid, rapidly and efficiently absorbed in the ileum.

• Paracetamol (Acetaminophen):

  • Inhibits the peroxidase function of COX and other enzymes.

  • Effective at low levels of arachidonic acid and peroxides; actions are reduced in severe inflammation where these levels are high.

  • Strong analgesic and antipyretic due to CNS effects, but a weak anti-inflammatory agent.

  • Well absorbed and metabolized in the liver.

  • Fewer side effects than aspirin with long-term use, but large doses cause kidney damage.

  • Hepatotoxicity: The metabolite N-acetyl-p-benzoquinone imine is hepatotoxic when it exists in an unconjugated form.

Clinical Summary and Applications

• Anti-thrombotic: Aspirin is used for patients at high risk of arterial thrombosis.

• Analgesia:

  • Short-term: Aspirin, paracetamol, ibuprofen.

  • Chronic pain: More potent, longer-lasting drugs such as Naproxen (sometimes combined with codeine).

  • Can be used to reduce the requirement for narcotic analgesics.

• Anti-inflammatory: Ibuprofen, naproxen.

• Antipyretic: Paracetamol.