Mechanism of Action and Pharmacodynamic Triad
The therapeutic effect of NSAIDs is based on the inhibition of the enzyme cyclooxygenase (COX), which disrupts the synthesis of prostaglandins (PGs). This provides three baseline clinical effects:
- Anti-inflammatory. Inhibition of COX-2 activity decreases the local production of key inflammatory mediators—prostaglandins $D_2$, $E_2$, and prostacyclin ($I_2$).
- Analgesic (pain-relieving). A reduction in $PGE_2$ concentration protects nociceptors (pain receptors) from sensitization (increased sensitivity). As a result, receptors stop reacting acutely to chemical stimuli, such as histamine and bradykinin, which are abundantly released in the focus of inflammation. These drugs are most effective specifically for pain of inflammatory origin.
- Antipyretic. The target is the thermoregulatory center in the anterior hypothalamus. Lowering $PGE_2$ levels triggers heat dissipation mechanisms: peripheral blood vessels dilate and sweating increases. Important nuance: NSAIDs reduce only febrile (elevated) temperature and have no effect on normal body temperature.
Classification by Selectivity
All drugs in this group are classified based on which specific COX isoforms they block, as well as the reversibility of their binding to the enzyme:
- Nonselective inhibitors (block both COX-1 and COX-2):
- Irreversible action: salicylates (acetylsalicylic acid).
- Reversible action: propionic acid derivatives (ibuprofen, ketoprofen, naproxen), acetic acid derivatives (diclofenac, indometacin, ketorolac), oxicams (piroxicam, meloxicam), pyrazolidines (phenylbutazone), and fenamates (mefenamic acid).
- Preferential COX-2 inhibitors: sulfonanilides (nimesulide).
- Selective COX-2 inhibitors: coxibs (celecoxib, rofecoxib).
Unique Properties of Aspirin
Acetylsalicylic acid is the historical standard and prototype of the entire NSAID group. Its predecessors (natural salicylates) were extracted from the bark of the willow tree (Salix) and meadowsweet (Spiraea, Filipendula ulmaria). The synthetic drug was developed by Felix Hoffmann in 1899.
The main pharmacological distinction of aspirin is its ability to inhibit COX irreversibly via acetylation. At low doses (80–325 mg/day), the drug exhibits a pronounced antiplatelet effect.
The mechanism is based on cellular differences:
- In platelets: aspirin irreversibly disables COX-1. Because platelets lack a nucleus, they cannot synthesize a new enzyme. The production of the pro-aggregant thromboxane $A_2$ is halted for the entire lifespan of the cell (3–7 days).
- In endothelial cells: vascular cells possess a nucleus. They rapidly synthesize COX de novo and restore the production of prostacyclin ($PGI_2$), which inhibits aggregation.
As a result, the balance of clotting factors shifts toward protecting blood vessels from thrombus formation.
Alternative Metabolic Pathway: Lipoxygenase
In addition to the classical cyclooxygenase pathway, arachidonic acid can serve as a substrate for another enzyme—5-lipoxygenase. This cascade synthesizes a distinct group of eicosanoids known as leukotrienes.
Leukotrienes are potent inflammatory mediators. They actively participate in the pathogenesis of allergic reactions and play a significant role in the development of respiratory and cardiovascular diseases.
Therapeutic Prescription Strategy
The fundamental principle of pharmacology states that inflammation is viewed as a protective, evolutionarily developed response of the organism. Therefore, the pharmacological suppression of this process requires strict justifications.
Indications for prescribing NSAIDs include:
- High symptom severity (the presence of severe, exhausting pain or dangerous hyperthermia).
- Protracted course (transition of the inflammatory process into a chronic form).
- Specific pathogenesis (autoimmune nature of inflammation, where the immune system mistakenly attacks its own tissues).