Indications
Aspirin, known chemically as acetylsalicylic acid (ASA), is primarily indicated for the relief of pain, fever, and inflammation associated with various conditions such as influenza, the common cold, neck and back pain, dysmenorrhea, headaches, toothaches, sprains, fractures, myositis, neuralgia, synovitis, arthritis, bursitis, burns, and other injuries. It provides symptomatic pain relief following surgical and dental procedures. Additionally, the extra-strength formulation is used for managing migraine pain accompanied by photophobia and phonophobia. Beyond pain relief, ASA plays a significant role in cardiovascular health, reducing the risk of cardiovascular death in suspected myocardial infarction cases and preventing non-fatal myocardial infarctions. It is also prescribed to minimize the risk of transient ischemic attacks and prevent atherothrombotic cerebral infarctions, often in conjunction with other therapies. Furthermore, ASA is utilized to prevent thromboembolism post-hip replacement surgery and reduce platelet adhesion post-carotid endarterectomy, aiding in the prevention of TIAs. It also helps prevent thrombosis in hemodialysis patients using a silicone rubber arteriovenous cannula.
Pharmacodynamics
Aspirin exerts its analgesic and antipyretic effects through the inhibition of cyclooxygenase enzymes COX-1 and COX-2, which disrupts the production of prostaglandins, substances known to enhance pain receptor sensitivity. This disruption results in decreased symptoms of pain and fever. As an antipyretic, ASA interferes with brain prostaglandin E1 production, a potent fever-inducing agent. In terms of platelet aggregation, ASA inhibits COX-1, which reduces thromboxane A2 production, a lipid responsible for platelet aggregation and clot formation. Recent studies have explored ASA's potential in cancer prevention, indicating its role in interfering with cancer signaling pathways and possibly upregulating tumor suppressor genes.
Absorption
The absorption of aspirin following oral administration is generally rapid and complete, although it can vary based on several factors, including the route of administration, dosage form, and gastric conditions such as contents, emptying time, and pH. Orally ingested ASA is absorbed through the stomach and proximal small intestine, with non-ionized ASA passing into the stomach lining by passive diffusion. Optimal salicylate absorption in the stomach occurs at pH values ranging from 2.15 to 4.10. Intestinal absorption proceeds at a faster rate, with at least half of the ingested ASA dose being hydrolyzed to salicylic acid within the first hour post-ingestion. Peak plasma concentrations of salicylate are typically reached 1 to 2 hours after administration.
Metabolism
Aspirin undergoes hydrolysis in the plasma, converting into salicylic acid. After administration, particularly with the extended-release formulation, plasma concentrations of ASA become largely undetectable 4 to 8 hours post-ingestion of a single dose. Salicylic acid is notably present 24 hours after a dose of extended-release ASA. While the liver is the primary site for salicylic acid metabolism, other tissues may also contribute to this process. The major metabolic products include salicyluric acid, phenolic glucuronide, and acyl glucuronide, with minor conversion to gentisic acid and other hydroxybenzoic acids.
Mechanism of Action
Aspirin, known chemically as acetylsalicylic acid (ASA), functions primarily through the inhibition of prostaglandin synthesis by targeting the cyclooxygenase (COX) enzymes, specifically COX-1 and COX-2. This inhibition occurs when the acetyl group of ASA covalently attaches to a serine residue on the COX-1 enzyme, resulting in an irreversible blockade. The suppression of COX-1 activity inhibits the formation of thromboxane A2 (TXA2), a key inducer of platelet aggregation, effectively reducing the potential for clot development and consequently lowering the risk of thromboembolic events such as pulmonary embolism and stroke. This effect on platelets persists for approximately 7 to 10 days, the typical lifespan of a platelet. Despite a 60% structural homology between COX-1 and COX-2 proteins, ASA demonstrates a higher affinity for COX-1 due to structural differences in their active sites; COX-2's slightly larger active site allows arachidonic acid to bypass ASA, necessitating higher concentrations of ASA to achieve significant COX-2 inhibition. Thus, while aspirin is non-selective, it preferentially inhibits COX-1 at standard doses.
