What is the mechanism of action of Aspirin?

• A. Irreversible COX 1 and COX 2 inhibitor
• B. Reversible COX 1 inhibitor
• C. Inhibits leukotriene synthesis
• D. Blocks prostaglandin receptors

Answer: (A)

Aspirin works by irreversibly inhibiting the cyclooxygenase enzymes (COX-1 and COX-2) through acetylation of a serine in the active site. This stops arachidonic acid from being converted into prostaglandin G2/H2 and subsequently reduces the production of prostaglandins and thromboxane A2. In platelets, which cannot synthesize new proteins, this inhibition lasts for the platelet’s lifetime (about 7–10 days), giving a lasting antiplatelet effect. At low doses, the effect is mainly on COX-1 in platelets, reducing thromboxane A2 and platelet aggregation; higher doses further suppress prostaglandin synthesis, contributing to anti-inflammatory and analgesic actions. The other options are not correct because leukotriene synthesis (5-lipoxygenase pathway) is unaffected by COX inhibitors, aspirin does not block prostaglandin receptors, and its inhibition of COX is irreversible, not reversible.