Aspirin: the Molecule of the 20th Century That Keeps Surprising

The willow tree and the modern molecule

The use of white willow (Salix alba) to relieve fevers and pain is recorded in Egyptian papyri some 3,500 years old. What the Egyptians did not know was why it worked: willow bark contains salicin, a glycoside that the body converts into salicylic acid, the active ingredient behind the effect.

Salicylic acid works, but it has a serious drawback: its acidity strongly irritates the lining of the mouth and stomach. In 1897 Felix Hoffmann, a chemist at Bayer, acetylated the phenolic –OH of salicylic acid to make acetylsalicylic acid, less irritating and just as effective. Bayer launched it as "Aspirin" in 1899, and today an estimated 100 billion tablets are taken every year.

Structure: the acetyl group that changes everything

Acetylsalicylic acid (C₉H₈O₄) is structurally simple: a benzene ring with a carboxyl group (–COOH) at position 1 and an acetyloxy group (–OCOCH₃) at position 2. This ortho arrangement has important consequences:

Limited stability. Aspirin's acetyl group is labile in water: it slowly hydrolyses to salicylic acid and acetic acid. That is why old aspirin tablets smell of vinegar (acetic acid) and lose potency. Hydrolysis is faster at physiological pH (7.4) than in acid, and in blood esterases speed it up further: aspirin itself has a half-life of only 15–20 minutes.

Yet the mechanism is irreversible. This paradox is fundamental: although aspirin is hydrolysed quickly, the damage it does to its molecular target is permanent. What matters is not its plasma half-life but how long it is in contact with the COX enzyme.

The mechanism: irreversible inhibition of COX

Aspirin's molecular target was identified by John Vane in 1971, work that earned him the 1982 Nobel Prize in Physiology or Medicine. Aspirin inhibits cyclooxygenase (COX), the enzyme that catalyses the first step in making prostaglandins from arachidonic acid.

The mechanism is unique among anti-inflammatories: aspirin covalently and irreversibly acetylates a serine residue (Ser-530 in COX-1, Ser-516 in COX-2) in the hydrophobic channel leading to the active site. The acetyl group of acetylsalicylic acid is transferred to the serine's –OH, physically blocking arachidonic acid from reaching the catalytic site.

This irreversible inhibition has huge practical consequences:

In cells with a nucleus: the acetylated COX is degraded and the cell can make new enzyme. The effect of a dose of aspirin lasts as long as that inhibited COX does, roughly 4–6 hours.

In platelets: platelets have no nucleus and cannot make new protein. A single dose of aspirin inhibits platelet COX-1 for the platelet's entire life (8–10 days). Production of thromboxane A₂ (which promotes platelet clumping) is permanently blocked. That is why low-dose aspirin (75–100 mg a day) is an effective antiplatelet drug at doses that barely touch COX-2 in other tissues.

Four effects, one enzyme

COX inhibition explains all four of aspirin's pharmacological effects:

Pain relief. Prostaglandins E₂ and I₂ sensitise peripheral pain receptors, lowering the activation threshold of C and Aδ fibres. By blocking their synthesis, aspirin raises the pain threshold.

Fever reduction. Fever is driven by prostaglandin E₂ in the hypothalamus, which acts on the preoptic area to raise the body's temperature set point. Aspirin reduces central PGE₂ synthesis and brings fever down without affecting normal body temperature.

Anti-inflammatory action. Prostaglandins are key mediators of inflammation: they cause vasodilation, increase vascular permeability and attract white blood cells. Aspirin cuts these effects off at the source.

Antiplatelet action. Thromboxane A₂ (TXA₂), made by platelets via COX-1, promotes platelet aggregation and vasoconstriction. Irreversible inhibition of platelet COX-1 blocks TXA₂ production for the platelet's whole life, an effect that accumulates with daily doses of 75–100 mg.

COX-1 vs COX-2: the stomach's Achilles heel

Aspirin inhibits both COX-1 (constitutive, present in the stomach, kidneys and platelets) and COX-2 (inducible, overexpressed in inflammation). Aspirin's stomach problem comes from inhibiting gastric COX-1, which makes protective prostaglandins (PGE₂ and PGI₂) that stimulate mucus and bicarbonate secretion, restrain acid secretion and maintain blood flow to the stomach lining. Without them, the lining is left exposed to stomach acid.

That insight drove the development of selective COX-2 inhibitors (celecoxib, rofecoxib) in the 1990s: keep the anti-inflammatory effect (COX-2) without the stomach damage (COX-1). The plan worked for the stomach, but rofecoxib (Vioxx) was withdrawn in 2004 for increasing cardiovascular risk, precisely because endothelial COX-2 makes prostacyclin (PGI₂), which counterbalances platelet TXA₂. Without endothelial COX-2, the balance tips towards clotting and thrombosis.

New uses: chemoprevention and more

Since the 1980s, epidemiological studies have consistently shown that regular aspirin users have a lower incidence of colorectal cancer. The proposed mechanism: COX-2 is overexpressed in cancerous colorectal tissue, and its prostaglandins promote cell proliferation; by inhibiting COX-2, aspirin suppresses this pathway.

Controlled clinical trials have confirmed a ~20 % reduction in colorectal adenomas with low-dose aspirin. The benefit is not risk-free, though: gastrointestinal bleeding increases with long-term use. Current guidelines no longer recommend aspirin to prevent colorectal cancer in the general population; it is considered case by case in high-risk people (for example, those with Lynch syndrome) who have a low bleeding risk.

The next article explores aspirin's modern heir: ibuprofen, a reversible COX inhibitor that is gentler on the stomach, and the chemistry behind its advantages and its risks.