Aspirin: Comprehensive Study Notes

Aspirin (Acetylsalicylic Acid)

Overview

Aspirin is a derivative of salicylic acid. It is classified as a Non-Steroidal Anti-Inflammatory Drug (NSAID). It was the first drug in this class to be discovered and remains one of the most widely used medications globally.

Preparation (Synthesis)

Aspirin is synthesized via the acetylation of salicylic acid. In this reaction, the phenolic hydroxyl group of salicylic acid reacts with acetic anhydride.

• Reagents: Salicylic acid and Acetic Anhydride.
• Catalyst: Concentrated Sulfuric acid H₂SO₄ or Phosphoric acid.
• By-product: Acetic acid.

Properties

Physical Properties

• Appearance: White crystalline solid.
• Melting Point: Approximately 136°C.
• Solubility: Low solubility in water, but dissolves readily in ethanol, ether and DMSO.

Chemical Properties

• Hydrolysis: In the presence of moisture, aspirin hydrolyzes back into salicylic acid and acetic acid, giving it a distinct vinegar-like odor.
• Acidity: It is a weak acid due to the presence of the carboxyl (—COOH) group.

Mechanism of Action

Aspirin works by irreversibly inhibiting the enzymes Cyclooxygenase-1 (COX-1) and Cyclooxygenase-2 (COX-2).

• Prostaglandins: By inhibiting COX enzymes, aspirin reduces the production of prostaglandins, which are mediators of inflammation, fever, and pain.
• Platelets: It inhibits the synthesis of Thromboxane A2. Since platelets cannot regenerate this enzyme, the "blood-thinning" effect lasts for the life of the platelet (7-10 days).

Medical Uses

• Analgesic: Relief of minor aches and pains (headache, muscle ache).
• Antipyretic: Reduction of fever.
• Anti-inflammatory: Treatment of arthritis and rheumatic fever.
• Anticoagulant: Used in low doses to prevent heart attacks and strokes.

Side Effects and Contraindications

While Aspirin is a versatile drug, it must be used with caution due to its systemic effects.

• Gastrointestinal Issues: Inhibition of COX-1 reduces the production of protective mucus in the stomach, which can lead to gastric ulcers and GI bleeding.
• Reye's Syndrome: A rare but fatal condition that can occur in children and teenagers recovering from viral infections (like flu or chickenpox) if they take aspirin. It causes swelling in the liver and brain.
• Anti-platelet Risk: Because it thins the blood, it can increase the risk of excessive bleeding during surgery or after an injury.
• Aspirin-Exacerbated Respiratory Disease (AERD): Some individuals with asthma may experience severe bronchospasms after taking aspirin.

Practice Questions

Q1. Aspirin is chemically known as:

• (A) 2-Acetyloxybenzoic acid
• (B) Methyl salicylate
• (C) Acetylsalicylaldehyde
• (C) Phenyl salicylate

Correct! Aspirin is the acetyl derivative of salicylic acid, officially named 2-acetyloxybenzoic acid.

Q2. Which enzyme does Aspirin inhibit to reduce pain and inflammation?

• (A) Lipoxygenase
• (B) Cyclooxygenase
• (C) Acetylcholinesterase
• (D) DNA Polymerase

Correct! Aspirin inhibits COX-1 and COX-2 enzymes, preventing prostaglandin synthesis.

Q3. Why is Aspirin generally avoided in children with viral fevers?

• (A) It causes permanent tooth discoloration
• (B) Risk of Reye's Syndrome
• (C) It is not effective against viral fever
• (D) It leads to sudden Vitamin D deficiency

Correct! Reye's Syndrome is a serious condition associated with aspirin use in children during viral illnesses. Acetaminophen (Paracetamol) is usually preferred instead.

Q4. Prolonged use of high-dose Aspirin can lead to stomach ulcers because it:

• (A) Increases the pH of stomach acid
• (B) Neutralizes digestive enzymes
• (C) Inhibits protective prostaglandin synthesis
• (D) Stimulates the growth of H. pylori bacteria

Correct! Prostaglandins (specifically PGE2) help maintain the stomach's protective lining. Inhibiting their synthesis leaves the stomach wall vulnerable to acid.