Temporal variations in paracetamol absorption and metabolism in man

Area of Science:

• Pharmacokinetics
• Chronopharmacology

Background:

• Paracetamol (acetaminophen) is a widely used analgesic and antipyretic.
• Understanding its absorption and metabolism is crucial for optimizing therapeutic use.

Purpose of the Study:

• To investigate the impact of oral administration timing on paracetamol absorption and metabolism in healthy subjects.
• To determine if temporal variations influence paracetamol glucuronide formation and excretion.

Main Methods:

• Oral administration of 1.5 g paracetamol to non-fasted healthy subjects at different times of day (8:00 a.m. vs. 8:00 p.m.).
• Analysis of paracetamol and its metabolites (paracetamol glucuronide) in urine and saliva.
• Intravenous administration of paracetamol to assess pharmacokinetic parameters independent of absorption.

Main Results:

• Significantly higher paracetamol glucuronide levels were found in 0-4 hour urine samples when paracetamol was administered at 8:00 a.m. compared to other times.
• Salivary paracetamol concentrations showed marked differences in the first 90 minutes, indicating decreased absorption at 8:00 p.m.
• No significant temporal variation was observed in the excretion of intact paracetamol or other metabolites via urine.
• Intravenous administration showed no difference in clearance, half-life, or volume of distribution between morning and evening administration.

Conclusions:

• Oral paracetamol absorption exhibits temporal variation, being faster in the morning than in the evening.
• The increased excretion of paracetamol glucuronide in the morning is likely due to enhanced absorption rates.
• These findings highlight the importance of administration timing for paracetamol's pharmacokinetic profile.