Oral Bioavailability of Azithromycin in Children
The oral bioavailability of azithromycin in pediatric patients is approximately 37%, similar to adults, with food reducing absorption by approximately 50% 1, 2.
Pharmacokinetic Profile in Children
The bioavailability characteristics are consistent across pediatric age groups:
- Absorption: Approximately 37% of an oral dose reaches systemic circulation 2
- Food effect: Concomitant food administration significantly decreases bioavailability by 50% 2
- Peak concentrations: Achieved within approximately 2 hours after oral administration 2
Age-Specific Pharmacokinetic Data
For children aged 1-5 years receiving the standard 10 mg/kg loading dose followed by 5 mg/kg maintenance dosing:
- Mean Cmax: 0.216 mcg/mL
- Mean Tmax: 1.9 hours
- Mean AUC0-24: 1.822 mcg·hr/mL 1
For children aged 5-15 years with the same dosing regimen:
- Mean Cmax: 0.383 mcg/mL
- Mean Tmax: 2.4 hours
- Mean AUC0-24: 3.109 mcg·hr/mL 1
Clinical Implications
Despite the relatively low oral bioavailability, azithromycin achieves therapeutic efficacy through:
- Extensive tissue distribution: Tissue concentrations far exceed plasma levels, with tissue-to-plasma ratios >100 in lung and tonsil tissue 1
- Prolonged tissue retention: Terminal elimination half-life of 2-4 days allows for once-daily dosing and short treatment courses 2
- Low plasma protein binding: <50% protein binding facilitates tissue penetration 2
Dosing Considerations
The standard pediatric dosing regimens account for the 37% bioavailability 3, 4:
- Loading dose: 10 mg/kg on day 1 (maximum 500 mg)
- Maintenance: 5 mg/kg once daily on days 2-5 (maximum 250 mg)
Azithromycin can be administered with or without food in children 1, though administration without food may optimize absorption. The clinical guidelines do not mandate fasting administration, recognizing that the extensive tissue distribution compensates for food-related bioavailability reduction 3.
The large volume of distribution (25-35 L/kg) and preferential tissue accumulation mean that despite modest oral bioavailability, azithromycin achieves concentrations at infection sites that are 10-100 times higher than plasma levels 1, 2.