Absorption
The peak serum concentrations (Cmax) of azithromycin after 500 mg oral suspension (40 mg/ml), 1000 mg powder for oral suspension, 500 mg (2 x 250 mg) tablets and 1000 mg (4 x 250 mg) capsules in healthy volunteers under fasted conditions were 0.29, 0.75, 0.34, and 1.07 mg/L respectively. The time-to-peak plasma (Tmax) concentrations of azithromycin after oral administration ranges from 2 to 3 hours. The mean absolute bioavailability in healthy volunteers after 500 mg oral suspension and 1000 mg powder for oral suspension in sachet was 37% and 44% in fasted conditions, respectively.
The effect of food on the relative oral bioavailability of azithromycin is formulation dependent. After the administration of 500 mg of an oral suspension (40 mg/ml), 1000 mg as powder for oral suspension and 500 mg oral dose of azithromycin tablets (2 x 250 mg), similar exposure was obtained with high-fat meal vs fasted conditions. Following the administration of a single dose of 500 mg (2 x 250 mg) capsule formulation with a high-fat meal vs fasted conditions, the mean ratio of Cmax and AUC0-24 was 52% and 43% lower.
Table 5 shows mean (SD) pharmacokinetic parameters in adult healthy volunteers after standard dosing regimens with tablets and capsules.
Table 5: AUC0-24 and Cmax of azithromycin for the 3-day and 5-day regimen at last day of dosing
| Dose regimen, formulation | AUC0-24 (μg•h/ml) | Cmax (μg/ml) |
| 3-day regimen (500 mg daily), tablet | 1.88 (0.96) | 0.42 (0.21) |
| 5-day regimen (500 mg D1, 250 mg D2 to D5), tablet | 0.80 (0.42) | 0.18 (0.10) |
| 5-day regimen (500 mg D1, 250 mg D2 to D5), capsule | 2.1 (0.6) | 0.24 (0.08) |
Distribution
Azithromycin is widely and rapidly distributed from plasma to the extravascular compartment, including tissues such as tonsil, lung and gynaecological tissues as well as the intracellular compartment, in particular to polymorphonuclear leukocytes, macrophages, and monocytes. Pharmacokinetic studies have shown considerably higher azithromycin concentrations in certain tissues (up to 50 times the maximum concentration observed in the plasma). This indicates an extensive binding to these tissues with a steady-state volume of distribution ranging from 23 to 31 L/kg. The redistribution phase from the intracellular to the extracellular compartment and to the plasma may result in prolonged low concentrations after treatment cessation.
Azithromycin shows low plasma protein binding, mainly to alpha 1-acid glycoprotein, and it decreases with increasing concentrations of antibiotic: 50%, 23% and 7% protein binding at concentrations of 0.05, 0.1 and 1 mg/L, respectively.
Biotransformation
Azithromycin is minimally metabolised in the liver. The primary route of biotransformation is N-demethylation of the desosamine sugar. Other pathways include O-demethylation, hydrolysis of cladinose (deconjugation of the cladinose sugar), and hydroxylation of desosamine sugar and macrolide ring.
There is no evidence of clinically relevant hepatic cytochrome CYP 3A4 induction or inhibition via the formation of a cytochrome-metabolite complex. Also, auto-induced metabolism of azithromycin by this pathway has not been detected.
Elimination
Azithromycin is mainly eliminated by (active) biliary excretion mostly as unchanged drug, but also as metabolites which are devoid of antibacterial activity. Urinary excretion represents a minor route of elimination with less than 6% of an oral dose and around 20% of the drug that reaches the systemic circulation excreted in urine. More than 50% of faecal, and 12% or urinary excretion is in the form of unchanged compound.
Following the administration of a single 500 mg azithromycin dose, a plasma clearance of 630 ml/min was estimated with a terminal half-life of approximately 68 hours. Renal clearance is generally in the range of 100-189 ml/min, substantially smaller than plasma clearance as expected due to the relatively poor contribution of the renal route to elimination.
Linearity/non-linearity
Following oral administration of an immediate release formulation, dose proportionality on AUC0-24 and Cmax was shown in the range of 250 mg to 1000 mg.
Special populations
Renal Impairment
Azithromycin pharmacokinetics was investigated in 43 adults (21 to 85 years of age) following the oral administration of a single 1.0 g dose of azithromycin (4 x 250 mg capsules) to subjects with GFR >80 ml/min (n =12), subjects with GFR between 10 and 80 ml/min (n = 12) and subjects with GFR <10 ml/min (n = 19).
The pharmacokinetics of azithromycin in subjects with GFR between 10 and 80 ml/min were not affected (mean Cmax and AUC0-120 increased by 5.1% and 4.2%, respectively compared to subjects with GFR >80 ml/min). The mean Cmax and AUC0-120 increased 61% and 35%, respectively, in subjects with GFR <10 ml compared to subjects with GFR >80 ml/min.
No data are available for subjects undergoing dialysis, but due to the elimination mechanism of azithromycin, dialysis is unlikely to result in significant removal of the active substance.
Hepatic Impairment
Azithromycin pharmacokinetics was investigated in 22 adults following the oral administration of a single 500 mg dose of azithromycin (2 x 250 mg capsules) to subjects with normal hepatic function (n = 6), Child-Pugh A (n = 10) and Child-Pugh B (n = 6). The pharmacokinetics of azithromycin in subjects with Child-Pugh A and B were 3% and 19% lower on AUC0-inf and 34% and 72% higher on Cmax, respectively, compared to subjects with normal hepatic function.
Elderly
In elderly volunteers (> 65 years) given azithromycin 500 mg (2 x 250 mg capsules) on day 1 followed by 250 mg from days 2 to 5 in the fasted state the AUC0-24 on Days 1 and 5 were 3.0 and 2.7 μg•h/ml, respectively. A 29% higher AUC0-24, a 8% higher Cmax and a 37.5% higher Tmax than in younger volunteers (<40 years) were observed at day 5. Since these differences are not considered clinically significant, no dose adjustment is required for elderly subjects with normal renal and hepatic function.
Paediatric population
The pharmacokinetics of azithromycin oral suspension have been characterised in 14 children aged 6 to15 years with pharyngitis and in 7 children aged 1 year to 5 years with otitis media. In these two studies, azithromycin oral suspension was dosed at 10 mg/kg on day 1, followed by 5 mg/kg on days 2 through 5. Following 5 days of treatment, mean AUC0-24 values were 3.1 μg•h/ml and 1.8 μg•h/ml, respectively. The mean Cmax value was 0.38 μg/ml and the corresponding mean Tmax value was 2.4 hours in children aged 6 to 15 years and 0.22 μg/ml and 1.9 hours for children 1 to 5 years of age. The mean Cmax and AUC0-24 values are 1.7 times greater in children 6 to 15 years of age than in children 1 to 4 years of age.
The PK of a 3-day course of azithromycin oral suspension at a dose of 10 mg/kg daily was also assessed in 16 children 6 months to 10 years with bacterial infections. The mean AUC0-24 for 7 children aged 2 to 4 years was 2.90 μg•h/ml while for the 8 children aged 5 to 10 years the value was 2.08 μg•h/ml. A low AUC0-24 value of 0.74 μg•h/ml was recorded for a single child in the 6 months to 2-year-old group.
Single dose pharmacokinetics of azithromycin in paediatric patients with given doses of 30 mg/kg have not been studied.