Distribution of Azithromycin in Plasma and Tonsil Tissue after Repeated Oral Administration of 10 or 20 Milligrams per Kilogram in Pediatric Patients

Abstract

Azithromycin concentrations in the tonsils of 56 pediatric patients, treated with 10 or 20 mg of the drug per kg of body weight for 3 days, were compared. Azithromycin levels in plasma and tonsil samples were determined up to 8.5 days after the last dose. The 20-mg/kg regimen resulted in an improved tonsillar distribution of azithromycin, suggesting the achievement of enhanced therapeutic concentrations at infective sites of the upper respiratory tract.

Azithromycin, the prototype of azalides, possesses peculiar pharmacokinetics, consisting of high distribution into target tissues (4), and displays a broad antimicrobial spectrum (12). Guidelines for azithromycin MICs have established that breakpoints of ≤2, 4, and ≥8 mg/liter indicate susceptible, intermediately resistant, and resistant bacteria, respectively (7).

Studies with adults have suggested that once-daily dosing with 500 mg for 3 days should maintain tonsillar azithromycin levels higher than 2 mg/kg of body weight for 10 days (3, 4). In pediatric patients, a 3-day treatment with 10 mg/kg allows a tonsillar azithromycin distribution similar to that of adults (15). Cure and improvement rates of 93 to 100% were obtained in children with bacterial pharyngitis and/or tonsillitis treated with 10 mg of azithromycin/kg for 3 days (7). However, lower success rates have also been reported (2, 11).

After oral administration, azithromycin undergoes a rapid uptake from systemic circulation into phagocytes, which then release the drug at infection sites, exposing local extracellular compartments to azithromycin concentrations higher than plasma levels (5, 6). On this basis, azithromycin administration at doses higher than standard regimens might improve eradication rates in pharyngitis and tonsillitis. Accordingly, this study compared azithromycin concentrations in tonsils collected from pediatric patients treated with 10 or 20 mg/kg daily for 3 days.

The study was performed with children scheduled to undergo surgical tonsil removal. The exclusion criteria were history of drug allergy; drug treatments, including antimicrobials, nonprescription drugs, or enzymatic inducers and/or inhibitors within the previous 2 weeks; conditions affecting drug absorption (vomiting, diarrhea, malabsorption); and major medical problems. Informed consent was obtained from parents or legal guardians. The investigation was approved by the local University Hospital Ethics Committee. Fifty-six patients (30 males, 26 females; age range, 4 to 12 years; mean age, 6.46 ± 0.32 years [mean ± standard deviation]) entered the study. They were assigned to two groups, balanced for sex and age, and were treated once daily with azithromycin in an oral suspension (Zitromax; Pfizer Italiana, Rome, Italy) at 10 or 20 mg/kg for 3 days. Patients underwent surgery from 0.5 to 8.5 days after the last dose. Standard anesthesiological procedures and perioperative medications were applied. All surgical procedures were performed by the same surgeon (P.B.). Plasma samples (2 ml) were collected before surgery. Specimens of normal tonsils (1,000 mg) were taken after surgery and washed in cold phosphate-buffered saline to remove blood. Adverse events were recorded with details of onset, duration, severity, outcome, and treatment. Physical examination, electrocardiogram, blood chemistry, hematology, and urine analysis were performed before enrollment and after completion of the study.

Azithromycin was extracted from plasma and tissue samples as previously described, and its concentration was measured by agar diffusion bioassay using Micrococcus luteus NCTC 8440 (8). Different dilutions of each extract were assayed. The lower limit of quantification was 0.01 mg/liter (plasma) or 0.01 mg/kg (tonsil). The assay was linear within 0.01 to 1 mg/liter or mg/kg, with correlation coefficients greater than 0.99. The drug percentage recovery was 96% for plasma and 94% for tonsils. Variations within or between days (n = 3) were less than 5.5% for plasma and 6.5% for tonsils. Data were validated by high-performance liquid chromatography (13). Results are given as means ± standard deviations. The significance was evaluated by the Student t test for unpaired data, and P levels of <0.05 were considered significant. The area under the concentration-time curve (AUC) was calculated by the trapezoidal method from day 0.5 to 8.5.

All patients completed the study. Two patients (one in the 10-mg/kg group, and one in the 20-mg/kg group) experienced mild nausea and abdominal pain that occurred at the third day of therapy and were likely to be induced by azithromycin. No abnormalities of electrocardiogram, blood chemistry, hematology, or urine analysis were detected.

At 10 mg/kg, the highest concentrations of azithromycin were measured in plasma and tonsil samples collected 0.5 and 2.5 days after the last dose (0.13 ± 0.027 mg/liter and 12.1 ± 4.5 mg/kg, respectively; Fig. 1). Consistent drug levels in tonsils were detected up to 8.5 days (4.6 ± 1.9 mg/kg; Fig. 1B). Plasma and tonsillar AUC values were determined to be 0.62 mg/ liter · day and 69.19 mg/kg · day, respectively.

FIG. 1.

Azithromycin concentrations assessed by microbiological assay in plasma samples (A) and tonsil tissues (B) of pediatric patients treated with 10 (▪) or 20 mg/kg/day (▴) for 3 days and subjected to surgical removal of tonsils on day 0.5, 2.5, 4.5, 6.5, or 8.5 after the end of treatment. Each point represents the mean value ± standard deviation (vertical bars) obtained from five to seven patients. ∗, P < 0.05 versus concentration values obtained after treatment with azithromycin at 10 mg/kg/day.

For patients treated with 20 mg/kg daily, the time courses of azithromycin disposition in plasma and tonsils were similar to those obtained with 10 mg/kg (Fig. 1). Azithromycin plasma levels after administration of 20 mg/kg did not differ from those measured following the 10-mg/kg regimen (Fig. 1A). Plasma AUC after 20 mg/kg was 0.72 mg/liter · day, and the ratio of AUC at 20 mg/kg over AUC at 10 mg/kg was 1.16. Azithromycin concentrations in tonsils were significantly higher after administration of 20 than 10 mg/kg up to day 6.5 (from +48.2 to +56.4%; Fig. 1B). A tonsillar AUC of 105.9 mg/kg · day was estimated for treatment with 20 mg/kg, and the ratio of tissue AUC at this dose over AUC at 10 mg/kg was 1.53.

A recurrence of streptococcal infections can occur in children with pharyngitis and/or tonsillitis after a 3-day treatment with azithromycin at 10 mg/kg (7). Since an increase in antibiotic dosage might reduce eradication failures, at least in the case of sensitive streptococci (7, 9), this study compared azithromycin concentrations in tonsils collected from pediatric patients treated with 10 or 20 mg/kg daily for 3 days.

After administration of 10 mg/kg, azithromycin penetrated extensively into the tonsils, and high drug levels were retained in this tissue up to 8.5 days after the last dose. These findings are consistent with previous data showing a favorable disposition of azithromycin into tonsils (3, 15). In particular, azithromycin levels ranging from 1.5 to 10 mg/kg were achieved in tonsillar tissue 1 to 8 days after a 3-day treatment of children given 10 mg of drug/kg (15).

It must be considered that the present azithromycin assay in tonsillar homogenates reflects the total concentration of this drug in both intracellular and extracellular fluids. On the other hand, while azithromycin concentrates extensively into intracellular compartments (5, 6), the bacteria involved in pharyngitis and/or tonsillitis are located in extracellular fluids, and therefore the clinical efficacy of antibiotics against these microorganisms is usually predicted by relating drug plasma concentrations to MICs (1, 10). However, due to the peculiar pharmacokinetic profile of azithromycin, its plasma levels do not allow to adequately predict the clinical efficacy, and concentrations achieved by azithromycin in the extracellular compartment at infection sites seem to reflect the extent of intracellular drug distribution (1, 14). On this basis, the present results suggest that tonsillar levels of azithromycin greater than the MICs for susceptible upper respiratory pathogens can be maintained for 8.5 days after standard treatment with azithromycin at 10 mg/kg. For the same reasons, the present data, indicating that a 3-day therapy with azithromycin at 20 mg/kg resulted in an improved tonsillar penetration, suggest that eradication failure in pediatric patients might decrease upon administration of azithromycin at doses higher than 10 mg/kg, even in the case of bacteria for which the MICs are >2 mg/liter. However, pharmacokinetic models, relating extracellular azithromycin concentrations with tissue penetration, are currently lacking. As a consequence, it remains undetermined to what extent the enhanced tonsillar distribution of azithromycin at 20 mg/kg may actually result in an improved clinical efficacy against intermediate or resistant microorganisms.

In conclusion, the present results indicate that a short-course treatment with azithromycin, at doses higher than those currently used, is well tolerated by pediatric patients and may ensure sustained therapeutic levels at infective sites of upper respiratory tract.