CLINICAL PHARMACOLOGY
Mechanism of Action
Azithromycin is an antimicrobial agent [See Clinical Pharmacology]
Pharmacodynamics
Cardiac Electrophysiology
QTc interval prolongation was studied in a randomized, placebo-controlled parallel trial in 116 healthy subjects who received either chloroquine (1000 mg) alone or in combination with azithromycin (500 mg, 1000 mg, and 1500 mg once daily). Co-administration of azithromycin increased the QTc interval in a dose- and concentration-dependent manner. In comparison to chloroquine alone, the maximum mean (95% upper confidence bound) increases in QTcF were 5 ms, 7 ms and 9 ms with the co-administration of 500 mg, 1000 mg and 1500 mg azithromycin, respectively.
Pharmacokinetics
Zmax is an extended release microsphere formulation. Based on data obtained from studies evaluating the pharmacokinetics of azithromycin in healthy adult subjects a higher peak serum concentration (Cmax) and greater systemic exposure (AUC 0–24) of azithromycin are achieved on the day of dosing following a single 2 g dose of Zmax versus 1.5 g of azithromycin tablets administered over 3 days (500 mg/day) or 5 days (500 mg on day 1, 250 mg/day on days 2–5) [Table 2]. Consequently, due to these different pharmacokinetic profiles, Zmax is not interchangeable with azithromycin tablet 3-day and 5-day dosing regimens.
Table 2. Mean (SD) Pharmacokinetic Parameters for Azithromycin on Day 1 Following the Administration of a Single Dose of 2 g Zmax or 1.5 g of Azithromycin Tablets Given over 3 Days (500 mg/day) or 5 Days (500 mg on Day 1 and 250 mg on Days 2–5) to Healthy Adult Subjects
Pharmacokinetic ParameterZmax, 3-day and 5-day regimen parameters obtained from separate pharmacokinetic studies
|
Azithromycin Regimen |
Zmax [N=41]N = 21 for AUC0–∞ and t1/2
|
3-day
[N=12] |
5-day
[N=12] |
SD = standard deviation Cmax = maximum serum concentration Tmax = time to Cmax
AUC = area under concentration vs. time curve t1/2 = terminal serum half-life |
Cmax (µg/mL)
|
0.821 (0.281) |
0.441 (0.223) |
0.434 (0.202) |
Tmax
Median (range) (hr)
|
5.0 (2.0–8.0) |
2.5 (1.0–4.0) |
2.5 (1.0–6.0) |
AUC0–24 (µg∙hr/mL)
|
8.62 (2.34) |
2.58 (0.84) |
2.60 (0.71) |
AUC0–∞
Total AUC for the 1-day, 3-day and 5-day regimens (µg∙hr/mL)
|
20.0 (6.66) |
17.4 (6.2) |
14.9 (3.1) |
t1/2 (hr)
|
58.8 (6.91) |
71.8 (14.7) |
68.9 (13.8) |
Absorption
The bioavailability of Zmax relative to azithromycin immediate release (IR) (powder for oral suspension) was 83%. On average, peak serum concentrations were achieved approximately 2.5 hours later following Zmax administration and were lower by 57%, compared to 2 g azithromycin IR. Thus, single 2 g doses of Zmax and azithromycin IR are not bioequivalent and are not interchangeable.
Effect of food on absorption: A high-fat meal increased the rate and extent of absorption of a 2 g dose of Zmax (115% increase in Cmax, and 23% increase in AUC0–72) compared to the fasted state. A standard meal also increased the rate of absorption (119% increase in Cmax) and with less effect on the extent of absorption (12% increase in AUC0–72) compared to administration of a 2 g Zmax dose in the fasted state.
Effect of antacids: Following the administration of Zmax with an aluminum and magnesium hydroxide antacid, the rate and extent of azithromycin absorption were not altered.
Distribution
The serum protein binding of azithromycin is concentration dependent, decreasing from 51% at 0.02 µg/mL to 7% at 2 µg/mL. Following oral administration, azithromycin is widely distributed throughout the body with an apparent steady-state volume of distribution of 31.1 L/kg.
Azithromycin concentrates in fibroblasts, epithelial cells, macrophages, and circulating neutrophils and monocytes. Higher azithromycin concentrations in tissues than in plasma or serum have been observed. White blood cell and lung exposure data in humans following a single 2 g dose of Zmax in adults are shown in Table 3. Following a 2 g single dose of Zmax, azithromycin achieved higher exposure (AUC0–120) in mononuclear leukocytes (MNL) and polymorphonuclear leukocytes (PMNL) than in serum. The azithromycin exposure (AUC0–72) in lung tissue and alveolar cells (AC) was approximately 100 times that in serum; and the exposure in epithelial lining fluid (ELF) was also higher (approximately 2–3 times) than in serum. The clinical significance of this distribution data is unknown.
Table 3. Azithromycin Exposure Data in White Blood Cells and Lung Following a 2 g Single Dose of Zmax in Adults
A single 2 g dose of Zmax |
Abbreviation: WBC: white blood cells; MNL: mononuclear leukocytes; PMNL: polymorphonuclear leukocytes; ELF: Epithelial lining fluid |
WBC |
Cmax (µg/mL) |
AUC0–24 (µg∙hr/mL) |
AUC0–120 (µg∙hr/mL) |
Ct=120
Azithromycin concentration at 120 hours after the start of dosing (µg/mL) |
MNL
|
116 (40.2) |
1790 (540) |
4710 (1100) |
16.2 (5.51) |
PMNL
|
146 (66.0) |
2080 (650) |
10000 (2690) |
81.7 (23.3) |
|
|
|
|
|
LUNG |
Cmax (µg/mL) |
AUC0–24 (µg∙hr/mL) |
AUC0–72 (µg∙hr/mL) |
|
ALVEOLAR CELL
|
669 |
7028 |
20403 |
- |
ELF
|
3.2 |
17.6 |
131 |
- |
|
|
|
|
|
| Cmax (µg/g) |
AUC0–24 (µg∙hr/g) |
AUC0–72 (µg∙hr/g) |
|
LUNG TISSUE
|
37.9 |
505 |
1693 |
- |
Following a regimen of 500 mg of azithromycin tablets on the first day and 250 mg daily for 4 days, only very low concentrations were noted in cerebrospinal fluid (less than 0.01 µg/mL) in the presence of non-inflamed meninges.
Metabolism
In vitro and in vivo studies to assess the metabolism of azithromycin have not been performed.
Excretion
Serum azithromycin concentrations following a single 2 g dose of Zmax declined in a polyphasic pattern with a terminal elimination half-life of 59 hours. The prolonged terminal half-life is thought to be due to a large apparent volume of distribution.
Biliary excretion of azithromycin, predominantly as unchanged drug, is a major route of elimination. Over the course of a week, approximately 6% of the administered dose appears as unchanged drug in urine.
Special Populations
Renal Impairment
Azithromycin pharmacokinetics were investigated in 42 adults (21 to 85 years of age) with varying degrees of renal impairment. Following the oral administration of a single 1.0 g dose of azithromycin (4 × 250 mg capsules), the mean Cmax and AUC0–120 were 5.1% and 4.2% higher, respectively in subjects with GFR 10 to 80 mL/min compared to subjects with normal renal function (GFR >80 mL/min). The mean Cmax and AUC0–120 were 61% and 35% higher, respectively in subjects with GFR <10 mL/min compared to subjects with normal renal function. (See Renal Impairment.)
Hepatic Impairment
The pharmacokinetics of azithromycin in subjects with hepatic impairment has not been established.
Pediatric Patients
The pharmacokinetics of azithromycin were characterized following a single 60 mg/kg dose of Zmax in pediatric patients aged 3 months to 16 years. Although there was high inter-patient variability in systemic exposure (AUC and Cmax) across the age groups studied, individual azithromycin AUC and Cmax values in pediatric patients were comparable to or higher than those following administration of 2 g Zmax in adults (Table 4). (See Pediatric Use.)
Table 4. Mean (SD) Pharmacokinetic Parameters for Azithromycin Following Administration of a Single Dose of Zmax (60 mg/kg, maximum dose of 2 g) to Pediatric Subjects Aged 3 Months to 16 Years
Treatment Group |
Pharmacokinetic Parameters |
Cmax
(µg/mL) |
Tmax
Median (range) presented only for Tmax
(hr) |
AUC(0–24)
(µg∙hr/mL) |
AUC(0–∞)
(µg∙hr/mL) |
Empty stomach = dosed with Zmax at least 1 hour before or 2 hours after a meal (Groups I–VI) Fed = dosed with Zmax within 5 minutes of consuming an age-appropriate high-fat breakfast (Group VII) |
Group 1 (N = 6) [3 to 18 months] |
0.74 (0.20) |
3 (3–3) |
6.29 (1.17) |
14.1 (2.16) (n = 3) |
Group 2
(N = 6) [>18 to 36 months] |
1.88 (0.50) |
3 (3–3) |
19.7 (5.35) |
37.3 (12.9) (n = 5) |
Group 3 (N = 6) [>36 to 48 months] |
1.23 (0.42) |
3 (3–6) |
12.9 (3.79) |
22.4 (5.96) |
Group 4 (N = 6) [>48 months to 8 years] |
1.13 (0.34) |
3 (3–6) |
13.0 (4.21) |
22.2 (6.89) |
Group 5 (N = 6) [>8 to 12 years] |
1.65 (0.38) |
3 (3–6) |
16.0 (4.99) |
30.1 (10.7) |
Group 6 (N = 6) [>12 to 16 years] |
0.98 (0.35) |
3 (3–6) |
11.0 (4.78) |
21.3 (9.37) |
Pooled 1–6 (N = 36) [On an empty stomach] |
1.27 (0.53) |
3 (3–6) |
13.1 (5.78) |
25.2 (10.7) (n = 32) |
Group 7One subject vomited immediately after dosing and discontinued from the study (N = 7) [Fed; 18 months to 8 years] |
1.41 (0.62) |
3 (1.5–3.1) |
7.43 (3.00) |
18.9 (3.57) (n = 3) |
Gender
The impact of gender on the pharmacokinetics of azithromycin has not been evaluated for Zmax. However, previous studies have demonstrated no significant differences in the disposition of azithromycin between male and female subjects.
Pharmacokinetic Interaction Studies
A drug interaction study was performed with Zmax and antacids. All other drug interaction studies were performed with azithromycin immediate release (IR) formulations (capsules and tablets, doses ranging from 500 to 1200 mg) and other drugs likely to be co-administered. The effects of co-administration of azithromycin on the pharmacokinetics of other drugs are shown in Table 5 and the effects of other drugs on the pharmacokinetics of azithromycin are shown in Table 6.
When used at therapeutic doses, azithromycin IR had a minimal effect on the pharmacokinetics of atorvastatin, carbamazepine, cetirizine, didanosine, efavirenz, fluconazole, indinavir, midazolam, nelfinavir, sildenafil, theophylline (intravenous and oral), triazolam, trimethoprim/sulfamethoxazole or zidovudine (Table 5). Although the drug interaction studies were not conducted with Zmax, similar modest effect as observed with IR formulation are expected since the total exposure to azithromycin is comparable for Zmax and other azithromycin IR regimens. Therefore, no dosage adjustment of drugs listed in Table 5 is recommended when co-administered with Zmax.
Nelfinavir significantly increased the Cmax and AUC of azithromycin following co-administration with azithromycin IR 1200 mg (Table 6). However, no dose adjustment of azithromycin is recommended when Zmax is co-administered with nelfinavir.
Pharmacokinetic and/or pharmacodynamic interactions with the drugs listed below have not been reported in clinical trials with azithromycin; however, no specific drug interaction studies have been performed to evaluate potential drug-drug interaction. Nonetheless, pharmacokinetic and/or pharmacodynamic interactions with these drugs have been observed with other macrolide products. Until further data are developed, careful monitoring of patients is advised when azithromycin and these drugs are used concomitantly: digoxin, ergotamine or dihydroergotamine, cyclosporine, hexobarbital and phenytoin.
Table 5. Drug Interactions: Pharmacokinetic Parameters of Co-administered Drugs in the Presence of Azithromycin
Co-administered Drug |
Dose of Co-administered Drug |
Dose of AzithromycinRefers to azithromycin capsules and tablets unless specified
|
n |
Ratio (with/without Azithromycin) of Co-administered Drug Pharmacokinetic Parameters (90% CI); No Effect = 1.00 |
Mean Cmax
|
Mean AUC |
Atorvastatin
|
10 mg/day × 8 days |
500 mg/day PO on days 6–8 |
12 |
0.83 (0.63 to 1.08) |
1.01 (0.81 to 1.25) |
Carbamazepine
|
200 mg/day × 2 days, then 200 mg BID × 18 days |
500 mg/day PO for days 16–18 |
7 |
0.97 (0.88 to 1.06) |
0.96 (0.88 to 1.06) |
Cetirizine
|
20 mg/day × 11 days |
500 mg PO on day 7, then 250 mg/day on days 8–11 |
14 |
1.03 (0.93 to 1.14) |
1.02 (0.92 to 1.13) |
Didanosine
|
200 mg PO BID × 21 days |
1,200 mg/day PO on days 8–21 |
6 |
1.44 (0.85 to 2.43) |
1.14 (0.83 to 1.57) |
Efavirenz
|
400 mg/day × 7 days |
600 mg PO on day 7 |
14 |
1.04
|
0.95
|
Fluconazole
|
200 mg PO single dose |
1,200 mg PO single dose |
18 |
1.04 (0.98 to 1.11) |
1.01 (0.97 to 1.05) |
Indinavir
|
800 mg TID × 5 days |
1,200 mg PO on day 5 |
18 |
0.96 (0.86 to 1.08) |
0.90 (0.81 to 1.00) |
Midazolam
|
15 mg PO on day 3 |
500 mg/day PO × 3 days |
12 |
1.27 (0.89 to 1.81) |
1.26 (1.01 to 1.56) |
Nelfinavir
|
750 mg TID × 11 days |
1,200 mg PO on day 9 |
14 |
0.90 (0.81 to 1.01) |
0.85 (0.78 to 0.93) |
Sildenafil
|
100 mg on days 1 and 4 |
500 mg/day PO × 3 days |
12 |
1.16 (0.86 to 1.57) |
0.92 (0.75 to 1.12) |
Theophylline
|
4 mg/kg IV on days 1, 11, 25 |
500 mg PO on day 7, then 250 mg/day on days 8–11 |
10 |
1.19 (1.02 to 1.40) |
1.02 (0.86 to 1.22) |
Theophylline
|
300 mg PO BID × 15 days |
500 mg PO on day 6, then 250 mg/day on days 7–10 |
8 |
1.09 (0.92 to 1.29) |
1.08 (0.89 to 1.31) |
Triazolam
|
0.125 mg on day 2 |
500 mg PO on day 1, then 250 mg/day on day 2 |
12 |
1.06
|
1.02
|
Trimethoprim/
Sulfamethoxazole
|
160 mg/800 mg/day PO × 7 days |
1,200 mg PO on day 7 |
12 |
0.85 (0.75 to 0.97)/ 0.90 (0.78 to 1.03) |
0.87 (0.80 to 0.95)/ 0.96 (0.88 to 1.03) |
Zidovudine
|
500 mg/day PO × 21 days |
600 mg/day PO × 14 days |
5 |
1.12 (0.42 to 3.02) |
0.94 (0.52 to 1.70) |
Zidovudine
|
500 mg/day PO × 21 days |
1,200 mg/day PO × 14 days |
4 |
1.31 (0.43 to 3.97) |
1.30 (0.69 to 2.43) |
Table 6. Drug Interactions: Pharmacokinetic Parameters of Azithromycin in the Presence of Co-administered Drugs
Co-administered Drug |
Dose of Co-administered Drug |
Dose of AzithromycinRefers to azithromycin capsules and tablets unless specified
|
n |
Ratio (with/without co-administered drug) of Azithromycin Pharmacokinetic Parameters (90% CI); No Effect = 1.00 |
Mean Cmax
|
Mean AUC |
Efavirenz
|
400 mg/day × 7 days |
600 mg PO on day 7 |
14 |
1.22 (1.04 to 1.42) |
0.9290% confidence interval not reported
|
Fluconazole
|
200 mg PO single dose |
1,200 mg PO single dose |
18 |
0.82 (0.66 to 1.02) |
1.07 (0.94 to 1.22) |
Nelfinavir
|
750 mg TID × 11 days |
1,200 mg PO on day 9 |
14 |
2.36 (1.77 to 3.15) |
2.12 (1.80 to 2.50) |
Aluminum and Magnesium hydroxide
|
20 mL regular strength, single dose |
2 g Zmax, single dose |
39 |
0.99 (0.93 to 1.06) |
0.99 (0.92 to 1.08) |
Microbiology
Mechanism of Action
Azithromycin acts by binding to the 50S ribosomal subunit of susceptible microorganisms and interferes with bacterial protein synthesis. Nucleic acid synthesis is not affected.
Cross Resistance
Azithromycin demonstrates cross-resistance with erythromycin-resistant Gram-positive isolates.
Azithromycin has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections as described in [see Indications and Usage (1) ].
Gram-Positive Bacteria
Streptococcus pneumoniae
Gram-Negative Bacteria
Haemophilus influenzae
Moraxella catarrhalis
"Other" Bacteria
Chlamydia pneumoniae
Mycoplasma pneumoniae
Susceptibility Testing Methods
When available, clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.
Dilution Techniques
Quantitative methods are used to determine minimal inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method 1,3,4 (broth and/or agar). The MIC values should be interpreted according to criteria provided in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters can provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using standardized methods 2,3,4. This procedure uses paper disk impregnated with 15 mcg azithromycin to test the susceptibility of bacteria to azithromycin. The disk diffusion interpretive criteria are provided in Table 7.
Table 7: Susceptibility Interpretive Criteria for Azithromycin
Pathogen |
Minimum Inhibitory Concentrations (mcg/mL) |
Disk Diffusion (zone diameter in mm) |
|
S |
I |
R |
S |
I |
R |
Haemophilus influenzae
|
≤4 |
- |
- |
≥12 |
- |
- |
Streptococcus pneumoniae
|
≤0.5
|
1 |
≥2 |
≥18 |
14–17 |
≤13 |
Moraxella catarrhalis
|
≤0.25
|
- |
- |
≥26 |
- |
- |
A report of "Susceptible" indicates that the pathogen is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration at the infection site necessary to inhibit growth of the pathogen. A report of "Intermediate" indicates that the result should be considered equivocal, and if the microorganism is not fully susceptible to alternative clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test 1,2,3,4. Standard azithromycin powder should provide the following range of MIC values provided in Table 8. For the diffusion technique using the 15-mcg azithromycin disk the criteria provided in Table 8 should be achieved.
Table 8: Acceptable Quality Control Ranges for Susceptibility Testing
Quality Control Organism |
Minimum Inhibitory Concentrations (mcg/mL) |
Disk Diffusion (zone diameters in mm) |
ATCC = American Type Culture Collection |
Staphylococcus aureus
ATCC 25923 |
Not Applicable |
21–26 |
Staphylococcus aureus
ATCC 29213 |
0.5–2 |
Not Applicable |
Haemophilus Influenzae
ATCC 49247 |
1.0–4.0 |
13–21 |
Streptococcus pneumoniae
ATCC 49619 |
0.06–0.25 |
19–25 |
The ability to correlate MIC values and plasma drug levels is difficult as azithromycin concentrates in macrophages and tissues. [See Clinical Pharmacology]
NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
Long-term studies in animals have not been performed to evaluate carcinogenic potential. Azithromycin has shown no mutagenic potential in standard laboratory tests: mouse lymphoma assay, human lymphocyte clastogenic assay, and mouse bone marrow clastogenic assay. No evidence of impaired fertility due to azithromycin was found in rats given daily doses up to 10 mg/kg (approximately 0.05 times the single 2 g oral adult human dose on a mg/m2 basis).
Animal Toxicology and/or Pharmacology
Phospholipidosis (intracellular phospholipid accumulation) has been observed in some tissues of mice, rats, and dogs given multiple doses of azithromycin. It has been demonstrated in numerous organ systems (e.g., eye, dorsal root ganglia, liver, gallbladder, kidney, spleen, and/or pancreas) in dogs treated with azithromycin at doses which, expressed on the basis of mg/m2, are approximately one-sixth the recommended adult dose, and in rats treated at doses approximately one-fourth the recommended adult dose. This effect has been shown to be reversible after cessation of azithromycin treatment. Based on the pharmacokinetic data, phospholipidosis has been seen in the rat (50 mg/kg/day dose) at the observed maximal plasma concentration of 1.3 µg/mL (1.6 times the observed Cmax of 0.821 µg/mL at the adult dose of 2 g.) Similarly, it has been shown in the dog (10 mg/kg/day dose) at the observed maximal serum concentration of 1 µg/mL (1.2 times the observed Cmax of 0.821 µg/mL at the adult dose of 2 g).
Phospholipidosis was also observed in neonatal rats dosed for 18 days at 30 mg/kg/day, which is less than the pediatric dose of 60 mg/kg on a mg/m2 basis, but was not observed in neonatal rats treated for 10 days at 40 mg/kg/day with mean maximal serum concentrations of 1.86 µg/ml, approximately 1.5 times the Cmax of 1.27 µg/ml at the pediatric dose. Phospholipidosis has been observed in neonatal dogs (10 mg/kg/day) at maximum mean whole blood concentrations of 3.54 µg/ml, approximately 3 times the pediatric dose Cmax.
The significance of the finding for animals and for humans is unknown.
|