Orally administered erythromycin base and its salts are readily absorbed in the microbiologically active form. Interindividual variations in the absorption of erythromycin are, however, observed, and some patients do not achieve acceptable serum levels. Erythromycin is largely bound to plasma proteins, and the freely dissociating bound fraction after administration of erythromycin base represents 90% of the total erythromycin absorbed. After absorption, erythromycin diffuses readily into most body fluids. In the absence of meningeal inflammation, low concentrations are normally achieved in the spinal fluid, but the passage of the drug across the blood-brain barrier increases in meningitis. The drug is excreted in human milk. The drug crosses the placental barrier, but fetal plasma levels are low. Erythromycin is not removed by peritoneal dialysis or hemodialysis.
In the presence of normal hepatic function erythromycin is concentrated in the liver and is excreted in the bile; the effect of hepatic dysfunction on biliary excretion of erythromycin is not known. After oral administration, less than 5% of the administered dose can be recovered in the active form in the urine.
The enteric coating of pellets in ERYC capsules protects the erythromycin base from inactivation by gastric acidity. Because of their small size and enteric coating, the pellets readily pass intact from the stomach to the small intestine and dissolve efficiently to allow absorption of erythromycin in a uniform manner. After administration of a single dose of a 250 mg ERYC capsule, peak serum levels in the range of 1.13 to 1.68 mcg/mL are attained in approximately 3 hours and decline to 0.30 to 0.42 mcg/mL in 6 hours. Optimal conditions for stability in the presence of gastric secretion and for complete absorption are attained when Erythromycin is taken on an empty stomach.
Erythromycin acts by inhibition of protein synthesis by binding 50 S ribosomal subunits of susceptible organisms. It does not affect nucleic acid synthesis. Antagonism has been demonstrated in vitro between erythromycin and clindamycin, lincomycin, and chloramphenicol.
Many strains of Haemophilus influenzae are resistant to erythromycin alone but are susceptible to erythromycin and sulfonamides used concomitantly.
Staphylococci resistant to erythromycin may emerge during a course of therapy.
Erythromycin has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.
- Corynebacterium diphtheriae
- Corynebacterium minutissimum
- Listeria monocytogenes
- Staphylococcus aureus (resistant organisms may emerge during treatment)
- Streptococcus pneumoniae
- Streptococcus pyogenes
- Bordetella pertussis
- Haemophilus influenzae
- Legionella pneumophila
- Neisseria gonorrhoeae
- Chlamydia trachomatis
- Entamoeba histolytica
- Mycoplasma pneumoniae
- Treponema pallidum
- Ureaplasma urealyticum
Quantitative methods are used to determine antimicrobial minimal inhibitory concentrations (MIC’s). These MIC’s provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MIC’s should be determined using a standardized procedure. Standardized procedures are based on a dilution method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of erythromycin powder. The MIC values should be interpreted according to the following criteria:
| MIC (μg/mL) || Interpretation |
|≤ 0.5||Susceptible (S)|
|1 - 4||Intermediate (I)|
|≥ 8||Resistant (R)|
A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. 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 or in situations where high dosage of drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard erythromycin powder should provide the following MIC values:
| Microorganism || MIC (μg/mL) |
| S. aureus ATCC 29213||0.12 - 0.5|
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 15 μg erythromycin to test the susceptibility of microorganisms to erythromycin.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 15 μg erythromycin disk should be interpreted according to the following criteria:
| Zone diameter (mm) || Interpretation |
|≥ 23||Susceptible (S)|
|14 - 22||Intermediate (I)|
|≤ 13||Resistant (R)|
Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for erythromycin.
As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 15 μg erythromycin disk should provide the following zone diameters in these laboratory test quality control strains:
| Microorganism || Zone Diameter (mm) |
| S. aureus ATCC 25923||22 - 30|