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 optimal serum levels. Erythromycin is largely bound to plasma proteins. 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. Erythromycin crosses the placental barrier, but fetal plasma levels are low. The drug is excreted in human milk. 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 erythromycin particles in PCE tablets are coated with a polymer whose dissolution is pH dependent. This coating allows for minimal release of erythromycin in acidic environments, e.g., stomach. This delivery system is designed for optimal drug release and absorption in the small intestine. In multiple-dose, steady-state studies, PCE tablets have demonstrated rapid and generally adequate drug delivery in both fasting and nonfasting conditions. However, the presence of food results in lower blood levels, and optimal blood levels are obtained when PCE tablets are given in the fasting state (at least ½ hour and preferably 2 hours before meals). Bioavailability data are available from Abbott Laboratories, Dept. 4PI.

Microbiology

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 erythromycin 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 section.

Gram-positive organisms

Corynebacterium diphtheriae
Corynebacterium minutissimum
Listeria monocytogenes
Staphylococcus aureus (resistant organisms may emerge during treatment)
Streptococcus pneumoniae
Streptococcus pyogenes

Gram-negative organisms

Bordetella pertussis
Legionella pneumophila
Neisseria gonorrhoeae

Other microorganisms

Chlamydia trachomatis
Entamoeba histolytica
Mycoplasma pneumoniae
Treponema pallidum
Ureaplasma urealyticum

The following in vitro data are available, but their clinical significance is unknown.

Erythromycin exhibits in vitro minimal inhibitory concentrations (MIC's) of 0.5 µg/mL or less against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of erythromycin in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.

Gram-positive organisms

Viridans group streptococci
Gram-negative organisms
Moraxella catarrhalis

Susceptibility Tests

Dilution Techniques

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