Erythromycin is a macrolide antibiotic that binds to the 50-S subunit of the ribosomes. It kills susceptible bacteria by interfering with their protein synthesis. Erythromycin is active against many aerobic gram-positive bacteria, selected gram-negative bacteria (including species of Legionella, N. meningitidis, H. influenzae, and Bordetella pertussis), and nonbacterial species (e.g., C. trachomatis, mycoplasma, and certain rickettsial species). There are three available oral preparations of erythromycin: erythromycin stearate, erythromycin ethylsuccinate, and erythromycin estolate. The first two preparations do not have intrinsic antibacterial activity until they dissociate or hydrolyze (respectively) to active compounds. They are routinely recommended to be taken orally one hour before meals, so that effective concentrations of drugs in plasma can be consistently achieved. The estolate form is associated with the highest concentrations in plasma.
The parenteral forms of erythromycin include an ethyl succinate form for IM administration and lactobionate or gluceptate forms for use intravenous. The severity of irritation of veins with erythromycin frequently requires large veins to be catheterized and low concentrations of lidocaine coadministered with the antibiotic to reduce pain. Excessively rapid intravenous infusion of erythromycin results in diffuse cramping and gastrointestinal discomfort due to contraction of intestinal smooth muscle.
Erythromycin is widely used as an alternative for β-lactam antibiotics in the patient who is allergic to penicillin and requires treatment for non-life-threatening gram-positive bacterial infection. These include streptococcal and pneumococcal systemic infections and staphylococcal skin infections. In more serious infections with these organisms, vancomycin is preferred over erythromycin. Erythromycin has been effective in treating Legionella pneumonia and H. influenzae respiratory and otitis media infections. It is effectively used as an alternative drug for a variety of sexually transmitted diseases, including gonorrhea, chlamydial infections, syphilis, and chancroid.
Clarithromycin has a similar spectrum of antimicrobial activity to erythromycin. However, clarithromycin is also active against M. avium complex. Clarithromycin has a longer half-life than erythromycin and can be given twice daily. Clarithromycin and azithromycin result in less gastrointestinal upset (nausea and vomiting) than erythromycin.
Azithromycin is usually discussed as a macrolide, but it is really an azalide. Like clarithromycin it is active against M. avium complex. Otherwise its spectrum of activity is similar to erythromycin. It can be given once weekly as prophylaxis against M. avium complex infections in human immunodeficiency virus-infected patients whose CD4 count is ≤50/mm3. Because of its long half-life, azithromycin is given once daily for 5 days to treat most infections.
Macrolides are metabolized by cytochromes P450 isoform CYP3A, and they (erythromycin and clarithromycin — very rarely azithromycin) interact with a number of drugs that are also metabolized via this system including carbamazepine, theophylline, phenytoin, warfarin, cyclosporin, colchicine, bromocriptine, valproic acid, terfenadine, cisapride, astemizole, triazolam, midazolarn, disopyramide, and acenocournarol. Erythromycin in combination with terfenadine, astemizole, or cisapride, should be avoided because of prolongation of the QT interval and the possibility of torsade des pointes. High-dose erythromycin (e.g., 4 g/day) in elderly patients or patients with renal failure can result in transient deafness.