eritromicin

Erythromycin is a macrolide antibiotic produced by the actinomycete Saccharopolyspora erythraea. It has a long history of clinical use for treating a range of bacterial infections. Available in several salt forms and as esters, erythromycin base is commonly taken orally, while estolate and other esters have been used to improve absorption in the past.

Mechanism and spectrum: Erythromycin binds reversibly to the 50S ribosomal subunit, blocking translocation during protein synthesis and thereby inhibiting bacterial growth. It is generally bacteriostatic, though it can be bactericidal for certain organisms at high concentrations. It covers many Gram-positive cocci and some Gram-negative pathogens such as Haemophilus influenzae and Moraxella catarrhalis, as well as atypical pathogens including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella. It is less reliable against most Enterobacteriaceae.

Pharmacokinetics and interactions: Erythromycin is variably absorbed from the gut; acid stability is improved by some ester forms. It is primarily metabolized in the liver and excreted in bile and urine. It inhibits cytochrome P450 3A4, leading to potential interactions with drugs such as statins and certain anticoagulants, and can prolong the QT interval in susceptible individuals.

Clinical uses and cautions: It is used for respiratory tract infections, skin and soft tissue infections, and as an alternative for patients with penicillin allergy for selected infections, including Chlamydia trachomatis. Topical erythromycin is used for acne, and ophthalmic ointment is given to newborns to prevent conjunctivitis. Historical formulations like erythromycin estolate carried a risk of cholestatic liver injury, especially in pregnancy.

Resistance and safety: Resistance arises mainly through methylation of the 23S rRNA or efflux mechanisms. Common adverse effects include gastrointestinal upset, with rare hepatotoxicity for certain formulations; hypersensitivity and, in predisposed patients, arrhythmias related to QT prolongation can occur.