azitromycin

Azitromycin, commonly spelled azithromycin, is a macrolide antibiotic of the azalide subclass used to treat a range of bacterial infections. It was developed to improve acid stability, tissue penetration, and a longer half-life compared with older macrolides.

Mechanism of action and spectrum: Azithromycin binds to the 50S ribosomal subunit and inhibits translocation during protein synthesis, tending to be bacteriostatic and sometimes bactericidal at high concentrations. It is active against many Gram-positive cocci, some Gram-negative pathogens such as Haemophilus influenzae and Moraxella catarrhalis, and atypical organisms including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila. Its utility covers a broad range of respiratory, soft tissue, and genitourinary infections.

Clinical uses: It is commonly used for community-acquired pneumonia, acute otitis media, sinusitis, pharyngitis/tonsillitis, and skin and soft tissue infections. It is also employed for certain sexually transmitted infections caused by Chlamydia trachomatis and for other indications where coverage of atypical pathogens is beneficial. In some regimens, it is used for traveler's diarrhea or as part of combination therapy for gonorrhea co-infection, depending on local guidelines.

Pharmacokinetics and dosing: Azithromycin exhibits extensive tissue distribution with high intracellular concentrations and a long elimination half-life, enabling convenient dosing. Oral bioavailability is moderate. Typical adult regimens include schedules such as a higher dose on day 1 followed by lower doses on subsequent days, with pediatric dosing being weight-based.

Safety and interactions: Common adverse effects include gastrointestinal upset, nausea, and diarrhea. Serious but rare risks include liver injury and QT interval prolongation, particularly with other QT-prolonging drugs or electrolyte disturbances. It has relatively limited cytochrome P450 interactions but caution is advised with drugs affected by QT prolongation. Safety in pregnancy is generally considered compatible (often category B).

Resistance and history: Resistance can arise via 23S rRNA methylation or efflux pumps, reducing efficacy in certain pathogens. Azithromycin was introduced in the 1980s and has since become widely used.