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Pharmacology: Pharmacokinetics: Following oral administration about 40% of a dose of azithromycin is bioavailable. Absorption from the capsule formulation, but not the tablet formulation, is reduced by food. Peak plasma concentrations are achieved 2 to 3 hours after a dose, but azithromycin is extensively distributed to the tissues, and tissue concentrations subsequently remain much higher than those in the blood; in contrast to most other antibacterials, plasma concentrations are therefore of little value as a guide to efficacy. High concentrations are taken up into white blood cells. There is little diffusion into the CSF when the meninges are not inflamed. Small amounts of azithromycin are demethylated in the liver, and it is excreted in bile as unchanged drug and metabolites. About 6% of an oral dose (representing about 20% of the amount in the systemic circulation) is excreted in the urine. The terminal elimination half-life is probably in excess of 40 hours.
Microbiology: Antimicrobial Action: Azithromycin has the same antimicrobial actions as erythromycin, only, azithromycin is less active than erythromycin against streptococci and staphylococci, but has greater activity than erythromycin in vitro against some Gram-negative pathogens such as Haemophilus influenzae and Moraxella catarrhalis (Branhamella catarrhalis), as well as having activity against some of the Enterobacteriaceae such as Escherichia coli and Salmonella and Shigella spp. Azithromycin is also more active than erythromycin against Chlamydia trachomatis and some opportunistic mycobacteria, including Mycobacterium avium complex. It has activity against the protozoa Toxoplasma gondii and Plasmodium falciparum.
