Clarithromycin is a macrolide antibacterial with a broad and essentially bacteriostatic action against many Gram-positive and to a lesser extent some Gram-negative bacteria, as well as other organisms including some Mycoplasma spp., Chlamydiaceae, Rickettsia spp., and spirochaetes. Clarithromycin and other macrolides bind reversibly to the 50S subunit of the ribosome, resulting in blockage of the transpeptidation or translocation reactions, inhibition of protein synthesis, and hence inhibition of cell growth. Because macrolides penetrate readily into white blood cells and macrophages there has been some interest in their potential synergy with host defence mechanisms in vivo.
Clarithromycin is rapidly absorbed from the gastro-intestinal tract following oral administration, and undergoes first-pass metabolism; the bioavailability of the parent drug is about 55%. The extent of absorption is relatively unaffected by the presence of food. Peak concentrations of Clarithromycin and its principal active metabolite 14-hydroxyclarithromycin are reported to be about 900 and 600 nanograms/mL respectively following a single 250mg dose by mouth; at steady state the same dose given every 12 hours as tablets produces peak concentrations of Clarithromycin of about 1microgram/mL. The drug and its principal metabolite are widely distributed, and tissue concentrations exceed those in serum, in part because of intracellular uptake. Clarithromycin has been detected in breast milk. It is extensively metabolised in the liver, and excreted in the faeces via the bile. The terminal half-life of Clarithromycin is reportedly about 3 to 4 hours in patients receiving 250mg doses twice daily, and about 5 to 7 hours in those receiving 500mg twice daily. The half-life is prolonged in renal impairment.