The Clinical Pharmacokinetics of Rifabutin

Rifabutin is structurally similar to rifampin, but there are important pharmacokinetic differences between the two drugs. Rifabutin is more lipid soluble than is rifampin, resulting in more-extensive tissue uptake, a larger volume of distribution, lower maximum plasma concentrations, lower trough concentrations, a longer terminal half-life, and higher tissue-to-plasma drug concentration ratios. The oral bioavailability of rifabutin is low. Like rifampin, rifabutin induces its own metabolism during multiple dosing. Rifabutin is extensively metabolized. The two major metabolites of rifabutin contribute to its antimicrobial activity. Rifabutin induces hepatic metabolism but is not as potent an inducer as is rifampin. Rifabutin does not affect the pharmacokinetics of antiretroviral drugs that are excreted in the urine. Although rifabutin decreases plasma concentrations of zidovudine, this finding does not appear to be clinically relevant. When administered during rifabutin prophylaxis, fluconazole decreases the incidence of Mycobacterium avium complex bacteremia. The coadministration of clarithromycin and rifabutin results in increased plasma concentrations of rifabutin and decreased plasma concentrations of clarithromycin; however, the plasma concentration of clarithromycin's active metabolite is increased.