Rifabutin has better activity against MAC organisms than rifampin and is active in vitro against MAC bacteria isolated from HIV-infected (typically M. avium) and non-HIV-infected individuals (~40% M. intracellular^). Rifabutin also inhibits the growth of many strains of M. tuberculosis. Cross-resistance between rifampin and rifabutin is common in M. tuberculosis, although some strains are resistant to rifampin yet sensitive to rifabutin. Most M. avium strains resistant to rifampin are still sensitive to rifabutin.
absorption, distribution, metabolism, and excretion Rifabutin is given orally, usually at 300 mg/day. Following GI absorption, it is metabolized by hepatic CYPs and eliminated in a biphasic manner with a mean terminal t1/2 of ~45 hours, predominantly in urine and bile. Dosage adjustment is not necessary in patients with impaired renal function.
therapeutic uses Rifabutin is effective for the prevention of MAC infection in HIV-infected individuals and can decrease the frequency of MAC bacteremia. Azithromycin or clarithromycin is more effective and less likely to interact with HAART drugs. Rifabutin also is commonly substituted for rifampin in the treatment of tuberculosis in HIV-infected patients, as it has a less profound CYP-dependent drug interaction. Rifabutin also is used in combination with clarithromycin and ethambutol for the therapy of MAC disease.
untoward effects Rifabutin generally is well tolerated in persons with HIV infection; primary reasons for discontinuation of therapy include rash (4%), GI intolerance (3%), and neutropenia (2%). Overall, neutropenia occurred in 25% of patients with severe HIV infection who received rifabutin. Uveitis and arthralgias have occurred in patients receiving rifabutin doses >450 mg daily in combination with clarithromycin or fluconazole. Patients should be cautioned to discontinue the drug if visual symptoms occur. Like rifampin, the drug causes an orange-tan discoloration. Rarely, thrombocytopenia, a flu-like syndrome, hemolysis, myositis, chest pain, and hepatitis have occurred.
Although less potent than rifampin, rifabutin does induce hepatic CYPs and can decrease the t1/2 of a number of drugs, including zidovudine, prednisone, digitoxin, quinidine, ketoconazole, propranolol, phenytoin, sulfonylureas, and warfarin.
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