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Managing Drug Interactions in the
Treatment of HIV-Related
Tuberculosis
Predicting Drug Interactions
Involving Rifamycins
Knowledge of the mechanisms of drug interactions can
help predict the likelihood of an interaction, if that specific combination
of drugs has not been formally evaluated. The rifamycin class upregulate (induce)
the synthesis of several classes of drug transporting and drug metabolizing
enzymes. With increased synthesis, there is increased total activity of the
enzyme (or enzyme system), thereby decreasing the serum half-life and serum
concentrations of drugs that are metabolized by that system. The most common
locus of rifamycin interactions is the cytochrome P450 enzyme system, particularly
the CYP3A4 and CYP2C8/9 isozymes. To a lesser extent, rifampin induces the
activity of the CYP2C19 and CYPD6 isozymes. The rifamycins vary in their potential
as CYP450 inducers, with rifampin being most potent, rifapentine intermediate,
and rifabutin being much less active. Rifampin also upregulates the synthesis
of cytosolic drug-metabolizing enzymes, including glucuronosyl transferase,
an enzyme involved in the metabolism of zidovudine 10 and raltegravir.
Last Reviewed: 05/18/2008 Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
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