 |
The rifamycins are bactericidal and inhibit bacterial DNA-dependent RNA polymerase, suppressing RNA synthesis (see Table 17: Bacteria and Antibacterial Drugs: Rifamycins ).
Rifampin and Rifabutin
Rifampin and rifabutin have similar pharmacology, antimicrobial spectra, and adverse effects.
Pharmacology
Oral absorption is good, producing wide distribution in body tissues and fluids, including CSF. Rifampin is concentrated in polymorphonuclear granulocytes and macrophages, facilitating clearance of bacteria from abscesses. It is metabolized in the liver and eliminated in bile and, to a much lesser extent, in urine.
Indications
Rifampin is active against
Resistance develops rapidly, so rifampin is rarely used alone. Rifampin is used with other antibiotics for
Rifampin can be used alone for prophylaxis of close contacts of patients with meningococcal or Haemophilus influenzae type b meningitis.
Rifabutin and rifampin are equally efficacious in regimens for TB in HIV-positive and HIV-negative patients.
Rifabutin is more active than rifampin against M. avium complex and is used preferentially in multidrug regimens for these infections, but otherwise, rifampin is preferred.
Contraindications
Rifampin and rifabutin are contraindicated in patients who have had an allergic reaction to them.
Use During Pregnancy and Breastfeeding
Rifabutin is in pregnancy category B (animal studies show no risk and human evidence is incomplete, or animal studies show risk but human studies do not). Safety during breastfeeding is unknown.
Rifampin is in pregnancy category C (animal studies show some risk [in this case, teratogenicity], evidence in human studies is inadequate, but clinical benefit sometimes outweighs risk). The drug crosses the placenta. Still, if risk of maternal TB is moderate or high, treatment is thought to be less harmful for the fetus than untreated maternal TB and is thus recommended. Because of potential tumorigenicity shown in animal studies, a decision to stop breastfeeding or to stop the drug should be made, depending on the importance of the drug to the mother.
Adverse Effects
Adverse effects include
Hepatitis occurs much more often when isoniazid or pyrazinamide is used concurrently with rifampin. During the first week of therapy, rifampin may cause a transient rise in unconjugated serum bilirubin, which results from competition between rifampin and bilirubin for excretion and which is not in itself an indication for interrupting treatment.
CNS effects may include headache, drowsiness, ataxia, and confusion. Rash, fever, leukopenia, hemolytic anemia, thrombocytopenia, interstitial nephritis, acute tubular necrosis, renal insufficiency, and interstitial nephritis are generally considered to be hypersensitivity reactions and occur when therapy is intermittent or when treatment is resumed after interruption of a daily dosage regimen; they are reversed when rifampin is stopped.
Less serious adverse effects are common; they include heartburn, nausea, vomiting, and diarrhea. Rifampin colors urine, saliva, sweat, sputum, and tears red-orange.
Dosing Considerations
If patients have a liver disorder, liver function tests should be done before rifampin therapy is started and every 2 to 4 wk during therapy, or an alternate drug should be used. Dose adjustments are unnecessary for renal insufficiency.
Rifampin interacts with many drugs because it is a potent inducer of hepatic cytochrome P-450 (CYP450) microsomal enzymes. Rifampin accelerates elimination and thereby may decrease the effectiveness of the following drugs: ACE inhibitors, atovaquone, barbiturates, β-blockers, Ca channel blockers, chloramphenicol, clarithromycin, oral and systemic hormone contraceptives, corticosteroids, cyclosporine, dapsone, digoxin, doxycycline, fluconazole, haloperidol, itraconazole, ketoconazole, the nonnucleoside reverse transcriptase inhibitors delavirdine and nevirapine, opioid analgesics, phenytoin, protease inhibitors, quinidine, sulfonylureas, tacrolimus, theophylline, thyroxine, tocainide, tricyclic antidepressants, voriconazole, warfarin, and zidovudine. To maintain optimum therapeutic effect of these drugs, clinicians may have to adjust the dosage when rifampin is started or stopped. Conversely, protease inhibitors, as well as other drugs (eg, azoles, the macrolide clarithromycin, nonnucleoside reverse transcriptase inhibitors) inhibit CYP450 enzymes and increase levels of rifamycins and thus potentially increase the frequency of toxic reactions. For example, uveitis occurs more commonly when rifabutin is used with clarithromycin or azoles.
Rifaximin
Rifaximin is a derivative of rifamycin that is poorly absorbed after oral administration; 97% is recovered primarily unchanged in feces. Rifaximin can be used for empiric treatment of traveler's diarrhea, which is caused primarily by enterotoxigenic and enteroaggregative Escherichia coli. Rifaximin is not known to be effective for diarrhea due to enteric pathogens other than E. coli. Because rifaximin is not systemically absorbed, it should not be used to treat infectious diarrhea caused by invasive enteric bacterial pathogens (eg, salmonellae, Campylobacter sp).
The dose is 200 mg q 8 h for 3 days in adults and children > 12 yr.
Adverse effects include nausea, vomiting, abdominal pain, and flatulence.
Last full review/revision July 2009 by Matthew E. Levison, MD
Content last modified February 2012
| |
|
