indication

For the treatment of patients (≥12 years of age) with travelers' diarrhea caused by noninvasive strains of Escherichia coli. Rifaximin is also designated an orphan drug by the Food and Drug Administration for the adjunctive treatment of hepatic encephalopathy to reduce blood ammonia concentrations and decrease severity of neurological manifestations.

pharmacology

Rifaximin is a structural analog of rifampin and a non-systemic, gastrointestinal site-specific antibiotic. This non-systemic property of the drug is due to the addition of a pyridoimidazole ring, which renders it non-absorbable. Rifaximin acts by inhibiting bacterial ribonucleic acid (RNA) synthesis and contributes to restore intestinal microflora imbalance. Other studies have also shown rifaximin to be an pregnane X receptor (PXR) activator. As PXR is responsible for inhibiting the proinflammatory transcription factor NF-kappa B (NF-κB) and is inhibited in inflammatory bowel disease (IBD), rifaximin may prove to be effective for the treatment of IBD.

mechanism of action

Rifaximin acts by inhibiting RNA synthesis in susceptible bacteria by binding to the beta-subunit of bacterial deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA) polymerase enzyme. This results in the blockage of the translocation step that normally follows the formation of the first phosphodiester bond, which occurs in the transcription process.

toxicity

LD₅₀ > 2 g/kg (orally, in rats)

biotransformation

In vitro drug interactions studies have shown that rifaximin, at concentrations ranging from 2 to 200 ng/mL, did not inhibit human hepatic cytochrome P450 isoenzymes: 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4. In an in vitro hepa-tocyte induction model, rifaximin was shown to induce cytochrome P450 3A4 (CYP3A4), an isoenzyme which rifampin is known to induce.

absorption

Low absorption in both the fasting state and when administered within 30 minutes of a high-fat breakfast.

half life

Approximately 6 hours.

route of elimination

In a mass balance study, after administration of 400 mg 14C-rifaximin orally to healthy volunteers, of the 96.94% total recovery, 96.62% of the administered radioactivity was recovered in feces almost exclusively as the unchanged drug and 0.32% was recovered in urine mostly as metabolites with 0.03% as the unchanged drug.Rifaximin accounted for 18% of radioactivity in plasma. This suggests that the absorbed rifaximin undergoes metabolism with minimal renal excretion of the unchanged drug