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Rifaximin, a rifamycin derivative, is a nonabsorbable antibiotic. Rifaximin inhibits RNA synthesis by binding the β subunit of the bacterial DNA-dependent RNA polymerase.

In vitro:Rifaximin, a gut-specific ligand for the human nuclear receptor pregnane-X receptor (PXR), contributes to the maintenance of the intestinal immune homeostasis. Rifaximin abrogates the binding of NF-κB caused by LPS. In human colon biopsies from inflammatory bowel diseases patients, exposure of rifaximin (100 μM) reduced mRNA levels of IL-8, Rantes, MIP-3α and TNFα induced by LPS stimulation [1]. Rifaximin acted on the β subunit of the deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA) polymerase enzyme of bacteria to inhibit bacterial RNA synthesis. The susceptibility of Gram-positive organisms to rifaximin was greater than that of Gram-negative organisms [2]. In the DPX2 cell line transfected with stable recombinant human PXR expression, hPXR was significantly activated at RIFax concentrations over 1 μM and the EC50 was about 20 μM[3].

In vivo:In the small intestine of hPXR mice treated with rifaximin, several PXR target genes such as CYP3A11, GSTA1, MRP2, and OATP2 were up-regulated. Rifaximin treatment demonstrated no significant effect on hepatic PXR target genes in wild-type, Pxr-null, and hPXR mice [3]. In PXR-humanized mice, long-term administration of rifaximin for 6 months on the liver up-regulated the expression of hepatic genes related to triglyceride synthesis and lipid accumulation [4].

Clinical trials: Over a 6-month period, treatment with rifaximin maintained remission from hepatic encephalopathy effectively. Rifaximin treatment also significantly reduced the risk of hospitalization involving hepatic encephalopathy [5]. Among patients with irritable bowel syndrome (IBS) without constipation, treatment with rifaximin for 2 weeks provided significant relief of IBS symptoms, bloating, abdominal pain, and loose or watery stools [6]. Rifaximin could improve the symptom in IBS patients, such as abdominal bloating and flatulence [7].

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