Rifamycin S

Rifamycin S (Rifaximin EP Impurity E) is a semi-synthetic antibiotic.

The rifamycins are a group of chemically related antibioticsobtained by fermentation from cultures of Streptomycesmediterranei. They belong to a class of antibiotics called theansamycins that contain a macrocyclic ring bridged acrosstwo nonadjacent positions of an aromatic nucleus. The termansa means “handle,” describing well the topography of thestructure. The rifamycins and many of their semisynthetic derivativeshave a broad spectrum of antimicrobial activity.They are most notably active against Gram-positive bacteriaand M. tuberculosis. However, they are also active againstsome Gram-negative bacteria and many viruses. Rifampin, asemisynthetic derivative of rifamycin B, was released as anantitubercular agent in the United States in 1971. A secondsemisynthetic derivative, rifabutin, was approved in 1992 forthe treatment of atypical mycobacterial infections.
The chemistry of rifamycins and other ansamycins hasbeen reviewed. All of the rifamycins (A, B, C, D, and E) arebiologically active. Some of the semisynthetic derivatives ofrifamycin B are the most potent known inhibitors of DNAdirectedRNA polymerase in bacteria, and their action isbactericidal. They have no activity against the mammalianenzyme. The mechanism of action of rifamycins as inhibitorsof viral replication appears to differ from that for their bactericidalaction. Their net effect is to inhibit the formation of thevirus particle, apparently by preventing a specific polypeptideconversion.77 Rifamycins bind to the β subunit of bacterialDNA-dependent RNA polymerases to prevent chain initiation.78 Bacterial resistance to rifampin has been associatedwith mutations leading to amino acid substitution in the subunit.78 A high level of cross-resistance between variousrifamycins has been observed.