The rifamycins are a group of antibiotics that are synthesized either naturally by the bacterium Amycolatopsis rifamycinica or artificially. They are a subclass of the larger family of ansamycins. Rifamycins are particularly effective against mycobacteria, and are therefore used to treat tuberculosis, leprosy, and mycobacterium avium complex (MAC) infections. The rifamycin group includes the classic rifamycin drugs as well as the rifamycin derivatives rifampicin (or rifampin), rifabutin, rifapentine, rifalazil and rifaximin. Rifamycin, sold under the trade name Aemcolo, is approved in the United States for treatment of travelers' diarrhea in some circumstances. The name "rifamycin" (originally "rifomycin") was derived from the 1955 French film Rififi.
Bacterium Streptomyces mediterranei was first isolated in 1957 from a soil sample collected near the beach-side town of St Raphael in southern France. The name was originally given by two microbiologists working with the Italian drug company Group Lepetit SpA in Milan, the Italian Grazia Beretta, and Pinhas Margalith of Israel. In 1969, the bacterium was renamed Nocardia mediterranei when another scientist named Thiemann found that it has a cell wall typical of the Nocardia species. Then, in 1986, the bacterium was renamed again Amycolatopsis mediterranei, as the first species of a new genus, because a scientist named Lechevalier discovered that the cell wall lacks mycolic acid and is not able to be infected by the Nocardia and Rhodococcus phages. Based on 16S ribosomal RNA sequences, Bala et al. renamed the species in 2004 Amycolatopsis rifamycinica.
First drugs Rifamycins were first isolated in 1957 from a fermentation culture of Streptomyces mediterranei at the laboratory of Gruppo Lepetit SpA in Milan by two scientist named Piero Sensi and Maria Teresa Timbal, working with the Israeli scientist Pinhas Margalith. Initially, a family of closely related antibiotics was discovered referred to as Rifamycin A, B, C, D, E. The only component of this mixture sufficiently stable to isolate in a pure form was Rifamycin B, which was poorly active. However, further studies showed that while Rifamycin B was essentially inactive, it was spontaneously oxidized and hydrolyzed in aqueous solutions to yield the highly active Rifamycin S. Simple reduction of Rifamycin S yielded the hydroquinone form called Rifamycin SV, which became the first member of this class to enter clinical use as an intravenous antibiotic. Further chemical modification of Rifamycin SV yielded an improved analog Rifamide, which was also introduced into clinical practice, but was similarly limited to intravenous use. After an extensive modification program, Rifampin was eventually produced, which is orally available and has become a mainstay of tuberculosis therapy
Lepetit filed for patent protection of Rifamycin B in the UK in August 1958, and in the US in March 1959. The British patent GB921045 was granted in March 1963, and U.S. Patent 3,150,046 was granted in September 1964. The drug is widely regarded as having helped conquer the issue of drug-resistant tuberculosis in the 1960s.
Clinical trials Rifamycins have been used for the treatment of many diseases, the most important one being HIV-related tuberculosis. A systematic review of clinical trials on alternative regimens for prevention of active tuberculosis in HIV-negative individuals with latent TB found that a weekly, directly observed regimen of rifapentine with isoniazid for three months was as effective as a daily, self-administered regimen of isoniazid for nine months. But the rifapentine-isoniazid regimen had higher rates of treatment completion and lower rates of hepatotoxicity. However, the rate of treatment-limiting adverse events was higher in the rifapentine-isoniazid regimen. The rifamycins have a unique mechanism of action, selectively inhibiting bacterial DNA-dependent RNA polymerase, and show no cross-resistance with other antibiotics in clinical use. However, despite their activity against bacteria resistant to other antibiotics, the rifamycins themselves suffer from a rather high frequency of resistance. Because of this, Rifampin and other rifamycins are typically used in combination with other antibacterial drugs. This is routinely practiced in TB therapy and serves to prevent the formation of mutants that are resistant to any of the drugs in the combination. Rifampin rapidly kills fast-dividing bacilli strains as well as "persisters" cells, which remain biologically inactive for long periods of time that allow them to evade antibiotic activity. In addition, rifabutin and rifapentine have both been used against tuberculosis acquired in HIV-positive patients. Although Tuberculosis therapy remains the most important use of Rifampin, an increasing problem with serious Multiple Drug Resistant bacterial infections has led to some use of antibiotic combinations containing Rifampin to treat them.
Mechanism of action The antibacterial activity of rifamycins relies on the inhibition of bacterial DNA-dependent RNA synthesis. This is due to the high affinity of rifamycins for the prokaryotic RNA polymerase. The selectivity of the rifamycins depends on the fact that they have a very poor affinity for the analogous mammalian enzyme. Crystal structure data of the antibiotic bound to RNA polymerase indicates that rifamycin blocks synthesis by causing strong steric clashes with the growing oligonucleotide ("steric-occlusion" mechanism). If rifamycin binds the polymerase after the chain extension process has started, no inhibition is observed on the biosynthesis, consistent with a steric-occlusion mechanism. Single step high level resistance to the rifamycins occurs as the result of a single amino acid change in the bacterial DNA dependent RNA polymerase.
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