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Fidaxomicin (OPT-80) was approved by the U.S. FDA in May 2011 for the treatment of Clostridium difficile-associated diarrhea (CDAD), joining metronidazole and vancomycin as drugs recommended for treatment of C. difficile infections (CDI). Fidaxomicin, also known as lipiarmycin and tiacumicin, is an 18-membered macrolide natural product that was first reported in mid-1970s and is produced by fermentation. Fidaxomicin and its primary metabolite OP-1118, which results from hydrolysis of the isobutyryl ester, are narrowspectrum antibacterial agents with activity against gram-positive aerobic and anaerobic organisms, but not against gram-negative organisms. Fidaxomicin and OP-1118 exert their antibacterial activity by inhibiting bacterial RNA polymerase, thereby inhibiting bacterial protein synthesis.
The MIC90 (minimum inhibitory concentration to kill 90% of bacteria) for fidaxomicin against C.difficile is 0.125–0.25 μg/mL; OP- 1118 is 4- to 16-fold less potent than the parent compound. Fidaxomicin has been reported to spare native intestinal flora such as Bacteroides spp. and as such, may prevent selection of drug-resistant bacteria. Fidaxomicin is bactericidal to C. difficile and has a low propensity for resistance development with no cross-resistance to existing antibiotics. Fidaxomicin shows minimal systemic absorption following oral administration in preclinical studies and humans.

Optimer Pharmaceuticals (United States)

Fidaxomicin is a recently marketed antibiotic with a confusing history dating back to its original isolation in 1975. Fidaxomicin is the major analogue of a family of macrocyclic lactones, isolated independently by three different groups from cultures belonging to three different genera (Actinoplanes, Dactylosporangium and Micromonospora) known as lipiarmycin A3, tiacumicin B and clostomicin B1, respectively. Fidaxomicin is a narrow spectrum antibiotic with excellent activity against Gram positive bacteria, notably Clostridium difficile. Fidaxomicin acts in the gastrointestinal tract without undue disruption to gut microbial flora.

Fidaxomycin is a natural macrocyclic antibiotic that inhibits RNA polymerase with selectivity for Gram-positive bacteria over Gram-negative bacteria (IC50s = 0.4 and 6 μM, respectively). It has potent antibacterial activity against most Gram-positive bacteria and effectively targets the Gram-positive C. difficile (MIC = 12 ng/ml). Orally administered fidaxomycin exhibits minimal systemic bioavailability resulting in maximal gastrointestinal tract distribution. Fidaxomycin is effective in clearing C. difficile infections while sparing Gram-negative bacteria in the gut.[Cayman Chemical]

ChEBI: An 18-membered macrolide that is a fermentation product obtained from the Actinomycete Dactylosporangium aurantiacum. A narrow spectrum antibiotic used for treatment of Clostridium difficile-related infections.

Dificid

Formerly known as difimicin. An 18-membered macrocyclic compound related to the tiacumicin group of antibiotics rather than conventional macrolides. It is active against staphylococci (MIC 0.5–2 mg/L) and most anaerobic Grampositive bacilli and cocci, but Gram-negative bacilli, including Gram-negative anaerobes, are resistant. It is very poorly absorbed when given orally and most interest surrounds its activity against C. difficile (MIC 0.12–0.25 mg/L). Such data as are presently available from clinical trials suggest that it is as safe and effective in the treatment of C. difficile-associated diarrhea as vancomycin.

Fidaxomicin is a first-in-class macrocyclic antibacterial agent for gram positive bacteria treatment, notably Clostridium difficile infections. Fidaxomicin produces its antibacterial effects by inhibiting bacterial RNA polymerase at transcription initiation. Furthermore, Fidaxomicin is an inhibitor of bacterial transcription. Fidaxomicin acts at an earlier step in the transcription initiation pathway. Specifically, Fidaxomicin binds to the DNA template-RNA polymerase complex and prevents the initial separation of DNA strands, which precedes messenger RNA synthesis by inhibiting the s subunit. Fidaxomicin′s unique target site may explain its limited spectrum of antimicrobial activity because s subunits differ among bacterial species.

Macrolide antibacterial agent
Treatment of Clostridium Difficile infection

Potentially hazardous interactions with other drugs
Anti-arryhthmics: avoid concomitant use with amiodarone and dronedarone.
Antibaterials: avoid concomitant use with clarithromycin and erythromycin.
Antifungals: avoid concomitant use with ketoconazole.
Calcium channel blockers: avoid concomitant use with verapamil.
Ciclosporin: increased fidaxomicin levels, avoid concomitant use.

Mainly metabolised by hydrolysis in the gut at the isobutyryl ester to form its main and microbiologically active metabolite, OP-1118. Over 92% of a dose is excreted in the faeces as either fidaxomicin or OP-1118, although very small amounts of OP-1118 have been recovered in the urine

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