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New Drug Approvals 2011 - Pt. XVII Fidaxomicin (Dificid TM)

ATC code (partial): A07A

On May 27th 2011, the FDA approved Fidaxomicin (Tradename: Dificid; Research Code: PAR-101, OPT-80, NDA 201699), a macrolide narrow spectrum antibacterial drug indicated for the treatment of Clostridium difficile-associated diarrhea (CDAD) in adults. Clostridium difficile (C. difficile) is an anaerobic, spore-forming Gram-positive bacteria, and overgrowth of this species can cause severe diarrhea and other more serious intestinal conditions, such as colitis.

Fidaxomicin is a fermentation product obtained from the Actinomycete Dactylosporangium aurantiacum. It exerts its therapeutic effect by inhibiting beta subunit of the bacterial enzyme DNA-directed RNA polymerase (RNAP) (UniProt:Q890N5), resulting in the death of C. difficile. Bacterial RNA polymerase is a large (~400 kDa) five subunit protein, and is the target of the already approved antibiotic rifampicin. Other treatments for CDAD already in the market include antibiotics such as Metronidazole (trade name Flagyl; ChEMBLID: CHEMBL137) and Vancomycin (ChEMBLID: CHEMBL262777). Patients generally respond to these antibiotic therapies, however there is a risk of recurrent infection associated with these treatments. Fidaxomicin has been shown to be more active in vitro than Vancomycin (minimum inhibitory concentration (MIC) of 0.12 µg/mL and 1.0 µg/mL, respectively) against C. difficile and also more selective, having limited activity in vitro and in vivo against components of the normal gut flora.

There are several known structures of bacterial RNA polymerases in complex with various antibiotics, typical is the structure of the Thermus aquaticus RNA polymerase in complex with sorangicin (PDBe:1ynn)

The recommended dose of Fidaxomicin is one 200 mg tablet twice daily for 10 days (equivalent to a daily dose of 380 umol). At therapeutic doses, Fidaxomicin has a minimal systemic absorption, with plasma concentrations of Fidaxomicin and OP-1118, its main and microbiologically active metabolite, in the ng/mL range. The mean terminal half-life (T1/2) of Fidaxomicin and OP-1118 is 11.7 and 11.2 hours, respectively. Fidaxomicin is primarily transformed by hydrolysis at the isobutyryl ester to form OP-1118. Metabolism of Fidaxomicin and formation of OP-1118 are not dependent on cytochrome P450 (CYP) enzymes. Fidaxomicin is mainly excreted in feces, with 92% of the dose recovered as either Fidaxomicin and OP-1118.

Fidaxomicin (IUPAC: [(2R,3S,4S,5S,6R)-6-[[(3E,5E,8S,9Z,11S,12R,13E,15E,18S)-12-[(2R,3S,4R,5S)-3,4-dihydroxy-6,6-dimethyl-5-(2-methylpropanoyloxy)oxan-2-yl]oxy-11-ethyl-8-hydroxy-18-[(1R)-1-hydroxyethyl]-9,13,15-trimethyl-2-oxo-1-oxacyclooctadeca-3,5,9,13,15-pentaen-3-yl]methoxy]-4-hydroxy-5-methoxy-2-methyloxan-3-yl]3,5-dichloro-2-ethyl-4,6-dihydroxybenzoate; SMILES: CC[C@H]1\C=C(/C)\[C@@H](O)C\C=C\C=C(/CO[C@H]2
[C@H](O)[C@@H]4O)[C@H](C)O; ChEMBL: CHEMBL485861; PubChem: 46174142) has a molecular weight of 1058 Da, an ALogP of 7.7, seven hydrogen bond donors and 18 acceptors, and thus is not rule of five compliant. A notable feature is the 18-member polyene macrolide ring.

The full prescribing information can be found here.

The license holder for Fidaxomicin is Optimer Pharmaceuticals, Inc. and the product website is


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