Fidaxomicin

Fidaxomicin is a novel macrolide antibiotic used in the treatment of diarrhea caused by Clostridioides (formerly Clostridium) difficile in adult and pediatric patients over the age of 6 months. Fidaxomicin is a naturally-occurring 18-member macrocycle derived from fermentation. Because fidaxomicin contains an 18-membered lactone ring in its structure, it is referred to as a macrocyclic lactone antibiotic drug. The antibacterial activity of fidaxomicin is distinct from macrolides and rifamycins, as the bactericidal activity is time-dependent, and not concentration-dependent. Fidaxomicin was the first macrocyclic lactone antibiotic with activity against C. difficile, and it displays a narrow spectrum of activity against gram-positive anaerobes. It mediates its potent bactericidal action on the bacteria by inhibiting the bacterial RNA synthase, thereby disrupting bacterial transcription. The minimum inhibitory concentration (MIC) for fidaxomicin is four times less than that of vancomycin, which was the primary drug of choice for C. difficile infection before the approval of fidaxomicin. Unlike vancomycin, however, fidaxomicin has a negligible effect on normal colonic microflora.

The FDA initially approved fidaxomicin in May 2011 for the treatment of C. difficile-associated diarrhea in adult patients over the age of 18. Later that year in December, the drug was also approved by the European Medicine Agency. In June 2012, fidaxomicin was also granted approval by Health Canada. The approved indication of fidaxomicin was expanded by the FDA in January 2020 to include pediatric patients over the age of 6 months in the treatment population.