Fidaxomicin was recently approved for the treatment of Clostridium difficile infection. It inhibits transcription by bacterial RNA polymerase. Because transcription is a multistep process, experiments were conducted in which fidaxomicin was added at different stages of transcriptional initiation to identify the blocked step. DNA footprinting experiments were also conducted to further elucidate the stage inhibited. Fidaxomicin blocks initiation only if added before the formation of the “open promoter complex,” in which the template DNA strands have separated but RNA synthesis has not yet begun. Binding of fidaxomicin precludes the initial separation of DNA strands that is prerequisite to RNA synthesis. These studies show that it has a mechanism distinct from that of elongation inhibitors, such as streptolydigin, and from the transcription initiation inhibitors myxopyronin and the rifamycins.

Whole-genome sequencing was used to determine whether the reductions in recurrence of Clostridium difficile infection observed with fidaxomicin in pivotal phase 3 trials occurred by preventing relapse of the same infection, by preventing reinfection with a new strain, or by preventing both outcomes. Paired isolates of C. difficile were available from 93 of 199 participants with recurrences (28 were treated with fidaxomicin, and 65 were treated with vancomycin). Given C. difficile evolutionary rates, paired samples ≤2 single-nucleotide variants (SNVs) apart were considered relapses, paired samples >10 SNVs apart were considered reinfection, and those 3–10 SNVs apart (or without whole-genome sequences) were considered indeterminate in a competing risks survival analysis. Fidaxomicin reduced the risk of both relapse (competing risks hazard ratio [HR], 0.40 [95% confidence interval {CI}, .25–.66]; P = .0003) and reinfection (competing risks HR, 0.33 [95% CI, 0.11–1.01]; P = .05).

The effects of the inoculum, pH, cation concentrations, and different lots of commercial media on the in vitro susceptibility of Clostridium difficile to fidaxomicin were examined. Of the factors evaluated, only pH alterations influenced the activity of fidaxomicin against C. difficile, noticeably reducing its activity at higher pH (>7.9).In recent years, the epidemiology of Clostridium difficile infections (CDIs) has been changing, and there are increasing numbers of CDI cases being reported each year (2,3,5,7,9,12,13,15,17,19,20,21). While standard therapies for CDIs reduce the rates of morbidity and mortality, they are known to lead to high rates of recurrence, with 15% to 30% of patients demonstrating relapses in symptoms in the first few weeks after treatment is discontinued (9). Fidaxomicin (formerly known as OPT-80 and PAR-101) is a novel and narrow-spectrum macrocyclic compound (1, 6, 10) that is in clinical development for the treatment of CDIs. In a recent phase 3 trial, fidaxomicintreated patients demonstrated better clinical outcomes; namely, they had significantly lower rates of recurrences than subjects who were treated with vancomycin (16).The site of action of C. difficile is the large intestine, a milieu filled with vast numbers of different species of anaerobic flora that, through their metabolites, maintain a physiological pH ranging from 5.5 to 7 (11). Disruption of the gut flora by antibiotic therapy can therefore lead to pH changes, which can affect the pH-dependent activities of many antibiotics (4,8,21). Other environmental variables, such as divalent cation concentrations (including calcium and magnesium) and bacterial density, can also influence the antimicrobial activities of compounds. The dependence of the antibacterial activity on these factors is an important consideration, particularly for an unabsorbed antibiotic such as fidaxomicin that localizes and targets bacteria in the gut, where these parameters can vary greatly with diet and disease state.The in vitro activities of antimicrobial compounds (expressed as the MICs) under conditions with such environmental variables are also important factors to be considered when a methodology for future in vitro testing is designed. Brucella agar, which is recommended by the Clinical and Laboratory Standards Institute (CLSI) (18) for use for MIC determination, is not standardized, and the consistency of the divalent cation concentrations has not been established. Moreover, the pH of the medium used under anaerobic conditions in a glove box may also vary with different gas mixtures, as the CO 2 concentration in the gas mixture has the propensity to acidify the medium and can thus be a significant source of variability. Macrolides, as an example, show elevated MICs in the presence of CO 2 (8). The inoculum size may also be difficult to standardize, given the variety of atmospheric conditions available for anaerobic susceptibility testing (H 2 /CO 2 generator, evacuation/replacement method, or anaerobic chamber) and the duration of organism ex...

Fidaxomicin (FDX) is a narrow-spectrum antibiotic for the treatment of Clostridium difficile-associated diarrhea. While FDX and rifamycins share the same target (RNA polymerase), FDX exhibits a unique mode of action distinct from that of rifamycins. In comparative microbiological studies with C. difficile, FDX interacted synergistically with rifamycins, demonstrated a lower propensity for the development of resistance to rifamycins, and exhibited no cross-resistance with rifamycins. These results highlight differences in the mechanisms of action of FDX and rifamycins. F idaxomicin (FDX) and rifaximin (RFX) are nonsystemic antibiotics that target RNA polymerase (RNAP) and inhibit bacterial transcription (1, 2). Unlike FDX, which is approved for treatment of Clostridium difficile-associated diarrhea, RFX is a broad-spectrum antibiotic indicated for treatment of travelers' diarrhea and for reducing the recurrence of overt hepatic encephalopathy (3, 4). Since RFX achieves high colonic concentrations, its use for treatment of other gastrointestinal diseases, including C. difficile infections (CDI), has been reported (5, 6). Most studies describe the use of RFX as a chaser following vancomycin therapy. While RFX demonstrated some success in treating recurrent CDI, rapid development of resistance has discouraged its use in patients with prior exposure to rifamycins (7-9).In clinical trials, FDX was superior to vancomycin, the standard comparator, in sustaining clinical cure without recurrence of CDI through 25 days posttreatment (10, 11). Enhanced performance may be ascribed to the favorable attributes of FDX against C. difficile, as described below. Foremost, the drug and its major metabolite, OP-1118, achieve high colonic concentrations and display narrow spectra of activity (12-17). While both FDX and OP-1118 exhibit potent bactericidal activity against C. difficile (18) and moderate activity against some Gram-positive bacteria, in vitro and in vivo studies have indicated that they are sparing of the normal gut flora, with no activity against Gram-negative bacteria (13-16). Additionally, FDX and its major metabolite demonstrate prolonged postantibiotic effects (PAE), suppressing C. difficile growth for time periods of up to 10 and 3 h, respectively, which are considerably longer than those of vancomycin (19). A prolonged PAE is indicative of slow organism recovery and may confer an advantage to patients with severe CDI by potentially extending the duration of inhibitory activity between doses. Finally, both FDX and OP-1118 inhibited in vitro toxin production and sporulation by C. difficile (20,21). In vitro findings are consistent with results of phase II stool analyses in which samples from FDX-treated subjects showed significantly lower spore counts and reduced incidences of toxin than samples from vancomycin-treated subjects (13).Although FDX and rifamycins are both inhibitors of bacterial transcription, FDX acts at an earlier step in the transcription initiation pathway. While rifamycins block extension of short...

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