Clinical efficacy 

In the pivotal clinical trials the rate of recurrence in the 30 days following treatment was assessed as a secondary endpoint. The rate of recurrence (including relapses) was significantly lower with fidaxomicin (14.1% versus 26.0% with a 95% CI of [-16.8%, -6.8%]), however these trials were not prospectively designed to prove prevention of reinfection with a new strain.

Description of the patient population in clinical trials 

In the two clinical trials of patients with CDI, 47.9% (479/999) of patients (per protocol population) were ≥65 years of age and 27.5% (275/999) of patients were treated with concomitant antibiotics during the study period. Twenty-four percent of patients met at least one of the following three criteria at baseline for scoring severity: body temperature >38.5°C, leukocyte count >15,000, or creatinine value ≥1.5 mg/dl. Patients with fulminant colitis and patients with multiple episodes (defined as more than one prior episode within the previous 3 months) of CDI were excluded in the studies.

Paediatric population 

The European Medicines Agency has deferred the obligation to submit the results of studies with fidaxomicin in one or more subsets of the paediatric population in enterocolitis caused by C. difficile (see section 4.2 for information on paediatric use).

5.2 Pharmacokinetic properties

Absorption 

The bioavailability in humans is unknown. In healthy adults, Cmax is approximately 9.88 ng/ml and AUC0-t is 69.5 ng•hr/ml following administration of 200 mg fidaxomicin, with a Tmax of 1.75 hours. In CDI patients, average peak plasma levels of fidaxomicin and its main metabolite OP-1118 tend to be 2- to 6-fold higher than in healthy adults. There was very limited accumulation of fidaxomicin or OP-1118 in plasma following administration of 200 mg fidaxomicin every 12 hours for 10 days.

Cmax for fidaxomicin and OP-1118 in plasma were 22% and 33% lower following a high fat meal vs fasting, but the extent of exposure (AUC0-t) was equivalent.

Fidaxomicin and the metabolite OP-1118 are substrates of P-gp.

In vitro studies showed that fidaxomicin and the metabolite OP-1118 are inhibitors of the transporters BCRP, MRP2 and OATP2B1, but were not found to be substrates. Under conditions of clinical use, fidaxomicin has no clinically relevant effect on the exposure of rosuvastatin, a substrate for OATP2B1 and BCRP (see section 4.5). The clinical relevance of MRP2 inhibition is not yet known.

Distribution 

The volume of distribution in humans is unknown, due to very limited absorption of fidaxomicin.

Biotransformation 

No extensive analysis of metabolites in plasma has been performed, due to low levels of systemic absorption of fidaxomicin. A main metabolite, OP-1118, is formed through hydrolysis of the isobutyryl ester. In vitro metabolism studies showed that the formation of OP-1118 is not dependent on CYP450 enzymes. This metabolite also shows antimicrobial activity (see section 5.1).

Fidaxomicin does not induce or inhibit CYP450 enzymes in vitro.

Elimination