CLINICAL PHARMACOLOGY
Mechanism Of Action
Fidaxomicin is an antibacterial drug [see Microbiology].
Pharmacodynamics
Fidaxomicin acts locally in the gastrointestinal tract on
C. difficile. In a dose-ranging trial (N=48) of fidaxomicin using 50 mg,
100 mg, and 200 mg twice daily for 10 days, a dose-response relationship was
observed for efficacy.
Pharmacokinetics
The pharmacokinetic parameters of fidaxomicin and its
main metabolite OP-1118 following a single dose of 200 mg in healthy adult
males (N=14) are summarized in Table 2.
Table 2: Mean (± Standard Deviation) Pharmacokinetic
Parameters of Fidaxomicin 200 mg in Healthy Adult Males
Parameter |
Fidaxomicin |
OP-1118 |
N |
Value |
N |
Value |
Cmax (ng/mL) |
14 |
5.20 ± 2.81 |
14 |
12.0 ± 6.06 |
Tmax (h)* |
14 |
2.00 (1.00-5.00) |
14 |
1.02 (1.00-5.00) |
AUC0.t (ng-h/mL) |
14 |
48.3 ± 18.4 |
14 |
103 ± 39.4 |
AUC0-∞ (ng-h/mL) |
9 |
62.9 ± 19.5 |
10 |
118 ± 43.3 |
t½ (h) |
9 |
11.7 ± 4.80 |
10 |
11.2 ± 3.01 |
* Tmax, reported as median (range).
Cmax, maximum observed concentration; Tmax, time to maximum observed
concentration; AUC0-t, area under the concentration-time curve from time 0 to
the last measured concentration; AUC0-∞, area under the
concentration-time curve from time 0 to infinity; t½, elimination half-life |
Absorption
Fidaxomicin has minimal systemic absorption following
oral administration, with plasma concentrations of fidaxomicin and OP-1118 in
the ng/mL range at the therapeutic dose. In fidaxomicin-treated patients from
controlled trials, plasma concentrations of fidaxomicin and OP-1118 obtained
within the Tmax window (1-5 hours) were approximately 2- to 6-fold higher than
Cmax values in healthy adults. Following administration of DIFICID 200 mg twice
daily for 10 days, OP-1118 plasma concentrations within the Tmax window were
approximately 50%-80% higher than on Day 1, while concentrations of fidaxomicin
were similar on Days 1 and 10.
In a food-effect study involving administration of
DIFICID to healthy adults (N=28) with a high-fat meal versus under fasting
conditions, Cmax of fidaxomicin and OP-1118 decreased by 21.5% and 33.4%,
respectively, while AUC0-t remained unchanged. This decrease in Cmax is not
considered clinically significant, and thus, DIFICID may be administered with
or without food.
Distribution
Fidaxomicin is mainly confined to the gastrointestinal
tract following oral administration. In selected patients (N=8) treated with
DIFICID 200 mg twice daily for 10 days from controlled trials, fecal
concentrations of fidaxomicin and OP-1118 obtained within 24 hours of the last
dose ranged from 639-2710 μg /g and 213-1210 μg /g, respectively. In contrast,
plasma concentrations of fidaxomicin and OP-1118 within the Tmax window (1-5
hours) ranged 2-179 ng/mL and 10-829 ng/mL, respectively.
Metabolism
Fidaxomicin is primarily transformed by hydrolysis at the
isobutyryl ester to form its main and microbiologically active metabolite,
OP-1118. Metabolism of fidaxomicin and formation of OP-1118 are not dependent
on cytochrome P450 (CYP) enzymes.
At the therapeutic dose, OP-1118 was the predominant
circulating compound in healthy adults, followed by fidaxomicin.
Excretion
Fidaxomicin is mainly excreted in feces. In one trial of
healthy adults (N=11), more than 92% of the dose was recovered in the stool as
fidaxomicin and OP-1118 following single doses of 200 mg and 300 mg. In another
trial of healthy adults (N=6), 0.59% of the dose was recovered in urine as
OP-1118 only following a single dose of 200 mg.
Specific Populations
Geriatric
In controlled trials of patients treated with DIFICID® 200
mg twice daily for 10 days, mean and median values of fidaxomicin and OP-1118
plasma concentrations within the Tmax window (1-5 hours) were approximately 2-
to 4- fold higher in elderly patients ( ≥ 65 years of age) versus non-elderly
patients ( < 65 years of age). Despite greater exposures in elderly patients,
fidaxomicin and OP-1118 plasma concentrations remained in the ng/mL range [see Use
in Specific Populations].
Gender
Plasma concentrations of fidaxomicin and OP-1118 within
the Tmax window (1-5 hours) did not vary by gender in patients treated with
DIFICID 200 mg twice daily for 10 days from controlled trials. No dose adjustment
is recommended based on gender.
Renal Impairment
In controlled trials of patients treated with DIFICID 200
mg twice daily for 10 days, plasma concentrations of fidaxomicin and OP-1118
within the Tmax window (1-5 hours) did not vary by severity of renal impairment
(based on creatinine clearance) between mild (51-79 mL/min), moderate (31-50 mL/min),
and severe ( ≤ 30 mL/min) categories. No dose adjustment is recommended
based on renal function.
Hepatic Impairment
The impact of hepatic impairment on the pharmacokinetics
of fidaxomicin has not been evaluated. Because fidaxomicin and OP-1118 do not
appear to undergo significant hepatic metabolism, elimination of fidaxomicin
and OP-1118 is not expected to be significantly affected by hepatic impairment.
Drug Interactions
In vivo studies were conducted to evaluate intestinal
drug-drug interactions of fidaxomicin as a P-gp substrate, P-gp inhibitor, and
inhibitor of major CYP enzymes expressed in the gastrointestinal tract (CYP3A4,
CYP2C9, and CYP2C19).
Table 3 summarizes the impact of a co-administered drug
(P-gp inhibitor) on the pharmacokinetics of fidaxomicin [see DRUG
INTERACTIONS].
Table 3: Pharmacokinetic Parameters of Fidaxomicin and
OP-1118 in the Presence of a Co-Administered Drug
Parameter |
Cyclosporine 200 mg + Fidaxomicin
200 mg*
(N=14) |
Fidaxomicin 200 mg Alone
(N=14) |
Mean Ratio of Parameters With/Without Co-Administered Drug (90% CI †) No Effect = 1.00 |
N |
Mean |
N |
Mean |
Fidaxomicin |
Cmax (ng/mL) |
14 |
19.4 |
14 |
4.67 |
4.15
(3.23-5.32) |
AUC0-∞ (ng-h/mL) |
8 |
114 |
9 |
59.5 |
1.92
(1.39-2.64) |
OP-1118 |
Cmax (ng/mL) |
14 |
100 |
14 |
10.6 |
9.51
(6.93-13.05) |
AUC0-∞ (ng-h/mL) |
12 |
438 |
10 |
106 |
4.11
(3.06-5.53) |
* Cyclosporine was administered 1 hour before
fidaxomicin.
† CI - confidence interval |
Fidaxomicin had no significant impact on the
pharmacokinetics of the following co-administered drugs: digoxin (P-gp
substrate), midazolam (CYP3A4 substrate), warfarin (CYP2C9 substrate), and
omeprazole (CYP2C19 substrate). No dose adjustment is warranted when
fidaxomicin is co-administered with substrates of P-gp or CYP enzymes.
Microbiology
Spectrum of Activity
Fidaxomicin is a fermentation product obtained from the
Actinomycete Dactylosporangium aurantiacum. In vitro, fidaxomicin is active
primarily against species of clostridia, including Clostridium difficile.
Mechanism of Action
Fidaxomicin is bactericidal against C. difficile
in vitro, inhibiting RNA synthesis by RNA polymerases.
Mechanism of Decreased Susceptibility to Fidaxomicin
In vitro studies indicate a low frequency of spontaneous
resistance to fidaxomicin in C. difficile (ranging from < 1.4 Ã 10-9
to 12.8 Ã 10-9). A specific mutation (Val-ll43-Gly) in the beta
subunit of RNA polymerase is associated with reduced susceptibility to fidaxomicin.
This mutation was created in the laboratory and seen during clinical trials in
a C. difficile isolate obtained from a subject treated with DIFICID who
had recurrence of CDAD. The C. difficile isolate from the treated
subject went from a fidaxomicin baseline minimal inhibitory concentration (MIC)
of 0.06 μg/mL to 16 μg/mL.
Cross-Resistance/Synergy/Post-Antibiotic Effect
Fidaxomicin demonstrates no in vitro cross-resistance
with other classes of antibacterial drugs. Fidaxomicin and its main metabolite
OP-1118 do not exhibit any antagonistic interaction with other classes of
antibacterial drugs. In vitro synergistic interactions of fidaxomicin and
OP-1118 have been observed in vitro with rifampin and rifaximin against C.
difficile (FIC values ≤ 0.5). Fidaxomicin demonstrates a
post-antibiotic effect vs. C. difficile of 6-10 hrs.
Susceptibility Testing
The clinical microbiology laboratory should provide
cumulative results of the in vitro susceptibility test results for antimicrobial
drugs used in local hospitals and practice areas to the physician as periodic
reports that describe the susceptibility profile of nosocomial and
community-acquired pathogens. These reports should aid the physician in selecting
appropriate antimicrobial drug therapy.
Dilution Techniques
Quantitative anaerobic in vitro methods can be used to
determine the MIC of fidaxomicin needed to inhibit the growth of the C.
difficile isolates. The MIC provides an estimate of the susceptibility of C.
difficile isolate to fidaxomicin. The MIC should be determined using
standardized procedures.{1} Standardized methods are based on an agar dilution
method or equivalent with standardized inoculum concentrations and standardized
concentration of fidaxomicin powder.
Susceptibility Test Interpretive Criteria
In vitro susceptibility test interpretive criteria for
fidaxomicin have not been determined. The relation of the in vitro fidaxomicin
MIC to clinical efficacy of fidaxomicin against C. difficile isolates
can be monitored using in vitro susceptibility results obtained from
standardized anaerobe susceptibility testing methods.
Quality Control Parameters for Susceptibility Testing
In vitro susceptibility test quality control parameters
were developed for fidaxomicin so that laboratories determining the
susceptibility of C. difficile isolates to fidaxomicin can ascertain
whether the susceptibility test is performing correctly. Standardized dilution
techniques require the use of laboratory control microorganisms to monitor the
technical aspects of the laboratory procedures. Standardized fidaxomicin powder
should provide the MIC with the indicated quality control strain shown in Table
4.
Table 4: Acceptable Quality Control Ranges for
Fidaxomicin
Microorganism |
MIC Range (μg/mL) |
C. difficile (ATCC 700057) |
0.03-0.25 |
Clinical Studies
In two randomized, double-blinded trials, a
non-inferiority design was utilized to demonstrate the efficacy of DIFICID® (200
mg twice daily for 10 days) compared to vancomycin (125 mg four times daily for
10 days) in adults with Clostridium difficile-associated diarrhea
(CDAD).
Enrolled patients were 18 years of age or older, and
received no more than 24 hours of pretreatment with vancomycin or
metronidazole. CDAD was defined by > 3 unformed bowel movements (or > 200
mL of unformed stool for subjects having rectal collection devices) in the 24
hours before randomization, and presence of either C. difficile toxin A
or B in the stool within 48 hours of randomization. Enrolled patients had
either no prior CDAD history or only one prior CDAD episode in the past three
months. Subjects with life-threatening/fulminant infection, hypotension, septic
shock, peritoneal signs, significant dehydration, or toxic megacolon were
excluded.
The demographic profile and baseline CDAD characteristics
of enrolled subjects were similar in the two trials. Patients had a median age
of 64 years, were mainly white (90%), female (58%), and inpatients (63%). The
median number of bowel movements per day was 6, and 37% of subjects had severe
CDAD (defined as 10 or more unformed bowel movements per day or WBC
≥ 15000/mm³). Diarrhea alone was reported in 45% of patients and 84% of
subjects had no prior CDAD episode.
The primary efficacy endpoint was the clinical response
rate at the end of treatment, based upon improvement in diarrhea or other
symptoms such that, in the investigator's judgment, further CDAD treatment was
not needed. An additional efficacy endpoint was sustained clinical response 25
days after the end of treatment. Sustained response was evaluated only for
patients who were clinical successes at the end of treatment. Sustained
response was defined as clinical response at the end of treatment, and survival
without proven or suspected CDAD recurrence through 25 days beyond the end of
treatment.
The results for clinical response at the end of treatment
in both trials, shown in Table 5, indicate that DIFICID is non-inferior to
vancomycin based on the 95% confidence interval (CI) lower limit being greater
than the noninferiority margin of -10%.
The results for sustained clinical response at the end of
the follow-up period, also shown in Table 5, indicate that DIFICID is superior
to vancomycin on this endpoint. Since clinical success at the end of treatment
and mortality rates were similar across treatment arms (approximately 6% in
each group), differences in sustained clinical response were due to lower rates
of proven or suspected CDAD during the follow-up period in DIFICID patients.
Table 5: Clinical Response Rates at End-of-Treatment
and Sustained Response at 25 days Post-Treatment
|
Clinical Response at End of Treatment |
Sustained Response at 25 days Post Treatment |
DIFICID % (N) |
Vancomycin % (N) |
Difference (95% CI)* |
DIFICID % (N) |
Vancomycin % (N) |
Difference (95% CI)* |
Trial 1 |
88% |
86% |
2.6% |
70% |
57% |
12.7% |
(N=289) |
(N=307) |
(-2.9%, 8.0%) |
(N=289) |
(N=307) |
(4.4%, 20.9%) |
Trial 2 |
88% |
87% |
1.0% |
72% |
57% |
14.6% |
(N=253) |
(N=256) |
(-4.8%, 6.8%) |
(N=253) |
(N=256) |
(5.8%, 23.3%) |
* Confidence interval (CI) was derived using Wilson's
score method. Approximately 5%-9% of the data in each trial and treatment arm
were missing sustained response information and were imputed using multiple
imputation method. |
Restriction Endonuclease Analysis (REA) was used to
identify C. difficile baseline isolates in the BI group, isolates
associated with increasing rates and severity of CDAD in the US in the years
prior to the clinical trials. Similar rates of clinical response at the end of
treatment and proven or suspected CDAD during the follow-up period were seen in
fidaxomicin-treated and vancomycin-treated patients infected with a BI isolate.
However, DIFICID did not demonstrate superiority in sustained clinical response
when compared with vancomycin (Table 6).
Table 6: Sustained Clinical Response at 25 Days after
Treatment by C. difficile REA Group at Baseline
Trial 1 |
Initial C. difficile Group |
DIFICID n/N (%) |
Vancomycin n/N (%) |
Difference (95% CI)* |
BI Isolates |
44/76 (58%) |
52/82 (63%) |
-5.5% (-20.3%, 9.5%) |
Non-BI Isolates |
105/126 (83%) |
87/131 (66%) |
16.9% (6.3%, 27.0%) |
Trial 2 |
Initial C. difficile Group |
DIFICID n/N (%) |
Vancomycin n/N (%) |
Difference (95% CI)* |
BI Isolates |
42/65 (65%) |
31/60 (52%) |
12.9% (-4.2%, 29.2%) |
Non-BI Isolates |
109/131 (83%) |
77/121 (64%) |
19.6% (8.7%, 30.0%) |
* Interaction test between the effect on sustained
response rate and BI versus non-BI isolates using logistic regression
(p-values: trial 1:0.009; trial 2: 0.29). Approximately 25% of the mITT
population were missing data for REA group. Confidence intervals (CI) were derived
using Wilson's score method. |
REFERENCES
1. Clinical and Laboratory Standards Institute (CLSI). Methods
for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved
Standard - 7th edition. CLSI document M11-A7. CLSI, 940 West Valley Rd., Suite
1400, Wayne, PA 19087-1898, 2007.