CLINICAL PHARMACOLOGY
Mechanism Of Action
Rifamycin is an antibacterial drug [see Microbiology].
Pharmacodynamics
AEMCOLO exposure-response relationships and time course of pharmacodynamic response are
unknown.
Pharmacokinetics
Plasma Concentrations
In healthy adults receiving the recommended dose of 388 mg rifamycin (taken as AEMCOLO) twice
daily for 3 days, the maximum observed rifamycin concentration in plasma was 8.72 ng/mL (6 hours
after the last dose). A majority (67%) of rifamycin concentrations in plasma were below the limit of
quantification (< 2 ng/mL) at this time point.
Absorption
Rifamycin (taken as AEMCOLO) has limited systemic exposure after oral administration of the
recommended dosage. Based on total urinary excretion data, bioavailability was < 0.1% under fasting
conditions.
Food Effect
A food-effect study involving administration of AEMCOLO to healthy volunteers under a fasted state
and with a meal (approximately 1,000 kcal including 500 kcal from fat) indicated that food decreased
systemic exposure of rifamycin. The decrease in systemic exposure of rifamycin is not expected to be
clinically relevant [see DOSAGE AND ADMINISTRATION].
Distribution
Plasma protein binding was approximately 80% in vitro. Binding was primarily to albumin and was
inversely proportional to concentration.
Elimination
The apparent half-life of orally administered rifamycin (taken as AEMCOLO) in plasma is unknown.
Metabolism
Cytochrome P450 (CYP) based metabolism of rifamycin was not observed in vitro.
Excretion
After a single oral dose of 400 mg AEMCOLO (388 mg rifamycin base) in fasting healthy adults, fecal
excretion of rifamycin was on average 86% of the nominal dose.
Specific Populations
The pharmacokinetics of rifamycin (taken as AEMCOLO) in patients with impaired renal or hepatic
function have not been studied.
Drug Interaction Studies
Clinical drug-drug interaction studies of rifamycin (taken as AEMCOLO) have not been conducted.
In Vitro Transporter Studies Where Drug Interaction Potential Was Not Further Evaluated Clinically
Rifamycin is a substrate of P-glycoprotein (P-gp) and anticipated to be an inhibitor of P-gp and breast
cancer resistant protein (BCRP) in the gut.
Rifamycin is an inhibitor of renal transporters organic anion transporter (OAT) 3, multidrug and toxin
extrusion (MATE) 1, and MATE2-K transporters in vitro, however, based on systemic concentrations
of rifamycin observed after administration of the recommended dose, clinically relevant inhibition of
these transporters in vivo is unlikely.
In Vitro Cytochrome P450 (CYP) Studies Where Drug Interaction Potential Was Not Further Evaluated
Clinically
Rifamycin is an inhibitor of CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6 and 3A4/5 in vitro, however, based
on systemic concentrations of rifamycin observed after administration of the recommended dose
clinically relevant inhibition of these enzymes in vivo is unlikely.
Rifamycin is an inducer of CYP3A4 and CYP2B6 but not CYP1A2 in vitro, however, based on
systemic concentrations of rifamycin observed after administration of the recommended dose,
clinically relevant induction of these enzymes in vivo is unlikely.
Rifamycin is not a substrate of CYPs 1A2, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4/5.
Microbiology
Mechanism Of Action
Rifamycin belongs to the ansamycin class of antibacterial drugs and acts by inhibiting the betasubunit
of the bacterial DNA-dependent RNA polymerase, blocking one of the steps in DNA
transcription. This results in inhibition of bacterial synthesis and consequently growth of bacteria.
Resistance
Resistance to rifamycin is associated with mutations in the RNA polymerase beta subunit. Among E.
coli strains, the spontaneous mutation frequency rate of rifamycin ranged from 10-6to 10-10 at 4x –
16x MIC; the mutation frequency was independent of rifamycin concentration. Increases in the
minimum inhibitory concentrations were observed both in vitro and while on treatment following
exposure to rifamycin. Cross-resistance between rifamycin and other ansamycins have been
observed.
Antibacterial Activity
Rifamycin has been shown to be active against most isolates of the following pathogen both in vitro and in clinical studies of travelers’ diarrhea:
Escherichia coli (enterotoxigenic and enteroaggregative isolates)
Clinical Studies
Travelers’ Diarrhea
The efficacy of AEMCOLO given as 388 mg orally, taken two times a day, for 3 days was evaluated in
one multi-center, randomized, double-blind, placebo-controlled trial in adults with travelers’ diarrhea.
Trial 1 (NCT01142089) was conducted at clinical sites in Guatemala and Mexico, and provides the
primary evidence for the efficacy of AEMCOLO. A second active-controlled trial (Trial 2 –
NCT01208922) conducted in India, Guatemala and Ecuador, provided supportive evidence for the
efficacy of AEMCOLO. Although patients with fever and/or bloody stool at baseline were to be
excluded from both trials, 18 subjects treated with AEMCOLO had fever and bloody diarrhea at
enrollment in Trial 2. Stool specimens were collected before treatment and 1 to 2 days following the
end of treatment to identify enteric pathogens. The predominant pathogen in both trials was E. coli.
The clinical efficacy of AEMCOLO was assessed using an endpoint of time to last unformed (watery
or soft) stool (TLUS) before achieving clinical cure. The endpoint of clinical cure was defined as two
or fewer soft stools and minimal enteric symptoms at the beginning of a 24-hour period or no
unformed stools at the beginning of a 48-hour period. Kaplan-Meier estimates of TLUS for the intentto-
treat (ITT) Population, which includes all randomized subjects, in Trial 1 (Figure 1) show that
AEMCOLO significantly reduced the TLUS compared to placebo (p=0.0008).
Figure 1: Kaplan-Meier Estimates of Time to Last Unformed Stool (TLUS) in Trial 1
(ITT Population)
Table 1 displays the median TLUS and the number of patients who achieved clinical cure for the ITT
population in Trial 1. The median duration of diarrhea was significantly shorter in patients treated with
AEMCOLO than in the placebo group. More patients treated with AEMCOLO were classified as
clinical cures than were those in the placebo group.
Table 1: Clinical Response in Trial 1 (ITT Population)
|
AEMCOLO
(N=199) |
Placebo
(N=65) |
Difference |
P value |
Median TLUS (hrs) |
46.0 |
68.0 |
-22.0 |
p = 0.0008a |
Clinical cure, n (%) |
162 (81.4%) |
37 (56.9%) |
24.5% |
p =0.0001b |
ITT = Intent-to-Treat; TLUS = time to last unformed stool (in hours)
alog-rank test. A 95% confidence interval for the difference in medians cannot be computed due to the amount of
censored observations in the placebo group.
bchi-square test, 95% confidence interval on the difference is (11.3, 37.7). |
The results of Trial 2 supported the results presented for Trial 1. In addition, this trial provided
evidence that AEMCOLO-treated subjects with fever and/or bloody diarrhea at baseline had
prolonged TLUS. [see WARNINGS AND PRECAUTIONS]