Download Pharmacokinetics of Erythromycin Ethylsuccinate

J. Lakritz, W. D. Wilson, A. E. Marsh, and J. E. Mihalyi
Pharmacokinetics of Erythromycin Ethylsuccinate
after Intragastric Administration to Healthy Foals*
Jeffrey Lakritz, DVM, PhDa
W. David Wilson, BVMS, MSb
Antoinette E. Marsh, PhDc
Judy E. Mihalyi, BSb
University of Missouri-Columbia
College of Veterinary Medicine
a
Department of Veterinary Medicine and Surgery
c
Department of Veterinary Pathobiology
Columbia, MO 65211
University of California, Davis
School of Veterinary Medicine
b
Department of Medicine and Epidemiology
One Shields Avenue
Davis, CA 95616
■ ABSTRACT
Plasma concentrations and pharmacokinetics
of erythromycin and related compounds were
determined after administration of erythromycin ethylsuccinate to six healthy male foals 3 to
5 months of age. Hay was withheld from the
foals overnight and erythromycin ethylsuccinate (25 mg/kg of body weight) was administered intragastrically. Plasma erythromycin concentrations were determined at specific times
after drug administration by high-performance
liquid chromatography assay. Maximum peak
plasma concentrations, time to maximum concentrations, area under plasma concentration
versus time curves, elimination half-life, and
mean residence time were determined from
concentration versus time curves. Maximum
peak concentration of erythromycin A (0.45 ±
0.27 µg/ml) after administration of erythromycin ethylsuccinate was observed at 2.38 ± 1.54
hours after treatment. Concentrations of anhy-
droerythromycin A were maximal at 2.2 ± 2.0
hours and reached a maximum of 2.6 ± 1.9
µg/ml. Plasma concentrations of the ester parent drug (erythromycin ethylsuccinate) were
below the limit of quantitation (0.1 µg/ml) at
all times except 2.5, 2.75, and 5.5 hours. Levels
at those times ranged from 0.1 to 0.144 µg/ml.
Erythromycin ethylsuccinate appears to be
poorly absorbed after oral administration to
fasted foals. Plasma concentrations of erythromycin A remained below 0.25 µg/ml (reported
minimum inhibitory concentration for Rhodococcus equi) for less than 4 hours after intragastric administration of erythromycin ethylsuccinate, suggesting that the recommended dosage
(25 mg/kg every 6 hours) would be suboptimal
for treatment of R. equi infections.
*This study was supported by the Center for Equine
Health, School of Veterinary Medicine, University
of California, Davis, CA, with funds provided by
the Oak Tree Racing Association, State of California
pari-mutuel wagering fund, and private donors.
■ INTRODUCTION
Pneumonia is a common cause of morbidity
and mortality in foals in the United States, and
Rhodococcus equi is one of the most common
and economically important causes.1 The
pneumonic process is often well advanced by
the time clinical signs are recognized by owners because most cases of R. equi pneumonia
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Veterinary Therapeutics • Vol. 3, No. 2, Summer 2002
represent the chronic form of the disease.
Therefore, the prognosis is guarded. The required course of antimicrobial treatment is
prolonged and expensive, and the most common treatment regimen used is associated with
important adverse effects.2 Erythromycin, a
macrolide antibiotic, is suitable for treatment
of pyogranulomatous lung inflammation such
as that caused by R. equi because it can be administered orally, has wide tissue distribution,
can reach high concentrations in lung tissue,
and accumulates in neutrophils at concentrations higher than those achieved in plasma.3
Rhodococcus equi is susceptible to some erythromycin formulations that achieve adequate
concentrations after oral or intragastric administration. When given in combination with rifampin, erythromycin has synergistic bactericidal activity against this organism.3
Because no erythromycin formulations are
approved for oral or intragastric administration
to horses and because there have been no controlled trials evaluating optimal therapy, the
pharmacokinetics of several different formulations of erythromycin have been studied in
foals within the age range in which pneumonia
commonly occurs.1 In these studies, concentrations of microbiologically active drug, as well
as degradation products or other compounds
that may enter the plasma of foals after oral or
intragastric administration of erythromycin,
have been studied. These studies were conducted in hopes of determining whether one
formulation is pharmacokinetically superior to
others based on plasma concentrations of active drug relative to established minimum inhibitory concentrations for commonly encountered foal pathogens. The purpose of the
study reported here was to determine the pharmacokinetics of erythromycin ethylsuccinate
and plasma concentrations of the inactive prodrug (erythromycin ethylsuccinate), the active
molecule (erythromycin A), and the degrada-
190
tion product (anhydroerythromycin A) after
intragastric administration to healthy foals.
The results provide valuable information about
the absorption of this formulation and its stability in the gut of foals. Other ester formulations (erythromycin estolate) are slowly hydrolyzed in foals, indicating greater stability of
this ester formulation in young horses. Furthermore, previous studies examining erythromycin estolate have indicated that less anhydroerythromycin A is absorbed by foals,
suggesting that the ester form is protected from
gastric acid degradation.
■ MATERIALS AND METHODS
Foals
Six healthy foals (three quarter horse, two
Thoroughbred, and one of mixed breeding), 3
and 5 months of age, were selected for the
study. Foals were determined to be healthy
based on results of physical examinations and
complete blood cell counts (CBC). Body
weights were recorded on the day before initiation of the study. On the night before dosing,
each foal and its dam were placed in a stall with
a small adjoining run. Mares and foals were fed
routinely the night before the study, but on
study days, hay was withheld until 1 hour after
treatment. The foals were allowed to nurse
their dams.
Treatment
Erythromycin ethylsuccinate (Barre-National, Baltimore, MD) was obtained as a suspension and administered without modification
after shaking thoroughly to ensure even suspension. Foals were then given erythromycin
ethylsuccinate (25 mg/kg of body weight) via
nasogastric tube.
Blood Samples
A Teflon® (E. I. du Pont de Nemours and
Company, Wilmington, DE) -coated jugular
J. Lakritz, W. D. Wilson, A. E. Marsh, and J. E. Mihalyi
catheter was placed for blood sampling. After
first withdrawing and discarding 5 ml of blood
from the intravenous catheter, blood was collected from each foal into tubes containing heparin as the anticoagulant immediately before
treatment (Time 0) and then 10, 20, 30, 45, 60
minutes and 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5,
3.75, 4, 4.5, 5, 5.5, 6, 6.5, 7, 8, 10, 12, and 24
hours after administration of erythromycin ethylsuccinate. Immediately after collection, blood
samples were centrifuged at 400 × g for 10
minutes, plasma was withdrawn, and duplicate
aliquots were stored at –20°C until they could
be analyzed by high-performance liquid chromatography (HPLC) assay. Samples were extracted and analyzed within 2 weeks of freezing.
Determination of Parent Drug and Related
Compounds
Plasma concentrations of erythromycin A
and anhydroerythromycin A were determined
by use of a previously described HPLC assay.4–6
Briefly, 500-µl aliquots of thawed plasma were
extracted by addition of 5 ml of methyl-tertbutyl ether followed by vortexing for 5 minutes
and centrifugation at 1000 × g for 30 minutes
at 4°C. The ether layer was removed, placed in
conical tubes, and dried under nitrogen at
room temperature (approximately 22°C).
Dried samples were reconstituted in 500 µl of
mobile phase and 50 µl was injected onto the
chromatographic system for analysis. The
HPLC system consisted of a pump, guard cell
(+0.950 V), autosampler, 15-cm (length) × 4.6mm (diameter) reverse-phase column (C18,
particle size 5 µm) and guard column, and analytical electrode (E1 = +0.75V; E2 = +0.88V)
connected to an electrochemical detector and
computer-controlled data acquisition and
analysis software. Chromatography was performed at room temperature, using a mobile
phase that consisted of 40% 50 mM ammonium acetate (pH 6.5), 50% acetonitrile, and
10% methanol. Flow was maintained at 1.0
ml/min during use. The HPLC system used
was the same as that used in previous studies,4–6
with the exception of the detector and electrode
settings (E1 = +0.75 V; E2 = +0.88 V).
Standard curves were prepared by adding
known concentrations of erythromycin A (erythromycin base, Lot L, USP Pharmacopoeia,
Rockville, MD), erythromycin ethylsuccinate
(erythromycin ethylsuccinate, Lot F-2, USP
Pharmacopoeia), and anhydroerythromycin A
(Anhydroerythromycin A, Abbott Laboratories,
North Chicago, IL) to pooled plasma from clinically normal foals. Standard samples were extracted and analyzed in the same manner as unknown
(test) samples. The limit of quantitation for
erythromycin A, erythromycin ethylsuccinate,
and anhydroerthromycin A by the methods in
the laboratory conducting these studies are
0.025, 0.1, and 0.01 µg/ml, respectively.
Pharmacokinetic Analyses
Concentration × time profiles of erythromycin in plasma after oral administration of
erythromycin ethylsuccinate to each foal were
subjected to statistical moment analysis to determine each of the following parameters: area
under the curve (AUC), area under the first
moment of the curve (AUMC), mean residence time (MRT), maximum concentration
in plasma (Cmax), time to maximum plasma
concentration (Tmax), and elimination half-life
(T1/2β) using a computer software program.4–7
Statistical moment analysis was used because
these parameters can be determined from early
time points, which are higher and relatively
more accurately determined. Additionally,
variables such as absorption, degradation,
and/or first-pass effects may complicate the
analysis of a drug administered extravascularly.
Pharmacokinetic variables for each foal were
used to determine the mean and standard deviation for AUC, AUMC, Cmax, and Tmax.
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Veterinary Therapeutics • Vol. 3, No. 2, Summer 2002
4.0
Plasma Concentration (µg/ml)
3.5
Erythromycin A
3.0
Anhydroerythromycin A
2.5
2.0
1.5
1.0
0.5
0.0
0
3
6
9
12
15
18
21
24
Time (hr)
Figure 1. Mean plasma erythromycin A and anhydroerythromycin A concentration versus time curves obtained after intragastric administration of erythromycin ethylsuccinate (25 mg/kg of body weight) to six healthy foals. The reference line (0.25 µg/ml) is the reported minimum inhibitory concentration for Rhodococcus equi. Error bars represent the standard deviation.
Mean residence time and T1/2β were recorded as
the median and median deviation.
■ RESULTS
Erythromycin ethylsuccinate was poorly absorbed after intragastric administration (Figure
1). The erythromycin A plasma concentration
versus time curve indicates plasma levels of
erythromycin were maintained above 0.25
µg/ml for approximately 2.75 hours, but in
only one foal was the maximum plasma concentration near 1.0 µg/ml (Figure 2). The
mean peak concentration of microbiologically
active erythromycin A, based on computer-fitted data, was 0.45 ± 0.27 µg/ml (range = 0.26
to 0.95). Computer-fitted mean peak plasma
192
concentration of anhydroerythromycin A (2.6
± 1.9 µg/ml; range=0.8 to 6.3) was achieved at
2.2 ± 0.2 (range=0.5 to 5.5) hours (Table 1).
With the exception of three time points (2.5,
2.75, and 5.5 hours), plasma concentrations of
parent erythromycin ethylsuccinate were below
or near the limit of quantitation (0.1 µg/ml) of
the assay method used. Plasma concentrations
of erythromycin ethylsuccinate were too low to
permit statistical moment analysis.
Results of noncompartmental statistical moment analysis for both erythromycin A and anhydroerythromycin A after intragastric administration of erythromycin ethylsuccinate are
reported in Table 1. Mean AUC of erythromycin A determined after administration of
J. Lakritz, W. D. Wilson, A. E. Marsh, and J. E. Mihalyi
Plasma Erythromycin A Concentration (µg/ml)
1.00
Foal 1
0.75
Foal 2
Foal 3
Foal 4
Foal 5
0.50
Foal 6
0.25
0.00
0
3
6
9
12
15
18
21
24
Time (hr)
Figure 2. Individual plasma erythromycin A concentration versus time curves obtained after intragastric administration of erythromycin ethylsuccinate (25 mg/kg) to six healthy foals.
erythromycin ethylsuccinate was low (1.62 ±
1.23 µg × hr/ml; range = 0.3 to 3.5). Mean
AUC for anhydroerythromycin A was higher
(9.25 ± 6.0 µg × hr/ml; range = 1.6 to 16.0)
than that observed for erythromycin A. Maximum plasma concentrations and Tmax for both
erythromycin A and anhydroerythromycin A
varied considerably among foals (Table 1).
■ DISCUSSION
These data indicate that erythromycin ethylsuccinate is poorly absorbed after intragastric
administration to foals that have been without
access to hay overnight. Poor absorption of
pro-drug results in low concentrations of both
erythromycin ethylsuccinate and active erythromycin A in plasma. After intragastric administration of erythromycin ethylsuccinate (25
mg/kg), mean peak concentration of microbiologically active erythromycin A (0.45 ± 0.27
µg/ml) was similar to that found in fed adult
horses given the same dose.8 However, peak
concentrations of erythromycin A were lower
than those reported for fasted foals given
crushed, enteric-coated erythromycin base (1.1
± 0.4 µg/ml; range = 0.6 to 1.6), erythromycin
phosphate (2.9 ± 1.1 µg/ml; range = 1.6 to
4.0), microencapsulated erythromycin base
(2.05 ± 1.3 µg/ml; range = 0.64 to 4.01), or
erythromycin estolate (1.0 ± 0.82 µg/ml; range
= 0.3 to 2.6) orally under the same experimental conditions as those used in this study.4–6
Similarly, concentrations of active erythromycin A remained detectable for a shorter duration after administration of erythromycin ethylsuccinate compared with administration of
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Veterinary Therapeutics • Vol. 3, No. 2, Summer 2002
TABLE 1. Pharmacokinetic Variables Determined from Analysis of Plasma Erythromycin
A and Anhydroerythromycin A Concentration Versus Time Curves after Intragastric
Administration of Erythromycin Ethylsuccinate (25 mg/kg) to Healthy Foals
Erythromycin
Formulation
AUC
AUMC
(µg × hr/ml)* (µg × hr2/ml)*
Erythromycin A
(n=6)
1.62 ± 1.23
(0.3–3.5)
MRT
(hr)†
7.34 ± 5.83 3.89 ± 2.18
(0.44–14.90) (1.5–7.4)
Anhydroerythromycin A 9.25 ± 6.00 57.0 ± 53.0
(n=6)
(1.60–16.00) (4.6–140.0)
5.0 ± 2.3
(2.8–8.9)
Cpmax
(µg/ml)
0.45 ± 0.27
(0.26–0.95)
2.6 ± 1.9
(0.8–6.3)
Tmax
(hr)*
T1/2 β
(hr)†
2.38 ± 1.54 1.52 ± 0.77
(1.0–5.0) (0.62–2.70)
2.2 ± 2.0
(0.5–5.5)
3.2 ± 1.7
(1.5–6.2)
*Data reported as mean ± SD (range).
†Data reported as median ± median deviation (range).
AUC = area under the plasma concentration versus time curve determined using the trapezoidal rule with extrapolation to infinity; AUMC = area under the first moment of the plasma concentration versus time curve; Cpmax = maximum plasma concentration determined by interpolation of three data points at the peak of the plasma concentration
versus time curve; MRT = mean residence time; Tmax = time to maximum plasma concentration determined based on
Cpmax; T1/2β = elimination half-life.
these formulations listed above.4–6 Poor absorption resulted in concentrations of microbiologically active erythromycin A remaining above
0.25 µg/ml (the minimum inhibitory concentration of R. equi) for less than 4 hours after
dosing with erythromycin ethylsuccinate.9
Care should be exercised when comparing
plasma concentrations of a drug with variable
absorption in different individuals and by different laboratories. However, with the exception of using different foals in some of these
studies, all other parameters that may affect interpretation of the data (laboratory, equipment, technician) were the same.
The mean area under the anhydroerythromycin plasma concentration versus time
curve was considerably larger than for erythromycin ethylsuccinate and erythromycin A, indicating that a substantial portion of the administered dose was degraded by gastric acid
prior to absorption and that anhydroerythromycin A is well absorbed from the gut. In
humans, the ethylsuccinate and estolate esters
of erythromycin are reported to be less susceptible than erythromycin base to degradation by
194
gastric acid.8,10 Consequently, the oral bioavailability of the ethylsuccinate ester is similar to
that of the estolate ester and is higher than that
of erythromycin base in people. The observed
elimination half-life of erythromycin ethylsuccinate is longer in comparison with crushed enteric-coated erythromycin base, erythromycin
phosphate, or erythromycin estolate but is similar to microencapsulated erythromycin base
when administered to comparable foals.4 In
contrast, the elimination half-life of erythromycin ethylsuccinate in fasted foals is shorter than
that observed in previously fed, adult horses.8
This suggests that foals are better capable of hydrolyzing this ester and differences in absorption or elimination of this drug exist in younger
animals or absorption was delayed in adult
horses resulting in longer elimination half-life.
Erythromycin ethylsuccinate is not recommended for administration in young humans.10
Comparison of the results of this study with
those of previous studies of erythromycin formulations in foals under identical conditions in
the authors’ laboratory indicates that the ethylsuccinate ester is considerably more susceptible
J. Lakritz, W. D. Wilson, A. E. Marsh, and J. E. Mihalyi
to acid degradation than either the estolate ester or microencapsulated erythromycin base
when comparing peak plasma concentrations of
anhydroerythromycin and the respective area
under the plasma concentration versus time
curves.4,6 Concentrations of anhydroerythromycin were considerably higher at all time
points after administration of erythromycin
ethylsuccinate compared with erythromycin estolate and microencapsulated base.4,6
It is possible (although unlikely) that absorption of erythromycin would have been enhanced if the experimental foals had been allowed to consume hay before dosing with
erythromycin ethylsuccinate. Additional studies will be necessary to document the effect of
feeding. However, plasma concentrations of
active erythromycin in adult horses given
erythromycin ethylsuccinate after feeding were
similar to those reported in the hay-fasted foals
in this study, suggesting that feeding is unlikely to enhance absorption.8 Furthermore, the
foals used in this study were subsequently used
in a study that demonstrated that feeding hay
prior to dosing caused a substantial reduction
in the bioavailability of microencapsulated
erythromycin base.4
Poor persistence of therapeutically effective
concentrations of erythromycin A, combined
with evidence of substantial degradation of
erythromycin ethylsuccinate to anhydroerythromycin A followed by absorption of this inactive product, suggest that erythromycin ethylsuccinate will likely be less effective than
equivalent doses of erythromycin estolate or
microencapsulated erythromycin base for the
treatment of R. equi infections in foals. Recently published evidence that oral administration
of erythromycin is associated with a significant
risk of adverse reactions in foals and concern
that some adverse reactions may be caused by
degradation products suggest that the microencapsulated base or estolate formulations should
be used in preference to erythromycin ethylsuccinate when treating foals with infections
caused by erythromycin-susceptible bacteria.2
■ ACKNOWLEDGMENTS
The authors thank Dr. Alan Buckpitt (Department of
Molecular Biosciences), Dr. Carter Judy (Veterinary Medical Teaching Hospital), Autumn Bernstein, Amber Steele,
Connie Littrell, Nettie Ehrke, Ben Ortega, and Trish Vogt
(Center for Equine Health) from the University of California, Davis, Davis, CA for technical assistance.
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