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Antimicrobial Agents and Chemotherapy, December 1999, p. 2855-2861, Vol. 43, No. 12
Glaxo Wellcome, Inc., Research Triangle Park,
North Carolina 27709,1 and Virginia
Commonwealth University/Medical College of Virginia, Richmond, Virginia
232192
Received 5 March 1999/Returned for modification 22 June
1999/Accepted 8 September 1999
Abacavir (1592U89) {( The current recommendation for
treatment of human immunodeficiency virus (HIV) infection is
combination therapy with antiretroviral drugs with different modes of
action, such as a protease inhibitor in combination with
nucleoside reverse transcriptase inhibitors (NRTIs) (10).
Problems with long-term antiretroviral therapy include (i) inadequate
antiviral suppression and development of drug-resistant variants
of HIV; (ii) adverse effects associated with drug toxicity, such
as peripheral neuropathy, anemia, and lipodystrophy; and (iii) the
failure of many antiretroviral drugs to penetrate the cerebrospinal
fluid (CSF) and cross the blood-brain barrier, thus enabling the virus
to replicate while sequestered in the central nervous system. Because
toxicity and loss of efficacy due to viral drug resistance necessitate
changes in drug regimens for HIV-infected patients, there is a
continuing clinical need for new antiretroviral agents, ideally, agents
with strong potencies, little or no cross-resistance to other
antiretroviral agents, good safety and tolerability profiles, and
adequate penetration into the CSF.
Abacavir (1592U89) is a 2'-deoxyguanosine analogue with
potent activity against HIV type 1 (HIV-1). In vitro
experiments with human T-lymphoblastoid CD4+ CEM cells have
shown that abacavir is rapidly phosphorylated by a unique metabolic
pathway to its active form, the triphosphate of the carbocyclic
guanosine analogue (1144U88 triphosphate [1144U88-TP]), which is
a potent inhibitor of viral reverse transcriptase (8). Preliminary data indicated that abacavir penetrates the CSF (7, 19), and in vitro experiments have shown abacavir to have potency against HIV strains resistant to other NRTIs (20).
Preclinical studies have shown that oral abacavir is primarily
eliminated by hepatic metabolism to two major metabolites: 2269W93, a
5'-carboxylate formed by cytosolic alcohol dehydrogenase, and 361W94, a
5'-glucuronide formed by uridine diphosphate glucuronyl transferase (11). In addition, 11 to 13% of oral abacavir
is recovered as unchanged drug in urine (11). In vitro
experiments have shown that clinically relevant concentrations of
abacavir do not inhibit human liver CYP3A4,
CYP2D6, or CYP2C9 activity and that abacavir is
not significantly metabolized by human liver CYP450 enzymes
(18).
In order to characterize in greater detail the metabolic profile
of abacavir in humans, this phase I mass balance study (Glaxo Wellcome protocol CNAA-1003) was conducted with radiolabeled
[14C]abacavir succinate administered as part of a single
600-mg oral abacavir dose to six HIV-infected volunteers. The purpose
of this study was to determine the major metabolites of abacavir,
describe pharmacokinetic parameters, estimate CSF penetration, and
determine the routes of elimination for abacavir and its metabolites.
Study design.
This study was an open-label, mass balance,
phase I trial conducted at a single study center, in which six
HIV-infected male volunteers each received a single 600-mg oral dose of
abacavir which contained 100 µCi of [14C]abacavir. The
study was approved by the Institutional Review Board and the Radiation
Safety/Radioactive Drug Research Committees at the Medical College of
Virginia/Virginia Commonwealth University, Richmond, clinical study
center, and the subjects provided written informed consent prior to
study participation. The study consisted of a screening evaluation, a
treatment phase with a single dosing period followed by a sample
collection period of up to 10 days' duration, and a follow-up
evaluation. Laboratory and clinical assessments were performed at the
screening evaluation, at check-in on the evening prior to dosing, and
at the follow-up evaluation. Laboratory assessments included
hematology, lymphocyte determination (screening only), clinical
chemistry analysis, dipstick urinalysis, and urine screen for illicit
drugs. Clinical assessments included physical examination,
determination of vital signs, and electrocardiogram analysis. Subjects
were monitored throughout the study for adverse experiences and for the
use of concurrent medications and other restricted substances. The AIDS
Clinical Trials Group toxicity grading scale was used for
classification of abnormal laboratory values as adverse experiences.
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Pharmacokinetics of [14C]Abacavir, a Human
Immunodeficiency Virus Type 1 (HIV-1) Reverse Transcriptase Inhibitor,
Administered in a Single Oral Dose to HIV-1-Infected Adults: a Mass
Balance Study
ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
)-(1S,
4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-methanol}
is a 2'-deoxyguanosine analogue with potent activity against human
immunodeficiency virus (HIV) type 1. To determine the metabolic
profile, routes of elimination, and total recovery of abacavir and
metabolites in humans, we undertook a phase I mass balance study in
which six HIV-infected male volunteers ingested a single 600-mg oral
dose of abacavir including 100 µCi of [14C]abacavir.
The metabolic disposition of the drug was determined through analyses
of whole-blood, plasma, urine, and stool samples, collected for a
period of up to 10 days postdosing, and of cerebrospinal fluid (CSF),
collected up to 6 h postdosing. The radioactivity from abacavir
and its two major metabolites, a 5'-carboxylate (2269W93) and a
5'-glucuronide (361W94), accounted for the majority (92%) of
radioactivity detected in plasma. Virtually all of the administered
dose of radioactivity (99%) was recovered, with 83% eliminated in
urine and 16% eliminated in feces. Of the 83% radioactivity dose
eliminated in the urine, 36% was identified as 361W94, 30% was
identified as 2269W93, and 1.2% was identified as abacavir; the
remaining 15.8% was attributed to numerous trace metabolites, of which
<1% of the administered radioactivity was 1144U88, a minor
metabolite. The peak concentration of abacavir in CSF ranged from 0.6 to 1.4 µg/ml, which is 8 to 20 times the mean 50% inhibitory concentration for HIV clinical isolates in vitro (0.07 µg/ml). In
conclusion, the main route of elimination for oral abacavir in humans
is metabolism, with <2% of a dose recovered in urine as unchanged
drug. The main route of metabolite excretion is renal, with 83% of a
dose recovered in urine. Two major metabolites, the 5'-carboxylate and
the 5'-glucuronide, were identified in urine and, combined, accounted
for 66% of the dose. Abacavir showed significant penetration into CSF.
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
200 CD4+ cells/mm3. In
addition, the following exclusion criteria were applied: no current
AIDS-defining conditions, no current treatment with protease inhibitors
or non-NRTIs; no current treatment with medications which could not be
withheld for 24 h (for antiretroviral or prophylactic agents) or
for 48 h (for other, nonessential medications) prior to dosing
until the end of the sample collection period; no current use of
illicit drugs; no history of clinically relevant hepatitis or
pancreatitis within the previous 6 months; no clinically significant neurological abnormality; absence of specified laboratory
abnormalities; no clinically significant history of systemic illness of
dysfunction which, in the opinion of the investigator, might have
compromised the safety of the subject; and no exposure to excess
radiation or participation in a study involving radioisotopes within
the previous 12 months.
Materials. The investigational drug was a mixture of radiolabeled [14C]abacavir succinate and unlabeled [12C]abacavir succinate powders, prepared by the Division of Bioanalysis and Drug Metabolism, Glaxo Wellcome, Inc. Radiolabeled drug was synthesized by the Chemical Development Division, Glaxo Wellcome, Inc. Drug was supplied in individual bottles containing 700 mg (base equivalent) of preweighed powder, comprising a nominal 0.82 mg (base equivalent) of [14C]abacavir and 699.18 mg (base equivalent) of abacavir. The batch number, specific activity, chemical purity, and radiochemical purity of [14C]abacavir were R675/136/1, 127 µCi/mg, 98.3%, and 99.6%, respectively. The batch number of abacavir succinate used in this study was 5X3226, and its chemical purity was approximately 98.1%.
Dosing.
For dose preparation for each subject, 70 ml of
sterile water for irrigation was added to the bottle containing the
700-mg mixture of [14C]abacavir succinate and
[12C]abacavir succinate powders. The mixture was gently
swirled and was then sonicated for 30 min in an ultrasonic bath to
ensure complete dissolution of the powder before removal of exactly 10 ml of solution. One 5-ml aliquot was used for liquid scintillation counting, to verify the amount of radioactivity in the solution prior
to administration of the dose to the subject. The other 5-ml aliquot
was stored in a non-self-defrosting freezer set at 20°C or lower
prior to shipment to Glaxo Wellcome, Inc., for analysis of the actual
concentrations of abacavir. The radioactivity contained in a 60-ml dose
ranged from 87 to 91 µCi. The subject ingested the solution under the
supervision of study site personnel. The bottle was rinsed twice with
50 ml of sterile water for irrigation, and the subject ingested both rinses.
Sample collections.
Samples that were collected included
urine and stool specimens, plasma and whole-blood samples, and CSF
samples. Stool and urine specimens were collected for a minimum 4-day
period, up to 10 days postdosing. For both urine and stool specimens,
collection continued (i) until 10 days postdosing or (ii) until
radioactivity levels in specimens from two consecutive collection
intervals were 
1% of the administered dose, whichever occurred first.
20°C prior to shipment to Glaxo Wellcome,
Inc., for analysis.
Stools were collected starting with a predose specimen (if available)
and thereafter over the following intervals: 0 to 24, 24 to 48, 48 to
72, and 72 to 96 h postdosing and for each subsequent 24-h period
until discontinuation as described above. After weighing, the stool
sample was homogenized in deionized water (volume of water equivalent
to four times the weight of the stool sample) with a model PT3000
Polytron homogenizer (Brinkmann Instrument Co., Westbury, N.Y.).
Triplicate portions of the homogenate were removed for liquid
scintillation counting, and a portion of the homogenate was transferred
to a 120-ml plastic bottle and stored in a non-self-defrosting freezer
set at approximately 20°C prior to shipment to Glaxo Wellcome,
Inc., for analysis.
Plasma and whole-blood samples were collected for a minimum 2-day
period. At each sampling time, 9 ml of blood was drawn into a syringe
with a peripheral venous catheter. For radioactivity determination, a
portion of the blood (1.0 to 1.5 ml) was immediately transferred to a
prelabeled, 5-ml, heparinized plastic tube. The remaining blood was
injected into two prelabeled, 4-ml, dipotassium EDTA-containing
VACUTAINER brand blood collection tubes, which were centrifuged for
separation of plasma. A portion (1 to 1.5 ml) of the plasma was taken
for liquid scintillation counting, and the remaining plasma was
transferred to prelabeled, 4-ml plastic tubes and stored in a
non-self-defrosting freezer set at approximately 20°C or lower
prior to shipment to Glaxo Wellcome Inc., for analysis. After samples
had been drawn for the first 48 h, plasma and whole blood-sample
collections were discontinued when one of the following occurred: (i)
assays indicated that radioactivity levels had decreased to less than
or equal to twice the level of background radioactivity (~30 dpm) in
two consecutive samples or (ii) the stool and urine specimen
collections were discontinued. Plasma and whole-blood samples were
taken starting 5 min prior to dosing, to establish a baseline, and
thereafter at 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 24, 36, and 48 h postdosing. If the radioactivity levels in
the samples retrieved at 36 and 48 h were each less than or equal
to twice the background radioactivity level, then sample collection was
discontinued; otherwise, sample collection was continued at 12-h
intervals up to 240 h postdosing, until a criterion for
discontinuation was met.
CSF samples were collected only on day 1. Three subjects who could be
catheterized received an indwelling lumbar catheter. A predose sample
was drawn when the catheter was inserted, with additional samples drawn
at 0.5, 1, 1.5, 2.5, 4, and 6 h postdosing. Subjects were
encouraged to remain semisupine from the time of catheterization until
at least 1 h after the lumbar catheter was removed. To prevent
dehydration, subjects were encouraged to drink 240 ml of water every 30 min. After retrieval of the sample at 6 h postdosing, the lumbar
catheter was removed.
Determination of total [14C]radioactivity. Samples were assayed with scintillation counters (models 4530 and 2100TR; Packard Instrument Co., Meridan, Conn.) at the Bioanalytical Analysis Laboratory of the Department of Pharmacy and Pharmaceutics, Medical College of Virginia/Virginia Commonwealth University, Richmond. Total 14C radioactivity was determined for dosing solutions prior to dosing of the subjects and for blood, plasma, CSF, stool, and urine samples throughout the study. The radioactivity was measured with triplicate aliquots for each sample by scintillation counting either for 10 min or until a 1% deviation was obtained. The radioactivity in the dosing solution, plasma, CSF, and urine samples was measured directly with Insta-Gel XF scintillation cocktail (Packard Instrument Co.); whole-blood and stool samples were assayed for radioactivity following combustion. Triplicate 0.25-ml blood samples were dried on absorption pads and were prepared for analysis by combustion in a sample oxidizer (model 306; Packard Instrument Co.) with 0.25 ml of Combustaid for 35 s (combustion cones, absorption pads, and Combustaid were from Packard Instrument Co.). Triple aliquots of 0.80 to 0.95 g of fecal homogenate were dried and were prepared similarly, with combustion for 45 s. After combustion, the resulting carbon dioxide was trapped with 10 ml of a CO2 fixer (Carbo-Sorb; Packard Instrument Co.) and was mixed with 10 ml of scintillation cocktail (Permafluor; Packard Instrument Co.).
Radiochemical profiling of urine and feces. Assays were performed with urine samples with detectable radiation and with fecal extracts with radioactivity >5% of the administered dose. Samples were assayed by the Division of Bioanalysis and Drug Metabolism, Glaxo Wellcome, Inc., with a reversed-phase high-performance liquid chromatography (HPLC) system (Waters Discovery HPLC System; Waters Inc., Milford, Mass.) with an on-line radiochemical detector (model LB 507A; Berthold, Wildbad, Germany). The HPLC system used a Kromasil octadecyl column (Phenomenex, Torrance, Calif.) at a flow rate of 0.7 ml/min, and the mobile phase was 25 mM ammonium acetate (pH 4.0)-methanol (95:5) with a linear acetonitrile gradient from 0 to 13% over 50 min. Fecal samples were extracted with methanol-water (3:1) prior to injection. The following analytical standards were injected: [14C]abacavir, abacavir, major metabolites 2269W93 and 361W94, and minor metabolites 1144U88 and 139U91. The range of the assay was 0.64 to 56.6 µg/ml for abacavir, 0.27 to 24.5 µg/ml for 2269W93, and 0.56 to 52.1 µg/ml for 361W94 in urine. Accuracy, expressed as mean percent difference from the nominal value, was demonstrated to be within 10% for abacavir and 2269W93 and within 3% for 361W94. Precision, expressed as a maximum coefficient of variation (CV), was demonstrated to be <10% for abacavir, <7% for 2269W93, and <8% for 361W94.
In profiling the urine and fecal samples, the percentage of the dose attributable to each chromatographic peak was estimated by using the total radioactivity data for each sample, as calculated by the Bioanalytical Analysis Laboratory at the Medical College of Virginia/Virginia Commonwealth University. The percentage of the administered dose attributable to each recovered analyte in each sample was estimated as [(peak chromatographic heightsample/sum total of peak heightssample) × percentage of dose in sample] × 100.Quantification of abacavir and metabolites. CSF, plasma, and urine samples were analyzed by the Division of Bioanalysis and Drug Metabolism, Glaxo Wellcome, Inc., by a validated HPLC method with UV detection at 295 nm for abacavir, 2269W93, and 361W94. The plasma samples were assayed over a quantifiable range of 25 to 5,000 ng/ml. The HPLC system and octadecyl column were identical to those used for radiochemical profiling, but the mobile phase was 25 mM ammonium acetate (pH 4.0)-methanol (95:5) with a linear methanol gradient from 0 to 50% over 30 min. For each analysis, calibration standards and quality control samples were included. Accuracy was demonstrated to be within 3% for abacavir, 6% for 2269W93, and 7% for 361W94. Precision (percent CV) was demonstrated to be <8% for abacavir and 2269W93 and <7% for 361W94. Because the study subjects were HIV positive, the samples were heated for 5 h at 58°C to inactivate virus prior to sample analysis. Following heat inactivation of HIV, the stabilities of abacavir and its metabolites were also assessed. Accuracy was demonstrated to be within 7% and precision (percent CV) was determined to be within 2% for all analytes.
CSF samples were injected directly into the HPLC system for analysis. For plasma sample analysis, sample protein was first removed by methanol precipitation prior to injection. The range of the assay was 0.06 to 5.13 µg/ml for abacavir, 0.06 to 5.10 µg/ml for 2269W93, and 0.06 to 5.13 µg/ml for 361W94 in CSF. Accuracy was demonstrated to be within 14% for abacavir, 9% for 2269W93, and 7% for 361W94. Precision (percent CV) was demonstrated to be <7% for abacavir, 2269W93, and 361W94. Urine samples were centrifuged to minimize particulates, and the supernatant was diluted 1:10 with the HPLC mobile phase prior to injection into the HPLC system. The range of the assay was 0.64 to 56.6 µg/ml for abacavir, 0.27 to 24.5 µg/ml for 2269W93, and 0.56 to 52.1 µg/ml for 361W94 in urine. Accuracy was demonstrated to be within 6% for abacavir, 10% for 2269W93, and 3% for 361W94. Precision (percent CV) was demonstrated to be <10% for abacavir, <7% for 2269W93, and <8% for 361W94. Following heat inactivation of HIV, the stabilities of abacavir and its metabolites were also assessed. Accuracy was demonstrated to be within 3% and precision was demonstrated to be <4% for all analytes.Pharmacokinetic analysis. Pharmacokinetic analyses were performed by Worldwide Clinical Pharmacology, Glaxo Wellcome, Inc. Pharmacokinetic parameters were calculated from plasma and CSF concentration-time data for abacavir, 2269W93, and 361W94 and from blood, plasma, and CSF radioactivity-time data by standard noncompartmental methodologies. The WinNonlin Noncompartmental Analysis Program, version 1.5 (Scientific Consulting, Inc., Cary, N.C.), with model 200 for oral input was used for data analyses.
The following parameters were derived for each subject: maximum measured concentration (Cmax), sample time associated with Cmax (Tmax), terminal elimination rate constant (
z) and the corresponding terminal half-life
(t1/2), area under the concentration-time curve
(AUC) from time zero to the last quantifiable concentration
(AUClast), area under the concentration-time curve extrapolated to infinite time (AUC0-
), calculated as
AUC0- = AUClast + Clast/z (where
Clast is the last measurable drug
concentration), and percentage of AUC0- extrapolated,
calculated as 100 · (AUC0- AUClast)/AUC0-.
AUC estimates were calculated by the log-linear trapezoidal rule.
Calculations of the penetration of abacavir into the CSF were based on
comparisons of the AUC0- for CSF to the
AUC0- for plasma, as described previously
(14). For presentation of data comparing pharmacokinetic
parameters of abacavir and the two major metabolites, metabolite
concentrations were converted to abacavir equivalents, based on the
following molecular weights: abacavir, 286.34; 2269W93, 300.32; and
361W94, 462.46.
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RESULTS |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Baseline characteristics. Six male HIV-seropositive subjects were enrolled in the study, and all subjects completed the study. The median age was 36.5 years (range, 31 to 47 years), the median weight was 74.25 kg (range, 59.4 to 112.6 kg), and the median height was 174.0 cm (range, 170.0 to 183.0 cm). Five subjects were black and one subject was white; four were in Centers for Disease Control and Prevention HIV classification category A (asymptomatic), and two were in category C (AIDS) (4). The median absolute CD4+ lymphocyte count was 478 cells/mm3 (range, 234 to 924 cells/mm3) and the median absolute CD8+ cell count was 654 cells/mm3 (range, 402 to 1,324 cells/mm3). Three subjects were enrolled at the discretion of the investigator, despite positive results of urine screens for illicit drug use (cannabis and cocaine) at screening or on the evening prior to study drug administration. These drugs were thought to be unlikely to affect the results of the study.
Pharmacokinetic parameter evaluations of plasma and CSF. HPLC analysis detected abacavir and its two major metabolites, 2269W93 and 391W94, in plasma from all six subjects, beginning with the first sample drawn at 0.25 h postdosing. Figure 1 shows a semilogarithmic plot of the concentrations in plasma of abacavir and its two major metabolites, expressed as abacavir equivalents, for the first 16 h postdosing.
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for abacavir was 12.81 µg · h/ml, and the geometric mean
AUC0- values, converted to abacavir equivalents, for
2269W93 and 361W94 were 8.80 and 15.27 µg · h/ml,
respectively. The corresponding geometric mean ratios of
AUC0-~ for 2269W93 and 361W94 metabolites to that for
abacavir (in abacavir equivalents) were calculated as 0.69 and 1.19, respectively.
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Recovery of abacavir and metabolites in urine.
A summary of
the amounts (in milligrams) and percentages of drug recovered in urine
is shown in Table 2. Recovery of abacavir in urine was low, with no measurable amounts detected in two
subjects (subjects 14102 and 14104). Measurable levels of both
2269W93 and 361W94 were recovered from all six subjects, and the median percentage of drug recovered as these three analytes was 46.26% of the
administered dose.
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Pharmacokinetic evaluation of radioactivity.
Summary pharmacokinetic data derived from assays of
radioactivity in plasma, blood, and CSF samples are shown in
Table 3. Data are presented as abacavir
equivalents. Measurements in whole blood are consistent with equivalent
measurements in plasma, with blood-to-plasma geometric mean
ratios of 97% for AUC0- and of 93% for
Cmax. Tmax and
t1/2 values are consistent between whole blood
and plasma, with overlap in the 95% confidence interval.
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in
abacavir equivalents derived from HPLC analysis (36.88 µg · h/ml) (Table 1) to the total geometric mean AUC0- in
abacavir equivalents derived from radioactivity measurements (40.10 µg · h/ml) (Table 3). On the basis of this ratio, the radioactivities of abacavir and its two major metabolites comprised 92% of the radioactivity assayed in plasma.
The mean AUC0- for abacavir in CSF determined by HPLC
was estimated to be 5.14 µg · h/ml (Table 1) and is 75% of
the mean AUC0- determined from radioactivity
measurements, estimated to be 6.87 µg · h/ml (Table 3). Mean
Cmax and median Tmax
estimates for abacavir in CSF were consistent between HPLC (Table 1)
and radioactivity measurements (Table 3). Cmax
values for abacavir in CSF ranged from 0.7 to 1.4 µg/ml (or ~2 to 5 µM). However, there are differences in t1/2
estimates, with the t1/2 estimate from total
radioactivity measurements (3.49 h) being approximately 1 h longer
than the t1/2 estimate for abacavir (2.32 h).
Radioactivity in urine and stool.
Virtually all administered
radioactivity was recovered during the study, with 83% recovered in
urine and 16% recovered in feces (Table
4). After the first 48 h, no urine
sample collected from any subject contained 1% of the radioactivity
in the dose administered to that subject. After 6 days (144 h), no
stool samples collected from any subjects contained 2% of the
radioactivity in the administered dose, with most of the recovered
radioactivity being collected from each subject within the first 4 days
(96 h).
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Radiochemical profiling of urine and feces. All analyte radioactivity comprising greater than 2% of the administered radioactivity was identified by radiochemical profiling of urine and feces. The only analytes in urine comprising greater than 2% of the administered dose, as determined by radiochemical profiling, were the two major metabolites of abacavir: 2269W93 accounted for 29.7% and 361W94 accounted for 35.8%. Abacavir accounted for 1.2% of the administered dose, 1144U88 accounted for 0.6%, and 1.7% was not accounted for. Thus, a total of 69% of the administered dose was detected by urine profiling, which also identified 67.3% of the administered dose. The difference between urinary recovery results detected by radioactivity measurements (83.1%) and those identified by radiochemical profiling (67.3%) indicates that approximately 15.8% of the radioactivity has not been identified and may be attributable to numerous minor metabolites, each of which accounted for <2% of the dose.
Radioactivity profiling of feces, based upon the only three samples which contained >5% of the administered radioactivity, showed a major peak identified by retention time as 2269W93. Minor peaks identified as abacavir and 1144U88 were present as trace components. No 361W94 was detected in feces.Safety. There were no serious adverse experiences reported during the study. Subjects reported a total of 26 adverse experiences during the study, only 1 of which, a single episode of nausea, was considered possibly attributable to the study drug. One subject experienced an adverse experience (back pain) graded as severe; all other adverse experiences were reported as mild (10 events) or moderate (1 event) or were graded in severity as AIDS Clinical Trials Group toxicity grade 1 (11 events) or grade 2 (3 events). All adverse experiences resolved prior to study completion. There were no consistent changes or abnormalities in clinical chemistry, hematology, or urinalysis parameters.
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DISCUSSION |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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This study provides a definitive analysis of abacavir metabolism
and elimination in humans and gives the first quantitation of the
two major abacavir metabolites in plasma by a validated assay. The
majority of radioactivity detected in plasma (92%) was attributed to
abacavir and its two major metabolites, 2269W93 and 361W94. The
metabolite profiles seen in human urine described in this study are
consistent with the profiles observed in preclinical studies
(12). The majority of radioactivity identified by urine profiling comprised a mixture of the radioactivity from abacavir and
its two major metabolites, leaving only 15.8% as the radioactivity from unidentified minor metabolites, each with <2% of the
administered radioactivity. This study demonstrated that abacavir has
significant CSF penetration, with the median ratio of the
AUC0- for CSF to the AUC0- for plasma
being 35% of total drug. Finally, a single 600-mg oral dose of
radiolabeled abacavir was well tolerated by all study subjects, with no
new safety concerns raised during the conduct of this study. The
majority of adverse experiences reported were associated with the
lumbar puncture procedure, and only a single episode of nausea was
considered possibly attributable to the study drug.
Pharmacokinetic studies show a wide range of oral bioavailabilities among different NRTIs. For example, oral bioavailabilities are 20 to 40% for didanosine (9), 60% for zidovudine (23), and 80% or greater for zalcitabine (1), stavudine (17), and lamivudine (15). In the present study, the extent of absorption following administration of a single oral dose of 600 mg of abacavir was at least 83% since that percentage of the administered radioactivity was recovered in the urine. This finding is consistent with results from an earlier study estimating 83% absolute bioavailability following administration of a 300-mg oral dose of abacavir (5). The pharmacokinetic parameters estimated in this study were generally consistent with estimates from similar studies in which an equivalent 600-mg dose was administered in solid tablet form rather than in oral solution form (13, 21). As expected, the Cmax estimate from the oral solution used in this study tended to be slightly higher than the Cmax estimates from the solid dosage forms used in previous studies.
Penetration of CSF is a critically important factor in antiretroviral
therapy for HIV infection, as HIV can continue to replicate sequestered
in central nervous system compartments, despite evidence of an
antiviral response in plasma (6, 22). Our results indicate that the Cmax of abacavir in CSF (0.6 to 1.4 µg/ml) was 8 to 20 times the mean 50% inhibitory concentration for
HIV clinical isolates in vitro (0.07 µg/ml), indicating significant
penetration into the CSF. Our results also suggest that very little
metabolism of abacavir occurs in the CSF and that the metabolites have
limited penetration into the CSF. The estimate of CSF penetration (31 to 44%) in this report is higher than that reported previously probably because of the multiple points used for analysis. Ravitch et
al. (19) reported CSF/plasma concentration ratios of 18% (range, 12 to 24%) at 1.5 to 2 h postdosing after administration of abacavir at 200 mg three times daily to HIV-infected patients (19). Since both drug entry and drug elimination are slower in CSF than in plasma (14; this study), calculations
of penetration into CSF based upon simultaneous sampling of plasma and
CSF tend to be inaccurate. Burger et al. (3) have
demonstrated that time after zidovudine administration strongly
influenced the calculated CSF/plasma concentration ratio. A more robust
method, such as the one used in the study described in this report, is
to describe the ratio of AUC0- for CSF to the
AUC0- for plasma, since this parameter is independent
of the kinetics of exchange between the body compartments
(14). Calculations by this method gave a median estimate of
35% CSF penetration. As 50% of abacavir is protein bound
(2a) and only unbound abacavir is available for CSF
penetration, it follows that 70% of the unbound drug is able to
penetrate into the CSF.
Results of the current study indicate that concentrations of abacavir in CSF (Cmax) are above the in vitro 50% inhibitory concentration reported previously (7), similar to reports for most other NRTIs (16). As a pharmacologic class, NRTIs have superior CSF penetration compared with the penetration of protease inhibitors and are widely variable in the degree to which they penetrate, with CSF/plasma concentration ratios ranging from 20% for zalcitabine to 60% for zidovudine (2). Thus, results from the present study indicate that abacavir compares favorably with other NRTIs in terms of CSF penetration (35%) and CSF drug concentrations.
Several reasons are likely to account for the difference noted in the amount of radioactivity in urine measured by scintillation counting versus the amount determined by radiochemical profiling. First, the volumes of urine produced by most subjects over the course of the study were somewhat higher than would normally be expected, but volumes for two subjects (subjects 14103 and 14104) over the first 24 h were very high (6.33 and 9.20 liters, respectively). The majority of samples for these subjects were much more dilute than would normally be expected for the collection intervals, resulting in most samples having less radioactivity than could be detected consistently by the on-line radiochemical detector. Second, radiochemical profiling can consistently identify only individual analytes that are greater than 2% of the dose in each radiochromatogram. As each sample contained multiple abacavir analytes, radioactivity in samples obtained after 8 h postdosing generally could not be profiled.
In summary, results from this study support previous evidence indicating that abacavir is well tolerated, that it has a high degree of oral bioavailability, that abacavir is metabolized to two primary metabolites, and that abacavir penetrates into the CSF effectively.
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ACKNOWLEDGMENTS |
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We are grateful to Yu Lou for statistical consultations, Veronica Parker for database management, Clark March for assistance with radioactivity studies, Michael O'Mara for assistance with plasma bioanalytical studies, and Belinda Ha and Barbara Rutledge for manuscript and writing assistance. We also thank the staff at Virginia Commonwealth University, in particular, Donna Francioni-Proffitt, Abraham Enfiedjian, Rajeev Menon, Varsa Natarajan, Ronald Lees, Linda Brown-Burton, and H. Thomas Karnes, for assistance in the conduct of the study.
This study was supported by a grant from Glaxo Wellcome, Inc.
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FOOTNOTES |
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* Corresponding author. Mailing address: Worldwide Clinical Pharmacology, Glaxo Wellcome, Inc., Five Moore Dr., Research Triangle Park, NC 27709. Phone: (919) 483-1102. Fax: (919) 483-6380. E-mail: JAM36914{at}glaxowellcome.com.
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REFERENCES |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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