| Research Article |
Open Access |
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| Monocarboxylate Transporter Inhibition with Osmotic Diuresis Increases
γ-Hydroxybutyrate Renal Elimination in Humans: A Proof-of-Concept
Study |
| Marilyn E. Morris1*, Bridget L. Morse1, Gloria J. Baciewicz1, Matthew M. Tessena2, Nicole M. Acquisto3, David J. Hutchinson4 and Robert
DiCenzo5 |
| 1Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, University at Buffalo, State University of New York, Buffalo, NY (Morris and
Morse) |
| 2Department of Psychiatry, University of Rochester Medical Center, University of Rochester, Rochester, NY (Baciewicz and Tessena) |
| 3Departments of Pharmacy and Emergency Medicine, University of Rochester Medical Center, University of Rochester, Rochester, NY (Acquisto) |
| 4Department of Pharmacy Practice, School of Pharmacy, St. John Fisher College, Rochester, NY (Hutchinson) |
| 5Department of Pharmacy Practice, Albany College of Pharmacy and Health Sciences, Albany, NY (DiCenzo) |
| *Corresponding author: |
Dr. Marilyn E. Morris
University at Buffalo
527 Hochstetter
Hallz Buffalo
NY 14260.
Tel: (716) 645-4839
Fax: (716) 645-3693
E-mail: memorris@buffalo.edu |
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| Received October 01, 2011; Accepted November 03, 2011; Published November
10, 2011 |
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| Citation: Morris ME, Morse BL, Baciewicz GJ, Tessena MM, Acquisto NM, et al.
(2011) Monocarboxylate Transporter Inhibition with Osmotic Diuresis Increases
?-Hydroxybutyrate Renal Elimination in Humans: A Proof-of-Concept Study. J
Clinic Toxicol 1:105. doi:10.4172/2161-0495.1000105 |
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| Copyright: © 2011 Morris ME, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
| |
| Abstract |
| |
| Background and objective: The purpose of the current study was to demonstrate proof-of-concept that
monocarboxylate transporter (MCT) inhibition with L-lactate combined with osmotic diuresis increases renal
clearance of γ-hydroxybutyrate (GHB) in human subjects. GHB is a substrate for human and rodent MCTs, which
are responsible for GHB renal reabsorption, and this therapy increases GHB renal clearance in rats |
| |
| Methods: Ten healthy volunteers were administered GHB orally as sodium oxybate 50 mg/kg (4.5 gm maximum
dose) on two different study days. On study day 1, GHB was administered alone. On study day 2, treatment of L-lactate
0.125 mmol/kg and mannitol 200 mg/kg followed by L-lactate 0.75 mmol/kg/hr was administered intravenously 30
minutes after GHB ingestion. Blood and urine were collected for 6 hours, analyzed for GHB, and pharmacokinetic
and statistical analyses performed. |
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| Results: L-lactate/mannitol administration significantly increased GHB renal clearance compared to GHB alone,
439 vs. 615 mL/hr (P=0.001), and increased the percentage of GHB dose excreted in the urine, 2.2 vs. 3.3%
(P=0.021). Total clearance was unchanged. |
| |
| Conclusions: MCT inhibition with L-lactate combined with osmotic diuresis increases GHB renal elimination in
humans. No effect on total clearance was observed in this study due to the negligible contribution of renal clearance
to total clearance at this low GHB dose. Considering the nonlinear renal elimination of GHB, further research in
overdose cases is warranted to assess the efficacy of this treatment strategy for increasing renal and total clearance
at high GHB doses. |
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| Keywords |
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| γ-hydroxybutyrate; Pharmacokinetics; Renal clearance;
Monocarboxylate transporter |
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| Abbreviations |
| |
| AUC: Area under the plasma concentration-time
curve; Cl: Clearance; ClR: Renal clearance; CV: Coefficient of variation;
F: Bioavailability; GHB: γ-hydroxybutyrate; MCT: Monocarboxylate
transporter |
| |
| Introduction |
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| Overdose of ã-hydroxybutyrate (GHB) and its precursors,
ã-butyrolactone and 1,4-butanediol, has recently been recognized as
a significant issue in public health. From 1990-2000, over 7100 GHB
overdoses including 65 deaths were reported in the U.S. [1], and in a
recent publication, 209 GHB-associated deaths were reported in the
U.S. from 1995-2005 [2]. Manifestations of GHB overdose include
sedation, coma, hypothermia, bradycardia, and respiratory arrest [3-5].
Although abuse of GHB has been recognized, there currently exists
no pharmacological treatment for the overdose of these compounds.
Current treatment consists primarily of supportive care and mechanical
ventilation in cases of significant respiratory depression [6,7]. |
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| GHB is currently used in the form of sodium oxybate (Xyrem®)
for the treatment of excessive daytime sleepiness and cataplexy in
patients with narcolepsy in the U.S. and in Europe. Clinical studies
with sodium oxybate demonstrate dose-dependent pharmacokinetics,
even at low, therapeutic doses [8,9]. Similar pharmacokinetic
properties are reported in rats, in which nonlinearity has been attributed to several concentration-dependent processes including
saturable metabolism, oral absorption, and renal reabsorption [10-12].
Saturable renal reabsorption in rats can be accounted for by saturable
transport by monocarboxylate transporters (MCTs), of which GHB is
a substrate [10,13]. In the kidney, MCTs act to conserve endogenous
monocarboxylates, such as lactate, from being cleared into the urine,
and serve a similar role in the conservation of GHB. Increasing GHB
renal elimination by inhibition of these transporters represents a
potential therapeutic strategy for the treatment of GHB overdose. This
strategy has been validated using rat kidney membrane vesicles and in
vivo rat studies to demonstrate that administration of MCT inhibitors
inhibits GHB transport in the kidney and effectively increases GHB total and renal clearance at high GHB doses [10,13-15]. Using a human
kidney cell line, MCT inhibitors similarly inhibited the transport of
GHB in human tissue [16]. |
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| Although well-established in rodents, the role of MCTs in the
renal elimination of GHB in humans has not been demonstrated in
vivo. The purpose of this study was to provide proof-of-concept that
administration of an MCT inhibitor, L-lactate, combined with osmotic
diuresis increases the renal clearance of GHB in human subjects. |
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| Materials and Methods |
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| Study design and selection of participants |
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| This was a prospective, open-label, crossover study performed
at a university hospital from 2009-2011. This study was approved by
the institutional review boards at the sponsoring institutions. Healthy
male and female volunteers, ages 21-55, were recruited for the study.
A screening visit was used to determine subjects in good health
considering medical history, physical examination, and laboratory
tests. Women of child-bearing age were administered a pregnancy test
at the screening visit and were required to use an acceptable method
of contraception throughout the study. Exclusion criteria included
evidence of organ dysfunction as determined by physical examination
and laboratory results, history of drug or alcohol abuse within 6 months
prior to the study, allergy to study medications, known succinic semialdehyde
dehydrogenase deficiency, women who were pregnant, breastfeeding
or unwilling to use an acceptable method of contraception, and
prescription or non-prescription drug use within 1 week prior to the
study, excluding oral contraceptives or other medications approved by
the investigator. |
| |
| On study day 1, subjects were administered sodium oxybate 50 mg/
kg (4.5 gm maximum dose) orally in water. Subjects were instructed
to fast overnight until they were served breakfast 2 hours after drug
administration and to withhold caffeine ingestion on study days.
On study day 2, subjects were administered sodium oxybate 50 mg/
kg (4.5 gm maximum dose) orally, and treatment was administered
intravenously at 30 minutes after GHB ingestion. Treatment consisted
of a 0.125 mmol/kg L-lactate bolus over 10 minutes and a 200 mg/kg
bolus of mannitol over 3 – 5 minutes, followed by a 0.75 mmol/kg/
hr L-lactate infusion for the duration of the study. The L-lactate bolus
and infusion were administered as sodium lactate 1/6 M solution for
infusion. Mannitol was administered as a 20% solution in normal
saline. In some subjects the study days were conducted in opposite
order. Regardless of order, a washout period of at least 1 week was
required between study days. |
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| Data collection |
| |
| Blood samples were taken on both study days directly before and
at 10, 15, 20, 25, 30, 45, 60, 90, 120, 150, 180, 210, 240, and 360 minutes
after GHB administration. A spot urine was collected predose and
urine samples were collected at intervals of 0-30, 30-60, 60-120, 120-
180, 180-240, and 240-360 minutes after GHB administration. GHB
plasma and urine concentrations were determined using an LC/MS/
MS assay [17,18]. Urine volume at urine collection intervals were
recorded and multiplied by urine concentrations to determine the total
amount of GHB recovered at each urine collection interval. On study
day 2, plasma lactate concentrations were also taken at baseline and at
180 minutes after GHB administration. Plasma lactate concentrations
were determined by a colorimetric lactate oxidase assay. Vital signs
and adverse events were continuously monitored on both study days.
On study day 2, an electrocardiogram was recorded via telemetry from time zero to 6 hours after study drug administration and a physical
examination was performed prior to release. |
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| Data and statistical analysis |
| |
| Pharmacokinetic parameters were determined by
noncompartmental analysis using WinNonlin 5.2 (Pharsight Corp.,
Palo Alto, CA). Primary outcome measurements included GHB renal
clearance (ClR) and the percentage of GHB excreted unchanged in the
urine. Other outcome measurements included total clearance (CL/F),
area under the plasma-concentration time curve (AUC), and maximum
plasma concentration (Cmax). Individual AUCs were determined by
the trapezoidal method, where the measured plasma concentrations
represented >95% the total AUC. Cl/F was determined as Dose/AUC.
ClR was determined by Ae/AUC where Ae represents the amount of
GHB recovered unchanged in the urine. Paired t-tests were used to
determine statistically significant differences in mean pharmacokinetic
parameters between study days. Statistical analyses were performed
using SigmaPlot 10.0 (Systat Software, Inc., San Jose, CA). In this study,
a 20% change in ClR was considered significant. Considering previous
reports of 25% coefficient of variation (CV) values in GHB clearance
[19] using α=0.05 and power of 0.8, 12 subjects were determined
necessary to detect significant differences in measured pharmacokinetic
parameters. |
| |
| Results |
| |
| A total of 21 subjects voluntarily gave informed consent for the
study. Of these, 15 subjects completed the screening visit and 10
subjects completed both study days and were included in the statistical
analyses. Primary outcome measurements were met with this sample
size; therefore recruitment was concluded. Demographics and baseline
characteristics of the 10 subjects are given in Table 1. No significant
differences in renal or liver function tests were detected between study
days. |
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|
Table 1: Baseline Characteristics of Study Population. |
|
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| Pharmacokinetic parameters determined for both study days are
given in Table 2. As shown, administration of L-lactate and mannitol
significantly increased GHB renal clearance and the percentage of
GHB dose excreted in the urine. Individual changes in renal clearance
are displayed in Figure 1. GHB plasma AUC and total clearance were
unchanged between study days. Administration of L-lactate on study
day 2 increased the plasma lactate level from 0.9 ± 0.2 mM at baseline
to 2.6 ± 0.8 mM at 180 minutes (mean ± SD, P<0.001). Adverse effects
were similar between study days and were limited to light sedation,
headache, dizziness, and nausea/vomiting. No effects on heart rate,
blood pressure, or ECG were observed. |
| |
|
Table 2: Effect of L-lactate/mannitol administration on GHB pharmacokinetics. |
|
| |
|
Figure 1: Individual renal clearances in 10 healthy volunteers administered
GHB 50 mg/kg alone and with L-lactate/mannitol. Renal clearance (ClR) of
each subject is represented by individual symbols. Heavy dashed line represents
mean. |
|
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| Discussion |
| |
| Although GHB abuse has resulted in an increasing incidence of
overdose case reports and fatalities, treatment for these cases remains
limited to supportive measures. Treatment with antidotes including
flumazenil, naloxone, and physostigmine has been attempted, with
little effect on clinical course [6]. Administration of GABAB antagonists
has been demonstrated effective for treating overdose in rodent studies
[20,21]; however, these agents are not currently available for use in
humans. This study was performed to assess the ability of a clinically
available treatment strategy of L-lactate and mannitol to increase GHB
renal elimination. The treatment strategy including MCT inhibition
and osmotic diuresis was chosen based upon results of previous animal
studies [10,14,15]. L-lactate was chosen as the MCT inhibitor based
upon its ability to increase renal and total clearances of GHB in rats
along with its demonstrated safety in humans at the effective doses
[15,22,23]. The addition of mannitol as an osmotic diuretic was based
upon animal experiments demonstrating an increased effect with the
combination of L-lactate and mannitol compared to L-lactate alone
[15]; however, the contribution of mannitol to overall effects on GHB
toxicokinetics remains uncertain. L-lactate effectively increased GHB
renal and total clearance in animal studies in the absence of mannitol
[10,15], as has also been demonstrated with other MCT inhibitors [14].
Ongoing animal studies are being conducted to determine the benefit of
mannitol co-administration on the improvement of GHB toxicokinetic
and toxicodynamic endpoints with MCT inhibitors. |
| |
| Although the renal clearance of GHB was increased in this study,
there was no effect of treatment on total clearance. At low, therapeutic
GHB doses, such as that studied currently, the primary route of
elimination is metabolism by GHB dehydrogenase to form succinic
semialdehyde, which is then metabolized by succinic semialdehyde
dehydrogenase to succinic acid, which enters the Krebs cycle at is
excreted as carbon dioxide [9]. Renal excretion represents a minor
route of elimination at low doses, as illustrated in this study with
only approximately 2% of the total dose excreted into the urine. In
both humans and rats, GHB metabolism is saturable [8,10,11], and
contributes less to overall clearance as doses are increased [10]. In rats,
nonlinear GHB renal clearance has also been well-characterized, and,
in contrast with metabolism, the contribution of renal elimination
to total clearance increases with dose [10]. In clinical studies, the
urinary recovery of GHB has also been demonstrated to increase with
dose [9], suggesting similar nonlinear renal elimination in humans.
Accordingly, extremely high urine concentrations have been reported
in overdose cases [24-26]. At low GHB doses, when renal elimination is
negligible, even a significant increase in renal clearance with treatment
administration would not be expected to significantly affect total
clearance, as was observed in this study. However, in clinical GHB
overdose, due to both concentration-dependent renal reabsorption and
saturation of metabolism, renal clearance may contribute significantly
to total drug elimination, allowing the increase in renal clearance
demonstrated in this study to translate to increased total clearance, a
concept which has been demonstrated in our animal studies of GHB
overdose. In animal studies, the increase in clearance and decrease
in GHB plasma concentrations with MCT inhibition also resulted in
improvement in the toxicodynamic endpoint of sedation [14,15]. In
the current study, the low dose and lack of effect on total clearance
limited the pharmacodynamic evaluation possible with this study,
as changes in total AUC would be necessary to expect differences
in pharmacodynamic endpoints. Animal studies are in progress to
further assess the effect of a clinically relevant dose of L-lactate with
and without mannitol on toxicodynamic endpoints of GHB overdose
including sedation, respiratory depression, and fatality. |
| |
| Although this study demonstrated a statistically significant increase
in renal clearance with L-lactate/mannitol administration, this increase
in renal clearance of approximately 40% is modest compared to that
observed in animal studies [10,15]. Since L-lactate is a competitive
inhibitor of MCTs, increasing the L-lactate dose may have greater
effects on GHB renal clearance. The dose of L-lactate administered
in this clinical study is moderate, and higher infusion rates have been
administered safely to human subjects [23]. Animal studies are being
conducted to compare a similar L-lactate/mannitol regimen as that
administered in this study with high-dose L-lactate/mannitol regimens. |
| |
| Limitations |
| |
| The primary limitation of this study was the low GHB dose used,
which reflects therapeutic dosing and not that in overdose cases. This
excluded the evaluation of possible effects of treatment on total GHB
clearance or pharmacodynamic endpoints. Due to the short half-life
of GHB at the low dose used, treatment was administered 30 minutes
after GHB administration in this study; in the clinic, treatment is
not likely to be available this quickly after GHB ingestion. However,
following overdoses, delayed peak plasma concentrations may occur,
perhaps hours after ingestion as observed in animal studies [11], which
may provide the opportunity for treatment at later time points. Finally,
with the administration of L-lactate and mannitol concomitantly in
this study, effects of MCT inhibition or osmotic diuresis alone on GHB
renal clearance cannot be determined. |
| |
| Conclusions |
| |
| This pilot study demonstrates the proof-of-concept that MCT
inhibition with L-lactate in combination with osmotic diuresis
increases GHB renal elimination in humans. Administration of
L-lactate and mannitol may represent a practical potential strategy for
GHB overdose due to the clinical availability and low risk associated
with these agents. Further research in GHB overdose cases is needed
to determine the efficacy of this treatment strategy for increasing GHB
total clearance and improving clinical course during GHB intoxication. |
| |
| Acknowledgements |
| |
| This research was supported by the National Institutes of Health National
Institute on Drug Abuse [grant DA023223] to MEM and by a fellowship from Pfizer
Global Research and Development to BLM. |
| |
| Sodium oxybate (Xyrem®) was provided by Jazz Pharmaceuticals, Inc., Palo
Alto, CA. |
| |
| The authors would like to acknowledge Alyse DiCenzo for protocol conduct
and data management during the study. |
| |
|
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