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1.  Overview of the Proton-coupled MCT (SLC16A) Family of Transporters 
The AAPS journal  2008;10(2):311-321.
The transport of monocarboxylates, such as lactate and pyruvate, is mediated by the SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). Fourteen MCT-related genes have been identified in mammals and of these seven MCTs have been functionally characterized. Despite their sequence homology, only MCT1-4 have been demonstrated to be proton-dependent transporters of monocarboxylic acids. MCT6, MCT8 and MCT10 have been demonstrated to transport diuretics, thyroid hormones and aromatic amino acids, respectively. MCT1-4 vary in their regulation, tissue distribution and substrate/inhibitor specificity with MCT1 being the most extensively characterized isoform. Emerging evidence suggests that in addition to endogenous substrates, MCTs are involved in the transport of pharmaceutical agents, including γ-hydroxybuytrate (GHB), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors (statins), salicylic acid, and bumetanide. MCTs are expressed in a wide range of tissues including the liver, intestine, kidney and brain, and as such they have the potential to impact a number of processes contributing to the disposition of xenobiotic substrates. GHB has been extensively studied as a pharmaceutical substrate of MCTs; the renal clearance of GHB is dose-dependent with saturation of MCT-mediated reabsorption at high doses. Concomitant administration of GHB and l-lactate to rats results in an approximately two-fold increase in GHB renal clearance suggesting that inhibition of MCT1-mediated reabsorption of GHB may be an effective strategy for increasing renal and total GHB elimination in overdose situations. Further studies are required to more clearly define the role of MCTs on drug disposition and the potential for MCT-mediated detoxification strategies in GHB overdose.
doi:10.1208/s12248-008-9035-6
PMCID: PMC2574616  PMID: 18523892
butyrate; gamma-hydroxybutyrate; lactate; monocarboxylate transporters; SLC16A
2.  Overview of the Proton-coupled MCT (SLC16A) Family of Transporters: Characterization, Function and Role in the Transport of the Drug of Abuse γ-Hydroxybutyric Acid 
The AAPS Journal  2008;10(2):311-321.
The transport of monocarboxylates, such as lactate and pyruvate, is mediated by the SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). Fourteen MCT-related genes have been identified in mammals and of these seven MCTs have been functionally characterized. Despite their sequence homology, only MCT1–4 have been demonstrated to be proton-dependent transporters of monocarboxylic acids. MCT6, MCT8 and MCT10 have been demonstrated to transport diuretics, thyroid hormones and aromatic amino acids, respectively. MCT1–4 vary in their regulation, tissue distribution and substrate/inhibitor specificity with MCT1 being the most extensively characterized isoform. Emerging evidence suggests that in addition to endogenous substrates, MCTs are involved in the transport of pharmaceutical agents, including γ-hydroxybuytrate (GHB), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors (statins), salicylic acid, and bumetanide. MCTs are expressed in a wide range of tissues including the liver, intestine, kidney and brain, and as such they have the potential to impact a number of processes contributing to the disposition of xenobiotic substrates. GHB has been extensively studied as a pharmaceutical substrate of MCTs; the renal clearance of GHB is dose-dependent with saturation of MCT-mediated reabsorption at high doses. Concomitant administration of GHB and l-lactate to rats results in an approximately two-fold increase in GHB renal clearance suggesting that inhibition of MCT1-mediated reabsorption of GHB may be an effective strategy for increasing renal and total GHB elimination in overdose situations. Further studies are required to more clearly define the role of MCTs on drug disposition and the potential for MCT-mediated detoxification strategies in GHB overdose.
doi:10.1208/s12248-008-9035-6
PMCID: PMC2574616  PMID: 18523892
butyrate; gamma-hydroxybutyrate; lactate; monocarboxylate transporters; SLC16A
3.  Mechanistic Models Describing Active Renal Reabsorption and Secretion: A Simulation-Based Study 
The AAPS Journal  2012;15(1):278-287.
The objective of the present study was to evaluate mechanistic pharmacokinetic models describing active renal secretion and reabsorption over a range of Michaelis–Menten parameter estimates and doses. Plasma concentration and urinary excretion profiles were simulated and renal clearance (CLr) was calculated for two pharmacokinetic models describing active renal reabsorption (R1/R2), two models describing active secretion (S1/S2), and a model containing both processes. A range of doses (1–1,000 mg/kg) was evaluated, and Vmax and Km parameter estimates were varied over a 100-fold range. Similar CLr values were predicted for reabsorption models (R1/R2) with variations in Vmax and Km. Tubular secretion models (S1/S2) yielded similar relationships between Michaelis–Menten parameter perturbations and CLr, but the predicted CLr values were threefold higher for model S1. For both reabsorption and secretion models, the greatest changes in CLr were observed with perturbations in Vmax, suggesting the need for an accurate estimate of this parameter. When intrinsic clearance was substituted for Michaelis–Menten parameters, it failed to predict similar CLr values even within the linear range. For models S1 and S2, renal secretion was predominant at low doses, whereas renal clearance was driven by fraction unbound in plasma at high doses. Simulations demonstrated the importance of Michaelis–Menten parameter estimates (especially Vmax) for determining CLr. Km estimates can easily be obtained directly from in vitro studies. However, additional scaling of in vitro Vmax estimates using in vitro/in vivo extrapolation methods are required to incorporate these parameters into pharmacokinetic models.
Electronic supplementary material
The online version of this article (doi:10.1208/s12248-012-9437-3) contains supplementary material, which is available to authorized users.
doi:10.1208/s12248-012-9437-3
PMCID: PMC3535105  PMID: 23196805
kidney transport parameters; models; reabsorption; renal clearance; secretion
4.  Use of a Local Sensitivity Analysis to Inform Study Design Based on a Mechanistic Toxicokinetic Model for γ-Hydroxybutyric Acid 
The AAPS Journal  2011;13(2):240-254.
γ-Hydroxybutyric acid (GHB), a drug of abuse, demonstrates complex toxicokinetics with capacity-limited metabolism and active renal reabsorption. The objectives of the present study were to conduct a local sensitivity analysis of a mechanistic model for the active renal reabsorption of GHB and to use the results to inform the design of future studies aimed at developing therapeutic strategies for treating GHB overdoses. A local sensitivity analysis was used to assess the influence of parameter perturbations on model outputs (plasma concentrations and urinary excretion of GHB). Further, a sensitivity index was calculated for each perturbed parameter to assess the specific segments of the time course that are critical to parameter estimation. Model outputs were simulated for rats dosed with 200, 400, 600, and 1,000 mg/kg GHB intravenously and individual parameters were perturbed by two-, five-, and tenfold higher and lower than the nominal value. Model outputs were sensitive to perturbations in clearance and volume parameters. In contrast, model outputs were found to be insensitive to changes in distributional parameters suggesting that additional tissue distribution data is required. Based on the sensitivity analysis the 1,000-mg/kg GHB dose can be eliminated from future studies as the parameters can be adequately estimated from the lower doses. To further validate the use of this model, dose-specific sampling schedules were designed based on model predictions for doses of 600 and 1,500 mg/kg. These sampling schedules were able to adequately capture the inflection point and terminal elimination phase of the plasma concentration–time profiles obtained.
doi:10.1208/s12248-011-9264-y
PMCID: PMC3085705  PMID: 21387146
active renal reabsorption; gamma-hydroxybutyric acid; monocarboxylate transporters; sensitivity analysis
5.  Mechanistic Toxicokinetic Model for γ-Hydroxybutyric Acid: Inhibition of Active Renal Reabsorption as a Potential Therapeutic Strategy 
The AAPS Journal  2010;12(3):407-416.
γ-Hydroxybutyric acid (GHB), a drug of abuse, exhibits saturable renal clearance and capacity-limited metabolism. The objectives of this study were to construct a mechanistic toxicokinetic (TK) model describing saturable renal reabsorption and capacity-limited metabolism of GHB and to predict the effects of inhibition of renal reabsorption on GHB TK in the plasma and urine. GHB was administered by iv bolus (200–1,000 mg/kg) to male Sprague-Dawley rats and plasma and urine samples were collected for up to 6 h post-dose. GHB concentrations were determined by LC/MS/MS. GHB plasma concentration and urinary excretion were well-described by a TK model incorporating plasma and kidney compartments, along with two tissue and two ultrafiltrate compartments. The estimate of the Michaelis-Menten constant for renal reabsorption (Km,R) was 0.46 mg/ml which is consistent with in vitro estimates of monocarboxylate transporter (MCT)-mediated uptake of GHB (0.48 mg/ml). Simulation studies assessing inhibition of renal reabsorption of GHB demonstrated increased time-averaged renal clearance and GHB plasma AUC, independent of the inhibition mechanism assessed. Co-administration of GHB (600 mg/kg iv) and l-lactate (330 mg/kg iv bolus plus 121 mg/kg/h iv infusion), a known inhibitor of MCTs, resulted in a significant decrease in GHB plasma AUC and an increase in time-averaged renal clearance, consistent with the model simulations. These results suggest that inhibition of renal reabsorption of GHB is a viable therapeutic strategy for the treatment of GHB overdoses. Furthermore, the mechanistic TK model provides a useful in silico tool for the evaluation of potential therapeutic strategies.
doi:10.1208/s12248-010-9197-x
PMCID: PMC2895455  PMID: 20461486
gamma-hydroxybutyrate; kidney reabsorption; pharmacokinetic model; renal clearance; toxicokinetics

Results 1-5 (5)