Changes In Plasma Protein Binding Research Articles

The effect of malaria-induced alteration of metabolism on piperaquine disposition in Plasmodium yoelii infected mice and predicted in malaria patients

ObjectivesMalaria-induced alteration of physiological parameters and pharmacokinetic properties of antimalarial drugs may be clinically relevant. Whether and how malaria alters the disposition of piperaquine (PQ) was investigated in this study. MethodsThe effect of malaria on drug metabolism-related enzymes and PQ pharmacokinetic profiles was studied in Plasmodium yoelii-infected mice in vitro/in vivo. Whether the malaria effect was clinically relevant for PQ was evaluated using a validated physiologically-based pharmacokinetic model with malaria-specific scalars obtained in mice. ResultsThe infection led to a higher blood-to-plasma partitioning (Rbp) for PQ, which was concentration-dependent and correlated to parasitemia. No significant change in plasma protein binding was found for PQ. Drug metabolism-related genes (CYPs/UDP-glucuronosyltransferase/nuclear receptor, except for CYP2a5) were downregulated in infected mice, especially at the acute phase. The plasma oral clearances (CL/F) of three probe substrates for CYP enzymes were significantly decreased (by ≥35.9%) in mice even with moderate infection. The validated physiologically-based pharmacokinetic model indicated that the hepatic clearance (CLH) of PQ was the determinant of its simulated CL/F, which was predicted to slightly decrease (by ≤23.6%) in severely infected mice but not in malaria patients. The result fitted well with the plasma pharmacokinetics of PQ in infected mice and literature data on malaria patients. The blood clearance of PQ was much lower than its plasma clearance due to its high Rbp. ConclusionsThe malaria-induced alteration of drug metabolism was substrate-dependent, and its impact on the disposition of PQ and maybe other long-acting aminoquinoline antimalarials was not expected to be clinically relevant.

Total and Free Blood and Plasma Concentration Changes in Pregnancy for Medicines Highly Bound to Plasma Proteins: Application of Physiologically Based Pharmacokinetic Modelling to Understand the Impact on Efficacy.

Free drug concentrations are generally considered the pharmacologically active moiety and are important for cellular diffusion and distribution. Pregnancy-related changes in plasma protein binding and blood partitioning are due to decreases in plasma albumin, alpha-1-acid glycoprotein, and haematocrit; this may lead to increased free concentrations, tissue distribution, and clearance during pregnancy. In this paper we highlight the importance and challenges of considering changes in total and free concentrations during pregnancy. For medicines highly bound to plasma proteins, such as tacrolimus, efavirenz, clindamycin, phenytoin, and carbamazepine, differential changes in concentrations of free drug during pregnancy may be clinically significant and have important implications for dose adjustment. Therapeutic drug monitoring usually relies on the measurement of total concentrations; this can result in dose adjustments that are not necessary when changes in free concentrations are considered. We explore the potential of physiologically based pharmacokinetic (PBPK) models to support the understanding of the changes in plasma proteins binding, using tacrolimus and efavirenz as example drug models. The exposure to either drug was predicted to be reduced during pregnancy; however, the decrease in the exposure to the total tacrolimus and efavirenz were significantly larger than the reduction in the exposure to the free drug. These data show that PBPK modelling can support the impact of the changes in plasma protein binding and may be used for the simulation of free concentrations in pregnancy to support dosing decisions.

Management of Psychiatric Disorders during the Perinatal Period.

Management of Psychiatric Disorders during the Perinatal Period.

Neglecting Plasma Protein Binding in COVID-19 Patients Leads to a Wrong Interpretation of Lopinavir Overexposure.

Boffito et al. recalled the critical importance to correctly interpret protein binding. Changes of lopinavir pharmacokinetics in coronavirus disease 2019 (COVID‐19) are a perfect illustration. Indeed, several studies described that total lopinavir plasma concentrations were considerably higher in patients with severe COVID‐19 than those reported in patients with HIV. These findings have led to a reduction of the dose of lopinavir in some patients, hypothesizing an inhibitory effect of inflammation on lopinavir metabolism. Unfortunately, changes in plasma protein binding were never investigated. We performed a retrospective cohort study. Data were collected from the medical records of patients hospitalized for COVID‐19 treated with lopinavir/ritonavir in intensive care units or infectious disease departments of Toulouse University Hospital (France). Total and unbound concentrations of lopinavir, C reactive protein, albumin, and alpha‐1‐acid glycoprotein (AAG) levels were measured during routine care on the same samples. In patients with COVID‐19, increased total lopinavir concentration is the result of an increased AAG‐bound lopinavir concentration, whereas the unbound concentration remains constant, and insufficient to reduce the severe acute respiratory syndrome‐coronavirus 2 (SARS‐CoV‐2) viral load. Although international guidelines have recently recommended against using lopinavir/ritonavir to treat severe COVID‐19, the description of lopinavir pharmacokinetics changes in COVID‐19 is a textbook case of the high risk of misinterpretation of a total drug exposure when changes in protein binding are not taken into consideration.

Insights into the Binding of Dietary Phenolic Compounds to Human Serum Albumin and Food-Drug Interactions.

The distribution of drugs and dietary phenolic compounds in the systemic circulation de-pends on, among other factors, unspecific/specific reversible binding to plasma proteins such as human serum albumin (HSA). Phenolic substances, present in plant-derived feeds, foods, beverages, herbal medicines, and dietary supplements, are of great interest due to their biological activity. Recently, considerable research has been directed at the formation of phenol–HSA complexes, focusing above all on structure–affinity relationships. The nucleophilicity and planarity of molecules can be altered by the number and position of hydroxyl groups on the aromatic ring and by hydrogenation. Binding affinities towards HSA may also differ between phenolic compounds in their native form and conjugates derived from phase II reactions. On the other hand, food–drug interactions may increase the concentration of free drugs in the blood, affecting their transport and/or disposition and in some cases provoking adverse or toxic effects. This is caused mainly by a decrease in drug binding affinities for HSA in the presence of flavonoids. Accordingly, to avoid the side effects arising from changes in plasma protein binding, the intake of flavonoid-rich food and beverages should be taken into consideration when treating certain pathologies.

A Physiologically Based Pharmacokinetic Perspective on the Clinical Utility of Albumin-Based Dose Adjustments in Critically Ill Patients.

In hypo-albuminemia, the extent of albumin binding of a drug decreases. The resulting change in plasma protein binding only rarely leads to clinically relevant changes in unbound drug exposure. Nevertheless, in the critically ill, a tendency to increase dosing of anti-infective therapy is seen in patients experiencing hypo-albuminemia. To reconcile basic pharmacological principles with current clinical practice, this work presents a pharmacologically-based pharmacokinetic simulation study to emphasize the (lack of) effect of altered plasma protein binding on a drug's concentration-time profile and associated pharmacokinetic parameters. Four virtual compounds, representing a broad chemical space (low/high clearance/volume of distribution), were created and administered to a virtual population of normal patients and three types of hypo-albuminemic patients in Simcyp®. The influence of decreased plasma protein binding in hypoalbuminemia on the pharmacokinetic parameters and profiles of these four compounds was investigated. Simulation results showed that while high-clearance compounds suffer from increased unbound exposure with decreased plasma protein binding, the unbound exposure of low-clearance compounds was unaffected. However, for the subset of low-clearance compounds with a small volume of distribution, it appeared that there were still alterations in their plasma concentration-time profiles. Since this may lead to different times above a minimum inhibitory concentration value, this might affect the bacterial killing for some anti-infective drugs. Overall, for any compound involved in the simulations, the unbound exposure did not decrease in plasma protein binding subjects relative to normal plasma protein binding subjects. This finding is in line with the few case-controlled studies in the literature. Hence, increasing the dose/dosing frequency seems futile and might reduce the benefit-risk ratio for narrow therapeutic index drugs. Moreover, these simulations indicate that when only total plasma concentrations and derived pharmacokinetic parameters are considered, incorrect conclusions will be drawn.

The significance of a new parameter – plasma protein binding – in therapeutic drug monitoring and its application to carbamazepine in epileptic patients

The primary cause of the variability of Cf in pharmacology is the change in plasma protein binding (PPB), thus PPB monitoring should be applied to a better individualization of drug dosage regimens in clinical patients.

The First-in-Class Potassium-Competitive Acid Blocker, Vonoprazan Fumarate: Pharmacokinetic and Pharmacodynamic Considerations.

Vonoprazan fumarate (Takecab) is a first-in-class potassium-competitive acid blocker that has been available in the market in Japan since February 2015. Vonoprazan is administered orally at 20 mg once daily for the treatment of gastroduodenal ulcer, at 20 and 10 mg once daily for the treatment and secondary prevention of reflux esophagitis, respectively, at 10 mg once daily for the secondary prevention of low-dose aspirin- or non-steroidal anti-inflammatory drug-induced peptic ulcer, and at 20 mg twice daily in combination with clarithromycin and amoxicillin for the eradication of Helicobacter pylori. It inhibits H(+),K(+)-ATPase activities in a reversible and potassium-competitive manner with a potency of inhibition approximately 350 times higher than the proton pump inhibitor, lansoprazole. Vonoprazan is absorbed rapidly and reaches maximum plasma concentration at 1.5-2.0 h after oral administration. Food has minimal effect on its intestinal absorption. Oral bioavailability in humans remains unknown. The plasma protein binding of vonoprazan is 80% in healthy subjects. It distributes extensively into tissues with a mean apparent volume of distribution of 1050 L. Being a base with pKa of 9.6 and with acid-resistant properties, vonoprazan is highly concentrated in the acidic canaliculi of the gastric parietal cells and elicited an acid suppression effect for longer than 24 h after the administration of 20 mg. The mean apparent terminal half-life of the drug is approximately 7.7 h in healthy adults. Vonoprazan is metabolized to inactive metabolites mainly by cytochrome P450 (CYP)3A4 and to some extent by CYP2B6, CYP2C19, CYP2D6, and SULT2A1. A mass balance study showed that 59 and 8% of the orally administered radioactivity was recovered in urine as metabolites and in an unchanged form, respectively, indicating extensive metabolism. Genetic polymorphism of CYP2C19 may influence drug exposure but only to a clinically insignificant extent (15-29%), according to the population pharmacokinetic study performed in Japanese patients. When vonoprazan was co-administered with clarithromycin, the mean AUC from time 0 to time of the next dose (dosing interval) of vonoprazan and clarithromycin were increased by 1.8 and 1.5 times, respectively, compared with the corresponding control values, indicating mutual metabolic inhibition. The mean area under the curve from time zero to infinity obtained from patients with severe liver and renal dysfunction were elevated by 2.6 and 2.4 times, respectively, compared with healthy subjects, with no significant changes in plasma protein binding. Vonoprazan increases intragastric pH above 4.0 as early as 4 h after an oral dose of 20 mg, and the extensive anti-secretory effect is maintained up to 24 h post-dose. During repeated dosing of 20 mg once daily, the 24-h intragastric pH >4 holding time ratios were 63 and 83 % on days 1 and 7, respectively. Because vonoprazan elicited a more extensive gastric acid suppression than the proton pump inhibitor, lansoprazole, it also gave rise to two to three times greater serum gastrin concentrations as compared with lansoprazole. In pre-approval clinical studies for the treatment of acid-related disorders, mild to moderate adverse drug reactions (mostly constipation or diarrhea) occurred at frequencies of 8-17%. Neither severe liver toxicity nor neuroendocrine tumor has been reported in patients receiving vonoprazan.

483P - Change in Plasma Protein Binding of Sn-38, an Active Metabolite of Irinotecan, in Cancer Patients with Severe Renal Failure (Srf)

ABSTRACT Aim: Our previous clinical study revealed that total plasma concentration of SN-38 was higher in cancer patients with SRF than those with normal kidney (NK), although SN-38 is mainly eliminated in the liver. Because SN-38 can bind to plasma protein, we hypothesized the possible increase in unbound fraction of SN-38 and area under the unbound plasma concentration-time curve (AUCu) caused by inhibition of plasma protein binding by uremic toxin(s) in patients with SRF. Methods: Total and unbound plasma concentrations of SN-38 in the cancer patients treated with 100 mg/m2 irinotecan alone were examined at 1 h after the end of irinotecan infusion to obtain unbound fraction of SN-38. Ex vivo analyses for SN-38 protein binding were performed with plasma samples obtained from 4 respective groups of patients (10-12 for each group) categorized by their renal function, estimated glomerular filtration (eGFR) 60 mL/min. The inhibition by 22 uremic toxins known to bind to plasma protein of SN-38 protein binding was also examined. Results: Unbound fraction of SN-38 in 3 cancer patients with SRF (0.023 ± 0.0060) was significantly higher than that in 5 of those with NK (0.0089 ± 0.0043) (P = 0.0339). AUCu in patients with SRF and those with NK were 0.030 ± 0.015 and 0.0069 ± 0.0043 µM·h, respectively (P = 0.0253). Ex vivo experiments revealed that unbound fractions of SN-38 was inversely correlated with eGFR (Spearman's rank correlation coefficient, -0.5207; P = 0.0003). The unbound fractions in patients with eGFR Conclusions: Unbound concentrations of SN-38 were higher in patients with SRF than those with NK. The inhibition of SN-38 protein binding by CMPF is a possible mechanism. Disclosure: All authors have declared no conflicts of interest.

Evaluations of In Vitro Metabolism, Drug–Drug Interactions Mediated by Reversible and Time-Dependent Inhibition of CYPs, and Plasma Protein Binding of MMB4 DMS

1,1'-Methylenebis[4-[(hydroxyimino)methyl]-pyridinium] (MMB4) dimethanesulfonate (DMS) is a bisquaternary pyridinium aldoxime that reactivates acetylcholinesterase inhibited by organophosphorus nerve agent. Drug metabolism and plasma protein binding for MMB4 DMS were examined using various techniques and a wide range of species. When (14)C-MMB4 DMS was incubated in liver microsomes, 4-pyridine aldoxime (4-PA) and an additional metabolite were detected in all species tested. Identity of the additional metabolite was postulated to be isonicotinic acid (INA) based on liquid chromatography with a tandem mass spectrometry analysis, which was confirmed by comparison with authentic INA. Formation of INA was dependent on species, with the highest level found in monkey liver microsomes. The MMB4 DMS exhibited reversible inhibition in a concentration-dependent manner toward cytochrome P450 1A2 (CYP1A2), CYP2C9, CYP2C19, CYP2D6, and CYP3A4 in human liver microsomes showing the highest inhibition for CYP2D6. Human recombinant CYPs were used to evaluate inhibitory curves more adequately and determine detailed kinetic constants for reversible inhibition and potential time-dependent inhibition (TDI). The MMB4 DMS exhibited reversible inhibition toward human-recombinant CYP2D6 with an inhibition constant (K i) value of 66.6 µmol/L. Based on the k inact/K I values, MMB4 DMS was found to exhibit the most potent TDI toward CYP2D6. The MMB4 DMS at 5 different concentrations was incubated in plasma for 5 hours using an equilibrium dialysis device. For all species tested, there were no concentration-dependent changes in plasma protein binding, ranging from 10% to 17%. These results suggest that MMB4 was not extensively bound to plasma protein, and there were no overt species-related differences in the extent of MMB4 bound to plasma protein.

The effect on the rat isolated atria of amiodarone in the presence of either ouabain or verapamil.

Amiodarone causes a decrease in the rate of contraction of the rat isolated atria and has a negative inotropic action in the paced preparation. Interactions occur between amiodarone and ouabain and amiodarone and verapamil. It is possible that the clinically reported drug interaction with amiodarone may have a component of direct interactions on the myocardium rather than solely changes in plasma protein binding.

Pharmacodynamic-Pharmacokinetic Integration as a Guide to Medicinal Chemistry

A primary objective of pharmacokinetic-pharmacodynamic (PKPD) reasoning is to identify key in vivo drug and system proper¬ties, enabling prediction of the magnitude and time course of drug responses under physiological and pathological conditions in animals and man. Since the pharmacological response generated by a drug is highly dependent on the actual system used to study its action, knowledge about its potency and efficacy at a given concentration or dose is insufficient to obtain a proper understanding of its pharmacodynamic profile. Hence, the output of PKPD activities extends beyond the provision of quantitative measures (models) of results, to the design of future protocols. Furthermore, because PKPD integrates DMPK (e.g. clearance) and pharmacology (e.g. potency),it provides an anchor point for compound selection, and, as such, should be viewed as an important weapon in medicinal chemistry. Here we outline key PK concepts relevant to PD, and then consider real-life experiments to illustrate the importance to the medicinal chemist of data obtained by PKPD. Useful assumptions and potential pitfalls are described, providing a holistic view of the plethora of determinants behind in vitro-in vivo correlations. By condensing complexity to simplicity, there are not only consequences for experimental design, and for the ranking and design of compounds, but it is also possible to make important predictions such as the impact of changes in drug potency and kinetics. In short, by using quantitative methods to tease apart pharmacodynamic complexities such as temporal differences and changes in plasma protein binding, it is possible to target the changes necessary for improving a compound's profile.

Binding Studies of Lamotrigine with Sera of Different Animal Species

Purpose: The serum protein-binding characteristic of lamotrigine with sera from various species (horse, rat, rabbit and goat) was studied at different dielectric constants and temperatures to obtain an insight into the mechanism of interaction, evaluate the effect of dielectric constant on binding affinity, and to determine the effect of species variation on drug plasma-protein interaction. Method: Binding data were generated by evaluating equilibrium dialysis of lamotrigine in methanolwatersolvent (1:1 and 1:3) with horse serum at 20, 28 and 37 0C The equilibrium dialysis evaluation of lamotrigine with sera of other animal species - rabbit, goat and rat - was also performed at 37 0C.Results: A Rosenthal plot obtained with the binding data at different temperatures showed that binding constant decreased with increasing temperature and that binding of lamotrigine to horse serumappeared to be saturation binding. The results also indicated that the binding process was characterized by negative DH0 value and a small positive DS0 values. The binding constant decreased as dielectricconstant rose. Conclusions: The results obtained suggest that hydrophobic interaction may have occurred and that van der Waals’ forces were responsible for binding in the hydrophobic region. Data obtained forlamotrigine binding with horse, goat, rabbit and rat sera show that there are no significant changes in plasma protein binding in the above-mentioned species.

Factors contributing to variability in drug pharmacokinetics. II. Drug distribution.

Many factors determine the distribution of a drug within the body. All factors may be altered in physiological and pathological states, and this may result in a need to change either the dose of the drug administered or the dosage interval. Changes in plasma protein binding may also alter the interpretation of the relationship between total plasma drug concentration and drug effect, and in such circumstances the free drug concentration may provide a better guide to optimal therapy.

Lopinavir/Ritonavir Pharmacokinetics in HIV/HCV-Coinfected Patients With or Without Cirrhosis

Liver disease may alter the pharmacokinetics of antiretrovirals and produce changes in plasma protein binding. The aim was to evaluate the pharmacokinetics of total and unbound lopinavir (LPV) in HIV-infected patients with and without hepatitis C virus (HCV) coinfection. Fifty-six HIV+ patients receiving lopinavir/ritonavir (LPV/r) (group I = 24 controls; II = 23 HIV/HCV-coinfected; III = 9 cirrhotic HIV/HCV-coinfected) were included. Total (n = 56) and unbound (n = 36) LPV pharmacokinetic parameters were determined at steady-state using validated high-performance liquid chromatography with ultraviolet detection and high-performance liquid chromatography-tandem mass spectrometry methods, respectively. Pharmacokinetic parameters (plasma concentration just before drug administration, peak concentrations in plasma, times to maximum plasma concentration, areas under the plasma concentration-time curve from 0 to 12 hours, and CL/F/kg) of both total and unbound LPV were calculated by standard noncompartmental methods and differences among groups evaluated (Kruskal-Wallis test).LPV apparent oral clearance normalized to body weight (median, interquartile range) was 55 (40-68), 59 (44-69), and 71 (53-78) mL/h/kg for groups I, II, and III, respectively (II vs. I, P = 0.52; III vs. I, P = 0.16). The areas under the plasma concentration-time curve from 0 to 12 hours were 110.4 (80.9-135.2), 103.4 (85.5-131.3), and 92.8 (87.4-116.3) microg h/mL for groups I, II, and III, respectively (II vs. I, P = 0.68; III vs. I, P = 0.71). Chronic liver impairment produced a slight, although not significant, decrease in plasma protein binding. The free-fraction of LPV increased ( approximately 21%) from 0.97% (0.80-1.06) in HIV+/HCV- patients to 1.18% (0.89-1.65) in HIV/HCV+ cirrhotic patients. The apparent oral clearance of unbound LPV (CLu/F/kg) in cirrhotic patients did not change significantly, supporting the concept that the clearance of unbound LPV in liver disease is not affected after being inhibited by low-dose ritonavir co-administration.LPV total and unbound pharmacokinetics were not affected by hepatic impairment, suggesting that no adjustment of LPV/r dose is required for HIV/HCV-coinfected patients with and without cirrhosis and moderate impairment of liver function.

All Dex’ed Out With Nowhere to Go?

All Dex’ed Out With Nowhere to Go?

Impact of pH on Plasma Protein Binding in Equilibrium Dialysis

Many pharmacokinetic analyses require unbound plasma concentrations, including prediction of clearance, volume of distribution, drug-drug interactions, brain uptake analysis, etc. It is most often more convenient to measure the total drug concentration in plasma rather than the unbound drug concentration. To arrive at unbound plasma concentrations, separate in vitro determinations of the plasma protein binding of a drug are usually carried out in serum or in plasma, and the plasma pharmacokinetic results are then mathematically adjusted by this fraction unbound ( f u,p). Plasma protein binding or the drug fraction unbound in plasma ( f u,p) is known to be affected by protein, drug, free fatty acid concentrations, lipoprotein partitioning, temperature, pH, and the presence or absence of other drugs/displacing agents within plasma samples. Errors in f u,p determination caused by lack of adequate pH control in newer assay formats for plasma protein binding (e.g., 96-well equilibrium thin walled polypropylene dialysis plates) will have significant drug-specific impact on these pharmacokinetic calculations. Using a diverse set of 55 drugs and a 96-well equilibrium dialysis plate format, the effect of variable pH during equilibrium dialysis experiments on measured values of f u,p was examined. Equilibrium dialysis of human plasma against Dulbecco's phosphate buffered saline at 37 degrees C under an air or 10% CO 2 atmosphere for 22 h resulted in a final pH of approximately 8.7 and 7.4, respectively. The ratio of f u,p at pH 7.4 (10% CO 2) vs pH 8.7 (air) was >or=2.0 for 40% of the 55 compounds tested. Only one of the 55 compounds tested had a ratio <0.9. Select compounds were further examined in rat and dog plasma. In addition, physicochemical properties were calculated for all compounds using ACD/Labs software or Merck in-house software and compared to plasma protein binding results. Changes in plasma protein binding due to pH increases which occurred during the equilibrium dialysis experiment were not species specific but were drug-specific, though nonpolar, cationic compounds had a higher likely hood of displaying pH-dependent binding. These studies underscore the importance of effectively controlling pH in plasma protein binding studies.

Changes in Pharmacokinetics of Anti-HIV Protease Inhibitors during Pregnancy: The Role of CYP3A and P-glycoprotein

Human immunodeficiency virus (HIV)-infected women have reduced exposure [area under the curve (AUC)] to anti-HIV protease inhibitors [e.g., nelfinavir (NFV)] during pregnancy. To determine the mechanistic basis of this phenomenon, we administered NFV mesylate orally (2.5 mg) or intravenously (0.625 mg) to timed pregnant (gestational age: 18-19 days) and non-pregnant FVB mice. After oral but not after i.v. administration, the plasma clearance of NFV was higher (by 134%, p < 0.05) and bioavailability was lower (by 32%, p < 0.05) in pregnant (n = 3) versus nonpregnant mice (n = 3). These effects of pregnancy were not due to changes in plasma protein binding of NFV. The half-life of NFV depletion in hepatic S-9 fractions of pregnant mice (n = 8) was 2.2-fold faster (p < 0.05) than that in nonpregnant mice (n = 7). Hepatic CYP3A activity (testosterone 6beta-hydroxylation, n = 4) and expression (n = 8) were significantly higher (by 138 and 49%, p < 0.05) in pregnant mice than that in nonpregnant mice. In the intestine, no CYP3A activity was detected and CYP3A protein expression (n = 6, p > 0.05) was not significantly different between the two groups. P-glycoprotein expression (n = 6) in hepatic and intestinal tissue of pregnant mice was not significantly different from that in nonpregnant mice. These changes in disposition of NFV during pregnancy are predominately due to a change in its bioavailability. An increase in hepatic CYP3A can explain the reduced bioavailability of NFV during pregnancy. If such upregulation of hepatic CYP3A activity occurs in pregnant women, it has important implications for dose adjustment of a variety of drugs ingested by pregnant women and cleared predominately via CYP3A metabolism.

Pharmacodynamic analysis of the interaction between tiagabine and midazolam with an allosteric model that incorporates signal transduction.

The objective of this study was to characterize quantitatively the pharmacodynamic interaction between midazolam (MDL), an allosteric modulator of the gamma-aminobutyric acid subtype A (GABAA) receptor, and tiagabine (TGB), an inhibitor of synaptic GABA uptake. The in vivo concentration-response relation of TGB was determined through pharmacokinetic/pharmacodynamic (PK/PD) modeling. Rats received a single intravenous dose of 10 mg/kg TGB in the absence and the presence of a steady-state plasma concentration of MDL. The EEG response in the 11.5- to 30-Hz frequency band was used as the pharmacodynamic end point. Infusion of MDL resulted in a mean steady-state plasma concentration of 66 +/- 3 ng/ml. A significant pharmacokinetic interaction with TGB was observed. MDL inhibited TGB clearance by 20 +/- 7 ml/min/kg from the original value of 89 +/- 6 ml/min/kg. However, no changes in plasma protein binding of both drugs were observed. The concentration-EEG relation of TGB was described by the sigmoid-Emax model. The pharmacodynamic parameter estimates of TGB were: Emax = 327 +/- 10 microV, EC50 = 392 +/- 20 ng/ml, and nH = 3.1 +/- 0.3. These values were not significantly different in the presence of MDL. Factors that may explain the lack of synergism were identified by a mechanism-based interaction model that separates the receptor activation from the signal-transduction process. High efficiency of signal transduction and the presence of a baseline response were shown to diminish the degree of synergism. We conclude that the in vivo pharmacodynamic interaction between MDL and TGB is additive rather than synergistic. This strongly suggests that allosteric modulation of the antiseizure activity of a GAT-1 inhibitor by a benzodiazepine does not offer a therapeutic advantage.

Changes in plasma protein binding have little clinical relevance.

Clinical Pharmacology &amp; Therapeutics (2002) 71, 115–121; doi: 10.1067/mcp.2002.121829