Concentration-dependent Protein Binding Research Articles

Surface-regulated injection dose response of ultrasmall luminescent gold nanoparticles.

With the rapid growth of the use of renal-clearable nanomedicines in disease targeting and therapy, a fundamental understanding of their injection dose responses is of great importance for future translation to clinical settings. Using glutathione-coated gold nanoparticles (GS-AuNPs) as a renal-clearable nanomedicine model for the construction of ultrasmall AuNPs with different serum protein binding abilities, we discover that the concentration-dependent serum protein binding capabilities endow GS-AuNPs with a more sensitive response to injection dose than NPs resistant to serum protein binding, resulting in greatly improved tumor-targeting efficiencies during both single and repeated low-dose injections; the performance is also distinct from nonrenal-clearable AuNPs coated with serum protein, which show decreased tumor-targeting efficiency with a decrease in the injection dose.

Detection, quantification, and investigation of the red blood cell partitioning of cryptolepine hydrochloride

Context: The fight against malaria is limited by the development of resistance of Plasmodium to medication. This has led to an urgent search for alternative medicinal agents. Aims: To determine the affinity of cryptolepine for the red blood cell. Methods: HPLC method for the identification and quantification of cryptolepine was developed. Lipid solubility for both quinine (control) and cryptolepine was determined. Partitioning and repartitioning of cryptolepine into RBCs were studied. Time, concentration, temperature and pH were varied to see their effect on the partitioning of cryptolepine. Plasma protein binding was determined by the red blood cell partitioning method. Results: An accurate, precise and robust HPLC method for cryptolepine hydrochloride was developed. Cryptolepine and quinine had lipophilicity of 0.91 ± 0.02 and 1.52 ± 0.27, respectively. The highest partitioning values of 2.02 ± 0.08 for cryptolepine and 0.93 ± 0.02 for quinine were obtained at 40 minutes. Concentration-dependent protein binding was observed for both compounds with cryptolepine having 0.43 and 0.38 for quinine. Partitioning was also found to be temperature dependent with the highest partitioning obtained at 370C for cryptolepine (1.56 ± 0.04) and quinine (0.78 ± 0.01). Partitioning of cryptolepine and quinine were inversely related to pH with R2 values of 0.94 and 0.96, respectively. P-values between partitioning and repartitioning for cryptolepine and quinine were 0.04 and 0.05, respectively. Conclusions: Partitioning was found to be time, temperature, concentration and pH dependent. Partitioning was irreversible for cryptolepine and reversible for quinine. Protein binding in both cases was moderate.

Monitoring Prednisolone and Prednisone in Saliva

Prednisolone (PLN) is a widely used corticosteroid in a variety of immune-mediated diseases. Treatment regimes generally consist of empirically derived treatment doses, whereas therapeutic response among patients is highly variable. Drug monitoring of serum PLN levels might support a more rational approach to dose selection, yet is invasive and laborious. In analogy to cortisol, salivary PLN may offer a good alternative for serum PLN, being a representative approximation of free serum PLN. The aims of this study were to evaluate the correlation between free serum and salivary PLN levels and to quantify this relationship within a population pharmacokinetic model. PLN and prednisone (PN) concentrations were measured in 396 samples from 19 healthy volunteers after oral ingestion of 80 mg PLN. Measurements in serum, ultrafiltrate, and saliva were performed with a recently validated liquid chromatography tandem mass spectrometry method. Population pharmacokinetic analysis was performed with nonlinear mixed effect modeling using NONMEM. Salivary PLN levels correlated well with free serum PLN levels (r = 0.931, P < 0.01). A weaker correlation was found for PN (r = 0.318, P < 0.01), which may be explained by the finding that salivary PN levels mainly seemed to consist of PLN enzymatically converted to PN. Total and free serum PLN concentrations decreased over time after drug administration and showed a nonlinear mutual relationship, consistent with concentration-dependent protein binding. Modeled PLN pharmacokinetics corresponded with previous reports. Low to moderate interindividual variability was found for V/F and CL/F (coefficients of variation were 13.8% and 14.6%, respectively). Free and salivary PLN showed a nonlinear relationship with total PLN. An equation predicting free serum levels from salivary levels was successfully derived from the data. This study is the first to describe the relationship between salivary and (free) serum PLN using a population pharmacokinetic model. Salivary PLN was found to be a reliable predictor of free and total serum PLN in healthy volunteers. The results of this study encourage further exploration of the use of saliva as a noninvasive and feasible method for drug monitoring of PLN.

Clinical Pharmacokinetics and Pharmacodynamics of Linagliptin

Linagliptin is an orally active small-molecule inhibitor of dipeptidyl peptidase (DPP)-4, which was first licensed in the US, Europe, Japan and other territories in 2011 to improve glycaemic control in adults with type 2 diabetes mellitus. Linagliptin is the first and thus far the only DPP-4 inhibitor, and oral antihyperglycaemic drug in general, to be approved as a single-strength once-daily dose (5 mg). Compared with other available DPP-4 inhibitors, linagliptin has a unique pharmacokinetic/pharmacodynamic profile that is characterized by target-mediated nonlinear pharmacokinetics, concentration-dependent protein binding, minimal renal clearance and no requirements for dose adjustment for any intrinsic or extrinsic factor. After single or multiple oral doses of 1-10 mg, linagliptin displays less than dose-proportional increases in maximum plasma concentration (C(max)) and area under the plasma concentration-time curve (AUC). Linagliptin is rapidly absorbed after oral administration, with C(max) occurring after approximately 90 minutes, and reaches steady-state concentrations within 4 days. With the therapeutic dose, steady-state C(max) (11-12 nmol/L) and AUC (∼150 nmol · h/L) are approximately 1.3-fold greater than after a single dose, indicating little drug accumulation with repeat dosing. Linagliptin exhibits concentration-dependent protein binding in human plasma in vitro (99% at 1 nmol/L to 75-89% at >30 nmol/L) and has a large apparent volume of distribution, demonstrating extensive distribution into tissues. The nonlinear pharmacokinetics of linagliptin are best described by a two-compartmental model that incorporates target-mediated drug disposition resulting from high-affinity, saturable binding to DPP-4. The oral bioavailability of linagliptin estimated with this model is approximately 30%. Linagliptin has a long terminal half-life (>100 hours); however, its accumulation half-life is much shorter (approximately 10 hours), accounting for the rapid attainment of steady state. The majority of linagliptin is eliminated as parent compound, demonstrating that metabolism plays a minor role in the overall pharmacokinetics in humans. The main, pharmacologically inactive S-3-hydroxypiperidinyl metabolite accounted for approximately 17% of the total drug-related compounds in plasma. Linagliptin is eliminated primarily in faeces, with only around 5% of the oral therapeutic dose excreted in the urine at steady state. Linagliptin potently inhibits DPP-4 (inhibition constant 1 nmol/L), and trough drug concentrations achieved with therapeutic dosing inhibit >80% of plasma DPP-4 activity, the threshold associated with maximal antihyperglycaemic effects in animal models. There are no clinically relevant alterations in linagliptin pharmacokinetics resulting from renal impairment, hepatic impairment, coadministration with food, race, body weight, sex or age. In vitro, linagliptin is a weak substrate and weak inhibitor of cytochrome P450 (CYP) 3A4 and permeability glycoprotein (P-gp) but not of other CYP isozymes or ATP-binding cassette transporters. Clinical studies have revealed no relevant drug interactions when coadministered with other drugs commonly prescribed to patients with type 2 diabetes, including the narrow therapeutic index drugs warfarin and digoxin. Linagliptin plasma exposure is reduced by potent inducers of CYP3A4 or P-gp. Linagliptin has demonstrated a large safety window (>100-fold the recommended daily dose) and clinically relevant antihyperglycaemic effects in patients with type 2 diabetes.

The concentration-dependent binding of linagliptin (BI 1356) and its implication on efficacy and safety

Linagliptin (BI 1356) is a dipeptidyl peptidase-4 (DPP-4) inhibitor for treatment of Type 2 diabetes which recently gained approval in the US, Europe, and Japan. Linagliptin showed nonlinear pharmacokinetics after intravenous and oral administration, which is due to a concentration-dependent protein binding of linagliptin to its target enzyme DPP-4. The aim of this analysis was to investigate this target-mediated binding of linagliptin and its implication on efficacy and safety. Pharmacokinetic modeling and simulations were performed using a two-compartment model with concentration-dependent binding in the central and in one peripheral compartment. The optimum therapeutic dose with minimal off-target side effects was simulated assuming that an antidiabetic effect of linagliptin was due to the linagliptin concentration bound to DPP-4 and that off-target side effects were related to free linagliptin. The difference between steady state AUCs of specifically bound and free linagliptin was maximized at oral doses of 2 - 5 mg. Since plasma DPP-4 inhibition increased slightly from 2.5 to 10 mg, pharmacokinetic simulations and the pharmacodynamic measurements taken together suggest that 5 mg linagliptin could be considered an optimum dose. Simulations with missed doses and additional doses at steady state showed the effect on DPP-4 bound linagliptin and change in DPP-4 inhibition was minimal after missing one 5 mg oral dose of linagliptin while two doses of 5 mg linagliptin resulted in a less than proportional increase of steady state AUC of free linagliptin. Results from modeling and simulation support a stable antidiabetic effect of linagliptin over 24 h at steady state and further indicate a low risk for off-target side effects.

Pharmacokinetics and Pharmacodynamics of Single Rising Intravenous Doses (0.5 mg–10 mg) and Determination of Absolute Bioavailability of the Dipeptidyl Peptidase-4 Inhibitor Linagliptin (BI 1356) in Healthy Male Subjects

Linagliptin (BI 1356) is a highly specific inhibitor of dipeptidyl peptidase (DPP)-4, which is currently in phase III clinical development for the treatment of type 2 diabetes mellitus. Linagliptin exhibits nonlinear pharmacokinetics after oral administration, which are mainly related to concentration-dependent binding of linagliptin to its target, DPP-4. The objectives of the study were to investigate the pharmacokinetics and pharmacodynamics after intravenous administration of linagliptin and to determine its absolute bioavailability (F). This was a single rising-dose, randomized, four-group, placebo-controlled, single-blind (within dose groups) study. Thirty-six healthy men aged 18-50 years were enrolled and randomized into four sequential treatment groups. Group 1 received linagliptin 0.5 mg intravenously, group 2 received 2.5 mg intravenously and group 4 received 10 mg intravenously. In group 3, subjects underwent a two-way randomized crossover, receiving 5 mg intravenously and a 10 mg oral tablet. Linagliptin concentrations in plasma and urine, as well as plasma DPP-4 activity, were determined by validated assays. Noncompartmental analysis and population pharmacokinetic modelling were performed. Linagliptin showed nonlinear pharmacokinetics after intravenous infusion of 0.5-10 mg, with a less than dose-proportional increase in exposure. Noncompartmental parameters were calculated on the basis of total (i.e. bound and unbound) plasma concentrations. The total clearance value was low and increased with dose from 2.51 to 14.3 L/h. The apparent steady-state volume of distribution (V(ss)) increased with dose from 380 to 1540 L. Renal excretion of the unchanged parent compound increased with increasing plasma concentrations from 2.72% in the 0.5 mg dose group to 23.0% in the 10 mg dose group. The terminal elimination half-life was comparable across dose groups (126-139 hours). Because of the nonlinear pharmacokinetics, the standard approach of comparing the area under the plasma concentration-time curve (AUC) after oral administration with the AUC after intravenous administration led to dose-dependent estimates of the absolute bioavailability. Therefore, a population pharmacokinetic model was developed, accounting for the concentration-dependent protein binding of linagliptin to its target enzyme, DPP-4. The model-derived estimates of the V(ss) and clearance of linagliptin not bound to DPP-4 were 402.2 L and 26.9 L/h, respectively. The absolute bioavailability was estimated to be about 30% for the linagliptin 10 mg tablet. The nonlinear pharmacokinetic characteristics and the pharmacokinetic/pharmacodynamic relationship of linagliptin were independent of the mode of administration (intravenous or oral). Because of the nonlinear pharmacokinetics, the standard approach of comparing the AUC after oral administration with the AUC after intravenous administration was inappropriate to determine the absolute bioavailability of linagliptin. By a modelling approach, the absolute bioavailability of the 10 mg linagliptin tablet was estimated to be about 30%.

Impact of Target‐Mediated Drug Disposition on Linagliptin Pharmacokinetics and DPP‐4 Inhibition in Type 2 Diabetic Patients

The pharmacokinetics of the novel dipeptidyl-peptidase 4 (DPP-4) inhibitor linagliptin is nonlinear. Based on in vitro experiments, concentration-dependent binding to DPP-4 is the most likely cause for the nonlinearity. Population pharmacokinetic/pharmacodynamic modeling was performed using linagliptin plasma concentrations and plasma DPP-4 activities from 2 phase 2a studies. In these studies, type 2 diabetic patients received either 1, 2.5, 5, or 10 mg of linagliptin once daily over 12 days (study 1) or 2.5, 5, or 10 mg of linagliptin once daily over 28 days (study 2). The modeling results supported the hypothesis that linagliptin exhibits target-mediated drug disposition. The linagliptin plasma concentrations were best described by a 2-compartment model including concentration-dependent protein binding in the central and peripheral compartment. The plasma DPP-4 activity was included in the model in a semi-mechanistic way by relating it to the model-calculated plasma DPP-4 occupancy with linagliptin. The target binding has a major impact on linagliptin pharmacokinetics. Although unbound linagliptin is cleared efficiently (CL/F 220 L/h), the concentration-dependent binding is responsible for the long terminal half-life (approximatelly 120 hours) of linagliptin and its nonlinear pharmacokinetics. The model allowed a comprehensive understanding of the impact of target-mediated drug disposition and provides a useful tool to support clinical development.

Pharmacokinetics and anti-inflammatory pharmacodynamics of prednisolone in cynomolgus monkey

The purpose was to investigate whether the pharmacokinetics and pharmacodynamics of prednisolone in the non-human primate was an appropriate surrogate for man.After single intravenous doses of 0.03, 0.3, and 3 mg kg−1, prednisolone demonstrated a dose-dependent clearance and volume of distribution. When corrected for concentration-dependent protein binding, the free clearance was linear at the tested dose levels. The protein binding-corrected volume of distribution was similar across doses. The serum half-life was estimated as being between 2 and 4 h. Prednisolone exhibits near complete inhibition of the cytokines TNF-α, IL-1β, IL-6 and IL-8 with very similar IC50 estimates from 0.09 to 0.16 μg ml−1 (from 0.24 to 0.44 μM).The monkey demonstrated a similar pharmacokinetics–pharmacodynamics profile of prednisolone when compared with man (from the literature).

Formation of the N-methylpyridinium ether derivative of propofol to improve sensitivity, specificity and reproducibility of its detection in blood by liquid chromatography–mass spectrometry

In spite of comparatively high therapeutic concentrations in blood, the quantitation of propofol may cause analytical challenges due to its pharmacokinetic (e.g. short half life, high distribution volume) and physicochemical (significant volatility) particularities. Moreover, a considerable and concentration dependent protein binding and a substance association to erythrocytes result in an irreproducible distribution between whole blood and serum. A possible redistribution in stored samples needs to be compensated during sample preparation or included into the result interpretation. The new analytical approach is based on a formation of N-methylpyridinium derivatives of propofol and corresponding internal standards and permits a significant (∼300 fold) increase of detection limits in LC–MS/MS. Derivatization is achieved by a direct conversion of the acetonitrile supernatant of a protein precipitation with 2-fluoro-1-methyl-pyridinium- p-toluene-sulfonate using triethylamine as catalyst. The derivative exhibits a high solvent stability and provides – in contrast to the unchanged parent compound – a sufficient number of diagnostic qualifier fragments to fulfil common identification criteria. By using 2-tert-butyl-6-methylphenol as internal standard (instead of the commonly applied thymol), a better compensation of matrix effects could be achieved. Owing to its high robustness, appropriate quantitation limits (LLOQ ∼ 13 ng/mL), minimum sample amount and preparation effort, the assay could efficiently be applied for quantitative propofol analyses in pharmacokinetic studies with high sampling rates.

Binding to dipeptidyl peptidase‐4 determines the disposition of linagliptin (BI 1356) – investigations in DPP‐4 deficient and wildtype rats

Linagliptin (BI 1356) is a novel dipeptidyl peptidase-4 (DPP-4) inhibitor in clinical development for the treatment of type 2 diabetes. It exhibits non-linear pharmacokinetics and shows concentration-dependent plasma protein binding to its target, DPP-4. The aim of this study was to investigate the impact of saturable binding of linagliptin to plasma and tissue DPP-4 by comparing the pharmacokinetics of linagliptin in wildtype and DPP-4 deficient Fischer rats using non-compartmental and model-based data analysis. The non-compartmental analysis revealed a significantly reduced AUC in DPP-4 deficient rats compared with wildtype rats when single intravenous doses <or=1 mg/kg were administered, but the exposure was similar in both strains at higher doses. The terminal half-lives were significantly shorter in DPP-4 deficient rats compared with wildtype rats. For doses <or=1 mg/kg, DPP-4 deficient rats exhibited linear pharmacokinetics, whereas the pharmacokinetics of wildtype rats was non-linear. In the model-based analysis these differences could be accounted for by assuming concentration-dependent protein binding in the central and one peripheral compartment in wildtype rats. In the model, disposition parameters for unbound linagliptin were assumed to be identical in both rat strains. Simulations with different doses of linagliptin and different concentrations of binding sites further illustrated that the interdependence of linagliptin and DPP-4 in plasma and in the periphery has a major influence on the disposition of linagliptin in wildtype rats. In conclusion, the study showed that the concentration-dependent binding of linagliptin to its target DPP-4 has a major impact on the plasma pharmacokinetics of linagliptin.

Contribution of Protein Binding to the Pharmacokinetics of the Ghrelin Receptor Agonist TZP-101 in Healthy Volunteers and Adults with Symptomatic Gastroparesis

TZP-101 is a selective, small molecule ghrelin receptor agonist in clinical development for the treatment of gastric motility disorders. The objectives of this study was to assess pharmacokinetic parameters of TZP-101 after multiple- and single-dose administration to healthy subjects and patients with gastroparesis, respectively, and to determine the contribution of protein binding to its pharmacokinetic behaviour. Pharmacokinetics following 30-minute intravenous infusions of single (160-600 microg/kg) doses of TZP-101 in patients with gastroparesis and multiple (80-600 mug/kg/day) doses of TZP-101 in healthy subjects were characterized. TZP-101 protein binding was measured in human, dog, rat, rabbit and monkey plasma using equilibrium dialysis. TZP-101 pharmacokinetic profiles were less than dose proportional in both healthy subjects and patients, most likely because of concentration-dependent protein binding. A small volume of distribution (99-180 mL/kg following single doses) and long half-life (10-20 hours) were concentration independent in both healthy subjects and patients. Systemic clearance increased with increasing dose. Incidence of adverse events was not related to dose or treatment (active vs placebo). TZP-101 binding to human plasma proteins (primarily alpha(1)-acid glycoprotein) was >/=99% between 5 and 15 mumol/L (2.7 and 8.1 microg/mL) and was significantly higher than in other species. The pharmacokinetic parameters of TZP-101 in patients with gastroparesis and healthy subjects are comparable and display a similar trend toward increased clearance at higher dose levels resulting in little accumulation of TZP-101 at high dose levels and after multiple dosing. Significant protein binding indicates that the fraction of free drug rather than the total plasma concentration should be taken into consideration for human risk assessment based on animal safety data. Furthermore, the concentration of unbound drug should be considered when optimizing the clinical dose.

Population pharmacokinetics (PK) of the vascular-disrupting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) in cancer patients

13076 Background: DMXAA (AS1404) is a small-molecule vascular-disrupting agent that selectively disrupts established tumor blood vessels. It is being evaluated in phase II trials in combination with cytotoxics. DMXAA is extensively metabolized, predominantly by UDP-glucuronosyltransferase (UGT) 2B7 and 1A9, and exhibits concentration-dependent protein binding. The study aim was to develop a population PK model for DMXAA and to examine covariates influencing drug disposition. Methods: 3,050 DMXAA total plasma concentrations were measured in 124 cancer patients receiving DMXAA monotherapy as a 20-min iv infusion weekly or every three weeks at doses from 6 to 4900 mg/m2 in 3 phase I trials. PK models were fitted to the data from all patients simultaneously using the program NONMEM. Potential covariates including age, body weight, height, body surface area (BSA), sex, and liver and kidney function tests were screened with general additive model analysis and tested in the PK model. Results: DMXAA concentration-time profiles after multiple-dose administration were well described by a 3-compartment model with saturable elimination (Michaelis-Menten kinetic) from the central compartment. BSA and sex were significant covariates on the volume of distribution of the central compartment (V1) and the maximum elimination capacity (Vm), respectively, accounting for 10 and 12% of interindividual variation (IIV) in these parameters. Population estimates for Vm, Km (concentration at which half-maximal elimination rate is achieved), and V1 were 122 × (1 + 0.502 × (2-SEX)) (μM/h) [females had lower Vm than males; SEX = 1 for males, 2 for females], 103 μM, and 8.15 × (BSA/1.8)0.69 (L), respectively. In the final model, IIV in Vm was 52%. Conclusions: DMXAA plasma disposition is characterized by a saturable elimination process. BSA-guided dosing for DMXAA is important. Evaluation of other covariates that reflect drug glucuronidation (e.g., UGT polymorphisms) may account for a greater fraction of the substantial unexplained PK variability. The population PK model will aid development of limited PK sampling strategies and investigation of PK-pharmacodynamic relationships in further trials. [Table: see text]

LBOVI-B-1Investigation of carbamazepine nonlinear protein binding in patients with epilepsy

Background: Carbamazepine (CBZ) disposition has been extensively investigated, but CBZ protein binding (PB) is not well characterized. As part of a CBZ pharmacokinetic study, we observed concentration-dependent PB. The purpose of this study was to further investigate CBZ PB.

Neutralization of Free Digoxin-like Immunoreactive Components of Oriental Medicines Dan Shen and Lu-Shen-Wan by the Fab Fragment of Antidigoxin Antibody (Digibind)

Dan Shen and Lu-Shen-Wan, traditional Chinese medicines used as remedies for heart diseases, demonstrate digoxin-like immunoreactivity. The digoxin-like immunoreactive components of Lu-Shen-Wan show approximately 55% protein binding, while Dan Shen demonstrates concentration-dependent protein binding (68% bound at lower concentrations but only 25% bound at higher concentrations). Because Dan Shen and Lu-Shen-Wan can cause substantial toxic effects in patients, we studied the potential use of Digibind (Fab fragment of polyclonal antidigoxin antibody; Burroughs Wellcome, Research Triangle Park, NC) for neutralizing the pharmacologically active free fractions of Dan Shen and Lu-Shen-Wan. Drug-free serum pools were supplemented with Dan Shen or Lu-Shen-Wan to achieve apparent digoxin concentrations expected in severe overdoses. Aliquots of supplemented serum pools were supplemented further with aqueous Digibind solution to achieve final Digibind concentrations between 5 and 20 microg/mL (expected in vivo range in patients overdosed with digoxin and being treated with Digibind). We observed complete removal of the free apparent digoxin in the presence of Digibind for Dan Shen and Lu-Shen-Wan. Digibind binds free digoxin-like immunoreactive components of Dan Shen and Lu-Shen-Wan in vitro.

Neutralization of Free Digoxin-like Immunoreactive Components of Oriental Medicines Dan Shen and Lu-Shen-Wan by the Fab Fragment of Antidigoxin Antibody (Digibind)

Dan Shen and Lu-Shen-Wan, traditional Chinese medicines used as remedies for heart diseases, demonstrate digoxin-like immunoreactivity. The digoxin-like immunoreactive components of Lu-Shen-Wan show approximately 55% protein binding, while Dan Shen demonstrates concentration-dependent protein binding (68% bound at lower concentrations but only 25% bound at higher concentrations). Because Dan Shen and Lu-Shen-Wan can cause substantial toxic effects in patients, we studied the potential use of Digibind (Fab fragment of polyclonal antidigoxin antibody; Burroughs Wellcome, Research Triangle Park, NC) for neutralizing the pharmacologically active free fractions of Dan Shen and Lu-Shen-Wan. Drug-free serum pools were supplemented with Dan Shen or Lu-Shen-Wan to achieve apparent digoxin concentrations expected in severe overdoses. Aliquots of supplemented serum pools were supplemented further with aqueous Digibind solution to achieve final Digibind concentrations between 5 and 20 microg/mL (expected in vivo range in patients overdosed with digoxin and being treated with Digibind). We observed complete removal of the free apparent digoxin in the presence of Digibind for Dan Shen and Lu-Shen-Wan. Digibind binds free digoxin-like immunoreactive components of Dan Shen and Lu-Shen-Wan in vitro.

Dose-related pharmacokinetics and pharmacodynamics of valproate in the elderly.

Valproate exhibits a complex pharmacokinetic profile due to concentration-dependent protein binding and clearance. It has been shown that the protein binding of valproate decreases as the serum concentration increases in a young adult population. Furthermore, the percentage of protein binding is lower in the elderly compared with young adults at comparable low therapeutic serum concentrations. The extent of valproate protein binding at higher concentrations in the elderly has not been described. Studies conducted in the elderly have found unbound valproate clearance to be decreased compared with younger adults, although these changes in clearance have not been evaluated at higher therapeutic serum concentrations. We evaluated the pharmacokinetics of valproate (protein binding and clearance) across a wide dosage range in the elderly and measured the impact of this on drug-related side effects using a single-blind within-subject study design in 6 healthy elderly volunteers (aged 65-76 years). Steady-state total and unbound serum valproate concentrations were assessed at 3 doses: 500, 1000, and 1500 mg/d. As doses and valproate serum concentrations increased, the unbound fraction (10.0%, 13.0%, 17.4%) and total clearance (4.8, 6.0, 6.7 mL/h/kg) increased, respectively. Unbound clearance decreased (49.4, 45.8, 39.4 mL/h/kg) with increasing valproate serum concentrations. Drug-induced CNS effects and nausea severity scores correlated with total and unbound serum valproate concentrations. Significant dose-dependent changes in valproate pharmacokinetics were observed in the elderly.

Pharmacodynamics of levofloxacin and ciprofloxacin in a murine pneumonia model: peak concentration/MIC versus area under the curve/MIC ratios.

During the last decade some studies have shown that the area under the curve (AUC)/MIC ratio is the pharmacodynamic index that best predicts the efficacies of quinolones, while other studies suggest that the predictive value of the peak concentration/MIC (peak/MIC) ratio is superior to the AUC/MIC ratio in explaining clinical and microbiological outcomes. In classical fractionated dose-response studies with animals, it is difficult to differentiate between the AUC/MIC ratio and the peak/MIC ratio because of colinearity. Three different levofloxacin and ciprofloxacin dosing regimens were studied in a neutropenic mouse pneumonia model. The different regimens were used with the aim of increasing the AUC/MIC ratio without changing the peak/MIC ratio and vice versa. The first regimen (RC) consisted of daily doses of 5 up to 160 mg/kg of body weight divided into one, two, or four doses. In the second regimen (R0), mice were given 1.25 mg/kg every hour from 1 to 23 h, while the dose given at 0 h was 2.5, 5, 10, 20, 40, or 80 mg/kg. In the third regimen (R11), mice also received 1.25 mg/kg every hour from 0 to 23 h; but in addition, they also received 2.5, 5, 10, 20, 40, or 80 mg/kg at 11 h. The level of protein binding was also evaluated. The results indicate that the unbound fraction (f(u)) was concentration dependent for both levofloxacin and ciprofloxacin and ranged from approximately 0.67 to 0.88 for both drugs between concentrations of 0.5 and 80 mg/liter. The relationships between the AUC/MIC ratio and the number of CFU were slightly better than those between the peak/MIC ratio and the number of CFU. There was no clear relationship between the amount of time that the concentration remained above the MIC and effect (R(2) < 0.1). For both drugs, the peak/MIC ratio that resulted in a 50% effective concentration was lower for the R0 and R11 dosing regimens, indicating the importance of the AUC/MIC ratio. The same was true for the static doses. Survival studies showed that for mice treated with the low doses the rate of survival was comparable to that for the controls, but with the higher doses the rate of survival was better for mice receiving the R0 regimen. We conclude that for quinolones the AUC/MIC ratio best correlates with efficacy against pneumococci and that the effect of the peak/MIC ratio found in some studies could be partly explained by concentration-dependent protein binding.

Validation of subcutaneous microdialysis sampling for pharmacokinetic studies of flurbiprofen in the rat

The objective of this study was to validate subcutaneous (sc) microdialysis sampling to study flurbiprofen pharmacokinetics and plasma protein binding in the awake freely moving rat. A linear microdialysis probe was manufactured using a Hemophane® hollow fiber which was tested in vitro and in vivo for the recovery of flurbiprofen and naproxen used as retrodialysis marker. Flurbiprofen was administered intraperitoneally and intravenously at a dose of 20 mg/kg in rats. In both cases, conventional blood sampling and sc microdialysis sampling were simultaneously performed. The microdialysates were analyzed on-line by high-pressure liquid chromatography. Naproxen, which was shown to have a similar in vivo loss by retrodialysis as flurbiprofen (71.5 ± 0.9% and 71.0 ± 0.8% respectively, n = 3), was used to continuously monitor probe recovery. Concentration-dependent protein binding of flurbiprofen was demonstrated in vivo based on experiments with a simultaneous sc microdialysis and blood sampling. Values of unbound fraction were similar to those reported previously by intravenous microdialysis sampling, demonstrating that the sc unbound concentrations are very similar to those in the central compartment. There was no significant difference among pharmacokinetic parameters (AUC, CL, t1/2z, Vd) for total or unbound flurbiprofen determined after intraperitoneal and intravenous administration. Subcutaneous microdialysis is a simple yet powerful tool to study the pharmacokinetics and the in vivo plasma protein binding of flurbiprofen in the awake unrestrained rat. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:1897–1906, 2001

Biopharmaceutical studies on drug/conjugated-metabolite interactions. II. Effect of acetaminophen sulfate on pharmacokinetics of acetaminophen in rats

The effect of conjugated-metabolite, acetaminophen sulfate (APAPS), on the pharmacokinetics of its parent drug, acetaimnophen (APAP), was examined in rats. Following the i.v. bolus administration of APAP with APAPS, the plasma elimination of APAP was delayed and the distribution volume of APAP was increased at the APAPS coadministration with 60 mg APAP equivalent per kg (eq/kg). The percentages of dose excreted in the urine and bile in 4 h as APAP and its conjugated metabolites, APAPS and acetaminophen glucuronide, were significantly decreased. On the other hand, following the i.v. bolus administration of APAP under the steady-state concentration of APAPS, the distribution volume and total body clearance of APAP were significantly increased. Competitive displacement in serum protein binding of APAP by APAPS was ascertained in vitro and in vivo. A part of the conflict between the bolus and infusion experiment may be explained by the changes in the distribution volume of APAP contributed to the APAPS concentration-dependent serum protein binding of APAP. It was speculated that the pharmacokinetics of APAP was partly interacted with APAPS by the displacement of serum protein binding.

Effect of Felbamate on the Plasma Protein Binding of Valproate

Coadministration of felbamate is known to raise plasma valproate concentrations. In an attempt to explain this interaction, the effect of felbamate (FBM) on the plasma protein binding of valproate (VPA) was investigated in vitro and in healthy subjects. In fresh drug-free human plasma spiked with 20.0, 60.0 and 100.0 mg NaVPA/L without and with 75.5 mg FBM/L, the overall mean (± SD) unbound VPA fraction was 10.3 ± 1.6% without and 11.7 ± 2.1% with FBM. Regression analysis, used to distinguish FBM effects on plasma protein binding from effects due to altered total VPA concentration (VPA shows concentrationdependent protein binding), indicated that the mean percentage unbound VPA rose by 1.44% (95% CI = 0.59 to 2.30%). In 12 subjects the mean (± SD) trough concentrations of total VPA in plasma rose from 21.0 ± 6.1 mg/L to 45.1 ± 9.5 mg/L (p < 0.01) when VPA was taken, and the overall mean (± SD) unbound VPA fraction increased from 10.5 ± 0.77% to 11.7 ± 1.17%. Regression analysis indicated that there was a mean increase of 0.68% (95% CI = 0.13 to 1.23%) in plasma-unbound VPA because of a direct effect of FBM on plasma protein binding, the remaining 0.5% increase being due to the altered VPA concentration. While FBM appears to displace VPA from plasma proteins, this interaction is small and unlikely to make any important contribution to the overall effect of adding FBM to VPA, although the total interaction between the two may be large enough to necessitate a lowering of VPA dose.