Nonlinear Protein Binding Research Articles

Population pharmacokinetics and pharmacodynamic target attainment analysis of cefazolin using total and unbound serum concentration in patients with prostatectomy or nephrectomy.

The aim of this study was to analyze the population pharmacokinetics of total and unbound concentrations of prophylactic cefazolin (CFZ) in patients with prostatectomy or nephrectomy. We also aimed to calculate a pharmacodynamics target unbound concentration that exceeded the minimum inhibitory concentration (MIC), to design an effective dosing regimen. Briefly, 614 total concentration and 610 unbound concentration samples from 152 individuals were evaluated, using a nonlinear mixed-effects model. The obtained pharmacodynamics index target value reflected the probability of maintaining CFZ unbound trough concentrations exceeding MIC90, 0.5 mg/L, and MIC50, and 1.0 mg/L, to account for methicillin-susceptible Staphylococcus aureus (MSSA) or Escherichia coli. Population pharmacokinetics were estimated using a two-compartment model with nonlinear protein binding. Unbound systemic clearance (CL) was significantly associated with creatinine clearance, while the maximum protein-binding constant was significantly associated with albumin levels. The probability of achieving an unbound concentration exceeding the MIC50 for E. coli or MIC90 for MSSA in a patient with normal renal function following a 1 g CFZ infusion over 15 min was above 90% at 3 h after the initial dose. Our findings indicated that population pharmacokinetic parameters are useful for determining unbound CFZ pharmacokinetics and evaluating intraoperative CFZ redosing intervals.

A simple and rapid HPLC-UV method for the determination of valproic acid in human plasma using microwave-assisted derivatization with phenylhydrazine hydrochloride

This study presents an efficient high-performance liquid chromatography with ultraviolet detection (HPLC-UV) method for monitoring valproic acid (VPA) level in human plasma. This method is distinguished by its simplicity, cost-effectiveness, and rapid execution, addressing the limitations associated with other advanced analytical techniques like liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), and immunoassays, which are generally complex and costly for routine application. A challenge in analyzing VPA is its non-linear protein binding profile and the absence of a chromophore in its structure, making direct detection difficult. To overcome this, the study developed an efficient HPLC-UV for VPA determination in human plasma, utilizing a simplified and rapid microwave-assisted derivatization process. Due to the lack of a chromophore in VPA structure, this work developed a microwave-assisted derivatization of VPA using phenylhydrazine hydrochloride (PH HCl). The process optimization was achieved at 450 W for 50 s, facilitating effective HPLC-UV detection. The derivatized product was characterized using 1H nuclear magnetic resonance (NMR) and Fourier transform infrared spectrometer (FT-IR). The derivative, identified as (Z)-N-phenyl-2-propylpentanehydrazonic acid, demonstrated specificity in plasma analysis with no detectable interference. The method exhibited a linear response for VPA concentrations ranging from 30 to 150 μg/mL, with a correlation coefficient exceeding 0.99. Recovery varied between 86.7% and 107%, with a maximum coefficient of variation (CV) of 10.0%. The findings suggest that the microwave-assisted derivatization technique substantially improves the feasibility and cost-effectiveness of HPLC-UV for the analysis of VPA in plasma. This method provides a viable alternative to conventional HPLC methodologies, offering a balance of efficiency and economic practicality for VPA quantification.

Population pharmacokinetics and pharmacodynamics of cefazolin using total and unbound serum concentrations in patients with high body weight.

The objective of this study was to evaluate the steady state pharmacokinetics and pharmacodynamics of cefazolin in patients with a high body weight. Cefazolin was administered by 0.5-h infusions to 11 patients with total body weight (TBW) ≥120 kg receiving 3 g q8h, and 12 patients with TBW <120 kg receiving 2 g q8h. Total and unbound serum concentration-time data obtained from serial blood samples were analysed simultaneously by population pharmacokinetic modelling using NONMEM. Probability of target attainment (PTA) was calculated for various dosing regimens through Monte Carlo simulations based on the cumulative percentage of the dosing interval that the unbound concentration exceeds the minimum inhibitory concentration (MIC) value for the pathogen at steady state (fTMIC) ≥40%, ≥60% and 100%. A two-compartment model with non-linear protein binding and allometric scaling of the central volume of distribution using TBW best characterized both total and unbound concentration-time data. Unbound clearance was significantly associated with creatinine clearance, and maximum protein binding constant was significantly associated with serum albumin concentration and body mass index (P <0.05). Based on unbound concentration-time profiles, all simulated regimens achieved PTA >90% at MIC values ≤2 mg/L using fTMIC ≥40%, at MIC values ≤1 mg/L using fTMIC ≥60%, and at MIC values ≤0.5 mg/L using fTMIC of 100%. At fTMIC ≥60%, 0.5-h infusion of cefazolin 1 g q8h achieved PTA <90% at MIC values ≥2 mg/L in patients with TBW≥120 kg; however, prolonged-infusion and higher-dose regimens improved PTA to >90%. Overall, cefazolin pharmacokinetics are altered considerably in obese patients. Higher-dose and/or prolonged-infusion cefazolin regimens should be considered in patients with TBW ≥120 kg, particularly those with less-susceptible Gram-negative infections.

Plasma and Interstitial Fluid Pharmacokinetics of Prophylactic Cefazolin in Elective Bariatric Surgery Patients.

Guidelines for surgical prophylactic dosing of cefazolin in bariatric surgery vary in terms of recommended dose. This study aimed to describe the plasma and interstitial fluid (ISF) cefazolin pharmacokinetics in patients undergoing bariatric surgery and to determine an optimum dosing regimen. Abdominal subcutaneous ISF concentrations (measured using microdialysis) and plasma samples were collected at regular time points after administration of cefazolin 2 g intravenously. Total and unbound cefazolin concentrations were assayed and then modeled using Pmetrics. Monte Carlo dosing simulations (n = 5,000) were used to define cefazolin dosing regimens able to achieve a fractional target attainment (FTA) of >95% in the ISF suitable for the MIC for Staphylococcus aureus in isolates of ≤2 mg · L-1 and for a surgical duration of 4 h. Fourteen patients were included, with a mean (standard deviation [SD]) bodyweight of 148 (35) kg and body mass index (BMI) of 48 kg · m-2. Cefazolin protein binding ranged from 14 to 36% with variable penetration into ISF of 58% ± 56%. Cefazolin was best described as a four-compartment model including nonlinear protein binding. The mean central volume of distribution in the final model was 18.2 (SD 3.31) L, and the mean clearance was 32.4 (SD 20.2) L · h-1. A standard 2-g dose achieved an FTA of >95% for all patients with BMIs ranging from 36 to 69 kg · m-2. A 2-g prophylactic cefazolin dose achieves appropriate unbound plasma and ISF concentrations in obese and morbidly obese bariatric surgery patients.

Seeking Nonspecific Binding: Assessing the Reliability of Tissue Dilutions for Calculating Fraction Unbound.

It has become commonplace (270+ article citations to date) to measure the fraction unbound (FrUn) of drugs in tissue homogenates and diluted plasma and then use a Correction Factor Equation (CFE) to extrapolate to the undiluted state. The CFE is based on assumptions of nonspecific binding with experimental use of very low drug concentrations. There are several possible determinants of apparent nonspecific binding as measured by methods such as equilibrium dialysis: true macromolecule binding and lipid partitioning along with receptor, enzyme, and transporter interactions. Theoretical calculations based on nonlinear protein binding indicate that the CFE will be most reliable to obtain FrUn when added drug concentration is small, binding constants are weak, protein concentrations are relatively high, and tissue dilution is minimal. When lipid partitioning is the sole factor determining apparent tissue binding, the CFE should be perfectly accurate. Use of very low drug concentrations, however, makes it more likely that specific binding to receptors and other targets may occur, and thus FrUn may reflect some binding to such components. Inclusion of trapped blood can clearly cause minor to marked discrepancies from purely tissue binding alone, which can be corrected. Furthermore, assessment of the occurrence of ionization/pH shifts, drug instability, and tissue metabolism may be necessary. Caution is needed in the use and interpretation of results from tissue dilution studies and other assessments of nonspecific binding, particularly for very strongly bound drugs with very small FrUn values and in tissues with metabolic enzymes, receptors, and trapped blood. SIGNIFICANCE STATEMENT: The use of tissue, plasma, and cell preparations to help obtain fraction unbound and tissue-to-plasma partition coefficients in pharmacokinetics has grown commonplace, especially for brain. This report examines theoretical, physiological, and experimental issues that need consideration before trusting such measurements and calculations.

Optimization of flucloxacillin dosing regimens in critically ill patients using population pharmacokinetic modelling of total and unbound concentrations.

BackgroundInitial appropriate anti-infective therapy is associated with improved outcomes in patients with severe infections. In critically ill patients, altered pharmacokinetic (PK) behaviour is common and known to influence the achievement of PK/pharmacodynamic targets.ObjectivesTo describe population PK and optimized dosing regimens for flucloxacillin in critically ill patients.MethodsFirst, we developed a population PK model, estimated between-patient variability (BPV) and identified covariates that could explain BPV through non-linear mixed-effects analysis, using total and unbound concentrations obtained from 35 adult critically ill patients treated with intermittent flucloxacillin. Second, we validated the model using external datasets from two different countries. Finally, frequently prescribed dosing regimens were evaluated using Monte Carlo simulations.ResultsA two-compartment model with non-linear protein binding was developed and validated. BPV of the maximum binding capacity decreased from 42.2% to 30.4% and BPV of unbound clearance decreased from 88.1% to 71.6% upon inclusion of serum albumin concentrations and estimated glomerular filtration rate (eGFR; by CKD-EPI equation), respectively. PTA (target of 100%fT>MIC) was 91% for patients with eGFR of 33 mL/min and 1 g q6h, 87% for patients with eGFR of 96 mL/min and 2 g q4h and 71% for patients with eGFR of 153 mL/min and 2 g q4h.ConclusionsFor patients with high creatinine clearance who are infected with moderately susceptible pathogens, therapeutic drug monitoring is advised since there is a risk of underexposure to flucloxacillin. Due to the non-linear protein binding of flucloxacillin and the high prevalence of hypoalbuminaemia in critically ill patients, dose adjustments should be based on unbound concentrations.

Nonlinear protein binding of phenytoin in clinical practice: Development and validation of a mechanistic prediction model.

To individualize treatment, phenytoin doses are adjusted based on free concentrations, either measured or calculated from total concentrations. As a mechanistic protein binding model may more accurately reflect the protein binding of phenytoin than the empirical Winter-Tozer equation that is routinely used for calculation of free concentrations, we aimed to develop and validate a mechanistic phenytoin protein binding model. Data were extracted from routine clinical practice. A mechanistic drug protein binding model was developed using nonlinear mixed effects modelling in a development dataset. The predictive performance of the mechanistic model was then compared with the performance of the Winter-Tozer equation in 5 external datasets. We found that in the clinically relevant concentration range, phenytoin protein binding is not only affected by serum albumin concentrations and presence of severe renal dysfunction, but is also concentration dependent. Furthermore, the developed mechanistic model outperformed the Winter-Tozer equation in 4 out of 5 datasets in predicting free concentrations in various populations. Clinicians should be aware that the free fraction changes when phenytoin exposure changes. A mechanistic binding model may facilitate prediction of free phenytoin concentrations from total concentrations, for example for dose individualization in the clinic.

Evaluation of non-invasive biomonitoring of 2,4-Dichlorophenoxyacetic acid (2,4-D) in saliva

The objective of this study was to evaluate the potential for non-invasive biomonitoring of 2,4-Dichlorophenoxyacetic acid (2,4-D) in saliva. Using an in vitro rat salivary gland epithelial cell (SGEC) system, a collection of experiments investigating chemical protein binding, temporal and directional transport, as well as competitive transport with para-aminohippuric acid (PAH), a substrate for renal organic anion transporters, was conducted to identify cellular transport parameters required to computationally model salivary transport of 2,4-D. Additionally, a physiological protein gradient was implemented to mimic physiologically relevant concentrations of protein in rat plasma and saliva, and under these conditions the transfer of 2,4-D was markedly slower, driven by increased protein binding (i.e. reduced free 2,4-D species available to cross salivary barrier). The rate of transfer was directly proportional to the amount of unbound 2,4-D and demonstrated no indication of active transport. An in vivo assessment of 2,4-D exposure in rats revealed non-linear protein binding in plasma, indicating saturated protein binding and increased levels of unbound 2,4-D species at higher doses. A strong correlation between 2,4-D concentrations in saliva and unbound 2,4-D in plasma was observed (Pearson correlation coefficient = 0.95). Saliva:plasma 2,4-D ratios measured in vivo (0.0079) were consistent within the linear protein binding range and expected 2,4-D levels from occupational exposures but were significantly different than ratios measured in vitro (physiological conditions) (0.034), possibly due to 2,4-D concentrations in saliva not being at equilibrium with 2,4-D concentrations in blood, as well as physiological features absent in in vitro settings (e.g. blood flow). We demonstrated that 2,4-D is consistently transported into saliva using both in vitro and in vivo models, making 2,4-D a potential candidate for human non-invasive salivary biomonitoring. Further work is needed to understand whether current sensor limits of detection are sufficient to measure occupationally relevant exposures.

Pharmacokinetics and pharmacodynamic target attainment of ceftriaxone in adult severely ill sub-Saharan African patients: a population pharmacokinetic modelling study.

In sub-Saharan Africa (SSA), the highly albumin-bound β-lactam ceftriaxone is frequently used for the empirical treatment of severe bacterial infections. Systemic drug exposure of β-lactams can be altered in critically ill ICU patients, but pharmacokinetic and pharmacodynamic data for non-ICU SSA populations are lacking. We performed a population pharmacokinetic study in an adult hospital population in Mozambique, treated with ceftriaxone for presumptive severe bacterial infection from October 2014 to November 2015. Four blood samples per patient were collected for total ceftriaxone (CEFt) and unbound ceftriaxone (CEFu) concentration measurement. We developed a population pharmacokinetic model through non-linear mixed effect analysis and performed simulations for different patient variable, dosing and pharmacodynamic target scenarios. Eighty-eight participants yielded 277 CEFt and 276 CEFu concentrations. The median BMI was 18.9 kg/m2 and the median albumin concentration was 29 g/L. In a one-compartment model with non-linear protein binding, creatinine clearance was positively correlated with CEFu clearance. For microorganisms with an MIC of 1 mg/L, simulations demonstrated that with a 1 g twice-daily regimen and a 2 g once-daily regimen, 95.1% and 74.8% would have a CEFu concentration > MIC during half of the dosing interval (fT>MIC = 50%), respectively, whereas this was only 58.2% and 16.5% for the fT>MIC = 100% target. Severely ill adult non-ICU SSA patients may be at substantial risk for underexposure to CEFu during routine intermittent bolus dosing, especially when their renal function is intact.

Effect of Disease-Related Changes in Plasma Albumin on the Pharmacokinetics of Naproxen in Male and Female Arthritic Rats.

Naproxen (NPX) is used in the treatment of rheumatoid arthritis (RA) for alleviation of pain and inflammation. In view of the extensive albumin binding of NPX, this study investigates whether chronic inflammation and sex influence the physiologic albumin concentrations, plasma protein binding, and pharmacokinetics (PK) of NPX. The PK of NPX was evaluated in a rat model of RA [collagen-induced arthritis (CIA) in Lewis rats] and in healthy controls. These PK studies included 1) NPX in female and male CIA rats that received 10, 25, or 50 mg/kg NPX i.p.; and 2) NPX in healthy female and male rats after i.p. dosing of NPX at 50 mg/kg. Plasma albumin concentrations were quantified by enzyme-linked immunosorbent assay, and protein binding was assessed using ultrafiltration. The NPX concentrations in plasma and filtrates were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Plasma concentration-time data of NPX were first assessed by noncompartmental analysis (NCA). Nonlinear PK as indicated by dose-dependent NCA clearances and distribution volumes was observed. A two-compartment model with a first-order absorption process incorporating nonlinear protein binding in plasma and tissues jointly described the PK data of all groups. Saturable albumin binding accounts for the nonlinearity of NPX PK in all rats as well as part of the PK differences in arthritic rats. The CIA rats exhibited reduced albumin concentrations, reduced overall protein binding, and reduced clearances of unbound NPX, consistent with expectations during inflammation. The net effect of chronic inflammation was an elevation of the Cmax and area under the plasma concentration-time curve (AUC) of unbound drug.

The complexity of minocycline serum protein binding.

Serum protein binding is critical for understanding the pharmacology of antimicrobial agents. Tigecycline and eravacycline were previously reported to have atypical non-linear protein binding; the percentage of free fraction decreased with increasing total concentration. In this study, we extended the investigation to other tetracyclines and examined the factors that might impact protein binding. Different minocycline concentrations (0.5-50 mg/L) and perfusion media (saline, 0.1 M HEPES buffer and 0.1 and 1 M PBS) were examined by in vitro microdialysis. After equilibration, two dialysate samples were taken from each experiment and the respective antimicrobial agent concentrations were analysed by validated LC-MS/MS methods. For comparison, the serum protein bindings of doxycycline and levofloxacin were also determined. The free fraction of minocycline decreased with increasing total concentration, and the results depended on the perfusion media used. The trends of minocycline protein binding in mouse and human sera were similar. In addition, serum protein binding of doxycycline showed the same concentration-dependent trend as minocycline, while the results of levofloxacin were concentration independent. The serum protein bindings of minocycline and doxycycline are negatively correlated with their total concentrations. It is possible that all tetracyclines share the same pharmacological property. Moreover, the specific perfusion media used could also impact the results of microdialysis. Additional studies are warranted to understand the mechanism(s) and clinical implications of serum protein binding of tetracyclines.

Development of a population pharmacokinetic model characterizing the tissue distribution of azithromycin in healthy subjects.

Recent clinical trials indicate that the use of azithromycin is associated with the emergence of macrolide resistance. The objective of our study was to simultaneously characterize free target site concentrations and correlate them with the MIC90s of clinically relevant pathogens. Azithromycin (500 mg once daily [QD]) was administered orally to 6 healthy male volunteers for 3 days. The free concentrations in the interstitial space fluid (ISF) of muscle and subcutaneous fat tissue as well as the total concentrations in plasma and polymorphonuclear leukocytes (PMLs) were determined on days 1, 3, 5, and 10. All concentrations were modeled simultaneously in NONMEM 7.2 using a tissue distribution model that accounts for nonlinear protein binding and ionization state at physiological pH. The model performance and parameter estimates were evaluated via goodness-of-fit plots and nonparametric bootstrap analysis. The model we developed described the concentrations at all sampling sites reasonably well and showed that the overall pharmacokinetics of azithromycin is driven by the release of the drug from acidic cell/tissue compartments. The model-predicted unionized azithromycin (AZM) concentrations in the cytosol of PMLs (6.0 ± 1.2 ng/ml) were comparable to the measured ISF concentrations in the muscle (8.7 ± 2.9 ng/ml) and subcutis (4.1 ± 2.4 ng/ml) on day 10, whereas the total PML concentrations were >1,000-fold higher (14,217 ± 2,810 ng/ml). The total plasma and free ISF concentrations were insufficient to exceed the MIC90s of the skin pathogens at all times. Our results indicate that the slow release of azithromycin from low pH tissue/cell compartments is responsible for the long terminal half-life of the drug and thus the extended period of time during which free concentrations reside at subinhibitory concentrations.

Population Pharmacokinetic and Pharmacodynamic Modeling of Acetazolamide in Peritoneal Dialysis Patients and Healthy Volunteers

To characterize the pharmacokinetics (PK) and pharmacodynamics (PD) of acetazolamide (ACTZ) in peritoneal dialysis patients, ACTZ 500 mg was administered intravenously to 7 healthy subjects (HV) and 8 peritoneal dialysis patients (CAPD). Population PK/PD modeling was performed with ACTZ serum (total and unbound), urine and dialysate concentrations, intra-ocular pressure (IOP) and covariates. A multi-compartment PK model (accounting for non-linear protein binding) and an inhibitory Emax (maximal change in IOP) PD model were selected. As expected, renal clearance (which almost equals total body clearance) was severely decreased in CAPD (1.2 vs 80.3 L/h) and the elimination half-life of total ACTZ was prolonged (20.6 vs 3.4 hours). The protein binding was significantly altered with a mean free fraction 4.2% in HV and 8.6% in CAPD. Moreover protein binding of ACTZ was concentration dependent in both HV and CAPD. Despite a higher free fraction of ACTZ, the Emax was lower in CAPD: 4.4±1.4 vs 7.4±2.8 mmHg. Both PK and PD are significantly altered in dialysis patients.

A Population Approach to Eplerenone Pharmacokinetics and Saturable Protein Binding

Eplerenone deviates from linear pharmacokinetics at doses above the therapeutic dose range. In addition, saturable protein binding of eplerenone is observed in in vitro plasma protein binding studies. The purpose of the present study was to clarify the factors contributing to the nonlinear pharmacokinetics of eplerenone. Plasma concentration data for eplerenone and its metabolite SC-70303, to which eplerenone is reversibly converted, obtained from four phase I studies were analyzed using NONMEM. A population pharmacokinetic model incorporating protein binding and the reversible relationship between eplerenone and SC-70303 was developed. Models with linear and nonlinear protein binding were fitted to the observed concentration data. The observed concentration data of eplerenone and SC-70303 were best described by a model with nonlinear protein binding. The area under the plasma concentration-time curve of eplerenone simulated by the model increased less than proportionally with increasing dose, whereas that of SC-70303 increased proportionally with increasing dose, consistent with observations from the non-compartmental analysis. In conclusion, the nonlinear pharmacokinetics of eplerenone and the apparently linear pharmacokinetics of SC-70303 were described by applying a model with nonlinear protein binding to observed plasma eplerenone and SC-70303 concentrations, suggesting that nonlinear protein binding plays a role in the nonlinear kinetics.

A Pharmacokinetic/Pharmacodynamic Approach to Predict Total Prednisolone Concentrations in Human Plasma

Prednisolone and prednisone are two widely used corticosteroids for various inflammatory and immune diseases. Prednisolone is the active form of prednisone in vivo. Total prednisolone in plasma exhibits nonlinear pharmacokinetics mainly due to its nonlinear protein binding. Other factors such as reversible metabolism (or interconversion between prednisolone and prednisone), competitive protein binding from endogenous cortisol, cortisol circadian rhythm, and prednisolone mediated cortisol suppression complicate prednisolone pharmacokinetics. This study was aimed to develop a new approach to describe the nonlinear pharmacokinetics of total prednisolone and predict total prednisolone concentrations in plasma. Based on literature datasets, a linear two-compartment pharmacokinetic model was developed to adequately describe the reversible metabolism between free prednisone and prednisolone. Cortisol and prednisolone protein binding were described via the sum of a Langmuir and linear type binding. The endogenous cortisol circadian rhythm and cortisol suppression during prednisone or prednisolone exposure were described with a previously reported linear release rate pharmacokinetic/pharmacodynamic (PK/PD) model. By combining the pharmacokinetic models for free prednisone and prednisolone, the linear release rate model for cortisol suppression, and competitive protein binding between cortisol and prednisolone, we were able to predict total prednisolone concentrations in plasma. The predicted total prednisolone concentrations in plasma were in good agreement with the literature reported data. Thus, this PK/PD approach shows that the combination of nonlinear protein binding, cortisol circadian rhythm, and cortisol suppression could account for the nonlinearity of total prednisolone. In addition, it also allows a valid prediction of total prednisolone in plasma after either prednisone or prednisolone administration.

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.

Predicted serum valproic acid concentrations in patients missing and replacing a dose of extended-release divalproex sodium

Computer simulations were used to analyze changes in steady-state total plasma valproic acid concentrations when a patient misses a dose of once-daily extended-release divalproex sodium, replaces it at a later time, and resumes scheduled therapy. Valproic acid concentration-time profiles were simulated for 1000 hypothetical patients for each of two missed-dose scenarios using a one-compartment (assumes rapid distribution) population kinetic model with nonlinear protein binding. For each scenario, a lognormal distribution of clearance of unbound valproic acid, volume of distribution of unbound valproic acid, protein-binding values, and albumin concentration was generated. All simulations incorporated 20% interpatient variability and 10% residual error. Our pharmacokinetic simulations predicted that the chance for high plasma valproic acid concentrations resulting in clinical toxicity is low when extended-release divalproex doses are replaced within 12 hours followed by resumption of scheduled administration, perhaps due to the controlled, near zero-order absorption characteristics of the extended-release formulation. If a patient misses a dose of extended-release divalproex, it should be replaced as soon as the patient remembers. The next dose should be taken at the regularly scheduled time. A missed dose of extended-release divalproex may be replaced up to 12 hours later without any clinically significant change in plasma valproic acid concentrations in a majority of the patients. Simulation of two scenarios suggested that a missed dose of extended-release divalproex sodium may be replaced up to 12 hours later without any clinically significant perturbation in plasma valproic acid concentrations in the majority of adolescent and adult patients with epilepsy.

Oral/intravenous maintenance dosing of valproate following intravenous loading: a simulation

Valproic acid (VPA) has a narrow therapeutic range (50–100 mg/l) and exhibits nonlinear protein binding. Additionally, VPA pharmacokinetics are dependent on age, induction status, and formulation; so titration and dosing vary between individuals. The aim of these simulations was to determine optimal intravenous (i.v.) loading dose, and i.v. and oral VPA maintenance regimens. A 5-min 15 mg/kg loading dose resulted in total and free plasma VPA concentrations of ∼65 and 7.5 mg/l in children, and ∼80 and 11 mg/l in adults, 1 h after the infusion; induction status had little effect. For uninduced children and adults, 7.5 and 3.5 mg/kg q6h i.v. valproate sodium, initiated 6 h after loading dose maintains therapeutic plasma VPA concentrations. The rapid decline of plasma VPA concentrations following an i.v. loading dose in combination with the delayed initial absorption of drug from delayed-release divalproex sodium tablets warrant beginning q12h oral maintenance regimens of delayed-release divalproex sodium within 2 h of a loading dose in the uninduced population. Plasma VPA concentrations can be sustained in the therapeutic range using once-daily maintenance regimens of extended-release divalproex sodium tablets if initiated concurrently with i.v. loading dose in the uninduced population. A two-fold higher i.v. and oral maintenance regimen dose may be required in induced patients.

Assay methodology for the quantitation of unbound ertapenem, a new carbapenem antibiotic, in human plasma

Ertapenem is a new once-a-day antibiotic with excellent coverage of common community gram negative and gram positive aerobes and anaerobes. It demonstrates nonlinear protein binding in human plasma (about 94% bound). An assay for unbound drug was developed to study the pharmacokinetics of unbound ertapenem in plasma. Unbound drug is separated from plasma samples (1.0 ml) by ultrafiltration using a Centrifree ® centrifugal filter device. Ertapenem (vulnerable to hydrolysis of the beta-lactam moiety) is stabilized in the filtrate by adding an equal volume of 0.1 M MES buffer, pH 6.5 and then is analyzed by reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet (UV) absorbance detection (300 nm). Non-specific binding to the Centrifree ® device is <3%. A suitable internal standard is not available. The assay is specific and linear over the concentration range of 0.25 to 100 μg/ml in plasma filtrate. The lower limit of quantitation (LLOQ) is 0.25 μg/ml. Intra-day precision is C.V.<10% and accuracy ranges from 97 to 101% of nominal concentration. Inter-day precision and accuracy were determined using quality control samples (QCs) prepared in plasma ultrafiltrate at 0.5, 12 and 80 μg/ml and stored at −70 °C with stabilizer. Inter-day assay accuracy and precision ranged from 100 to 111% of nominal concentration and 1.8 to 5.3% C.V. ( n=40), respectively. The assay has been used to analyze plasma samples from subjects receiving 500 and 2000 mg i.v. doses of ertapenem (30 min infusion).

SB-242235, a selective inhibitor of p38 mitogenactivated protein kinase. I: Preclinical pharmacokinetics

1. SB-242235 (1-(4-piperidinyl)-4-(4-fluorophenyl)-5-(2-methoxy-4-pyrimidinyl) imidazole) is a potent and selective p38 MAP kinase inhibitor that may be an effective therapy for cytokine-mediated diseases such as autoimmune or inflammatory diseases. The present studies were conducted to evaluate the pharmacokinetics of SB-242235 in several preclinical species, including rat, dog and monkey. 2. SB-242235 demonstrates generally favourable pharmacokinetic properties in all species examined. Systemic plasma clearance was high in rat, but in the non-rodent species SB-242235 demonstrated low to moderate clearance with plasma half-lives > 4 h. Oral bioavailability in each preclinical species was high. In rat and monkey, SB-242235 demonstrated non-linear elimination kinetics that manifested as a decrease in clearance with increasing dose and apparent oral bioavailability >100% at high oral doses. Furthermore, SB-242235 displayed concentration-dependent plasma protein binding over a concentration range of 1000-10,000 ng ml -1. 3. In conclusion, SB-242235 demonstrates high oral bioavailability across the major preclinical species, and may thus be a useful tool compound for investigation of the role of p38 inhibition in various disease states. However, the observations of non-linear protein binding and disposition also suggest the need for caution in the design of and data interpretation from such studies.