Determination of unbound drug concentration and protein-drug binding fraction in plasma

Reproducible Quantification of Unbound Fractions of Four Beta-Lactam Antibiotics: Ultrafiltration Versus Microdialysis of Spiked Healthy Donor Plasma

BACKGROUND

Ultrafiltration (UF) is a conventional method for isolating the protein-unbound plasma fractions of therapeutic drugs. However, the ideal UF conditions for specific compounds remain largely unexplored. By comparing UF-derived unbound concentrations with the corresponding results obtained using a reference method, the authors sought to identify appropriate UF conditions for cefotaxime, cloxacillin, flucloxacillin, and piperacillin.

METHODS

In vitro microdialysis (MD) with a no-net-flux approach was used as a reference method for plasma protein separation, for which UF performance was assessed. Four levels of relative centrifugal force (2500-11,290 g ) and 2 levels of temperature (37 vs. 22°C) during 10 minutes of UF centrifugation were evaluated. Ultrafiltrates and reference microdialysates were analyzed using liquid chromatography-tandem mass spectrometry to obtain unbound concentrations. After identifying the appropriate UF conditions in the spiked plasma samples, exploratory analyses of clinical samples (n = 10 per analyte) were performed.

RESULTS

Of the evaluated UF alternatives, the best overall agreement with the MD-derived reference concentrations was obtained with 11,290 g UF performed at 22°C. For cloxacillin specifically, 37°C UF yielded better agreement than 22°C UF at 11,290 g. Clinical sample analyses indicated minimal differences between 22°C and 37°C at 11,290 g UF for cefotaxime and piperacillin. However, consistently lower levels of unbound cloxacillin (median: -23%, IQR: -19% to -24%) and flucloxacillin (median: -27%, IQR: -21 to -34%) were observed after UF at 22°C versus 37°C.

CONCLUSIONS

For the evaluated UF device, 10 minutes of 11,290 g UF at 22°C is appropriate for flucloxacillin, cefotaxime, and piperacillin, and can arguably be justified for cloxacillin as well for laboratory practice purposes. Maintenance of 37°C during high-centrifugal UF may lead to overestimation, particularly for unbound flucloxacillin.

Fresh Molecular Concepts to Extend the Lifetimes of Old Antimicrobial Drugs

Antimicrobial drug development generally initiates with target identification and mode of action studies. Often, emergence of resistance and/or undesired side effects that are discovered only after prolonged clinical use, result in discontinuation of clinical use. Since the cost and time required for improvement of existing drugs are considerably lower than those required for the development of novel drugs, academic and pharmaceutical company researchers pursue this direction. In this account we describe selected examples of how chemical probes generated from antimicrobial drugs and chemical and enzymatic modifications of these drugs have been used to modify modes of action, block mechanisms of resistance, or reduce side effects, improving performance. These examples demonstrate how new and comprehensive mechanistic insights can be translated into fresh concepts for development of next-generation antimicrobial agents.

Serum Prevents Interactions between Antimicrobial Amphiphilic Aminoglycosides and Plasma Membranes

Antimicrobial cationic amphiphiles have broad-spectrum activity, and microbes do not readily develop resistance to these agents, highlighting their clinical and industrial potential. Cationic amphiphiles perturb the integrity of membranes leading to cell death, and the lack of discrimination between microbial and mammalian plasma membranes is thought to be one of the main barriers of using these agents for the treatment of systemic infections. Here, we describe the synthesis and study of 20 antimicrobial cationic amphiphiles that are derivatives of the aminoglycoside nebramine with different numbers of alkyl chain ethers that differ in length and degree of unsaturation. We determined antifungal activities and evaluated hemoglobin release from red blood cells as a measure of membrane selectivity and analyzed how serum influences these activities. Microscopic images revealed morphological transformations of red blood cells from the normal double-disc shape to empty ghost cells upon treatment with the cationic amphiphiles. Antifungal activity, hemolysis, and morphological changes in red blood cells decreased as the percentage of serum in the culture medium was increased. In images of red blood cells treated with fluorescently labeled amphiphilic nebramine probes, the accumulation of the cationic amphiphiles in the membranes decreased as serum concentration increased. This suggests that, in addition to its known effect of preventing the deformability of red blood cells, serum prevents interactions between cationic amphiphiles and the plasma membrane. The results of this study indicate that biological activities of cationic amphiphiles are abrogated in serum. Thus, these agents are suitable for external and industrial uses but probably not for effective treatment of systemic infections.

Electromembrane extraction as a new approach for determination of free concentration of phenytoin in plasma using capillary electrophoresis

PURPOSE

Electromembrane extraction is a new membrane-based extraction method in which charged compounds are extracted by an electric field. So far, this method has been used to extract and isolate a variety of acidic and basic drugs from various samples, including blood and plasma. However, in this procedure, it is not yet clear whether only unbound fraction of a drug is extracted or the total drug. The aim of this study is to reveal the nature of drug extraction in the presence of plasma proteins.

METHODS

To determine the nature of the extraction, the electromembrane extraction was performed from plasma solutions of phenytoin with concentrations 0.03 and 1.0 μg/mL, then the result was compared with the values obtained from the electromembrane extraction of ultrafiltrate of the same solutions (free concentration) and protein-free ultrafiltrate of plasma with final concentration of 0.03 and 1.0 μg/mL (total concentration). For this purpose, EME followed by capillary electrophoresis coupled with diode array detection was optimized and validated.

RESULTS

The results showed that the electromembrane extraction method was only able to extract the unbound fraction of phenytoin from plasma samples. The method was validated over a concentration range of 0.03-4 μg/mL. The inter and intra-assay precisions were less than 6.7%. The phenytoin protein binding was also determined to be in agreement with the literature data and confirms the validity of this method.

CONCLUSION

This sensitive and quick EME approach for determining the free concentration of a phenytoin, can be a good alternative to classic methods for therapeutic drug monitoring and pharmacokinetic studies.

How to Determine the Role of the Microbiome in Drug Disposition

With a paradigm shift occurring in health care toward personalized and precision medicine, understanding the numerous environmental factors that impact drug disposition is of paramount importance. The highly diverse and variant nature of the human microbiome is now recognized as a factor driving interindividual variation in therapeutic outcomes. The purpose of this review is to provide a practical guide on methodology that can be applied to study the effects of microbes on the absorption, distribution, metabolism, and excretion of drugs. We also highlight recent examples of how these methods have been successfully applied to help build the basis for researching the intersection of the microbiome and pharmacology. Although in vitro and in vivo preclinical models are highlighted, these methods are also relevant in late-phase drug development or even as a part of routine after-market surveillance. These approaches will aid in filling major knowledge gaps for both current and upcoming therapeutics with the long-term goal of achieving a new type of knowledge-based medicine that integrates data on the host and the microbiome.

Analysis of free drug fractions in serum by ultrafast affinity extraction and two-dimensional affinity chromatography using α1-acid glycoprotein microcolumns

In the circulatory system, many drugs are reversibly bound to serum proteins such as human serum albumin (HSA) and alpha1-acid glycoprotein (AGP), resulting in both free and protein-bound fractions for these drugs. This report examined the use of microcolumns containing immobilized AGP for the measurement of free drug fractions by ultrafast affinity extraction and a two-dimensional affinity system. Several drugs known to bind AGP were used as models to develop and evaluate this approach. Factors considered during the creation of this method included the retention of the drugs on the microcolumns, the injection flow rate, the microcolumn size, and the times at which a second AGP column was placed on-line with the microcolumn. The final system had residence times of only 110-830ms during sample passage through the AGP microcolumns and allowed free drug fractions to be determined within 10-20min when using only 3-10μL of sample per injection. This method was used to measure the free fractions of the model drugs at typical therapeutic levels in serum, giving good agreement with the results obtained by ultrafiltration. This approach was also used to estimate the binding constants for each drug with AGP in serum, even for drugs that had significant interactions with both AGP and HSA in such samples. These results indicated that AGP microcolumns could be used with ultrafast affinity extraction to measure free drug fractions in a label-free manner and to study the binding of drugs with AGP in complex samples such as serum.

Interpreting physicochemical experimental data sets

With the wealth of experimental physicochemical data available to chemoinformaticians from the literature, commercial, and company databases an increasing challenge is the interpretation of such datasets. Subtle differences in experimental methodology used to generate these datasets can give rise to variations in physicochemical property values. Such methodology nuances will be apparent to an expert experimentalist but not necessarily to the data analyst and modeller. This paper describes the differences between common methodologies for measuring the four most important physicochemical properties namely aqueous solubility, octan-1-ol/water distribution coefficient, pK(a) and plasma protein binding highlighting key factors that can lead to systematic differences. Insight is given into how to identify datasets suitable for combining.

Importance of relating efficacy measures to unbound drug concentrations for anti-infective agents

For the optimization of dosing regimens of anti-infective agents, it is imperative to have a good understanding of pharmacokinetics (PK) and pharmacodynamics (PD). Whenever possible, drug efficacy needs to be related to unbound concentrations at the site of action. For anti-infective drugs, the infection site is typically located outside plasma, and a drug must diffuse through capillary membranes to reach its target. Disease- and drug-related factors can contribute to differential tissue distribution. As a result, the assumption that the plasma concentration of drugs represents a suitable surrogate of tissue concentrations may lead to erroneous conclusions. Quantifying drug exposure in tissues represents an opportunity to relate the pharmacologically active concentrations to an observed pharmacodynamic parameter, such as the MIC. Selection of an appropriate specimen to sample and the advantages and limitations of the available sampling techniques require careful consideration. Ultimately, the goal will be to assess the appropriateness of a drug and dosing regimen for a specific pathogen and infection.

Interactions between thrombin and natural products of Millettia nitita var. hirsutissima using capillary zone electrophoresis

A sensitive and selective high-performance analytical method based on capillary zone electrophoresis (CZE) was developed for investigating interactions between natural products isolated from Millettia nitita var. hirsutissima and thrombin qualitatively and quantitatively for the first time. The results showed that, compared with positive and negative control, the compounds ZYY-5 (genistein-8-C-beta-d-apiofuranosyl-(1-->6)-O-beta-d-glucopyranoside), ZYY-6 (calycosin), ZYY-8 (isoliquiritigenin), ZYY-9 (formononetin), ZYY-12 (gliricidin), ZYY-13 (8-O-methylretusin), FJ-2 (dihydrokaempferol), FJ-3 (biochanin), FJ-5 (afromosin) and XC-2 (hirsutissimiside F) interacted with thrombin, while ZYY-1 (sphaerobioside), ZYY-2 (formononetin-7-O-beta-d-apiofuranosyl-(1-->6)-O-beta-d-glucopyranoside), ZYY-3 (genistein-5-methylether-7-O-alpha-l-rhamnopyranosyl-(1-->6)-O-beta-d-glucopyranoside), ZYY-4 (retusin-7,8-O-beta-d-diglucopyranoside), ZYY-7 (symplocoside), ZYY-10 (ononin), ZYY-11 (genistin), ZYY-14 (afromosin-7-O-beta-d-glucopyranoside), ZYY-15 (lanceolarin), FJ-1 (liquiritigenin), FJ-4 (7,2-dihydroxy,4-methoxyisoflavan) and XC-1 (sphaerobioside) had no binding to thrombin. This indicated that the reported CZE method for the determination of compound-thrombin interactions is powerful, sensitive and fast, and requires less amounts of reagents, and further, it can be employed as a reliable alternative to other methods.

Evaluation of alternatives to warfarin as probes for Sudlow site I of human serum albumin: characterization by high-performance affinity chromatography

Warfarin is often used as a site-specific probe for examining the binding of drugs and other solutes to Sudlow site I of human serum albumin (HSA). However, warfarin has strong binding to HSA and the two chiral forms of warfarin have slightly different binding affinities for this protein. Warfarin also undergoes a slow change in structure when present in common buffers used for binding studies. This report examined the use of four related, achiral compounds (i.e., coumarin, 7-hydroxycoumarin, 7-hydroxy-4-methylcoumarin, and 4-hydroxycoumarin) as possible alternative probes for Sudlow site I in drug binding studies. High-performance affinity chromatography and immobilized HSA columns were used to compare and evaluate the binding properties of these probe candidates. Binding for each of the tested probe candidates to HSA was found to give a good fit to a two-site model. The first group of sites had moderate-to-high affinities for the probe candidates with association equilibrium constants that ranged from 6.4 x 10(3)M(-1) (coumarin) to 5.5 x 10(4)M(-1) (4-hydroxycoumarin) at pH 7.4 and 37 degrees C. The second group of weaker, and probably non-specific, binding regions, had association equilibrium constants that ranged from 3.8 x 10(1)M(-1) (7-hydroxy-4-methylcoumarin) to 7.3 x 10(2)M(-1) (coumarin). Competition experiments based on zonal elution indicated that all of these probe candidates competed with warfarin at their high affinity regions. Warfarin also showed competition with coumarin, 7-hydroxycoumarin and 7-hydroxy-4-methycoumarin for their weak affinity sites but appeared to not bind and/or compete for all of the weak sites of 4-hydroxycoumarin. It was found from this group that 4-hydroxycoumarin was the best alternative to warfarin for examining the interactions of drugs at Sudlow site I on HSA. These results also provided information on how the major structural components of warfarin contribute to the binding of this drug at Sudlow site I.

Validated assay for the simultaneous quantification of total vincristine and actinomycin-D concentrations in human EDTA plasma and of vincristine concentrations in human plasma ultrafiltrate by high-performance liquid chromatography coupled with tandem mass spectrometry

A sensitive, specific and efficient high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) assay for the simultaneous determination of total vincristine and actinomycin-D concentrations in human plasma and an assay for the determination of unbound vincristine are presented. Electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and heated electrospray ionization (H-ESI) were tested as ionization interfaces. For reasons of robustness ESI was chosen followed by tandem mass spectrometry (ESI-MS/MS). For the plasma assay a 30 microL aliquot was protein precipitated with acetonitrile/methanol (50:50, v/v) containing the internal standard vinorelbine and 10 microL volumes were injected onto the HPLC system. To determine unbound vincristine, ultrafiltrate was produced from plasma using 30 kDa centrifugal filter units. The plasma ultrafiltrate was mixed with methanol (50:50, v/v), internal standard vinorelbine was added and 20 microL aliquots were injected onto the HPLC system. Separation was achieved on a 50x2.1 mm i.d. Xbridge C18 column using 1 mM ammonium acetate/acetonitrile (30:70, v/v) adjusted to pH 10.5 with ammonia, run in a gradient with methanol at a flow rate of 0.4 mL/min. HPLC run time was 6 min. The assay quantifies in plasma vincristine from 0.25 to 100 ng/mL and actinomycin-D from 0.5 to 250 ng/mL using plasma sample volumes of only 30 microL. Vincristine in plasma ultrafiltrate can be quantified from 1 to 100 ng/mL. Validation results demonstrate that vincristine and actinomycin-D can be accurately and precisely quantified in human plasma and plasma ultrafiltrate with the presented methods. The assays are now in use to support clinical pharmacological studies in children treated with vincristine and actinomycin-D.

Chemiluminescence detection coupled to high-performance frontal analysis for the determination of unbound concentrations of drugs in protein binding equilibrium

High-performance frontal analysis coupled with chemiluminescence detection (HPFA-CL) was developed for the determination of unbound oxacillin concentration in human serum albumin solution. The HPFA system consisted of an ISRP column and a mobile phase of 67 mM potassium phosphate buffer of pH 7.4 and ionic strength of 0.17. The luminol-H2O2-Co2+ system was used in the chemiluminescence detection. An enhancement of luminol chemiluminescence by oxacillin was investigated and employed for determining the concentration of oxacillin in the HPFA eluate. Sample solutions were directly injected onto the column; the drug was eluted as a zonal peak with a plateau region. The unbound drug concentrations were determined by using the height of the plateau. The results agreed with those obtained with conventional ultrafiltration-HPLC method. Good reproducibility was confirmed by the within run and between run RSD < or = 7.4%. HPFA-CL provided a selective method for determination of unbound drug concentration in protein binding equilibrium.

Determination of nelfinavir free drug concentrations in plasma by equilibrium dialysis and liquid chromatography/tandem mass spectrometry: important factors for method optimization

A method was developed and validated for measuring the free fraction of nelfinavir in plasma employing equilibrium dialysis for the separation of free (unbound) drug and liquid chromatography/tandem mass spectrometry for quantitation. Nelfinavir, widely used to treat HIV infection, is a highly bound HIV protease inhibitor with the fraction bound in plasma being greater than 98%. Thus variations in the free fraction may be clinically important when interpreting total drug concentrations. Optimization of the method was carried out considering the influence of sample matrix and physicochemical and absorptive properties of nelfinavir. Nelfinavir free fraction averaged 0.41 +/- 0.094, 0.43 +/- 0.087 and 0.41 +/- 0.063% at nelfinavir plasma concentrations of 1000, 2000 and 3000 ng/ml, respectively. Free nelfinavir concentrations were underestimated with this assay by approximately 25% because of unavoidable losses to adsorption. However, the adsorptive loss was reproducible and consistent across the concentration range of the assay. Within-day and between-day precisions ranged from 6.0 to 9.4% and 15.2 to 27.3%, respectively. The lower limit of quantitation of the unbound concentration of nelfinavir was 1.0 ng/ml, permitting analysis of samples with total concentrations of nelfinavir in plasma that are > or = 400 ng/ml. This developed method proves reproducible and sensitive and its application to patient plasma samples is also reported.

Simultaneous determination of total and free drug plasma concentrations combined with batch-wise pH-adjustment for the free concentration determinations

Batch-wise pH-adjustment of plasma samples for free concentration determinations can be performed directly in the ultrafiltration devices using a CO(2) incubator. The pH of the samples could be adjusted to within a sufficiently narrow interval though the initial differences in pH were large. The possibilities for simultaneous determination of free and total concentrations were studied using NAD-299 as a model compound. By adding a known concentration of a (13)C-labelled isotope of the studied drug to the sample prior to ultrafiltration it was possible to calculate the total concentration from the ratio of the drug peak area to the isotope peak area while the free concentration was calculated from the drug peak area. Initial experiments showed good precision and accuracy as well as a good correlation with concentration data acquired in the conventional way.

Determination of unbound cefamandole in rat blood by microdialysis and microbore liquid chromatography

To analyze unbound cefamandole in rat blood, a method combing microdialysis with microbore liquid chromatography has been developed. A microdialysis probe was inserted into the jugular vein/right atrium of male Sprague-Dawley rats to examine the unbound cefamandole level in the rat blood following cefamandole administration (50 mg/kg, i.v.). The dialysates were directly submitted to a liquid chromatographic system. Samples were eluted with a mobile phase containing acetonitrile-methanol-100 mM monosodium phosphate (pH 5.0; 15:20:65, v/v). The UV wavelength was set at 270 nm for monitoring the analyte. Using the retrograde method, at infusion concentrations of 1 microg/mL of cefamandole, the in vivo microdialysis recoveries were 55.44% for the rat blood (n = 6). Intra- and inter-assay accuracy and precision of the analyses were < or = 10% in the range of 0.1-10 microg/mL. Pharmacokinetic parameters were calculated from the recovery-corrected dialysate concentrations of cefamandole vs time data. The elimination half-life (t1/2,beta) was 21.6 +/- 1.6 min. The results suggest that the pharmacokinetics of unbound cefamandole in blood following cefamandole administration (50 mg/kg, i.v., n = 5) fit best to the two-compartmental model.