Therapeutic monitoring of free (unbound) drug levels: Analytical aspects

Stable SERS substrate based on highly reflective metal liquid-like films wrapped hydrogels for direct determination of small molecules in a high protein matrix

The study of the interaction between small molecules and proteins is important. Surface-enhanced Raman spectroscopy (SERS) is suitable for such applications since it has the power of detecting a molecule based on its intrinsic nature and without labeling. Herein, the MeLLFs@PAAG SERS substrate supporting highly reflective metal liquid-like films (MeLLFs) with polyacrylamide hydrogels (PAAG) has high-density "hot spots" to provide excellent SERS activity. The MeLLFs@PAAG formed by AgNPs only has less than 15% SERS activity loss when stored in the air for more than three weeks. By using rhodamine 6G (R6G) as a model analyte, the AgNPs based MeLLFs@PAAG SERS substrate exhibits an enhancement factor (EF) as high as 8.0 × 10⁶, a limit of detection (LOD) of 76.8 pM (S/N = 3). Also, the formed PAAG provided a 3D molecular network to orderly secure the assembled nanoparticles (NPs), which not only improves the stability of NPs but also shields the Raman signal of proteins as high as 45 g/L allowing the direct determination of the binding rate of human serum albumin (HSA) and doxorubicin (DOX). A binding rate of about 70% was detected, which is consistent with previous reports. Thus, proposed the MeLLFs@PAAG SERS substrate can be used as a promising candidate for SERS measurement in complex biological samples.

A new application of micellar liquid chromatography in the determination of free ampicillin concentration in the drug-human serum albumin standard solution in comparison with the adsorption method

The determination of free drug concentration is a very important issue in the field of pharmacology because only the unbound drug fraction can achieve a pharmacological effect. Due to the ability to solubilize many different compounds in micellar aggregates, micellar liquid chromatography (MLC) can be used for direct determination of free drug concentration. Proteins are not retained on the stationary phase probably due to the formation of protein - surfactant complexes which are excluded from the pores of stationary phase. The micellar method is simple and fast. It does not require any pre-preparation of the tested samples for analysis. The main aim of this paper is to demonstrate a completely new applicability of the analytical use of MLC concerning the determination of free drug concentration in the standard solution of human serum albumin. The well-known adsorption method using RP-HPLC and the spectrophotometric technique was applied as the reference method. The results show that the free drug concentration value obtained in the MLC system (based on the RP-8 stationary phase and CTAB) is similar to that obtained by the adsorption method: both RP-HPLC (95.83μgmL(-1), 79.86% of free form) and spectrophotometry (95.71μgmL(-1), 79.76%). In the MLC the free drug concentration was 93.98μgmL(-1) (78.3%). This indicates that the obtained results are within the analytical range of % of free ampicillin fraction and the MLC with direct sample injection can be treated like a promising method for the determination of free drug concentration.

Influence of intralipid on free propofol fraction assayed in human serum albumin solutions and human plasma

AIM

It is generally assumed that only unbound drugs can reach the site of action by diffusing across the membranes and exerting pharmacological effects by interacting with receptors. Recent research has shown that the percentage of free drugs may depend on the total drug concentration. The aim of the paper is to verify whether the mentioned dependence reported for propofol also takes place in plasma and human serum albumin samples in the presence of intralipid-the medium used as a vehicle for propofol infusions and a parenteral nutrition agent.

METHODS

Artificial plasma samples and human plasma were spiked with intralipid or ethanolic solutions of propofol. The samples were then assayed for free propofol concentration using ultrafiltration and high performance liquid chromatography with fluorimetric detection.

RESULTS

The decrease of the total drug concentration results in free propofol fraction increase, irrespectively of the used type of propofol solvent and sample type. The addition of intralipid causes the lowering of the overall free drug fraction with respect to the samples spiked with ethanolic solutions of the drug.

CONCLUSION

The presence of intralipid does not influence the phenomenon of free propofol fraction rise at low total drug concentration. Such a rise cannot be ignored in clinical conditions when the drug is applied for sedative, antiemetic or other low-dosage purposes.

Application of adsorption method to the chromatographic analysis of free drug concentration

The development of analytical techniques for estimation of free drug concentration is crucial for the needs of modern pharmacology. Up to now, many different methods of free drug assay have been used. The methods involving isolation of free drug from a sample are most frequently applied because they can be easily adopted for the processing of real samples. Among the methods that do not require free drug isolation only the adsorption method could be promising in application to samples of this kind. The successful use of the adsorption method is possible only if two requirements are fulfilled: (I) the chosen adsorbent does not bind proteins from the sample and (II) the processes of adsorption of the assayed drug on the adsorbent used and on the protein should be independent. This paper discusses the possibilities of application of the adsorption method for determination of free drug concentration using propofol as the model drug. The presented results show that the fulfillment of the above conditions may be very difficult if not impossible not only for propofol, but for other drugs as well.

Influence of propofol concentration in human plasma on free fraction of the drug

Drugs exist in blood in two forms: free and bound to proteins and blood cells. It is generally assumed that only the unbound form of a drug exerts pharmacological activity as it is able to diffuse across the membranes and reach the site of action. Since for the majority of drugs their free fraction is usually constant, the therapeutic effect of the drug is most often correlated with its total concentration. However, in case of some disease states (e.g. renal or hepatic disorders) the protein concentration may change dramatically, resulting in clinically significant change of free drug fraction. The results presented in the paper prove that, in case of propofol, an increase of free fraction occurs with a decrease of total drug concentration. This dependence is observed both in vitro (in artificial and native human plasma) and in vivo. Free propofol fraction, which in clinical conditions ranges from 1 to 3%, at very low total propofol concentrations (below 0.01 microgml(-1)) tends to reach 100%. This increase of free drug percentage is discussed in terms of its possible reasons as well as its potential clinical relevance.

Free and bound propofol concentrations in human cerebrospinal fluid

AIMS

The aim of this study was to define the relationship between unbound propofol concentrations in plasma and total drug concentrations in human cerebrospinal fluid (CSF), and to determine whether propofol exists in the CSF in bound form.

METHODS

Forty-three patients (divided into three groups) scheduled for elective intracranial procedures and anaesthetized by propofol target control infusion (TCI) were studied. Blood and CSF samples (taken from the radial artery, and the intraventricular drainage, respectively) from group I (17 patients) were used to investigate the relationship between unbound propofol concentration in plasma and total concentration of the drug in CSF. CSF samples taken from group II (18 patients) were used to confirm the presence of the bound form of propofol in this fluid. The CSF and blood samples taken from group III (eight patients) were used to monitor the course of free and bound CSF propofol concentrations during anaesthesia.

RESULTS

For group I patients the mean (and 95% confidence interval) total plasma propofol concentration was 6113 (4971, 7255) ng ml(-1), the mean free propofol concentration in plasma was 63 (42, 84) ng ml(-1), and the mean total propofol concentration in CSF was 96 (76, 116) ng ml(-1) (P < 0.05 for the difference between the last two values). For group II patients the fraction of free propofol in CSF was 31 (26, 37)%. For group III patients the fraction of free propofol in CSF during TCI was almost constant (about 36%).

CONCLUSIONS

The unbound propofol concentration in plasma was not equal to its total concentration in CSF and cannot be directly related to the drug concentration in the brain. Binding of propofol to components of the CSF may be an additional mechanism regulating the transport of the drug from blood into CSF.

HPLC investigation of free and bound propofol in human plasma and cerebrospinal ?uid

The paper compares the total propofol concentration in the cerebrospinal fluid (CSF) with the free drug concentration in plasma measured in 35 humans scheduled for elective neurosurgical procedures during propofol anaesthesia. The concentrations of total and free propofol in the blood and CSF samples were measured by means of HPLC using liquid-liquid extraction and ultrafiltration in the sample preparation procedure. The arterial blood and CSF samples (collected from intraventricular drainage) were taken at the same time. According to the obtained results, the usually expected equality between free drug concentration in plasma and its total concentration in CSF is not valid for propofol: the unbound propofol concentration in plasma is not equal to its total concentration in CSF (p < 0.05). This difference suggests a substantial contribution of active transport in propofol transfer from blood into CSF. Moreover, the paper shows the presence of bound propofol in CSF, which is a novel finding.

The validation criteria for analytical methods used in pharmacy practice research

An integral part of analytical method development is validation, i.e. once the method has been devised it is necessary to evaluate it under the conditions expected for real samples before being used for a specific purpose. Although the validation stage is crucial in method development, the importance of this step is often overlooked. This paper attempts to clarify the nomenclature of method validation and describes the validation procedure for analytical methods used for the determination of drugs in biological fluids and formulations.

Preanalytical considerations in drug assays in clinical pharmacokinetic studies

Many hospital pharmacy laboratories undertake drug analysis in biological fluids for the production of pharmacokinetic data. The success of such an undertaking very much depends on the selection of a suitable analytical method and a proper approach to sample collection and handling. This paper surveys the main types of biological specimens taken from the patients for pharmacokinetic drug analysis and discusses factors that affect them during or subsequent to their removal. Guidelines are provided in specimen handling and dealing with many problems which could arise prior to actual analysis. By its very nature this paper brings in many disciplines, the full details of which are well beyond its scope, however, some discussion on pharmacokinetic and bioavailability methods in relation to sampling procedures is included to put the matter into a proper perspective.