Issues in evaluation of bioanalytical method selectivity and drug stability.

Evaluation of selectivity is one of the most controversial aspects of method validation and application of methods to studies. The focus of selectivity testing should address the question: Above what level will interference significantly impact on study conclusions? Four key issues will be addressed: the statistical relevance of any selectivity test performed; a criterion for significant interference; experimental methods to establish selectivity; and criteria for acceptance. To ensure that compound integrity is maintained throughout the work-up process, statistically meaningful methods of stability evaluation which are associated with specific acceptance criteria are required. Suitable methods for evaluating stability of analyte and/or solutions of analyte, in process stability, processed sample stability, long term stability and freeze-thaw stability, as well as meaningful acceptance criteria, are presented.

Application issues in bioanalytical method validation, sample analysis and data reporting.

Although some degree of consensus has been reached concerning the requirements for acceptable method validation, the procedures used to establish them vary significantly between laboratories. Also, issues arising from application of these requirements during validation and subsequent sample analysis need to be addressed. The purpose of this paper is to discuss application issues concerning prerequisites to method validation, and all validation criteria for evaluation of method reliability and overall performance. Other poorly addressed issues such as re-validation, cross-validation, partial sample volume, multicomponent analysis and reporting will also be discussed. Although many issues discussed are of a general nature, the scope of this presentation is primarily to address issues arising from the validation and routine application of chromatographic methods.

High-performance liquid chromatographic method for the determination of indapamide in human whole blood.

A sensitive, accurate, and reproducible high-performance liquid chromatographic procedure for the analysis of indapamide in human whole blood is reported. After a single-step liquid-liquid extraction at pH 6.6 using diethyl ether, indapamide was eluted from a Nucleosil C18 5-microns column with 80 mM ammonium acetate, pH 3.5-acetonitrile-2-propanol (65:30:5, v/v/v). The peak height versus whole blood concentration was linear over the range 10.0-500 ng/ml using ultraviolet detection. The mean absolute recovery of indapamide using the described assay was 87.4%. The inter- and intra-day accuracy and precision were within 9.6% of the actual values for all concentrations investigated. Furthermore, this procedure was applied to the analysis of whole blood samples from healthy subjects receiving a single 2.5-mg oral dose of indapamide.

Determination of amphetamine, methamphetamine and desmethyldeprenyl in human plasma by gas chromatography/negative ion chemical ionization mass spectrometry.

A sensitive and specific assay for the quantitative determination of amphetamine, methamphetamine and desmethyldeprenyl in human plasma specimens is described. Electron capture/negative ion chemical ionization gas chromatography/mass spectrometry is used to determine the extracted plasma concentrations of the three target compounds as their N-heptafluorobutyryl derivatives. Quantitation is performed by stable isotope dilution using d6-amphetamine and d6-methamphetamine as internal standards. Selected ion monitoring of the [M-HF]- ions of both the analytes and internal standards results in minimum quantifiable limits of 0.10 ng ml-1 for both amphetamine and methamphetamine and 0.25 ng ml-1 for desmethyldeprenyl. Excellent linearity (r = 0.998) up to at least 5.00 ng ml-1 is demonstrated.

Determination of selenium in blood plasma and serum by flow injection hydride generation atomic absorption spectrometry.

A flow injection hydride generation atomic absorption spectrometric (AAS) method has been used to determine the selenium concentrations of human serum and plasma samples following digestion with nitric, sulphuric and perchloric acids. In the hydride generation process, reduction was carried out by sodium tetrahydroborate to produce a hydride that was atomized in a flame-heated atomisation cell. The method had a detection limit of 1.2 ng ml-1 and a sensitivity of 2.1 ng ml-1. Within-run precisions of 5.8% at 20 ng ml-1 and 4.5% at 80 ng ml-1, and between-run precisions of 4.8% at 69 ng ml-1 and 3.4% at 80 ng ml-1 were obtained. An inter-laboratory comparison study with a graphite furnace AAS method was carried out and the results showed excellent agreement. The flow injection method of sample introduction allowed the use of a sample volume of 330 microliters with an injection rate of 90 injections per hour.

Retention characteristics in high-performance liquid chromatography of basic drugs and plasma extracts on an alumina column.

The retention characteristics of 35 drugs on an alumina column using buffered aqueous methanolic mobile phases have been investigated. It has been shown that the effect of pH on retention depends on the pKa of the drug and the type of buffer ion used; that there is an inverse relation between ionic strength and solute retention; and that a decrease in the amount of methanol in the mobile phase causes the drugs to be held longer on the column. The chromatographic system was also coupled to an on-line column-switching assembly to facilitate extraction of the drugs from plasma, and results are presented which indicate that the alumina column lends itself well to this convenient method of plasma clean-up.

Applications of column-switching technique in biopharmaceutical analysis. I. High-performance liquid chromatographic determination of amitriptyline and its metabolites in human plasma.

A new high-performance liquid chromatographic method for the determination of amitriptyline and its metabolites, nortriptyline, 10-hydroxynortriptyline and 10-hydroxyamitriptyline, in plasma is described which uses direct injection and a column-switching valve. The method is based on the enrichment of drugs on a reversed-phase concentration column, packed with Corasil RP. The enriched drugs were then separated, using back-flush mode on a bonded-phase CN column using an isocratic acetonitrile-acetate buffer (60:40, v/v) mobile phase. The validation of the method showed excellent sensitivity, precision and reproducibility. The limit of detection, using a 250-microliter direct injection of plasma, was between 5 and 10 ng/ml for each of the four drugs. The mean coefficient of variation for intra- and inter-assay was better than 5%. The method showed obvious advantages over conventional extraction procedures in terms of speed and ease of sample handling. The method has been successfully applied to the samples from patients receiving oral doses of amitriptyline.

High-performance liquid chromatographic determination of certain salicylates and their major metabolites in plasma following topical administration of a liniment to healthy subjects.

The liniment used is a topical analgesic and anti-inflammatory preparation containing two active constituents, 3-phenylpropylsalicylate and ethyl-5-methoxysalicylate, in solution in isobutyl decanoate. It is known that 3-phenylpropylsalicylate is metabolised to salicylic acid and salicyluric acid and ethyl-5-methoxysalicylate is metabolised to 5-methoxysalicylic acid and gentisic acid. In the present study the separation of the salicylates and their metabolites was carried out on a Waters mu Bondapak C18 column using two different mobile phases, methanol-water (80:20) for the parent drugs and methanol-5% aqueous acetic acid (27:73) for their metabolites. The salicylates and their metabolites were detected by absorption at 310 nm. The limits of detection for parent drugs and metabolites were respectively 0.2 and 0.1 microgram/ml in plasma, using a 1-ml plasma sample and a 20-microliter injection from a reconstituted volume of 250 microliter. Mean percentage coefficients of variation for intra-assay and inter-assay precision were between 3.3 +/- 1.9% to 9.1 +/- 3.7% and 6.8 +/- 2.2% to 15.7 +/- 10.1%, respectively. Linearity, as measured by the correlation coefficient of intra-assay linear regression curves, was better than 0.998 in all cases.

Gas chromatographic determination of mefloquine in human and dog plasma using electron-capture detection.

A sensitive and selective gas-liquid chromatographic method for the determination of plasma levels of mefloquine in human and dog plasma is described. The drug and internal standard were extracted from plasma at pH 9.0 into isopropyl acetate. After evaporation of the solvent, the residue was taken up in toluene and derivatised with heptafluorobutyrylimidazole. The derivative was quantified by gas-liquid chromatography on a 3% GC GE-SE30 column with electron-capture detection. The limit of detection for mefloquine in plasma was 10 ng/ml. The mean overall recovery from plasma was 102.7 +/- 3.3%. The method was shown to be specific for mefloquine without any interference from endogenous compounds in plasma or from the drugs pyrimethamine and sulfadoxine (compounds often administered in combination with mefloquine). The assay described was successfully applied to the determination of plasma levels of mefloquine in man and dog following oral and intravenous administration, respectively.