The measurement of vitamins D2 and D3 and seven major metabolites in a single sample of human plasma using gas chromatography/mass spectrometry

Vitamin D metabolites and analytical challenges

Vitamin D is an essential micronutrient for bone health and the general cellular functions of the body. Its insufficiency/deficiency leads to the pathophysiology of disorders like diabetes, cancer, autoimmune, neurodegenerative, and cardiovascular diseases. Clinical interest in Vitamin D metabolites and their role in various medical disorders have contributed to an increase in laboratory demands for vitamin D measurements. For clinical and research laboratories worldwide, analysis of vitamin D and associated metabolites is a significant problem. The best way for determining vitamin D levels is constantly being debated. Various methods such as immunoassays and chromatographic techniques are available for determining vitamin D levels. Additionally, biosensors have recently been considered promising options for routine vitamin D analysis. The existing methods and other developments in the measurement of vitamin D metabolites and associated analytical challenges are discussed in this review.

Assessing vitamin D status: pitfalls for the unwary

The use of vitamin D testing has grown rapidly in the recent times as a result of increased interest in the role of vitamin D in health. Although the generally accepted measure of vitamin D status is circulating 25(OH)D concentration, there is little consensus on which assay method should be used. Commonly used assays include competitive protein-binding assay, RIA, enzyme immunoassay, chemiluminescence immunoassays, HPLC, and LC-MS/MS, each with its own advantages and disadvantages. However, there is significant interassay and interlaboratory variability in measurements. Our simulation of the published data showed that using a deficiency cut-point of 50 nmol/L, 57% of samples assessed using a chemiluminescence immunoassay were classified as deficient compared with 41% of samples assessed using LC-MS/MS; a 20% misclassification rate. Similar rates of misclassification were seen at 75 nmol/L. This has implications for clinical practice and decision limits for vitamin D supplementation, suggesting that cut-points should be assay specific rather than universal and that greater harmonization between laboratories is required. Newer assays using alternative biological samples to determine the circulating 25(OH)D have been proposed and advances in the genetics of vitamin D and the role of vitamin D-binding protein may improve future assay accuracy.

Degradation chemistry of a Vitamin D analogue (ecalcidene) investigated by HPLC–MS, HPLC–NMR and chemical derivatization

Ecalcidene (1-[(1alpha,3beta,5Z,7E,20S)-1,3-dihydroxy-24-oxo-9,10-secochola-5,7,10(19)-trien-24-yl]-piperidine) is a new 1-hydroxyvitamin D analogue. In this report, the thermal degradation, acid induced degradation and iodine induced degradation of ecalcidene were investigated using HPLC-MS, HPLC-NMR and chemical derivatization. In solution ecalcidene was thermally and reversibly transformed to a pre-Vitamin D type isomer 1 which subsequently produced the dehydrated pyrocalciferol and isopyrocalciferol type isomers 2 and 3 by cyclization and dehydration at elevated temperatures. Acidic conditions resulted in the formation of a novel C9-hydroxylated isomer 4 of ecalcidene, possibly via a tachysterol type intermediate, followed by the acid facilitated nucleophilic addition of water. In the presence of iodine, cis/trans isomerization of both ecalcidene and its pre-Vitamin D type isomer 1 occurred. The results may shed light on the stability and metabolism of ecalcidene, provide useful information for its potential pharmaceutical development, and enrich the knowledge of Vitamin D chemistry.

Review: derivatization in mass spectrometry-6. Formation of mixed derivatives of polyfunctional compounds

The review describes chemical transformations of multifunctional compounds (amino acids and peptides, amino alcohols, amino thiols, hydroxy acids, oxo acids, oxo alcohols, compounds containing simultaneously three or more different groups etc.) by using step-wise or one-step modification or protection of functional groups. Some chemical aspects of mixed derivatization performed for improving the physical-chemical properties and mass spectral characteristics are discussed. Application of mixed derivatization to qualitative and quantitative analysis of various multifunctional compounds mainly in biological fluids and other matrices by gas chromatography/mass spectrometry in electron ionization, chemical ionization, negative-ion chemical ionization and selected ion monitoring modes is considered.

Review: Derivatization in mass spectrometry--4. Formation of cyclic derivatives

This fourth in a series of reviews describes a further common derivatization approach, namely, the formation of cyclic derivatives (cyclic acetals and ketals, boronates, siliconides, carbonates and other miscellaneous derivatives) that can be used to increase volatility and to improve chromatographic and, if possible, the mass spectral properties of various di- and polyfunctional compounds. Some chemical aspects of this type of derivatization are briefly discussed. Characteristic mass spectral features of various cyclic derivatives that are helpful in the structure determination, profiling and quantitation of multifunctional organic compounds are presented. Some recent analytical applications of mass spectrometry in conjunction with preliminary cyclic derivative formation are given.

Rapid and sensitive high-performance liquid chromatographic method for simultaneous determination of retinol, alpha-tocopherol, 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2 in human plasma with photodiode-array ultraviolet detection

A new rapid and sensitive high-performance liquid chromatographic method using 0.5 ml of plasma has been developed for the simultaneous determination of retinol (vitamin A), alpha-tocopherol (vitamin E), 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3. The eluate was monitored with a photodiode-array detector with two fixed wavelengths (267 nm for vitamin D, 292 nm for alpha-tocopherol and retinol). For all compounds, including internal standards, the method provides extraction recoveries greater than 81%. Detection limits were equal to or lower than 1.5 microg/l for the 4 vitamins. Linearity of standards was excellent (r>0.999 in all cases). Intra-day and inter-day precision were generally acceptable; the intra-dayassay C.V. was 3/4 7.7 for all compounds and the inter-day-assay C.V. was <9.2% except for the lower concentrations of 25-hydroxyvitamin D3, 25-hydroxyvitamin D2 and alpha-tocopherol (10.8, 11.8 and 11.9, respectively). The important properties of the present method are its ease of use, its rapidity, since sample preparation was achieved in 15 min and all the compounds were eluted in less than 15 min, and its small sample volume required (=0.5 ml), which enables it to be used in pediatric practice.

Continuous cleanup/preconcentration procedure of hydroxyvitamin D3 metabolites in plasma as an alternative to batch solid-phase extraction

A continuous automatable cleanup procedure coupled on-line with a liquid chromatograph and UV detector for hydroxyvitamin D3 metabolites [24,25-(OH)2, 1,25-(OH)2 and 25-(OH)] as an alternative to batch solid-phase extraction is reported. The method, based on continuous solid-phase cleanup/preconcentration of the analytes, requires only the passage of the sample through a single minicolumn, which also results in a preconcentration effect which increases the sensitivity. The proposed method is also compared with a conventional batch, two-step solid-phase extraction method previously improved by the authors. The method has been checked by applying it to plasma samples spiked with the target analytes (linear range between 0.05 and 100 ng/ml with coefficient of variation values lower than 6.5%) and acceptable recoveries ranging between 94.6 and 101% have been obtained. The sampling frequency was 4 h-1.

The use of octadecyl-bonded microparticulate silica in the separation of free and bound fractions during saturation analysis of vitamin D metabolites

The use of octadecyl-bonded microparticulate silica to separate free and bound fractions during the saturation analysis of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D has been investigated. A slurry of octadecyl-bonded silica in an appropriate incubation buffer was prepared and used in parallel with a conventional dextran-coated charcoal suspension in several assay procedures. Standard curves, non-specific binding and plasma values were compared. A competitive protein binding assay for 25-hydroxyvitamin D and two radioreceptor assays and one radioimmunoassay for 1,25-dihydroxyvitamin D were investigated. In most cases the octadecyl-bonded silica preparation gave the more favourable results; its action was rapid, time- and temperature-independent, and it produced low non-specific binding and higher B0 values in all the assays examined. It was in our hands easier to use than dextran-coated charcoal. The use of octadecyl-bonded silica is recommended as an efficient agent for the separation of free and bound fractions in the saturation analysis of vitamin D metabolites.

Stable isotope-labeled vitamin D, metabolites and chemical analogs: synthesis and use in mass spectrometric studies

Methods for the measurement of vitamin D and its metabolites using stable isotope-labeled internal standards and mass spectrometry are reviewed. The synthesis of both labeled and unlabeled standards is illustrated, and details of the synthesis of (26,26,27,27,27(-2)H5)-25,26-dihydroxyvitamin D3 and (28,28,28(-2)H3)-24,25-dihydroxyvitamin D2 are given. The use of in vitro biologic systems for the production of further metabolites of deuterated 25-hydroxyvitamin D3 is discussed. Use of deuterated 25-hydroxydihydrotachysterol3 as a substrate in the isolated perfused rat kidney has provided valuable data for the assignment of structure to a number of metabolites of 25-hydroxydihydrotachysterol3 formed in this system.