Comparative evaluation of new Cookson-type reagents for LC/ESI-MS/MS assay of 25-hydroxyvitamin D3 in neonatal blood samples

Accurate Clinical Detection of Vitamin D by Mass Spectrometry: A Review

Vitamin D deficiency is thought to be associated with a wide range of diseases, including diabetes, cancer, depression, neurodegenerative diseases, and cardiovascular and cerebrovascular diseases. This vitamin D deficiency is a global epidemic affecting both developing and developed countries and therefore qualitative and quantitative analysis of vitamin D in a clinical context is essential. Mass spectrometry has played an increasingly important role in the clinical analysis of vitamin D because of its accuracy, sensitivity, specificity, and the ability to detect multiple substances at the same time. Despite their many advantages, mass spectrometry-based methods are not without analytical challenges. Front-end and back-end challenges such as protein precipitation, analyte extraction, derivatization, mass spectrometer functionality, must be carefully considered to provide accurate and robust analysis of vitamin D through a well-designed approach with continuous control by internal and external quality control. Therefore, the aim of this review is to provide a comprehensive overview of the development of mass spectrometry methods for vitamin D accurate analysis, including emphasis on status markers, deleterious effects of biological matrices, derivatization reactions, effects of ionization sources, contribution of epimers, standardization of assays between laboratories.

Determination of vitamin D metabolites in various biological samples through an improved chemical derivatization assisted liquid chromatography-tandem mass spectrometry approach

Vitamin D (VD) metabolites are involved in a variety of important metabolic processes and physiological effects in organisms. Profiling of VD metabolites favors a deep understanding of the physiological role of VD. However, VD metabolites are difficult to detect due to their high chemical structural rigidity, structural similarity, and low sensitivities under liquid chromatography-tandem mass spectrometry (LC-MS). Herein, we present a chemical derivatization assisted LC-MS/MS strategy for the detection of VDs, in which 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) is employed to derivatize the conjugated diene of VD metabolites and provides sensitizing reporters for MS detection. After PTAD derivatization, the sensitivities of seven VD metabolites increased by 24-276 folds, with the limits of detection ranging from 3 to 20 pg mL-1. Using this method, we achieved a sensitive and accurate quantification of 7 VD metabolites (vitamin D2, vitamin D3, 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, 1,25-dihydroxyvitamin D2, 1,25-dihydroxyvitamin D3, and 1,24,25-trihydroxyvitamin D3) of the VD metabolic pathway in different trace biological samples, including human serum, mouse tissues (namely liver, kidney, lung, and spleen), and cells. We believe that the present method can provide a promising tool for an in-depth analysis of VD metabolism.

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.

Analysis of vitamin D metabolic markers by mass spectrometry: Recent progress regarding the "gold standard" method and integration into clinical practice

Liquid chromatography/tandem mass spectrometry is firmly established today as the gold standard technique for analysis of vitamin D, both for vitamin D status assessments as well as for measuring complex and intricate vitamin D metabolic fingerprints. While the actual mass spectrometry technology has seen only incremental performance increases in recent years, there have been major, very impactful changes in the front- and back-end of MS-based vitamin D assays; for example, the extension to new types of biological sample matrices analyzed for an increasing number of different vitamin D metabolites, novel sample preparation techniques, new powerful chemical derivatization reagents, as well the continued integration of high resolution mass spectrometers into clinical laboratories, replacing established triple-quadrupole instruments. At the same time, the sustainability of mass spectrometry operation in the vitamin D field is now firmly established through proven analytical harmonization and standardization programs. The present review summarizes the most important of these recent developments.

A novel caged Cookson-type reagent toward a practical vitamin D derivatization method for mass spectrometric analyses

RATIONALE

25-Hydroxylated vitamin D is the best marker for vitamin D (VD). Due to its low ionization efficiency, a Cookson-type reagent, 1,2,4-triazoline-3,5-dione (TAD), is used to improve the detection/quantification of VD metabolites by liquid chromatography/tandem mass spectrometry (LC/MS/MS). However, the high reactivity of TAD makes its solution stability low and inconvenient for practical use. We here describe the development of a novel caged Cookson-type reagent, and we assess its performances in the quantitative and differential detection of four VD metabolites in serum using LC/MS/MS.

METHODS

Caged 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD) analogues were prepared from 4-(4'-dimethylaminophenyl)-1,2,4-triazolidine-3,5-dione. Their stability and reactivity were examined. The optimized caged DAPTAD (14-(4-(dimethylamino)phenyl)-9-phenyl-9,10-dihydro-9,10-[1,2]epitriazoloanthracene-13,15-dione, DAP-PA) was used for LC/MS/MS analyses of VD metabolites.

RESULTS

The solution stability of DAP-PA in ethyl acetate dramatically improved compared with that of the non-caged one. We measured the thermal retro-Diels-Alder reaction enabling the release of DAPTAD and found that the derivatization reaction was temperature-dependent. We also determined the detection limit and the lower limit of quantifications for four VD metabolites with DAPTAD derivatization.

CONCLUSIONS

DAP-PA was stable enough for mid- to long-term storage in solution. This advantage shall contribute to the detection and quantification of VD in clinical laboratories, and as such to the broader use of clinical mass spectrometry.

Quantification of the 3α and 3β epimers of 25-hydroxyvitamin D3 in dried blood spots by LC-MS/MS using artificial whole blood calibration and chemical derivatization

While the biological function of the 3α epimer of 25-hydroxyvitamin D₃ (25(OH)D₃) remains unknown, its presence needs to be accurately captured and separated from the main 3β epimer, to avoid positive bias in vitamin D status analyses. Several recent LC-MS/MS assays for 25(OH)D₃ successfully separate the 3α and 3β epimers by chromatography. Unfortunately, none of the existing LC-MS/MS assays, which utilize dried blood spots (DBS) as sampling/storage vessels, is able to quantify the individual epimers. DBS are often used for analysis of infant blood, however, and these samples are particularly likely to contain significant levels of interfering 3α epimer. Furthermore, proper calibration of DBS samples is much more difficult to achieve than for liquid serum or plasma samples. We addressed this important issue by creating an artificial vitamin D-free whole blood for calibration and then quantified 3α- and 3β-25(OH)D₃ levels from DBS. After chemical derivatization, the vitamin D epimers were separated on a PFP column and concentrations determined by electrospray ionization LC-MS/MS on a triple quadrupole mass spectrometer. Calibration with artificial whole blood showed improved precision over standard addition (7.6 versus 31.5% RSD for 3β-25(OH)D₃). The limits of quantification for 3β-25(OH)D₃ and for 3α-25(OH)D₃ were 1.0 and 0.1ng/mL, respectively. Excellent intra/interday precisions between 2.1 and 2.2% CV (intra) and 4.4-5.3% CV (inter) were established for 3β-25(OH)D₃ and 3α-25(OH)D₃. For 3β-25(OH)D₃, only small concentration-independent bias and deviation of <3.3ng/mL were seen between serum LC-MS/MS and DBS-LC-MS/MS measurements; analyses of 3α-25(OH)D₃ showed deviations of <0.8ng/mL in all experiments.

Enhancing analysis throughput, sensitivity and specificity in LC/ESI-MS/MS assay of plasma 25-hydroxyvitamin D3 by derivatization with triplex 4-(4-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD) isotopologues

The plasma/serum concentration of 25-hydroxyvitamin D₃ [25(OH)D₃] is a diagnostic index for vitamin D deficiency/insufficiency, which is associated with a wide range of diseases, such as rickets, cancer and diabetes. We have reported that the derivatization with 4-(4-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD) works well in the liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) assay of the serum/plasma 25(OH)D₃ for enhancing the sensitivity and the separation from a potent interfering metabolite, 3-epi-25-hydroxyvitamin D₃ [3-epi-25(OH)D₃]. However, enhancing the analysis throughput remains an issue in the LC/ESI-MS/MS assay of 25(OH)D₃. The most obvious restriction of the LC/MS/MS throughput is the chromatographic run time. In this study, we developed an enhanced throughput method for the determination of the plasma 25(OH)D₃ by LC/ESI-MS/MS combined with the derivatization using the triplex (²H₀-, ²H₃- and ²H₆-) DAPTAD isotopologues. After separate derivatization with 1 of 3 different isotopologues, the 3 samples were combined and injected together into LC/ESI-MS/MS. Based on the mass differences between the isotopologues, the derivatized 25(OH)D₃ in the 3 different samples were quantified within a single run. The developed method tripled the hourly analysis throughput without sacrificing assay performance, i.e., ease of pretreatment of plasma sample (only deproteinization), limit of quantification (1.0ng/mL when a 5μL-plasma was used), precision (intra-assay RSD≤5.9% and inter-assay RSD≤5.5%), accuracy (98.7-102.2%), matrix effects, and capability of separating from an interfering metabolite, 3-epi-25(OH)D₃. The multiplexing of samples by the isotopologue derivatization was applied to the analysis of plasma samples of healthy subjects and the developed method was proven to have a satisfactory applicability.