Characterization of urinary metabolites of vitamin D(3) in man under physiological conditions using liquid chromatography-tandem mass spectrometry

Advances and challenges in the measurement of 1,25-dihydroxyvitamin D: a comprehensive review

Vitamin D has received significant attention from clinical societies, researchers, and the general population in recent years. While 25-hydroxyvitamin D (25(OH)D) is the most commonly-used biomarker of vitamin D status, 1α,25-dihydroxyvitamin D (1,25(OH)2D), its bioactive form, plays a critical role in regulating calcium and phosphorus homeostasis and is also involved in the immune system and cellular differentiation. Consequently, accurate measurements of 1,25(OH)2D can aid in the differential diagnosis of calcium-related disorders such as hypocalcemia in vitamin D-dependent rickets and hypercalcemia due to inappropriate increase of serum 1,25(OH)2D in granulomatous diseases. However, due to its lipophilicity and very low circulating concentration, the measurement of 1,25(OH)2D is particularly challenging. Over the past several decades, numerous efforts have been made to develop sensitive, specific, and practical laboratory methods for measuring 1,25(OH)2D. Methods using radioreceptor assay, radioimmunoassay, enzyme immunoassay, enzyme-linked immunosorbent assay, automated chemiluminescent immunoassay, and liquid chromatography-tandem mass spectrometry have been described. Each of these methods has unique advantages and limitations, and some are no longer used. Despite the sophisticated methods in use today, substantial variations between methods still exist. A concerted effort toward standardization of 1,25(OH)2D measurement is needed to ensure accurate and reliable results across laboratories and methods.

Analysis of vitamin D metabolites in biological samples using a nanoluc-based vitamin D receptor ligand sensing system: NLucVDR

Many assays are currently being developed to measure the levels of vitamin D metabolites in various samples (such as blood, urine, and saliva). This study focused on the measurement of vitamin D metabolites in serum and urine using the NLucVDR assay system, which consists of a split-type nanoluciferase and ligand-binding domain (LBD) of the human vitamin D receptor. Blood and urine samples were collected from 23 participants to validate the NLucVDR assay. The 25(OH)D3 levels in the serum and urine determined by the NLucVDR assay showed good correlations with those determined by standard analytical methods (ECLIA for serum and LC-MS/MS for urine), with correlation coefficients of 0.923 and 0.844 for serum and urine samples, respectively. In the case of serum samples, 25(OH)D3 levels determined by the NLucVDR assay were in good agreement with those determined by ECLIA. Therefore, the NLucVDR assay is a useful tool for measuring serum 25(OH)D3 levels. The contribution of each vitamin D metabolite to the luminescence intensity obtained during the NLucVDR assay depends on its concentration and affinity for NLucVDR. Thus, the contribution of 25(OH)D3 in serum appears to be much higher than that of the other metabolites. In contrast, the 25(OH)D3 levels in the urine determined by the NLucVDR assay were more than 20-fold higher than those determined by a standard analytical method (LC-MS/MS), suggesting that some vitamin D metabolite(s) in the urine remarkably increased the luminescence intensity of the NLucVDR assay. Notably, the 25(OH)D3 concentration in the urine determined by the NLucVDR assay and the serum 25(OH)D3 concentration determined by standard analytical methods showed a significant positive correlation (r = 0.568). These results suggest that the analysis of a small amount of urine using the NLucVDR assay may be useful for predicting the serum 25(OH)D3 levels.

2-fluoro-1-methylpyridinium p-toluene sulfonate: a new LC-MS/MS derivatization reagent for vitamin D metabolites

Vitamin D analysis by MS faces several analytical challenges, including inefficient ionization, nonspecific fragmentation, interferences from epimers, isomers, and isobars, as well as very low concentration levels. In this study, we used 2-fluoro-1-methylpyridinium (FMP) p-toluene sulfonate for derivatization of vitamin D3 metabolites to increase detection sensitivity and allow for full chromatographic separation of vitamin D isomers and epimers. UHPLC-MS/MS was used for measurement of five vitamin D3 metabolites in human serum. Compared with Amplifex and 4-phenyl-1,2,4-triazolin-3,5-dion, the FMP p-toluene sulfonate reaction required less time to be performed. The method was optimized and validated to ensure accuracy, precision, and reliability. In-house and commercial quality control samples were used to assure the quality of the results for 25-hydroxyvitamin D3. The method showed very good linearity and intraday and interday accuracy and precision; coefficients of determination (r2) ranged between 0.9977 and 0.9992, relative recovery from 95 to 111%, and coefficient of variation from 0.9 to 11.3. Stability tests showed that the extracted derivatized serum samples were stable for 24 h after storage at -20°C; 24,25-dihydroxyvitamin D3 and 1,25-dihydroxyvitamin D3-FMP derivatives were stable for 1 week at -80°C. The method was applied to samples of healthy individuals for quantitative determination of vitamin D3, the two epimers of 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3.

Separation and identification of monoglucuronides of vitamin D3 metabolites in urine by derivatization-assisted LC/ESI-MS/MS using a new Cookson-type reagent

A liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) method was developed using a new Cookson-type reagent, 4-[4-(1-pipelidinyl)phenyl]-1,2,4-triazoline-3,5-dione (PIPTAD), to analyze the monoglucuronides (Gs) of vitamin D₃ metabolites in human urine. The G of 23S,25-dihydroxyvitamin D₃ [23,25(OH)₂D₃] was previously found as a major metabolite of vitamin D₃ in the urine, but its conjugation position remained undetermined. Determination of the position was an important research issue to clarify the whole picture of the excretion of surplus 25-hydroxyvitamin D₃ [25(OH)D₃, the circulating form of vitamin D₃] in humans. After the pretreated urine sample was derivatized with PIPTAD, the peak corresponding to the G of 23,25(OH)₂D₃ was satisfactorily separated from the urine-derived interfering substances on reversed-phase LC, which could not be achieved by using the previous analogous reagent, DAPTAD. The PIPTAD-derivatized Gs of the vitamin D₃ metabolites provided characteristic product ions useful for identifying the conjugation positions during the MS/MS. Accordingly, we successfully determined the glucuronidated position of 23,25(OH)₂D₃ to be the C23-hydroxy group. The developed method also enabled the simultaneous detection of Gs of 25(OH)D₃ and 24R,25-dihydroxyvitamin D₃ as well as 23,25(OH)₂D₃-23-G without interference from the urine components.

Determination of vitamin D3 conjugated metabolites: a complementary view on hydroxylated metabolites

Experts typically define vitamin D deficiency levels by the determination of a circulating 25-hydroxyvitamin D₃-calcifediol prohormone. A large part of the population is characterized by deficient vitamin D levels (calcifediol < 20 ng mL^-1) despite individuals not being affected by any disorder. Cholecalciferol (vitamin D₃) and/or calcifediol supplementation is a common practice for vitamin D-deficient individuals as recommended by international scientific societies and official agencies. In the last few years, several studies have reported the presence of conjugated vitamin D₃ metabolites, mainly glucuronidation and sulfation derivatives, although simultaneous quantitative measurements involving phase I and II vitamin D metabolites have not been carried out. A quantitative method based on tandem mass spectrometry detection is proposed here for the combined determination of phase I and phase II vitamin D₃ metabolites in human serum. As phase I and phase II metabolites are preferentially ionized in different modes, a switching polarity mode was adopted to determine both groups of compounds in serum at high sensitivity levels (pg mL^-1). The validation of this proposal was successfully accomplished by following the Center for Drug Evaluation and Research (CDER) guidelines. Its applicability was tested in a cohort of volunteers with mostly deficient baseline levels. Considering the sulfated form of calcifediol, the sum of its concentrations showed sufficient baseline vitamin D levels in all individuals, suggesting that this could be a novel strategy for vitamin D deficiency definition. Therefore, phase II metabolites are proposed to be included when evaluating the vitamin D status since they provide more information about the overall status of the vitamin D endocrine system. Nevertheless, further studies are required to confirm the biological activity of these conjugated metabolites and the suitability of this strategy for the description of vitamin D deficiency.

23,25-Dihydroxyvitamin D&lt;sub&gt;3&lt;/sub&gt; is liberated as a major vitamin D&lt;sub&gt;3&lt;/sub&gt; metabolite in human urine after treatment with β-glucuronidase: Quantitative comparison with 24,25-dihydroxyvitamin D&lt;sub&gt;3&lt;/sub&gt; by LC/MS/MS

The complete understanding of the excretion of surplus 25-hydroxyvitamin D&lt;sub&gt;3&lt;/sub&gt; [25(OH)D&lt;sub&gt;3&lt;/sub&gt;] in humans remains to be accomplished. In our previous study, 24,25-dihydroxyvitamin D&lt;sub&gt;3&lt;/sub&gt; [24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt;] 24-glucuronide was identified as a major urinary vitamin D&lt;sub&gt;3&lt;/sub&gt; metabolite, while the glucuronide of 23,25-dihydroxyvitamin D&lt;sub&gt;3&lt;/sub&gt; [23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt;] is another metabolite of interest but has not been sufficiently evaluated. Although the quantitative analysis of 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; liberated in urine by the treatment with β-glucuronidase (GUS) has been conducted, no information was provided about the amount of the glucuronidated 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; in the urine. In this study, we first developed and validated a liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS)-based method for the simultaneous quantification of 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; and 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; liberated in urine by GUS. The analysis of the urine samples revealed that the amount of 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; was almost as much as that of 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt;, in contrast to the fact that the plasma concentration of 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; was much lower than that of 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt;. These results strongly suggested that 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; is more susceptible to glucuronidation and more promptly excreted into urine than 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt;. Furthermore, the amount ratios of 23,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; to 24,25(OH)&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;3&lt;/sub&gt; in the urine samples did not markedly vary during the day (morning/evening) and even by a week-long vitamin D&lt;sub&gt;3&lt;/sub&gt; supplementation (1000 IU/body/day). We concluded that the C-23 hydroxylation plays a crucial role in the urinary excretion of surplus 25(OH)D&lt;sub&gt;3&lt;/sub&gt;.

Sample preparation techniques for extraction of vitamin D metabolites from non-conventional biological sample matrices prior to LC-MS/MS analysis

The determination of vitamin D metabolites as status marker or for diagnostic purposes is almost entirely conducted from blood serum or plasma. Other biological matrices, however, have also interested researchers, for two main reasons: (1) alternative matrices may allow non-invasive sampling, permit easier sample transfer and require less demanding storage conditions; and (2) the levels of vitamin D metabolites in other body compartments may further aid the understanding of vitamin D metabolism and function. Thus, the development of reliable and efficient sample preparation protocols for sample matrices other than serum/plasma, which will remove potential interferences and selectively extract the targeted metabolites, is of great importance. This review summarizes sample preparation methods for measurement of vitamin D metabolites using liquid chromatography-(tandem)mass spectrometry in more than ten different human tissues, including hair, saliva, adipose tissue, brain and others.

Circulating Conjugated and Unconjugated Vitamin D Metabolite Measurements by Liquid Chromatography Mass Spectrometry

CONTEXT

Vitamin D status is conventionally defined by measurement of unconjugated circulating 25-hydroxyvitamin D (25OHD), but it remains uncertain whether this isolated analysis gives sufficient weight to vitamin D's diverse metabolic pathways and bioactivity. Emerging evidence has shown that phase II endocrine metabolites are important excretory or storage forms; however, the clinical significance of circulating phase II vitamin D metabolites remains uncertain.

OBJECTIVE

In this study we analyzed the contribution of sulfate and glucuronide vitamin D metabolites relative to unconjugated levels in human serum.

METHODS

An optimized enzyme hydrolysis method using recombinant arylsulfatase (Pseudomonas aeruginosa) and beta-glucuronidase (Escherichia coli) was combined with liquid chromatography mass spectrometry (LC-MS/MS) analysis to measure conjugated and unconjugated vitamin D metabolites 25OHD3, 25OHD2, 3-epi-25OHD3, and 24,25(OH)2D3. The method was applied to the analysis of 170 human serum samples from community-dwelling men aged over 70 years, categorized by vitamin D supplementation status, to evaluate the proportions of each conjugated and unconjugated fraction.

RESULTS

As a proportion of total circulating vitamin D metabolites, sulfate conjugates (ranging between 18% and 53%) were a higher proportion than glucuronide conjugates (ranging between 2.7% and 11%). The proportion of conjugated 25OHD3 (48 ± 9%) was higher than 25OHD2 conjugates (29.1 ± 10%) across all supplementation groups. Conjugated metabolites correlated with their unconjugated forms for all 4 vitamin D metabolites (r = 0.85 to 0.97).

CONCLUSION

Sulfated conjugates form a high proportion of circulating vitamin D metabolites, whereas glucuronide conjugates constitute a smaller fraction. Our findings principally in older men highlight the differences in abundance between metabolites and suggest a combination of both conjugated and unconjugated measurements may provide a more accurate assessment of vitamin D status.

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.

Vitamin D Metabolism and Profiling in Veterinary Species

The demand for vitamin D analysis in veterinary species is increasing with the growing knowledge of the extra-skeletal role vitamin D plays in health and disease. The circulating 25-hydroxyvitamin-D (25(OH)D) metabolite is used to assess vitamin D status, and the benefits of analysing other metabolites in the complex vitamin D pathway are being discovered in humans. Profiling of the vitamin D pathway by liquid chromatography tandem mass spectrometry (LC-MS/MS) facilitates simultaneous analysis of multiple metabolites in a single sample and over wide dynamic ranges, and this method is now considered the gold-standard for quantifying vitamin D metabolites. However, very few studies report using LC-MS/MS for the analysis of vitamin D metabolites in veterinary species. Given the complexity of the vitamin D pathway and the similarities in the roles of vitamin D in health and disease between humans and companion animals, there is a clear need to establish a comprehensive, reliable method for veterinary analysis that is comparable to that used in human clinical practice. In this review, we highlight the differences in vitamin D metabolism between veterinary species and the benefits of measuring vitamin D metabolites beyond 25(OH)D. Finally, we discuss the analytical challenges in profiling vitamin D in veterinary species with a focus on LC-MS/MS methods.

Quantification of Alfacalcidol Tablets Dissolution Content by Chemical Derivatization and LC-MS

Application of liquid chromatography-mass spectrometry (LC-MS) in analyzing the content of alfacalcidol tablets dissolution faces big challenges due to the low amount of alfacalcidol in each tablet and the low ionization efficacy of the compound with electrospray ionization (ESI) or atmospheric-pressure chemical ionization (APCI). Here, extraction, derivatization, and LC-MS quantitation method have been developed and validated for measuring alfacalcidol tablets dissolution content. After alfacalcidol dissolution solution was extracted with dichloromethane to remove surfactant and inorganic salts, alfacalcidol was then derivatized via a Cookson reagent, 4-phenyl-1, 2, 4-triazoline-3, 5-dione (PTAD), under ambient conditions. Alfacalcidol derivative was successfully analyzed by LC-MS. Limit of detection (LOD) of the derivatized alfacalcidol was improved 100 times (0.01 μg/mL) compared with the nontreated compound (1 μg/mL). The new method was then validated following International Conference on Harmonization (ICH) guidance. The method shows a good linearity with r ² > 0.99. Interday and intraday reproducibility was 3.3% and 7.9%, respectively. This procedure can be used in quantification of alfacalcidol tablets dissolution content and corresponding pharmaceutical quality control.

The vitamin D metabolome: An update on analysis and function

Current understanding of vitamin D tends to be focussed on the measurement of the major circulating form 25-hydroxyvitamin D3 (25OHD3) and its conversion to the active hormonal form, 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂ D3) via the enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1). However, whilst these metabolites form the endocrine backbone of vitamin D physiology, it is important to recognise that there are other metabolic and catabolic pathways that are now recognised as being crucially important to vitamin D function. These pathways include C3-epimerization, CYP24A1 hydroxylase, CYP11A1 alternative metabolism of vitamin D3, and phase II metabolism. Endogenous metabolites beyond 25OHD3 are usually present at low endogenous levels and may only be functional in specific target tissues rather than in the general circulation. However, the technologies available to measure these metabolites have also improved, so that measurement of alternative vitamin D metabolic pathways may become more routine in the near future. The aim of this review is to provide a comprehensive overview of the various pathways of vitamin D metabolism, as well as describe the analytical techniques currently available to measure these vitamin D metabolites.

The serum vitamin D metabolome: What we know and what is still to discover

Vitamin D, referring to the two forms, D2 from the diet and D3 primarily derived from phototransformation in the skin, is a prohormone important in human health. The most hormonally active form, 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), formed from vitamin D via 25-hydroxyvitamin D (25(OH)D), is not only important for regulating calcium metabolism, but has many pleiotropic effects including regulation of the immune system and has anti-cancer properties. The major circulating form of vitamin D is 25(OH)D and both D2 and D3 forms are routinely measured by LC/MS/MS to assess vitamin D status, due to their relatively long half-lives and much higher concentrations compared to 1α,25(OH)₂D. Inactivation of both 25(OH)D and 1α,25(OH)₂D is catalyzed by CYP24A1 and 25-hydroxyvitamin D3 3-epimerase. Initial products from these enzymes acting on 25(OH)D3 are 24R,25(OH)₂D3 and 3-epi-25(OH)D3, respectively, and both of these can also be measured routinely in some clinical laboratories to further document vitamin D status. With advances in LC/MS/MS and its increased availability, and with the help of studies with recombinant vitamin D-metabolizing enzymes, many other vitamin D metabolites have now been detected and in some cases quantitated, in human serum. CYP11A1 which catalyzes the first step in steroidogenesis, has been found to also act on vitamins D3 and D2 hydroxylating both at C20, but with some secondary metabolites produced by subsequent hydroxylations at other positions on the side chain. The major vitamin D3 metabolite, 20S-hydroxyvitamin D3 (20S(OH)D3), shows biological activity, often similar to 1α,25(OH)₂D3 but without calcemic effects. Using standards produced enzymatically by purified CYP11A1 and characterized by NMR, many of these new metabolites have been detected in human serum, with semi-quantitative measurement of 20S(OH)D3 indicating it is present at comparable concentrations to 24R,25(OH)₂D3 and 3-epi-25(OH)D3. Recently, vitamin D-related hydroxylumisterols derived from lumisterol3, a previtamin D3 photoproduct, have also been measured in human serum and displayed biological activity in initial in vitro studies. With the current extensive knowledge on the reactions and pathways of metabolism of vitamin D, especially those catalyzed by CYP24A1, CYP27A1, CYP27B1, CYP3A4 and CYP11A1, it is likely that many other of the resulting hydroxyvitamin D metabolites will be measured in human serum in the future, some contributing to a more detailed understanding of vitamin D status in health and disease.

Concise synthesis of 23-hydroxylated vitamin D3 metabolites

Three 23-hydroxylated vitamin D₃ derivatives, which are metabolites of 25-hydroxyvitamin D₃ produced by CYP24A1 and a related diastereomer, were efficiently synthesized. Each C23 hydroxy unit was constructed by the Claisen condensation reaction with ethyl acetate or the Grignard reaction with 2-methylallymagnesium chloride. Stereochemistry at the C23 position was determined by a modified Mosher's method. The triene structures were constructed by the Wittig-Horner reaction utilizing the A-ring phosphine oxide moiety.

Simultaneous quantification of 25-hydroxyvitamin D3-3-sulfate and 25-hydroxyvitamin D3-3-glucuronide in human serum and plasma using liquid chromatography-tandem mass spectrometry coupled with DAPTAD-derivatization

25-hydroxyvitamin D₃-3-sulfate (25-OHD₃-S) and 25-hydroxyvitamin D₃-3-glucuronide (25-OHD₃-G) are major conjugative metabolites of vitamin D₃ found in the systemic circulation and potentially important reservoirs for 25-hydroxyvitamin D₃. Simultaneous and accurate quantification of these metabolites could advance assessment of the impact of vitamin D₃ on health and disease. In this study, a highly sensitive and accurate liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneous quantification of 25-OHD₃-S and 25-OHD₃-G in human serum or plasma. Following protein precipitation, the analytes of interest were partially purified by solid-phase extraction and subjected to derivatization with 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD). Quantification of the analytes was based on multiple reaction monitoring (MRM) operated in the positive ion mode, and deuterated internal standards were used for each conjugative metabolite. Applying this method to the analysis of 25-OHD₃-S and 25-OHD₃-G concentrations in human serum or plasma samples achieved satisfactory reproducibility, accuracy and sensitivity. We subsequently used this method to simultaneously determine serum concentrations of the two metabolites in archived samples from a rifampin treatment study. Drug treatment had no effect on metabolite concentrations, but significantly increased the 25-OHD₃-S/25-OHD₃ concentration ratio (p=0.01). The availability of this new method should improve sample throughput and our ability to quantify and monitor circulating 25-OHD₃-S and 25-OHD₃-G concentrations.

Human UGT1A4 and UGT1A3 conjugate 25-hydroxyvitamin D3: metabolite structure, kinetics, inducibility, and interindividual variability

25-Hydroxyvitamin D3 (25OHD3) is used as a clinical biomarker for assessment of vitamin D status. Blood levels of 25OHD3 represent a balance between its formation rate and clearance by several oxidative and conjugative processes. In the present study, the identity of human uridine 5'-diphosphoglucuronyltransferases (UGTs) capable of catalyzing the 25OHD3 glucuronidation reaction was investigated. Two isozymes, UGT1A4 and UGT1A3, were identified as the principal catalysts of 25OHD3 glucuronidation in human liver. Three 25OHD3 monoglucuronides (25OHD3-25-glucuronide, 25OHD3-3-glucuronide, and 5,6-trans-25OHD3-25-glucuronide) were generated by recombinant UGT1A4/UGT1A3, human liver microsomes, and human hepatocytes. The kinetics of 25OHD3 glucuronide formation in all systems tested conformed to the Michaelis-Menten model. An association between the UGT1A4*3 (Leu48Val) gene polymorphism with the rates of glucuronide formation was also investigated using human liver microsomes isolated from 80 genotyped livers. A variant allele dose effect was observed: the homozygous UGT1A4*3 livers (GG) had the highest glucuronidation activity, whereas the wild type (TT) had the lowest activity. Induction of UGT1A4 and UGT1A3 gene expression was also determined in human hepatocytes treated with pregnane X receptor/constitutive androstane receptor agonists, such as rifampin, carbamazepine, and phenobarbital. Although UGT mRNA levels were increased significantly by all of the known pregnane X receptor/constitutive androstane receptor agonists tested, rifampin, the most potent of the inducers, significantly induced total 25OHD3 glucuronide formation activity in human hepatocytes measured after 2, but not 4 and 24 hours, of incubation. Finally, the presence of 25OHD3-3-glucuronide in both human plasma and bile was confirmed, suggesting that the glucuronidation pathway might be physiologically relevant and contribute to vitamin D homeostasis in humans.

Derivatization methods for quantitative bioanalysis by LC–MS/MS

LC with atmospheric pressure ionization MS is essential to a large number of quantitative bioanalyses for a variety of compounds, especially nonvolatile or highly polar compounds. However, in many instances, weak ionization, poor LC retention and instability of certain analytes hinder the development of the LC–MS/MS method. Chemical derivatization has been used for different classes of analytes to improve their ionization efficiency, chromatographic separation and chemical stability. This work presents an overview of chemical derivatization methods that have been applied to the quantitative LC–MS/MS analyses of nine classes of molecules, including aldehydes, amino acids, bisphosphonate drugs, carbohydrates, carboxylic acids, nucleosides and their associated analogs, steroids, thiol-containing compounds and vitamin D metabolites, in biological matrices.

Enhancement of the capabilities of liquid chromatography-mass spectrometry with derivatization: general principles and applications

The integration of liquid chromatography-mass spectrometry (LC-MS) with derivatization is a relatively new and unique strategy that could add value and could enhance the capabilities of LC-MS-based technologies. The derivatization process could be carried out in various analytical steps, for example, sampling, storage, sample preparation, HPLC separation, and MS detection. This review presents an overview of derivatization-based LC-MS strategy over the past 10 years and covers both the general principles and applications in the fields of pharmaceutical and biomedical analysis, biomarker and metabolomic research, environmental analysis, and food-safety evaluation. The underlying mechanisms and theories for derivative reagent selection are summarized and highlighted to guide future studies.

Vitamin D and cancer: clinical aspects

There are substantial preclinical and epidemiologic data that suggest that vitamin D plays a role in the prevention and treatment of cancer. Numerous observational studies have shown that low blood levels of 25(OH) vitamin D (cholecalciferol), estimated by geographical location, diet and activity assessment or measured serum levels are associated with a higher risk of cancer and worse cancer-specific survival as well as numerous morbidities to e.g. cardiovascular disease, stroke, infection, autoimmune disease, and neuromuscular dysfunction among large populations. A considerable number of in vitro and in vivo studies indicate that the most active metabolite of vitamin D--1,25-dihydroxycholecalciferol or calcitriol--has anti-proliferative, pro-apoptotic, pro-differentiating, and anti-angiogenic properties. Combined treatment of calcitriol and many types of cytotoxic agents has synergistic or at least additive effects. However, clinical trials testing these hypotheses have been less encouraging, though a number of methodological, pharmacological, and pharmaceutical issues confound all trials ever conducted. In order to properly assess the clinical value of vitamin D, its metabolites and analogs in cancer prevention and treatment, more studies are needed.

Misleading measures in Vitamin D analysis: a novel LC-MS/MS assay to account for epimers and isobars

Background

Recently, the accuracies of many commercially available immunoassays for Vitamin D have been questioned. Liquid chromatography tandem mass spectrometry (LC- MS/MS) has been shown to facilitate accurate separation and quantification of the major circulating metabolite 25-hydroxyvitamin-D3 (25OHD3) and 25-hydroxyvitamin-D2 (25OHD2) collectively termed as 25OHD. However, among other interferents, this method may be compromised by overlapping peaks and identical masses of epimers and isobars, resulting in inaccuracies in circulating 25OHD measurements. The aim of this study was to develop a novel LC-MS/MS method that can accurately identify and quantitate 25OHD3 and 25OHD2 through chromatographic separation of 25OHD from its epimers and isobars.

Methods

A positive ion electrospray ionisation (ESI) LC-MS/MS method was used in the Multiple Reaction Monitoring (MRM) mode for quantification. It involved i) liquid-liquid extraction, ii) tandem columns (a high resolution ZORBAX C18 coupled to an ULTRON chiral, with guard column and inlet filter), iii) Stanozolol-D3 as internal standard, and iv) identification via ESI and monitoring of three fragmentation transitions. To demonstrate the practical usefulness of our method, blood samples were collected from 5 healthy male Caucasian volunteers; age range 22 to 37 years and 25OHD2, 25OHD3 along with co-eluting epimers and analogues were quantified.

Results

The new method allowed chromatographic separation and quantification of 25OHD2, 25OHD3, along with 25OHD3 epimer 3-epi-25OHD3 and isobars 1-α-hydroxyvitamin-D3 (1αOHD3), and 7-α-hydroxy-4-cholesten-3-one (7αC4). The new assay was capable of detecting 0.25 ng/mL of all analytes in serum.

Conclusions

To our knowledge, this is the first specific, reliable, reproducible and robust LC-MS/MS method developed for the accurate detection of 25OHD (Vitamin D). The method is capable of detecting low levels of 25OHD3 and 25OHD2 together with chromatographic separation from the co-eluting epimers and isobars and circumvents other instrumental/analytical interferences. This analytical method does not require time-consuming derivatisation and complex extraction techniques and could prove very useful in clinical studies.

Serum vitamin D and risk of bladder cancer

Vitamin D may protect against several cancers, but data about the association between circulating vitamin D and bladder cancer are limited. Within the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study, a randomized controlled trial conducted to determine the effects of α-tocopherol and β-carotene supplements on cancer incidence in male smokers, 250 bladder cancer cases were randomly sampled by month of blood collection. Controls were matched 1:1 to cases on age at randomization and date of blood collection. Conditional logistic regression was used to estimate odds ratios (OR) and 95% confidence intervals (CI) of bladder cancer by a priori categories of baseline serum 25-hydroxyvitamin D [25(OH)D; i.e., <25, 25 to <37.5, 37.5 to <50, ≥50 nmol/L] and by season-specific quartiles. After multivariable adjustment, we found that lower 25(OH)D was associated with a statistically significantly increased risk of bladder cancer (versus ≥50 nmol/L; <25 nmol/L: OR, 1.73; 95% CI, 1.03-2.91; 25 to <37.5 nmol/L: OR, 1.81; 95% CI, 1.05-3.14; 37.5 to <50 nmol/L: OR, 1.76; 95% CI, 1.02-3.02; P trend=0.04). Similarly, increased risks for the lowest vitamin D category were observed when season-specific quartiles were used (Q1 versus Q4: OR, 1.63; 95% CI, 0.96-2.75; P trend=0.03). In this prospective study of male smokers, lower serum 25(OH)D was associated with an increased risk of bladder cancer. Future studies should examine the association in other populations, especially nonsmokers and women.

Quantitative determination of vitamin D metabolites in plasma using UHPLC-MS/MS

Vitamin D is an important determinant of bone health at all ages. The plasma concentrations of 25-hydroxy vitamin D (25-OH D) and other metabolites are used as biomarkers for vitamin sufficiency and function. To allow for the simultaneous determination of five vitamin D metabolites, 25-OH D(3), 25-OH D(2), 24,25-(OH)(2) D(3), 1,25-(OH)(2) D(3), and 1,25-(OH)(2) D(2), in low volumes of human plasma, an assay using ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) was established. Plasma samples were spiked with isotope-labeled internal standards and pretreated using protein precipitation, solid-phase extraction (SPE) and a Diels-Alder derivatization step with 4-phenyl-1,2,4-triazoline-3,5-dione. The SPE recovery rates ranged from 55% to 85%, depending on the vitamin D metabolite; the total sample run time was <5 min. Mass spectrometry was conducted using positive ion electrospray ionization in the multiple reaction monitoring mode on a quadrupole-quadrupole-linear ion trap instrument after pre-column addition of methylamine to increase the ionization efficiency. The intra- and inter-day relative standard deviations were 1.6-4.1% and 3.7-6.8%, respectively. The limit of quantitation for these compounds was determined to be between 10 and 20 pg/mL. The 25-OH D results were compared with values obtained for reference materials (DEQAS). In addition, plasma samples were analyzed with two additional Diasorin antibody assays. All comparisons with conventional methods showed excellent correlations (r(2) = 0.9738) for DEQAS samples, demonstrating the high degree of comparability of the new UHPLC-MS/MS technique to existing methods.

A sensitive LC/MS/MS assay of 25OH vitamin D3 and 25OH vitamin D2 in dried blood spots

BACKGROUND

Low levels of 25 hydroxyvitamin D (25OHD) during early development is associated with a range of adverse health outcomes. While a number of methods exist to measure 25OHD in sera, none have been specifically developed to examine dried blood spots (DBS).

METHODS

We describe an assay where 25 hydroxyvitamin D(3) (25OHD3) and 25 hydroxyvitamin D(2) (25OHD2) are extracted from 3.2 mm DBS punches, derivatised with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) prior to analysis with LC/MS/MS. We assessed assay precision, relative accuracy and examined the impact of storage conditions in samples stored for up to 22 years.

RESULTS

The new assay had good accuracy and precision, and was highly sensitive, being capable of detecting <1 nmol/l 25OHD3 and 2 nmol/l 25OHD2. CDER sensitivity criteria were slightly higher at 7.7 nmol/l for 25OHD3 and 10.7 nmol/l for 25OHD2. The mean 25OHD3 concentration in 118 archived DBS was 20.8+/-11.4, (4.8 to 67.8 nmol/l). 25OHD2 was detected in only two of these samples. 25OHD3 concentrations were significantly higher in DBS collected in summer compared to winter (p<0.0001).

CONCLUSION

Both 25OHD3 and 25OHD2 can be reliably quantified in archived 3.2 mm dried blood spots. We can not be certain that the levels we measure in archived samples are exactly the same as when they were collected. However, the fact that the DBS levels reflect the well-known seasonal variation in this vitamin and when corrected for sera, values fall within the normal range for 25OHD3, means that DBS are a useful tissue repository for testing a range of hypotheses linking developmental hypovitaminosis D and adverse health outcomes.

Metabolic profiling of major vitamin D metabolites using Diels-Alder derivatization and ultra-performance liquid chromatography-tandem mass spectrometry

Biologically active forms of vitamin D are important analytical targets in both research and clinical practice. The current technology is such that each of the vitamin D metabolites is usually analyzed by individual assay. However, current LC-MS technologies allow the simultaneous metabolic profiling of entire biochemical pathways. The impediment to the metabolic profiling of vitamin D metabolites is the low level of 1alpha,25-dihydroxyvitamin D(3) in human serum (15-60 pg/mL). Here, we demonstrate that liquid-liquid or solid-phase extraction of vitamin D metabolites in combination with Diels-Alder derivatization with the commercially available reagent 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) followed by ultra-performance liquid chromatography (UPLC)-electrospray/tandem mass spectrometry analysis provides rapid and simultaneous quantification of 1alpha,25-dihydroxyvitamin D(3), 1alpha,25-dihydroxyvitamin D(2), 24R,25-dihydroxyvitamin D(3), 25-hydroxyvitamin D(3) and 25-hydroxyvitamin D(2) in 0.5 mL human serum at a lower limit of quantification of 25 pg/mL. Precision ranged from 1.6-4.8 % and 5-16 % for 25-hydroxyvitamin D(3) and 1alpha,25-dihydroxyvitamin D(3), respectively, using solid-phase extraction.

Identification of human UDP-glucuronosyltransferases catalyzing hepatic 1alpha,25-dihydroxyvitamin D3 conjugation

The biological effects of 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) are terminated primarily by P450-dependent hydroxylation reactions. However, the hormone is also conjugated in the liver and a metabolite, presumably a glucuronide, undergoes enterohepatic cycling. In this study, the identity of human enzymes capable of catalyzing the 1,25(OH)2D3 glucuronidation reaction was investigated in order to better understand environmental and endogenous factors affecting the disposition and biological effects of vitamin D3. Among 12 different UGT isozymes tested, only UGT1A4 >> 2B4 and 2B7 supported the reaction. Two different 1,25(OH)2D3 monoglucuronide metabolites were generated by recombinant UGT1A4 and human liver microsomes. The most abundant product was identified by mass spectral and NMR analyses as the 25-O-glucuronide isomer. The formation of 25-O-glucuronide by UGT1A4 Supersomes and human liver microsomes followed simple hyperbolic kinetics, yielding respective Km and Vmax values of 7.3 and 11.2 microM and 33.7 +/- 1.4 and 32.9 +/- 1.9 pmol/min/mg protein. The calculated intrinsic 25-O-glucuronide M1 formation clearance for UGT1A4 was 14-fold higher than the next best isozyme, UGT2B7. There was only limited (four-fold) inter-liver variability in the 25-O-glucuronidation rate, but it was highly correlated with the relative rate of formation of the second, minor metabolite. In addition, formation of both metabolites was inhibited >80% by the selective UGT1A4 inhibitor, hecogenin. If enterohepatic recycling of 1,25(OH)2D3 represents a significant component of intestinal and systemic 1,25(OH)2D3 disposition, formation of monoglucuronides by hepatic UGT1A4 constitutes an important initial step.

Trace determination of steroids causing age-related diseases using LC/MS combined with detection-oriented derivatization

With the rapid shift to an aging society in Japan, age-related diseases, such as osteoporosis, dementia and cancer, are sharply increasing. The measurement of steroids related to these diseases in biological fluids and tissues is useful for elucidation of the nature, diagnosis and treatment of such diseases. LC/MS is considered to be the most promising method for this purpose due to its specificity and versatility, but it sometimes does not demonstrate the required sensitivity for trace amounts of steroids, because steroids have a rather low response using either electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI). To overcome this problem, the author developed detection-oriented derivatization procedures for steroids in LC/MS. For ESI-MS, introduction of a permanently charged moiety is effective. Based on this, 2-hydrazino-1-methylpyridine was developed and used in monitoring prostatic 5alpha-dihydrotestosterone, a good index for the follow-up of patients affected by prostate cancer under androgen deprivation therapy and salivary dehydroepiandrosterone, which is now often designated as an anti-aging hormone. A proton-affinitive Cookson-type reagent, 4-[2-(6,7-dimethoxy-4-methyl-3-oxo-3,4-dihydroquinoxalyl)ethyl]-1,2,4-triazoline-3,5-dione, was used for the determination of 1alpha-hydroxyvitamin D3 [1alpha(OH)D3], a synthetic prodrug of the active form of vitamin D3, in human plasma, and this new LC/positive-APCI-MS method enabled the pharmacokinetic study of 1alpha(OH)D3 in humans. Electron-capture APCI-MS based on derivatization with 2-nitro-4-trifluoromethylphenylhydrazine was used for the analysis of neurosteroids, which affect brain excitability through action at the neurotransmitter receptors. With this method, the stress-induced rapid biosynthesis of pregnane-type neurosteroids in rat brains was demonstrated.

Strategies for characterization of drug metabolites using liquid chromatography–tandem mass spectrometry in conjunction with chemical derivatization and on-line H/D exchange approaches

Strategies using high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) in conjunction with techniques such as chemical derivatization and on-line hydrogen/deuterium (H/D) exchange for structural elucidation of drug metabolites in crude samples are reviewed. Useful mass spectrometric scan techniques discussed include product ion scan, constant neutral-loss scan, precursor ion scan, multistage MS(n), and accurate mass measurements. In biological systems, xenobiotics are transformed into metabolites, which usually involves introduction of one or more polar functional groups or removal or blockage of such structural moieties. Therefore, chemical derivatization strategies for determination of functional groups and on-line H/D exchange approaches for probing number of exchangeable hydrogens are powerful tools for structural elucidation of drug metabolites in drug metabolism studies. More importantly, these experiments can be carried out on crude samples in microscale, providing sufficient material for LC-MS/MS analysis. Therefore, labor intensive and technically challenging purification of low levels of drug metabolites from complex biological matrices can be avoided. It is the authors' conclusion that strategies such as chemical derivatization and on-line H/D exchange should be used more routinely in drug metabolism studies in order to facilitate metabolite identification.

Application of 4-(4-nitrophenyl)-1,2,4-triazoline-3,5-dione to analysis of 25-hydroxyvitamin D3 in human plasma by liquid chromatography/electron capture atmospheric pressure chemical ionization-mass spectrometry

The utility of 4-(4-nitrophenyl)-1,2,4-triazoline-3,5-dione (NPTAD) as a derivatization reagent in the analysis of 25-hydroxyvitamin D3 [25(OH)D3] using liquid chromatography/electron capture atmospheric pressure chemical ionization-mass spectrometry (LC/ECAPCI-MS) was examined. The derivatives of 25(OH)D3 with NPTAD underwent electron capture in the APCI source in the negative-ion mode and provided 30-fold higher sensitivity compared to an intact compound. This derivatization-LC/ECAPCI-MS method was applied to a plasma 25-hydroxyvitamin D3 assay, and gave satisfactory results in sensitivity, specificity, measurable range and throughput of the analysis.

An update on the therapeutic potential of vitamin D analogues

The review provides an evaluation of the therapeutic potential of vitamin D analogues in the context of the current understanding of vitamin D biochemistry, molecular biology and physiology. Vitamin D activity results from several circulating and intracellular physiological metabolites acting simultaneously through at least three receptors. Common analogues are reviewed. Although most vitamin D analogues have traditionally been analogues of 1,25-dihydroxyvitamin D, it may be better to deliver high doses of base vitamin or (analogues) of 25-hydroxyvitamin D. This would permit physiological endocrine, paracrine and autocrine vitamin D metabolism. Agonists or antagonists of tissue-specific vitamin D metabolic pathways could be coadministered. The importance of measuring endogenous vitamin D metabolites during in vivo studies and the pitfalls of extending data across species and time are emphasised. Human vitamin D analogue trials should include direct comparison against the related endogenous metabolite.