Chemical Characterization and Quantification of Circulating Intact PTH and PTH Fragments by High-Resolution Mass Spectrometry in Chronic Renal Failure

LC-HRMS and GC-MS Profiling of Urine Free Cortisol, Cortisone, 6Β-, and 18-Hydroxycortisol for the Evaluation of Glucocorticoid and Mineralocorticoid Disorders

INTRODUCTION

Urine free cortisol measurements are routinely performed to evaluate hypercortisolism. Despite their analytical inaccuracy, immunoassay-based methods are frequently used. Advances in liquid chromatography-high-resolution mass spectrometry (LC-HRMS) facilitate the incorporation of powerful diagnostic tools into clinical laboratories. In addition to its high analytical specificity and simultaneous analysis of different metabolites, accurate mass measurement allows for untargeted compound identification, which may help to identify clinically relevant metabolites or drugs.

METHODS

The present study aimed to validate a simple routine LC-HRMS method to quantify cortisol, cortisone, 6β-hydroxycortisol, and 18-hydroxycortisol simultaneously in human urine. Additionally, the study also validated a GC-MS method for the same steroids, evaluated their cross-reactivity with commercial cortisol immunoassays, and quantified the 24 h urine excretion in patients under clinical suspicion or follow-up for hypercortisolism.

RESULTS

The LC-HRMS method involved liquid-liquid extraction using dichloromethane, micro-LC for chromatographic separation and detection using the accurate masses of the steroids, and simultaneous high-resolution full scan acquisition. The method presented acceptable linearity, precision, and accuracy. Significant interference from 6β-hydroxycortisol and cortisone was demonstrated in the cortisol immunoassays, which impacted their reliability in the follow-up of patients with hypercortisolism and significant changes in these cortisol metabolites (i.e., due to drug-induced changes in CYP3A4 activity).

CONCLUSION

A rapid and accurate routine LC-HRMS method was validated, which is useful for the evaluation of hypercortisolism and other disorders of glucocorticoid and mineralocorticoid metabolism.

Unveiling a new era with liquid chromatography coupled with mass spectrometry to enhance parathyroid hormone measurement in patients with chronic kidney disease

Precise determination of circulating parathyroid hormone (PTH) concentration is crucial to diagnose and manage various disease conditions, including the chronic kidney disease-mineral and bone disorder. However, the lack of standardization in PTH assays is challenging for clinicians, potentially leading to medical errors because the different assays do not provide equivalent results and use different reference ranges. Here, we aimed to evaluate the impact of recalibrating PTH immunoassays by means of a recently developed LC-MS/MS method as the reference. Utilizing a large panel of pooled plasma samples with PTH concentrations determined by the LC-MS/MS method calibrated with the World Health Organization (WHO) 95/646 International Standard, five PTH immunoassays were recalibrated. The robustness of this standardization was evaluated over time using different sets of samples. The recalibration successfully reduced inter-assay variability with harmonization of PTH measurements across different assays. By recalibrating the assays based on the WHO 95/646 International Standard, we demonstrated the feasibility for standardizing PTH measurement results and adopting common reference ranges for PTH assays, facilitating a more consistent interpretation of PTH values. The recalibration process aligns PTH results obtained from various immunoassays with the LC-MS/MS method, providing more consistent and reliable measurements. Thus, establishing true standardization across all PTH assays is crucial to ensure consistent interpretation and clinical decision-making.

Standardization of PTH measurement by LC-MS/MS: a promising solution for interassay variability

Parathyroid hormone measurement is critical in managing chronic kidney disease-mineral bone disorders. However, there are several commercially available immunoassays with interassay variability. To address this, Cavalier et al. developed standardization of parathyroid hormone measurement by establishing regression equations of each assay against the liquid chromatography coupled to tandem mass spectrometry method. The recalibration successfully reduced interassay variability, allowing for more consistent interpretation. The proposed approach may pave the way for accurate interpretation of parathyroid hormone in clinical practice.

Concise review on the combined use of immunocapture, mass spectrometry and liquid chromatography for clinical applications

Immunocapture is now a well-established method for sample preparation prior to quantitation of peptides and proteins in complex matrices. This short review will give an overview of some clinical applications of immunocapture methods, as well as protocols with and without enzymatic digestion in a clinical context. The advantages and limitations of both approaches are discussed in detail. Challenges related to the choice of mass spectrometer are also discussed. Top-down, middle-down, and bottom-up approaches are discussed. Even though immunocapture has its limitations, its main advantage is that it provides an additional dimension of separation and/or isolation when working with peptides and proteins. Overall, this short review demonstrates the potential of such techniques in the field of proteomics-based clinical medicine and paves the way for better personalized medicine.

Tutorial review for peptide assays: An ounce of pre-analytics is worth a pound of cure

The recent increase in peptidomimetic-based medications and the growing interest in peptide hormones has brought new attention to the quantification of peptides for diagnostic purposes. Indeed, the circulating concentrations of peptide hormones in the blood provide a snapshot of the state of the body and could eventually lead to detecting a particular health condition. Although extremely useful, the quantification of such molecules, preferably by liquid chromatography coupled to mass spectrometry, might be quite tricky. First, peptides are subjected to hydrolysis, oxidation, and other post-translational modifications, and, most importantly, they are substrates of specific and nonspecific proteases in biological matrixes. All these events might continue after sampling, changing the peptide hormone concentrations. Second, because they include positively and negatively charged groups and hydrophilic and hydrophobic residues, they interact with their environment; these interactions might lead to a local change in the measured concentrations. A phenomenon such as nonspecific adsorption to lab glassware or materials has often a tremendous effect on the concentration and needs to be controlled with particular care. Finally, the circulating levels of peptides might be low (pico- or femtomolar range), increasing the impact of the aforementioned effects and inducing the need for highly sensitive instruments and well-optimized methods. Thus, despite the extreme diversity of these peptides and their matrixes, there is a common challenge for all the assays: the need to keep concentrations unchanged from sampling to analysis. While significant efforts are often placed on optimizing the analysis, few studies consider in depth the impact of pre-analytical steps on the results. By working through practical examples, this solution-oriented tutorial review addresses typical pre-analytical challenges encountered during the development of a peptide assay from the standpoint of a clinical laboratory. We provide tips and tricks to avoid pitfalls as well as strategies to guide all new developments. Our ultimate goal is to increase pre-analytical awareness to ensure that newly developed peptide assays produce robust and accurate results.

Determination of parathyroid hormone: from radioimmunoassay to LCMS/MS

Parathyroid hormone (PTH) determination is of paramount importance for the exploration of diseases related with calcium metabolism and for the follow-up of patients suffering from bone and mineral disorders associated with chronic kidney diseases (CKD-MBD). Unfortunately, the biologically active form of PTH, i.e. 1-84 PTH, circulates in the blood stream with many fragments and post-translationally modified forms, which decreases the specificity of immunoassays. The assays used to measure PTH, either from 2nd or 3rd generation, are not standardised, which may lead to interpretation errors and clinical consequences. Reference ranges for PTH have neither been always correctly established and the stability of the peptide is also a matter of concern. Fortunately, these last years, newer techniques using mass spectrometry (either high resolution or triple quadripole) coupled with liquid chromatography have been developed, which will help to standardise the different assays. Indeed, PTH assays standardisation is one of the task of the IFCC Committee for Bone Metabolism. Such standardisation will allow a better consistency in the interpretation of the results and will promote studies aiming at the establishment of correct reference ranges.

Preanalytical considerations in parathyroid hormone measurement

Automated immunoassays used to evaluate parathyroid function are vulnerable to different types of interference, which can affect clinical practices. This review provides a detailed overview of the six main types of interference known to affect the measurement of parathyroid hormone (PTH): heterophilic antibodies, biotin, PTH fragments, oxidized PTH (oxPTH), phosphorylated PTH, and some preanalytical factors. Because the prevalence of some of these conditions has been reported to approach 11.7%, and the frequency of testing for parathyroid function is important, the scale of the problem might be tremendous. Potential interference in parathyroid function testing should always be suspected whenever clinical or biochemical discrepancies arise. Their identification typically relies on additional laboratory tests, including method comparison, serial dilution, blocking reagent studies, affinity adsorption, and polyethylene glycol precipitation. Moreover, some of these issues can be mitigated with the development of mass spectrometry. This review also evaluated the clinical impact of parathyroid interference on immunoassays, including misdiagnosis, inappropriate parathyroidectomy; and delay in receiving appropriate therapy. Hence, strong communication should be maintained between the clinician and laboratory to avoid such scenarios.

Validation of an LC-MS/MS Method Using Solid-Phase Extraction for the Quantification of 1-84 Parathyroid Hormone: Toward a Candidate Reference Measurement Procedure

BACKGROUND

Parathyroid hormone (PTH) measurement is important for patients with disorders of calcium metabolism, including those needing bone-turnover monitoring due to chronic kidney disease-mineral bone disorder. There are currently 2 generations of PTH immunoassays on the market, both having cross-reactivity issues and lacking standardization. Therefore, we developed an LC-MS/MS higher-order method for PTH analysis.

METHODS

The method was calibrated against the international standard for 1-84 PTH (WHO 95/646). Antibody-free sample preparation with the addition of an isotope-labeled internal standard was performed by solid-phase extraction. Extracts were analyzed by LC-MS/MS. EDTA-K2 plasma was used throughout the development and validation. Bias and uncertainty sources were tested according to ISO 15193. Clinical Laboratory Standards Institute guidelines and reference measurement procedures were consulted for the design of the validation. Patient samples and external quality controls were compared between LC-MS/MS and 2 third-generation immunoassays.

RESULTS

The method was validated for 1-84 PTH from 5.7 to 872.6 pg/mL. The interassay imprecision was between 1.2% and 3.9%, and the accuracy ranged from 96.2% to 103.2%. The measurement uncertainty was <5.6%. The comparison between LC-MS/MS and the immunoassays showed a proportional bias but moderate to substantial correlation between methods.

CONCLUSIONS

This LC-MS/MS method, which is independent of antibodies, is suitable for a wide range of PTH concentrations. The obtained analytical performance specifications demonstrate that development of a reference measurement procedure will be possible once a higher order reference standard is available.

Quantification of Parathyroid Hormone and its Fragments in Serum by Liquid Chromatography-High-Resolution Mass Spectrometry

Parathyroid hormone (PTH), an 84-amino acid peptide hormone, is a major regulator of calcium homeostasis. Quantification of PTH in serum is used clinically to investigate calcium imbalances and for monitoring osteodystrophy in patients with renal failure. In addition to intact PTH, several PTH fragments are found in circulation. Recent studies have shown that accurate quantification of PTH fragments may provide valuable clinical information in certain scenarios. In this chapter, a high-resolution mass spectrometry-based method for quantification of PTH (1-84) and its fragments is described. This method involves immunoaffinity capture of intact PTH and PTH-fragments followed by liquid chromatography-high-resolution mass spectrometry (LC-HRMS).

Fibroblast growth factor 23: translating analytical improvement into clinical effectiveness for tertiary prevention in chronic kidney disease

OBJECTIVES

Fibroblast growth factor 23 (FGF23) plays a key role in the pathophysiology of chronic kidney disease (CKD) and of the associated cardiovascular diseases, ranking on the crossroads of several evolving areas with a relevant impact on the health-care system (ageing, treatment of CKD and prevention from cardiovascular and renal events). In this review, we will critically appraise the overall issues concerning the clinical usefulness of FGF23 determination in CKD, focusing on the analytical performances of the methods, aiming to assess whether and how the clinical introduction of FGF23 may promote cost-effective health care policies in these patients.

CONTENT

Our comprehensive critical appraisal of the literature revealed that we are currently unable to establish the clinical usefulness of FGF23 measured by ELISA in CKD, as stability issues and suboptimal analytical performances are the major responsible for the release of misleading results. The meta-analytical approach has failed to report unambiguous evidence in face of the wide heterogeneity of the results from single studies.

SUMMARY AND OUTLOOK

Our review has largely demonstrated that the clinical usefulness depends on a thorough analytical validation of the assay. The recent introduction of chemiluminescent intact-FGF23 (iFGF23) assays licensed for clinical use, after passing a robust analytical validation, has allowed the actual assessment of preliminary risk thresholds for cardiovascular and renal events and is promising to capture the iFGF23 clinically relevant changes as a result of a therapeutic modulation. In this perspective, the analytical optimization of FGF23 determination may allow a marriage between physiology and epidemiology and a merging towards clinical outcomes.

High-Resolution Mass Spectrometry for the Measurement of PTH and PTH Fragments: Insights into PTH Physiology and Bioactivity

Full-length parathyroid hormone (PTH 1-84) is crucial for the regulation of calcium and phosphate homeostasis and bone remodeling. PTH 1-84 is metabolized into various PTH fragments, which are measured with varying levels of efficiency by PTH immunoassays. These PTH fragments, which increase in serum as CKD progresses, could potentially modulate the effects of PTH 1-84 and contribute to CKD-associated bone disorders. To obtain a true biologic representation of total PTH bioactivity, it is necessary to measure not only PTH 1-84 but also PTH fragments that are present in circulation. Traditional second-generation PTH immunoassays collectively measure PTH 1-84, PTH fragments, and post-translationally modified PTH 1-84, making it difficult to accurately predict the character of underlying renal osteodystrophy. This review highlights current advances in methods available for PTH measurement and the clinical relevance of PTH fragments in CKD. We emphasize the usefulness of mass spectrometry as a potential reference method for PTH measurement.

Qualitative and quantitative comparison of different commercially available 1-84 parathyroid hormone proteins to the WHO international standard 95/646 using orthogonal methods

Relative quantitation methods rely on the use of reference substances to determine the content of samples. The aim of this study was to compare 1-84 parathyroid hormone (PTH) standards from different manufacturers to the WHO international standard 95/646. CE and LC with UV detection were investigated as quick and inexpensive quantitation methods, with an emphasis on selectivity between intact 1-84 PTH and its oxidized forms. Both methods were fully validated according to ICH Q2R1. Moreover, method performance was also evaluated according to guidelines defining the maximum allowable measurement uncertainty (MU) of a biological parameter from its intraindividual variation (CV_I), as well as the proportion of that MU devoted to the reference material. This study highlighted the fact that some 1-84 PTH standards have a content that is actually twice as high as the one stated on the label, which was confirmed by an amino acid analysis investigation. Our approach offers a quick and inexpensive way to estimate the content of 1-84 PTH standards.

Proteoforms and their expanding role in laboratory medicine

The term “proteoforms” describes the range of different structures of a protein product of a single gene, including variations in amino acid sequence and post-translational modifications. This diversity in protein structure contributes to the biological complexity observed in living organisms. As the concentration of a particular proteoform may increase or decrease in abnormal physiological states, proteoforms have long been used in medicine as biomarkers of health and disease. Notably, the analytical approaches used to analyze proteoforms have evolved considerably over the years. While ligand binding methods continue to play a large role in proteoform measurement in the clinical laboratory, unanticipated or unknown post-translational modifications and sequence variants can upend even extensively tested and vetted assays that have successfully made it through the medical regulatory process. As an alternate approach, mass spectrometry—with its high molecular selectivity—has become an essential tool in detection, characterization, and quantification of proteoforms in biological fluids and tissues. This review explores the analytical techniques used for proteoform detection and quantification, with an emphasis on mass spectrometry and its various applications in clinical research and patient care including, revealing new biomarker targets, helping improve the design of contemporary ligand binding in vitro diagnostics, and as mass spectrometric laboratory developed tests used in routine patient care.

Short carboxyl terminal parathyroid hormone peptides modulate human parathyroid hormone signaling in mouse osteoblasts

BACKGROUND

Novel human parathyroid hormone (hPTH) peptides of unknown biological activity have recently been identified in the serum of subjects with normal renal function, chronic renal failure, and end-stage renal disease through the application of liquid chromatography-high resolution mass spectrometry.

PURPOSE

of experiments: To determine the bioactivity of these peptides, we synthesized hPTH28-84, hPTH38-84, and hPTH45-84 peptides by solid phase peptide synthesis and tested their bioactivity in MC3T3-E1 mouse osteoblasts, either individually or together with the native hormone, hPTH1-84, by assessing the accumulation of 3´,5´-cyclic adenosine monophosphate (cAMP) and the induction of alkaline phosphatase activity.

RESULTS

Increasing doses of hPTH1-84 (1-100 nM) increased the accumulation of cAMP and alkaline phosphatase activity in osteoblasts. hPTH28-84, hPTH38-84, and hPTH45-84 in concentrations of 1-100 nM were biologically inert. Surprisingly, 100 nM hPTH38-84 and hPTH45-84 increased the accumulation of cAMP in osteoblasts treated with increasing amounts of hPTH1-84. Human PTH28-84 had no effects on cAMP activity alone or in combination with hPTH1-84. Conversely, 100 nM hPTH38-84, hPTH45-84, and hPTH28-84 blocked the activation of alkaline phosphatase activity by hPTH1-84.

CONCLUSIONS

The data show that the short carboxyl-terminal hPTH peptides, hPTH38-84 and hPTH45-84, increase the amount of cellular cAMP generated in cultured osteoblasts in response to treatment with full-length hPTH1-84 when compared to full-length hPTH1-84 alone. Human PTH28-84 had no effect on cAMP activity alone or in combination with hPTH1-84. Human PTH28-84, hPTH38-84 and hPTH45-84 reduced the effects of hPTH1-84 in osteoblasts with respect to the induction of alkaline phosphatase activity compared to hPTH1-84 alone. Short carboxyl peptides of human PTH are biologically inert but when administered together with full-length hPTH1-84 modulate the bioactivity of hPTH1-84 in osteoblasts.