Development of an absolute quantification method targeting growth hormone biomarkers using liquid chromatography coupled to isotope dilution mass spectrometry

Challenges of insulin-like growth factor-1 testing

Insulin-like growth factor 1 (IGF-1), primarily synthesized in the liver, was initially discovered due to its capacity to replicate the metabolic effects of insulin. Subsequently, it emerged as a key regulator of the actions of growth hormone (GH), managing critical processes like cell proliferation, differentiation, and apoptosis. Notably, IGF-1 displays a longer half-life compared to GH, making it less susceptible to factors that may affect GH concentrations. Consequently, the measurement of IGF-1 proves to be more specific and sensitive when diagnosing conditions such as acromegaly or GH deficiency. The recognition of the existence of IGFBPs and their potential to interfere with IGF-1 immunoassays urged the implementation of various techniques to moderate this issue and provide accurate IGF-1 results. Additionally, in response to the limitations associated with IGF-1 immunoassays and the occurrence of discordant IGF-1 results, modern mass spectrometric methods were developed to facilitate the quantification of IGF-1 levels. Taking advantage of their ability to minimize the interference caused by IGF-1 variants, mass spectrometric methods offer the capacity to deliver robust, reliable, and accurate IGF-1 results, relying on the precision of mass measurements. This also enables the potential detection of pathogenic mutations through protein sequence analysis. However, despite the analytical challenges, the discordance in IGF-1 reference intervals can be attributed to a multitude of factors, potentially leading to distinct interpretations of results. The establishment of reference intervals for each assay is a demanding task, and it requires nationwide multicenter collaboration among laboratorians, clinicians, and assay manufacturers to achieve this common goal in a cost-effective and resource-efficient manner. In this comprehensive review, we examine the challenges associated with the standardization of IGF-1 measurement methods, the minimization of pre-analytical factors, and the harmonization of reference intervals. Particular emphasis will be placed on the development of IGF-1 measurement techniques using "top-down" or "bottom-up" mass spectrometric methods.

The impact of different calibration matrices on the determination of insulin-like growth factor 1 by high-resolution-LC-MS in acromegalic and growth hormone deficient patients

OBJECTIVES

Calibration is an important source of variability in liquid chromatography mass spectrometry (LC-MS) methods for insulin-like growth factor 1 (IGF-1). This study investigated the impact of different calibrator matrices on IGF-1 measurements by LC-MS. Moreover, the comparability of immunoassays and LC-MS was assessed.

DESIGN & METHODS

Calibrators from 12.5 to 2009 ng/ml were prepared by spiking WHO international Standard (ID 02/254 NIBSC, UK) into the following matrices: native human plasma, fresh charcoal-treated human plasma (FCTHP), old charcoal-treated human plasma, deionized water, bovine serum albumin (BSA), and rat plasma (RP). A validated in-house LC-MS method was calibrated repeatedly with these calibrators. Then, serum samples from 197 growth hormone excess and deficiency patients were analysed with each calibration.

RESULTS

The seven calibration curves had different slopes leading to markedly different patient results. The largest differences in IGF-1 concentration from the median (interquartile range) was observed with the calibrator in water and the calibrator in RP (336.4 [279.6-417.0] vs. 112.5 [71.2-171.2], p < 0.001). The smallest difference was observed with calibrators in FCTHP and BSA (141.8 [102.0-198.5] vs. 127.9 [86.9-186.0], p < 0.049). Compared to LC-MS with calibrators in FCTHP, immunoassays showed relevant proportional bias (range: -43% to -68%), constant bias (range: 22.84 to 57.29 ng/ml) and pronounced scatter. Comparing the immunoassays with each other revealed proportional bias of up to 24%.

CONCLUSIONS

The calibrator matrix is critical for the measurement of IGF-1 by LC-MS. Regardless of the calibrator matrix, LC-MS shows poor agreement with immunoassays. Also, the agreement between different immunoassays is variable.

Updates on the study of lysosomal protein dynamics: possibilities for the clinic

INTRODUCTION

The lysosome is the main degradative organelle of almost all mammalian cells, fulfilling important functions in macromolecule recycling, metabolism, and signaling. Lysosomal dysfunction is connected to a continuously growing number of pathologic conditions, and lysosomal proteins present potential biomarkers for a variety of diseases. Therefore, there is an increasing interest in their analysis in patient samples.

AREAS COVERED

We provide an overview of OMICs studies which identified lysosomal proteins as potential biomarkers for pathological conditions, covering proteomics, genomics, and transcriptomics approaches, identified through PubMed searches. With respect to discovery proteomics analyses, mainly lysosomal luminal and associated proteins were detected, while membrane proteins were found less frequently. Comprehensive coverage of the lysosomal proteome was only achieved by ultra-deep-coverage studies, but targeted approaches allowed for the reproducible quantification of lysosomal proteins in diverse sample types.

EXPERT OPINION

The low abundance of lysosomal proteins complicates their reproducible analysis in patient samples. Whole proteome shotgun analyses fail in many instances to cover the lysosomal proteome, which is due to under-sampling and/or a lack of sensitivity. With the current state of the art, targeted proteomics assays provide the best performance for the characterization of lysosomal proteins in patient samples.

Procollagen type III amino-terminal propeptide and insulin-like growth factor I as biomarkers of growth hormone administration

The acceptance in 2012 by the World Anti‐Doping Agency (WADA) of the biomarker test for human growth hormone (hGH) based on procollagen type III amino‐terminal propeptide (P‐III‐NP) and insulin‐like growth factor I (IGF‐I) was perhaps the first time that such a method has been used for forensic purposes. Developing a biomarker test to anti‐doping standards, where the strict liability principle applies, is discussed. An alternative WADA‐accepted approach is based on the measurement of different hGH isoforms, a method that suffers from the very short half‐life of hGH limiting the detection period. Modification or withdrawal of the immunoassays, on which the biomarker measurements largely depend, has necessitated revalidation of the assays, remeasurement of samples and adjustment of the decision limits above which an athlete will be assumed to have administered hGH. When a liquid chromatography coupled mass spectrometry (LC–MS) method became a reality for the measurement of IGF‐I, more consistency of results was assured. Measurement of P‐III‐NP is still dependent on immunoassays although work is underway to develop an LC–MS method. The promised long‐term detection time for the biomarker assay does not appear to have been realised in practice, and this is perhaps partly the result of decision limits being set too high. Nevertheless, more robust assays are needed before a further adjustment of the decision limit is warranted. In the meantime, WADA is considering using P‐III‐NP and IGF‐I as components of a biomarker passport system recording data from an individual athlete, rather than the population. Using this approach, smaller perturbations in the growth hormone (GH) score would mandate an investigation and possible action for hGH administration.

An antibody-free LC-MS/MS method for the quantification of intact insulin-like growth factors 1 and 2 in human plasma

Insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) are important biomarkers in research and diagnosis of growth disorders. Quantitative analysis is performed using various ligand-binding assays or enzymatic digestion LC-MS/MS methods, whose widespread adoption is hampered by time-consuming sample preparation procedures. We present a simple and fast antibody-free LC-MS/MS method for the quantification of intact IGF-1 and IGF-2 in human plasma. The method requires 50 μL of plasma and uses fully ¹⁵N-labelled IGF-1 as internal standard. It features trifluoroethanol (TFE)-based IGF/IGF-binding protein complex dissociation and a two-step selective protein precipitation workflow, using 5% acetic acid in 80/20 acetone/acetonitrile (precipitation 1) and ice-cold ethanol (precipitation 2). Detection of intact IGF-1 and IGF-2 is performed by means of a Waters XEVO TQ-S triple quadrupole mass spectrometer in positive electrospray ionisation (ESI+) mode. Lower limits of quantification were 5.9 ng/mL for IGF-1 and 8.4 ng/mL for IGF-2. Intra-assay imprecision was below 4.5% and inter-assay imprecision was below 5.8% for both analytes. An excellent correlation was found between nominal and measured concentrations of the WHO reference standard for IGF-1. Comparison with the IDS-iSYS IGF-1 immunoassay showed good correlation (R² > 0.97), although a significant bias was observed with the immunoassay giving substantially higher concentrations. The LC-MS/MS method described here allows for reliable and simultaneous quantification of IGF-1 and IGF-2 in plasma, without the need for enzymatic digestion. The method can be readily implemented in clinical mass spectrometry laboratories and has the potential to be adapted for the analysis of different similarly sized peptide hormones.

Inter-Laboratory Agreement of Insulin-like Growth Factor 1 Concentrations Measured Intact by Mass Spectrometry

BACKGROUND

Insulin-like growth factor-I (IGF-1) is measured mainly by immunoassay for the diagnosis and treatment of growth hormone (GH) disorders, and to detect misuse of GH in sport. Immunoassays often have insufficient inter-laboratory agreement, especially between commercial kits. Over the expected range of IGF-1 in blood (∼50-500 ng/mL), in an inter-laboratory study we previously established a measurement imprecision of 11% (%CV) for the digested protein analyzed by LC-MS. Measuring intact IGF-1 by LC-MS should be simpler. However, no inter-laboratory agreement has been published.

METHODS

Intact and trypsin-digested IGF-1 in 32 serum samples from healthy volunteers and human growth hormone administration studies were analyzed by LC-MS using different instruments in five laboratories, as well as by immunoassay in a single laboratory. Another 100 samples were analyzed for IGF-1, both intact and after trypsin-digestion, in each laboratory by LC-MS. The statistical relationship between measurements and the imprecision of each assay group was assessed.

RESULTS

An intra-laboratory variability of 2-4% CV was obtained. Inter-laboratory variability was greater at 14.5% CV. Orthogonal regression of intact versus trypsin-digestion methods (n = 646) gave a slope of 1.01 and intercept of 2.05 ng/mL.

CONCLUSIONS

LC-MS measurements of IGF-1 by intact and trypsin-digestion methods are not statistically different and each is similar to immunoassay. The two LC-MS approaches may be used interchangeably or together to eliminate concerns regarding an immunoassay IGF-1 measurement. Because intact and digested IGF-1 measurements generally agreed within 20% of each other, we propose this as a criterion of assay acceptability.

A quantitative LC-MS/MS method for insulin-like growth factor 1 in human plasma

BACKGROUND

Insulin-like growth factor 1 (IGF1) is a biomarker with various applications in medicine and also in doping control.

METHODS

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed that employs 15N-IGF1 as an internal standard. The method features urea-based IGF1/IGFBP-complex dissociation which is directly followed by tryptic digestion. Following solid-phase extraction (SPE) sample clean-up of the digest, IGF1 is detected by means of two signature peptides that enable quantification of total IGF1 as well as discrimination between IGF1 proteoforms with 'native' and modified or extended N-terminal sequences.

RESULTS

Our method is capable of measuring plasma IGF1 concentrations over the clinically relevant range of 10-1000 ng/mL and was validated according to regulatory guidelines. Comparison with the IDS-iSYS IGF1 immunoassay revealed good correlation (R2>0.97) and no proportional bias between both assays was observed after normalizing the results against the WHO reference standard for IGF1 (02/254). Evaluation of several commercially available IGF1 preparations showed varying responses which were due to inconsistencies in purity and absolute amount of IGF1 present in these products.

CONCLUSIONS

Our LC-MS/MS method introduces urea-based dissociation of IGF1/IGFBP-complexes to enable reliable quantification of IGF1 in plasma. Furthermore, the method is able to detect clinically relevant IGF1 levels without an enrichment procedure at the protein-level and thereby minimizes the risk of losing IGF1 proteoforms during sample preparation.

Review of approaches and examples for monitoring biotransformation in protein and peptide therapeutics by MS

Biotherapeutic drugs have emerged in quantity in pharmaceutical pipelines, and increasingly diverse biomolecules are progressed through preclinical and clinical development. As purification, separation, mass spectrometer detection and data processing capabilities improve, there is opportunity to monitor drug concentration by traditional ligand-binding assay or MS measurement and to monitor metabolism, catabolism or other biomolecular mass variants present in circulation. This review highlights approaches and examples of monitoring biotransformation of biotherapeutics by MS as these techniques are poised to add value to drug development in years to come. The increased use of such approaches, and the successful quantitation of biotherapeutic structural modifications, will provide insightful data for the benefit of both researchers and patients.

Development of a Parallel Reaction Monitoring-MS Method To Quantify IGF Proteins in Dogs and a Case of Nonislet Cell Tumor Hypoglycemia

Nonislet-cell tumor hypoglycemia (NICTH) is a rare paraneoplastic phenomenon well described in dogs and humans. Tumors associated with NICTH secrete incompletely processed forms of insulin-like growth factor-II (IGF-II), commonly named big IGF-II. These forms have increased bioavailability and interact with the insulin and IGF-I receptor causing hypoglycemia and growth-promoting effects. Immunoassays designed for human samples have been used to measure canine IGF-I and -II, but they possess some limitations. In addition, there are no validated methods for measurement of big IGF-II in dogs. In the present study, a targeted parallel reaction monitoring MS-based method previously developed for cats has been optimized and applied to simultaneously quantify the serum levels of IGF-I, IGF-II, and IGFBP-3, and for the first time, the levels of big IGF-II in dogs. This method allows the absolute quantification of IGF proteins using a mixture of QPrEST proteins previously designed for humans. The method possesses good linearity and repeatability and has been used to evaluate the IGF-system in a dog with NICTH syndrome. In this dog, the levels of big IGF-II decreased by 80% and the levels of IGF-I and IGFBP-3 increased approximately 20- and 4-times, respectively, after removal of the tumor.

Detection and confirmation of α-cobratoxin in equine plasma by solid-phase extraction and liquid chromatography coupled to mass spectrometry

α-Cobratoxin (CTX) is a large peptide (71 amino acids) with strong analgesic effect and may be misused in sports such as horse racing. To prevent such misuse, a sensitive method is required for detection and confirmation of the toxin in equine samples. CTX was extracted from equine plasma using an optimized mixed-mode solid-phase extraction (SPE) procedure. Extracted CTX was reduced with dithiothreitol and alkylated with iodoacetamide, and then was digested by trypsin at 56°C for 30min. The digest was analysed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and tryptic peptides T2 (³CFITPDITSK¹²) and T4 (²⁴TWCDAFCSIR³³) were monitored for detection and confirmation of CTX. The limit of detection (LOD) was 0.05ng/mL for CTX in plasma, and the limit of confirmation (LOC) 0.2ng/mL. Unlike small peptides consisting of the 20 canonical amino acids, CTX was stable in equine plasma at ambient temperature for at least 24h. The developed analytical method was successfully applied to analysis of incurred plasma samples; CTX was detected in plasma collected 15min through 36h post subcutaneous administration of CTX (2.0mg dose) to a research horse, and confirmed 30min through 24h. Additionally, an approach named "reliable targeted SEQUEST search" has been proposed for assessing the specificity of T2 at product ion spectrum level for confirmation of CTX. T2 is uniquely specific for CTX, as evaluated with this approach and BLAST search. Furthermore, the effect of dimethyl sulfoxide (DMSO) as a mobile phase additive on electrospray (ESI) response of T2 and T4, background noise level and signal to noise ratio (S/N) was examined; DMSO increased signal intensity of T2 and T4 by a factor of less than 2. It is the first report that DMSO raised background noise level and did not improve S/N for the peptides, to the authors' knowledge. The developed analytical method may be applicable for analysis of CTX in plasma from other species such as greyhound dogs or even human beings.

Partial phenotype conversion and differential trait response to conditions of husbandry in mice

Functional genome analysis usually is performed on the level of genotype–phenotype interaction. However, phenotypes also depend on the relations between genomes and environment. In our experimental system, we observed differential response to environmental factors defined by different conditions of husbandry in a semi-barrier unit or in a SPF (specific pathogen free) barrier unit, which resulted in partial reversal of phenotypes previously observed under semi-barrier conditions. To provide an update of basic phenotypes in unselected and randomly mated controls (DUC) and long-term selected DUhTP (Dummerstorf high treadmill performance) mice in the SPF facility, we compared growth parameters, reproductive performance, the accretion of muscle and fat mass, physical activity, and running performance as well as food intake in all experimental groups. For selected parameters, the comparative analysis spans more than 30 generations. In DUC mice, under SPF conditions a more than threefold (P < 0.0001) higher subcutaneous fat mass, higher muscle mass by about 25% (P < 0.0001), but lower epididymal fat mass in DUhTP mice by about 20% (P < 0.0001) were observed. In SPF husbandry, body weight increased to a stronger extent in adult DUC mice (≈ 20%; P < 0.0001) than in DUhTP mice (≈ 8%; P = 0.001). The concentrations of IGF-1 and IGFBPs in the serum as well as the liver weights were similar in all experimental groups, indicating growth effects independent of the somatotropic axis. Under SPF conditions the litter size at birth increased in DUC mice (P < 0.001) but not in DUhTP mice. The differential effect of husbandry on body weights at day 21 and concentrations of triglycerides in the serum of our model were due to the different diets used in the semi-barrier and in the SPF facility. Our results demonstrate differential trait response to environmental factors resulting in partial phenotype conversion in our experimental system. The existence of conditional phenotypes as a result of genotype–environment interactions points to the importance of environmental factors in functional genome analysis.

Role of Mass Spectrometry in Clinical Endocrinology

The advent of mass spectrometry into the clinical laboratory has led to an improvement in clinical management of several endocrine diseases. Liquid chromatography tandem mass spectrometry found some of its first clinical applications in the diagnosis of inborn errors of metabolism, in quantitative steroid analysis, and in drug analysis laboratories. Mass spectrometry assays offer analytical sensitivity and specificity that is superior to immunoassays for many analytes. This article highlights several areas of clinical endocrinology that have witnessed the use of liquid chromatography tandem mass spectrometry to improve clinical outcomes.

Quantitative and Selective Analysis of Feline Growth Related Proteins Using Parallel Reaction Monitoring High Resolution Mass Spectrometry

Today immunoassays are widely used in veterinary medicine, but lack of species specific assays often necessitates the use of assays developed for human applications. Mass spectrometry (MS) is an attractive alternative due to high specificity and versatility, allowing for species-independent analysis. Targeted MS-based quantification methods are valuable complements to large scale shotgun analysis. A method referred to as parallel reaction monitoring (PRM), implemented on Orbitrap MS, has lately been presented as an excellent alternative to more traditional selected reaction monitoring/multiple reaction monitoring (SRM/MRM) methods. The insulin-like growth factor (IGF)-system is not well described in the cat but there are indications of important differences between cats and humans. In feline medicine IGF–I is mainly analyzed for diagnosis of growth hormone disorders but also for research, while the other proteins in the IGF-system are not routinely analyzed within clinical practice. Here, a PRM method for quantification of IGF–I, IGF–II, IGF binding protein (BP) –3 and IGFBP–5 in feline serum is presented. Selective quantification was supported by the use of a newly launched internal standard named QPrEST™. Homology searches demonstrated the possibility to use this standard of human origin for quantification of the targeted feline proteins. Excellent quantitative sensitivity at the attomol/μL (pM) level and selectivity were obtained. As the presented approach is very generic we show that high resolution mass spectrometry in combination with PRM and QPrEST™ internal standards is a versatile tool for protein quantitation across multispecies.

Current trends in mass spectrometry of peptides and proteins: Application to veterinary and sports-doping control

Detection of misuse of peptides and proteins as growth promoters is a major issue for sport and food regulatory agencies. The limitations of current analytical detection strategies for this class of compounds, in combination with their efficacy in growth-promoting effects, make peptide and protein drugs highly susceptible to abuse by either athletes or farmers who seek for products to illicitly enhance muscle growth. Mass spectrometry (MS) for qualitative analysis of peptides and proteins is well-established, particularly due to tremendous efforts in the proteomics community. Similarly, due to advancements in targeted proteomic strategies and the rapid growth of protein-based biopharmaceuticals, MS for quantitative analysis of peptides and proteins is becoming more widely accepted. These continuous advances in MS instrumentation and MS-based methodologies offer enormous opportunities for detection and confirmation of peptides and proteins. Therefore, MS seems to be the method of choice to improve the qualitative and quantitative analysis of peptide and proteins with growth-promoting properties. This review aims to address the opportunities of MS for peptide and protein analysis in veterinary control and sports-doping control with a particular focus on detection of illicit growth promotion. An overview of potential peptide and protein targets, including their amino acid sequence characteristics and current MS-based detection strategies is, therefore, provided. Furthermore, improvements of current and new detection strategies with state-of-the-art MS instrumentation are discussed for qualitative and quantitative approaches.

Direct-S: a directed mass spectrometry method for biomarker verification in native serum

Serum has been the logical choice and most-used bio-specimen for monitoring biomarkers. However, direct analysis of low-abundance biomarkers in serum is still a problem. Here, we have established a directed mass spectrometry (inclusion list driven MS) method, Direct-S, for direct quantification of protein biomarkers in native serum samples without high-abundance protein depletion or pre-fractionation. In Direct-S, an (18)O-labeling technique was used to produce internal standards of the targeted peptides, and only targeted peptides were selected for tandem mass spectrometry (MS/MS) fragmentation to increase sensitivity and efficiency. The (16)O/(18)O ion pairs of target peptides and the elution time/fragmental pattern of the internal standards were used to facilitate the identification of the low-abundance peptides. Using Direct-S, three candidate biomarkers, α1-antitrypsin (A1AT), galectin-3 binding protein (LG3BP) and cathepsin D (CTSD), which represent different abundance levels, were quantified in serum samples of colorectal cancer (CRC) patients and healthy candidates. Direct-S exhibited good linearity of response from 20 fmol to 0.5 nmol (r > 0.9845). Reliable quantification across five orders of magnitude and as low as 71 pg μL(-1) was achieved in serum samples. In conclusion, Direct-S is a low cost, convenient and accurate method for verifying serum biomarkers.

Clinical assays for quantitation of insulin-like-growth-factor-1 (IGF1)

Insulin-like growth factor 1 (IGF1), a 70 amino acid peptide hormone is the principal mediator of effects of growth hormone (GH). Since GH secretion is pulsatile in nature and is affected by many factors including sleep, feeding and exercise it is not a reliable marker for diagnosis of GH related disorders. On the other hand, IGF1 levels does not undergo short-term fluctuations in the manner that GH does making it the preferred IGF1 biomarker for the diagnosis of growth related disorders. There are several immunoassays available for IGF1 determination. Since majority (>90%) of IGF1 circulates as a ternary complex bound to its principal carrier/binding protein, IGF binding protein 3 (IGFBP3) and acid labile subunit (ALS), the assay methodology used to quantitate IGF1 has to dissociate IGF1 from IGFBPs prior to quantitation. IGFBPs are known to be a source of interference in immunoassays and many techniques have been employed to circumvent this issue. Immunoassays rely on antibody specificity towards IGF1 and differential cross reactivity towards IGFBPs. Mass spectrometry (MS) has also been employed for quantitation of IGF1. Liquid chromatography tandem mass spectrometry (LC-MS/MS) assays for IGF1 rely on generating tryptic peptides followed by selective reaction monitoring (SRM) while LC high resolution accurate-mass mass spectrometry (LC-HRAMS) approaches for intact IGF1 rely on mass accuracy for reliable, robust and accurate quantitation. This review article will focus on the clinical assays available and the clinical utility of quantitative assessment of IGF1. IGF1 quantitation using diverse assay platforms including immunoassay, LC-MS/MS and LC-HRAMS are discussed in detail.

Antibody-free workflows for protein quantification by LC–MS/MS

Antibody-free approaches for quantitative LC–MS/MS-based protein bioanalysis are reviewed and critically evaluated, and compared with the more widely used immunoaffinity-based approaches. Antibody-free workflows will be divided into four groups and discussed in the following order: direct analysis of signature peptides after proteolytic digestion; enrichment of target proteins and signature peptides by fractionated protein precipitation; enrichment of target proteins and signature peptides by reversed-phase and ion-exchange solid-phase extraction; and enrichment of target proteins and signature peptides by (antibody-free) affinity-solid-phase extraction.

Liquid chromatography tandem mass spectrometry in the clinical laboratory

Clinical laboratory medicine has seen the introduction and evolution of liquid chromatography tandem mass spectrometry in routine clinical laboratories over the last 10–15 years. There still exists a wide diversity of assays from very esoteric and highly specialist manual assays to more simplified kit-based assays. The technology is not static as manufacturers are continually making improvements. Mass spectrometry is now commonly used in several areas of diagnostics including therapeutic drug monitoring, toxicology, endocrinology, paediatrics and microbiology. Some of the most high throughput analyses or common analytes include vitamin D, immunosuppressant monitoring, androgen measurement and newborn screening. It also offers flexibility for the measurement of analytes in a variety of different matrices which would prove difficult with immunoassays. Unlike immunoassays or high-pressure liquid chromatography assays using ultraviolet or fluorescence detection, mass spectrometry offers better specificity and reduced interferences if attention is paid to potential isobaric compounds. Furthermore, multiplexing, which enables multiple analytes to be measured with the same volume of serum is advantageous, and the requirement for large sample volumes is decreasing as instrument sensitivity increases. There are many emerging applications in the literature. Using mass spectrometry to identify novel isoforms or modified peptides is possible as is quantification of proteins and peptides, with or without protein digests. Future developments by the manufacturers may also include mechanisms to improve the throughput of samples and strategies to decrease the level of skill required by the operators.

Quantification of peptides from immunoglobulin constant and variable regions by LC-MRM MS for assessment of multiple myeloma patients

PURPOSE

Quantitative MS assays for Igs are compared with existing clinical methods in samples from patients with plasma cell dyscrasias, for example, multiple myeloma (MM).

EXPERIMENTAL DESIGN

Using LC-MS/MS data, Ig constant region peptides, and transitions were selected for LC-MRM MS. Quantitative assays were used to assess Igs in serum from 83 patients. RNA sequencing and peptide-based LC-MRM are used to define peptides for quantification of the disease-specific Ig.

RESULTS

LC-MRM assays quantify serum levels of Igs and their isoforms (IgG1-4, IgA1-2, IgM, IgD, and IgE, as well as kappa (κ) and lambda (λ) light chains). LC-MRM quantification has been applied to single samples from a patient cohort and a longitudinal study of an IgE patient undergoing treatment, to enable comparison with existing clinical methods. Proof-of-concept data for defining and monitoring variable region peptides are provided using the H929 MM cell line and two MM patients.

CONCLUSIONS AND CLINICAL RELEVANCE

LC-MRM assays targeting constant region peptides determine the type and isoform of the involved Ig and quantify its expression; the LC-MRM approach has improved sensitivity compared with the current clinical method, but slightly higher inter-assay variability. Detection of variable region peptides is a promising way to improve Ig quantification, which could produce a dramatic increase in sensitivity over existing methods, and could further complement current clinical techniques.

Targeted selected reaction monitoring mass spectrometric immunoassay for insulin-like growth factor 1

Insulin-like growth factor 1 (IGF1) is an important biomarker of human growth disorders that is routinely analyzed in clinical laboratories. Mass spectrometry-based workflows offer a viable alternative to standard IGF1 immunoassays, which utilize various pre-analytical preparation strategies. In this work we developed an assay that incorporates a novel sample preparation method for dissociating IGF1 from its binding proteins. The workflow also includes an immunoaffinity step using antibody-derivatized pipette tips, followed by elution, trypsin digestion, and LC-MS/MS separation and detection of the signature peptides in a selected reaction monitoring (SRM) mode. The resulting quantitative mass spectrometric immunoassay (MSIA) exhibited good linearity in the range of 1 to 1,500 ng/mL IGF1, intra- and inter-assay precision with CVs of less than 10%, and lowest limits of detection of 1 ng/mL. The linearity and recovery characteristics of the assay were also established, and the new method compared to a commercially available immunoassay using a large cohort of human serum samples. The IGF1 SRM MSIA is well suited for use in clinical laboratories.

Fully validated LC-MS/MS assay for the simultaneous quantitation of coadministered therapeutic antibodies in cynomolgus monkey serum

An LC-MS/MS assay was developed and fully validated for the simultaneous quantitation of two coadministered human monoclonal antibodies (mAbs), mAb-A and mAb-B of IgG4 subclass, in monkey serum. The total serum proteins were digested with trypsin at 50 °C for 30 min after methanol denaturation and precipitation, dithiothreitol reduction, and iodoacetamide alkylation. The tryptic peptides were chromatographically separated with a C18 column (2.1 × 100 mm, 1.7 μm) with mobile phases of 0.1% formic acid in water and acetonitrile. Four peptides, a unique peptide for each mAb and two confirmatory peptides from different antibody domains, were simultaneously quantified by LC-MS/MS in the multiple reaction-monitoring mode. Stable isotopically labeled peptides with flanking amino acids on C- and N-terminals were used as internal standards to minimize the variability during sample processing and detection. The LC-MS/MS assay showed lower limit of quantitation (LLOQ) at 5 μg/mL for mAb-A and 25 μg/mL for mAb-B. The intra- and interassay precision (%CV) was within 10.0% and 8.1%, respectively, and the accuracy (%Dev) was within ±5.4% for all the peptides. Other validation parameters, including sensitivity, selectivity, dilution linearity, processing recovery and matrix effect, autosampler carryover, run size, stability, and data reproducibility, were all evaluated. The confirmatory peptides played a critical role in confirming quantitation accuracy and the integrity of the drugs in the study samples. The robustness of the LC-MS/MS assay and the data agreement with the ligand binding data demonstrated that LC-MS/MS is a reliable and complementary approach for the quantitation of coadministered antibody drugs.

Current status and advances in quantitative proteomic mass spectrometry

The accurate quantitation of proteins and peptides in complex biological systems is one of the most challenging areas of proteomics. Mass spectrometry-based approaches have forged significant in-roads allowing accurate and sensitive quantitation and the ability to multiplex vastly complex samples through the application of robust bioinformatic tools. These relative and absolute quantitative measures using label-free, tags, or stable isotope labelling have their own strengths and limitations. The continuous development of these methods is vital for increasing reproducibility in the rapidly expanding application of quantitative proteomics in biomarker discovery and validation. This paper provides a critical overview of the primary mass spectrometry-based quantitative approaches and the current status of quantitative proteomics in biomedical research.

Analytical challenges in the detection of peptide hormones for anti-doping purposes

Although significant progress has been achieved during the past few years with the introduction of new assays and analytical methodologies, the detection and quantification of protein analytes, in particular of peptide hormones, continues to pose analytical challenges for the World Anti-Doping Agency-accredited anti-doping laboratories. In this article, the latest achievements in the application of MS-based methodologies and specific biochemical and immunological assays to detect some of the prohibited substances listed in section S2 of the World Anti-Doping Agency List of Prohibited Substances and Methods are reviewed. In addition, we look towards the future by focusing on some of the most promising analytical approaches under development for the detection of so-called ‘biomarkers of doping’.

Functional and total IGFBP3 for the assessment of disorders of the GH/IGF1 axis in children with chronic kidney disease, GH deficiency, or short stature after SGA status at birth

OBJECTIVE

IGFBP3 immunoreactivity may appear elevated in patients with chronic kidney disease (CKD), in part due to accumulation of low molecular fragments. The importance of these IGFBP3 variants for binding and inactivation of IGF1 and their relevance for the impaired growth of uremic children are unclear. Nevertheless, IGFBP3, measured as total (t-)IGFBP3, is frequently used as a diagnostic parameter in pediatric CKD patients. A new assay for functional (f-)IGFBP3 exclusively detects IGFBP3 capable of IGF binding. The aim of the study was to evaluate the significance of f-IGFBP3 measurements for the assessment of uremic abnormalities of the GH/IGF1 axis.

DESIGN

Prospective cross-sectional study.

METHODS

t-IGFBP3, f-IGFBP3, and IGF1 were measured in pediatric CKD patients, including patients with CKD stage 3-4 not on dialysis (CKD, n=33), on dialysis treatment (DT, n=26), patients after renal transplantation (RTx, n=89), healthy children (n=29), children with GH deficiency (GHD, n=42), and small for gestational age (SGA) children (SGA, n=34).

RESULTS

Mean t-IGFBP3 SDS was elevated in CKD, DT, and RTx children compared with controls and GHD patients (P≤0.0004). Highest values were reached in DT (P<0.0001 vs all groups). In contrast, mean f-IGFBP3 was similar in all groups (P=0.30).

CONCLUSIONS

Pediatric CKD patients displayed elevated serum concentrations of t-IGFBP3 but not f-IGFBP3, supporting the hypothesis that IGFBP3 fragments not binding IGF1 accumulate during uremia. f-IGFBP3 is an indicator of IGFBP3 fragmentation and seems to reflect IGF1 binding in CKD better than t-IGFBP3. However, the role of f-IGFBP3 for the diagnosis of disturbances of the GH/IGF hormonal axis appears to be limited.

Growth hormone doping in sports: a critical review of use and detection strategies

GH is believed to be widely employed in sports as a performance-enhancing substance. Its use in athletic competition is banned by the World Anti-Doping Agency, and athletes are required to submit to testing for GH exposure. Detection of GH doping is challenging for several reasons including identity/similarity of exogenous to endogenous GH, short half-life, complex and fluctuating secretory dynamics of GH, and a very low urinary excretion rate. The detection test currently in use (GH isoform test) exploits the difference between recombinant GH (pure 22K-GH) and the heterogeneous nature of endogenous GH (several isoforms). Its main limitation is the short window of opportunity for detection (~12-24 h after the last GH dose). A second test to be implemented soon (the biomarker test) is based on stimulation of IGF-I and collagen III synthesis by GH. It has a longer window of opportunity (1-2 wk) but is less specific and presents a variety of technical challenges. GH doping in a larger sense also includes doping with GH secretagogues and IGF-I and its analogs. The scientific evidence for the ergogenicity of GH is weak, a fact that is not widely appreciated in athletic circles or by the general public. Also insufficiently appreciated is the risk of serious health consequences associated with high-dose, prolonged GH use. This review discusses the GH biology relevant to GH doping; the virtues and limitations of detection tests in blood, urine, and saliva; secretagogue efficacy; IGF-I doping; and information about the effectiveness of GH as a performance-enhancing agent.

Using quantitative proteomic analysis to understand genotype specific intrinsic drug resistance in melanoma

The discovery of activating BRAF V600E mutations in 50% of all melanoma patients and the development of small molecule BRAF inhibitors looks set to revolutionize the therapy of disseminated melanoma. However, in the recent clinical trial of the BRAF inhibitor, vemurafenib (PLX4032), a significant percentage of BRAF V600E mutant melanoma patients did not meet the RECIST criteria for a response. Recent work from our lab identified loss of the tumor suppressor phosphatase and tensin homolog (PTEN) as being a possible mediator of intrinsic BRAF inhibitor resistance. In this commentary, we describe the development of a novel mass spectrometry based proteomic screen of Bcl-2 family proteins that was used to delineate the PTEN-dependent differences in apoptosis signaling observed when BRAF was inhibited. We further discuss how use of these sensitive quantitative proteomic methods gives unique insights into the signaling of cancer cells that are not captured through routine biochemical techniques and how this may lead to the development of combination therapy strategies for overcoming intrinsic BRAF inhibitor resistance.

Monitoring a nuclear factor-κB signature of drug resistance in multiple myeloma

The emergence of acquired drug resistance results from multiple compensatory mechanisms acting to prevent cell death. Simultaneous monitoring of proteins involved in drug resistance is a major challenge for both elucidation of the underlying biology and development of candidate biomarkers for assessment of personalized cancer therapy. Here, we have utilized an integrated analytical platform based on SDS-PAGE protein fractionation prior to liquid chromatography coupled to multiple reaction monitoring mass spectrometry, a versatile and powerful tool for targeted quantification of proteins in complex matrices, to evaluate a well-characterized model system of melphalan resistance in multiple myeloma (MM). Quantitative assays were developed to measure protein expression related to signaling events and biological processes relevant to melphalan resistance in multiple myeloma, specifically: nuclear factor-κB subunits, members of the Bcl-2 family of apoptosis-regulating proteins, and Fanconi Anemia DNA repair components. SDS-PAGE protein fractionation prior to liquid chromatography coupled to multiple reaction monitoring methods were developed for quantification of these selected target proteins in amounts of material compatible with direct translation to clinical specimens (i.e. less than 50,000 cells). As proof of principle, both relative and absolute quantification were performed on cell line models of MM to compare protein expression before and after drug treatment in naïve cells and in drug resistant cells; these liquid chromatography-multiple reaction monitoring results are compared with existing literature and Western blots. The initial stage of a systems biology platform for examining drug resistance in MM has been implemented in cell line models and has been translated to MM cells isolated from a patient. The ultimate application of this platform could assist in clinical decision-making for individualized patient treatment. Although these specific assays have been developed to monitor MM, these techniques are expected to have broad applicability in cancer and other types of disease.

Multiplexed protein quantification in maize leaves by liquid chromatography coupled with tandem mass spectrometry: an alternative tool to immunoassays for target protein analysis in genetically engineered crops

A multiplexing liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) method to quantify three proteins in maize leaves was developed and validated. For each protein, a hybrid Q-TRAP mass spectrometer was operated in the information-dependent acquisition (IDA) mode to select optimal potential signature peptides. The respective signature peptides were then further optimized and quantified as protein surrogates by multiple reaction monitoring (MRM). Leaf crude extracts were subject to microwave-assisted trypsin digestion for 30 min and then injected directly onto a high-performance liquid chromatography (HPLC) column without further separation or enrichment. The minimum sample process enabled us to achieve high recovery and good reproducibility, with a throughput of 200 samples per day. Using recombinant proteins as standards, a linear dynamic quantitative range of 2 orders of magnitude was obtained (correlation coefficient > 0.997) with good accuracy (deviation from nominal concentration < 15%) for all three proteins. Our study demonstrates that LC-MS/MS can be used as an alternative to immunoassays to quantify multiple low abundant proteins in genetically engineered crops.

Development and validation of an enzyme-linked immunosorbent assay for the detection of circulating antibodies raised against growth hormone as a consequence of rbST treatment in cows

The recombinant bovine growth hormone (rbST) is used to increase lactating performances of dairy cows. Administration of rbST is banned in the European Union; nevertheless, its use is probable. Until now, efficient analytical strategies to detect such practice are based on the direct detection by mass spectrometry of the presence of rbST in biological fluids, which suits for confirmatory purposes. Current screening strategies do not offer satisfactory performances; therefore, alternative screening strategies are required. The aim of the present work is to develop and validate an ELISA to measure the production of specific antibodies upon rbST in bovine sera. In this immunoassay, rbST is absorbed onto microtiter plate. After specific purification of the antibodies in serum, samples are analysed and the presence of antibodies anti-rbST is detected by Protein G peroxidase conjugate and 2-2'-azino di-ethyl benz-thiazoline-6-sulphonic acid (ABTS). The mean reproducibility of the OD (λ=405 nm) measurement was calculated with a CV of 13%. The intra- and inter-assay CVs ranged from 0.79% to 7.91% and from 2.69% to 20% respectively. The test presents cross-reaction with other growth hormones such as the recombinant equine (reST) and porcine (pST) (100% and 80% respectively). The specificity of the test toward rbST anabolic treatment was confirmed through the analysis of sera samples collected on animals administered with other anabolic compounds (steroids). The performances of the present anti-rbST ELISA proves its efficiency as a new screening tool to highlight illegal administration of rbST in cattle up to at least 3 weeks after treatment.

Quantitative liquid chromatography tandem mass spectrometry analysis of macromolecules using signature peptides in biological fluids

Targeted protein quantification using peptide surrogates has increasingly become important to the validation of biomarker candidates and development of protein therapeutics. These approaches have been proposed and employed as alternatives to immunoassays in biological fluids. Technological advances over the last 20 years in biochemistry and mass spectrometry have prompted the use of peptides as surrogates to quantify enzyme digested proteins using triple quadrupole mass spectrometers. Multiple sample preparation processes are often incorporated to achieve quantification of target proteins using these signature peptides. This review article focuses on these processes or hyphenated techniques for quantification of proteins with peptide surrogates. The most recent advances and strategies involved with hyphenated techniques are discussed.