High throughput quantification of apolipoproteins A-I and B-100 by isotope dilution MS targeting fast trypsin releasable peptides without reduction and alkylation

Proteomic studies on apoB-containing lipoprotein in cardiovascular research: A comprehensive review

The complexity of cardiovascular diseases (CVDs), which remains the leading cause of death worldwide, makes the current clinical pathway for cardiovascular risk assessment unsatisfactory, as there remains a substantial unexplained residual risk. Simultaneous assessment of a large number of plasma proteins may be a promising tool to further refine risk assessment, and lipoprotein-associated proteins have the potential to fill this gap. Technical advances now allow for high-throughput proteomic analysis in a reproducible and cost-effective manner. Proteomics has great potential to identify and quantify hundreds of candidate marker proteins in a sample and allows the translation from isolated lipoproteins to whole plasma, thus providing an individual multiplexed proteomic fingerprint. This narrative review describes the pathophysiological roles of atherogenic apoB-containing lipoproteins and the recent advances in their mass spectrometry-based proteomic characterization and quantitation for better refinement of CVD risk assessment.

Fast and straightforward simultaneous quantification of multiple apolipoproteins in human serum on a high-throughput LC-MS/MS platform

PURPOSE

Apolipoprotein monitoring is useful for diagnosing cardiovascular diseases, as they are risk factors of arteriosclerosis and other neutral fat-related diseases. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is advantageous for simultaneous apolipoprotein quantification, differentiation, and standardization including their isoforms. However, fast and straightforward sample preparation that retains quantification accuracy remains challenging in clinical MS.

EXPERIMENTAL DESIGN

We developed a simultaneous assay for serum apolipoprotein A-I (ApoA-I), apolipoprotein B100 family, and apolipoprotein C-III (ApoC-III) using a high-throughput LC-MS/MS platform coupled with a BRAVO system. The assay was simplified by using sodium deoxycholate and trypsin/lys-C without reduction and alkylation steps.

RESULTS

Simple sample preparation reduced turnaround time by 1.5 h and neat goat serum was chosen as an optimal calibration matrix for accurate protein quantification. Assay precision, linearity, correlation, accuracy, limit of detection (LOD), limit of quantitation (LOQ), and carryover were validated according to CLSI guidelines over 41 days using more than 100 human serum samples. Good correlation compared with turbidimetric immunoassay (TIA) was observed by Deming regression for all analytes.

CONCLUSIONS AND CLINICAL RELEVANCE

A high-throughput LC-MS/MS and BRAVO assay for simultaneous apolipoprotein analysis was validated using a simple preparation method with a human serum calibrator in goat serum matrix. The assay is readily expandable to include other target serum proteins and/or their isoforms.

Use of an alternative signature peptide during development of a LC-MS/MS assay of plasma nivolumab levels applicable for multiple species

Recently, immune checkpoint inhibitors, including anti-programmed cell death protein 1 (PD-1) antibodies, have dramatically changed treatment strategies for several cancers. In pharmacokinetic/pharmacodynamic studies, experiments using a variety of animal species are assumed. We have identified optimal multiple reaction monitoring transitions for signature candidate peptides of nivolumab in human, mouse, and rat plasma and developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to quantify nivolumab (an anti-PD-1 antibody) using trastuzumab as the internal standard. Calibration curves were linear in the range of 1-200 µg/mL. The intra- and inter-day precision and accuracy in human plasma fulfilled Food and Drug Administration guideline criteria for bioanalytical validation. There was no need to change the measurement method in mouse plasma. On the other hand, in rat plasma, an interference peak was observed at a retention time similar to that of the surrogate peptide ASGITFSNSGMHWVR (550.75 > 661.50) employed in human and mouse plasma. Therefore, we confirmed that ASQSVSSYLAWYQQKPGQAPR (785.0 > 940.2) can be used as an alternate nivolumab surrogate peptide in rat plasma at the same concentration range as used in human and mouse plasma. Using our method, the concentration range and a gradual increase in trough value were confirmed in clinical samples from two antibody-treated patients, including one with gastric cancer and one with non-small-cell lung cancer. The time course and blood concentration transition also were evaluated in nivolumab administration experiments in mouse and rat. The present study showed that the selection of the optimal peptide is essential for accurate LC-MS/MS measurement of nivolumab concentration in human, mouse, and rat plasma. The method developed here is expected to be of use in non-clinical and clinical pharmacokinetic studies.

Comparability of Lipoprotein Particle Number Concentrations Across ES-DMA, NMR, LC-MS/MS, Immunonephelometry, and VAP: In Search of a Candidate Reference Measurement Procedure for apoB and non-HDL-P Standardization

BACKGROUND

Despite the usefulness of standard lipid parameters for cardiovascular disease risk assessment, undiagnosed residual risk remains high. Advanced lipoprotein testing (ALT) was developed to provide physicians with more predictive diagnostic tools. ALT methods separate and/or measure lipoproteins according to different parameters such as size, density, charge, or content, and equivalence of results across methods has not been demonstrated.

METHODS

Through a split-sample study, 25 clinical specimens (CSs) were assayed in 10 laboratories before and after freezing using the major ALT methods for non-HDL particles (non-HDL-P) or apolipoprotein B-100 (apoB-100) measurements with the intent to assess their comparability in the current state of the art.

RESULTS

The overall relative standard deviation (CV) of non-HDL-P and apoB-100 concentrations measured by electrospray differential mobility analysis, nuclear magnetic resonance, immunonephelometry, LC-MS/MS, and vertical autoprofile in the 25 frozen CSs was 14.1%. Within-method comparability was heterogeneous, and CV among 4 different LC-MS/MS methods was 11.4% for apoB-100. No significant effect of freezing and thawing was observed.

CONCLUSIONS

This study demonstrates that ALT methods do not yet provide equivalent results for the measurement of non-HDL-P and apoB-100. The better agreement between methods harmonized to the WHO/IFCC reference material suggests that standardizing ALT methods by use of a common commutable calibrator will improve cross-platform comparability. This study provides further evidence that LC-MS/MS is the most suitable candidate reference measurement procedure to standardize apoB-100 measurement, as it would provide results with SI traceability. The absence of freezing and thawing effect suggests that frozen serum pools could be used as secondary reference materials.

Core lipid, surface lipid and apolipoprotein composition analysis of lipoprotein particles as a function of particle size in one workflow integrating asymmetric flow field-flow fractionation and liquid chromatography-tandem mass spectrometry

Lipoproteins are complex molecular assemblies that are key participants in the intricate cascade of extracellular lipid metabolism with important consequences in the formation of atherosclerotic lesions and the development of cardiovascular disease. Multiplexed mass spectrometry (MS) techniques have substantially improved the ability to characterize the composition of lipoproteins. However, these advanced MS techniques are limited by traditional pre-analytical fractionation techniques that compromise the structural integrity of lipoprotein particles during separation from serum or plasma. In this work, we applied a highly effective and gentle hydrodynamic size based fractionation technique, asymmetric flow field-flow fractionation (AF4), and integrated it into a comprehensive tandem mass spectrometry based workflow that was used for the measurement of apolipoproteins (apos A-I, A-II, A-IV, B, C-I, C-II, C-III and E), free cholesterol (FC), cholesterol esters (CE), triglycerides (TG), and phospholipids (PL) (phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and lysophosphatidylcholine (LPC)). Hydrodynamic size in each of 40 size fractions separated by AF4 was measured by dynamic light scattering. Measuring all major lipids and apolipoproteins in each size fraction and in the whole serum, using total of 0.1 ml, allowed the volumetric calculation of lipoprotein particle numbers and expression of composition in molar analyte per particle number ratios. Measurements in 110 serum samples showed substantive differences between size fractions of HDL and LDL. Lipoprotein composition within size fractions was expressed in molar ratios of analytes (A-I/A-II, C-II/C-I, C-II/C-III. E/C-III, FC/PL, SM/PL, PE/PL, and PI/PL), showing differences in sample categories with combinations of normal and high levels of Total-C and/or Total-TG. The agreement with previous studies indirectly validates the AF4-LC-MS/MS approach and demonstrates the potential of this workflow for characterization of lipoprotein composition in clinical studies using small volumes of archived frozen samples.

Targeted On-line SPE-LC-MS/MS Assay for the Quantitation of 12 Apolipoproteins from Human Blood

Laborious sample pretreatment of biological samples represents the most limiting factor for the translation of targeted proteomics assays from research to clinical routine. An optimized method for the simultaneous quantitation of 12 major apolipoproteins (apos) combining on-line SPE and fast LC-MS/MS analysis in 6.5 min total run time was developed, reducing the manual sample pretreatment time of 3 μL serum or plasma by 60%. Within-run and between-day imprecisions below 10 and 15% (n = 10) and high recovery rates (94-131%) were obtained applying the high-throughput setup. High-quality porcine trypsin was used, which outperformed cost-effective bovine trypsin regarding digestion efficiency. Comparisons with immunoassays and another LC-MS/MS assay demonstrated good correlation (Pearson's R: 0.81-0.98). Further, requirements on sample quality concerning sampling, processing, and long-term storage up to 1 year were investigated revealing significant influences of the applied sampling material and coagulant on quantitation results. Apo profiles of 1339 subjects of the LIFE-Adult-Study were associated with lifestyle and physiological parameters as well as establish parameters of lipid metabolism (e.g., triglycerides, cholesterol). Besides gender effects, most significant impact was seen regarding lipid-lowering medication. In conclusion, this novel highly standardized, high-throughput targeted proteomics assay utilizes a fast, simultaneous analysis of 12 apos from least sample amounts.

Advances in quantifying apolipoproteins using LC-MS/MS technology: implications for the clinic

INTRODUCTION

Apolipoproteins play a key role in pre-, pro-, and anti-atherosclerotic processes and have become important circulating biomarkers for the prediction of cardiovascular disease (CVD) risk. Whereas currently clinical immunoassays are not available for most apolipoproteins and lack the capacity for multiplexing, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) allows simultaneous, highly-specific, and precise quantification of multiple apolipoproteins. Areas covered: We discuss LC-MS/MS methods for quantification of apolipoproteins reported in the literature and highlight key requirements for clinical use. Besides the advances in sample preparation and LC-MS/MS technologies, this overview also discusses advances in proteoform analysis and applications of dried blood/plasma collection. Expert commentary: Standardized quantification using LC-MS/MS technology has been demonstrated for apolipoprotein A-I and B. However, for implementation in clinical CVD risk assessment, LC-MS/MS must bring significant added clinical value in comparison to fast, standardized, and straightforward clinical (immuno)assays. Ongoing advances in accuracy and multiplexing capacity of LC-MS/MS, nonetheless, bear potential to enable standardized and interpretable personalized profiling of a patient's CVD risk by simultaneous quantification of multiple apolipoproteins and -variants. We, moreover, anticipate further personalization of CVD risk assessment by the potential of LC-MS/MS to enable simultaneous genotyping and remote monitoring using dried blood/plasma collection devices.

High throughput quantification of apolipoproteins A-I and B-100 by isotope dilution MS targeting fast trypsin releasable peptides without reduction and alkylation

PURPOSE

Apolipoprotein A-I (ApoA-I) and apolipoprotein B-100 (ApoB-100) are amphipathic proteins that are strong predictors of cardiovascular disease risk. The traceable calibration of apolipoprotein assays is a persistent challenge, especially for ApoB-100, which cannot be solubilized in purified form.

EXPERIMENTAL DESIGN

A simultaneous quantitation method for ApoA-I and ApoB-100 was developed using tryptic digestion without predigestion reduction and alkylation, followed by LC separation coupled with isotope dilution MS analysis. The accuracy of the method was assured by selecting structurally exposed signature peptides, optimal choice of detergent, protein:enzyme ratio, and incubation time. Peptide calibrators were value assigned by isobaric tagging isotope dilution MS amino acid analysis.

RESULTS

The method reproducibility was validated in technical repeats of three serum samples, giving 2-3% intraday CVs (N = 5) and <7% interday CVs (N = 21). The repeated analysis of interlaboratory harmonization standards showed -1% difference for ApoA-I and -12% for ApoB-100 relative to the assigned value. The applicability of the method was demonstrated by repeated analysis of 24 patient samples with a wide range of total cholesterol and triglyceride levels.

CONCLUSIONS AND CLINICAL RELEVANCE

The method is applicable for simultaneous analysis of ApoA-I and ApoB-100 in patient samples, and for characterization of serum pool calibrators for other analytical platforms.