Ultra-fast quantitation of saquinavir in human plasma by matrix-assisted laser desorption/ionization and selected reaction monitoring mode detection

Rapid determination of antiviral drugs in yellow catfish (Pelteobagrus fulvidraco) using graphene/silica nanospheres (G/KCC-1) based pipette tip solid-phase extraction with ultra-performance liquid chromatography-tandem mass spectrometry

Yellow catfish (Pelteobagrus fulvidraco) is commonly contaminated by protease inhibitors because of the illegal use of antiviral drugs in aquaculture, so the determination of antiviral drugs is essential in food safety supervision. In this study, a novel sorbent, graphene and silica nanospheres composite (G/KCC-1), was synthesized for pipette-tip-based solid-phase extraction (PT-SPE) and purification of ritonavir, saquinavir, and indinavir in yellow catfish, followed by ultra-performance liquid chromatography and tandem mass spectrometry (UPLC-MS/MS) analysis. The wrinkly structure of G/KCC-1 with center-radial nanowrinkles enlarged the surface area and increased the capacity of absorbing the target analyte. With the proposed G/KCC-1 based PT-SPE UPLC-MS/MS method, the pH of sample solution, aspirating/dispensing cycles for extraction and elution were optimized to be 4.0, 25, and 10 respectively, and the eluting solvent was methanol/ammonia (95:5, v/v) with 0.02 M sodium chloride. This new method was further validated to be linear (correlation coefficient R², 0.9993-0.9996), sensitive (limit of detection, LOD ≤ 0.8 ng mL^-1), accurate (89.3-114.2%), and precise (relative standard deviation, RSD ≤ 6.23%). These results indicated that the proposed method is qualified in bioanalytical method validation and meets the requirements for detecting illegally used antiviral drugs in yellow catfish. The demonstrated G/KCC-1 based PT-SPE UPLC-MS/MS method is a potential analytical method in food and drug administration.

Quantification of low molecular weight compounds by MALDI imaging mass spectrometry - A tutorial review

Matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) permits label-free in situ analysis of chemical compounds directly from the surface of two-dimensional biological tissue slices. It links qualitative molecular information of compounds to their spatial coordinates and distribution within the investigated tissue. MALDI-MSI can also provide the quantitative amounts of target compounds in the tissue, if proper calibration techniques are performed. Obviously, as the target molecules are embedded within the biological tissue environment and analysis must be performed at their precise locations, there is no possibility for extensive sample clean-up routines or chromatographic separations as usually performed with homogenized biological materials; ion suppression phenomena therefore become a critical side effect of MALDI-MSI. Absolute quantification by MALDI-MSI should provide an accurate value of the concentration/amount of the compound of interest in relatively small, well-defined region of interest of the examined tissue, ideally in a single pixel. This goal is extremely challenging and will not only depend on the technical possibilities and limitations of the MSI instrument hardware, but equally on the chosen calibration/standardization strategy. These strategies are the main focus of this article and are discussed and contrasted in detail in this tutorial review. This article is part of a Special Issue entitled: MALDI Imaging, edited by Dr. Corinna Henkel and Prof. Peter Hoffmann.

MALDI-MS drug analysis in biological samples: opportunities and challenges

Drug analysis represents a large field in different disciplines. Plasma is commonly considered to be the biosample of choice for that purpose. However, concentrations often do not represent the levels present within deeper compartments and therefore cannot sufficiently explain efficacy or toxicology of drugs. MALDI-MS in drug analysis is of great interest for high-throughput quantification and particularly spatially resolved tissue imaging. The current perspective article will deal with challenges and opportunities of MALDI-MS drug analysis in different biological samples. A particular focus will be on hair samples. Recent applications were included, reviewed for their instrumental setup and sample preparation and pros and cons as well as future perspectives are critically discussed.

The use of mass spectrometry to analyze dried blood spots

Dried blood spots (DBS) typically consist in the deposition of small volumes of capillary blood onto dedicated paper cards. Comparatively to whole blood or plasma samples, their benefits rely in the fact that sample collection is easier and that logistic aspects related to sample storage and shipment can be relatively limited, respectively, without the need of a refrigerator or dry ice. Originally, this approach has been developed in the sixties to support the analysis of phenylalanine for the detection of phenylketonuria in newborns using bacterial inhibition test. In the nineties tandem mass spectrometry was established as the detection technique for phenylalanine and tyrosine. DBS became rapidly recognized for their clinical value: they were widely implemented in pediatric settings with mass spectrometric detection, and were closely associated to the debut of newborn screening (NBS) programs, as a part of public health policies. Since then, sample collection on paper cards has been explored with various analytical techniques in other areas more or less successfully regarding large-scale applications. Moreover, in the last 5 years a regain of interest for DBS was observed and originated from the bioanalytical community to support drug development (e.g., PK studies) or therapeutic drug monitoring mainly. Those recent applications were essentially driven by improved sensitivity of triple quadrupole mass spectrometers. This review presents an overall view of all instrumental and methodological developments for DBS analysis with mass spectrometric detection, with and without separation techniques. A general introduction to DBS will describe their advantages and historical aspects of their emergence. A second section will focus on blood collection, with a strong emphasis on specific parameters that can impact quantitative analysis, including chromatographic effects, hematocrit effects, blood effects, and analyte stability. A third part of the review is dedicated to sample preparation and will consider off-line and on-line extractions; in particular, instrumental designs that have been developed so far for DBS extraction will be detailed. Flow injection analysis and applications will be discussed in section IV. The application of surface analysis mass spectrometry (DESI, paper spray, DART, APTDCI, MALDI, LDTD-APCI, and ICP) to DBS is described in section V, while applications based on separation techniques (e.g., liquid or gas chromatography) are presented in section VI. To conclude this review, the current status of DBS analysis is summarized, and future perspectives are provided.

Regulated bioanalysis of conformers - A case study with ASP2151 in dog plasma and urine

We developed and validated bioanalytical methods for a potent helicase-primase inhibitor ASP2151 that has two conformers. These conformers elute as unseparated broad peaks under ordinary high-performance liquid chromatographic conditions, indicating discernable differences in hydrophobicity. We observed that column temperature and mobile phase pH have no effect on these peaks and that conformers form a single symmetrical peak when tetrahydrofuran is added to the mobile phase. In addition, we needed to develop semi-automated methods where inter-conversion of the conformers is unlikely to cause sample-to-sample extraction variability. Briefly, following the addition of deuterium-labeled ASP2151 as an internal standard (IS), dog plasma samples or acetonitrile-added urine samples were filtrated. The filtrates were then injected into a column-switching liquid chromatography-tandem mass spectrometry (LC-MS/MS) system and trapped onto an extraction column. Extracts were back-flushed onto an analytical C18 column (4.6×50mm, 3μm) with a mobile phase consisting of methanol, tetrahydrofuran, and 20mmol/L ammonium acetate (45:5:50, v/v/v). The eluent was monitored in the negative atmospheric pressure chemical ionization mode. The calibration curve was linear over a range of 5-1000ng/mL for plasma and 0.5-100μg/mL for urine. Validation data met the acceptance criteria in accordance with regulatory guidance and demonstrated that these methods were selective, accurate, and reproducible. In addition, the present methods were successfully applied to a pharmacokinetic study in dogs.

Fast quantitative determination of methylphenidate levels in rat plasma and brain ex vivo by MALDI-MS/MS

This study presents a simple and sensitive high-throughput matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry (MALDI-MS/MS) method for ex vivo quantification of methylphenidate (MPH) in rat plasma and brain. The common MALDI matrix alpha-cyano-4-hydroxycinnamic acid was used to obtain an optimal dried droplet preparation. For method validation, standards diluted in plasma and brain homogenate prepared from untreated (control) rats were used. MPH was quantified within a concentration range of 0.1-40 ng/ml in plasma and 0.4-40 ng/ml in brain homogenate with an excellent linearity (R²  ≥ 0.9997) and good precision. The intra-day and inter-day accuracies fulfilled the FDA's ±15/20 critera. The recovery of MPH ranged from 93.8 to 98.5% and 87.2 to 99.8% in plasma and homogenate, respectively. We show that MPH is successfully quantified in plasma and brain homogenate of rats pre-treated with this drug using the internal standard calibration method. By means of this method, a linear correlation between plasma and brain concentration of MPH in rodents pre-treated with MPH was detected. The simple sample preparation based on liquid-liquid extraction and MALDI-MS/MS measurement requires approximately 10 s per sample, and this significantly reduces analysis time compared with other analytical methods. To the best of our knowledge, this is the first MALDI-MS/MS method for quantification of MPH in rat plasma and brain. Copyright © 2015 John Wiley & Sons, Ltd.

Quantification in MALDI-MS imaging: what can we learn from MALDI-selected reaction monitoring and what can we expect for imaging?

Quantification by mass spectrometry imaging (Q-MSI) is one of the hottest topics of the current discussions among the experts of the MS imaging community. If MSI is established as a powerful qualitative tool in drug and biomarker discovery, its reliability for absolute and accurate quantification (QUAN) is still controversial. Indeed, Q-MSI has to deal with several fundamental aspects that are difficult to control, and to account for absolute quantification. The first objective of this manuscript is to review the state-of-the-art of Q-MSI and the current strategies developed for absolute quantification by direct surface sampling from tissue sections. This includes comments on the quest for the perfect matrix-matched standards and signal normalization approaches. Furthermore, this work investigates quantification at a pixel level to determine how many pixels must be considered for accurate quantification by ultraviolet matrix-assisted laser desorption/ionization (MALDI), the most widely used technique for MSI. Particularly, this study focuses on the MALDI-selected reaction monitoring (SRM) in rastering mode, previously demonstrated as a quantitative and robust approach for small analyte and peptide-targeted analyses. The importance of designing experiments of good quality and the use of a labeled compound for signal normalization is emphasized to minimize the signal variability. This is exemplified by measuring the signal for cocaine and a tryptic peptide (i.e., obtained after digestion of a monoclonal antibody) upon different experimental conditions, such as sample stage velocity, laser power and frequency, or distance between two raster lines. Our findings show that accurate quantification cannot be performed on a single pixel but requires averaging of at least 4-5 pixels. The present work demonstrates that MALDI-SRM/MSI is quantitative with precision better than 10-15 %, which meets the requirements of most guidelines (i.e., in bioanalysis or toxicology) for quantification of drugs or peptides from tissue homogenates.

Quantification of saquinavir from lysates of peripheral blood mononuclear cells using microarrays and standard MALDI-TOF-MS

Drug monitoring is usually performed by liquid chromatography coupled with optical detection or electrospray ionization mass spectrometry. More recently, matrix-assisted laser desorption/ionization (MALDI) in combination with triple quadrupole or Fourier-transform (FT) mass analyzers has also been reported to allow accurate quantification. Here, we present a strategy that employs standard MALDI time-of-flight (TOF) mass spectrometry (MS) for the sensitive and accurate quantification of saquinavir from an extract of blood peripheral mononuclear cells. Unambiguous identification of saquinavir in the mass spectra was possible because of using internal mass calibration and by an overall low chemical noise in the low mass range. Exact mass determination of the constant background peaks of the cell extract, which were used for recalibration, was performed by an initial MALDI-FT-MS analysis. Fast and multiplexed sample analysis was enabled by microarray technology, which provided 10 replicates in the lower nL range for each sample in parallel lanes on a chip. In order to validate the method, we employed various statistical tests, such as confidence intervals for linear regressions, three quality control samples, and inverse confidence limits of the estimated concentration ratios.

Advancing the sensitivity of selected reaction monitoring-based targeted quantitative proteomics

Selected reaction monitoring (SRM) - also known as multiple reaction monitoring (MRM) - has emerged as a promising high-throughput targeted protein quantification technology for candidate biomarker verification and systems biology applications. A major bottleneck for current SRM technology, however, is insufficient sensitivity for, e.g. detecting low-abundance biomarkers likely present at the low ng/mL to pg/mL range in human blood plasma or serum, or extremely low-abundance signaling proteins in cells or tissues. Herein, we review recent advances in methods and technologies, including front-end immunoaffinity depletion, fractionation, selective enrichment of target proteins/peptides including posttranslational modifications, as well as advances in MS instrumentation which have significantly enhanced the overall sensitivity of SRM assays and enabled the detection of low-abundance proteins at low- to sub-ng/mL level in human blood plasma or serum. General perspectives on the potential of achieving sufficient sensitivity for detection of pg/mL level proteins in plasma are also discussed.

Characterization of β-lactamase enzyme activity in bacterial lysates using MALDI-mass spectrometry

Plasmid-encoded β-lactamases are a major reason for antibiotic resistance in gram negative bacteria. These enzymes hydrolyze the β-lactam ring structure of certain β-lactam antibiotics, consequently leading to their inactivation. The clinical situation demands for specific first-line antibiotic therapy combined with a quick identification of bacterial strains and their antimicrobial susceptibility. Strategies for the identification of β-lactamase activity are often cumbersome and usually lack sensitivity and specificity. The current work demonstrates that matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is an ideal tool for these analytical investigations. Herein, we describe a fast and specific assay to determine β-lactamase activity in bacterial lysates. The feasibility of the analytical read-out was demonstrated on a MALDI-triple quadrupole (QqQ) and a MALDI time-of-flight (TOF) instrument, and the results allow the comparison of both approaches. The assay specifically measures enzyme-mediated, time-dependent hydrolysis of the β-lactam ring structure of penicillin G and ampicillin and inhibition of hydrolysis by clavulanic acid for clavulanic acid susceptible β-lactamases. The assay is reproducible and builds the basis for future in-depth investigations of β-lactamase activity in various bacterial strains by mass spectrometry.

Assessment of intracellular methotrexate and methotrexate-polyglutamate metabolite concentrations in erythrocytes by ultrafast matrix-assisted laser desorption/ionization triple quadrupole tandem mass spectrometry

A new ultrafast quantitative and high-throughput mass spectrometric method using matrix-assisted laser desorption/ionization triple quadrupole tandem mass spectrometry has been developed and validated for determination of intracellular erythrocyte concentrations of the antifolate drug methotrexate (MTX) and its polyglutamate metabolites. The method consists of a solid-phase extraction of MTX and MTX-polyglutamate metabolites from deproteinized erythrocyte lysates spiked with aminopterin as internal standard. The newly developed method was validated according to the most recent FDA guidelines on linearity, recovery, within-run and between-run accuracy and precision and stability of the analytes. The low limit of quantification (LLOQ) was 10 nmol/L for all analytes while the limit of detection (LOD) determined at a signal-to-noise (S/N) ratio = 3:1 in drug- free erythrocyte lysate was on average 0.3 nmol/L. After validation, the new method was used in the measurement of intracellular erythrocyte concentrations of MTX and MTX-polyglutamate metabolites (MTXPG2 to MTXPG7) in packed human erythrocyte samples collected from patients with rheumatoid arthritis receiving low-dose oral methotrexate therapy. Mean (SD) intracellular erythrocyte concentrations observed in patient samples were 12.8 (12.6), 12.4 (9.4), 44.4 (30.0), 33.6 (35.9) and 9.4 (8.2) nmol/L for MTX to MTXPG5, respectively, in 10(6) erythrocytes. The highest observed glutamylation degree of MTX was MTXPG5, the very long chain MTX-polyglutamate metabolites MTXPG6 and MTXPG7 were not detected in the packed erythrocyte pellets collected from rheumatoid arthritis patients.

Fast quantitative determination of melamine and its derivatives by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A simple, sensitive and fast method for the determination of melamine and its derivatives in milk powder using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was developed. Neither time-consuming sample preparation, nor special target plates, or other extra equipment are necessary. The common matrix sinapinic acid (SA) was used with a dried-droplet preparation. Detection limits (signal-to-noise (S/N) ratio = 3) for standard solutions of melamine, ammeline and cyanuric acid were 10, 25 and 10 µg/L, respectively. The limit of quantification (LOQ) for melamine was 25 µg/L and excellent linearity (R(2): 0.9990) was maintained over the range of 10-2000 µg/L. Ammeline and cyanuric acid were analyzed with an LOQ of 50 µg/L and also excellent linearity (R(2): 0.9997 and R(2): 0.9998). Good accuracy and precision were obtained for all concentrations within the range of the standard curve. The developed method was successfully used for the determination of melamine, ammeline and cyanuric acid in milk powder samples with a simple sample preparation. The LOQ of melamine was 0.5 µg/g. Ammeline and cyanuric acid were detectable at 0.5 and 5 µg/g. This method showed excellent accuracy, precision and linearity and significantly reduces the needed analysis time, as only approximately 10 s/sample measuring time is required. To the authors' knowledge, this is the first published method to quantify melamine and derivatives by MALDI-TOF-MS.

Imaging of intact tissue sections: moving beyond the microscope

MALDI-imaging MS is a new molecular imaging technology for direct in situ analysis of thin tissue sections. Multiple analytes can be monitored simultaneously without prior knowledge of their identities and without the need for target-specific reagents such as antibodies. Imaging MS provides important insights into biological processes because the native distributions of molecules are minimally disturbed, and histological features remain intact throughout the analysis. A wide variety of molecules can be imaged, including proteins, peptides, lipids, drugs, and metabolites. Several specific examples are presented to highlight the utility of the technology.

Single hair cocaine consumption monitoring by mass spectrometric imaging

Matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) was used to image the distribution of cocaine and its metabolites in intact single hair samples from chronic users down to a concentration of 5 ng/mg. Acquisitions were performed in rastering mode, at a speed of 1 mm/s and in the selected reaction monitoring (SRM) mode on a MALDI triple quadrupole linear ion trap fitted with a high repetition rate laser (1 kHz). Compared to traditional methods based on LC-MS/MS or GC-MS(/MS) which require to segment the hair to obtain spatial resolution, MALDI-MSI, with a straightforward sample preparation beforehand, allowed obtaining a spatial resolution of 1 mm and thus the chronological information about cocaine consumption contained in a single intact hair over several months could be monitored. The analysis time of an intact single hair sample of 6 cm is approximately of 6 min. Cocaine and its metabolites benzoylecgonine, ethylcocaine, and norcocaine were investigated in nine sets of hair samples for forensic purposes. The analyses were accomplished by spraying α-cyano-4-hydroxycinnamic acid (CHCA), 4-chloro-α-cyano-cinnamic acid (Cl-CCA), or (E)-2-cyano-3-(naphthalen-2-yl)acrylic acid (NpCCA) as MALDI matrices. We also propose a rapid strategy for sensitive confirmatory analyses with both MS/MS and MS(3) experiments performed directly on intact hair samples. Since only part of the hair strand is analyzed, additional analyses are possible at any time on the remaining hair from the strand.

Determination of the antiretroviral drug tenofovir in plasma from HIV-infected adults by ultrafast isotope dilution MALDI-triple quadrupole tandem mass spectrometry

A new and reliable mass spectrometric method using an isotope dilution method in combination with matrix-assisted laser desorption/ionization-triple quadrupole tandem mass spectrometry (ID-MALDI-QqQ-MS/MS) has been developed and validated for the determination of concentrations of the antiretroviral drug tenofovir (TNV) in plasma from HIV-infected adults. The advantage of this new method is that (1) the method is ultrafast and (2) can be applied for high-throughput measurement of TNV in plasma. The method is based on a simple plasma deproteinization step in combination with the use of [adenine-(13) C(5) ]-TNV as the internal standard. TNV and [adenine-(13) C(5) ]-TNV were monitored by multiple reaction monitoring using the transition m/z 288.0 → 176.2 and m/z 293.2 → 181.2 for TNV and [adenine-(13) C(5) ]-TNV, respectively. The method was validated according to the most recent FDA guidelines for the development and validation of (new) bio-analytical assays. Validated method parameters were: linearity, accuracy, precision and stability of the method. The lowest limit of quantification was 0.10 µmol/l, whereas the limit of detection determined at a signal-to-noise ratio (S/N = 3:1) in pooled drug free human control plasma was 0.04 µmol/l. The validated method was successfully applied and tested for its clinical feasibility by the analysis of plasma samples from selected HIV-infected adults receiving the prodrug tenofovir disoproxil fumarate. Observed plasma TNV concentrations ranged between 0.11 and 0.76 µmol/l and measured plasma TNV concentrations were within the therapeutically relevant concentration range.

Alternative CHCA-based matrices for the analysis of low molecular weight compounds by UV-MALDI-tandem mass spectrometry

Analysis of low molecular weight compounds (LMWC) in complex matrices by vacuum matrix-assisted laser desorption/ionization (MALDI) often suffers from matrix interferences, which can severely degrade limits of quantitation. It is, therefore, useful to have available a range of suitable matrices, which exhibit complementary regions of interference. Two newly synthesized α-cyanocinnamic acid derivatives are reported here; (E)-2-cyano-3-(naphthalen-2-yl)acrylic acid (NpCCA) and (2E)-3-(anthracen-9-yl)-2-cyanoprop-2enoic acid (AnCCA). Along with the commonly used α-cyano-4-hydroxycinnamic acid (CHCA), and the recently developed 4-chloro-α-cyanocinnamic acid (Cl-CCA) matrices, these constitute a chemically similar series of matrices covering a range of molecular weights, and with correspondingly differing ranges of spectral interference. Their performance was compared by measuring the signal-to-noise ratios (S/N) of 47 analytes, mostly pharmaceuticals, with the different matrices using the selected reaction monitoring (SRM) mode on a triple quadrupole instrument equipped with a vacuum MALDI source. AnCCA, NpCCA and Cl-CCA were found to offer better signal-to-noise ratios in SRM mode than CHCA, but Cl-CCA yielded the best results for 60% of the compounds tested. To better understand the relative performance of this matrix series, the proton affinities (PAs) were measured using the kinetic method. Their relative values were: AnCCA > CHCA > NpCCA > Cl-CCA. This ordering is consistent with the performance data. The synthesis of the new matrices is straightforward and they provide (1) tunability of matrix background interfering ions and (2) enhanced analyte response for certain classes of compounds.

Biomedical application of MALDI mass spectrometry for small-molecule analysis

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is an emerging analytical tool for the analysis of molecules with molar masses below 1,000 Da; that is, small molecules. This technique offers rapid analysis, high sensitivity, low sample consumption, a relative high tolerance towards salts and buffers, and the possibility to store sample on the target plate. The successful application of the technique is, however, hampered by low molecular weight (LMW) matrix-derived interference signals and by poor reproducibility of signal intensities during quantitative analyses. In this review, we focus on the biomedical application of MALDI-MS for the analysis of small molecules and discuss its favorable properties and its challenges as well as strategies to improve the performance of the technique. Furthermore, practical aspects and applications are presented.

MALDI imaging mass spectrometry--painting molecular pictures

MALDI Imaging Mass Spectrometry is a molecular analytical technology capable of simultaneously measuring multiple analytes directly from intact tissue sections. Histological features within the sample can be correlated with molecular species without the need for target-specific reagents such as antibodies. Several studies have demonstrated the strength of the technology for uncovering new markers that correlate with disease severity as well as prognosis and therapeutic response. This review describes technological aspects of imaging mass spectrometry together with applications in cancer research.

Ultrafast and high-throughput mass spectrometric assay for therapeutic drug monitoring of antiretroviral drugs in pediatric HIV-1 infection applying dried blood spots

Kaletra® (Abott Laboratories) is a co-formulated medication used in the treatment of HIV-1-infected children, and it contains the two antiretroviral protease inhibitor drugs lopinavir and ritonavir. We validated two new ultrafast and high-throughput mass spectrometric assays to be used for therapeutic drug monitoring of lopinavir and ritonavir concentrations in whole blood and in plasma from HIV-1-infected children. Whole blood was blotted onto dried blood spot (DBS) collecting cards, and plasma was collected simultaneously. DBS collecting cards were extracted by an acetonitrile/water mixture while plasma samples were deproteinized with acetone. Drug concentrations were determined by matrix-assisted laser desorption/ionization-triple quadrupole tandem mass spectrometry (MALDI-QqQ-MS/MS). The application of DBS made it possible to measure lopinavir and ritonavir in whole blood in therapeutically relevant concentrations. The MALDI-QqQ-MS/MS plasma assay was successfully cross-validated with a commonly used high-performance liquid chromatography (HPLC)–ultraviolet (UV) assay for the therapeutic drug monitoring (TDM) of HIV-1-infected patients, and it showed comparable performance characteristics. Observed DBS concentrations showed as well, a good correlation between plasma concentrations obtained by MALDI-QqQ-MS/MS and those obtained by the HPLC-UV assay. Application of DBS for TDM proved to be a good alternative to the normally used plasma screening. Moreover, collection of DBS requires small amounts of whole blood which can be easily performed especially in (very) young children where collection of large whole blood amounts is often not possible. DBS is perfectly suited for TDM of HIV-1-infected children; but nevertheless, DBS can also easily be applied for TDM of patients in areas with limited or no laboratory facilities.

Ultra-fast analysis of plasma and intracellular levels of HIV protease inhibitors in children: a clinical application of MALDI mass spectrometry

HIV protease inhibitors must penetrate into cells to exert their action. Differences in the intracellular pharmacokinetics of these drugs may explain why some patients fail on therapy or suffer from drug toxicity. Yet, there is no information available on the intracellular levels of HIV protease inhibitors in HIV infected children, which is in part due to the large amount of sample that is normally required to measure the intracellular concentrations of these drugs. Therefore, we developed an ultra-fast and sensitive assay to measure the intracellular concentrations of HIV protease inhibitors in small amounts of peripheral blood mononuclear cells (PBMCs), and determined the intracellular concentrations of lopinavir and ritonavir in HIV infected children. An assay based on matrix-assisted laser desorption/ionization (MALDI) - triple quadrupole mass spectrometry was developed to determine the concentrations of HIV protease inhibitors in 10 µL plasma and 1×10⁶ PBMCs. Precisions and accuracies were within the values set by the FDA for bioanalytical method validation. Lopinavir and ritonavir did not accumulate in PBMCs of HIV infected children. In addition, the intracellular concentrations of lopinavir and ritonavir correlated poorly to the plasma concentrations of these drugs. MALDI-triple quadrupole mass spectrometry is a new tool for ultra-fast and sensitive determination of drug concentrations which can be used, for example, to assess the intracellular pharmacokinetics of HIV protease inhibitors in HIV infected children.

Rapid simultaneous quantitative determination of different small pharmaceutical drugs using a conventional matrix-assisted laser desorption/ionization time-of-flight mass spectrometry system

The present study establishes a simple, rapid and sensitive method for the simultaneous quantification of different small pharmaceutical drugs using a matrix-assisted laser desorption/ionization source (MALDI) coupled with a time-of-flight (TOF) mass analyzer. Neither time-consuming sample preparation, nor special target plates, isotopically labelled internal standards or other extra equipment are necessary. A simple standard dried-droplet preparation with the common matrix alpha-cyano-4-hydroxycinnamic acid (CHCA) was used. The background signals of CHCA in the low-mass region did not pose the presumed problem, because the sensitivity, resolution and mass accuracy of a modern MALDI-TOF MS system is sufficient to overcome this difficulty. Four experiments were performed in order to verify the quantification method. First, ten different phenothiazines were quantified in the range of 5-2000 nM (1-880 ng/mL). A good precision (relative standard deviation (RSD) 4.4-9.3%), linearity (R2 >0.99) and accuracy (error 4.7-11%) was obtained in all cases. Additionally, simultaneous quantification of these ten phenothiazines was carried out in human plasma without prior chromatographic separation in the range of 2-1750 ng/mL yielding good linearity, precision and accuracy (mean RSD 7.6%; R2 >0.99, mean error 8.0%). Accordingly, a quantitative analysis of ten chemically and pharmaceutically unrelated drugs was performed in the same way. A comparable linearity (R2 >0.99), precision (mean RSD 7.6%) and accuracy (mean error 8.3%) was obtained in the range of 5-2000 nM. Finally, the prazosin content of a commercial tablet was directly determined without further purification steps.

Bioanalysis of recombinant proteins and antibodies by mass spectrometry

In recent years, biotechnologically-derived drugs have been a major focus of research and development in the pharmaceutical industry. Their pharmacokinetics and pharmacokinetic/pharmacodynamic relationships impact every stage of the development process and require their assessment in the circulation in preclinical species and in humans. To this end, immunoassays are a reference, but standardisation remains an issue owing to the restricted pattern of antibody specificity and interference with endogenous components. As an alternative, we report here analytical strategies involving liquid chromatography coupled to mass spectrometry (LC-MS) for the accurate quantification of therapeutic proteins and antibodies in biological fluids.

Quantitative matrix-assisted laser desorption/ionization mass spectrometry

This review summarizes the essential characteristics of matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOF MS), especially as they relate to its applications in quantitative analysis. Approaches to quantification by MALDI-TOF MS are presented and published applications are critically reviewed.