Measurement of enkephalin peptides in canine brain regions, teeth, and cerebrospinal fluid with high-performance liquid chromatography and mass spectrometry

Mass Spectrometry-Based Biomarkers in Drug Development

Advances in mass spectrometry, proteomics, protein bioanalytical approaches, and biochemistry have led to a rapid evolution and expansion in the area of mass spectrometry-based biomarker discovery and development. The last decade has also seen significant progress in establishing accepted definitions, guidelines, and criteria for the analytical validation, acceptance and qualification of biomarkers. These advances have coincided with a decreased return on investment for pharmaceutical research and development and an increasing need for better early decision making tools. Empowering development teams with tools to measure a therapeutic interventions impact on disease state and progression, measure target engagement and to confirm predicted pharmacodynamic effects is critical to efficient data-driven decision making. Appropriate implementation of a biomarker or a combination of biomarkers can enhance understanding of a drugs mechanism, facilitate effective translation from the preclinical to clinical space, enable early proof of concept and dose selection, and increases the efficiency of drug development. Here we will provide descriptions of the different classes of biomarkers that have utility in the drug development process as well as review specific, protein-centric, mass spectrometry-based approaches for the discovery of biomarkers and development of targeted assays to measure these markers in a selective and analytically precise manner.

Mass spectrometry-based biomarkers in drug development

Advances in mass spectrometry, proteomics, protein bioanalytical approaches, and biochemistry have led to a rapid evolution and expansion in the area of mass spectrometry-based biomarker discovery and development. The last decade has also seen significant progress in establishing accepted definitions, guidelines, and criteria for the analytical validation, acceptance, and qualification of biomarkers. These advances have coincided with a decreased return on investment for pharmaceutical research and development and an increasing need for better early decision making tools. Empowering development teams with tools to measure a therapeutic interventions impact on disease state and progression, measure target engagement, and to confirm predicted pharmacodynamic effects is critical to efficient data-driven decision making. Appropriate implementation of a biomarker or a combination of biomarkers can enhance understanding of a drugs mechanism, facilitate effective translation from the preclinical to clinical space, enable early proof of concept and dose selection, and increase the efficiency of drug development. Here we will provide descriptions of the different classes of biomarkers that have utility in the drug development process as well as review specific, protein-centric, mass spectrometry-based approaches for the discovery of biomarkers and development of targeted assays to measure these markers in a selective and analytically precise manner.

Mass spectrometry based targeted protein quantification: methods and applications

The recent advance in technology for mass spectrometry-based targeted protein quantification has opened new avenues for a broad range of proteomic applications in clinical research. The major breakthroughs are highlighted by the capability of using a "universal" approach to perform quantitative assays for a wide spectrum of proteins with minimum restrictions and the ease of assembling multiplex detections in a single measurement. The quantitative approach relies on the use of synthetic stable isotope labeled peptides or proteins, which precisely mimic their endogenous counterparts and act as internal standards to quantify the corresponding candidate proteins. This report reviews recently developed platform technologies for emerging applications of clinical proteomics and biomarker development.

Quantitative, multiplexed assays for low abundance proteins in plasma by targeted mass spectrometry and stable isotope dilution

Biomarker discovery produces lists of candidate markers whose presence and level must be subsequently verified in serum or plasma. Verification represents a paradigm shift from unbiased discovery approaches to targeted, hypothesis-driven methods and relies upon specific, quantitative assays optimized for the selective detection of target proteins. Many protein biomarkers of clinical currency are present at or below the nanogram/milliliter range in plasma and have been inaccessible to date by MS-based methods. Using multiple reaction monitoring coupled with stable isotope dilution mass spectrometry, we describe here the development of quantitative, multiplexed assays for six proteins in plasma that achieve limits of quantitation in the 1-10 ng/ml range with percent coefficients of variation from 3 to 15% without immunoaffinity enrichment of either proteins or peptides. Sample processing methods with sufficient throughput, recovery, and reproducibility to enable robust detection and quantitation of candidate biomarker proteins were developed and optimized by addition of exogenous proteins to immunoaffinity depleted plasma from a healthy donor. Quantitative multiple reaction monitoring assays were designed and optimized for signature peptides derived from the test proteins. Based upon calibration curves using known concentrations of spiked protein in plasma, we determined that each target protein had at least one signature peptide with a limit of quantitation in the 1-10 ng/ml range and linearity typically over 2 orders of magnitude in the measurement range of interest. Limits of detection were frequently in the high picogram/milliliter range. These levels of assay performance represent up to a 1000-fold improvement compared with direct analysis of proteins in plasma by MS and were achieved by simple, robust sample processing involving abundant protein depletion and minimal fractionation by strong cation exchange chromatography at the peptide level prior to LC-multiple reaction monitoring/MS. The methods presented here provide a solid basis for developing quantitative MS-based assays of low level proteins in blood.

Protein biomarker discovery and validation: the long and uncertain path to clinical utility

Better biomarkers are urgently needed to improve diagnosis, guide molecularly targeted therapy and monitor activity and therapeutic response across a wide spectrum of disease. Proteomics methods based on mass spectrometry hold special promise for the discovery of novel biomarkers that might form the foundation for new clinical blood tests, but to date their contribution to the diagnostic armamentarium has been disappointing. This is due in part to the lack of a coherent pipeline connecting marker discovery with well-established methods for validation. Advances in methods and technology now enable construction of a comprehensive biomarker pipeline from six essential process components: candidate discovery, qualification, verification, research assay optimization, biomarker validation and commercialization. Better understanding of the overall process of biomarker discovery and validation and of the challenges and strategies inherent in each phase should improve experimental study design, in turn increasing the efficiency of biomarker development and facilitating the delivery and deployment of novel clinical tests.

Absolute quantification strategies in proteomics based on mass spectrometry

The strong need for quantitative information in proteomics has fueled the development of mass spectrometry-based analytical methods that are able to determine protein abundances. This article reviews mass spectrometry experiments aimed at providing an absolute quantification of proteins. The experiments make use of the isotope-dilution concept by spiking a known amount of synthetic, isotope-labeled reference peptide into the analyte sample. Quantification is achieved by comparing the mass spectrometry signal intensities of the reference with an endogenous peptide that is generated upon proteolytic cleavage of the target protein. In an analogous manner, the level of post-translational modification at a distinct residue within a target protein can be determined. Among the strengths of absolute quantification are low detection limits reaching subfemtomole levels, a high dynamic range spanning approximately five orders of magnitude, low requirements for sample clean-up, and a fast and straightforward method development. Recent studies have demonstrated the compatibility of absolute quantification with various mass spectrometry readout techniques and sample purification steps such as 1D gel electrophoresis, size-exclusion chromatography, isoelectric peptide focusing, strong cation exchange and reversed phase or affinity chromatography. Under ideal conditions, quantification errors and coefficients of variation below 5% have been reported. However, the fact that at the start of the experiment the analyte is a protein and the internal standard is a peptide, severe quantification errors may result due to the selection of unsuitable reference peptides and/or imperfect protein proteolysis. Within the ensemble of mass spectrometry-based quantification methods, absolute quantification is the method of choice in cases where absolute numbers, many repetitive experiments or precise levels of post-translational modifications are required for a few, preselected species of interest. Consequently, prominent application areas include biomarker quantification, the study of post-translational modifications such as phosphorylation or ubiquitination and the comparison of concentrations of interacting proteins.

Separation and determination of enkephalin-related peptides using capillary electrophoresis

CZE with UV-absorption detection has been used for the separation and determination of enkephalin-related peptides. The experimental conditions, such as pH and concentration of running buffer, applied voltage, injection method, and time, were investigated in detail. Excellent separation efficiency could be obtained for ten enkephalin-related peptides with a 50 microm (ID) x 58 cm capillary using sodium dihydrogen phosphate as the running buffer (pH 3.11) when 20 kV of applied voltage was used. The concentration detection limits were found to be in the range of 0.31-1.94 microg/mL (defined as S/N = 3). The proposed method has been applied to analyze the spiked cerebrospinal fluid (CSF) sample, and the results showed that CZE is a powerful technique for separation and detection of the above biological peptides.

Absolute Quantification of the Model Biomarker Prostate-Specific Antigen in Serum by LC−MS/MS Using Protein Cleavage and Isotope Dilution Mass Spectrometry

Protein cleavage-isotope dilution mass spectrometry (PC-IDMS) can be used to quantify proteins, with an isotope-labeled analogue of the peptide fragment used as an internal standard. Here, we investigate use of a standard LC-MS/MS platform for quantifying a model biomarker directly from serum by this technique. We synthesized a peptide (IVGGWECEK) identical to the N-terminal tryptic fragment of PSA but with each glycine containing two 13C atoms and one 15N atom. PSA-free human serum was denatured with urea followed by the introduction of PSA standard and the stable isotope labeled internal standard peptide. The sample was then proteolyzed with trypsin and subjected to quantification using LC-MS/ MS on a triple quadrupole mass spectrometer. A linear least squares calibration curve made from five different concentrations of PSA added to serum and digested (each made in triplicate and randomly injected three times) had a mean slope of 0.973 (SE = 0.023), intercept of -0.003 (SE = 0.022), and R2 of 0.971. Recovery of calibrators ranged from 70 to 85% with a mean run-to-run CV of 13% and a mean within-run CV of 5.7%. PC-IDMS is a promising technique for quantifying proteins covering a broad range of applications from standardizing immunoassays to monitoring post-translational modifications to quantifying newly discovered biomarkers prior to the development and implementation of an immunoassay, just to name a few. Issues surrounding the application of PC-IDMS for the absolute quantification of proteins include selection of a proteolytic fragment for quantification that can be cleaved and isolated reproducibly over a broad dynamic range, stable isotope labeled synthetic peptide standards that give consistent results, and LC-MS/MS methods that provide adequate sensitivity and reproducibility without creating impractical analysis times. The results presented here show that absolute quantification can be performed on the model biomarker PSA introduced into denatured serum when analyzed by LC-MS/MS. However, concerns still exist regarding sensitivity compared to existing immunoassays as well as the reproducibility of PC-IDMS performed in different matrixes.

Determination of vasoactive intestinal peptide in rat brain by high-performance capillary electrophoresis

A high-performance capillary electrophoresis (HPCE) method for determining vasoactive intestinal peptide (VIP) in rat brain was developed. Cerebral cortex was first extracted by solid-phase extraction and purified by reversed-phase high-performance liquid chromatography. The VIP-rich fraction was further analysed by capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography using a commercial HPCE instrument with UV detection. The identity of the peak of endogenous VIP was confirmed by performing multiple CZE analyses at different pH values. This HPCE method allows VIP to be detected and measured with good molecular specificity and could represent a reference method to validate data obtained by radioimmunoassay.

Methionine enkephalin-like, substance P-like, and beta-endorphin-like immunoreactivity in human parotid saliva

These three neuropeptides were measured at daily baseline values by radioimmunoassay. Stimulated parotid saliva was collected from 31 subjects using a modified Carlson-Crittenden device affixed over Stenson's duct. Methionine enkephalin-like immunoreactivity ranged from 6.6 to 11.7 fmol/ml, with a mean of 9.3 fmol/ml. Substance P-like immunoreactivity ranged from 6.1 to 12.6 fmol/ml, with a mean of 9.3 fmol/ml. beta-Endorphin-like immunoreactivity ranged from 1.2 to 3.6 fmol/ml, with a mean of 2.6 fmol/ml. This is believed to be the first documentation of methionine enkephalin- and substance P-like activities in human parotid saliva and the first demonstration of beta-endorphin-like activity in any type of human saliva. Substance P-like activity was significantly higher in morning than evening samples; beta-endorphin-like activity also tended to be higher in the morning samples. Substance P and beta-endorphin-like immunoreactivities covaried in a significant positive manner, suggesting either common control mechanisms or similar responses to physiological variables.

Fluorogenic derivatization of peptides with naphthalene-2,3-dicarboxaldehyde/cyanide: optimization of yield and application in the determination of leucine-enkephalin spiked human plasma samples

Initial attempts to derivatize the alpha-amino site of several tripeptides with naphthalene-2,3-dicarboxaldehyde/cyanide (NDA/CN) resulted in poor yields of the expected N-substituted 1-cyanobenz[f]isoindole (CBI) products. Examination of the CBI-formation mechanism, in conjunction with knowledge of the general structure-reactivity properties of the tripeptides, led to the recognition of a competing non-productive reaction pathway. Through the use of model reactions and the isolation and structural elucidation of a predicted side-product the viability of the competing pathway was confirmed. From an understanding of the key features of both the productive and non-productive reaction pathways, a rational approach for the optimization of CBI-derivative yield was proposed and confirmed experimentally. This information led, in turn, to the development of HPLC methodology suitable for the determination of leu-enkephalin spiked into human plasma; fluorescence detection was used in conjunction with leu-enkephalin amide as the internal standard. The method enabled leu-enkephalin to be determined at a concentration of 0.31 nmol ml-1 with an error of less than 4% for 25 pmol injected.

Determination of leucine enkephalin and methionine enkephalin in equine cerebrospinal fluid by microbore high-performance liquid chromatography and capillary zone electrophoresis coupled to tandem mass spectrometry

The performance of microbore high-performance liquid chromatography and capillary zone electrophoresis, both equipped with on-line tandem mass spectrometric detection capability, was evaluated critically for the determination of endogenous amounts of leucine enkephalin and methionine enkephalin in equine cerebrospinal fluid. Using an identical sample clean-up and enrichment procedure, capillary zone electrophoresis-mass spectrometry is limited in its concentration detection capacity owing to its much smaller injection volume. Leucine enkephalin was identified in post-mortem equine cerebrospinal fluid at the 1-5 ng/ml level by liquid chromatography-mass spectrometry.

High-performance liquid chromatographic determination of leucine-enkephalin-like peptide in rat brain by pre-column fluorescence derivatization involving formylation followed by reaction with 1,2-diamino-4,5-dimethoxybenzene

Fluorimetrically reactive leucine-enkephalin, one of the opioid peptides in rat brain tissues such as the striatum, cortex and hypothalamus, was assayed by reversed-phase high-performance liquid chromatography with fluorescence detection based on pre-column derivatization of the tyrosyl residue. The tyrosine-containing peptides extracted from the tissue were first formylated with chloroform in an alkaline medium, and the resulting aldehydes were then converted into highly fluorescent derivatives by reaction with 1,2-diamino-4,5-dimethoxybenzene. The derivative of leucine-enkephalin-like peptide in tissue was separated from various other fluorescent compounds on a reversed-phase column (TSK gel ODS-120T) by isocratic elution and detected by fluorimetry. The concentrations of the leucine-enkephalin-like peptide in the tissues were 20-245 pmol/g. The method is sensitive enough to permit the quantitative determination of the endogenous peptide at concentrations as low as 5.6 pmol/g in brain tissues.

Detection of enkephalin-like immunoreactive material in human saliva

1. A specific radioimmunoassay (RIA) was used to detect the presence of enkephalin-like activity in mixed saliva from healthy male and female human subjects, whose ages ranged from 20 to 30 and from 55 to 65 years. 2. Enkephalin-like immunoreactivity was measured in saliva from all subjects. Within the limits posed by the immunoassay no significant differences in levels were evident among the various groups. 3. The results of this study provide the first demonstration of opioid-like substances in saliva. 4. As the collection of saliva involves non-invasive techniques, this source may prove useful for monitoring enkephalin levels during various clinical procedures or experimental treatments.

High-performance liquid chromatography of N-terminal tyrosine-containing oligopeptides by pre-column fluorescence derivatization with hydroxylamine, cobalt (II) and borate reagents

A pre-column fluorescence derivatization method is described for the high-performance liquid chromatographic determination of N-terminal tyrosine-containing oligopeptides involving methionine-enkephalin and leucine-enkephalin. The peptides are converted into fluorescent derivatives by heating in a weakly alkaline medium (pH 8.5) containing hydroxylamine, cobalt(II) ion and borate. The derivatives are separated on a reversed-phase column (TSKgel ODS-120T) by gradient elution of acetonitrile in a mobile phase containing borate buffer (pH 8.5) and tetra-n-butylammonium chloride, and then determined by fluorimetry. The derivatization provides a single fluorescent product for each N-terminal tyrosine-containing oligopeptide, but does not allow the production of fluorescent derivatives for peptides having no tyrosyl residue at the N-terminal. The method is selective and sensitive; the lower limits of detection for the N-terminal tyrosine-containing oligopeptides tested were 140-310 fmol per 100 microliters injected.

On-line post-column fluorescence detection for N-terminal tyrosine-containing peptides in high-performance liquid chromatography

A detection system based on on-line post-column fluorescence derivatization is described for the determination of N-terminal tyrosine-containing peptides by reversed-phase high-performance liquid chromatography. The peptides are automatically converted into fluorescent derivatives by reaction with hydroxylamine, cobalt (II) and borate after peptide separation on a reversed-phase column (TSKgel ODS-120T) followed by passage through an ultraviolet absorbance detector. The reaction system permits the fluorescence detection at 435 nm (emission) with excitation at 335 nm for N-terminal tyrosine-containing synthetic peptides in as little as picomole amounts. The facile fluorescence detection of N-terminal tyrosine-containing fragments produced from methionine-enkephalin by enzymatic degradation using a rat brain homogenate was achieved by comparison with the ultraviolet absorption detection at 215 nm.

Reverse phase liquid chromatography of beta-endorphins and related peptides

Anomalous band broadening of beta-endorphin related polypeptides chromatographed on hydrophobic high performance stationary phases can be attributed to ligand induced conformational changes associated with polypeptide folding. In the presence of anionic lipids the size exclusion chromatographic behaviour of members of the beta-endorphin family also exhibits similar behaviour. These structure-retention and band broadening behaviour of these polypeptides were in accord with predictions made by hydropathy algorithms and amphipathic helix representations. These observations on surface accessibility of key amino acid residues and their interaction as a conformationally induced domains with stationary phase ligands are equally relevant to other peptidic solutes and neurotransmitters.

HPLC receptorassay of opioid peptides in the cerebrospinal fluid of lower back pain patients

Total opioid peptide receptoractivity in human cerebrospinal fluid is measured in patients who are experiencing lower back pain. Desalted CSF is eluted from a C18 Sep-Pak and is subjected to a radioreceptorassay (RRA) that employs tritiated etorphin, which is a ligand that is effectively displaced by opioids from several different types of opioid receptors. Three clinical groups have significantly different endogenous levels of 2.4, 4.5, and 6.4 pmol of methionine enkephalin-equivalents per mL CSF. Those three levels indicate that more opioid activity is correlated with the amount of drug to relieve the patient's perception of pain. When the total opioid content exceeds an empirical threshold, the sample is further fractionated with gradient reversed phase HPLC, and the opioid receptoractivity in each HPLC fraction is measured to determine the characteristic pattern of those receptoractive opioid peptides present in that patient's CSF. Different HPLC RRA patterns are found for different clinical categories. A possible interpretation of these two different sets of data.is that a lesion exists in one or several of the opioid peptidergic systems (metabolism, receptors) in this particular patient population.

Opioid peptides in the cerebrospinal fluid of Alzheimer patients

Endogenous opioid receptoractive peptides in the cerebrospinal fluid (CSF) of human controls and in those patients diagnosed as having senile dementia of the Alzheimer's type (SDAT) are measured with a radioreceptorassay following HPLC separation. [3H]Etorphine is the ligand used to detect in the HPLC fractions the presence of those endogenous peptides that preferentially interact with several opioid receptors. The RRA uses a receptor-rich P2 fraction extracted from a canine limbic system. The total opioid peptide content found in the HPLC fractions 6-20 (to avoid salts in fractions 1-5) of SDAT CSF (383 +/- 187 pmol ME-equivalents per ml CSF) is significantly higher than the corresponding total from patients with no known neurological disorders (89.1 +/- 46.3 pmol ME-equivalents per ml).

Metabolic profiling of opioid peptides in canine pituitary and selected brain regions using HPLC with a radioreceptor assay detector

A combination of gradient reversed-phase high performance liquid chromatography (RP/HPLC) with a radioreceptor assay detector that uses two ligands is used to obtain effectively the metabolic profile of endogenous receptoractive opioid peptides in the canine pituitary and in seven selected brain regions including the hypothalamus, caudate nucleus, mid-brain, amygdala, thalamus, pons-medulla, and the hippocampus. Gradient RP/HPLC separates a mixture of endogenous peptides over a wide range of hydrophobicities. A novel opioid preparation from canine limbic system synaptosomes is utilized in a radioreceptorassay screen; tritiated etorphine (ET) or D-2ala, D-5leuleucine enkephalin (DADL) is used as the competitively displaced ligand. This receptor-rich preparation contains several receptor types, and thus serves well as a screen with the required low level of specificity. Subsequent analysis with other detectors of high specificity (MS, RIA) will follow this screen in other studies. Etorphine interacts with several of the opioid peptide-preferring receptors, whereas DADL is more specific towards the delta receptor that preferentially binds the smaller pentapeptides of the enkephalin family. The highest amount of peptide receptor activity found in this study is in the pituitary tissue, a smaller amount in the hypothalamus and caudate nucleus, and still lower amounts in the other five brain tissue extracts. This variation in peptide concentration most probably reflects three separate factors that operate in this biologic system: differential tissue-specific processing patterns of the large peptide precursors; distribution of the three opioid peptide systems; and the receptor preparation and the radioligand used in the assay. The structures of the receptoractive compounds in each RP/HPLC peak await mass spectrometric confirmation.

Metabolic profiling of radioreceptor-assayable opioid peptides in a human pituitary ACTH-secreting tumor

The profile of endogenous opioid peptides in the peptide-rich fraction obtained from a homogenate of an ACTH-secreting human pituitary tumor is presented. Gradient RP-HPLC is used to separate the mixture into peptide constituents. A preparation of opioid receptors is used in a radioreceptor assay with ethorphine - a relatively non-specific ligand that is used as a screen because it interacts with mu, sigma, and delta receptors - as the HPLC detector to detect a range of peptides that derive from proenkephalin A and POMC.

Metabolic profiling of opioid peptides in tooth pulp by HPLC and radioreceptor assay

A analytical system using a combination of gradient RP-HPLC and radioreceptor assay as the HPLC-detector is used to analyze the peptide-rich fraction extracted from a canine tooth pulp homogenate and to provide a metabolic profile of endogenous receptoractive peptides. The gradient RP-HPLC effectively separates the endogenous peptide mixture into a range of hydrophobicities that corresponds to a spectrum of peptide sizes. The receptor preparation is derived from a canine limbic system synaptosome fraction. 3H-DADL serves as the ligand in the RRA. The RP-HPLC/RRA data indicate canine tooth pulp contains a wide range of peptides that interact with the opioid peptide receptor preparation and displace the delta-receptor preferring ligand D2-ala, D5-Leu leucine enkephalin.