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PURPOSE: Mass spectrometry-based serum peptidome profiling is a promising tool to identify novel disease-associated biomarkers, but is limited by preanalytic factors and the intricacies of complex data processing. Therefore, we investigated whether standardized sample protocols and new bioinformatic tools combined with external data validation improve the validity of peptidome profiling for the discovery of pancreatic cancer-associated serum markers. EXPERIMENTAL DESIGN: For the discovery study, two sets of sera from patients with pancreatic cancer (n = 40) and healthy controls (n = 40) were obtained from two different clinical centers. For external data validation, we collected an independent set of samples from patients (n = 20) and healthy controls (n = 20). Magnetic beads with different surface functionalities were used for peptidome fractionation followed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). Data evaluation was carried out by comparing two different bioinformatic strategies. Following proteome database search, the matching candidate peptide was verified by MALDI-TOF MS after specific antibody-based immunoaffinity chromatography and independently confirmed by an ELISA assay. RESULTS: Two significant peaks (m/z 3884; 5959) achieved a sensitivity of 86.3% and a specificity of 97.6% for the discrimination of patients and healthy controls in the external validation set. Adding peak m/z 3884 to conventional clinical tumor markers (CA 19-9 and CEA) improved sensitivity and specificity, as shown by receiver operator characteristics curve analysis (AUROC(combined) = 1.00). Mass spectrometry-based m/z 3884 peak identification and following immunologic quantitation revealed platelet factor 4 as the corresponding peptide. CONCLUSIONS: MALDI-TOF MS-based serum peptidome profiling allowed the discovery and validation of platelet factor 4 as a new discriminating marker in pancreatic cancer.

Peptidome profiling of human cerebrospinal fluid (CSF) is a promising tool to identify novel disease-associated biomarkers. Our aim was to develop a standardized protocol for reproducible peptidome profiling of CSF using magnetic bead (MB) separation followed by MALDI-TOF MS. Peptidome fractionation and profiling of CSF were performed using MBs with different surface functionalities. We investigated exogenous variables (storage conditions, freeze-thaw-cycles) and endogenous interferences (albumin, immunoglobulin, blood, leukocytes) in pooled CSF samples. We detected approximately 500 signals with an S/N ratio > 10 and an overlap frequency of about 40% in non-pathological CSF. Within- and between-day imprecisions in relative signal intensities ranged from 3 to 28% and 7 to 47%, respectively. CSF storage at room temperature for up to 6 h and at 4 °C for up to 3 days did not significantly influence the mass spectra. Consecutive freeze-thaw-cycles significantly affected the mass spectra. High albumin and immunoglobulin content altered the CSF preparation using MB-HIC C8 beads. Blood contamination showed no effect on mass spectra up to a hemoglobin concentration of 0.075 µmol/L. The presence of leukocytes up to a cell number of 30 Mpt/L did not affect mass spectra. Our reliable pretreatment protocol allows standardization of preanalytical modalities and thereby enables reproducible peptidome profiling of human CSF using MB separation followed by MALDI-TOF MS.