B-318 Serum PSA in Extracellular Vesicles: Immunoreactivity to Commercial Kits and Elimination Kinetics

Abstract

Abstract Background Prostate Specific Antigen (PSA) also circulates in blood bound to extracellular vesicles (EVs), and observed higher ratio of PSA in EVs respect total serum PSA when serum concentrations are <4 μg/L, and can represent up to 59% of total circulating PSA. Moreover, this molecular form of PSA (ev-PSA) is recognized and can be quantified with commercial methodologies designed for serum quantification. We aimed to characterize ev-PSA studying its immunoreactivity to different commercial assays and analyzing its presence in international standards used for assays standardization. Serum half-life and elimination kinetics of ev-PSA were also studied. Methods Serum EVs from 32 prostate cancer (PCa) patients were isolated by ultracentrifugation (100,000xg, 90 minutes). PSA was assessed in serum (srm-), serum supernatant free of EVs (sn-) and EVs (ev-) with total PSA (T-PSA) commercial immunoassays (Elecsys® from Roche Diagnostics and Immulite® 2000 from Siemens Healthineers), and Immulite/Elecsys ratio was calculated in all samples. The analysis of the differences between methods was performed with Bland-Altman test, representing the percentage of the difference respect the mean. WHO 96/670 PSA Standard was ultracentrifuged and analyzed by Nanoparticle Tracking Analysis (NTA) in a NanoSight LM20.For half-life estimation, serum samples from 10 localized PCa patients were collected basal and 2 days after radical prostatectomy, and EVs were isolated by Size Exclusion Chromatography using Exo-spinTM kit (Cell Guidance System). The Research Ethic Committee approved the study. Results Significant differences were observed between Elecsys and Immulite immunoassays in ev-T-PSA (p<0.001) concentrations. ev-T-PSA was quantified in all samples with Elecsys (median: 0.134 μg/L; Q1-Q3: 0.033-0.391 μg/L), but only in 22 patients (68.75%) with Immulite (median: 0.206 μg/L; Q1-Q3: 0.074-0.340 μg/L). In those 22 quantifiable patients, the median Immulite/Elecsys ratio was similar (p=0.057) between sn-T-PSA and srm-T-PSA (0.96 and 0.92, respectively), but much lower for ev-T-PSA (0.77; p<0.001). Bland-Altman graphical analysis showed that Elecsys overestimates the concentrations of T-PSA in all samples compared to Immulite, being this overestimation more marked in the case of PSA contained in EVs, and amplified at lower PSA concentrations. Besides, NTA analysis of WHO 96/670 PSA Standard showed that it does not contain EVs, and therefore, ev-PSA. The percentage of remnant PSA calculated in samples obtained post-surgery respect to baseline concentration in prostatectomized patients, was significantly higher for ev-T-PSA compared to soluble srm-T-PSA (p=0.014) and srm-F-PSA (p=0.002). The elimination half-life of ev-T-PSA was 3.99 days, much higher than those estimated for serum T-PSA and F-PSA, that were 1.76 days and 0.64 days, respectively. Conclusions The quantification of ev-PSA is affected by the different immunoreactivity of the diverse commercial assays. Thus, Elecsys T-PSA immunoassay shows higher immunoreactivity compared to Immulite technique, being this different reactivity more marked at lower PSA concentrations. ev-PSA is not considered in the International WHO 96/670 PSA Standard used for methods standardization. PSA bound to EVs remains longer in circulation than soluble serum PSA.