Academy of Diagnostics & Laboratory Medicine - Scientific Short
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Prostate cancer (PCa) is the second most common cancer in men, accounting for around 7% of cancer deaths globally, and about half of men over 70 will eventually be diagnosed with PCa. Prostate-specific antigen (PSA) is a key factor in managing PCa. It is known that PSA circulates as free PSA (F-PSA), or bound to either alpha1-antichimiotrypsin (C-PSA) or alpha2-macroglobulin, being the latter an immunologically hidden isoform, not recognized by routine analytical methods. Most total PSA tests are designed to equally detect F-PSA and C-PSA concentrations. Recent studies have shown that PSA also circulates bound to extracellular vesicles (EVs), which are nano-sized vesicles secreted, containing molecules selectively loaded onto them. We also observed that although PSA commercial kits aren't designed to react with this molecular form of PSA (ev-PSA), they do recognize it. Also, the proportion of ev-PSA is especially high when total serum PSA is less than 4 μg/L and in cancer patients compared to healthy controls or patients with benign hyperplasia. In some patients, ev-PSA represented more than 30% of srm-T-PSA.
We wanted to get a better understanding of how ev-PSA reacted to different commercial assays, and to assess whether the presence can cause bias in the PSA quantification. Consequently, we isolated EVs from serum PCa patients using ultracentrifugation. We measured total PSA (T-PSA) concentration in serum (srm-), serum supernatant free of EVs (sn-), and EVs (ev-) using commercial immunoassays (Elecsys® from Roche Diagnostics and Immulite® 2000 from Siemens Healthineers). We also analyzed the presence of EVs in the WHO 96/670 PSA Standard, used to calibrate the different assays.
Results were presented at the ADLM 2024 congress. We observed that there were significant differences between the Elecsys and Immulite immunoassays regarding their ability to react with ev-T-PSA. Effectively, Elecsys could quantify ev-T-PSA in all samples, but Immulite in only 68.9% of samples. In these samples, the Elecsys/Immulite ratio was similar for sn-T-PSA and srm-T-PSA, but much higher for ev-T-PSA. Bland-Altman graphical analysis also showed that Elecsys overestimates T-PSA concentrations in all samples compared to Immulite. This overestimation is more marked for ev-T-PSA and much higher at lower PSA concentrations. Also, Nanoparticle Tracking Analysis showed that the WHO 96/670 PSA Standard doesn't contain EVs, so there's no ev-PSA in this standard. The percentage of remnant PSA in samples taken after radical prostatectomy surgery was significantly higher for ev-T-PSA compared to soluble srm-T-PSA and srm-F-PSA. This longer elimination half-life of ev-PSA compared to soluble PSA could affect the proportion of the PSA isoforms during follow-up. These results suggest that the presence of ev-PSA can cause biases between different assays kits and that it would be necessary to standardized PSA considering this circulating PSA form. Also, the lack of standardization of ev-PSA could affect patients follow-up when the proportion of ev-PSA might change.
