Characterization and determination of the complex between prostate-specific antigen and alpha(1)-protease inhibitor in benign and malignant prostatic diseases

Prostate-specific antigen (PSA) is a tissue-specific serine protease which forms complexes with protease inhibitors such as alpha (1)-antichymotrypsin and alpha (2)-macroglobulin. We have studied the interaction between PSA and al-protease inhibitor (API) in vitro and found that 15% of the added PSA binds to API while the majority of API is cleaved between Met358 and Ser359 when PSA is incubated with a 5-fold excess of API at 37 degreesC for 7 days. The complex between PSA and API (PSA-API) formed in vitro displays the same chromatographic behavior, molecular size and immunoreactivity as endogenous PSA-API occurring in serum, indicating that they are identical. PSA-API can be detected in serum by a time-resolved immunofluorometric assay (IFMA), in which a monoclonal antibody to PSA is used as a catcher and a polyclonal antibody to API labeled with a Eu-chelate is used as a tracer. Purified PSA-API formed in vitro is used as a calibrator. PSA-API in serum represents 1.0-7.9% (median 2.4%) of total PSA (tPSA) in prostate cancer (PCa, n=82) and 1.3-12.2% (median 3.6%, p<0.01) in patients with benign prostatic hyperplasia (BPH, n=66). The IFMA for PSA-API in serum is hampered by a variable background, which is caused by non-specific adsorption of the huge excess of API in serum to the solid phase. The background can be determined by an assay using the same tracer as in the IFMA for PSA-API but PSA-unrelated antibody on the solid phase. The background signal is subtracted from the PSA-API signal. The clinical utility of PSA-API in serum has been evaluated in PSA-positive subjects from the Finnish PCa screening trial. After subtraction of the background, the proportion of PSA-API in relation to tPSA is lower in PCa than in controls, 0.9% vs. 1.6%, respectively (p<0.001). Logistic regression analysis showed that the concentration of PSA-API was independent of the proportion of free PSA as a diagnostic variable among subjects with a tPSA of 4-10 mug/l (p=0.009). The probability of PCa calculated by logistic regression using the concentration of PSR-API and the proportion of free PSA in serum significantly improved cancer specificity at high sensitivity levels (85-95%) as compared to the proportion of free PSA alone.