Clinical Utility of Mutant Antibody-Based Assays for Determination of Internally Cleaved and Intact Forms of Free Prostate-Specific Antigen

Abstract

Background: Subforms of prostate-specific antigen (PSA) have been a subject of intensive research, and use of multikallikrein immunoassays can add clinical value to the early detection of prostate cancer, overcoming known limitations of PSA. In this study, we evaluated mutant 4D4 (L3-2) antibody-assisted assay constructs against reference wild-type (wt)-4D4-based assays for determination of intact PSA (iPSA) and nicked PSA (nPSA) in plasma samples. Methods: Perioperative plasma samples obtained from 105 men who underwent biopsy (73 cancer, 32 noncancer) were analyzed with sandwich immunoassays for total PSA (tPSA), free PSA (fPSA), iPSA (3 constructs), and measured nPSA (2 constructs). Calculated nPSA (CN) was obtained from total fPSA - iPSA. Results: Mutant-assisted iPSA assays measured lower concentrations than the reference in both patient groups. CN separated the 2 groups with the iPSA using the mutant for capture (I-MC) performing the best (P = 0.008). In prostate volume group > median, only measured nPSA provided significant discrimination [area under the curve (AUC), 0.71; P = 0.016] but equally using mutant and wt antibodies. In the whole cohort, all ratios to tPSA performed well (AUC, 0.819-0.870; P ≤ 0.0001) with CN based on I-MC scoring highest (AUC, 0.870). Importantly, in the ≤ median volume group, the I-MC/F and CN(I-MC)/T ratios stand out as the best performing parameters (AUC, 0.825 and 0.861; P = 0.001 and P = 0.0003, respectively). Conclusions: The new assay construct using the mutant 4D4 (L3-2) as a capture provides clear improvement in separating cancer from noncancer in all subgroups analyzed but especially in patients with prostate volume ≤ median. Clinical Trial Registration: ClinicalTrials.gov Identifier NCT01864135.