Proximity ligation measurement of the complex between prostate specific antigen and alpha1-protease inhibitor.
BACKGROUND
Prostate specific antigen (PSA)-alpha1-protease inhibitor complex (PSA-API) is a minor form of PSA in serum. It may be useful for prostate cancer (PCa) diagnosis, but its specific detection is hampered by nonspecific background. To avoid this, we developed an immunoassay for PSA-API based on proximity ligation.


METHODS
We used a monoclonal antibody (mAb) to total PSA (tPSA) to capture PSA, while using another anti-tPSA mAb together with an anti-API mAb as probes. We measured PSA-API by quantification of amplified DNA strands conjugated to the probes. We measured serum PSA-API in 84 controls and 55 men with PCa who had PSA concentrations of 4.0-10 microg/L.


RESULTS
The detection limit of the assay was 6.6 ng/L. The proportion of PSA-API to tPSA (%PSA-API) tended to be lower in men with PCa (2.8%) than without cancer (3.3%) but was not statistically significant (P = 0.363). When used alone, %PSA-API [area under the curve (AUC) 0.546] did not improve detection of PCa, whereas %fPSA (AUC 0.710) and the sum of %fPSA and %PSA-API (AUC 0.723) did. At 90% diagnostic sensitivity, the diagnostic specificity for cancer was not significantly better for %f PSA + %PSA-API than for %fPSA alone (36% vs 30%).


CONCLUSIONS
Proximity ligation eliminated nonspecific background, enabling accurate measurement of PSA-API in serum specimens with moderately increased tPSA. The combined use of %PSA-API and %fPSA provided a modest improvement for PCa detection, but based on the current study cohort, it is uncertain whether the improvement has clinical utility. .
Zhu
Lei
L
Department of Clinical Chemistry, Biomedicum Helsinki, University of Helsinki and Helsinki University Central Hospital, Helsinki, Finland.
Koistinen
Hannu
H
Landegren
Ulf
U
Stenman
Ulf-Håkan
UH
D000368:Aged; D000911:Antibodies, Monoclonal / Q000276:immunology; D002852:Chromatography, Ion Exchange; D006801:Humans; D007118:Immunoassay / Q000379:methods*; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D017430:Prostate-Specific Antigen / Q000097:blood* / Q000276:immunology / Q000302:isolation & purification; D011471:Prostatic Neoplasms / Q000097:blood / Q000175:diagnosis*; D011485:Protein Binding; D012680:Sensitivity and Specificity; D000515:alpha 1-Antitrypsin / Q000097:blood* / Q000276:immunology
D000911:Antibodies, Monoclonal; D000515:alpha 1-Antitrypsin; D017430:Prostate-Specific Antigen