Development of an ultrasensitive immunoassay for human glandular kallikrein with no cross-reactivity from prostate-specific antigen

Background: Studies demonstrating that human glandular kallikrein (hK2) is increased in prostate cancer patients have prompted speculation that this marker may of use in addition to prostate-specific antigen (PSA). Methods: An ultrasensitive hK2 sandwich immunoassay was developed, and its detection limit, cross-reactivity, analytical recovery, precision, and linearity of dilution were evaluated, hK2 was measured in seminal plasma and sera from healthy males, females, and prostatectomized patients. Results: Our assay has an excellent detection limit (6 ng/L) and precision (>90%). Recovery studies indicated that hK2 binds to serum protease inhibitors. All sera from healthy males had measurable hK2 concentrations (median, 402 ng/L). Almost all female sera had undetectable hK2. Serum hK2 and PSA in males correlated positively (r = 0.44), but hK2 was present at concentrations similar to 2.5-fold lower than PSA. The PSA/hK2 ratio in male sera was 0.1-34 with a median of 2.6. In seminal plasma, this ratio was 100-500. More than 94% of immunoreactive hK2 in serum was in the free form (similar to 30 kDa); traces of hK2 complexed to alpha(1-)antichymotrypsin were present. Conclusions: The limit of detection of the method for hK2 measurement described here (similar to 20-fold lower than any other reported assay for hK2) allows the generation of new clinical information. When combined with a previously described method for PSA measurement that has no cross-reactivity from hK2, this methods allows the relative proportions of hK2 and PSA in biological fluids to be measured. (C) 1999 American Association for Clinical Chemistry.