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 m arker 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 line arity of dilution were evaluated, hK2 was measured in seminal plasma and se ra 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 inhibit ors. 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 concentra tions similar to 2.5-fold lower than PSA. The PSA/hK2 ratio in male sera wa s 0.1-34 with a median of 2.6. In seminal plasma, this ratio was 100-500. M ore than 94% of immunoreactive hK2 in serum was in the free form (similar t o 30 kDa); traces of hK2 complexed to alpha(1-)antichymotrypsin were presen t. Conclusions: The limit of detection of the method for hK2 measurement descr ibed here (similar to 20-fold lower than any other reported assay for hK2) allows the generation of new clinical information. When combined with a pre viously described method for PSA measurement that has no cross-reactivity f rom hK2, this methods allows the relative proportions of hK2 and PSA in bio logical fluids to be measured. (C) 1999 American Association for Clinical C hemistry.