Rapid communication: Concentration of enzymatically active prostate-specific antigen (PSA) in the extracellular fluid of primary human prostate cancers and human prostate cancer xenograft models

BACKGROUND. Prostate-specific antigen (PSA) targeted prodrugs are under development in our laboratory. Concentrations of total PSA and enzymatically active PSA produced by various human prostate cancer xenograft models have not been well characterized. METHOD. The concentration of PSA secreted into the extracellular fluid (ECF) in normal human prostate tissue, primary prostate cancers obtained directly from patients, and serially passageable human prostate cancer xenografts (PC-82, LNCaP, LAPC-4) were determined using Tandem assays. Percent enzymatically active PSA in the ECF and in conditioned media was also determined using a previously validated assay employing a monoclonal antibody to the PSA catalytic site. In addition, the concentration and activity of PSA within sera from men with and without prostate cancer, as well as from tumor-bearing animals, was likewise assayed. RESULTS. Normal human prostate tissue and primary human prostate cancers have high concentrations of PSA in the ECF (i.e., 1600-2100 nM). The majority of this PSA is enzymatically active (i.e., 80-90%). Human PC-82 prostate cancer xenografts also have high concentrations of PSA in the ECF (624 ± 360 nM), and the majority of this PSA is also enzymatically active (i.e., 66 ± 4%). In contrast, much lower concentrations of PSA are found in the ECF from LNCaP (45 ± 9 nM) and LAPC-4 (7.3 ± 0.6 nM). Only a small portion of the total PSA isolated from DHT-containing, serum-free, conditioned media from these cell lines is enzymatically active (i.e., ∼18%). While PSA was detected in all serum samples regardless of the type of host, no enzymatically active PSA was detected in any of these serum samples. CONCLUSIONS. Prostate cancers obtained directly from patients produce and secrete large amounts of PSA, the majority of which is highly enzymatically active. In contrast, while PSA was detected in the sera, none of this PSA was enzymatically active. This is also the case for the human PC-82 prostate cancer xenografts. In contrast, LNCaP and LAPC-4 human prostate cancer xenograft models secrete ∼70-300-fold less PSA in the ECF than prostate cancers from patients and the majority of this PSA is enzymatically inactive. Also, the serum from these animals had detectable PSA, but none of this PSA was enzymatically active. Thus, these latter two prostate cancer models define the least and the PC-82, the most, optimized xenograft model for screening PSA targeted prodrugs.