Prostate cancer cells, like normal prostate epithelial cells, produce high levels of the differentiation marker and serine protease prostate-specific antigen (PSA). PSA is used extensively as a biomarker to screen for prostate cancer, to detect recurrence following local therapies, and to follow response to systemic therapies for metastatic disease. While much is known about PSA's role as a biomarker, only a relatively few studies address the role played by PSA in the pathobiology of prostate cancer. Autopsy studies have documented that not only do prostate cancer cells maintain production of high amounts of PSA but they also maintain the enzymatic machinery required to process PSA to an enzymatically active form. A variety studies performed over the last 10 years have hinted at a role for PSA in growth, progression, and metastases of prostate cancer. A fuller understanding of PSA's functional role in prostate cancer biology, however, has been hampered by the lack of appropriate models and tools. Therefore, the purpose of this review is not to address issues related to PSA as a biomarker. Instead, by reviewing what is known about the genetics, biochemistry, and biology of PSA in normal and malignant prostate tissue, insights may be gained into the role PSA may be playing in the pathobiology of prostate cancer that can connect measurement of this biomarker to an understanding of the underlying etiology and progression of the disease.

Prostate-specific membrane antigen (PSMA) is highly expressed by both normal and malignant prostate epithelial cells and by the neovasculature of many tumor types; however, it is not expressed by normal endothelial cells or other normal tissues. PSMA, therefore, represents an attractive candidate for selectively targeted therapies for prostate and/or other solid tumors. As an alternative approach to antibody-based anti-PSMA therapies, small peptides that bind selectively to PSMA-producing cells can be used to deliver cytotoxic drugs, protein toxins, and viruses selectively to malignant sites while minimizing systemic toxicity to normal tissues. Small peptides are relatively inexpensive to produce, not immunogenic, and easily coupled to cytotoxic agents. In the present study, a random phage library consisting of linear 12 amino acid peptides was used to identify peptides that bound selectively to PSMA. From a series of monomeric peptides, one with the sequence WQPDTAHHWATL was used to show binding of soluble peptide to PSMA. A dimeric version of this peptide showed markedly enhanced binding to soluble PSMA and an IC 50 of 2.2 Mmol/L for inhibition of PSMA enzymatic activity. Fluorescently labeled dimeric peptide bound selectively to PSMA-producing prostate cancer cells in vitro with no significant binding to non-PSMA-producing cells. Molecular modeling of the dimeric peptide revealed that histidine residues in close vicinity can efficiently coordinate a divalent ion and hold the peptide in a favorable configuration for binding and subsequent inhibition. These dimeric peptides, therefore, represent putative PSMA-selective targeting agents that are currently being evaluated for selective binding in vivo. (Cancer Res 2006; 66(18): 9171-7)

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Starburst dendrimers are novel, water-soluble polymeric materials, with a well-defined composition and structure. In our application, we used dendrimers composed of poly(amidoamine) groups to which we coupled several specific antibodies, to investigate potential formats based on radial partition immunoassay. The coupled antibodies have retained their stability and immunological binding after coupling, both in solution and when immobilized onto a solid support. On the basis of our feasibility studies with model systems, we conclude that immunoassays can be developed with performance equivalent to or better than that in many established systems. By application of a mixture of the dendrimer-coupled antibody and the analyte of interest to the solid phase, we have investigated the performance characteristics of solution-phase immunoassays. Our experiments demonstrate enhanced sensitivity for creatine kinase MB isoenzyme (CKMB), thyrotropin, and myoglobin assays and reduced instrumental analysis time for the CKMB assay.

SUMMARY Prostate cancer cells produce high (microgram to milligram/milliliter) levels of the serine protease Prostate-Specific Antigen (PSA). PSA is enzymatically active in the extracellular fluid surrounding prostate cancers but is found at 1,000- to 10,000-fold lower concentrations in the circulation, where it is inactivated due to binding to abundant serum protease inhibitors. The exclusive presence of high levels of active PSA within prostate cancer sites makes PSA an attractive candidate for targeted imaging and therapeutics. A synthetic approach based on a peptide substrate identified first peptide aldehyde and then boronic acid inhibitors of PSA. The best of these had the sequence Cbz-Ser-Ser-Lys-Leu-(boro)Leu, with a Ki for PSA of 65 nM. The inhibitor had a 60-fold higher Ki for chymotrypsin. A validated model of PSA’s catalytic site confirmed the critical interactions between the inhibitor and residues within the PSA enzyme.