Development and application of a reporter-probe system based on the Prostate Specific Membrane Antigen

• Main Author: Castanares, Mark A
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2012
• Subjects: Antigens; Medical imaging; Membranes; Oncology; Pharmacology; Prostate cancer
• ISBN: 1267613823; 9781267613820

Molecular-genetic imaging offers a non-invasive method to monitor and evaluate a population of cells. Because of this, much effort is dedicated to the development or reporter genes and corresponding imaging probes. The limitations of current gene reporter-probe systems include biocompatibility due to immunogenicity, tissue specificity, and low sensitivity. Due to these limitations, much effort has been dedicated to finding a genetic reporter of human origin that is able to efficiently sequester an imaging probe. The Prostate Specific Membrane Antigen (PSMA) is a biomarker for prostate cancer which is being developed for use in diagnosis and therapy. PSMA is located on the cell membrane, has a very selective expression pattern and possesses enzymatic and transporter activity. Because of these properties, we hypothesize that PSMA can be utilized as the basis of a reporter-probe system. A panel of novel PSMA inhibitors developed in the Pomper lab was screened using an enzymatic inhibition assay. Many compounds were able to inhibit the enzymatic activity of PSMA in the low nanomolar range and were developed as successful imaging agents. We then constructed reporter adenoviruses for PSMA and two genetic reporters currently in clinical trials, the human sodium iodide symporter (hNIS) and the mutant type 1 herpes simplex virus thymidine kinase (HSV-Sr39TK). These reporters were characterized for expression and functionality by in vitro assays in two cell lines, HCT116 and PC3-CAR. Once the adenoviral reporters were validated, each reporter-probe system was compared using a non-biased matrigel suspension model which we developed. Dynamic PET scans from this study revealed that the PSMA reporter-probe system had the highest absolute signal of all reporters as well as the best signal to noise ratio. To apply PSMA as a reporter gene, we imaged two populations, (1) metastatic cancer cells in the lymph nodes of nude mice and (2) the livers of nude mice which were infected with adenovirus. Using MR optical imaging, we were able to image lymph node metastasis in a novel model of metastatic prostate cancer. Secondly, using both SPECT and NIR optical ligands, we were able to detect adenovirus infection in the liver of nude mice. These studies validate the feasibility of PSMA as a reporter and support further development of PSMA as a clinically viable genetic reporter.