Robert E. Reiter

Xenograft Models and the Molecular Biology of Human Prostate Cancer

The study of human prostate cancer has historically lagged behind that of other common malignancies such as breast and colon cancer. Although the reasons for this “lag” are multiple, a major obstacle to progress has been the relative lack of human prostate cancer model systems available to study the disease and to test new potential therapeutic strategies. Tissue specimens of primary prostate tumors, although common, are often too small or heterogeneous to be useful for molecular studies. Metastatic specimens are rare. Even when surgical material is available, it has been difficult to establish permanent prostate cancer cell lines. To date, most of the studies in the field of human prostate cancer have focused on just three human prostate cancer cell lines—PC-3, DU-145, and LNCaP. DU-145 and PC-3 do not express prostate specific antigen (PSA) or androgen receptor (AR), raising questions about the relevance of these two cell lines to most cases of clinical prostate cancer. LNCaP is androgen responsive and produces PSA, but it contains a mutation in the androgen receptor which alters ligand specificity and may limit its utility for molecular analysis. More important, it is unlikely that a single PSA-positive cell line can represent the heterogeneous disease we call prostate cancer.

Dual-Modality Immuno-PET and Near-Infrared Fluorescence Imaging of Pancreatic Cancer Using an Anti–Prostate Stem Cell Antigen Cys-Diabody

Pancreatic cancer has a high mortality rate due to late diagnosis and the tendency to invade surrounding tissues and metastasize at an early stage. A molecular imaging agent that enables both presurgery antigen-specific PET (immuno-PET) and intraoperative near-infrared fluorescence (NIRF) guidance might benefit diagnosis of pancreatic cancer, staging, and surgical resection, which remains the only curative treatment. Methods: We developed a dual-labeled probe based on A2 cys-diabody (A2cDb) targeting the cell-surface prostate stem cell antigen (PSCA), which is expressed in most pancreatic cancers. Maleimide-IRDye800CW was site-specifically conjugated to the C-terminal cys-tag (A2cDb-800) without impairing integrity or affinity (half-maximal binding, 4.3 nM). Direct radioiodination with 124I (124I-A2cDb-800) yielded a specific activity of 159 ± 48 MBq/mg with a radiochemical purity exceeding 99% and 65% ± 4.5% immunoreactivity (n = 3). In vivo specificity for PSCA-expressing tumor cells and biodistribution of the dual-modality tracer were evaluated in a prostate cancer xenograft model and compared with single-labeled 124I-A2cDb. Patient-derived pancreatic ductal adenocarcinoma xenografts (PDX-PDACs) were grown subcutaneously in NSG mice and screened for PSCA expression by immuno-PET. Small-animal PET/CT scans of PDX-PDAC–bearing mice were obtained using the dual-modality 124I-A2cDb-800 followed by postmortem NIRF imaging with the skin removed. Tumors and organs were analyzed ex vivo to compare the relative fluorescent signals without obstruction by other organs. Results: Specific uptake in PSCA-positive tumors and low nonspecific background activity resulted in high-contrast immuno-PET images. Concurrent with the PET studies, fluorescent signal was observed in the PSCA-positive tumors of mice injected with the dual-tracer 124I-A2cDb-800, with low background uptake or autofluorescence in the surrounding tissue. Ex vivo biodistribution confirmed comparable tumor uptake of both 124I-A2cDb-800 and 124I-A2cDb. Conclusion: Dual-modality imaging using the anti-PSCA cys-diabody resulted in high-contrast immuno-PET/NIRF images of PDX-PDACs, suggesting that this imaging agent might offer both noninvasive whole-body imaging to localize PSCA-positive pancreatic cancer and fluorescence image–guided identification of tumor margins during surgery.