Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, is highly expressed by virtually all prostate cancers and is currently the focus of several diagnostic and therapeutic strategies. We have previously reported on the generation of several monoclonal antibodies (mAb) and antibody fragments that recognize and bind with high affinity to the extracellular domain of cell-adherent PSMA. This article reports the in vivo behavior and tumor uptake of the radiolabeled anti-PSMA mAb 3/A12 and its potential as a tracer for PET. Methods: The mAb 3/A12 was conjugated with the chelating agent 1,4,7,10-tetraazacyclododecane-N,N9,N$,N9$-tetraacetic acid (DOTA) and radiolabeled with 64 Cu. Severe combined immunodeficient mice bearing PSMA-positive C4-2 prostate carcinoma xenografts were used for small-animal PET imaging. Mice with PSMA-negative DU 145 tumors served as controls. For PET studies, each animal received 20-30 mg of radiolabeled mAb corresponding to an activity of 7.6-11.5 MBq. Imaging was performed 3, 24, and 48 h after injection. After the last scan, the mice were sacrificed and tracer in vivo biodistribution was measured by g-counting. Results: Binding of the mAb 3/A12 on PSMA-expressing C4-2 cells was only minimally influenced by DOTA conjugation. The labeling efficiency using 64 Cu and DOTA-3/A12 was 95.3% 6 0.3%. The specific activity after 64 Cu labeling was between 327 and 567 MBq/mg. After tracer injection, static small-animal PET images of mice with PSMA-positive tumors revealed a tumor-to-background ratio of 3.3 6 1.3 at 3 h, 7.8 6 1.4 at 24 h, and 9.6 6 2.7 at 48 h. In contrast, no significant tracer uptake occurred in the PSMA-negative DU 145 tumors. These results were confirmed by direct counting of tissues after the final imaging. Conclusion: Because of the high and specific uptake of 64 Cu-labeled mAb 3/A12 in PSMA-positive tumors, this ligand represents an excellent candidate for prostate cancer imaging and potentially for radioimmunotherapy.
Prostate cancer is the most common cancer in men from Western industrialized countries and a significant proportion of patients progress to advanced metastatic disease, for which currently no curative treatment exists. Therefore, new therapeutic approaches need to be considered. Prostate specific membrane antigen (PSMA) is an integral, non-shed type 2 membrane protein that is highly and specifically expressed on prostate epithelial cells and strongly upregulated in prostate cancer. PSMA is also present in the neovasculature of other solid tumors. These findings have spurred the development of PSMA-targeted therapies and first-generation products have entered clinical testing. The proposed strategies range from targeted toxins and radionuclides to immunotherapeutic agents. The present review provides an overview of these approaches.
Our results suggest that only the mAbs and scFvs, that were elicited with unpurified LNCaP lysate and not with purified PSMA will be useful agents for diagnostic imaging and therapeutic applications of prostate cancer.
Background Although cancer of the prostate is one of the most commonly diagnosed cancers in men, no curative treatment currently exists after its progression beyond resectable boundaries. Therefore, new agents for targeted treatment strategies are needed. Cross-linking of tumor antigens with T-cell associated antigens by bispeciWc monoclonal antibodies have been shown to increase antigen-speciWc cytotoxicity in T-cells. Since the prostate-speciWc membrane antigen (PSMA) represents an excellent tumor target, immunotherapy with bispeciWc diabodies could be a promising novel treatment option for prostate cancer. Methods A heterodimeric diabody speciWc for human PSMA and the T-cell antigen CD3 was constructed from the DNA of anti-CD3 and anti-PSMA single chain Fv fragments (scFv). It was expressed in E. coli using a vector containing a bicistronic operon for co-secretion of the hybrid scFv V H CD3-V L PSMA and V H PSMA-V L CD3. The resulting PSMAxCD3 diabody was puriWed from the periplasmic extract by immobilized metal aYnity chromatography (IMAC). The binding properties were tested on PSMA-expressing prostate cancer cells and PSMA-negative cell lines as well as on Jurkat cells by Xow cytometry.
Due to their specific binding characteristics, the anti-PSMA mAbs 3/A12, 3/E7, and 3/F11 show great promise for diagnostic and therapeutic applications in prostate cancer.
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