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125I-Labeled monoclonal antibody PSA30.

Authors

Shan L1.

Source

Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2012 Nov 09 [updated 2012 Dec 05].

Author information

1
National Center for Biotechnology Information, NLM, NIH

Excerpt

125I-Labeled monoclonal antibody (mAb) PSA30, abbreviated as 125I-PSA30, was developed by Evans-Axelsson et al. for use in prostate cancer imaging by targeting the free, unbound form (fPSA) of prostate-specific antigen (PSA) (1). The human kallikrein-related peptidases (KLKs) are a group of 15 secreted serine proteases with different expression patterns and physiological roles (2, 3). These KLKs exist as single-chain preproenzymes of 30–40 kDa and are overproduced in various malignancies, where they are involved in diverse cancer-related processes such as cell growth regulation, angiogenesis, invasion, and metastasis (2, 4). Several members of the KLKs have been shown to be useful as prognostic biomarkers in various malignancies, with PSA being the most widely accepted and broadly used in clinical practice (4). PSA is encoded by the KLK3 gene and produced in abundance at almost all clinical stages and grades of prostate cancer (1, 2). An important finding is that, after synthesis in the cells, PSA is then released into the blood and exists in a noncatalytic form by forming stable covalent complexes (cPSA) with extracellular protease inhibitors, particularly a-1-antichymotrypsin (ACT) (5, 6). A smaller percentage (5%–40%) of the noncatalytic PSA in the blood also presents as a free, unbound form, which is unable to form complexes with inhibitors despite ~104-fold excess presence of inhibitors in the blood (1, 7). Because of its small size (28.4 kDa), fPSA is eliminated via glomerular filtration by the kidneys, with a half-life time of 12–18 h (7). In contrast, cPSA (~90 kDa) is eliminated very slowly, possibly via the liver. Measurements of the serum fPSA and cPSA are used for screening, diagnosis, and prognostic prediction of prostate cancer as well as for monitoring cancer recurrence. Prostate cancer imaging with radiolabeled antibodies by targeting PSA was tested as early as 15–20 years ago (8-10). However, these studies were in general unable to delineate tumors with a good signal/noise ratio because of the high background activity. One reason for the failure might be the use of polyclonal antibodies as imaging agents, which not only cross-react with other antigens but also fail to discriminate fPSA from cPSA. Evans-Axelsson et al. hypothesized that in vivo imaging of prostate cancer, especially the disseminated cancer, might be feasible by targeting the fPSA based on the facts that fPSA, rather than cPSA, is present at high abundance in close proximity to its local site of production, and that fPSA is cleared rapidly from the blood by the kidneys (1). The investigators tested their hypothesis by radiolabeling the PSA30 mAb and imaging tumors in animal models of prostate cancer (1). PSA30 recognizes an epitope that is covered by the ACT and other inhibitors, but is accessible on fPSA (11, 12).

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