Background: Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein defined by the monoclonal antibody 7E11.C5. The 7E11.C5 antibody forms the basis of an in vivo diagnostic imaging agent (ProstaScint, Cyt-356) for identification of metastatic prostate cancer. The epitope on PSMA recognized by 7E11.C5 has been determined to be the first 6 amino acids from the N-terminal, expressed on the cytoplasmic side of the plasma membrane. Thus, the basis for 7E11.C5 specificity in imaging studies remains unclear.
Methods: Fluorescence-activated cell sorter (FACS) analysis of fixed and viable cultured cells was used to determine the staining intensity with FITC-labeled antibodies.
Results: The results indicate that FITC-labeled 7E11.C5 antibody is taken up and specifically labels viable LNCaP cells in vitro. Labeling intensity of viable cells after 2 hr of antibody incubation was similar to that of fixed cells. No labeling of cells that do not express PSMA was observed, nor was labeling observed with LNCaP cells treated with an isotype-matched irrelevant antibody.
Conclusions: Uptake and labeling of PSMA by FITC-labeled 7E11.C5 in viable cells in vitro strongly suggest that this is a major basis for effectiveness of the 7E11.C5 antibody during in vivo imaging applications with 111In-labeled antibody (ProstaScint, Cyt-356).