SaporinToxin-Conjugated Monoclonal Antibody Targeting Prostate-Specific Membrane Antigen Has Potent Anticancer Activity

BACKGROUND. Prostate-specific membrane antigen (PSMA) provides an attractive target for monoclonal antibody targeted therapies in the treatment of prostate cancer (PC). In this study, we generated an immunotoxin by linking a humanized anti-PSMA monoclonal antibody (hJ591) to the ribosome-inactivating protein toxin saporin. The hJ591 saporin immunoconjugate was evaluated for antitumor activity against PC cells. METHODS. PSMA-positive cell lines, LNCaP and CWR22Rv1 and a PSMA-negative cell line, PC-3, were used in these experiments. The hJ591 was biotinylated and mixed with streptavidin saporin (SAZAP). The binding ability of hJ591 SAZAP and the extent of internalization into the cells were tested. The viability of cells treated with hJ591 SAZAP was also examined and the apoptotic cells were measured. Lastly, the anticancer effect of hJ591 SAZAP was investigated in vivo. RESULTS. The binding ability of hJ591 SAZAP to PSMA was equivalent to that of unconjugated J591. Internalization of hJ591 SAZAP was clearly detected in PSMA-positive, but not in PSMA-negative cell lines. IC(50) of hJ591 SAZAP was 0.14 nM, 1.99 nM, and more than 100 nM in LNCaP, CWR22Rv1, and PC-3 cells, respectively. After 72 hr of hJ591 SAZAP treatment, the percentage of apoptotic cells was 60.29% and 40.73% in LNCaP and CWR22Rv1 cells, respectively, compared to 4.70% in PC-3 cells. The hJ591 SAZAP also had anticancer activity in a LNCaP xenograft model. CONCLUSIONS. Our findings show that hJ591 SAZAP conjugate has potent and selective antitumor effects on PSMA-positive PC cells in vitro and in vivo. This study supports development of PSMA antibody toxin conjugates for therapy of PC. Prostate 70: 1286-1294, 2010. (C) 2010 Wiley-Liss, Inc.