Development and characterization of a novel 177Lu-MeO-DOTA-7E11 antibody construct (CYT-500) for the treatment and imaging of prostate adenocarcinoma
Authors: John Wolodzko, John Tedesco, William Goeckeler, Mei-Hsiu Pan, Dong-Wei Gao, Jinjin Feng, Jiang He, Bruce Hasegawa, Benjamin Franc
Journal: Cancer Research (2007)

4745    Background:  The effective treatment of prostate cancer is a significant clinical problem. Prostate-specific membrane antigen (PSMA) is overexpressed by prostate cancer cells. The anti-PSMA monoclonal antibody 7E11 labelled with  111 In  via  the chelator GYK-DTPA (CYT-356) is currently FDA approved for clinical imaging. We have developed a stable methoxy-DOTA-linked conjugate to 7E11 (CYT-500), labelled with the beta and gamma emitter  177 Lu, and compared it to CYT-356 in a LNCaP-bearing mouse model. Compared to CYT-356, CYT-500 exhibits favorable biodistribution and may be an effective radioimmunotherapeutic for prostate cancer.      Methods:  DOTA-7E11 immunoconjugates were formed under various conditions and characterized using SDS-PAGE and HPSEC for purity and molecular weight and MALDI-MS for chelator:antibody ratio. Specificity and biological activity were evaluated with immunoaffinity binding ELISA targeting the PSMA N-terminal epitope recognized by 7E11. Optimal  177 Lu radiolabelling conditions were determined. PK studies were performed in mice. Acute and expanded acute toxicology studies were conducted in rats and dogs, and safety pharmacology studies in dogs. Pharmacokinetics and biodistribution studies of CYT-500 and CYT-356 were performed in LNCaP xenograft mice at 2, 8, 12, 24, 72 and 168 hrs. Studies of CYT-500 were performed in non-tumor-bearing controls at 8, 24 and 72 hrs. Optical imaging and single-photon emission computed tomography were performed, followed by autoradiography and tissue counting to assess the distribution of the agents in tumor and normal tissues.  Results:  Conjugation of me-O-DOTA to 7E11 (CYT-500) has been achieved using conditions that preserve the integrity of 7E11 and its PSMA binding ability. Molar ratios of linker to antibody range from 0.2 to 5.3. Radiolabelling to activities in excess of 20 mCi/mg is achieved in 10-20 minutes at room temperature. Purity is 96% to 98%. There is little radiolabel loss  in vitro  or  in vivo . PK studies in mice show a favorable clearance pattern. Biodistribution and imaging studies demonstrate significant tumor uptake (injected radioactivity of 0.3% to 2.2% in 12 animals ranging from 6 hrs to 13 days). Acute and expanded acute toxicology studies in rats and dogs, respectively, and safety pharmacology studies in dogs showed no significant adverse reactions at doses up to 20 times the anticipated human dose. In the imaging studies, both  177 Lu- and  111 In-7E11 conjugates demonstrated an early blood pool phase with uptake dominated by the blood, lung, spleen and liver, followed by prominent tumor uptake and retention after 24 hours. Uptake of  177 Lu MAb in tumor was notably higher than that of  111 In MAb at 24 hours. Only the liver differed significantly, with the  111 In-7E11 demonstrating continued uptake after 72 hours.   Conclusions:  A  177 Lu-labeled radioimmunoconjugate has been developed. The biological and physical characteristics of this agent support its use for therapy and imaging of prostate cancer.