Development of optimal Lu-177 labeled monoclonal antibody (7E11) constructs (CYT-500) for radioimmunotherapy of hormone refractory prostate cancer

B45 Background: Prostate-specific membrane antigen (PSMA) is highly over expressed on the surface of prostate cancer cells. Radioimmunoimaging using the anti-PSMA monoclonal antibody 7E11 coupled to indium-111 via the bifunctional chelating agent (BFC) GYK-DTPA is currently FDA approved. In prior clinical studies evaluating this antibody and BFC labeled with Y-90, the dose that could be safely administered was limited by toxicity secondary to loss of radionuclide from the BFC. We have investigated the use of kinetically inert macrocyclic BFCs (of the DOTA family) conjugated to this antibody and labeled with lutetium-177 to produce immunoconjugates that reduce radiation doses to bone marrow as compared to Y-90 thereby potentially allowing administration of higher and more efficacious doses. Lutetium-177 emits low energy beta particles and has the added benefit of also emitting imageable gamma radiation.Methods: A number of DOTA-based 7E11 immunoconjugates were generated using a variety of pH, ionic and temperature conditions. Characterization of the resultant immunoconjugates was performed using SDS-PAGE and HPSEC to assess purity and apparent molecular weight and MALDI-MS to determine ratios of BFC to antibody. Specificity and biological activity of the resultant conjugates were evaluated using an immunoaffinity binding ELISA that targets the N-terminal epitope on PSMA recognized by 7E11. Optimal radiolabeling conditions using Lu-177 have also been determined. PK studies have been performed in mice. Biodisitribution has been determined using Lu-177 labeled 7E11-meO-DOTA immunoconjugates in LNCaP xenograph-bearing nude mice. Acute and expanded acute toxicology studies in rats and dogs, respectively, and safety pharmacology studies in dogs have been completed.Results: Conjugation of DOTA based BFC to 7E11 (CYT-500) has been achieved using conditions that preserve the integrity of 7E11 and its ability to specifically bind PSMA. Molar ratios of linker to antibody ranged from 0.2 to 5.3. Radiolabeling to specific activities in excess of 20 mCi/mg has been carried out in 10-20 minutes at room temperature. Purity of the immunoconjugates ranged from 96% to 98%. These conjugates have also been shown to be more stable than previous constructs resulting in lower levels of radionuclide loss in both in vitro and in vivo studies. PK studies in mice demonstrate an acceptable clearance pattern. Biodisitribution and imaging studies demonstrate a significant amount of label localized in the tumors (the average % injected dose ranged from 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 did not reveal any significant adverse reactions at doses up to 20 times the anticipated human dose. Imaging studies using LNCaP xenografts in nude mice demonstrate selectivity of CYT-500 for the PSMA bearing tumor cells, with biodistribution and organ clarance that are similar to that obtained with commercial preparations of ProstaScint. An IND has been submitted to and cleared by FDA to initiate clinical studies which are scheduled to begin in 2006.Conclusions: Based on these studies, a lutetium-177 labeled radioimmunoconjugate that should allow administration of higher doses than previous constructs has been developed and is being scaled up for human clinical testing. The demonstrated biological and physical characteristics of this agent support its use for both therapy and simultaneous imaging.