320 Comparative Pharmacokinetics of Trastuzumab-US and Trastuzumab-EU and the Potential Biosimilar Trastuzumab-Pfizer in Male CD-1 Mice

Background: Prostate cancer is the most common cancer in men. Although screening, radiation, surgery and hormone ablation therapy have greatly improved the detection and treatment of early stage prostate cancer, few options exist to treat metastatic, castrate-resistant prostate cancer (CRPC). We have developed a therapeutic approach that uses a bispecific SCORPION (multi-specific protein therapeutic) molecule that redirects T cell cytotoxicity against cells expressing a common prostate cancer antigen, PSMA (Prostate Specific Membrane Antigen). SCORPION molecules are antibody-like therapeutics containing two sets of binding domains linked to immunoglobulin Fc domains, which extends the half-life of the molecule in vivo. Materials and Methods: Cytotoxic activity was examined in vitro by treating PSMA(+) cell lines (LNCaP, C4−2B, MDA-PCa-2b) and PSMA(−) cell lines (DU-145, BxPC-3) with SCORPION molecules in the presence of purified human T cells or human peripheral blood mononuclear cells (PBMCs). Cytotoxic activity was determined by chromium release assays or by incorporation of a label such as propidium iodide or 7-aminoactinomycin D. CFSE-labeled T cells were assessed for activation and proliferation in a similar setting using multi-color flow cytometry. To evaluate serum stability, SCORPION proteins were incubated in mouse serum and monitored for functional activity. Results: SCORPION molecules targeting PSMA and CD3 efficiently redirected T cell cytotoxicity against PSMA(+) cell lines in vitro at low picomolar concentrations. Cytotoxic activity was target dependent and required the presence of T cells. T cells were activated and proliferated in response to the SCORPION therapeutic in the presence of PSMA(+) target cells; no proliferation was observed in response to PSMA(−) cells. SCORPION proteins were stable to freeze/thaw in mouse serum and showed stable functional activity in mouse serum when incubated at 37oC for up to one week. Conclusions: These studies show that anti-PSMA× anti-CD3 SCORPION therapeutics may be efficient agents for redirecting T cell cytotoxicity and show promise as potential therapeutics in CRPC.