Abstract 5000: Immunotherapy with anti-PSMA x anti-CD3 bispecific antibody stimulates potent killing of a human prostate cancer cell line and target-mediated T cell activation in monkeys: A potential therapy for prostate cancer

PSMA (Prostate-Specific Membrane Antigen) is a promising therapeutic target in prostate cancer. Exploiting the high and selective expression of PSMA in the prostate, capromab and J591 antibodies are used as imaging or therapeutic agents, and antibody-toxin conjugates such as MLN2704 are being developed. Such antibodies do not stimulate T cell-mediated killing of prostate cancer cells; however, promising clinical data with T cell-recruiting bispecific agents and chimeric antigen receptor (CAR) T cells have created new immunotherapy paradigms that may also have potential in prostate cancer. We designed long-acting humanized bispecific antibodies that coengage PSMA+ cells and CD3+ T cells to stimulate redirected T cell-mediated cytotoxicity (RTCC) of prostate cancer cells. Unlike other bispecific formats, our antibodies possess an Fc domain and form stable heterodimers that are easily manufactured. Binding to Fcγ receptors was also abolished (reducing the potential for nonselective T cell activation), yet binding to human FcRn was preserved to maintain long serum half-life. We screened several anti-PSMA x anti-CD3 bispecific antibodies in vitro, and selected XmAb14484 based on its potent (< 1ng/ml) stimulation of human T cell killing of the LNCaP prostate tumor cell line. RTCC activity required PSMA binding, because a non-specific control (XENP13245, anti-RSV x anti-CD3) was inactive. XmAb14484 crossreacts with monkey but not mouse targets; therefore, we evaluated its effects in cynomolgus monkeys. Peripheral blood cells do not express PSMA, so we were unable to monitor target cell depletion. However, T cell engagement of target cells induces distinct effects on peripheral T cells, which can be used as surrogate markers for activity in the prostate. After treatment with 0.03 mg/kg XmAb14484, CD4+ and CD8+ T cells were rapidly activated (quantified by CD69 upregulation) and redistributed from the periphery. Serum cytokines including IL-6 and TNF were also strongly induced. T cell and cytokine responses returned to baseline within 2-3 days. In marked contrast, a 100-fold higher dose (3 mg/kg) of the anti-RSV x anti-CD3 control antibody induced minimal effects, demonstrating that PSMA+ target cells are required for T cell activation by XmAb14484. In summary, the pharmacologic activities of XmAb14484 on human cells and in monkeys support its clinical assessment in prostate cancer. We also demonstrate that T cell effects readily measured in peripheral blood (T cel