P1.12-18 Nonclinical Safety Assessment of AMG 757, a DLL3 Bispecific T Cell Engager, in the Cynomolgus Monkey

Bispecific T-cell engager (BiTE®) antibody constructs, which direct T cell killing of tumor cells by bridging the T cell receptor CD3 subunit and a tumor-associated antigen, are a clinically validated therapeutic modality in hematologic malignancies and have the potential to be effective in solid tumors. Delta-like 3 (DLL3) is overexpressed in small cell lung cancer (SCLC), an aggressive neuroendocrine tumor with a poor prognosis and limited therapeutic options. AMG 757 is a half-life extended BiTE® molecule that binds DLL3 and is in clinical development for the treatment of SCLC. The nonclinical safety assessment of AMG 757 included an evaluation of DLL3 expression and a repeat dose toxicology study in the cynomolgus monkey. DLL3 mRNA and protein expression were evaluated in an extensive set of normal cynomolgus monkey and human tissues using RNA sequencing, in situ hybridization, quantitative polymerase chain reaction, Western blot, and immunohistochemistry. In the cynomolgus monkey toxicology study, 3 animals/sex/group received AMG 757 weekly by intravenous infusion at 0, 50, 500, or 4500 μg/kg for 28 days during the dosing phase. An additional 2 animals/sex/group received 0 or 500 μg/kg weekly during the dosing phase and were retained for a 28-day recovery phase. The expression data indicate that DLL3 is expressed at low levels only in the pituitary, pancreas, and brain. In an immunohistochemistry assay, immunoreactivity with an anti-DLL3 antibody was cytoplasmic and generally weak in intensity. In the 28-day toxicology study, there were no AMG 757-related changes in clinical signs, body weight, food consumption, ophthalmic examinations, respiration rates, body temperature, neurologic examinations, clinical pathology parameters, cytokines, organ weights, or macroscopic tissue observations. AMG 757-related changes were limited to a transient, slightly higher heart rate at ≥500 μg/kg, a transient, minor decrease in lymphocyte populations at 4500 μg/kg, and a minimal to mild mixed cell infiltrate comprised of lymphocytes and eosinophils in the pituitary at 50 and 500 μg/kg. These changes were considered reversible and the highest non-severely toxic dose was determined to be 4500 μg/kg. DLL3 is expressed at low levels in the cytoplasm of a few normal tissues, where it would not be accessible to AMG 757. The nonclinical safety profile of AMG 757 was consistent with this expression pattern and the drug’s mechanism of action. These results suggest that limited on-target, off-tumor toxicity is anticipated, and that it may be possible to achieve high clinical exposures that maximize T cell-redirected responses in patients.