Subcutaneous (SC) Administration of Mosunetuzumab with Cycle 1 Step-up Dosing Is Tolerable and Active in Patients with Relapsed/Refractory B-Cell Non-Hodgkin Lymphomas (R/R B-NHL): Initial Results from a Phase I/II Study

Background: Cytokine release syndrome (CRS) is a potentially serious complication of T-cell engaging immunotherapy. Effective measures are needed to reduce the rate and severity. In a multicenter Phase I/II study (NCT02500407), the CD20xCD3 bispecific antibody mosunetuzumab (Mosun) showed durable complete responses (CR) and had manageable safety in patients (pts) with late-line R/R B-NHL (Schuster et al. ASH 2019). IV administration with Cycle (C) 1 step-up dosing was an effective strategy for mitigating CRS during C1 (Bartlett et al. ASCO 2019). Fixed-dose SC administration was also a viable strategy for CRS mitigation, owing to the slower rate of Mosun absorption compared with IV (Matasar et al. ASH 2020). A combination of both strategies could further improve the CRS profile. We present safety and efficacy data from the initial cohorts investigating SC Mosun administration with C1 step-up dosing in the Phase I/II study.