As of July 28, 2020, 20 subjects have enrolled, and 14 subjects have received CT053 infusion in the phase 1b portion of the study, including 8 subjects who received 1.5-1.8×10⁸ CAR+ T cells while 6 subjects received 2.5-3.0×10⁸ CAR+ T cells. The treated subjects had a median age of 59 years (range 42-73), had received a median of 6 (range 3-11) prior lines of treatment, 93% were triple refractory to a PI, IMiD, and anti-CD38 antibody, and 64% were penta-refractory. In addition, 36% of the subjects had extramedullary disease at baseline, and 64% had high-risk cytogenetics. All subjects received bridging therapy.

The most common ≥ grade 3 AE was hematological toxicity. All subjects experienced ≥ grade 3 neutropenia (100%) and leukopenia (100%), and 36% of subjects had ≥ grade 3 thrombocytopenia within 30 days of treatment. No grade 3 or higher CRS or neurotoxicity was observed. Twelve of 14 subjects (86%) experienced grade 1 or 2 CRS, including 2 subjects who experienced grade 2 CRS and 1 subject who had grade 2 neurotoxicity. CRS events had a generally predictable time to onset, occurring at a median of 2 days post infusion with a median duration of 4 days (range 1-6). Two subjects received tocilizumab, and one subject with grade 2 CRS received both tocilizumab and steroids. One subject experienced grade 2 neurotoxicity with complete resolution within 24 hours upon administration of dexamethasone.

At the data cutoff, 10 subjects were evaluable for at least two months of efficacy assessment with a median follow-up of 4.5 months (range 2-8). A 100% ORR was observed, with 2 stringent complete responses, 2 CRs, 1 very good partial response, and 5 partial responses. Of the 12 subjects with evaluable bone marrow samples, 11 were MRD-negative at the 10^-5 sensitivity level. Responses were independent of baseline bone marrow BCMA expression.