Results: As of June 2, 2023, 40 pts; median age 66 years (range: 44-76) were enrolled; 38 received CART-ddBCMA (32, DL1; 6, DL2) & 38 were evaluable for initial safety & clinical response. Two pts who were not dosed had cell product manufactured but were not eligible for cell infusion due to medical complications. Pts had a median of 4 (range: 3-16) prior lines of therapy. All infused pts (100%) were triple-refractory, & 26 (68%) were penta-refractory; 34 pts (89%) were refractory to last-line of treatment; 9 pts (24%) had high tumor burden with ≥60% bone marrow plasma cells; 13 (34%) pts had extramedullary disease; & 11 (29%) pts had high-risk cytogenetics (Del 17p, t(14;16), t(4;14)) at baseline. Median follow-up after CART-ddBCMA infusion was 22 months (range: 9-40 months). CAR+ cells comprised a median 70% (range: 48-87%) of total CD3+ T cells; median vector copy number was 2.2 copies/cell (range: 1.1-3.5); median cell viability was 98% (range: 90-100%), & median cell manufacturing yield was 1174 ×106 CAR+ cells (range: 470-1626 ×106). CART-ddBCMA product characteristics were consistent with the specifications in all the lots, & there were no manufacturing failures. CRS occurred in 36/38 (95%) pts but only 1 pt in DL2 had grade (Gr) 3 CRS & all other cases were Gr≤2. ICANS occurred in 7 pts (5, Gr≤2; 2, Gr3), with 1 Gr3 case in each DL. All cases of CRS & ICANS resolved without further sequalae with management. No cases of off-tumor cell mediated toxicity, delayed neurotoxicity events (i.e., occurring after day 28), or Parkinsonian-like symptoms were observed. All 38 evaluable pts demonstrated investigator-assessed clinical response per 2016 IMWG criteria (ORR, 100%) with 22 sCR, 7 CR (≥CR rate, 76%), 6 VGPR (≥VGPR rate, 92%), & 3 PR. Responses deepened over time & conversion to CR/sCR was observed with longer follow-up (as late as month 12). Of those evaluable for MRD testing to date (n=29), 25 (86%) were MRD-neg at 10-5. Median duration of response, PFS, & OS were not reached at the time of data-cut because 25 of 38 evaluable pts (66%) had ongoing response. The Kaplan-Meier estimated PFS rates for 6, 12 & 18 months were 92%, 74%, & 67% respectively. Durable responses were also observed in patients with high-risk features (EMD, BMPC ≥ 60%, or B2M ≥ 5.5 at baseline) & high-risk cytogenetics. PFS rates at 6-, 12-, & 18-months are shown in Table 1. Based on the results from the study, a dose of 115 ± 10 ×106 cells, consistent with DL1, was recommended for the phase 2 study.

Conclusions: Adverse events with CART-ddBCMA, including CRS & ICANS, were manageable & no off-tumor tissue-targeted toxicity, delayed neurotoxicity, or Parkinsonian-like events were observed in the entire cohort at the time of data-cut. Ongoing efficacy results are encouraging, with 100% ORR, including 35 (92%) response of VGPR or better & 29 (76%) with CR/sCR. More importantly, clinical responses were durable with an overall estimated 18-mo PFS rate of 67% with comparable clinical responses seen in ‘high-risk’ patients known to have poor prognosis. Updated data with additional follow-up based on later data-cut will be presented.