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e20028 Background: Tec is a B-cell maturation antigen × CD3 bispecific antibody being evaluated in MajesTEC-1 (NCT04557098), a single-arm, phase 1/2 study in pts with RRMM who were exposed to ≥3 lines of therapy (LOT), including an immunomodulatory drug, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. While there is currently no standard of care for treatment of pts with TCE RRMM, sel-dex is a recently approved, novel therapeutic option. Given the absence of a control arm in MajesTEC-1, we used an unanchored MAIC to compare efficacy outcomes of pts who received tec in MajesTEC-1 vs pts treated with sel-dex in the single-arm, phase 2b STORM Part 2 trial (NCT02336815). Methods: An unanchored MAIC was performed with individual pt-level data (IPD) from pts treated with tec (1.5 mg/kg weekly) in MajesTEC-1 at a clinical cutoff of Sep 7, 2021 (N = 150) and published summary-level data from pts who received sel-dex in STORM Part 2 (N = 122). After applying the STORM Part 2 eligibility criteria (penta-exposed, triple-class refractory, and refractory to last LOT), IPD from pts in MajesTEC-1 (N = 69) were weighted to match the aggregated baseline pt characteristics from STORM Part 2. Baseline characteristics of prognostic significance (refractory status, cytogenetic profile, revised International Staging System stage, presence of extramedullary disease, and number of prior LOT) were adjusted for in the analysis. Comparative efficacy of tec vs sel-dex was estimated for overall response rate (ORR), complete response or better (≥CR) rate, progression-free survival (PFS), duration of response (DOR), and overall survival (OS). For binary endpoints (ORR and ≥CR rate), the relative effects of tec vs sel-dex were estimated using an odds ratio (OR) and 95% CI derived from a weighted logistic regression. Time-to-event endpoints (PFS, OS, and DOR) were estimated using a weighted Cox proportional hazards model. Results: After adjustment, the effective sample size (ESS) of the MajesTEC-1 cohort was 37. Baseline characteristics were balanced between the 2 cohorts. Pts treated with tec had improved ORR (OR 3.14; 95% CI 1.48–6.69; P= 0.0029), ≥CR rate (OR 16.3; 95% CI 3.5–77.1; P= 0.0004), PFS (HR 0.58; 95% CI 0.30–1.11; P= 0.1007), DOR (hazard ratio [HR] 0.04; 95% CI 0.01–0.10; P< 0.0001), and OS (HR 0.52; 95% CI 0.28–0.95; P= 0.0344) compared with sel-dex. Despite a reduced ESS that reduced limited power to detect statistically significant differences, the majority of outcomes was statistically significant in favor of tec. Conclusions : In this MAIC, tec showed significantly improved efficacy over sel-dex for all outcomes except PFS, which was numerically in favor of tec, highlighting its potential as a highly effective treatment option for pts with TCE RRMM who received ≥3 prior LOT.

8035 Background: Pts with RRMM who are TCE to immunomodulatory drugs, proteasome inhibitors, and anti-CD38 antibodies have limited treatment options. While there is no standard of care for treatment of pts with TCE RRMM, belamaf is a recently approved, novel therapeutic option. MajesTEC-1 (NCT04557098) is a single-arm phase 1/2 study evaluating tec, a B-cell maturation antigen × CD3 bispecific antibody in pts with TCE RRMM who received ≥3 prior lines of therapy (LOT). Given the absence of a control arm in MajesTEC-1, we compared efficacy outcomes of pts who received tec at the recommended phase 2 dose in MajesTEC-1 with those of pts treated with belamaf in the phase 2 DREAMM-2 trial (NCT03525678). Methods: An unanchored MAIC was performed using individual pt-level data (IPD) from MajesTEC-1 (tec 1.5 mg/kg weekly; N = 150) at a clinical cutoff of Sep 7, 2021, and published summary-level data from pts who received the approved dose of belamaf in DREAMM-2 (2.5 mg/kg every 3 weeks; N = 97). The DREAMM-2 eligibility criteria were applied to pts from the intent-to-treat population of MajesTEC-1. IPD from MajesTEC-1 were weighted to match the aggregated DREAMM-2 baseline pt characteristics. Baseline characteristics of prognostic significance (refractory status, cytogenetic profile, International Staging System stage, presence of extramedullary disease, and number of prior LOT) were adjusted for in the analysis. Comparative efficacy of tec vs belamaf was estimated for overall response rate (ORR), complete response or better (≥CR) rate, progression-free survival (PFS), overall survival (OS), and duration of response (DOR). For binary endpoints (ORR and ≥CR rate), the relative effects of tec vs belamaf were quantified using an odds ratio (OR) and 95% CI derived from a weighted logistic regression analysis, while time-to-event endpoints (DOR, PFS, OS) were estimated using a weighted Cox proportional hazards model. Results: After adjustment, the effective sample size (ESS) of the MajesTEC-1 cohort was 33 and baseline characteristics for the reweighted MajesTEC-1 population were balanced with the DREAMM-2 population. Pts treated with tec had an improved ORR (OR 2.05; 95% CI 0.92–4.57; P= 0.0786), ≥CR rate (OR 2.13; 95% CI 0.80–5.65; P= 0.1283), PFS (HR 0.63; 95% CI 0.34–1.15; P= 0.1338), OS (HR 0.95; 95% CI 0.47–1.92; P= 0.8897), and DOR (hazard ratio [HR] 0.19; 95% CI 0.05–0.73; P= 0.0149) compared with belamaf. The reduced ESS following adjustment may account for the lack of statistical significance for most outcomes. Conclusions: These analyses demonstrated statistically improved DOR for tec vs belamaf and numerically favorable results for other outcomes, highlighting its potential as a treatment for pts with TCE RRMM who received ≥3 prior LOT.