Wang, Fan

Abstract Background B-cell maturation antigen (BCMA)-targeting immunotherapies represent a promising class of treatment for the management of immunoglobulin light chain (AL) amyloidosis, a disease that is related to clonal plasma cells. Etentamig is a 2nd generation, off-the-shelf, fully human BMCA X CD3 bispecific antibody with bivalent BCMA domain and low CD3 affinity that targets both BCMA-expressing plasma cells and CD3-expressing T cells. Etentamig monotherapy is being studied in multiple myeloma (MM) in an ongoing phase 3 study (NCT06158841). Here, we report the first safety and efficacy results from M24-209, an open-label phase 1/2 study evaluating etentamig monotherapy in patients (pts) with relapsed/refractory (RR) AL amyloidosis from dose level 1 (DL1). Methods The phase 1 dose escalation (ESC) portion of the study (NCT06158854) with etentamig enrolled pts with RR AL amyloidosis with ≥1 prior lines of therapy (LOT), including a proteasome inhibitor and an anti-CD38 monoclonal antibody with no prior BCMA exposure. Eligible pts were required to have a difference in involved and uninvolved serum free light chain (dFLC) of ≥50 mg/L, and pts with Mayo Clinic 2004 cardiac risk stage IIIb with NT-proBNP ≥8500 pg/ml were excluded. Three dose groups were explored in dose ESC (etentamig dose levels [DL] 1, 2, and 3). In all cohorts, pts will receive up to 24 cycles of treatment and will be followed until disease progression. In dose ESC, the primary objectives are to determine safety/tolerability, pharmacokinetic/pharmacodynamic, and recommended phase 2 dose. The secondary objectives include evaluation of efficacy as measured by hematologic response and organ response. Responses are investigator assessed per consensus guidelines (Comenzo, et al. Leukemia. 2012. 26[11]:2317-25; Palladini G, et al. Amyloid. 2021 [Epub];28[1]:1-2). Results As of 17 July 2025, all DLs in dose ESC have been fully enrolled, and 12 pts have been treated with etentamig at DL1. Due to short follow-up, this initial report only includes data from DL1, with further follow-up data from ESC to be included at the time of presentation. Median duration of follow up in DL1 was 9.4 months (8.3–13.8) with 83% of pts continuing treatment. Median age for pts was 72 years (range, 56–85 years). Median number of prior LOT was 2 (range, 1–6), and median time from initial diagnosis was 3.6 years. Thirty-three percent have received a prior stem cell transplant and 50% previously received the regimen of daratumumab with cyclophosphamide, bortezomib, and dexamethasone in combination. Median dFLC at study baseline was 110. 5 mg/L (range, 56.6–617.5). At baseline, 83% of pts had cardiac involvement, 25% had renal involvement, and 8% had hepatic involvement. Eighty-three percent of pts had lambda AL amyloidosis. Treatment-emergent adverse events were reported in 100% (42% Grade ≥3) of pts at DL1. Cytokine release syndrome (CRS) occurred in 2 pts (17%, all Grade 1), with no instances of any grade immune effector cell-associated neurotoxicity syndrome (ICANS). CRS occurring after the first dose had median time to onset of 6.8 hours (range, 6.7–6.9). Grade ≥3 infections occurred in 1 pt (8%, all Grade 3). Grade ≥3 neutropenia also occurred in 1 pt (8%, all Grade 3), and no Grade ≥3 events of thrombocytopenia or anemia occurred. No Grade 5 events have occurred. Within the dose ESC DL1 cohort, the overall hematologic response rate was 100%, with all pts achieving ≥VGPR and 83% of pts achieving complete response (CR). The median time to CR was 0.9 months (0.3–9.3). The median duration of response has not yet been reached and 100% of pts remain in their best overall response by investigator assessment. To date, no pts have experienced hematologic progression on study. Cardiac response was observed in 30% of evaluable pts. Renal response was seen in 67% of evaluable pts. Conclusions In the DL1 cohort, etentamig showed a manageable safety profile in AL amyloidosis with low incidence and severity of CRS, no ICANS, and limited Grade ≥3 cytopenia events. Deep and rapid hematologic responses were observed with 100% ≥VGPR rate and 83% hematologic CR rate. Additional dose-level data will be presented at the time of the meeting. All pts remain in ongoing follow-up and evaluation, and the study will begin the dose expansion (EXP) portion after dose selection.

Introduction ABBV-383 is a differentiated BCMA x CD3 bispecific antibody (fully humanized IgG4) T-cell engager composed of a bivalent BCMA binding domain with high avidity, a low-affinity CD3-binding domain, and a present but silenced-Fc tail retaining the FcRn binding allowing for an extended half-life and thereby, enabling dosing convenience. ABBV-383 monotherapy has shown promising activity and improved CRS safety profile with modified dexamethasone (Dex) premedication in patients (pts) with RRMM (NCT03933735; Rodriguez et al., JCO 2024;42[suppl 16]:7531). Here, ER and CRS PK/PD analyses supporting the selection of optimal SUD for ABBV-383 are reported. Methods ER-efficacy (N=286) and -safety (N=290) analyses were conducted using logistic regression methodology (R version 4.3.2) combining data from escalation (0.025-120 mg Q3W) and expansion (20, 40, 60 mg Q3W and 60 mg Q4W) cohorts of phase 1 study NCT03933735 and Arm A of phase 1b study NCT05650632 evaluating step-up dose (2 or 4 mg on Day 1) with a full dose (60 mg on Day 4) in pts with RRMM. Pts received modified Dex with full dose in expansion cohorts of 60 mg Q4W and 2/60 mg Q4W. In all other cohorts with or without SUD, pts received low Dex. Based on prior knowledge (JCO 2024;42[suppl 16]:7541), the safety endpoints (CRS: any-grade and ≥ G2) were modeled with Cycle 1 total (unbound + bound to soluble BCMA) Cmax, while objective response rate (ORR) was modeled with free (unbound + partially bound to soluble BCMA) Cavg. CRS ER models included a priori selected Dex (low vs. modified) and SUD (yes vs. no) as predictors. Relevant pt-specific covariates were tested in all models. CRS PK/PD analyses included total PK, IL-6, and CRS data (N=199) from the first 3 cycles of the above phase 1/1b studies. A semi-mechanistic indirect response PK/IL-6 model was developed to correlate total PK and IL-6 (NONMEM 7.5). Maximal IL-6 concentrations were derived using post hoc estimates from the PK/IL-6 model and correlated with CRS events (logistic regression; R version 4.3.2). The models included effects of time-dependent Dex (low vs. modified) and SUD (yes vs. no). The overall modeling approach was assessed by comparing observed and predicted CRS rates. Simulations were conducted for various dosing regimens, including one or two SUD and Dex premedication scenarios to support the selection of the optimal SUD. Results Consistent with clinical observations (separate abstract submission), any-grade CRS model predicted probabilities suggested two key outcomes: 1) premedication with modified Dex improves the CRS safety profile compared to the low Dex (42% vs 69% for 60 mg Q4W); 2) modified Dex with SUD further improves the CRS safety profile of ABBV-383 compared to low Dex with SUD (29% vs 43% for 2/60 mg Q4W). Consistent with prior work, no ER relationship was observed for ≥ G2 CRS (p > 0.05). The ORR model resulted in a predicted probability of 68% ORR (closely matching with overall observed 64% ORR) with no impact of SUD and modified Dex. Only soluble BCMA was selected as a covariate in the ORR ER model. The final semi-mechanistic PK/IL-6 model well captured the observed IL-6 data, and the CRS rates (predicted by the logistic regression models) closely matched with the observed data. The CRS PK/PD analyses findings were consistent with the clinical observations and ER analyses key outcomes 1 and 2. The CRS PK/PD analyses predictions comparing the 1-SUD vs 2-SUD (untested) regimens with modified Dex suggested that 1) among the 2-SUD regimens with modified Dex, the 2/3/60 mg regimen would have the maximum reduction of 4% points in any-grade CRS compared to the 2/60 mg 1-SUD regimen, however, with substantial overlap of confidence intervals and with no decrease in ≥ G2 CRS (< 0.5% point); 2) 4 mg as an intermediate SUD in a 2-SUD regimen would lead to higher any-grade and ≥ G2 CRS rates (~9% and 3% points higher, respectively), compared to the 2/60 mg Q4W regimen. Conclusions ER and CRS PK/PD analyses suggested that modified Dex and SUD improve the CRS safety profile of ABBV-383 with no impact on efficacy (ORR) in pts with RRMM. The CRS PK/PD predictions indicated that implementation of 2-SUDs provide minimal additional or no improvement in CRS safety profile compared to 1-SUD. Overall, the 1-SUD dosing regimen of 2 mg with modified premedication prior to the full dose of 60 mg Q4W showed promising results and can be considered for future evaluation and studies.