Zhang, Catherine

Abstract Background Etentamig is a second-generation B-cell maturation antigen (BCMA) x CD3 bispecific antibody that has shown promising efficacy and a favorable safety profile in heavily pretreated patients with relapsed/refractory multiple myeloma (RRMM). Etentamig possesses a unique combination of 2 high-affinity BCMA binding domains, coupled to a low-affinity CD3 binding domain that have been shown to reduce the negative impact of soluble BCMA (sBCMA), decrease risk for cytokine release syndrome (CRS), and drive sustained T-cell activation with reduced immune exhaustion in preclinical models of MM (Blood 2023;142 [suppl 1]:4666). Here, we describe correlative biomarker analyses from Phase 1 (NCT03933735) and Phase 1b (NCT05650632) clinical trials of etentamig in RRMM designed to assess the impact of sBCMA on clinical response and pharmacodynamic changes in proinflammatory cytokines, and T-cell redistribution, activation, proliferation, and exhaustion. Methods Peripheral blood and serum samples from patients with RRMM enrolled in Phase 1 and Phase 1b open-label studies who received etentamig monotherapy were collected at baseline, on treatment, and at disease progression. The Phase 1b study consisted of dose-optimization (DO) and dose-expansion (DE) phases. During DO, patients received a single step-up dose (SUD) of intravenous etentamig (2 or 4 mg) on Cycle (C) 1 Day (D) 1, followed by the target dose of 60 mg on D4. In DE, based on DO findings, patients received the recommended etentamig SUD (2 mg) on C1D1, followed by 60 mg on D4 with modified dexamethasone predosing. Samples were analyzed by flow cytometry for immune cell populations, Luminex® for cytokines, and electrochemiluminescence for sBCMA. Peripheral blood mononuclear cells and bone marrow mononuclear cells (BMMC) at baseline and on-treatment were subjected to single-cell Cellular Indexing of Transcriptomes and Epitopes sequencing (scCITEseq) and single-cell T-cell receptors (scTCR) / B-cell receptors clonality analyses. Results While baseline serum sBCMA levels did not correlate with clinical response to etentamig at the selected dose level of 60 mg every 4 weeks (Q4W) with 2 mg SUD, a reduction in sBCMA levels over time was associated with best International Myeloma Working Group response. Etentamig treatment led to a rapid and transient increase of proinflammatory cytokines and promoted T-cell redistribution, activation, and proliferation. Maximum reduction in peak levels of CRS-related cytokines, including (interleukin [IL]-6, IL-8, IL-10, tumor necrosis factor–a), were observed in DE compared with DO and 60 mg Q4W without SUD. Peak induction levels of proinflammatory cytokines during C1 correlated with occurrence of CRS (≥Grade [G] 1) as well as clinical response across dose levels. A modified dexamethasone premedication schedule of 2 mg/60 mg Q4W cohort for C1 resulted in significantly reduced baseline levels of IL-6 and key macrophage-associated cytokine/chemokines (eg, monocyte chemoattractant protein-1 and macrophage inflammatory protein-3 beta). Incidence of CRS was lowest in the 2 mg/60 mg Q4W cohort (26% G1, 4% G2, 0% ≥G3). Higher T-cell to myeloma cell ratio in BMMCs at baseline correlated with better response to etentamig. scCITEseq and scTCR analysis revealed that at baseline, responders also had greater CD8+ T-cell cytotoxic activities, increased TCR clonality, and elevated inflammatory signatures in T-cells from both peripheral blood and bone marrow. Etentamig at a dosage of 60 mg Q4W resulted in significant clonal expansion of CD8+ effector T-cells and enhanced gene expression signatures related to activation and cytotoxicity in both CD8+ T effector and natural killer cells. Along treatment cycles, responders maintained their T-cell fitness, high clonality, and low T-cell exhaustion levels. Frequency of baseline CD4+ or CD8+ T-cell exhaustion (programmed cell death receptor 1+/ T cell immunoglobulin and mucin-domain containing-3 +) did not impact clinical response at optimal dose level of 2 mg/60 mg Q4W etentamig. Conclusions Responses to etentamig were independent of baseline sBCMA levels and resulted in rapid and transient proinflammatory cytokine production that promoted robust T-cell redistribution, activation, and proliferation indicating that the optimal dose of etentamig (2 mg/60 mg Q4W) maximizes its clinical potential as a convenient, safe, and effective therapy for MM.

7532 Background: ABBV-383 is a BCMA x CD3 bispecific antibody with 2 high-affinity BCMA-binding domains and a low-affinity CD3 engaging arm that has been shown to decrease the negative impact of soluble BCMA (sBCMA) and drive sustained T-cell activation with reduced cytokine release in preclinical models of MM. Here, we describe correlative biomarker results of baseline and longitudinal sBCMA levels and immune profiles in the ongoing first-in-human trial that supports 60 mg Q4W as the optimal therapeutic dose of ABBV-383 monotherapy. Methods: Peripheral blood and serum samples from patients with RRMM enrolled in the phase 1 open-label, dose-expansion study (NCT03933735) who received ABBV-383 at 20, 40, or 60 mg Q3W or 60 mg Q4W were collected at baseline (post-dexamethasone, pre–ABBV-383), on treatment, and at disease progression. Samples were analyzed by flow cytometry for immune cell populations, Luminex for cytokines, and electrochemiluminescence ligand binding for sBCMA. Results: Median sBCMA levels at baseline were highly variable among patients, but did not associate with clinical response (≥ partial response [PR]) to ABBV-383 at the selected optimal dose level of 60 mg Q4W (<PR: 244.7 ng/mL; ≥PR: 52.2 ng/mL; P=0.13). Reduction in sBCMA levels over time associated with clinical response (Cmin at cycle 5: <PR, 244.7 ng/mL; ≥PR, 4.7 ng/mL; P=0.04). ABBV-383 treatment led to a rapid and transient increase of proinflammatory cytokines, including IL-6, IL-8, IL-10, TNF-α (Cmax within cycle 1: 29, 349.5, 1375, 456.5 pg/mL, respectively), and promoted T-cell redistribution (89% reduction of CD8+ T cells from periphery), activation (1.9-fold increase in CD69+CD8+ T cells), and proliferation (2-fold increase in Ki67+CD8+ T cells) within cycle 1 in 60-mg Q4W-treated patients. The frequency of baseline CD8+ T-cell exhaustion (PD-1/TIM3) did not impact clinical response at the optimal dose level of 60 mg Q4W ABBV-383 (<PR: 3.5%; ≥PR: 3.9%; P=0.9). Peak induction levels of proinflammatory cytokines during cycle 1 correlated with occurrence of cytokine release syndrome (≥G1) as well as clinical response across dose levels. Conclusions: High-avidity BCMA binding coupled to low-affinity T-cell engagement distinguishes the resulting mechanism of action for ABBV-383. Responses to 60 mg Q4W ABBV-383 were independent of baseline sBCMA levels and resulted in rapid and transient proinflammatory cytokine production that promoted robust T-cell redistribution, activation, and proliferation, indicating that the optimal dose of ABBV-383 maximizes its clinical potential as a convenient, safe, and effective therapy for MM. Clinical trial information: NCT03933735 .