Abstract 5099: Notch inhibition as a therapeutic approach to eliminate dormant cancer cells in multiple myeloma

Abstract Multiple myeloma (MM) remains incurable due to disease relapses, thought to stem from the activation of dormant MM cells. Preventing the awakening or eliminating dormant MM cells is crucial to avoiding disease relapse. In this study, we sought to identify targetable vulnerabilities to eliminate or control dormant cells. First, we used a scRNAseq dataset of dormant MM cells and found that dormant MM cells exhibit upregulation of Notch pathway genes compared to proliferating MM cells. Based on this, we hypothesized that inhibiting Notch signals in the tumor niche should decrease dormant MM cells. To test this notion, we injected immunocompetent mice i.v. with murine 5TGM1 MM cells double-labeled with a fixed genetic reporter and a membrane dye (DiD) retained by non-dividing cells, allowing us to distinguish dormant (GFP+DID+) from proliferating cells (GFP+DID-). After 2wks, we randomized mice to groups receiving vehicle or a bone-targeted Notch inhibitor (BT-GSI). We detected a population of GFP+DiD+ dormant cells that expressed high levels of Axl and were growth arrested throughout the life span of the mice. Short-term treatment with BT-GSI (1wk) reduced tumor burden (75%) and decreased the number of dormant MM cells (70%) via apoptosis. Prolonged BT-GSI treatment (7 wks) eliminated all dormant MM cells and further reduced proliferating cells. These effects were replicated in immunodeficient mice injected with RFP+DID+ human OPM2 MM cells and in human bones infiltrated by OPM2 RFP+DID+ cultured ex vivo. Next, we explored if modulating the tumor niche could influence MM cell dormancy. Since dormant MM cells colocalize with osteoblasts in the endosteal bone surface, we assessed if increasing osteoblasts numbers and their activity prevents the awakening of MM dormant cells or enhances their engagement in dormancy programs. To test this, we injected GFP+DiD+ MM cells and, after two weeks, randomized mice to groups receiving vehicle or Scl-Ab, an anabolic agent that rapidly increases osteoblasts. Scl-Ab (2wks) increased osteoblasts but did not affect the proportion of proliferating or dormant MM cells. Similar results were observed in human bones infiltrated with human MM cells cultured ex vivo treated with Scl-Ab. Next, we pre-treated mice with Scl-Ab (2wks) to generate a microenvironment loaded with osteoblasts before MM cell injection. Despite the increase in osteoblasts, Scl-Ab did not impact the number of dormant or proliferating MM cells. Further, co-administration of Scl-Ab did not interfere with BT-GSI’s ability to eliminate proliferating or dormant MM cells in any of these models. In conclusion, our findings show that 1) Notch signals promote the survival of dormant MM cells, 2) manipulating osteoblasts in the tumor niche does not affect MM cell dormancy, and 3) bone-targeted Notch inhibition is a promising approach to eradicate dormant tumor cells and potentially delay/prevent disease relapse in MM patients. Citation Format: Hayley M. Sabol, Aric Anloague, Japneet Kaur, Sharmin Khan, Bethany Paxton, James Smith, Michelle McDonald, Peter Croucher, C. Lowry Barnes, Elena Ambrogini, Alison Frontier, Frank H. Ebetino, Jesus Delgado-Calle. Notch inhibition as a therapeutic approach to eliminate dormant cancer cells in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5099.