CSF-1R inhibition and lenalidomide synergize to promote myeloma control after autologous stem cell transplantation

Abstract Background: Autologous stem cell transplantation (ASCT) remains the standard of care consolidation in eligible patients with multiple myeloma. Typically, this is followed by lenalidomide maintenance therapy until progression or intolerance. This combinatorial approach results in improved progression-free survival compared to drug-based consolidation. However, the majority of patients ultimately relapse, consistent with myeloma progression and immune escape. Increasing evidence suggests that myeloma may invoke a suppressive tumor microenvironment (TME), principally in the bone marrow (BM), which may impair endogenous T cells and/or therapeutic T cell engagers or chimeric antigen receptor T cells targeting tumor antigen. Immunosuppressive myeloid cells are known to be abundant in the TME, and we and others have previously implicated macrophages in this process. To examine potential cellular sources of immunosuppression in the TME, we used flow cytometry to analyze myeloid cells in the BM from patients undergoing ASCT for myeloma. BM mononuclear cells were isolated from aspirates drawn at approximately 2-6 weeks prior to ASCT (n=13), 90 days post-transplant, and at relapse or 1-year post transplant. Post-transplant samples were categorized as relapsed (n=12) or controlled for >1 year (n=8). BM from patients with progressive myeloma was characterized by a population of CD64+CD172+ macrophages that express CD169, putatively marking BM residency, and exhibiting a highly immunosuppressive phenotype evidenced by expression of CD163, CD155, PD-L1, and CSF-1R. Notably, the frequency of CD169+CD163+ macrophages (within CD64+ cells) but not the CD64+CD172+ macrophage population as a whole were expanded in patients at the time of relapse (mean frequency pre-ASCT: 3.46%, D+90: 2.96%, controlled: 4.02%, relapse: 13.83%; P<0.05 relapse vs. controlled; P<0.01 relapse vs. pre-ASCT or D+90), and correlated with multiple myeloma burden in the BM (r2 = 0.477, P=0.0103). Using a preclinical system of ASCT in mice bearing VK*MYC myeloma, we identified a similar macrophage subset in the BM, characterized as CD11b+Ly6GnegCD11c+CD64+ with high expression of CSF-1R, PD-L1, and F4/80. We sought to target these putatively immunosuppressive macrophages, using antibody-based CSF-1R inhibition, to determine whether we could augment lenalidomide maintenance after ASCT. We used a suboptimal T cell dose wherein endogenous anti-myeloma immunity is limited and assessed each therapy alone and in combination. We used cereblon (CRBN)-transgenic mice that are genetically modified to allow engagement of thalidomide and its derivatives (including lenalidomide), resulting in degradation of the IMiD targets, Aiolos and Ikaros. As expected, CSF-1R inhibition efficiently ablated the CX3RC1+Ly6Clo monocyte population in the blood and BM (mean frequency in lineage negative in blood: 2.9% vs. 18.1% in isotype-treated; in BM: 0.4% vs. 5.8% in isotype-treated; p<0.001 for both sites). While single agent CSF-1R inhibition or lenalidomide had no significant effect on disease progression, the combination of the two agents had a synergistic effect in attenuating myeloma relapse after ASCT. At 108 days follow-up, median overall survival for mice receiving CSF-1R inhibition and lenalidomide was not reached, while median survival was 65 days for either isotype- or lenalidomide-treated mice, and 77 days for mice receiving only CSF-1R inhibition (P<0.05). Myeloma M band progression was also significantly reduced with combination therapy (p<0.001). Conclusion: The ability of CSF-1R inhibition to deplete CSF-1R+ macrophages and prevent tissue fibrosis has previously been demonstrated in preclinical models of chronic graft-versus-host disease (GVHD). Subsequent clinical trials have proven axatilimab, a human anti-CSF-1R antibody, to be well tolerated and effective for the treatment of chronic GVHD, leading to FDA approval. Given lenalidomide is already a standard maintenance therapy, combination therapy with axatilimab represents a readily testable clinical strategy for augmenting progression-free survival in patients with multiple myeloma after ASCT.