High-Parameter Mass Cytometry (CyTOF) Evaluation of Relapsed/Refractory Multiple Myeloma (MM) Pts (Pts) Treated with Daratumumab Supports Immune Modulation As a Novel Mechanism of Action

Introduction: Daratumumab (DARA) is a human CD38-targeting monoclonal antibody that induces deep clinical responses in MM pts through multifaceted mechanisms of action (MOA) including complement-dependent cytotoxicity, antibody-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis and induction of apoptosis. Flow cytometry analysis revealed a previously unknown immunomodulatory role of DARA, via T-cell induction expansion, T-cell activity enhancement, and reduction of immune suppressive cell populations including CD38⁺ myeloid-derived suppressor cells, CD38⁺ regulatory T cells (T_Regs), and CD38⁺ regulatory B cells (B_Regs). Next-generation mass cytometry (CyTOF), which allows high parameter evaluation of the immune system, was used to assess the effects of DARA alone or in combination on a more comprehensive profile of immune cell subpopulations.

Methods: Relapsed/refractory MM pt samples from a subset of single agent studies; SIRIUS (32 pts; whole blood [WB] only; Lonial S et al. The Lancet, 2016) and GEN501 (5 pts; WB and bone marrow [BM], Lokhorst HM et al. NEJM, 2015) along with GEN503, a study of DARA plus lenalidomide and dexamethasone (9 pts; WB and BM; Plesner T et al. ASH 2015) were analyzed. Fluorochrome or metal-conjugated antibody panel stained samples were evaluated by flow cytometry or cytometry by time-of-flight (CyTOF^®) platforms, respectively. FACS analyses were performed and analyzed by FACS Canto II flow cytometers and FACSDiva software. For CyTOF analysis, events were clustered by phenotype by a spanning tree progression of density normalized events (SPADE) algorithm, and each cluster was associated with an immune population via Cytobank^® software. Differential analysis of population fractions and marker intensity, over time and between response groups, derived raw P values from t-tests and single cell level bootstrap adjusted P values corrected for multiple dependent hypothesis testing. Results were visualized using SPADE trees (Figure) and Radviz projections, a new method that allows for the comparison of populations and conditions while preserving the relation to original dimensions.

Results: Flow cytometry and high-dimensional CyTOF analyses confirmed previous findings including higher CD38 expression on plasma cells compared with other immune populations of natural killer (NK), monocytes, B and T cells, and depletion of both plasma cells and NK cells upon DARA treatment. Interestingly, while NK cells were significantly reduced with DARA treatment, remaining active NK cells (CD16+CD56^dim) demonstrated increased expression of activation markers CD69, CD25 and CD137 while also decreasing granzyme B and increasing naive marker CD27. Though functionality tests weren't performed, the ability to evaluate several markers simultaneously suggests these cells possess limited cytotoxicity. Additionally, these studies indicated depletion of CD38 positive immune suppressive subsets of T_regs and B_regs. CD38⁺ basophil reductions occurred independent of response and may provide insight to short-lived infusion related reactions. Several observations within the T-cell compartment were indicative of a DARA-mediated adaptive response in both WB and BM samples. T cells displayed increases in total numbers and shifted towards higher CD8:CD4 and effector:naïve ratios after 2 months of DARA treatment. Responders had higher expression levels of several activation markers including CD69 and HLA-DR along with increased production of cytolytic enzyme granzyme B in CD8⁺ T cells following DARA treatment. Interestingly, in the GEN503 sample set, pts who achieved a complete response presented with a distinct BM CD4 T-cell phenotype of high granzyme B positivity versus those that achieved a partial response or very good partial response. This observation suggests pts with an active immune phenotype may achieve deeper responses to DARA in combination with standard of care agents lenalidomide and dexamethasone.

Conclusion: CyTOF analysis of pt samples from both single agent and combination DARA studies agree with flow cytometry and support the pharmacodynamics and immune modulatory MOA of DARA while providing additional insight into changes in T-cell subtypes and activation status. Future CyTOF analyses of clinical samples from phase 3 combination studies aim to confirm these observations and expand the understanding of the MOA of DARA.

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