Mezagitamab Induces Immunomodulatory Effect in Patients with Relapsed/Refractory Multiple Myeloma (RRMM)
Authors: Abadier Michael , Estevam Jose , Berg Deborah , Fedyk Eric Robert 
Published: November 2020

Background
Mezagitamab is a fully human immunoglobulin (Ig) G1 monoclonal antibody with high affinity to CD38 that depletes tumor cells expressing CD38 by antibody- and complement-dependent cytotoxicity. CD38 is a cell surface molecule that is highly expressed on myeloma cells, plasma cells, plasmablasts, and natural killer (NK) cells, and is induced on activated T cells and other suppressor cells including regulatory T (Tregs) and B (Bregs) cells.
Data suggest that immune landscape changes in cancer patients and this may correlate with disease stage and clinical outcome. Monitoring specific immune cell subsets could predict treatment responses since certain cell populations either enhance or attenuate the anti-tumor immune response.
Method
To monitor the immune landscape changes in RRMM patients we developed a mass cytometry panel that measures 39-biomarkers to identify multiple immune cell subsets, including T cells (naïve, memory, effector, regulatory), B cells (naïve, memory, precursors, plasmablasts, regulatory), NK cells, NKT cells, gamma delta T cells, monocytes (classical, non-classical and intermediate), dendritic cells (mDC; myeloid and pDC; plasmacytoid) and basophils.
After a robust analytical method validation, we tested cryopreserved peripheral blood and bone marrow mononuclear cells from 19 RRMM patients who received ≥ 3 prior lines of therapy. Patients were administered 300 or 600 mg SC mezagitamab on a QWx8, Q2Wx8 and then Q4Wx until disease progression schedule (NCT03439280). We compared the percent change in immune cell subsets at baseline versus week 4 and week 16.
Results
CD38 is expressed at different levels on immune cells and sensitivity to depletion by mezagitamab generally correlates positively with the density of expression. CD38 is expressed at high densities on plasmablasts, Bregs, NK-cells, pDC and basophils at baseline and this was associated with reductions in peripheral blood and bone marrow (plasmablasts, 95%, Bregs, 90%, NK-cells, 50%, pDC, 55% and basophils, 40%) at week 4 post treatment. In contrast, no changes occurred in the level of total T-cells and B-cells, which is consistent with low expression of CD38 on most cells of these large populations.
Among the insensitive cell types, remaining NK-cells acquired an activated, proliferative and effector phenotype. We observed 60-150% increase in activation (CD69, HLA-DR), 110-200% increase in proliferation (Ki-67), and 40-375% increase in effector (IFN-γ) markers in peripheral blood and bone marrow. Importantly, NK-cells which did not express detectable CD38, also exhibited a similar phenotype possibly by a mechanism independent of CD38. Consistent with these data, the remaining CD4 and CD8 T-cell populations exhibited an activated effector phenotype as observed by 40-200% increase in activation, 60-200% increase in proliferation and 40-90% increase in effector markers in peripheral blood.
A potential explanation for this acquisition of activated effector phenotypes could be a reduction in suppressive regulatory lymphocytes. Next, we measured levels of Tregs and Bregs, and observed that Bregs which are CD24hiCD38hi were reduced to 60-90% in peripheral blood and bone marrow. In contrast, total Tregs were reduced by only 5-25% because CD38 expression in Tregs appears as a spectrum where only ~10-20% are CD38+, and thus CD38+ Tregs were reduced more significantly (45-75%), reflecting the selectively of mezagitamab to cells expressing high levels of CD38.
CD38+ Tregs are induced in RRMM patients, thus we looked at the phenotype of CD38-, CD38mid, and CD38high -expressing Tregs. We observed higher level of markers that correlate with highly suppressive Tregs such as Granzyme B, Ki-67, CTLA-4 and PD-1 in CD38high Tregs. Accordingly, the total Treg population exhibited a less active phenotype after exposure to mezagitamab, which selectively depleted the highly suppressive CD38+ Tregs.
Conclusions
Chronic treatment with mezagitamab is immunomodulatory in patients with RRMM, which is associated with reductions in tumor burden, subpopulations of B and T regulatory cells, and characterized by conventional NK and T cells exhibiting an activated, proliferative and effector phenotype. The immune landscape changes observed is consistent with the immunologic concept of converting the tumor microenvironment from cold-to-hot and highlights a key mechanistic effect of mezagitamab.

Disclosures
Berg: Takeda Pharmaceuticals Inc: Current Employment.