Salvage with Sequential T Cell Redirection for Myeloma Patients Who Have Previously Progressed on T Cell Directed Therapy Is Associated with Significant Cellular Activation of the Immune Microenvironment

Background: With the recent approval of two anti-BCMA CAR-Ts and multiple anti-BCMA and anti-GPRC5D bispecific antibodies (BiAbs) in clinical trials, rational salvage strategies for patients who progress after these therapies is an area of unmet need in multiple myeloma. We have previously reported favorable outcomes for patients receiving sequential T cell directed therapies with depth of response and progression free survival (PFS) (Mouhieddine et al Blood (2021) 138 (Supplement 1: 821). However, the underlying immune mechanisms are not well known and would be essential to design sequential strategies combining different T cell directed therapies. Here we show that sequential T cell mediated therapies promote not only T cell engagement but also immune activation in natural killer (NK) and dendritic cells (DC) cellular subsets in patients with relapsed/refractory multiple myeloma (RRMM).

Methods: We identified 15 RRMM patients who had progressed on a T cell directed therapy, either anti-BCMA CAR-T (n=10) or BiAbs (n=5) and then received another BiAb as a subsequent line of therapy. 70% of patients who progressed on anti-BCMA CAR-T (7/10) and 20% who progressed on BiAbs had interim therapies before receiving subsequent BiAbs therapy. The median time between the first T cell directed therapy and subsequent therapy was 5 months (range 0- 26 months). We profiled immune cell activation in peripheral blood (PB) by high dimensional flow cytometry and Olink Proteomics. Clinical data such as patient demographics, disease characteristics and post-clinical trial outcomes were collected retrospectively until July of 2022. Data and sample collection for this study was approved by the institutional review board (IRB) and follows the Declaration of Helsinki and International Conference on Harmonization Guidelines for Good Clinical Practice (IRB: GCO#: 11-1433 & GCO #: 18-00456).

Results: We compared baseline immune cell populations of 15 RRMM patients in the PB post relapse on T cell mediated therapy and during subsequent BiAb treatment (Figure 1a). 93% of patients (14/15) received BiAbs therapy with a different target than their first T cell directed regimen. 73% of patients (11/15) received sequential BiAbs therapy in combination with an anti-CD38 monoclonal antibody. Patients had a median age of 63 years (range 43-89) at time of progression after the first T cell directed therapy. MM patients were heavily pre-treated with a median of 6 prior lines of therapy (range 3-16). All patients were triple-class refractory while 66% were penta-drug refractory.

Immune profiling at time of progression on the first T cell directed therapy, was associated with increased expression of exhaustion markers on CD4+ and CD8+ T cells. Treatment with a second BiAb was associated with significant re-activation of endogenous T cells. Specifically, CD8+ T cells demonstrated increased expression of hallmark surface activation markers such as HLA-DR and CD69+ (p<0.05, Figure 1b), and this increase was consistent in CD4+ T cells as well (p<0.01, Figure 1b). Alongside T cell engagement we saw an increase in surface activation markers CD69+ and HLA-DR on NK and DC populations respectively (p<0.01). This indicates that the application of the second T cell directed therapy not only reactivates T cell compartment but potentiates widespread immune cell activation. Plasma cytokine analysis from these patients revealed elevated CXCL10, CXCL9, and TNFa during bispecific therapy; reinforcing the activation state of the patients during treatment. Although the CD4:CD8 ratios remain conserved, we observed a significant differentiation of CD8+ T cells through sequential therapy, with a reduction of naïve CD8+ T cells (p=0.022) and increase of effector memory CD8+ T cells (p<0.01).

Conclusion: Our data suggests that patients who progressed on T cell directed therapy and went on to receive T cell mediated BiAbs, not only respond clinically, but also have hallmark expression of activation markers. T cells can be reactivated post relapse from T cell mediated therapies, indicating that T cell targeted treatments can be applied successfully in sequential order with favorable patient outcomes. We are currently performing immune profiling on more PB and bone marrow samples to further confirm these findings in a larger patient population.