Preliminary Translational Results from an Ongoing Phase 1 Study of Flotetuzumab, a CD123 x CD3 Dart®, in AML/MDS: Rationale for Combining Flotetuzumab and Anti-PD-1/PD-L1 Immunotherapies

Background:

Immunotherapy has transformed cancer treatment; in particular, checkpoint inhibitors (CPI) are now approved for the treatment of several solid tumors and in patients with Hodgkin's disease. In hematological malignancies, including acute myeloid leukemia (AML), the utility of CPIs may be more limited due to less mutations observed in their genomes. Immunologic treatments of hematologic malignancies targeting "shared" epitopes differentially overexpressed by hematologic cancers also represents a rational approach to the immunotherapy of AML.

Methods:

Translational studies were performed in an ongoing phase 1 study in patients with AML and MDS treated with flotetuzumab (MGD006/S80880), a novel CD123 x CD3 bispecific DART^® protein, to assess the potential role of combining PD-1/PD-L1 inhibitors with flotetuzumab.

Results:

Flotetuzumab has anti-leukemic activity in patients with relapsed/refractory AML (ESMO 2017). Inherent to flotetuzumab's mode of action is redirecting T-cells to lyse AML cells in conjunction with enhancement of T-cell activation, proliferation and cytokine production. Multispectral immunohistochemistry analysis of bone marrow samples from our study showed flotetuzumab in situ with a ~2-fold increase in CD8 T-cells upon treatment; consistent with this, we observe up-regulation of PD-1 on both CD4 and CD8 T-cells along with dose dependent secretion of IFNɣ in patient samples. In line with reports of PD-L1 induction by IFNɣ, we observed PD-L1 upregulation on AML blasts incubated with IFNɣ. Furthermore, circulating levels of IFNɣ secretion in flotetuzumab-treated patients correlated with PD-L1 expression on AML blasts. Notably, primary AML samples with higher levels of PD-L1 on malignant blasts were less susceptible to flotetuzumab-mediated killing in vitro. Furthermore, patients that progressed early (within two-weeks) on flotetuzumab treatment had higher baseline levels of PD-L1 on AML cells. Interestingly, a number of patients with residual disease on flotetuzumab showed higher proportion of PD-L1 positive AML blasts vs. basal levels.

Conclusions:

Flotetuzumab has shown evidence of activity against CD123+ AML blasts in vivo, associated with an increase in T-cell infiltration and activation in the bone marrow, with enhanced IFNɣ secretion, which in turn may lead to upregulation of PD-1 on T cells and PD-L1 expression by the AML blasts. Synergistic cytotoxicity was observed after treatment of AML cell lines with both flotetuzumab and CPI in vitro. Furthermore, AML cells in patients that progressed on flotetuzumab or with residual disease tend to be PD-L1 positive. Taken together, these data suggest that combination with anti-PD1/PD-L1 therapy may enhance the effect of flotetuzumab in selected patients with relapsed or refractory AML. clinicaltrials.gov NCT02152956