Preclinical Characterization of Combinability and Potential Synergy of Anti-CD20/CD3 T-Cell Dependent Bispecific Antibody with Chemotherapy and PD-1/PD-L1 Blockade

The anti-CD20/CD3 T-cell recruiting bispecific antibody (CD20-TDB) is a full-length, fully humanized IgG1 molecule currently under clinical investigation in B-cell malignancies. Previously we have shown that CD20-TDB is highly active in killing CD20-expressing B cells, including primary patient leukemia and lymphoma cells both in vitro and in vivo (Sun et.al. STM 2015). The current standard therapy in B-cell malignancies often contains anti-CD20 based monoclonal antibody and various chemo reagents such as the R-CHOP regimen in Non-Hodgkin's' Lymphoma. Previously we have shown that CD20-TDB can be potentially combined with rituximab as very low level of antigen expression or antigen receptor occupancy is needed for CD20-TDB activity. As many chemo reagents have non-targeted, anti-proliferative activity or immune suppressive activity such as glucocorticoids, it's conceivable that they could potentially interfere with T-cell activation and the subsequent T-cell proliferation and therefore negatively affect CD20-TDB activity. In addition, as a T-cell recruiting bispecific reagent, cell killing activity of CD20-TDB is dependent on T-cell activation which can be subject to negative regulation posed by checkpoint molecules such as PD-1/PD-L1. Here in an effort to better understand the clinical applicability and to improve upon single-agent activity of CD20-TDB, we evaluated the combinability of CD20-TDB with standard-of-care chemo reagents as well as potential synergy of CD20-TDB with PD-1/PD-L1 blockade in vitro and in vivo.

B-cell killing activity of CD20-TDB was not significantly impacted by high concentration of chemo reagents including cyclophosphamide, hydroxydaunorubicin, vincristine, and dexamethasone individually in vitro. In vivo in human CD20/CD3 double transgenic mice, no apparent inhibitory effect on CD20-TDB activity in T-cell activation and B-cell depletion was observed with cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone either individually or in combination.

In vitro, PD-1 and PD-L1 expression appeared to be upregulated on T-cells and B-cells respectively upon CD20-TDB treatment, though the expression of PD-1/PD-L1 didn't appear to inhibit the B-cell killing activity of CD20-TDB significantly. The in vivo anti-tumor activity of the combination of CD20-TDB and anti-PD-L1, as well as CD20-TDB and anti-PD-1, was evaluated in an A20-human CD20 syngeneic mouse lymphoma model. In the A20-human CD20 mouse B-lymphoma tumor model, where the target B lymphoma cells uniformly express high level of PD-L1, single-agent CD20-TDB did not significantly inhibit tumor growth. Treatment with single-agent anti-PD-L1 inhibited tumor growth and resulted in three partial responses (tumor regression of more than 50% but less than 100% of the starting tumor volume) out of nine treated animals. The combination of CD20-TDB and anti-PD-L1 resulted in substantially greater tumor growth inhibition compared to either agent alone and resulted in tumor regression in the majority of the nine animals tested, achieving eight partial responses and one complete response (100% tumor regression, no measurable tumor). Similar results were observed with the combination of CD20-TDB and anti-PD-1.

Together, these results suggest that CD20-TDB can have broad clinical applicability, either combining with chemo reagents to enable flexible treatment strategies to incorporate CD20-TDB into current standard of therapy for B cell malignancies or with immune checkpoint inhibitors such as anti-PD-L1/PD-1 to improve upon single-agent efficacy.