IMGN529, a Novel Antibody-Drug Conjugate (ADC) Targeting CD37 Shows Synergistic Activity with Rituximab in Non-Hodgkin Lymphoma (NHL) Models

Introduction: Relapsed/refractory B-cell NHL remains an area of significant medical need. CD37 is highly expressed in many B-cell malignancies, including NHL, making it an ideal target for ADC-based therapy. IMGN529 is a CD37-targeting ADC consisting of a CD37-binding antibody conjugated to the maytansinoid anti-mitotic, DM1. IMGN529 has been shown to have potent, targeted activity against NHL cell lines and xenograft models via antibody-mediated direct cell-killing, effector function and the anti-mitotic activity of the DM1 payload. IMGN529 has shown early signs of clinical activity at tolerable doses in an ongoing phase I trial in adult patients with relapsed/refractory NHL (R/R-NHL) (NCT01534715) (Blood 2014 124:1760). Rituximab, an anti-CD20 monoclonal antibody, is widely used for NHL therapy and remains a component of both front-line (with chemotherapy combinations) and late-line (both as a monotherapy and in combination) regimens. We have previously shown data from an in vitro synergy screen which identified strong anti-NHL synergy for IMGN529 used in combination with anti-CD20 antibodies (Hematol Oncol 2015; 33: 181–243).

Methods: The activity and mechanism-of-action of IMGN529 in combination with rituximab was further evaluated in clinically relevant preclinical models of NHL: Cell viability in response to single agents and combinations was tested using the WST-8 assay. In vivo combination studies were carried out using human xenograft models of DLBCL implanted in SCID mice. Induction of apoptosis was measured via Annexin-V flow cytometry and caspase 3/7 cleavage assays. Changes in molecular signaling in response to treatment were measured using western blotting and ELISA.

Results: Combination of IMGN529 and the anti-CD20 antibodies rituximab, obinutuzumab and ofatumumab resulted in high synergy scores, identifying a potential class-effect of synergy between IMGN529 and anti-CD20 antibodies. The notable activity of an IMGN529/ rituximab combination was confirmed both in vitro and in vivo using cell line viability and xenograft models of DLBCL (both ABC and GCB subtypes). In these models, the activity of the IMGN529/ rituximab combination was consistently greater than either agent administered as a monotherapy. We examined whether this synergistic reduction in cell viability was due to a reduction in cell growth or an induction of cell death. In multiple NHL cell lines, we found that the combination of IMGN529 and rituximab induced significantly higher levels of Annexin-V positivity and caspase 3/7 activity than either single agent alone, consistent with the pro-apoptotic mechanism of action proposed for IMGN529. To further elucidate the mechanisms underlying the synergy of the combination, we are monitoring the effect on key components of upstream signaling pathways responsible for cell survival and induction of apoptosis, including: the apoptotic inhibitors Bcl-2, Bcl-xL and Mcl-1; MAP-kinase signaling, and the NF-kB and AKT/mTOR axes, all of which have been linked to treatment sensitization by rituximab in NHL cell lines.

Conclusions: IMGN529 demonstrates synergistic activity in combination with CD20-targeting antibodies including rituximab. In in vitro and in vivo models of NHL, the combination of IMGN529 and rituximab is more active than either agent alone, and this enhanced activity is associated with an increase in the induction of apoptosis and apoptotic signaling pathways. These results support clinical assessment of IMGN529 in combination with rituximab, and a phase II trial assessing safety and efficacy in R/R-NHL is planned.