CD20xCD3 Bispecific Fully Human Antibody Induces Potent Anti-Tumor Activity Against CD20-Expressing Tumors in Immune Competent Mice Humanized for CD20 and CD3

Bispecific antibodies targeting T cells to a tumor antigen represent a promising new therapeutic approach for the treatment of hematologic and solid tumors.  Numerous studies have demonstrated the efficacy of such bispecific antibodies in inhibiting tumor growth in preclinical models.  However, these studies are usually restricted to in vitro studies or have typically utilized in vivo models in which human cancer cell lines are mixed with pre-activated human T cells or peripheral blood mononuclear cells (PBMCs), implanted into immune-deficient mice, and immediately dosed with antibody before the formation of tumors. 

REGN1979, a fully human bispecific antibody targeted to both CD20 on B cells and CD3 on T cells has demonstrated potent anti-tumor activity against large established tumors in three different preclinical models: 1) human PBMC-reconstituted NOD-SCID-IL2R-gamma deficient (NSG) mice, 2) human CD34+ hematopoietic stem cell immune-reconstituted Balb/c RAG2-IL2R-gamma deficient hSIRP-alpha mice, and 3) immune competent C57BL/6 mice humanized for CD20 and CD3.  Importantly, this third model allows investigation of clinical bispecific antibodies in tumor-bearing immune competent mice.  This platform allows us to answer crucial questions regarding antibody pharmacokinetics in a tumor setting, understand the impact of CD3+ targeted bispecifics on immune cells in the tumor microenvironment, and understand which cytokines and cells are crucial for mediating potent anti-tumor mediated bispecific activity. 

Pharmacokinetic studies in human PBMC-immune reconstituted Raji tumor-bearing NSG mice show that REGN1979 maintains high circulating levels even seven days after a single dose. However, the bispecific antibody is cleared considerably faster from the circulation in mice expressing human CD20 presumably due to consumption of the antibody by target-expressing normal B cells.  Notably, subsequent dosing of the antibody in the humanized CD20 mice maintains higher serum antibody levels that are associated with antibody-mediated depletion of normal B cells from circulation.

REGN1979 bispecific antibody exhibits potent anti-tumor activity against established B16F10.9 melanoma tumors expressing human CD20 in humanized CD20 and CD3 mice.  In addition, these studies demonstrate clear and immediate depletion of normal circulating B cells, in addition to transient reduction and expansion of the number of circulating T cells.  Specific depletion of CD8+ T cells, but not CD4+ T cells, prior to therapy completely abrogated the REGN1979 anti-tumor activity.

Administration of REGN1979 induced the production of TNF-alpha, IL-2 and IL-4 in the serum at 3 hours post-dose in tumor bearing humanized CD20 and CD3 mice, and IFN-gamma levels were increased at 6 hours post-dose. IL-6 and IL-10 were not significantly increased.  All cytokines returned to control levels at 1, 3, 7 or 14 days post-dosing.

These studies show potent activity of a new class of fully human bispecific antibodies for treating tumors and provide strong support for clinical testing of REGN1979 in patients with CD20+ cancers.