Relative Target Affinities of T Cell-Dependent Bispecific Antibodies Determine Biodistribution in a Solid Tumor Mouse Model

Anti-HER2/CD3, a T cell-dependent bispecific antibody (TDB) construct, induces T cell-mediated cell death in cancer cells expressing HER2 by cross-linking tumor HER2 with CD3 on cytotoxic T cells, thereby creating a functional cytolytic synapse. TDB design is a very challenging process that requires consideration of multiple parameters. While therapeutic antibody design strategy is commonly driven by striving for the highest attainable antigen binding affinity, little is known about how the affinity of each TDB arm can affect the targeting ability of the other arm and the consequent distribution and efficacy. To our knowledge, no distribution studies have been published using preclinical models wherein the T cell-targeting arm of the TDB is actively bound to T cells. We used a combined approach involving radiochemistry, invasive biodistribution and non-invasive single-photon emission tomographic (SPECT) imaging to measure TDB distribution and catabolism in transgenic mice with human CD3 expression on T cells. Using CD3 affinity variants, we assessed the impact of CD3 affinity on short term pharmacokinetics, tissue distribution and cellular uptake. Our experimental approach determined the relative effects of (i) CD3 targeting to normal tissues, (ii) HER2 targeting to HER2-expressing tumors and (iii) relative HER2/CD3 affinity, all as critical drivers for TDB distribution. We observed a strong correlation between CD3 affinity and distribution to T cell-rich tissues, with higher CD3 affinity reducing systemic exposure and shifting TDB distribution away from tumor to T cell-containing tissues. These observations have important implications for clinical translation of bispecific antibodies for cancer immunotherapy.

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