Generation of FcRH5xCD28 Bispecific Antibodies Synergizing with T-Cell Engagers for Enhanced Multiple Myeloma Treatment

The approval of several bispecific T-cell engagers (TCEs) in recent years has contributed to revolutionizing the treatment of multiple myeloma (MM). However, not all patients benefit from these new treatments and acquired resistance eventually occurs. Consequently, strategies to deepen the extent of the response to TCEs in MM patients are needed. Here we describe the generation of FcRH5-targeted CD28 bispecific antibodies (bsAbs) designed to co-stimulate T cells specifically in presence of FcRH5-positive MM cells. FcRH5 is a B-cell lineage marker whose extracellular domain (ECD) consists of 9 Ig-like domains (D1 to D9) that is often overexpressed by MM cells. Importantly, MM patients display high levels of soluble FcRH5 (consisting of ECD D1 to D8) and preferential binding of the bsAbs to membrane-bound FcRH5 via domain 9 (D9) is highly desirable to minimize the potential loss of efficacy due to the sink effect. However, the amino acid sequence of D9 shares a high degree of similarity with some of the other FcRH5 ECD domains (intramolecular similarity) and with some of the Ig-like domains present in the ECD of the other FcRH family members (intermolecular similarity). Consequently, the generation of FcRH5-specific antibodies that bind uniquely to membrane-bound FcRH5 is a challenging process. Using our proprietary fully human κλ body antibody platform, which relies on a common heavy chain and on two distinct light chains driving specificity and affinity (i.e., one kappa and one lambda), we have identified a panel of cyno cross-reactive, FcRH5 specific (i.e., not binding to other FcRH family members) antibody arms that preferentially bind to membrane-bound FcRH5. Binding experiments using PEAK cells expressing at the cell surface the different FcRH family members or truncated version of the FcRH5 protein confirmed the desired features of the identified antibody arms. These arms were paired to an agonist (but not superagonist) anti-CD28 arm to generate FcRH5-CD28 κλ bodies capable of bridging T cells to MM cells to deliver signal 2. In the absence of signal 1, FcRH5xCD28 κλ bodies showed no sign of T cell activation, potentially limiting peripheral toxicity. FcRH5xCD28 κλ bodies strongly synergized with TCEs such as Teclistamab (BCMAxCD3 bsAb) and Talquetamab (GPRC5DxCD3 bsAb) resulting in enhanced killing of MM cell lines in in vitro T-cell dependent cellular cytotoxicity assays. Of note, in some cases, the bsAbs could even rescue the activity of TCEs (i.e. in settings where the TCEs alone displayed limited or no activity). Importantly, significant activity is retained in the presence of soluble FcRH5 in vitro. Therefore, FcRH5xCD28 bispecific antibodies have the potential to expand the number of patients who benefit from TCE-based therapies, possibly leading to deeper and more durable responses.