Bi-specific antibody engagers for cancer immunotherapy

Bispecific antibody engagers are fusion proteins composed of a nanobody that recognizes immunoglobulin kappa light chains ( ${\text{VHH}}_{\text{kappa}}$ ) and a nanobody that recognizes either CTLA-4 or PD-L1. These fusions show strong antitumor activity in mice through recruitment of polyclonal immunoglobulins independently of specificity or isotype. In the MC38 mouse model of colorectal carcinoma, the anti-CTLA-4 ${\text{VHH-VHH}}_{\text{kappa}}$ conjugate eradicates tumors and reduces the number of intratumoral regulatory T cells. The anti-PD-L1 ${\text{VHH-VHH}}_{\text{kappa}}$ conjugate is less effective in the MC38 model, whilst still outperforming an antibody of similar specificity. The potency of the anti-PD-L1 ${\text{VHH-VHH}}_{\text{kappa}}$ conjugate was strongly enhanced by installation of the cytotoxic drug maytansine or a STING agonist. The ability of such fusions to engage the Fc-mediated functions of all immunoglobulin isotypes is an appealing strategy to further improve on the efficacy of immune checkpoint blockade, commonly delivered as a monoclonal immunoglobulin of a single defined isotype.