Combination of short-lived and extended half-life target modules for optimized UniCAR T cell therapy

Background: Chimeric antigen receptor (CAR) T cells are powerful living drugs to fight against cancer. However, they also possess the capacity to elicit moderate to severe toxicities that might be even fatal. Thus, one major issue of CAR T cell engineering is to reduce the risk for side effects while maintaining high anti-tumor activity. In order to improve the safety profile of CAR, we developed the so-called UniCAR system. In this modular platform technology, soluble, tumor-specific target modules (TM) act as molecular switches of per se inactive universal (Uni)CAR T cells. TM consist of tumor-specific binding domains fused to the E5B9 peptide epitope that is recognized by the UniCAR. All so far developed TMs have a low molecular weight and are therefore rapidly eliminated. This allows to specifically and repeatedly turn on/off UniCAR T cell activity via TM dosing.
Aims: Tumor patients with bulky disease present the highest risk for CAR T cell-related toxicities. At this stage, a high level of safety and therefore controllability of (Uni)CAR T cells is required. However, for convenient treatment of patients with lower tumor burden, we intended to develop extended half-life TM to foster anti-tumor responses and to ease the clinical TM administration at later stages of tumor therapy.
Methods: Based on the human IgG4 Fc-domain, we engineered a set of novel extended half-life TM each consisting of tumor-specific single-chain fragments variable (scFv), the IgG4 hinge and Fc domain as well as the E5B9 peptide epitope. Functionality of these IgG4-based TMs was analyzed in vitro and in vivo in comparison to originally developed scFv-based TM. Pharmacokinetic properties were studied in experimental mice.
Results: In presence of extended half-life TM, UniCAR T cells are able to efficiently mediate tumor cell lysis in vitro and in vivo. Anti-tumor responses are comparable or even improved in comparison to smaller TM, whereas bioavailability and plasma half-life are prolonged.
Summary: Overall, combination of both short-lived and longer lasting (IgG4-based) TM is a highly promising approach for redirection of UniCAR T cells to various cancer cells. At the beginning of tumor treatment, rapidly eliminated TM should be chosen to provide a fast safety switch. After significant reduction in tumor burden, IgG4-based TM with increased serum half-lives could be administered to avoid continuous TM infusions and to improve the elimination of residual tumor cells. This strategy might allow a more convenient, individualized and safe treatment of cancer patients.