Background
CD3 engaging bispecific antibodies (bsAbsCD3) have emerged as a promising class of anti-tumor therapeutics that effectively trigger T-cell activity resulting in tumor cell destruction. Nevertheless, their effectiveness is constrained by T-cell exhaustion resulting from strong TCR/CD3 signaling and lack of costimulatory ‘signal 2’ to sustain T-cell functionality. Recently, we introduced a PSMAxCD3 bsAb (CC-1), undergoing monotherapy in castration-resistant prostate carcinoma patients (NCT04104607). While CC-1 treatment reduced PSA levels in all but one of the heavily pretreated patients, the induced T-cell activation was rather short-lived. To sustain T-cell antitumor-reactivity, we developed a combinatorial approach using bispecific costimulators for activation of CD28 (BiCos) to be used in combination with bsAbCD3 including CC-1. Our selected lead construct is directed to Endoglin and was designated BiCo-1.
Methods
Various bispecific formats containing different proprietary target binders that allow for target restricted activation of CD28 were designed. The capacity to reinforce T-cell activation and antitumor efficacy in combination with CC-1 was investigated in multiple in vitro assays and xenograft mouse models. Moreover, function of T-cells derived from patients undergoing CC-1 therapy, as well as exhausted T-cells derived from healthy donors was investigated.
Results
An optimal BiCo format was selected based on its superior affinity, good producibility, low aggregation tendency and, most importantly, fully target cell-restricted CD28 activity. Unlike CD28 superagonist molecules, which caused profound toxicity in early clinical trials, and in line with the fact that physiologically CD28 stimulation alone does not induce T-cell activation, but is strictly dependent on the presence of T-cell-signal 1, our BiCo format induces neither therapeutic activity nor toxicity as single agent. In combination with bsAbCD3 like CC-1, BiCo-1 allowed for increased selectivity of the induced T-cell response, sustained T-cell proliferation, long-lasting T-cell memory formation and prevention/breaking of anergy, resulting in significantly improved antitumor activity. This is demonstrated in multiple in vitro assays as well as in various xenograft mouse models in which the prevention of metastasis and the eradication of large established tumors was evaluated. The combinatorial approach relying on our BiCo format thus allows for reduction of side effects while at the same time improving efficacy of T-cell-based immunotherapy for solid cancers.
Conclusions
Our data demonstrate that target-restricted CD28 costimulation can significantly improve efficacy of bsAbCD3 therapies. GMP-compliant production of BiCo-1 has meanwhile been completed and a combinatorial clinical ‘first in man’ trial with CC-1 in prostate cancer patients will start recruitment in 2024.
Ethics Approval
The study was approved by IRB (ethics committee of the Faculty of Medicine of the Eberhard Karls Universität Tübingen) at the University Hospital Tübingen and was conducted in accordance with the Declaration of Helsinki; reference number 13/2007 V. Human material was collected after obtaining informed consent.