(T-092) Mechanistic Modeling of Bispecific Antibody Combination Therapy to Optimize TumorKilling in Cancer Immunotherapy

Kayden Tang – Central Bucks HS East; Xiaozhi Liao – University of North Carolina - Chapel Hill; Yanguang Cao – University of North Carolina - Chapel Hill

Objectives: Bispecific T-cell engagers (bsTCEs) represent a promising class of cancer immunotherapies. However, challenges remain in optimizing dosing regimen and combination strategies due to complex dynamics of immune–tumor interactions. This study aims to develop a mechanistic framework to quantify tumor killing effect when combining a CD3×PSMA bsTCE with another costimulatory CD28×PSMA bsTCE and identify key factors that influence the effectiveness of bsTCE combination therapy.

Methods: A mechanistic mathematical model was developed to describe the dynamic interactions between T cells and tumor cells mediated by bsTCEs. The modeling process followed a staged approach: first, a base model characterizing the tumor killing effect with CD3×PSMA monotherapy was first established, then extended to incorporate CD28×PSMA bsTCE, which acts as a CD28-mediated co-stimulation without direct cytotoxicity. Both bsTCE target the same tumor-associated antigen–PSMA, introducing competitive binding dynamics. Literature in vitro cytotoxicity data were used for model calibration. Tumor killing effect was assessed by evaluating the CD3-bsTCE-PSMA trimer-cytotoxicity relationship using Emax model. Key parameter EC50 (CD3 occupancy for half-maximal cytotoxicity) was estimated and compared between monotherapy and combination therapy. Sensitivity analyses were performed to evaluate the impact of binding kinetics (kon, koff, KD) on trimer formation and efficacy.

Results: The model successfully captured in vitro tumor-killing data for both monotherapy and combination therapy. The estimated CD3 occupancy by CD3-PSMA trimer (%) for combination therapy was 217,000-fold lower than that for monotherapy, indicating a strong synergistic effect from the costimulatory bispecific antibody, aligned with the observed shift in potency value from the concentration-cytotoxicity data. Sensitivity analyses revealed that enhancing CD3 affinity of CD3×PSMA improves trimer formation and tumor killing efficacy. In contrast, the excessively high PSMA affinity of the costimulatory bsTCE reduce CD3×PSMA antibody binding and diminishing overall efficacy. These findings highlight the importance of finely tuning binding affinities to optimize bispecific T cell engagers in combination settings.

Conclusions: This work demonstrates the utility of mechanistic modeling to guide bispecific antibody combination strategies in cancer immunotherapy. By integrating biological mechanisms with quantitative analysis, the developed framework supports rational antibody design and potentially dose selection. While this study focused on efficacy, future extensions will address safety and therapeutic index assessment. The approach is adaptable across cancer types, combination regimens (e.g., triple-BiTEs), and other disease areas such as autoimmune disorders.

Citations: [1] Dimitris Skokos et al. ,A class of costimulatory CD28-bispecific antibodies that enhance the antitumor activity of CD3-bispecific antibodies. Sci. Transl. Med.12,eaaw7888(2020).DOI:10.1126/scitranslmed.aaw7888
[2] Jiang, Xiling, et al. "Development of a Target Cell-Biologics-Effector Cell (TBE) Complex-based Cell Killing Model to Characterize Target Cell Depletion by T Cell Redirecting Bispecific Agents." MAbs, vol. 10, no. 6, 18 Aug. 2018, pp. 876-89, https://doi.org/10.1080/19420862.2018.1480299.
[3] Lotze, Michael T., et al. "CD28 Co-stimulation: Novel Insights and Applications in Cancer Immunotherapy." Nature Reviews Immunology, 25 July 2024, https://doi.org/10.1038/s41577-024-01061-1.

Keywords: Bispecific TCE, Combination Therapy