Intratumoral B7H3:CD3 Bispecific T-cell Engager Drives Localized T-cell Accumulation in Canine Sarcoma Patients

Suita, Y.; Ang, L. S.; Brasel, K.; Morris, S. M.; Girard, E. J.; Williams, A. M.; Chen, S. C.; Blumenthal, I.; Hottmann, N. M.; Heusser, J.; Mhyre, D. J.; DeForest, C. A.; Moore, P. F.; Price, J. P.; Fidel, J.; Olson, J. M.

doi:10.64898/2026.05.27.728355

bioRxiv · 10.64898/2026.05.27.728355

Intratumoral B7H3:CD3 Bispecific T-cell Engager Drives Localized T-cell Accumulation in Canine Sarcoma Patients

Suita, Y.·Ang, L. S.·Brasel, K.·Morris, S. M.·Girard, E. J.·Williams, A. M.·Chen, S. C.·Blumenthal, I.·Hottmann, N. M.·Heusser, J.·Mhyre, D. J.·DeForest, C. A.·Moore, P. F.·Price, J. P.·Fidel, J.·Olson, J. M.

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Abstract

BackgroundBispecific T-cell Engagers (TCEs) targeting B7H3 (CD276) show promise for solid tumors but are limited by systemic toxicities and poor tumor penetration. Intratumoral (IT) delivery is proposed as a solution, but the safety and spatial pharmacodynamics (PD) remain poorly defined in these malignancies. Spontaneous canine tumors serve as a highly translatable model for human therapeutic development due to its clinical, genetic, and immunological similarities to human patients. This study evaluates the feasibility of an IT-delivered B7H3:CD3 TCE in a trial that enrolls companion dogs with solid tumors. MethodsWe engineered a canine B7H3:CD3 TCE and validated its ability to induce T-cell activation and T-cell mediated cytotoxicity in vitro on several B7H3-expressing canine tumor cell lines. Two STS canine patients received intratumoral columnar injections of the TCE and saline (internal control) at fixed distance of 1.5cm using a custom-engineered multi-needle assembly. Safety was evaluated by physical examinations and hematological and biochemical changes in peripheral blood. PD response was analyzed by H&E and immunohistochemistry. ResultsIn vitro assays validated the cytotoxicity of the B7H3:CD3 TCE on B7H3+ canine tumor cell lines. TCE IT administration (7.83 g / 148.2 pmol) was well tolerated with no adverse events greater than Grade 1 and no evidence of systemic cytokine release or organ toxicity. Immunohistochemistry of tumors collected 7 days after TCE administration revealed a significant five-fold increase in CD3+ T-cell density at the TCE injection site (within 0.5 cm radius) compared to internal saline controls. ConclusionsThis study demonstrated the feasibility of evaluating pharmacodynamic response to IT delivery of B7H3:CD3 TCE, namely local T-cell accumulation. T-cell localization around the TCE injection site supports our hypothesis that effective IT immunotherapy might require enhanced volumetric coverage using multi-needle injections and/or co-stimulatory strategies to convert T-cell localization into a robust, sustained anti-tumor response.

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Suita, Y., Ang, L. S., Brasel, K., Morris, S. M., Girard, E. J., Williams, A. M., Chen, S. C., Blumenthal, I., Hottmann, N. M., Heusser, J., Mhyre, D. J., DeForest, C. A., Moore, P. F., Price, J. P., Fidel, J., Olson, J. M.. 2026-05-31. Intratumoral B7H3:CD3 Bispecific T-cell Engager Drives Localized T-cell Accumulation in Canine Sarcoma Patients. https://doi.org/10.64898/2026.05.27.728355

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