Work

825 Intratumoral B7H3:CD3 bispecific T-cell engager drives localized T-cell accumulation in canine sarcoma patients

Year: 2026

Type: conference-abstract

Abstract: Background Bispecific T-cell engagers (TCE) are a promising strategy to treat solid tumors but systemically delivered TCEs frequently suffer from dose-limiting toxicity and poor tumor penetration, thus narrowing their therapeutic window. Intratumoral injection offers a viable approach to overcome this limitation; however, the safety and pharmacodynamic response of TCEs in solid tumors remains poorly characterized. Spontaneous canine tumor patients represent a highly translatable model that closely mirrors the clinical, genetic, and immunological features of human cancer. The objective of this study was to assess the feasibility of an intratumorally administered B7H3:CD3 TCE in a clinical trial involving companion dogs with soft tissue sarcomas (STS).Methods We engineered several canine B7H3:CD3 TCEs incorporating multiple CD3 clones (2A12, 6F9, and 8D4) in two formats (canonical and silenced-Fc) and compared their T-cell activation and cytotoxicity in vitro. Using the top candidate (canonical TCE with CD3 clone 2A12), we evaluated its cytotoxicity across B7H3-expressing canine tumor cell lines. This TCE was then intratumorally injected into two STS patients using a custom-engineered multi-needle assembly that enabled columnar injections of TCE and saline (internal control) at a fixed spacing of 1.5 cm. Safety was assessed by physical examination and hematological and serum chemistry analysis. Pharmacodynamic response was evaluated on tissues harvested at day 7 post-injection by H&E and immunohistochemistry (IHC) for CD3 and CD25 at fluorescent beads-defined injection sites across five tissue slices (100 µm apart).Results In vitro, the canonical TCE with CD3 clone 2A12 demonstrated the highest potency against B7H3+ canine STS cell line and showed cytotoxic activity across multiple B7H3+ canine malignancies. Intratumoral injection with the canonical TCE (7.83µg / 148.2pmol) was well tolerated with no adverse events greater than Grade 1 and no evidence of systemic cytokine release or organ toxicity. IHC analysis across five tissue slices revealed a significant 5-fold increase in CD3+ T-cell density at the TCE injection site (within 0.5cm radius) compared to internal saline controls (**p < 0.01 for patient 1, *p < 0.05 for patient 2), despite the relatively small molecule size of the canonical TCE.Conclusions This study demonstrates the feasibility of the canonical B7H3:CD3 TCE intratumoral injection, with observed T cell accumulation at the TCE injection site and an absence of significant adverse events. These findings suggest that, despite the small size of the canonical TCE, achieving effective volumetric tumor coverage will require multi-needle injection strategies and/or co-stimulatory approaches to promote a durable anti-tumor response.

Source: Journal for ImmunoTherapy of Cancer

DOI: 10.1136/jitc-2026-sitc2026.0825