Background Immune-stimulating antibody conjugates (ISACs) are comprised of immune stimulants conjugated to tumor-targeting antibodies. Trastuzumab-T785 ISAC is a murine surrogate of BDC-1001, a HER2-targeting ISAC currently under evaluation in Phase 2 studies. Trastuzumab-T785 consists of trastuzumab conjugated with a non-cleavable linker to a TLR7/8 agonist. Trastuzumab-T785 elicits myeloid activation and tumor eradication in trastuzumab-resistant HER2 IHC3+ models. Given that the activity of trastuzumab-T785 is dependent on FcgR-mediated phagocytosis, we hypothesized that trastuzumab-T785 and pertuzumab, which binds a distinct HER2 epitope from trastuzumab, would enhance anti-tumor efficacy by increasing Fc clustering and promoting phagocytosis.

Methods The effect of combining pertuzumab with ISAC therapy was explored in nine different xenograft tumor models. SCID/beige mice bearing HER2-expressing tumors (IHC3+, 2+, or 1+) were treated with trastuzumab-T785 at multiple dose levels administered Q5Dx4 in combination with pertuzumab or an isotype control; tumor growth inhibition (TGI) was assessed approximately 20 days after the initial treatment. Intra-tumoral cytokines and chemokines were measured using multiplex ELISA to examine myeloid activation. The role of phagocytes was assessed through in vivo depletion with an anti-CSF1R antibody. The requirement of Fc-mediated effector function was explored using a pertuzumab variant containing Fc effector-attenuating substitutions.

Results Trastuzumab-T785 ISAC monotherapy improved TGI compared to trastuzumab and pertuzumab combination, and trastuzumab-T785 and pertuzumab combination further enhanced TGI across multiple tumor models and dose levels. Moreover, the addition of pertuzumab lowered the quantity of ISAC required for efficacy in the JIMT-1 HER2 IHC2+ model. Anti-tumor efficacy depended on phagocytes, as depletion with an anti-CSF1R antibody significantly reduced TGI by ~50%. Treatment with ISAC and a pertuzumab variant with a non-functional Fc region also reduced TGI by ~50%, demonstrating a functional Fc was required for optimal efficacy. The combination of pertuzumab with trastuzumab-T785 ISAC significantly increased the amplitude of the cytokine and chemokine response relative to ISAC monotherapy or antibody combination therapy, indicating enhanced myeloid activation in the tumor.

Conclusions The combination of trastuzumab-T785 and pertuzumab significantly enhanced efficacy in multiple HER2-expressing tumors, including those with lower HER2 expression. ISAC therapy utilizes phagocytosis to initiate anti-tumor responses, and pertuzumab serves as an additional source of ‘eat me’ signals that likely enhance phagocytosis and deepen anti-tumor efficacy. These studies suggest that this combination may enhance the clinical activity of trastuzumab-based ISACs. This hypothesis is being assessed in a randomized Phase 2 clinical trial with BDC-1001 and pertuzumab in patients with HER2+ breast cancer post trastuzumab deruxtecan (BBI-20231001).