T cell-engaging bispecific antibodies (TCBs) are highly potent therapeutics that direct the activity of cytotoxic T cells to tumors. TCBs have shown clinical activity in hematologic malignancies, but development of TCBs for solid tumor indications is proving more challenging. Due to their high potency, TCBs can target normal tissues with low antigen expression, resulting in significant on-target, off-tumor toxicity that can limit dosing to low levels. As a result, it has been difficult to reach the level of drug exposure required for efficacy without excessive toxicity. Therefore, novel methods are needed to enable the potent antitumor activity of TCBs while minimizing toxicity due to cytokine release and damage to healthy tissues. CytomX has developed a new class of recombinant, proteolytically activated antibody prodrugs (ProbodyTM therapeutics) that are “masked” to prevent binding to antigen in healthy tissue, but can become “unmasked” by proteases that are preferentially activated in the tumor microenvironment. In this way, Probody therapeutics are designed to increase therapeutic index by maximizing efficacy and minimizing on-target toxicity in normal tissues. Here we describe a T cell-engaging Bispecific Probody therapeutic (Pb-TCB) targeting Epidermal Growth Factor Receptor (EGFR) and CD3 that has been optimized for affinity, effector function, masking, and cleavability. In vitro, under protease-deficient conditions, we demonstrate that the unmasked EGFR-CD3 TCB has potent, EGFR-dependent tumor cell killing, while the doubly-masked EGFR-CD3 Pb-TCB reduces target-dependent cytotoxicity by more than 100,000-fold. However, in established tumor models where tumor-resident proteases are expected to be active, we demonstrate that Pb-TCBs potently induce tumor regressions. In nonhuman primates, the maximum tolerated dose (MTD) of the EGFR-CD3 Pb-TCB is more than 60-fold higher than the MTD of the unmasked TCB, and the tolerated exposure (AUC) is more than 10,000-fold higher. Finally, despite the 60-fold dose differential at the MTDs, transient serum cytokine and AST/ALT increases observed in nonhuman primates treated with the Pb-TCB are still lower than those induced by the TCB. By localizing activity to the tumor microenvironment, Pb-TCBs have the potential to expand clinical opportunities for T cell-engaging bispecific therapies that are limited by on-target toxicities, especially in solid tumors. Moreover, an EGFR-CD3 Pb-TCB has the potential to address EGFR-expressing tumors that are poorly responsive to existing EGFR-directed therapies. PROBODY is a trademark of CytomX Therapeutics, Inc.

Citation Format: Leila M. Boustany, Laurie Wong, Clayton W. White, Linnea Diep, Yuanhui Huang, Shouchun Liu, Jennifer H. Richardson, W. Michael Kavanaugh, Bryan A. Irving. EGFR-CD3 bispecific Probody™ therapeutic induces tumor regressions and increases maximum tolerated dose >60-fold in preclinical studies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A164.