Abstract A081: A PD-L1-targeted Probody provides antitumor efficacy while minimizing induction of systemic autoimmunity
Authors: Wong Chihunt , Mei Li , Wong Kenneth R. , Menendez Elizabeth E.M. , Vasiljeva Olga , Richardson Jennifer H. , West James W. , Kavanaugh Michael , Irving Bryan A. 
Published: May 2016

Abstract
Antibodies to T cell checkpoint molecules like PD-1, PD-L1 and CTLA-4 can unleash potent and durable anti-tumor immunity in many different cancer types. These drugs induce anti-tumor immune responses by activating T cells in tumors, but because similar mechanisms control anti-tumor immunity and self-tolerance, they can also induce systemic autoimmunity by activating autoreactive T cells in normal tissues. Combinations of checkpoint inhibitors greatly increase clinical responses, but similarly increase these toxicities, thereby limiting their clinical utility. New methods are therefore needed that provide anti-tumor activity without deregulating systemic immunity.
Probody™ therapeutics are recombinant, proteolytically-activated antibody prodrugs, designed to widen therapeutic index by minimizing interaction with normal tissue and maximizing interaction with tumor. Probodies exploit the fundamental dysregulation of extracellular protease activity that exists in tumors relative to healthy tissue. The amino terminus of the antibody light chain is extended with a protease-cleavable linker and a masking peptide designed to block antigen binding to the antibody. Cleavage of the linker by specific tumor-associated proteases leads to dissociation of the mask and release of an antibody competent to bind to antigen in the tumor. Probodies bind only minimally to antigen in healthy tissue because there are insufficient active proteases present to remove the mask. The feasibility of the Probody approach to checkpoint targets is supported by preclinical studies in which intratumoral delivery of low dose immune modulators enables durable, systemic anti-tumor responses despite negligible systemic exposure.
Here we demonstrate the ability of a Probody targeting PD-L1 to provide equivalent anti-tumor efficacy in mice to that of its antibody parent, while minimizing induction of systemic autoimmunity. A mouse/human cross-reactive anti-PD-L1 antibody was generated by phage display, and engineered into a Probody. Binding to mouse PD-L1 by the Probody in its masked state was significantly reduced relative to its binding following proteolytic removal of the mask. The presence of active proteases capable of releasing the Probody's mask was demonstrated in frozen sections of MC38 and CT26 murine syngeneic tumors. Systemic administration of the PD-L1 Probody to mice bearing established MC38 syngeneic tumors induced anti-tumor efficacy comparable to the unmasked parental antibody at the same dose. In contrast, the PD-L1 Probody provided protection from induction of autoimmune diabetes in NOD mice at doses sufficient to induce maximal diabetes with the parental antibody. Finally, PD-L1 occupancy by the Probody on peripheral blood and splenic T cells was significantly reduced compared to that of the parental antibody at the same dose. This finding is consistent with the reduced ability of the Probody in its masked form to bind to PD-L1 and to induce autoimmunity outside of the tumor. By demonstrating equivalent efficacy with an improved safety profile, these results provide validation of the Probody concept for a T cell checkpoint target. Probodies should expand opportunities for immunotherapies, particularly those that are limited by enhanced toxicities when used in combination.
Citation Format: Chihunt Wong, Li Mei, Kenneth R. Wong, Elizabeth E.M. Menendez, Olga Vasiljeva, Jennifer H. Richardson, James W. West, Michael Kavanaugh, Bryan A. Irving. A PD-L1-targeted Probody provides antitumor efficacy while minimizing induction of systemic autoimmunity. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A081.