Abstract 3211: PD-1-targeted Probody therapeutics provide anti-tumor efficacy and a 10-fold dose protection against systemic autoimmunity in preclinical studies
Authors: Tipton Kimberly A. , Chan Chanty , Wong Kenneth R. , Singson Victoria , Richardson Jennifer H. , Kavanaugh W Michael , Irving Bryan A. , West James W. 
Published: July 2016

Abstract
Immuno-oncology approaches have transformed the treatment of advanced melanoma, NSCLC and an increasing number of other cancers. Antibodies targeting the T cell checkpoint molecules CTLA-4 and PD-1 can reverse tumor suppression of T cell activation that prevents effective anti-tumor immunity. However, blockade of checkpoint molecules, particularly in combination, in non-tumor tissues can result in autoimmune side effects that can limit therapeutic utility. Combination of PD-1 and CTLA-4 blockade provides increased response rates but with concomitant increases in immune-related adverse events. Here we demonstrate that Probody™ therapeutics targeting PD-1 synergize with a CTLA-4 checkpoint inhibitor to eradicate established tumors in a mouse model while protecting against PD1-mediated systemic autoimmunity.
Probody therapeutics are recombinant, proteolytically-activated antibody prodrugs that have the potential to meaningfully widen therapeutic index by minimizing interactions with normal tissue while retaining anti-tumor activity. Probody therapeutics take advantage of protease activities that are abundant in tumors but suppressed in normal tissues. An extension of the antibody light chain with a masking peptide blocks binding of the antibody to antigen in normal tissue. Tumor-associated proteases cleave and release the mask, enabling the active antibody to bind antigen preferentially in the tumor. The feasibility of the Probody approach to checkpoint targets is supported by preclinical and clinical studies in which intratumoral delivery of low dose immune modulators enables anti-tumor responses despite negligible systemic exposure.
For preclinical assessment of PD-1 as a Probody target, a family of Probody therapeutics derived from the anti-mouse PD-1 antibody J43 was developed. The intact Probody therapeutics demonstrated reduced binding to mouse PD-1 relative to the parental antibody that was completely restored following proteolytic activation. The anti-tumor efficacy of the J43 antibody and Probody therapeutics was assessed following systemic administration as single agents or in combination with an anti-CTLA-4 antibody to mice bearing established MC38 syngeneic tumors. As single agents, the PD-1 antibody and Probody therapeutics reduced tumor growth relative to an isotype antibody control and synergized with CTLA-4 blockade. Consistent with their reduced ability to bind target in the absence of proteolytic activation, the PD-1 Probody therapeutics were up to 10 times less potent than the parental anti-PD-1 antibody in inducing autoimmune diabetes in NOD mice. Our preclinical findings demonstrate that PD-1 Probody therapeutics retain anti-tumor efficacy with improved safety profiles and therefore have the promise to enable better tolerated PD-1 combination immunotherapies.
PROBODY is a trademark of CytomX Therapeutics, Inc.
Citation Format: Kimberly A. Tipton, Chanty Chan, Kenneth R. Wong, Victoria Singson, Jennifer H. Richardson, W Michael Kavanaugh, Bryan A. Irving, James W. West. PD-1-targeted Probody therapeutics provide anti-tumor efficacy and a 10-fold dose protection against systemic autoimmunity in preclinical studies. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3211.