Abstract B116: Optimizing a CD71-targeting Probody drug conjugate (PDC) for activity in multiple solid tumor and lymphoma models and for tolerability in nonhuman primates (2018)

Abstract ProbodyTM therapeutics are antibody prodrugs designed to remain largely inactive until proteolytically activated in the tumor microenvironment (TME), potentially enabling the safer targeting of antigens that are highly expressed in both tumor and normal tissue. CD71 (transferrin receptor) is an example of an ideal Probody Drug Conjugate (PDC) target, not only because it efficiently internalizes and can deliver a cytotoxic payload intracellularly, but also because it is expressed at high levels both in many different tumor types as well as in dividing normal cells. We have previously demonstrated that while an anti-CD71 antibody drug conjugate (ADC) is highly toxic, a CD71-targeting PDC is both efficacious in mouse tumor models and well tolerated in nonhuman primates. Two key components of a Probody therapeutic prodomain that reduce its binding to normal tissue and allow for its tumor-specific activation are 1) a mask that reduces the ability of the antibody binding site to interact with target antigen and 2) a protease-activatable substrate that is cleaved in the TME, resulting in removal of the mask. Here, we demonstrate how modulating mask strength and substrate cleavability can optimize efficacy and safety of a CD71-targeting PDC in preclinical models. Through this process, we have selected a lead molecule, CX-2029, for further development. CX-2029 is a CD71-targeting PDC conjugated to vcMMAE with a Drug to Probody Ratio (DPR) of 2, achieved by purification. At dose levels consistent with those expected in humans, a more strongly masked PDC, CX-2018, was less efficacious in a mouse xenograft tumor model compared to PDC CX-2016, which has a weaker mask, demonstrating that mask strength affects antitumor activity. Further, PDC CX-2019, which has the same mask but a less cleavable substrate than CX-2016, was similarly efficacious in a mouse xenograft tumor model, demonstrating that both substrates are sufficiently cleaved in the TME to activate the PDCs. However, CX-2019 was better tolerated in NHP at 6 mg/kg than CX-2016, suggesting that the less cleavable substrate in CX-2019 leads to a better therapeutic index. Using a LC/MS/MS method, we showed lower levels of circulating activated CX-2019 compared with circulating activated CX-2016, which is consistent with CX-2019’s improved tolerability. Lead CX-2029 contains the same mask and substrate as CX-2019 but differs in having a DPR of 2 versus ~3 for CX-2019. Up to 6 mg/kg of CX-2029 as a single dose produced complete regressions and durable responses in mouse xenograft tumor models encompassing multiple indications, and was tolerated in monkeys at doses of up to 12 mg/kg. These data demonstrate that, in preclinical models, tuning of mask strength and substrate cleavability can optimize the efficacy and tolerability of Probody Therapeutics and have the potential to enable the safe and effective targeting of highly expressed tumor antigens like CD-71. CX-2029 is currently under development, with an IND filing expected in 2018. PROBODY is a trademark of CytomX Therapeutics, Inc. Citation Format: Shweta Singh, Laura Serwer, Niharika Chauhan, Amy DuPage, Michael Krimm, Ken Wong, Yuanhui Huang, Andrew Jang, Eric Ureno, Adam Miller, Sarah Patrick, Shanti Duvur, Fritz Buchanan, Matthew M. Ravn, Rob Leanna, Ilaria Badagnani, Tracy Henriques, Shouchun Liu, Claus Krebber, Sridhar Viswanathan, Jennifer Richardson, Susan Morgan-Lappe, Michael Kavanaugh. Optimizing a CD71-targeting Probody drug conjugate (PDC) for activity in multiple solid tumor and lymphoma models and for tolerability in nonhuman primates [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 B116.