Abstract 1201: Anti-B7-H3 antibody-drug conjugates as potential therapeutics for solid cancer
Authors: Loo Deryk , Scribner Juniper A. , Son Thomas , Hooley Jeff , Hotaling Timothy , Chiechi Michael , Li Pam , De Costa Anushka , Chen Yan , Easton Ann , Chen Francine Z. , Barat Bhaswati , Ciccarone Valentina , Tamura James , Kubik Mark , Koenig Scott , Johnson Syd , Moore Paul A. , Bonvini Ezio 
Published: July 2016

Introduction: Monoclonal antibodies (mAbs) were generated via a target-unbiased approach based on intact cell immunization with cell lines, fetal progenitor cells, and cancer stem cells. An immunohistochemical screen for cancer-specific candidates identified a panel of anti-B7-H3 (CD276) mAbs with highly differential tumor-versus-normal tissue binding. B7-H3 expression was observed in tumor epithelium as well as tumor-associated vasculature and stroma. Consistent with our findings, B7-H3 has been reported to be overexpressed in a growing number of solid cancers, including breast, lung, pancreatic, prostate, kidney, and colon cancer, as well as melanoma and glioblastoma. Furthermore, overexpression of B7-H3 has been correlated with disease severity and poor outcome in a number of these cancer types. A humanized version of an anti-B7-H3 mAb engineered with an enhanced Fc domain (enoblituzumab or MGA271) and a humanized Dual-Affinity Re-Targeting (DART®) protein that recognizes both B7-H3 and CD3 and redirects T cells to kill B7-H3-expressing cells (MGD009) are being investigated in Phase 1 clinical studies. In this nonclinical study, we evaluated the therapeutic potential of anti-B7-H3 antibody-drug conjugates (ADCs) toward B7-H3-expressing solid cancers.

Methods: A panel of anti-B7-H3 mAbs was screened for internalization and a subset of mAbs that were efficiently internalized by tumor cells was identified. These mAbs were converted to ADCs via chemical conjugation; in vitro and in vivo activity studies were then conducted with a range of tumor cell lines representing human cancer types that overexpress B7-H3.

Results: The anti-B7-H3 ADCs exhibited specific, dose-dependent cytotoxicity toward B7-H3-positive tumor cell lines in vitro, including breast, lung, ovarian, pancreatic, and prostate cancer lines, with IC50 values generally in the sub-nM range. Cytotoxicity was not observed with cell lines lacking B7-H3 expression. The anti-B7-H3 ADCs exhibited potent antitumor activity in vivo, resulting in tumor stasis and tumor regression in mice bearing B7-H3-positive human breast, lung, and ovarian tumor xenografts.

Conclusion: Anti-B7-H3 ADCs exhibited dose-dependent cytotoxicity in vitro and potent antitumor activity in vivo toward a range of B7-H3-expressing tumor cell lines representing cancer types that overexpress B7-H3. Our findings demonstrate that ADCs targeting B7-H3 may serve as potential therapeutics for B7-H3-expressing solid cancers.

Citation Format: Deryk Loo, Juniper A. Scribner, Thomas Son, Jeff Hooley, Timothy Hotaling, Michael Chiechi, Pam Li, Anushka De Costa, Yan Chen, Ann Easton, Francine Z. Chen, Bhaswati Barat, Valentina Ciccarone, James Tamura, Mark Kubik, Scott Koenig, Syd Johnson, Paul A. Moore, Ezio Bonvini. Anti-B7-H3 antibody-drug conjugates as potential therapeutics for solid cancer. [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 1201.