Abstract 1198: Characterization of a novel maytansinoid-antibody-drug conjugate targeting LAMP1 expressed at the surface of tumor cells
Authors: Baudat Yves , Cameron Beatrice , Dabdoubi Tarik , Lefebvre Anne-Marie , Merino-Trigo Ana , Thomas Corinne , Pecheux Veronique , Genet Bruno , Calvet Loreley , Blot Lydia , Mathieu Magali , Gauzy Laurence , Berthou-Soulie Laurence , Prades Catherine , Amara Celine , Nunes Manoel , Henry Christophe , Combeau Cecile , Blanche Francis , Mayaux Jean-Francois , Garcia-Echeverria Carlos , Naimi Souad , Blanc Veronique 
Published: July 2016

Abstract
Lysosome-associated membrane protein 1 (LAMP1), one of the most abundant protein expressed at the membrane of lysosomes, is a type I transmembrane protein involved in the maintenance of lysosome membrane integrity and phagolysosome formation. In activated lymphocytes LAMP1 is described as a marker of degranulation.
Unexpectedly, by immunizing mice with a colon patient-derived xenograft (PDX) followed by a screening of monoclonal antibodies (mAb) by immunohistochemistry (IHC) for selection of antibodies that specifically stain tumor plasma membrane and de-orphaning by Immunoprecipitation-Mass spectrometry, we identified LAMP1 as the target of several antibodies. One of them, Ab-1, showed binding to the luminal domain of human LAMP1 with nM affinity. Crystal structure of its Fab with LAMP1 extracellular domain, showed that the epitope was non-linear, not a glycotope and spanned between position 29 to195.
LAMP1 expression was further documented by IHC with Ab-1, showing limited cell surface expression in normal tissues while moderate to high plasma membrane expression was found in a number of breast, including TNBC, colorectal, gastric, prostate, lung and ovary tumors.
The humanized Ab-1 mAb, humAb-1, was shown to display rapid cycling after binding to LAMP1 at the surface of colo205 cell line, allowing internalization and processing of a number of LAMP1/antibody complex 10 folds higher than the number of LAMP1 molecules at the cell surface.
humAb-1 was conjugated to DM4 maytansinoid derivative using an SPDB cleavable linker to generate a new antibody-drug conjugate, SAR428926, for the treatment of patients with cell surface LAMP1-positive tumors.
Conjugated and naked antibody displayed similar affinities for LAMP1. SAR428926 killed tumor cell lines (engineered to express cell surface LAMP1) in the sub-nM range. In contrast, no target-mediated cytotoxicity was observed when SAR428926 was incubated with normal cells, including resting or activated lymphocytes.
PDX models reflecting the pattern and level of LAMP1 expression at the surface of human tumors were selected to evaluate SAR428926 in vivo efficacy. Outstanding in vivo activity was observed in different indications, including TNBC, lung and colon PDXs, with complete regressions after a single administration at 5 mg/kg.
These encouraging preclinical data have prompted the initiation of IND-enabling studies with the goal to progress humAb-1-ADC to the clinic in patients with tumors expressing LAMP1 at the cell surface. The First-In-Human trial has been initiated in October 2015.
Citation Format: Yves Baudat, Beatrice Cameron, Tarik Dabdoubi, Anne-Marie Lefebvre, Ana Merino-Trigo, Corinne Thomas, Veronique Pecheux, Bruno Genet, Loreley Calvet, Lydia Blot, Magali Mathieu, Laurence Gauzy, Laurence Berthou-Soulie, Catherine Prades, Celine Amara, Manoel Nunes, Christophe Henry, Cecile Combeau, Francis Blanche, Jean-Francois Mayaux, Carlos Garcia-Echeverria, Souad Naimi, Veronique Blanc. Characterization of a novel maytansinoid-antibody-drug conjugate targeting LAMP1 expressed at the surface of tumor cells. [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 1198.