Abstract 1197: Outstanding preclinical efficacy of a novel maytansinoid-antibody-drug conjugate targeting LAMP1 in patient-derived xenograft solid tumors

Lysosome-associated membrane protein 1 (LAMP1) is involved in the maintenance of lysosome membrane integrity and phagolysosome formation. LAMP1 shows very limited expression at the cell surface in normal tissues while, a moderate to high expression was found in a number of solid tumors. We generated SAR428926, an antibody drug conjugate (ADC) comprising an anti-LAMP1 humanized monoclonal antibody conjugated via a cleavable SPDB linker, to the maytansinoid derivative DM4, a tubulin interfering molecule. Here we report the annotation of LAMP1 expression in a large panel of human tumors and correlate it to the in vivo efficacy of SAR428926 in a panel of over 50 human patient-derived xenograft (PDX) solid tumors. Indications of interest were identified by immunohistochemistry (IHC) conducted on a large panel of frozen tumor samples using the murine version of the therapeutic anti-human LAMP1 antibody. The prevalence of tumor samples with positive membrane staining was 49% in breast invasive lobular or ductal carcinoma, and in particular in a TNBC subset (87%), 25% in gastric adenocarcinoma, 52% in colon/rectum adenocarcinoma, 22% in lung adenocarcinoma, 20% in lung squamous cell carcinoma, 25% in prostate adenocarcinoma and 31% in ovary adenocarcinoma. In vivo efficacy of SAR428926 was evaluated in PDX models engrafted subcutaneously into immunocompromised SCID mice. PDX models retain the molecular diversity, cellular heterogeneity, and histology typically seen in patient tumors and offer a distinct advantage over cell line models. The PDX models used in the study, which include colon, breast, lung, prostate, gastric and ovarian cancer, were selected by IHC based on LAMP1 expression levels distribution and the frequency of LAMP1 membrane positive cells, which varied between 5% and 100% of the tumor cells, allowing coverage of a wide variety of cases. Outstanding efficacy with complete regressions was observed at 5 mg/kg iv single administration, a dose that gives an exposure tolerated in toxicology species. Efficacy was also observed at lower doses of 2.5 mg/kg and 1.25 mg/kg after single administration. Our results show that while the presence of the antigen is required as there is no efficacy in LAMP1-negative PDX, there is no linear correlation between the level of antigen expression and the antitumor activity across the panel of PDX tested. In addition to membrane LAMP1 expression, a key driver for efficacy was the intrinsic sensitivity of the PDX models t