ALPPL2 is a highly specific and targetable tumor cell surface antigen

Abstract

It has been challenging to identify tumor-specific cell surface antigens as the vast majority of tumor-associated antigens are also expressed by some normal tissues. In the course of our study on mesothelioma, we identified a highly specific tumor cell surface antigen that can be targeted for therapy development. Mesothelioma is caused by malignant transformation of the mesothelium, incurable and categorized into three histological subtypes, epithelioid, biphasic and sarcomatoid. To identity novel mesothelioma cell surface antigens with broad subtype coverage and high tissue specificity, we have previously selected phage antibody display libraries on live mesothelioma cells and tissues following counter-selection on normal cells, and identified a panel of human antibodies that bind all subtypes of mesothelioma but not normal mesothelium. One of the antibodies, M25, showed high specificity, and we hereby report the identification of the M25 antigen as ALPPL2. We performed immunohistochemistry on normal human tissues and found that ALPPL2 is expressed only on placental trophoblasts but not any other normal tissues. This exquisite tissue specificity and broad tumor type coverage suggests that ALPPL2 could be an excellent cell surface target for therapeutic development against mesothelioma. To evaluate therapeutic potential of ALPPL2 targeting, we developed an ALPPL2-targeted antibody-drug conjugate and demonstrated potent and specific tumor killing in vitro and in vivo against both epithelioid and sarcomatoid mesothelioma. Thus ALPPL2 belongs to a rare class of cell surface antigens that can be said as being truly tumor specific and is well suited for therapy development against ALPPL2 expressing tumors.