Abstract 6318: Acquired resistance in a malignant pleural mesothelioma preclinical model after treatment with Anetumab ravtansine

Mesothelin (MSLN) is a cell surface glycoprotein with high expression in various cancers and only limited expression in normal tissues. It is expressed in the majority of malignant pleural mesothelioma (MPM), a rare but very aggressive disease. Thus, several anti-MSLN-directed therapies including antibody-drug conjugates (ADCs), immunotoxins, targeted thorium-227 conjugates (TTCs), and CAR-T are currently being explored in MPM and further cancer indications. We developed an anti-mesothelin antibody-drug conjugate, anetumab ravtansine (BAY 94-9343, ARav) in which the anti-MSLN antibody is conjugated via an SPDB linker to the microtubule disrupting payload DM4, a derivative of maytansine. In order to explore potential mechanisms of acquired resistance to treatment with ARav in MPM, the cell line-derived MSLN-positive tumor model NCI-H226 was selected. We had previously shown that this model expresses high levels of MSLN and that ARav dosed for two cycles with 11.2 mg/kg Q3Dx3 inhibited tumor growth by 94% as compared to the vehicle control and achieved a 63% response rate (partial regression in 5 out of 8 mice). NCI-H226 xenograft tumors were transplanted subcutaneously onto mice and treated with 10 mg/kg ARav Q3Dx3 for 3 cycles. A tumor on one mouse that initially responded and afterwards started to regrow despite ARav treatment was excised and tumor tissue was transplanted onto 10 new mice (first in vivo passage). These mice were treated with ARav at 10 mg/kg (Q3Dx3) in the 1rst and 20 mg/kg in the 2nd cycle (n=5; Q3Dx3) or vehicle (n=5). From this approach, 2 tumors (A1, A4) with progressive disease under ARav treatment were obtained. These tumors were each transplanted onto 6 mice (second in vivo passage) which were treated with 20 mg/kg ARav (Q3Dx3) or vehicle. After implantation onto a new host, tumors were not sensitized to ARav treatment indicating that fundamental changes in the tumor cells had happened. To analyze potential molecular changes, tumors from ARav treated and vehicle treated mice were collected and FFPE tumors were analyzed by immunohistochemistry for MSLN and P-gp (MDR1) expression. In addition, MSLN mRNA levels were analyzed by qRT-PCR. In the NCI-H226 tumors serially passaged and with increased growth in the presence of ARav, MSLN protein was reduced to background level when compared to the MSLN Level in freshly established NCI-H226 xenograft tumors. The decrease of MSLN expression was also observed on the RNA level. The experimental data from serial passaging of the NCI-H226 cell line-derived MPM in vivo model with reduced response to ARav show that loss of the MSLN antigen occurs on the mRNA and protein level. Reduction of MSLN expression may decrease binding and internalization of ARav and may thus be a mechanism for acquired resistance in MSLN-expressing tumor models. Citation Format: Christoph Schatz, Sabine Zitzmann-Kolbe, Reimo Tetzner, Marlen Keil, Jens Hoffmann, Dominik Mumberg, Anette Sommer. Acquired resistance in a malignant pleural mesothelioma preclinical model after treatment with Anetumab ravtansine [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6318.