Abstract 5355: MSLN-targeted thorium-227 conjugate demonstrates increased antitumor activity in combination with bevacizumab and regorafenib

Abstract

Abstract Mesothelin-targeted thorium-227 conjugate (MSLN-TTC) is a targeted alpha therapeutic consisting of a MSLN-targeting antibody anetumab covalently attached to a 3,2-HOPO chelator complexed with the alpha emitter thorium-227. It has potent in vivo activity in cell and patient-derived xenograft (PDX) models in monotherapy and in combination with DNA damage repair inhibitors. We evaluated the antitumor efficacy of MSLN-TTC in a mesothelin overexpressing ovarian PDX model (ST206B) in monotherapy and in combination bevacizumab or the multikinase inhibitor regorafenib which both have anti-angiogenic activity. The biodistribution of MSLN-TTC was studied in parallel. Athymic mice were transplanted s.c. with ST206B-derived tumor fragments and treated with MSLN-TTC (250 kBq/kg, Q7Dx2, 0.43 mg/kg) alone or in combination with bevacizumab (1.25 or 5 mg/kg, i.p., Q5D; or 5 mg/kg, single dose) or regorafenib (7.5, 15, or 30 mg/kg, QD, p.o. for 28 days). The biodistribution of the respective treatment groups was analyzed using a germanium detector. For molecular analyses, tumors were stained for γ-H2AX, Ki67, CD31 and α-SMA. MSLN-TTC monotherapy showed high in vivo potency at 2 × 250 kBq/kg with a Treated/Control (T/C) ratio of 0.28 (n=5) on day 29. Monotherapy treatment with bevacizumab and regorafenib showed dose-dependent antitumor activity with T/C ratios ranging from 0.31 (regorafenib 30 mg/kg) to 0.33 (bevacizumab 5 mg/kg, single dose). The in vivo efficacy of MSLN-TTC was further enhanced by combination therapy with bevacizumab (T/C 0.21; n=5) or regorafenib (T/C 0.15; n=5). Combination of MSLN-TTC and regorafenib (30 mg/kg) resulted in more complete and partial responses (3/4 CRs; 1/4 PR) as compared to MSLN-TTC (2/3 PRs; 1/3 PD) at day 141. All treatments were well-tolerated based on body weight assessment; a reversible myelosuppression of white blood cells was observed for MSLN-TTC monotherapy and combination with bevacizumab or regorafenib did not worsen it. In the biodistribution study, tumor-specific accumulation of MSLN-TTC was observed over 336 h (14 days) with lower accumulation in the MSLN-TTC + bevacizumab group (1.25 mg/kg, Q5D, p<0.05; 5 mg/kg, single dose, p<0.05). On the molecular level, MSLN-TTC- and MSLN-TTC/ regorafenib (15 and 30 mg/kg) -treated tumors showed an increase in the DNA damage marker γ-H2AX compared to vehicle. The endothelial cell marker CD31 was decreased in tumors treated with MSLN-TTC and bevacizumab (1.25 mg/kg, Q5D; 5 mg/kg, single dose), but not in MSLN-TTC monotherapy (p<0.05). In summary, MSLN-TTC effectively reduced the growth of primary tumors in the ST206B ovarian cancer model which was further enhanced by addition of bevacizumab and regorafenib. Further studies are needed to investigate the underlying molecular mechanisms. Citation Format: Urs B. Hagemann, Christine Ellingsen, Claudia Kamfenkel, Dieter Zopf, Jesper Fonslet, Carsten Nielsen, Hartwig Hennekes, Alan S. Cuthbertson, Dominik Mumberg. MSLN-targeted thorium-227 conjugate demonstrates increased antitumor activity in combination with bevacizumab and regorafenib [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 5355.