Mesothelin-Targeted Thorium-227 Conjugate (MSLN-TTC): Preclinical Evaluation of a New Targeted Alpha Therapy for Mesothelin-Positive Cancers.

Targeted thorium-227 conjugates (TTC) represent a new class of molecules for targeted alpha therapy (TAT). Covalent attachment of a 3,2-HOPO chelator to an antibody enables specific complexation and delivery of the alpha particle emitter thorium-227 to tumor cells. Because of the high energy and short penetration range, TAT efficiently induces double-strand DNA breaks (DSB) preferentially in the tumor cell with limited damage to the surrounding tissue. We present herein the preclinical evaluation of a mesothelin (MSLN)-targeted thorium-227 conjugate, BAY 2287411. MSLN is a GPI-anchored membrane glycoprotein overexpressed in mesothelioma, ovarian, pancreatic, lung, and breast cancers with limited expression in healthy tissue. The binding activity and radiostability of BAY 2287411 were confirmed bioanalytically. The mode-of-action and antitumor potency of BAY 2287411 were investigated in vitro and in vivo in cell line and patient-derived xenograft models of breast, colorectal, lung, ovarian, and pancreatic cancer. BAY 2287411 induced DSBs, apoptotic markers, and oxidative stress, leading to reduced cellular viability. Furthermore, upregulation of immunogenic cell death markers was observed. BAY 2287411 was well-tolerated and demonstrated significant antitumor efficacy when administered via single or multiple dosing regimens in vivo. In addition, significant survival benefit was observed in a disseminated lung cancer model. Biodistribution studies showed specific uptake and retention of BAY 2287411 in tumors and enabled the development of a mechanistic pharmacokinetic/pharmacodynamic model to describe the preclinical data. These promising preclinical results supported the transition of BAY 2287411 into a clinical phase I program in mesothelioma and ovarian cancer patients (NCT03507452).