Purpose: 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. Experimental Design: 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. Results: 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. Conclusions: These promising preclinical results supported the transition of BAY 2287411 into a clinical phase I program in mesothelioma and ovarian cancer patients (NCT03507452).

The clinical efficacy of the first approved alpha pharmaceutical, Xofigo (radium-223 dichloride, 223 RaCl 2 ), has stimulated significant interest in the development of new alpha-particle emitting drugs in oncology. Unlike radium-223 ( 223 Ra), the parent radionuclide thorium-227 ( 227

◥Purpose: Prostate-specific membrane antigen (PSMA) is an attractive target for radionuclide therapy of metastatic castrationresistant prostate cancer (mCRPC). PSMA-targeted alpha therapy (TAT) has shown early signs of activity in patients with prostate cancer refractory to beta radiation. We describe a novel, antibody-based TAT, the PSMA-targeted thorium-227 conjugate PSMA-TTC (BAY 2315497) consisting of the alpha-particle emitter thorium-227 complexed by a 3,2-HOPO chelator covalently linked to a fully human PSMA-targeting antibody.Experimental Design: PSMA-TTC was characterized for affinity, mode of action, and cytotoxic activity in vitro. Biodistribution, pharmacokinetics, and antitumor efficacy were investigated in vivo using cell line and patient-derived xenograft (PDX) models of prostate cancer.Results: PSMA-TTC was selectively internalized into PSMApositive cells and potently induced DNA damage, cell-cycle arrest, and apoptosis in vitro. Decrease in cell viability was observed dependent on the cellular PSMA expression levels. In vivo, PSMA-TTC showed strong antitumor efficacy with T/C values of 0.01 to 0.31 after a single injection at 300 to 500 kBq/kg in subcutaneous cell line and PDX models, including models resistant to standard-of-care drugs such as enzalutamide. Furthermore, inhibition of both cancer and cancer-induced abnormal bone growth was observed in a model mimicking prostate cancer metastasized to bone. Specific tumor uptake and efficacy were demonstrated using various PSMA-TTC doses and dosing schedules. Induction of DNA double-strand breaks was identified as a key mode of action for PSMA-TTC both in vitro and in vivo.Conclusions: The strong preclinical antitumor activity of PSMA-TTC supports its clinical evaluation, and a phase I trial is ongoing in mCRPC patients (NCT03724747).

The second phase is independent of concentration and is exponential. In contrast, in the unfolding direction, all molecules display two-state kinetics. Collectively this implies a transition state between unfolded helices and dimeric intermediate that is readily traversed in both directions. We demonstrate that the added stability of cFos-JunW Ph1 relative to cFos-JunW is achieved via a combination of kinetic rate changes; cFosJunW E23K has an increased initial dimerization rate, prior to the major transition state barrier while cFos-JunW Q21R displays a decreased unfolding rate. The former implies that improved hydrophobic burial and helix-stabilizing mutations exert their effect on the initial, rapid, monomer-collision event. In contrast, electrostatic interactions exert their effect late in the folding pathway. Although our focus is the leucine zipper region of the oncogenic transcription factor Activator Protein-1, coiled coils are ubiquitous and highly specific in their recognition of partners. Consequently, generating kinetic-based rules to predict and engineer their stability is of major significance in peptidebased drug design and nano-biotechnology.

The cell surface receptor CD70 has been previously reported as a promising target for B-cell lymphomas and several solid cancers including renal cell carcinoma. We describe herein the characterization and efficacy of a novel CD70 targeted thorium-227 conjugate (CD70-TTC) comprising the combination of the three components, a CD70 targeting antibody, a chelator moiety and the short-range, high-energy alpha-emitting radionuclide thorium-227 (227Th). In vitro analysis demonstrated that the CD70-TTC retained binding affinity to its target and displayed potent and specific cytotoxicity compared to an isotype control-TTC. A biodistribution study in subcutaneous tumor-bearing nude mice using the human renal cell carcinoma cell line 786-O demonstrated significant uptake and retention with 122 ± 42% of the injected dose of 227Th per gram (% ID/g) remaining in the tumor seven days post dose administration compared to only 3% ID/g for the isotype control-TTC. Tumor accumulation correlated with a dose dependent and statistically significant inhibition in tumor growth compared to vehicle and isotype control-TTC groups at radioactivity doses as low as 50 kBq/kg. The CD70-TTC was well tolerated as evidenced by only modest changes in hematology and normal gain in body weight of the mice. To our knowledge, this is the first report describing molecular targeting of CD70 expressing tumors using a targeted alpha-therapy (TAT).