Solution Thermodynamics and Kinetics of Metal Complexation with a Hydroxypyridinone Chelator Designed for Thorium-227 Targeted Alpha Therapy

Abstract

The solution chemistry of a chelator developed for 227 Th targeted alpha therapy was probed. The compound of interest is an octadentate ligand comprising four N-methyl-3-hydroxy-pyridine-2-one metal-binding units, two tertiary amine groups, and one carboxylate arm appended for bioconjugation. The seven pK a values of the ligand and the stability constants of complexes formed with Th(IV), Hf(IV), Zr(IV), Gd(III), Eu(III), Al(III), and Fe(III) were determined. The ligand exhibits extreme thermodynamic selectivity towards tetravalent metal ions, with a ca. 20 orders of magnitude difference between the formation constant of the Th(IV) species formed at physiological pH, namely [ThL]-, and that of its Eu(III) analogue. Likewise, log β 110 values of 41.7 ± 0.3 and 26.9 ± 0.3 (T = 25°C) were measured for [ThL]-and [Fe III L] 2-, respectively, highlighting the high affinity and selectivity of the ligand for Th ions over potentially competing endogenous metals. Single-crystal structural analysis of the Fe(III) complex revealed a dinuclear 2:2 metal:chelator complex crystallizing in the P-1 space group. The formation of this dimeric species is likely favored by several intramolecular hydrogen bonds and the protonation state of the chelator in acidic media. L III edge EXAFS data on the Th(IV) complexes of both the ligand and a monoclonal antibody conjugate revealed the expected mononuclear 1:1 metal:chelator coordination environment. This was also confirmed by high resolution mass spectrometry. Finally, kinetic experiments demonstrated that labeling the bioconjugated ligand with Th(IV) could be achieved and Page 2 of 40 completed after1 hour at room temperature, reinforcing the high suitability of this chelator for 227 Th targeted alpha therapy.

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

“…Therefore, the development of novel modalities offering new treatment opportunities to patients is constantly needed. We describe herein, the preclinical efficacy of a MSLN-targeted thorium-227 conjugate (BAY 2287411) comprising the fully human IgG1 antibody BAY 86-1903, covalently conjugated to a 3,2-HOPO chelator (4,5) enabling stable complexation of the alpha particle emitter thorium-227. The radiolabeling is high yielding and robust and can be performed by simply mixing the conjugate with thorium-227 at ambient temperature.…”

“…The inherent boneseeking accumulation of radium-223 (3) allows for effective delivery to bone metastases, but the lack of suitable chelating agents has hindered the development of radioimmunoconjugates using this radionuclide. On the other hand, the progenitor of radium-223, thorium-227, can be directly complexed at ambient temperature to antibody conjugates bearing chelators of the octadentate 3,2-hydroxypyridinone (3,2-HOPO) class (4,5). Fortuitously, the half-life of thorium-227 (18.7 days) is compatible with the blood half-life of therapeutic antibodies in humans, the decay chain resulting in the generation of a total of five highenergy alpha and two beta particles ending with the nonradioactive element lead-207 (6).…”

“…We present herein the preclinical evaluation of BAY 2287411, a MSLN-targeting thorium-227 conjugate (MSLN-TTC), which is based on the fully human IgG1 antibody BAY 86-1903 (anetumab) covalently attached to a thorium-227 complexing 3,2-HOPO chelator (4,5). BAY 2287411 demonstrated potent in vitro and in vivo activity in cellular and patient-derived xenograft (PDX) models of breast, lung (mesothelioma), ovarian, and pancreatic cancer.…”

Advances in Precision Oncology: Targeted Thorium-227 Conjugates As a New Modality in Targeted Alpha Therapy

“…2). 25,31,32 Compared with the tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator, which often requires heating, the HOPO chelators are superior due to efficient radiolabeling at ambient temperatures and high stability of formed complexes. 33,34 Mode of Action of TTCs a-Particle emitters exert their effects independently of cell proliferative state and cell cycle phase.…”

Preclinical Efficacy of a PSMA-Targeted Thorium-227 Conjugate (PSMA-TTC), a Targeted Alpha Therapy for Prostate Cancer

“…PSMA-TTC uses the alpha-particle emitter thorium-227 (half-life of 18.7 days), which is the parent nuclide of radium-223. Thorium-227 can be efficiently complexed by the 3,2-hydroxypyridinone (3,2-HOPO) octadentate chelator (10,11) covalently linked to antibodies (12)(13)(14)(15) at ambient temperature. Strong antitumor efficacy of PSMA-TTC is shown in a panel of prostate cancer models mimicking different clinical stages of prostate cancer including castration-resistant and bone metastatic disease.…”