Abstract 6033: Preclinical evaluation of an actinium-225 labeled PSMA-targeting small molecule (225Ac-PSMA-Trillium (BAY 3563254)) for the treatment of metastatic castration resistant prostate cancer (mCRPC)
Authors: Zitzmann-Kolbe Sabine , Papple Alex , Poethko Thorsten , Moen Ingrid , Schatz Christoph , Hillier Shawn , Babich John W. , Cuthbertson Alan , Hagemann Urs B. 
Published: March 2024

Abstract
PSMA is a clinically validated target for the treatment of mCRPC. Since the linear energy transfer and degree of induced DNA damage of alpha particles far exceeds that of beta particles, several investigational PSMA-targeted small molecules labeled with the alpha-emitter actinium-225 (225Ac) have shown encouraging clinical signs of efficacy. However, xerostomia is often found as a dose-limiting side effect, presumably due to low levels of PSMA expression and efficient uptake of small molecules in salivary glands. To reduce these unwanted side effects a PSMA-targeted radioligand comprising a triad of important features was investigated. It consists of a high-affinity PSMA binder for specific tumor-targeting; a customized albumin-binding moiety designed to prolong plasma residence time and increase the therapeutic index by improving tumor uptake while reducing salivary gland uptake; and a chelator for efficient complexation of 225Ac. From a series of compounds containing the same high-affinity PSMA-targeting moiety, but different chain lengths and albumin-binding domains, compound RPS-072 containing a PEG-3 and PEG-8 chain and an iodobenzyl moiety as albumin-binder was selected for further evaluation (J Nucl Med 2019; 60:656-663). This compound was the basis for the PSMA-Trillium with a DOTA chelator for imaging applications in humans (NCT05773703) or a macropa chelator for 225Ac labeling resulting in 225Ac-PSMA-Trillium. In vitro preclinical characterization of 225Ac-PSMA-Trillium was performed to test affinities for PSMA and albumin. In a SPR assay, Kd was measured as 4.88x1011 M, Kon as 1.4x107 1/Ms and Koff as 6.6x10-4 1/Ms. 225Ac-PSMA-Trillium demonstrated good stability up to 48 h and induced potent in vitro cytotoxicity in the LNCaP subclone C4-2 (0.114 kBq/ml). In vivo biodistribution of 225Ac-PSMA-Trillium showed consistent with the albumin binding property a relatively slow pharmacokinetics profile with 5 % ID/g still in the blood after 24 h and tumor accumulation peaking at ~ 20 % ID/g after 5-7 days. Tumor penetration was investigated using autoradiography which demonstrated fast, homogeneous, and even distribution across the tumor within 15 mins after injection. This property was also reflected by strong therapeutic efficacy in several PrCa models. In the LNCaP model a tumor growth inhibition was observed at a single dose dependent efficacy at 150 kBq/kg and 300 kBq/kg and increased the time to reach 400 mm3 by 35 days for the 300 kBq/kg group. In the androgen independent PDX KuCaP-1 model, a single dose of 250 kBq/kg 225Ac-PSMA-Trillium induced strong tumor growth inhibition over the course of 35 days. A first-in-human trial with 225Ac-PSMA-Trillium is anticipated to start in the first half of 2024.
Citation Format: Sabine Zitzmann-Kolbe, Alex Papple, Thorsten Poethko, Ingrid Moen, Christoph Schatz, Shawn Hillier, John W. Babich, Alan Cuthbertson, Urs B. Hagemann. Preclinical evaluation of an actinium-225 labeled PSMA-targeting small molecule (225Ac-PSMA-Trillium (BAY 3563254)) for the treatment of metastatic castration resistant prostate cancer (mCRPC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6033.