Abstract 3318: PSMA-targeted thorium-227 conjugate (PSMA-TTC) inhibits tumor growth and abnormal bone changes in the intratibial LNCaP xenograft model of bone-metastatic prostate cancer
Authors: Schatz Christoph , Suominen Mari I. , Knuuttila Matias , Zitzmann-Kolbe Sabine , Rissanen Jukka , Käkönen Sanna-Maria , Hagemann Urs , Scholz Arne 
Published: June 2022

Abstract
Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein specifically overexpressed on the cell membrane of prostate cancer cells. PSMA-targeted thorium-227 conjugate (PSMA-TTC, BAY 2315497) consists of a human PSMA-targeting antibody covalently linked to a 3,2-HOPO (hydroxypyridinone) chelator moiety, radiolabeled with alpha-particle emitting thorium-227. Previous studies have demonstrated PSMA-TTC to be an attractive therapy option for patients with metastatic castration-resistant prostate cancer (mCRPC). In this study, we investigated the efficacy of PSMA-TTC in the LNCaP intratibial model mimicking prostate cancer metastasized to bone.
LNCaP prostate cancer cells were inoculated into the right tibiae of male NOD.scid mice. The mice were randomized to treatment groups (n=12/group) based on serum prostate-specific antigen (PSA) and treated with a single dose of vehicle or PSMA-TTC (300 kBq/kg, 0.75 mg/kg, i.v.). Tumor growth was followed by biweekly serum PSA measurements for 42 days. Tumor-induced abnormal bone growth, PSMA-TTC uptake, bone microarchitecture, bone formation rate and tumor area in tumor-bearing tibiae were determined by radiography, gamma counter, microCT, and dynamic and static histomorphometry, respectively.
A single injection of PSMA-TTC efficiently inhibited intratibial tumor growth (p&lt;0.001) as evidenced by a 82.6% decrease from baseline PSA value in the treatment group at the end of the study, whereas the vehicle group increased by 380% from baseline. Furthermore, the tumor-bearing tibiae were heavier (138%) than the non-tumor-bearing tibiae in the vehicle group, whereas in the PSMA-TTC-treated mice no such difference was observed (103%). PSMA-TTC decreased tumor-induced abnormal bone area in the tumor-bearing tibiae (43.5% of the vehicle). PSMA-TTC uptake relative to tibia weight was higher in the tumor-bearing tibiae compared with the non-tumor-bearing tibiae, resulting in higher levels of thorium-227 and its daughter isotype radium-223 both 3 days and 3 weeks post injection.
In this preclinical study, PSMA-TTC showed robust antitumor efficacy, for the first time, in a model mimicking prostate cancer metastasized to bone as evidenced by a decrease in serum PSA levels, tumor-induced abnormal bone changes, and tumor-bearing tibia weights in the PSMA-TTC-treated mice. PSMA-TTC binds to PSMA-expressing cancer cells, delivering a cytotoxic dose of alpha radiation. The decay product of thorium-227, radium-223 binds to bone matrix and this could also contribute to the antitumor efficacy in this bone-metastatic model. Taken together, our data indicate activity of PSMA-TTC in bone-metastatic CRPC. PSMA-TTC as a monotherapy as well as in combination with darolutamide is currently being investigated in patients with mCRPC in a phase 1 clinical trial (NCT03724747).
Citation Format: Christoph Schatz, Mari I. Suominen, Matias Knuuttila, Sabine Zitzmann-Kolbe, Jukka Rissanen, Sanna-Maria Käkönen, Urs Hagemann, Arne Scholz. PSMA-targeted thorium-227 conjugate (PSMA-TTC) inhibits tumor growth and abnormal bone changes in the intratibial LNCaP xenograft model of bone-metastatic prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3318.