A Next Generation Theranostic PSMA Ligand for ⁶⁴Cu and⁶⁷Cu-Based Prostate Cancer Imaging and Therapy

Objectives: The concept of theranostics describes the use of a matched pair of radioisotopes to enable quantification of the distribution of radioactivity in the body, followed by radioligand therapy using the same delivery vector [1]. ⁶⁴Cu (t_1/2 = 12.7 h, β⁺ = 17.5%) and ⁶⁷Cu (t_1/2 = 2.58 d, β^- = 100%) form a unique theranostic pair of radioisotopes suitable for this purpose. We have recently introduced a number of trifunctional PSMA-targeting ligands bearing the DOTA macrocycle for chelation of trivalent metals [2]. The in vivo tissue distribution of these ligands is highly promising, but the complexation of copper by DOTA is suboptimal. Sarcophagine chelators such as MeCOSar complex copper ions efficiently and with high stability [3,4]. Our aim was develop a ⁶⁴Cu/⁶⁷Cu theranostic ligand with high stability and optimal in vivo properties for PSMA-targeted imaging and therapy of prostate cancer. Methods: RPS-085 was synthesized by conjugation of a PSMA-targeting triazolylphenylurea-derivatized glutamate-urea-lysine moiety, an N^ε-(2-(4-iodophenyl)acetyl)lysine albumin binding group, and a bifunctionalized MeCOSar chelator via a PEG3 spacer. The PSMA affinity of the metal-free ligand was determined in a competition binding assay against [^99mTc]MIP-1427 in LNCaP cells. [⁶⁴Cu]RPS-085 was synthesized in 20 min at 25 °C in either 10X PBS, 0.5 M NH₄OAc, pH 5-6, or 3 N NaOAc, pH 5. Imaging by microPET/CT was performed up to 48 h post injection (p.i.) of 21.5 ± 0.5 MBq [⁶⁴Cu]RPS-085in nu/nu mice bearing LNCaP tumor xenografts. Results: The PSMA affinity of RPS-085 was high (IC₅₀ <30 nM). The radiosynthesis of [⁶⁴Cu]RPS-085 was nearly quantitative in both 10X PBS and 0.5 M NH₄OAc. When the latter buffer was used, radiochemical yield was 98.3%, radiochemical purity exceeded 99%, and the molar activity was 117 GBq/µmol. The radioligand was reformulated in 10% v/v EtOH/saline, and was >99% stable over 24 h at 25 °C. The tumor uptake of [⁶⁴Cu]RPS-085 reached a maximum at 3 h p.i. (28.1 ± 8.5 %ID/cm³), and was persistent out to 48 h p.i. (23.3 ± 8.2 %ID/cm³). Kidney uptake was high initially, but cleared rapidly. As a result, the tumor-to-kidney ratio was 3.3 ± 1.3 at 6 h, and 45.6 ± 17.2 at 48 h p.i. No accumulation of activity was observed in other tissues, including liver.

Conclusions: Complexation of ⁶⁴Cu²⁺ by the MeCOSar chelator is rapid and efficient, and complex stability is excellent both in vitro and in vivo. Furthermore, the in vivo tissue profile of [⁶⁴Cu]RPS-085, including high and sustained tumor uptake, and excellent tumor-to-background ratios even at early time points, is promising. The ligand shows great potential for PET imaging using ⁶⁴Cu, and its tissue distribution projects favorably for therapeutic applications. Further studies will evaluate the efficacy of [⁶⁷Cu]RPS-085. References: [1] Rösch F, et al. The Beginning and Development of the Theranostic Approach in Nuclear Medicine, as Exemplified by the Radionuclide Pair ⁸⁶Y and ⁹⁰Y. Pharmaceuticals (Basel). 2017;10:56. [2] Kelly JM, et al. Trifunctional PSMA-targeting constructs for prostate cancer with unprecedented localization to LNCaP tumors. Eur J Nucl Med Mol Imaging. 2018;45:1841-1851. [3] Paterson BM, et al. PET imaging of tumours with a ⁶⁴Cu labeled macrobicyclic cage amine ligand tethered to Tyr³-octreotate. Dalton Trans. 2014;43:1386-1396. [4] Gourni E, et al. Copper-64 Labeled Macrobicyclic Sarcophagine Coupled to a GRP Receptor Antagonist Shows Great Promise for PET Imaging of Prostate Cancer. Mol Pharm. 2015;12:2781-2790.