Development of Novel PSMA Ligands for Imaging and Therapy with Copper Isotopes

Abstract

Prostate-specific membrane antigen (PSMA)-binding tracers have been shown to be promising agents for the specific targeting of prostate tumors. On labeling with the short-lived isotopes 18 F and 68 Ga, excellent molecular imaging performance is achieved. This potential could be further exploited using long-lived isotopes. Because of the favorable half-life of 64 Cu, tracers labeled with this PET nuclide could solve logistic problems. Moreover, this isotope provides a theranostic pair with the therapeutic copper isotope 67 Cu. Hence, 9 novel tracers that combine dedicated copper chelators with the PSMA-specific urea-based binding motif were developed. Methods: The precursors were obtained by solid-phase synthesis. The purity and molecular weight of the PSMA ligands were confirmed by high-performance liquid chromatography and liquid chromatography-mass spectrometry. The compounds were labeled with 64 Cu, with a radiolabeling yield of more than 99%. Competitive cell binding assays and internalization assays were performed with C4-2 cells, a subline of the PSMA-positive cell line LNCaP (human lymph node carcinoma of the prostate). In vitro serum stability, the stability of 64 Cu-CA003 in blood, and the in vivo fate of neat 64 Cuchloride or 64 Cu-CA003 were determined to prove whether the stability of the radiolabeled compounds is sufficient to ensure no significant loss of copper during the targeting process. For PET imaging and biodistribution studies, a C4-2 tumor-bearing mouse model was used. Results: The radiolabeled 64 Cu-PSMA ligands showed high serum stability. All PSMA ligands showed high inhibition potencies, with equilibrium inhibition constants in the low nanomolar range. 64 Cu-CA003 and 64 Cu-CA005 showed high internalization ratios (34.6% ± 2.8 and 18.6% ± 4.4, respectively). Both the in vitro serum stability determination and the in vivo characterization of the main radiolabeled compounds confirmed that, except for 64 Cu-PSMA-617, all compounds showed high serum stability within the observation period of 24 h. Small-animal PET imaging demonstrated high tumor uptake within 20 min. Organ distribution studies confirmed high specific uptake in the tumor, with 30.8 ± 12.6 percentage injected dose (%ID)/g at 1 h after injection. Rapid clearance from the kidneys was observed-a decrease from 67.0 ± 20.9 %ID/g at 1 h after injection to 7.5 ± 8.51 %ID/g at 24 h after injection (in the case of CA003). The performance of CA003, the compound with the best preclinical properties, was assessed in a first patient. In line with its preclinical data, PET imaging resulted in clear visualization of the cancer lesions, with high contrast. Conclusion: The 64 Culabeled PSMA ligands are promising agents to target PSMA and visualize PSMA-positive tumor lesions as shown in preclinical evaluation by small-animal PET studies, organ distribution, and a patient application. Most importantly, the images obtained at 20 h enabled delineation of unclear lesions, showing that the compounds fulfill the prerequisite for dosimetry in the co...

An Albumin-Binding PSMA Ligand with Higher Tumor Accumulation for PET Imaging of Prostate Cancer

“…Compared with 18 F (T 1/2 , 109.77 min; β + , 96.7%), 64 Cu (T 1/2 , 12.7 h; β + , 17.4% [E max , 0.656 MeV]; β − , 39% [E max , 0.573 MeV]) was considered to be a diagnostic nuclide with its unique decay characteristics of the matched pair 64 Cu/ 67 Cu for theranostic applications. In addition, the longer half-life of 64 Cu provides an advantage for delay imaging, which may effectively monitor the uptake of radiotracers in tumors [ 28 ]. Hence, in order to match the prolonged half-life of PSMA-CM, we radiolabeled PSMA-CM with 64 Cu and evaluated 64 Cu-PSMA-CM in mice bearing 22Rv1 tumors, showing great potential for detecting prostate lesions in delayed imaging.…”

Refined Chelator Spacer Moieties Ameliorate the Pharmacokinetics of PSMA-617

“…The linker modification with the respective benzyl moiety connected to the DOTA chelator was found to further improve the tumor targeting characteristics, and in CA027, CA029, and CA030, further fine tuning was accomplished by using DOTA variants which contained reduced numbers of polar side chains. More lipophilic chelators were found promising to improve pharmacokinetics when they were introduced to realize copper (34) or lead (35) complexed PSMA ligands ( Persson et al, 2013 ; Hao et al, 2017 ; Dos Santos et al, 2019 ; Dos Santos et al, 2020 ). However, it is relevant to emphasize that the presence of the benzyl group connected directly or via a linker to the chelator can be assumed to interact with the rigid part of the PSMA binding pocket.…”

An in silico study on the effect of the radionuclide half-life on PET/CT imaging with PSMA-targeting radioligands

“…64 Cu-labelled compounds have shown superior performance in particular in patients with low PSA level [6]. In addition, there is the possibility for a theranostic application using 67 Cu [12]. Clearly, the use of imaging agents with longer half-life and high affinity would lead to a higher activity concentration in the tumour.…”