Preclinical Evaluation of a Tailor-Made DOTA-Conjugated PSMA Inhibitor with Optimized Linker Moiety for Imaging and Endoradiotherapy of Prostate Cancer

Abstract

Despite many advances in the past years, the treatment of metastatic prostate cancer still remains challenging. In recent years, prostate-specific membrane antigen (PSMA) inhibitors were intensively studied to develop low-molecular-weight ligands for imaging prostate cancer lesions by PET or SPECT. However, the endoradiotherapeutic use of these compounds requires optimization with regard to the radionuclide-chelating agent and the linker moiety between chelator and pharmacophore, which influence the overall pharmacokinetic properties of the resulting radioligand. In an effort to realize both detection and optimal treatment of prostate cancer, a tailor-made novel naphthyl-containing DOTA-conjugated PSMA inhibitor has been developed. Methods: The peptidomimetic structure was synthesized by solid-phase peptide chemistry and characterized using reversedphase high-performance liquid chromatography and matrix-assisted laser desorption/ionization mass spectrometry. Subsequent 67/68 Ga and 177 Lu labeling resulted in radiochemical yields of greater than 97% or greater than 99%, respectively. Competitive binding and internalization experiments were performed using the PSMA-positive LNCaP cell line. The in vivo biodistribution and dynamic small-animal PET imaging studies were investigated in BALB/c nu/nu mice bearing LNCaP xenografts. Results: The chemically modified PSMA inhibitor PSMA-617 demonstrated high radiolytic stability for at least 72 h. A high inhibition potency (equilibrium dissociation constant [K i ] 5 2.34 ± 2.94 nM on LNCaP; K i 5 0.37 ± 0.21 nM enzymatically determined) and highly efficient internalization into LNCaP cells were demonstrated. The small-animal PET measurements showed high tumor-tobackground contrasts as early as 1 h after injection. Organ distribution revealed specific uptake in LNCaP tumors and in the kidneys 1 h after injection. With regard to therapeutic use, the compound exhibited a rapid clearance from the kidneys from 113.3 ± 24.4 at 1 h to 2.13 ± 1.36 percentage injected dose per gram at 24 h. The favorable pharmacokinetics of the molecule led to tumor-to-background ratios of 1,058 (tumor to blood) and 529 (tumor to muscle), respectively, 24 h after injection. Conclusion: The tailor-made DOTA-conjugated PSMA inhibitor PSMA-617 presented here is sustainably refined and advanced with respect to its tumor-targeting and pharmacokinetic properties by systematic chemical modification of the linker region. Therefore, this radiotracer is suitable for a first-in-human theranostic application and may help to improve the clinical management of prostate cancer in the future.

Lutetium 177 PSMA radionuclide therapy for men with prostate cancer: a review of the current literature and discussion of practical aspects of therapy

“…This is a small molecule peptide, rather than an antibody, chemically conjugated with 177 Lutetium. In an initial study in mice, it has a highly efficient internalisation into prostate cancer cells with approximately 75% of the peptide bound to the cell internalised after 3 h of incubation 22. A similar small molecule PSMA peptide ending with a different chemical conjugation ( 177 Lu PSMA‐I&T) also appears effective as a therapeutic agent in a number of published studies 3, 21…”

The Theranostic PSMA Ligand PSMA-617 in the Diagnosis of Prostate Cancer by PET/CT: Biodistribution in Humans, Radiation Dosimetry, and First Evaluation of Tumor Lesions

“…The precursor compound PSMA-617 was produced at the German Cancer Research Center as recently published (24). However, the compound is commercially available from ABX advanced biochemical compounds.…”

“…In preclinical studies, tumor-to-background ratios of up to 1,058 were observed at 24 h after injection. In addition, the internalization of the PSMA-617 into the PCa cells is highly effective (internalized fraction: 17.67% 6 4.34% injected activity/10 6 LNCaP cells [PSMA-11, 9.47% 6 2.56% injected activity/10 6 LNCaP cells]) (18,24).…”

“…In preclinical studies, this ligand showed a significantly improved binding affinity to PSMA and a highly efficient internalization into PCa cells (24). PSMA-617 can be labeled with 68 Ga, 177 Lu, 111 In, and 90 Y and, therefore, be used for PET imaging as well as for radioligand-based therapy.…”

Repeated 177Lu-Labeled PSMA-617 Radioligand Therapy Using Treatment Activities of Up to 9.3 GBq

“…The Glu-urea based ligand PSMA-617 was pre-clinically optimized for low kidney uptake and improved ligand induced cellular internalization. Coupling with DOTA enables labeling with several diagnostic and therapeutic radionuclides (5). Different centers (6)(7)(8) reported favorable dosimetry for 177 Lu-PSMA-617, which in regard to kidney (approx.…”