Heterodimeric Radiotracer Targeting PSMA and GRPR for Imaging of Prostate Cancer-Optimization of the Affinity towards PSMA by Linker Modification in Murine Model.

Fanny Lundmark,Mats Larhed,Zohreh Varasteh,Sara S Rinne,Anna Orlova,Ulrika Rosenström,Vladimir Tolmachev,Bogdan Mitran,Ayman Abouzayed

doi:10.3390/pharmaceutics12070614

Abstract

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [111In]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [111In]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.

Highlights

Prostate cancer (PCa) remains one of the most frequently diagnosed cancers worldwide, with almost 1.3 million new cases in 2018 [1]
Identity was confirmed by liquid chromatography-mass spectrometry (LC-MS); calculated [M + H]1+ 489.27, observed [M + H]1+ 489.2 (Figure S3) and nuclear magnetic resonance (NMR) spectroscopy; 1H NMR (400 MHz, Chloroform-d) δ 5.86 (d, 1H, NH), 5.43 (d, 1H, NH), 4.48–4.40 (m, 1H, CH), 4.36–4.29 (m, 1H, CH), 2.44–2.38 (m, 2H, CH2), 2.37–2.26 (m, 2H, CH2), 2.19–2.06 (m, 2H, CH2), 1.93–1.81 (m, 2H, CH2), 1.48 (s, 9H, CH3), 1.46 (s, 9H, CH3), 1.43 (s, 9H, CH3) (Figure S4). 13C NMR (101 MHz, Chloroform-d) δ 175.6, 172.7, 171.8, 158.1, 82.9, 82.4, 53.6, 53.3, 31.7, 30.8, 28.5, 28.4, 28.2, 28.2, 28.1 (Figure S5)
The bispecific Prostate-specific membrane antigen (PSMA)/gastrin-releasing peptide receptor (GRPR)-targeting heterodimers BQ7810, BQ7812 and BQ7813 were successfully synthesized by manual solid-phase peptide synthesis (SPPS) and purified using reversed-phase high performance liquid chromatography (RP-HPLC)

Summary

Introduction

Prostate cancer (PCa) remains one of the most frequently diagnosed cancers worldwide, with almost 1.3 million new cases in 2018 [1]. Since the choice of patient management is determined by the stage of PCa, accurate diagnosis is crucial for effective and successful treatment of the patient [3,4,5]. New approaches such as molecular imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT) have gained a lot of focus since they are non-invasive and repetitive methods with high potential to improve diagnostic accuracy

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Conclusion