Fanny Lundmark: Design, Synthesis, and Preclinical Evaluation of Peptide-based Radioligands Targeting PSMA and GRPR in Prostate Cancer

Abstract

Prostate cancer remains the most frequently diagnosed cancer among men worldwide. Enhanced diagnostic strategies are crucial for better patient management and outcomes, ultimately increasing overall survival. Over the past decade, radionuclide-based molecular imaging has emerged as a significant advancement in prostate cancer management. This technique involves the use of radioligands—molecules labelled with radioactive isotopes—that target specific biomarkers associated with the disease. Two key biomarkers are prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) which are overexpressed in prostate cancer tissues and can be effectively targeted using radioligands. Ideal radioligands exhibit high specificity and affinity for their targets, strong retention in tumour tissues, minimal off-target uptake, rapid clearance from healthy organs, and can be synthesised and labelled efficiently. This thesis comprises four original articles focused on enhancing the diagnostic potential of peptide-based radioligands for imaging prostate cancer by optimising the targeting properties.

Papers I and II focus on the development of PSMA/GRPR-targeting heterodimeric radioligands. In particular, Paper I investigate the length and hydrophobicity of the functional linkers in indium-111 labelled radioligands for SPECT imaging. It was shown that compound BQ7812 consisting of a shorter GRPR linker in combination with a more hydrophobic PSMA linker was beneficial for the targeting properties. These findings led to Paper II which covers the preclinical evaluation of BQ7812 labelled with gallium-68 for PET imaging. PET imaging provides higher sensitivity compared to SPECT, increasing the possibility to visualise small metastases. Results confirmed [68Ga]Ga-BQ7812 as a promising radioligand for PET imaging of prostate cancer. Paper III covers a SAR study of PSMA-targeting radioligands focusing on the molecular structure of the functional linker. Modifications of the functional linker have been shown to enhance the targeting properties significantly. Radioligand BQ7859 consisting of a 2-naphthyl-L-alanine-L-tyrosine linker, demonstrated improved tumour retention and increased tumour-to-blood ratio. It was concluded that 2-naphthyl-L-alanine was crucial for high affinity while the subsequent linker position was more acceptable for structural changes and could be used for optimising targeting properties. The last study, Paper IV, explores the development of a GRPR-targeting radioligand labelled with fluorine-18 for PET imaging. Fluorine-18 exhibits favourable radionuclide properties well-suited peptide-based radioligands. This study demonstrates the efficient labelling of GRPR-targeting radioligands using the Tz-TCO IEDDA click chemistry approach. This method occurs at physiological pH and room temperature without the need for further purification of the final product, which is favourable compared to other labelling techniques. [18F]F-MeTz-PEG2-RM26 demonstrated specific binding to the target in vivo with stable retention in GRPR-targeted organs and tumours, leading to increased tumour-to-organ ratios over time.

In conclusion, the papers included in this thesis demonstrate that careful optimisation of the functional linkers in PSMA and GRPR-targeting radioligands could lead to improved targeting properties for diagnostic imaging of prostate cancer.