Hybrid Multimodal Imaging Synthons for Chemoselective and Efficient Biomolecule Modification with Chelator and Near-Infrared Fluorescent Cyanine Dye

Ralph Hübner,Carmen Wängler,Ralf Schirrmacher,Vanessa Benkert,Björn Wängler,Roland Krämer,Valeska Von Kiedrowski

doi:10.3390/ph13090250

Abstract

The development of hybrid multimodal imaging synthons (MIS), carrying in addition to a chelator for radiometal labeling also a near-infrared (NIR) fluorescent cyanine dye was the aim of this work. The MIS should be introducible into biomolecules of choice via an efficient and chemoselective click chemistry reaction. After chemical optimization, a successful synthetic strategy towards such hybrid MIS was developed, based on solid phase-based synthesis techniques and applying different near-infrared fluorescent cyanine dyes. The developed hybrid agents were shown to be easily introducible into a model homobivalent peptidic gastrin-releasing peptide receptor- (GRPR)-specific carrier without forming any side products and the MIS as well as their bioconjugates were radiolabeled with the positron-emitter 68Ga3+. The hybrid multimodal agents were characterized with regard to their logDs, GRPR target affinities and photophysical characteristics. It could be shown that the properties of the bioconjugates were not per se affected by the introduction of the MIS but that the cyanine dye used and specifically the number of comprised negative charges per dye molecule can have a considerable influence on target receptor binding. Thus, the molecular toolbox described here enables the synthesis of tailored hybrid multimodal imaging synthons for biomolecule modification, meeting the specific need and envisioned application of the combined imaging agent.

Highlights

Multimodal imaging for medical diagnosis is beneficial as it enables the confirmation of results obtained from one imaging modality by another method, providing complementary imaging results [1].If the strengths of individual modalities are combined, it allows for synergistic imaging thereby enhancing the accuracy of diagnosis in a shorter time period [2].Nowadays, combinations of imaging modalities are widespread in clinical diagnostic imaging such as positron emission tomography/computed tomography (PET/CT), which has been used since the early 2000s to provide both morphological and metabolic information
Combinations of imaging modalities are widespread in clinical diagnostic imaging such as positron emission tomography/computed tomography (PET/CT), which has been used since the early 2000s to provide both morphological and metabolic information
When combining PET and NIR-optical imaging (OI), the PET-radionuclide in the molecular probe defines the synthetic strategy, because radiolabeling will always be the last step for molecules prepared for diagnostic PET imaging [7]

Summary

Introduction

Multimodal imaging for medical diagnosis is beneficial as it enables the confirmation of results obtained from one imaging modality by another method, providing complementary imaging results [1].If the strengths of individual modalities are combined, it allows for synergistic imaging thereby enhancing the accuracy of diagnosis in a shorter time period [2].Nowadays, combinations of imaging modalities are widespread in clinical diagnostic imaging such as positron emission tomography/computed tomography (PET/CT), which has been used since the early 2000s to provide both morphological and metabolic information. In addition to the design and synthesis of the hybrid multimodal imaging synthons, their introduction into biomolecules is of interest to determine the efficiency of conjugation and their general applicability for biomolecule modification. We aimed to demonstrate the introduction of the developed MIS 10–12 into a model bioactive compound to show their applicability for the synthesis of multimodal target-specific imaging agents. As the hybrid multimodal imaging synthons are primarily intended to be used for the synthesis of tumor-specific imaging agents, a tumor targeting vector was chosen as a model biomolecule for MIS conjugation. Mainly peptides and antibodies are used as they exhibit a high target affinity and specificity Antibodies with their large size and complex molecular structure usually tolerate chemical modifications without considerably decreased target affinity. A peptide-based carrier seemed to be a reasonable choice for a model biomolecule to study MIS conjugation and determination of the influence of the MIS on the chemical, biological and photophysical characteristics of the resulting conjugates

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