Synthesis and Characterization of a "Clickable" PBR28 TSPO-Selective Ligand Derivative Suitable for the Functionalization of Biodegradable Polymer Nanoparticles.

Renato Auriemma,Davide Moscatelli,Alessandro Sacchetti,Marco Peviani,Mattia Sponchioni,Umberto Capasso Palmiero,Arianna Rossetti,Giacomo Rossino,Andrea Marsala,Simona Collina

doi:10.3390/nano11071693

Abstract

Reactive microgliosis is a pathological hallmark that accompanies neuronal demise in many neurodegenerative diseases, ranging from acute brain/spinal cord injuries to chronic diseases, such as amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD) and age-related dementia. One strategy to assess and monitor microgliosis is to use positron emission tomography (PET) by exploiting radioligands selective for the 18 kDa translocator protein (TSPO) which is highly upregulated in the brain in pathological conditions. Several TSPO ligands have been developed and validated, so far. Among these, PBR28 has been widely adopted for PET imaging at both preclinical and clinical levels, thanks to its high brain penetration and high selectivity. For this reason, PBR28 represents a good candidate for functionalization strategies, where this ligand could be exploited to drive selective targeting of TSPO-expressing cells. Since the PBR28 structure lacks functional moieties that could be exploited for derivatization, in this work we explored a synthetic pathway for the synthesis of a PBR28 derivative carrying an alkyne group (PBR-alkyne), enabling the fast conjugation of the ligand through azide-alkyne cycloaddition, also known as click-chemistry. As a proof of concept, we demonstrated in silico that the derivatized PBR28 ligand maintains the capability to fit into the TSPO binding pocked, and we successfully exploited PBR-alkyne to decorate zwitterionic biodegradable polymer nanoparticles (NPs) resulting in efficient internalization in cultured microglia-like cell lines.

Highlights

Assessment of microglial activation in neurodegenerative conditions can be performed through neuroimaging of the 18 kDa translocator protein (TSPO) using selective TSPO radioligands which allow the exploration of microgliosis in neurological disorders [1,2,3].TSPO, formerly named peripheral benzodiazepine receptor [4,5], is part of a multimeric protein complex associated with the outer mitochondrial membrane
To confirm that the triazole formed upon reaction of the alkyne moiety with the azide group does not interfere with binding with the receptor, we adopted an in silico approach using the TSPO crystal structure in complex with PK11195 (PDB ID: 4RYI) as a test platform to perform docking studies
This work, we successfully added of a functional moiety tosuitable the high-affinity ligandInPBR28 allowing for the generation a PBR28-derivative for conjugation ligand

Summary

Introduction

TSPO, formerly named peripheral benzodiazepine receptor [4,5], is part of a multimeric protein complex associated with the outer mitochondrial membrane. Nanomaterials 2021, 11, 1693 expressed in the adrenals, kidneys, lungs and spleen. It is hardly detectable in the healthy brain whereas it becomes overexpressed in activated glial cells. The localization of TSPO in these cell types is still debated. Increased expression of TSPO was reported in several cancer cell lines as well as in tumor-associated macrophages, suggesting that TSPO could play a role in tumorigenesis. These observations prompted the attempts to exploit

Methods

Results

Conclusion