Synthesis of [18F]2B-SRF101: A Sulfonamide Derivative of the Fluorescent Dye Sulforhodamine 101

Dissecting the neurovascular unit in physiology and Alzheimer's disease: Functions, imaging tools and genetic mouse models

The neurovascular unit (NVU) plays an essential role in regulating neurovascular coupling, which refers to the communication between neurons, glia, and vascular cells to control the supply of oxygen and nutrients in response to neural activity. Cellular elements of the NVU coordinate to establish an anatomical barrier to separate the central nervous system from the milieu of the periphery system, restricting the free movement of substances from the blood to the brain parenchyma and maintaining central nervous system homeostasis. In Alzheimer's disease, amyloid-β deposition impairs the normal functions of NVU cellular elements, thus accelerating the disease progression. Here, we aim to describe the current knowledge of the NVU cellular elements, including endothelial cells, pericytes, astrocytes, and microglia, in regulating the blood-brain barrier integrity and functions in physiology as well as alterations encountered in Alzheimer's disease. Furthermore, the NVU functions as a whole, therefore specific labeling and targeting NVU components in vivo enable us to understand the mechanism mediating cellular communication. We review approaches including commonly used fluorescent dyes, genetic mouse models, and adeno-associated virus vectors for imaging and targeting NVU cellular elements in vivo.

PET/Fluorescence Imaging: An Overview of the Chemical Strategies to Build Dual Imaging Tools

Molecular imaging is a biomedical research discipline that has quickly emerged to afford the observation, characterization, monitoring, and quantification of biomarkers and biological processes in living organism. It covers a large array of imaging techniques, each of which provides anatomical, functional, or metabolic information. Multimodality, as the combination of two or more of these techniques, has proven to be one of the best options to boost their individual properties, hence offering unprecedented tools for human health. In this review, we will focus on the combination of positron emission tomography and fluorescence imaging from the specific perspective of the chemical synthesis of dual imaging agents. Based on a detailed analysis of the literature, this review aims at giving a comprehensive overview of the chemical strategies implemented to build adequate imaging tools considering radiohalogens and radiometals as positron emitters, fluorescent dyes mostly emitting in the NIR window and all types of targeting vectors.

Biological Assessment of a 18F-Labeled Sulforhodamine 101 in a Mouse Model of Alzheimer's Disease as a Potential Astrocytosis Marker

Neurodegenerative diseases have mainly been associated with neuronal death. Recent investigations have shown that astroglia may modulate neuroinflammation in the early and late stages of the disease. [¹¹C]Deuterodeprenyl ([¹¹C]DED) is a tracer that has been used for reactive astrocyte detection in Alzheimer’s disease, Creutzfeldt–Jakob disease and amyotrophic lateral sclerosis, among others, with some limitations. To develop a new radiotracer for detecting astrocytosis and overcoming associated difficulties, we recently reported the synthesis of a sulfonamide derivative of Sulforhodamine 101 (SR101), labeled with ¹⁸F, namely SR101 N-(3-[¹⁸F]Fluoropropyl) sulfonamide ([¹⁸F]2B-SRF101). The red fluorescent dye SR101 has been used as a specific marker of astroglia in the neocortex of rodents using in vivo models. In the present work we performed a biological characterisation of the new tracer including biodistribution and micro-PET/computed tomography (CT) images. PET/CT studies with [¹¹C]DED were also done to compare with [¹⁸F]2B-SRF101 in order to assess its potential as an astrocyte marker. Biodistribution studies with [¹⁸F]2B-SRF101 were carried out in C57BL6J black and transgenic (3xTg) mice. A hepatointestinal metabolization as well as the pharmacokinetic profile were determined, showing appropriate characteristics to become a PET diagnostic agent. Dynamic PET/CT studies were carried out with [¹⁸F]2B-SRF101 and [¹¹C]DED to evaluate the distribution of both tracers in the brain. A significant difference in [¹⁸F]2B-SRF101 uptake was especially observed in the cortex and hippocampus, and it was higher in 3xTg mice than it was in the control group. These results suggested that [¹⁸F]2B-SRF101 is a promising candidate for more extensive evaluation as an astrocyte tracer. The difference observed for [¹⁸F]2B-SRF101 was not found in the case of [¹¹C]DED. The comparative studies between [¹⁸F]2B-SRF101 and [¹¹C]DED suggest that both tracers have different roles as astrocytosis markers in this animal model, and could provide different and complementary information at the same time. In this way, by means of a multitracer approach, useful information could be obtained for the staging of the disease.

Dosimetry and Toxicity Studies of the Novel Sulfonamide Derivative of Sulforhodamine 101([18F]SRF101) at a Preclinical Level

BACKGROUND

The SR101 N-(3-[18F]Fluoropropyl) sulfonamide ([18F]SRF101) is a Sulforhodamine 101 derivative that was previously synthesised by our group. The fluorescent dye SR101 has been reported as a marker of astroglia in the neocortex of rodents in vivo.

OBJECTIVE

The aim of this study was to perform a toxicological evaluation of [18F]SRF101 and to estimate human radiation dosimetry based on preclinical studies.

METHODS

Radiation dosimetry studies were conducted based on biokinetic data obtained from a mouse model. A single-dose toxicity study was carried out. The toxicological limit chosen was <100 μg, and allometric scaling with a safety factor of 100 for unlabelled SRF101 was selected.

RESULTS

The absorbed and effective dose estimated using OLINDA/EXM V2.0 for male and female dosimetric models presented the same tendency. The highest total absorbed dose values were for different sections of the intestines. The mean effective dose was 4.03 x10-3 mSv/MBq and 5.08 x10-3 mSv/MBq for the male and female dosimetric models, respectively, using tissue-weighting factors from ICRP-89. The toxicity study detected no changes in the organ or whole-body weight, food consumption, haematologic or clinical chemistry parameters. Moreover, lesions or abnormalities were not found during the histopathological examination.

CONCLUSION

The toxicological evaluation of SRF101 verified the biosafety of the radiotracer for human administration. The dosimetry calculations revealed that the radiation-associated risk of [18F]SRF101 would be of the same order as other 18F radiopharmaceuticals used in clinical applications. These study findings confirm that the novel radiotracer would be safe for use in human PET imaging.