18F-Sulforhodamine 101 derivative as a potential agent for astrocytosis diagnosis: labelling and preliminary biological studies

Objectives Alzheimer´s disease (AD) is the most common form of dementia in the aging population. It is a complex disease that affects many functions and is characterized by a progressive impairment of cognitive abilities. The pathological features of AD include neuritic plaques composed of amyloid-β peptide fibrils, neurofibrillary tangles of hyperphosphorylated tau, and neurotransmitter deficits. As the widespread cellular degeneration and the neuronal loss in AD progress, the astrocytes become reactive, surrounding amyloid plaques. New astrocytosis tracers could contribute to the AD diagnosis and eventually, to the evaluation of treatments. The aim of this work is the development of new PET radiotracers for astrocytosis detection, using 11C- Deuterodeprenyl (11C-DED) as “gold standard”. For this purpose Sulforhodamine 101 (SR101), a specific marker of astroglia, was labelled with 18F and biologic evaluation was performed.

Methods Chemical derivatization of the SR101 sulfonate group was performed, reacting SR101 sulfonyl chloride with 3-bromopropylamine, which results in the formation of two derivatives of SR101 N-(3-Bromopropyl)-sulfonamide. The synthetic process was carried out in the automated synthetic platform GE TRACERlab® FX-FN, using 18F-fluoride (solubilized in H218O) produced in cyclotron (GE PETtrace 16.5MeV) via the nuclear reaction 18O(p,n)18F. In order to label with 18F the two sulfonamide isomers, different labelling conditions were tested, varying precursor mass, solvent, reaction time and temperature. Crude product was purified and formulated in physiological saline solution. Quality control analyses included appearance of solution, pH, residual solvents control (GC), chemical and radiochemical purity control (HPLC), radionucleidic purity and identity. Biological characterization of 18F-labelled SR101 sulfonamide was performed with the following studies: a) dynamic PET/CT scans in healthy Black C57BL6J mice and in a transgenic mice model of AD (9 months old), in the preclinical Triumph Tri-modality Scanner (Gamma Medica, Inc.); b) biodistribution studies in the same mice model at 10, 30, 60 and 90 min. PET/CT studies were analyzed and quantified using the PMOD software. Different brain regions were normalized against cerebellum for the uptake quantification: striatum, cortex, hippocampus, hipothalamus, thalamus, amygdala, olfactory bulb and midbrain.

Results Labelling with 18F was achievable only with one of the isomers and the optimum condition was 1 mg of precursor, DMSO as solvent, 10 min of reaction time and heating at 160ºC. With this condition SR101 N-(3-18F-Fluropropyl)-sulfonamide was obtained with radiochemical purity of (97.1 ± 0.8) % and in accordance with the established quality control specifications. PET/CT studies showed a higher uptake in all the analyzed brain regions in transgenic mice compared to control mice with the same age. Biodistribution studies showed a hepatobiliary metabolization of the labelled compound.

Conclusions A SR101 sulfonamide derivative was labelled with 18F, purified and formulated, obtaining a radiotracer suitable to perform biological studies. Preliminary biological data showed that the compound could be useful to detect pathological processes, such as actrocytosis, that take place in different regions of the brain depending on the disease.