Optimization of solid phase extraction (SPE) in preparation of D3-[18F]FSP: A new PET imaging agent for mapping Aβ plaques (2019)

485 Introduction: Accumulation of β-amyloid (Aβ) plaques in the brain is considered one of the major biomarkers of Alzheimer9s disease (AD). The first FDA approved Aβ imaging probe [18F]AV-45 (florbetapir F18, Amyvid) provides pathological information otherwise not easily accessible. One of the major metabolites of AV-45 is the N-demethylated product, which shows a lower binding affinity towards Aβ and thus increases the non-specific binding in the brain. In an attempt to reduce this metabolite in vivo by taking advantage of employing more stable C-D bonds, a deuterated N-D3-methyl derivative of AV-45, D3-[18F]FSP, was developed to improve Aβ selectivity and image contrast. In developing a convenient preparation method for clinical trials of D3-[18F]FSP, we report herein optimization of radiosynthesis using solid phase extraction (SPE) purification, instead of preparative high-performance liquid chromatography (HPLC) purification.
 Methods: The preparation of D3-[18F]FSP was evaluated under different fluorination conditions (temperature, time and solvent), using different deprotection acid solutions and SPE purification methods (cartridges and elution). All but a small amount of the pseudo carrier, the corresponding OH derivative, could be removed by this unique SPE purification. In vitro binding assays were performed to measure the binding affinity to Aβ plaques of this pseudo-carrier. In vitro autoradiography studies were employed to compare HPLC-purified and SPE-purified D3-[18F]FSP doses using post mortem AD brain sections.
 Results: Radiofluorination was successfully accomplished under optimized conditions (DMSO, 130 oC and heating for 10 minutes). The yield of the intermediate, D3-[18F]Boc-FSP, was 73.9 ± 14.3% as determined by HPLC of the crude product. Deprotection was successfully carried out with a 3 M HCl/H2O solution at 100 oC for 5 min. A fast and simple SPE-purification method using combinations of solvents was developed to reduce all chemical impurities. Total amount of chemical impurities (OH-pseudo carrier and D3-FSP) was 2.1 ± 0.5 μg/mL as determined by analytical HPLC, using UV absorption at λ = 350 nm and the desired product D3-[18F]FSP was obtained in 44.4 ± 5.7% radiochemical yield (decay corrected). The radiochemical purity (RCP) was >95%, and the whole process was accomplished in 50 min using radioactive levels ranging from 18 - 444 MBq. In vitro binding assay showed that the OH-pseudo carrier displayed slightly lower binding affinity to Aβ aggregates (IC50 = 42.6 ± 1.1 nM) than AV-45 (IC50 = 18.8 ± 1.1 nM) and D3-FSP (IC50 = 21.4 ± 1.1 nM). However, in vitro autoradiography studies suggested that the concentrations of the residual chemical impurities in SPE-purified D3-[18F]FSP doses were not sufficient to saturate the Aβ plaque-binding sites in the brain.
 Conclusions: An optimized radiosynthesis and fast SPE purification method for D3-[18F]FSP were achieved. The results of quality control tests, binding assay and autoradiography studies suggest that SPE-purified doses of D3-[18F]FSP are suitable for human clinical studies.