The incorporation of silicon fluoride acceptor (SiFA) moieties into a variety of molecules, such as peptides, proteins and biologically relevant small molecules, has improved the generation of 18F-radiopharmaceuticals for medical imaging. The efficient isotopic exchange radiofluorination process, in combination with the enhanced [18F]SiFA in vivo stability, make it a suitable strategy for fluorine-18 incorporation. This review will highlight the clinical applicability of [18F]SiFA-labeled compounds and discuss the significant radiotracers currently in clinical use.
Phosphodiesterase
5 (PDE5) is a clinically relevant biomarker and
therapeutic target for many human pathologies, yet a noninvasive agent
for the assessment of PDE5 expression has yet to be realized. Such
agents would improve our understanding of the nitric oxide (NO)/cyclic
guanosine 3′,5′-monophosphate (cGMP)/PDE5 pathway in
human pathologies and potentially lead to novel uses of PDE5 inhibitors
to manage lung conditions like SARS-CoV-2-mediated pulmonary inflammatory
responses. In this study, efforts were made to produce an
18
F-labeled analogue of the PDE5 inhibitor tadalafil to visualize PDE5
expression
in vivo
with positron emission tomography
(PET). However, during the late-stage fluorination step, quantitative
epimerization of the tadalafil
C
12a stereocenter
occurred, yielding a less active epi-isomer.
In vivo
dynamic microPET images in mice revealed that the epimerized radiotracer,
[
18
F]
epi-18
, rapidly accumulated in the liver
with negligible uptake in tissues of known PDE5 expression.
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