Abstract
Background18F-N-[3-bromo-4-(3-fluoro-propoxy)-benzyl]-guanidine (18F-LMI1195) is a new class of PET tracer designed for sympathetic nervous imaging of the heart. The favorable image quality with high and specific neural uptake has been previously demonstrated in animals and humans, but intracellular behavior is not yet fully understood. The aim of the present study is to verify whether it is taken up in storage vesicles and released in company with vesicle turnover.ResultsBoth vesicle-rich (PC12) and vesicle-poor (SK-N-SH) norepinephrine-expressing cell lines were used for in vitro tracer uptake studies. After 2 h of 18F-LMI1195 preloading into both cell lines, effects of stimulants for storage vesicle turnover (high concentration KCl (100 mM) or reserpine treatment) were measured at 10, 20, and 30 min. 131I-meta-iodobenzylguanidine (131I-MIBG) served as a reference. Both high concentration KCl and reserpine enhanced 18F-LMI1195 washout from PC12 cells, while tracer retention remained stable in the SK-N-SH cells. After 30 min of treatment, 18F-LMI1195 releasing index (percentage of tracer released from cells) from vesicle-rich PC12 cells achieved significant differences compared to cells without treatment condition. In contrast, such effect could not be observed using vesicle-poor SK-N-SH cell lines. Similar tracer kinetics after KCl or reserpine treatment were also observed using 131I-MIBG. In case of KCl exposure, Ca2+-free buffer with the calcium chelator, ethylenediaminetetracetic acid (EDTA), could suppress the tracer washout from PC12 cells. This finding is consistent with the tracer release being mediated by Ca2+ influx resulting from membrane depolarization.ConclusionsAnalogous to 131I-MIBG, the current in vitro tracer uptake study confirmed that 18F-LMI1195 is also stored in vesicles in PC12 cells and released along with vesicle turnover. Understanding the basic kinetics of 18F-LMI1195 at a subcellular level is important for the design of clinical imaging protocols and imaging interpretation.
Highlights
18F-N-[3-bromo-4-(3-fluoro-propoxy)-benzyl]-guanidine (18F-LMI1195) is a new class of positron emission tomography (PET) tracer designed for sympathetic nervous imaging of the heart
Treatment of PC12 cells with high concentration KCl buffer induced robust tracer depletion of both 18F-LMI1195 and 131I-MIBG in a time-dependent manner, leading to 88 ± 4% of 18F-LMI1195 and 70 ± 2% of 131I-MIBG total uptake released from cells (p < 0.001, vs. untreated controls, Fig. 1a)
Tracer depletion in PC12 cells 30 min after treatment with high concentration KCl could be overturned by using Ca2+-free buffer containing ethylenediaminetetraacetic acid (EDTA) (Fig. 2)
Summary
18F-N-[3-bromo-4-(3-fluoro-propoxy)-benzyl]-guanidine (18F-LMI1195) is a new class of PET tracer designed for sympathetic nervous imaging of the heart. The single-photon emission computed tomography (SPECT) tracer 123I-meta-iodobenzylguanidine (MIBG) targeting norepinephrine transporter (NET) is currently the most widely used clinical tracer for sympathetic nervous imaging with well-established protocols and mature. Thereby, these 18F-labeled tracers provide a unique opportunity to further enhance the development and application of PET imaging in terms of reduction of the financial burden of hospitals, flexible novel tracer design, and labeling procedure with improved stabilities [4]. A couple of 18F-labeled tracers targeting the NET are available: N-[3-bromo-4-(3-18F-fluoropropoxy]benzyl]-guanidine (18F-LMI1195) is designed for assessment of sympathetic innervation of the heart and has successfully passed through phase I clinical trial, which confirmed its tolerance in human subjects along with favorable biodistribution for cardiac imaging [5]. Our group has demonstrated that the retention of 18F-LMI1195 is resistant to desipramine chase (desipramine added after tracer delivery), which emphasizes its potential of mimicking the physiological norepinephrine turnover [11]
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