Introduction: Radioiodinated meta-iodobenzylguanidine (MIBG), a norepinephrine transporter (NET) substrate, has been extensively used as an imaging agent to study the pathophysiology of the heart and for the diagnosis and treatment of neuroendocrine tumors. The goal of this study was to develop an (18)F-labeled analogue of MIBG that like MIBG itself could be synthesized in a single radiochemical step. Towards this end, we designed 4-fluoropropoxy-3-iodobenzylguanidine (FPOIBG).
Methods: Standards of FPOIBG and 4-fluoropropoxy-3-bromobenzylguanidine (FPOBBG) as well as their tosylate precursors for labeling with (18)F, and a tin precursor for the preparation of radioiodinated FPOIBG were synthesized. Radiolabeled derivatives were synthesized by nucleophilic substitution and electrophilic iododestannylation from the corresponding precursors. Labeled compounds were evaluated for NET transporter recognition in in vitro assays using three NET-expressing cell lines and in biodistribution experiments in normal mice, with all studies performed in a paired-label format. Competitive inhibition of [(125)I]MIBG uptake by unlabeled benzylguanidine compounds was performed in UVW-NAT cell line to determine IC50 values.
Results: [(18)F]FPOIBG was synthesized from the corresponding tosylate precursor in 5.2 ± 0.5% (n = 6) overall radiochemical yields starting with aqueous fluoride in about 105 min. In a paired-label in vitro assay, the uptake of [(18)F]FPOIBG at 2h was 10.2 ± 1.5%, 39.6 ± 13.4%, and 13.3 ± 2.5%, in NET-expressing SK-N-SH, UVW-NAT, and SK-N-BE(2c) cells, respectively, while these values for [(125)I]MIBG were 57.3 ± 8.1%, 82.7 ± 8.9%, and 66.3 ± 3.6%. The specificity of uptake of both tracers was demonstrated by blocking with desipramine. The (125)I-labeled congener of FPOIBG gave similar results. On the other hand, [(18)F]FPOBBG, a compound recently reported in the literature, demonstrated much higher uptake, albeit less than that of co-incubated [(125)I]MIBG. IC50 values for FPOIBG were higher than those obtained for MIBG and FPOBBG. Unlike the case with [(18)F]FPOBBG, the heart uptake [(18)F]FPOIBG in normal mice was significantly lower than that of MIBG.
Conclusion: Although [(18)F]FPOIBG does not appear to warrant further consideration as an (18)F-labeled MIBG analogue, analogues wherein the iodine in it is replaced with a chlorine, fluorine or hydrogen might be worth pursuing.
Advances in knowledge and implications for patient care: An (18)F-labeled analogue of the well-known radiopharmaceutical MIBG could have significant impact, potentially improving imaging of NET related disease in cardiology and in the imaging of neuroendocrine tumors. Although (18)F-labeled analogues of MIBG have been reported including LMI1195, we undertook this work hypothesizing that based on its greater structural similarity to MIBG, FPOIBG might be a better analogue than LMI1195.
Keywords: Fluorine-18; MIBG; Neuroendocrine tumors; Norepinephrine transporter; PET imaging.
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