Norepinephrine transporter (NET) ligand [131/123I]MIBG (131/123I-meta-iodobenzylguanidine) is widely used for imaging neuroendocrine tumours (NETs) and sympathetic activity, especially cardiac sympathetic innervation (Verberne et al., 2008; Sisson & Yanik, 2012; Nakajima et al., 2017; Agrawal et al., 2018). Fluorine-18-labelled version, [18F]MFBG (18F-meta-fluorobenzylguanidine) (Garg et al., 1994; Hu et al., 2015) is safer, allows same-day imaging, and better image quality than what is possible with [124I]MIBG PET (Zhang et al., 2014a and 2014b; Pandit-Taskar et al., 2018).
Myocardial [123I]MIBG data is often analyzed simply by calculating heart-to-mediastinum ratio (H/M). Late-time H/M reflects overall sympathetic function, including the effects of NE uptake, storage, and release. Early H/M (15 min) is a measure of the anatomical distribution of functioning adrenergic neurons. [123I]MIBG washout rate (WOR) reflects NE retention in adrenergic neurons, and is associated with sympathetic tone (Laursen et al., 2018). Background-corrected WOR can be calculated from decay-corrected early and late data as:
[18F]MFBG is more hydrophilic than [123I]MIBG, and its plasma protein binding is lower: ∼30% of [18F]MFBG was bound to human and murine serum protein (Zhang et al., 2014a). Competitive affinity measured using glioma and glioblastoma cells was ∼3 fold lower than that of [123I]MIBG, but in in vivo imaging the xenografts were visible already 1 h p.i. while [124I]MIBG required at least 24 h waiting (Zhang et al., 2014a and 2014b). [18F]MFBG was also accumulated in the urine bladder and salivary glands of the mice (Zhang et al., 2014a).
Pandit-Taskar et al (2018) conducted the first-in-human study in patients with either recurrent neuroblastoma or progressive paraganglioma/pheochromocytome. The in vivo stability of [18F]MFBG was good: ∼90% of plasma activity at 1 and 2 h was due to authentic radiotracer. Prominent uptake in normal bone cannot be seen in the published images, indicating low defluorination. Blood SUV stayed above 1 for 4 hours. Highest uptake was seen in the salivary glands and adrenal glands. Liver and myocardial uptake was prominent in 30-60 min scan. Pancreas and thyroid were also well visible. Activity in urine was high: 45% of activity was excreted within 1 h and 61-95% within 3-4 h after administration. Uptake in the kidneys was low. Average tumour-to-normal tissue ratios were ∼6-8 already during the 30-60 min whole-body scan, and stayed at the same level until the last scan at 3-4 h. About two times more lesions were detected with [18F]MFBG PET than with [123I]MIBG SPECT.
Christensen TE, Kjaer A, Hasbak P. The clinical value of cardiac sympathetic imaging in heart failure. Clin Physiol Funct Imaging 2014; 34(3): 178-182. doi: 10.1111/cpf.12091.
Updated at: 2020-01-06
Created at: 2020-01-04
Written by: Vesa Oikonen