Radiosynthesis and preclinical evaluation of [18F]FEM as a potential novel PET probe for tumor imaging

China’s radiopharmaceuticals on expressway: 2014–2021

This review provides an essential overview on the progress of rapidly-developing China’s radiopharmaceuticals in recent years (2014–2021). Our discussion reflects on efforts to develop potential, preclinical, and in-clinical radiopharmaceuticals including the following areas: (1) brain imaging agents, (2) cardiovascular imaging agents, (3) infection and inflammation imaging agents, (4) tumor radiopharmaceuticals, and (5) boron delivery agents (a class of radiopharmaceutical prodrug) for neutron capture therapy. Especially, the progress in basic research, including new radiolabeling methodology, is highlighted from a standpoint of radiopharmaceutical chemistry. Meanwhile, we briefly reflect on the recent major events related to radiopharmaceuticals along with the distribution of major R&D forces (universities, institutions, facilities, and companies), clinical study status, and national regulatory supports. We conclude with a brief commentary on remaining limitations and emerging opportunities for China’s radiopharmaceuticals.

Radioiodinated Ethinylestradiol Derivatives for Estrogen Receptor Targeting Breast Cancer Imaging

Two novel PEGylated ethinylestradiol (PEG = poly(ethylene glycol)) estrogen receptor (ER) targeting probes [¹³¹I]EITE and [¹³¹I]MITE were synthesized and evaluated. Both probes had a nanomolar binding affinity to the ER receptor (36.47 nM for [¹³¹I]EITE and 61.83 nM for [¹³¹I]MITE). They showed high uptake in ER-positive MCF-7 cells and tumors, which could be significantly blocked by a coinjection of excess estradiol. Their ER specificities were further demonstrated by the low uptake in ER-negative MDA-MB-231 cells and tumors. The maximum tumor-to-muscle (T/M) ratios reach to 6.59 for [¹³¹I]EITE at 1 h postinjection (p.i.) and to 3.69 for [¹³¹I]MITE at 2 h p.i. in MCF-7 tumors. Among these two probes, [¹³¹I]EITE showed a faster tumor accumulation and a higher T/M ratio indicating it could be a better candidate for the potential diagnosis of ER-positive breast cancers.

Novel [18F]-Labeled Meta-Bromobenzylguanidine Derivatives: Potential Positron Emission Tomography Imaging Probes for the Norepinephrine Transporter

To develop novel norepinephrine transporter (NET)-targeting positron emission tomography (PET) probes with optimal pharmacokinetic properties, a series of meta-bromobenzylguanidine derivatives was synthesized. 4-Fluorodiethoxyethane-3-bromobenzylguanidine (compound 12) showed relatively good affinity for the NET (IC50 = 1.00 ± 0.04 μM). The corresponding radiotracer 18F-12 was prepared in high radiochemical purity (>98%) via a three-step method. The in vitro cellular uptake results demonstrated that 18F-12 was specifically taken up by NET-expressing SK-N-SH cells by the uptake-1 mechanism. Biodistribution studies in mice showed that 18F-12 exhibited high cardiac uptake (10.45 ± 0.66 %ID/g at 5 min p.i. and 6.44 ± 0.40 %ID/g at 120 min p.i.), faster liver clearance, and a lower dose of absorbed radiation than [123I]-labeled meta-iodobenzylguanidine ([123I]MIBG). Small animal PET imaging confirmed the high heart-to-background ratio of 18F-12 and the uptake-1 mechanism specific for the NET in rats, indicating its potential as a promising PET radiotracer for cardiac sympathetic nerve imaging.