A radioiodinated FR-β-targeted tracer with improved pharmacokinetics through modification with an albumin binder for imaging of macrophages in AS and NAFL

Purpose The formation of advanced plaques, which is characterized by the uninterrupted aggregation of macrophages with high expression of folate receptor-beta (FR-beta), is observed in several concomitant metabolic syndromes. The objective of this study was to develop a novel FR-beta-targeted single-photon emission computed tomography (SPECT) radiotracer and validate its application to the noninvasive detection of atherosclerosis (AS) plaque and non-alcoholic fatty liver (NAFL). Methods Two radioiodinated probes, [I-131]IPBF and [I-131]IBF, were developed, and cell uptake studies were used to identify their specific targets for activated macrophages. Biodistribution in normal mice was performed to obtain the pharmacokinetic information of the probes. Apolipoprotein E knockout (ApoE(-/-)) mice with atherosclerotic aortas were induced by a high-fat and high-cholesterol (HFHC) diet. To investigate the affinity of radiotracers to FR-beta, K-d values were determined using in vitro assays. In addition, the assessments of the aorta in the ApoE(-/-) mice at different stages were performed using in vivo SPECT/CT imaging, and the findings were compared by histology. Results Both [I-131]IPBF and [I-131]IBF were synthesized with > 95% radiochemical purity and up to 3 MBq/nmol molar activity. In vitro assay of [I-131]IPBF showed a moderate binding affinity to plasma proteins and specific uptake in activated macrophages. The prolonged blood elimination half-life (t(1/2z)) of [I-131]IPBF (8.14 h) was observed in a pharmacokinetic study of normal mice, which was significantly longer than that of [I-131]IBF (t(1/2z) = 2.95 h). As expected, the K-d values of [I-131]IPBF and [I-131]IBF in the Raw 264.7 cells were 43.94 +/- 9.83 nM and 61.69 +/- 15.19 nM, respectively. SPECT imaging with [I-131]IPBF showed a high uptake in advanced plaques and NAFL. Radioactivity in excised aortas examined by ex vivo autoradiography further confirmed the specific uptake of [I-131]IPBF in high-risk AS plaques. Conclusions In summary, we reported a proof-of-concept study of an albumin-binding folate derivative for macrophage imaging. The FR-beta-targeted probe, [I-131]IPBF, significantly prolongs the plasma elimination half-life and has the potential for the monitoring of AS plaques and concomitant fatty liver.

Automated summary

A novel FR-beta-targeted radiotracer was developed and shown to have potential for noninvasive detection of atherosclerosis and non-alcoholic fatty liver. Cell uptake studies and animal experiments demonstrated the high affinity and signal specificity of the tracer, suggesting its utility for imaging macrophages in advanced plaques.