Synthesis and Evaluation of [18F]FEtLos and [18F]AMBF3Los as Novel 18F-Labelled Losartan Derivatives for Molecular Imaging of Angiotensin II Type 1 Receptors

Isotope Exchange-Based 18F-Labeling Methods

¹⁸F-Labeling methods for the preparation of ¹⁸F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and ¹⁸F/¹⁹F isotope exchange. Isotope exchange-based ¹⁸F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct ¹⁸F-labeling of substrates. This paper systematically reviews isotope exchange-based ¹⁸F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the ¹⁸F-product are mainly discussed for each isotope exchange-based ¹⁸F-labeling method as well as the cutting-edge application of the corresponding ¹⁸F-labeled molecular probes.

Development of 18F-Labeled Vinyl Sulfone-PSMAi Conjugates as New PET Agents for Prostate Cancer Imaging

Radiolabeled prostate-specific membrane antigen (PSMA) ligands have been rapidly adopted as part of patient care for prostate cancer. In this study, a new series of ¹⁸F-labeled PSMA-targeting agents was developed based on the high-affinity Glu-ureido-Lys scaffold and ¹⁸F-vinyl sulfones (VSs), the tumor uptake and tumor/major organ contrast of which could be tuned by pharmacokinetic linkers within the molecules. In particular, ¹⁸F-PEG₃-VS-PSMAi showed the highest tumor uptake (12.1 ± 2.2%ID/g at 0.5 h p.i.) and ¹⁸F-PEG₂-VS-PSMAi showed the highest tumor-to-liver ratio (T/L = 3.7 ± 1.0, 4.8 ± 1.2, and 6.3 ± 1.1 at 0.5, 1.5, and 3 h p.i. respectively). Significantly, compared with the FDA-approved ⁶⁸Ga-PSMA-11, the newly developed ¹⁸F-PEG₃-VS-PSMAi has an almost double tumor uptake (P < 0.0001) when tested in the same animal model. In conclusion, ¹⁸F-VS-labeled PSMA ligands are promising PET agents with prominent tumor uptake and high contrast. The lead agents ¹⁸F-PEG₂-VS-PSMAi and ¹⁸F-PEG₃-VS-PSMAi warrant further evaluation in prostate cancer patients.

Evaluation of the high affinity [18F]fluoropyridine-candesartan in rats for PET imaging of renal AT1 receptors

INTRODUCTION

Alterations in the expression of the Angiotensin II type 1 receptors (AT₁R) have been demonstrated in the development of several heart and renal diseases. The aim of this study was to evaluate the novel compound [¹⁸F]fluoropyridine-candesartan as a PET imaging tracer of AT₁R in rat kidneys.

METHODS

Competition binding assays were carried out with membranes from CHO-K1 cells expressing human AT₁R. Binding to plasma proteins was assessed by ultrafiltration. Radiolabeled metabolites in rat plasma and kidneys of control and pretreated animals (candesartan 10 mg/kg or losartan 30 mg/kg) were analyzed by column-switch HPLC. Dynamic PET/CT images of [¹⁸F]fluoropyridine-candesartan in male Sprague-Dawley rats were acquired for 60 min at baseline, pre-treatment with the AT₁R antagonist losartan (30 mg/kg) or the AT₂R antagonist PD123,319 (5 mg/kg).

RESULTS

Fluoropyridine-candesartan bound with a high affinity for AT₁R (Ki = 5.9 ± 1.1 nM), comparable to fluoropyridine-losartan but lower than the parent compound candesartan (Ki = 0.4 ± 0.1 nM). [¹⁸F]Fluoropyridine-candesartan bound strongly to plasma proteins (99.3%) and was mainly metabolized to radiolabeled hydrophilic compounds, displaying minimal interference on renal AT₁R binding with 82% of unchanged tracer in the kidneys at 20 min post-injection. PET imaging displayed high renal and liver accumulations and slow clearances, with maximum tissue-to-blood ratios of 14 ± 3 and 54 ± 12 in kidney cortex and liver, respectively, at 10 min post-injection. Binding specificity for AT₁R was demonstrated with marked reductions in kidney cortex (-84%) and liver (-93%) tissue-to-blood ratios at 20 min post-injection, when blocking with AT₁R antagonist losartan (30 mg/kg). No change was observed in kidney cortex of rats pre-treated with AT₂R antagonist PD 123,319 (5 mg/kg), confirming binding selectivity for AT₁ over AT₂ receptors.

CONCLUSION

High kidney-to-blood ratios and binding selectivity to renal AT₁R combined with tracer in vivo stability displaying minimal interference from labeled metabolites support further PET imaging studies with [¹⁸F]fluoropyridine-candesartan.