Development of fibroblast activation protein-α radiopharmaceuticals: Recent advances and perspectives.

Fibroblast activation protein-α (FAP) has emerged as a promising target in the field of radiopharmaceuticals due to its selective expression in cancer-associated fibroblasts (CAFs) and other pathological conditions involving fibrosis and inflammation. Recent advancements have focused on developing FAP-specific radioligands for diagnostic imaging and targeted radionuclide therapy. This perspective summarized the latest progress in FAP radiopharmaceutical development, highlighting novel radioligands, preclinical evaluations, and potential clinical applications. Additionally, we analyzed the advantages and existing problems of targeted FAP radiopharmaceuticals, and discussed the key breakthrough directions of this target, so as to improve the development and conversion of FAP-targeted radiopharmaceuticals.

Design, Synthesis, and Evaluation of [18F]BIBD-300 as a Positron Emission Tomography Tracer for Poly(ADP-Ribose) Polymerase-1.

Compounds 8a-j were designed to adjust the mode of interaction and lipophilicity of FTT by scaffold hopping and changing the length of the alkoxy groups. Compounds 8a, 8d, 8g, and BIBD-300 were screened for high-affinity PARP-1 through enzyme inhibition assays and are worthy of further evaluation. PET imaging of MCF-7 subcutaneous tumors with moderate expression of PARP-1 showed that compared to [18F]FTT, [18F]8a, [18F]8d, and [18F]8g exhibited greater nonspecific uptake, a lower target-to-nontarget ratio, and severe defluorination, while [18F]BIBD-300 exhibited lower nonspecific uptake and a greater target-to-nontarget ratio. PET imaging of 22Rv1 subcutaneous tumors, which highly express PARP-1, confirmed that the uptake of [18F]BIBD-300 in normal organs, such as the liver, muscle, and bone, was lower than that of [18F]FTT, and the ratio of tumor-to-muscle and tumor-to-liver [18F]BIBD-300 was greater than that of [18F]FTT. The biodistribution results in mice with MCF-7 and 22Rv1 subcutaneous tumors further validated the results of PET imaging. Unlike [18F]FTT, which mainly relies on hepatobiliary clearance, [18F]BIBD-300, which has lower lipophilicity, undergoes a partial shift from hepatobiliary to renal clearance, providing the possibility for [18F]BIBD-300 to indicate liver cancer. The difference in the PET imaging results for [18F]FTT, [18F]BIBD-300, and [18F]8j in 22Rv1 mice and the corresponding molecular docking results further confirmed that subtle structural modifications in lipophilicity greatly optimize the properties of the tracer. Cell uptake experiments also demonstrated that [18F]BIBD-300 has a high affinity for PARP-1. Metabolized and unmetabolized [18F]FTT and [18F]BIBD-300 were detected in the brain, indicating that they could not accurately quantify the amount of PARP-1 in the brain. However, PET imaging of glioma showed that both [18F]FTT and [18F]BIBD-300 could accurately localize both in situ to C6 and U87MG tumors. Based on its potential advantages in the diagnosis of breast cancer, prostate cancer, and glioma, as well as liver cancer, [18F]BIBD-300 is a new option for an excellent PARP-1 tracer.

18F-Labeled PET Tracers Specific for Adenosine A2A Receptor: Design, Synthesis, and Biological Evaluation.

By modifying the structures of targeted A2AR antagonists and tracers, novel compounds 3, 7a, 9, 12c, and BIBD-399 were designed and synthesized. In vitro inhibition experiments demonstrated that 3, 12c, and BIBD-399 have high affinity for A2AR. [18F]3 and [18F]BIBD-399 were successfully synthesized. In terms of biological distribution, the brain uptake of [18F]MNI-444 exhibits greater than that of [18F]3 and [18F]BIBD-399. PET imaging shows that [18F]3 is off-target in the brain, while [18F]BIBD-399 and [18F]MNI-444 can be specifically imaged in regions with high A2AR expression. Differently, [18F]BIBD-399 could quickly reach equilibrium in the targeted region within 10 min after administration, while [18F]MNI-444 shows a slowly increasing trend within 2 h of administration. [18F]BIBD-399 is mainly metabolized by the liver and kidney, and there is no obvious defluorination in vivo. Additional in vitro autoradiography showed that the striatal signals of [18F]BIBD-399 and [18F]MNI-444 were inhibited by the A2AR antagonist SCH442416 but not by the A1R antagonist DPCPX, demonstrating the high A2AR binding specificity of [18F]BIBD-399. Molecular docking further confirms the high affinity of MNI-444 and BIBD-399 for A2AR. Further tMCAo imaging showed that [18F]BIBD-399 can sensitively distinguish between infarcted and noninfarcted sides, a capability not observed with [18F]MNI-444. Given its pharmacokinetic properties and the ability to identify lesion regions, [18F]BIBD-399 has potential advantages in monitoring A2AR changes, meriting further clinical investigation.

Design of Novel 18F-Labeled Amino Acid Tracers Using Sulfur 18F-Fluoride Exchange Click Chemistry.

[18F]Gln-OSO2F, [18F]Arg-OSO2F, and [18F]FSY-OSO2F were designed by introducing sulfonyl 18F-fluoride onto glutamine, arginine, and tyrosine, respectively. [18F]FSY-OSO2F can be prepared directly by sulfur 18F-fluoride exchange, while [18F]Gln-OSO2F and [18F]Arg-OSO2F require a two-step labeling method. Those tracers retain their typical transport characteristics for unmodified amino acids. Both PET imaging and biodistribution confirmed that [18F]FSY-OSO2F visualized MCF-7 and 22Rv1 subcutaneous tumors with high contrast, and its tumor-to-muscle ratio was better than that of [18F]FET. However, [18F]Gln-OSO2F and [18F]Arg-OSO2F poorly image MCF-7 subcutaneous tumors, possibly due to differences in the types and amounts of transporters expressed in tumors. All three tracers can visualize the U87MG glioma. According to our biological evaluation, none of the tracers evaluated in this study exhibited obvious defluorination, and subtle structural changes led to different imaging characteristics, indicating that the application of sulfur 18F-fluoride exchange click chemistry in the design of radioactive sulfonyl fluoride amino acids is feasible and offers significant advantages.

Design, Synthesis, and Evaluation of 18F-Labeled Tracers Targeting Fibroblast Activation Protein for Brain Imaging.

Fibroblast activation protein (FAP) is closely related to central nervous system diseases such as stroke and brain tumors, but PET tracers that can be used for brain imaging have not been reported. Here, we designed, synthesized, and evaluated 18F-labeled UAMC1110 derivatives suitable for brain imaging targeting FAP. By substituting the F atom for the H atom on the aromatic ring of compound UAMC1110, 1a-c were designed and prepared. 1a-c were confirmed to have a high affinity for FAP through molecular docking and enzyme assay. [18F]1a-c were successfully prepared and confirmed to have high affinity. The stability in vivo indicates that no obvious metabolites of [18F]1a,b were found in the plasma 1 h after injection, which is beneficial for brain imaging. In vitro cell uptake experiments showed that [18F]1a,b and [68Ga]FAPI04 exhibited similar uptake and internalization rates. PET imaging of U87MG subcutaneous tumor showed that [18F]1a,b could penetrate the blood-brain barrier with higher uptake and longer retention time than [68Ga]FAPI04 (uptake at 62.5 min, 1.06 ± 0.23, 1.09 ± 0.25% ID/g vs 0.21 ± 0.10% ID/g, respectively). The brain-to-blood ratios of [18F]1a,b were better than [68Ga]FAPI04. Biodistribution and PET imaging showed that [18F]1a had better uptake on tumors and a higher tumor-to-muscle ratio than [18F]1b and [68Ga]FAPI04. Further imaging of U87MG intracranial glioma showed that [18F]1a outlined high-contrast gliomas in a short period of time compared to [18F]1b. Therefore, [18F]1a is expected to be useful in the diagnosis of FAP-related brain diseases.

Synthesis and In Vitro and In Vivo Evaluation of 18F-Labeled Positron Emission Tomography Tracers for Imaging Aß Plaques.

Accurate quantification of amyloid beta (Aß) plaques is an important indicator for Alzheimer's disease diagnosis and treatment. For this purpose, new highly sensitive Aß tracers were designed by regulating the position and number of nitrogen atoms. A series of derivatives of florbetapir (AV45) containing different numbers and positions of N atoms were synthesized and evaluated for in vitro affinity and in vivo biodistribution. Preliminary study results showed that [18F]BIBD-124 and [18F]BIBD-127 had better clearance rates and less in vivo defluorination than AV45 in ICR (ICR = Institute of Cancer Research) mice. Autoradiography and molecular docking indicated that the binding sites of [18F]BIBD-124/127 were similar to that of [18F]AV45. Micro-positron emission tomography-computed tomography imaging further demonstrated that [18F]BIBD-124 could monitor Aß plaques similar to [18F]AV45. Besides, the imaging contrast of [18F]BIBD-124 is better than that of [18F]AV45. Mass spectrometric metabolic analysis showed that BIBD-124 was less demethylated than AV45 without subsequent acetylation, which might explain its less non-specific uptake and higher imaging contrast. Gauss calculations further confirmed that the introduction of N5 in [18F]BIBD-124 decreased demethylation. Considering imaging contrast and in vivo defluorination, [18F]BIBD-124 is expected to be a promising radiotracer of Aß plaques for further clinical trials.

[18F]BIBD-239: 18F-Labeled ER176, a Positron Emission Tomography Tracer Specific for the Translocator Protein.

[11C]ER176 has adequate sensitivity to image the human brain translocator protein (TSPO) in all three genotypes by positron emission tomography (PET). However, its clinical application is limited by the short half-life of 11C (20.38 min). To overcome the deficiency of [11C]ER176 and keep the pharmacophore features of ER176 to the maximum extent, we designed four fluorine-labeled ER176 derivatives using the deuterium method. In vitro competition binding confirmed that the designed compounds had high affinity for TSPO. Biodistribution experiments showed that tissues with high expression of TSPO had high uptake of these compounds, as well as that the compound showed high brain penetration and mild defluorination in vivo. Therefore, [18F]BIBD-239 with simple synthesis conditions was selected for further biological evaluation. Theoretical simulations showed that BIBD-239 and ER176 have similar binding modes and sites to Ala147-TSPO and Thr147-TSPO, which indicated that the tracers may have consistent sensitivity to the three affinity genotypes. In vitro autoradiography and in vivo PET studies of the ischemic rat brain showed dramatically higher uptake of [18F]BIBD-239 on the lesion site compared to the contralateral side with good brain kinetics. Additionally, [18F]BIBD-239 provided clear tumor PET images in a GL261 glioma model. Importantly, PET imaging and liquid chromatography-high-resolution mass spectrometry (LC-HRMS) results showed that in vivo defluorination and other metabolites of [18F]BIBD-239 did not interfere with brain imaging. Conclusively, [18F]BIBD-239, similar to ER176 with low polymorphism sensitivity, has simple labeling conditions, high labeling yield, high affinity, and high specificity for TSPO, and it is planned for further evaluation in higher species.

Novel 18F-Labeled PET Tracers Specific to Aromatase: Design, Synthesis, and Biological Evaluation.

The abnormal expression of aromatase is associated with the occurrence and development of a variety of neurological diseases and tumors. A series of 18F-labeled and 68Ga-labeled potential aromatase-binding candidate compounds were designed and synthesized based on the structures of aromatase inhibitors. Competitive inhibition experiments in vitro and molecular docking showed that BIBD-069 and BIBD-071 have high affinity for aromatase. The radiolabeling conditions of [18F]BIBD-069 and [18F]BIBD-071 were simple, and the yields were high. Biodistribution and in vivo inhibition experiments confirmed that [18F]BIBD-069 and [18F]BIBD-071 specifically bind to aromatase. [18F]BIBD-069 and [18F]BIBD-071 selectively imaged the amygdala and nucleus of the stria terminalis, which is similar to the imaging result of [11C]vorozole. Radiometabolites of [18F]BIBD-069 and [18F]BIBD-071 did not bind to aromatase and interfered with brain imaging. MicroPET-CT imaging further confirmed that [18F]BIBD-069 and [18F]BIBD-071 can specifically bind to aromatase and were not defluorinated in vivo. Given that [18F]BIBD-069 and [18F]BIBD-071 exhibit excellent aromatase binding affinities, mild radiolabeling conditions, and good pharmacokinetics, they can be important tools for the diagnosis and treatment of aromatase-related diseases.