[68Ga]Ga-HBED-CC-FAPI Derivatives with Improved Radiolabeling and Specific Tumor Uptake

Radiosynthesis and preclinical evaluation of [18F]AlF-labeled HBED-CC-FAPI derivatives for imaging of cancer-associated fibroblasts

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177Lu Radiolabeled Polydopamine Decorated with Fibroblast Activation Protein Inhibitor for Locoregional Treatment of Glioma

Radionuclide therapy is expected to be a powerful tool for glioma treatment. Here, we introduced a novel nuclear nanomedicine based on polydopamine (PDA), incorporating fibroblast activation protein inhibitor (FAPI) and macrocyclic chelator (DOTA) for specific cancer targeting and 177Lu labeling. The synthesized nanoradiopharmaceutical, 177Lu-DOTA-PEG-PDA-FAPI, exhibits good stability in serum, saline and PBS over 5 days. 177Lu-DOTA-PEG-PDA-FAPI shows efficient specific uptake and internalization when incubated with U87MG cells. In vivo distribution visualized prominent accumulation and long retention ability of 177Lu-DOTA-PEG-PDA-FAPI at tumor sites after local administration. Moreover, 177Lu-DOTA-PEG-PDA-FAPI has satisfactory antitumor ability without apparent toxic and side effects observed from therapy assay and H&E staining. This study highlights the feasibility of using PDA as a nanocarrier for glioma endoradiotherapy by targeting fibroblast activation protein.

PSMA and SSTR2 Dual-Targeting Theranostic Agents for Neuroendocrine-Differentiated Prostate Cancer (NEPC)

Radioactive prostate-specific membrane antigen (PSMA)-targeting agents are clinically useful for the diagnosis and treatment of patients with PSMA-positive metastatic castration-resistant prostate cancer (mCRPC). Neuroendocrine-differentiated prostate cancer (NEPC), a highly aggressive subtype that is strongly associated with a poor clinical prognosis, may present with reduced PSMA expression and evade detection with PSMA-targeted agents. Several studies have shown elevated uptake of somatostatin receptor 2 (SSTR2) ligands in PSMA-negative NEPC. By combining a SSTR2-targeting peptide, JR11, with previously reported PSMA-targeting ligands, P16-093 and P17-087, [68Ga]Ga-1 and [68Ga]Ga/[177Lu]Lu-2 were designed and synthesized. The cell uptake of [68Ga]Ga-1 was comparable to [68Ga]Ga-P16-093 in PSMA-positive cell lines, while [68Ga]Ga-1 and [68Ga]Ga-2 showed a positive but slightly lower uptake than [68Ga]Ga-DOTA-TATE in SSTR2-positive cell lines. In vivo studies in SSTR2+ or PSMA+ tumor-bearing mice demonstrated that [68Ga]Ga-1 and [68Ga]Ga/[177Lu]Lu-2 showed positive uptake for both SSTR2+ and PSMA+ tumors. These dual-targeting radiotracers are potentially valuable for the diagnosis and radioligand therapy of NEPC.

Advances in Preclinical Research of Theranostic Radiopharmaceuticals in Nuclear Medicine

Theranostics of nuclear medicine refers to the combination of radionuclide imaging and internal irradiation therapy, which is currently a research hotspot and an important direction for the future development of nuclear medicine. Radiopharmaceutical is a vital component of nuclear medicine and serves as one of the fundamental pillars of molecular imaging and precision medicine. At present, a variety of radiopharmaceuticals have been developed for various targets such as fibroblast activation protein (FAP), prostate-specific membrane antigen (PSMA), somatostatin receptor 2 (SSTR2), C-X-C motif chemokine receptor 4 (CXCR4), human epidermal growth factor-2 (HER2), and integrin αvβ3, and some of them have been successfully applied in clinical practice. The radiopharmaceutical with theranostic function plays an important role in the diagnosis, treatment, efficacy evaluation, and prognosis prediction of cancers and is the key to realize the personalized treatment of tumors. This Review summarizes the preclinical research progress of theranostic radiopharmaceuticals toward the above targets in the field of nuclear medicine and discusses the prospects and development directions of radiopharmaceuticals in the future.

Development of [68Ga]Ga/[177Lu]Lu-DOTA-NI-FAPI-04 Containing a Nitroimidazole Moiety as New FAPI Radiotracers with Improved Tumor Uptake and Retention

Fibroblast activation protein (FAP), which is overexpressed in cancer-associated fibroblasts (CAFs), represents a promising target for cancer diagnosis and therapy. Hypoxia is a common feature of solid tumors. A bivalent agent, DOTA-NI-FAPI-04 (1), was developed by incorporating hypoxia-sensitive nitroimidazole (NI) into the FAP-targeting agent FAPI-04. Compound 1 exhibited a strong FAP binding affinity with an IC50 of 7.44 nM. Radiolabeled [68Ga]Ga-1 and [177Lu]Lu-1 demonstrated enhanced in vitro cell uptake. In vivo positron emission tomography/computed tomography (PET/CT) imaging showed that [68Ga]Ga-1 displayed significantly higher specific uptake and retention in U87MG tumor-bearing mice compared to [68Ga]Ga-FAPI-04 (SUVavg: 7.87 vs 1.99% ID/mL at 120 min). Biodistribution studies confirmed superior tumor uptake of [68Ga]Ga-1 (48.15 vs 5.72% ID/g at 120 min). Similarly, [177Lu]Lu-1 exhibited higher tumor uptake than [177Lu]Lu-FAPI-04 (50.75 vs 20.48% ID/g at 120 min). These preliminary results suggest that a nitroimidazole-containing bivalent-targeting agent, [68Ga]Ga/[177Lu]Lu-1, is a promising candidate for tumor theranostics.

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.

Development of an FAP-Targeted PET Probe Based on a Novel Quinolinium Molecular Scaffold

Fibroblast activation protein (FAP) has recently gained significant attention as a promising tumor biomarker for both diagnosis and therapeutic applications. A series of radiopharmaceuticals based on fibroblast activation protein inhibitors (FAPIs) have been developed and translated into the clinic. Though some of them such as radiolabeled FAPI-04 probes have achieved favorable in vivo imaging performance, further improvement is still highly desired for obtaining radiopharmaceuticals with a high theranostics potential. In this study, we innovatively designed an FAPI ligand SMIC-3002 by changing the core quinoline motif of FAPI-04 to the quinolinium scaffold. The engineered molecule was further radiolabeled with 68Ga to generate a positron emission tomography (PET) probe, [68Ga]Ga-SMIC-3002, which was then evaluated in vitro and in vivo. [68Ga]Ga-SMIC-3002 demonstrated high in vitro stability, nanomolar affinity for FAP (8 nM for protein, 23 nM for U87MG cells), and specific uptake in FAP-expressing tumors, with a tumor/muscle ratio of 19.1 and a tumor uptake of 1.48 ± 0.03 ID/g% at 0.5 h in U87MG tumor-bearing mice. In summary, the quinolinium scaffold can be successfully used for the development of the FAP-targeted tracer. [68Ga]Ga-SMIC-3002 not only shows high potential for clinical translation but also offers insights into designing a new generation of FAPI tracers.

68Ga/177Lu-Labeled Bivalent Agents for Targeting Hypoxia and PSMA-Binding in Prostate Cancer

Prostate-specific membrane antigen (PSMA) is an excellent target for cancer detection and therapy. Hypoxia is prevalent in solid tumors, and various nitroimidazole (NI) radioligands can be trapped inside hypoxic cells for diagnosis and therapy. To enhance tumor uptake and retention, we designed bivalent agents (compounds 1-8) incorporating a hypoxia-sensitive NI-moiety and a PSMA-targeting group. Ligands 1-8 were successfully prepared and labeled with 68Ga or 177Lu. Among them, [68Ga]Ga-8 ([68Ga]Ga-AAZTA-NI-PSMA-093) demonstrated significantly higher cellular accumulation under hypoxic conditions than under normoxic conditions, suggesting hypoxia-selective trapping by the introduction of NI group. PET/CT imaging at 60 min postinjection of [68Ga]Ga-8 revealed high tumor uptake (SUVmax: 10.68%ID/mL) in the tumor-bearing mice model. SPECT/CT imaging of [177Lu]Lu-8 at 24 and 48 h postinjection demonstrated excellent accumulation and retention. Preliminary studies indicate that [68Ga]Ga/[177Lu]Lu-8 may be promising bivalent agents targeting hypoxia and PSMA binding for diagnosis and radiotherapy.

Synthesis and Evaluation of Radiogallium Labeled Bone-Imaging Probes Using Oligo-γ-Carboxy Glutamic Acid Peptides as Carriers to Bone

We investigated the importance of the carboxy group density in bone affinity during the development of peptide-based bone-seeking radiopharmaceuticals and carriers. Oligo-γ-carboxy glutamic acid peptides [(Gla)n] with higher carboxy group density than oligo-glutamic acid peptides [(Glu)n] and oligo-aspartic acid peptides [(Asp)n] were chosen. Using the radiogallium chelator N,N'-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC), we synthesized [67Ga]Ga-HBED-CC-(Gla)n (n = 1, 2, 5, 8, 11, or 14) with high yields. Hydroxyapatite-binding assays, biodistribution, and SPECT imaging showed higher affinity and bone accumulation for [67Ga]Ga-HBED-CC-(Gla)n compared to [67Ga]Ga-HBED-CC-(Glu)n. Notably, [67Ga]Ga-HBED-CC-(Gla)8 and [67Ga]Ga-HBED-CC-(Gla)11 exhibited superior bone accumulation and rapid blood clearance. SPECT/CT imaging with [67Ga]Ga-HBED-CC-(Gla)8 exclusively visualized the bone tissue. These findings support the potential use of [67Ga]Ga-HBED-CC-(Gla)n as excellent bone-imaging PET probes, suggesting (Gla)n peptides are superior bone-seeking carriers.

Synthesis and Preclinical Evaluation of Novel 99mTc-Labeled FAPI-46 Derivatives with Significant Tumor Uptake and Improved Tumor-to-Nontarget Ratios

Fibroblast activation protein (FAP), which is expressed on the cell membranes of fibroblasts in most solid tumors, has become an important target for tumor diagnosis and treatment. However, previously reported 99mTc-labeled FAPI-04 complexes have high blood uptake, limiting their use in the clinic. In this work, six 99mTc-labeled FAPI-46 derivatives with different linkers (different amino acids, peptides, or polyethylene glycol) were prepared and evaluated. They had good in vitro stability, hydrophilicity, and good specificity for FAP. The biodistribution and MicroSPECT images revealed that they all had high specific tumor uptake for FAP, and their blood uptake was significantly decreased. Among them, [99mTc]Tc-6-1 exhibited the highest target-to-nontarget ratios (tumor/blood: 6.06 ± 1.19; tumor/muscle: 10.26 ± 0.44) and good tumor uptake (16.15 ± 0.83%ID/g), which also had significantly high affinity for FAP, good in vivo stability, and safety. Therefore, [99mTc]Tc-6-1 holds great potential as a promising molecular tracer for FAP tumor imaging.