Synthesis and Preclinical Evaluation of Three Novel 68Ga-Labeled Bispecific PSMA/FAP-Targeting Tracers for Prostate Cancer Imaging

Current Status and Perspectives of Novel Radiopharmaceuticals with Heterologous Dual-targeted Functions: 2013-2023

Radiotracers provide molecular- and cellular-level information in a noninvasive manner and have become important tools for precision medicine. In particular, the successful clinical application of radioligand therapeutic (RLT) has further strengthened the role of nuclear medicine in clinical treatment. The complicated microenvironment of the lesion has rendered traditional single-targeted radiopharmaceuticals incapable of fully meeting the requirements. The design and development of dual-targeted and multitargeted radiopharmaceuticals have rapidly emerged. In recent years, significant progress has been made in the development of heterologous dual-targeted radiopharmaceuticals. This perspective aims to provide a comprehensive overview of the recent progress in these heterologous dual-targeted radiopharmaceuticals, with a special focus on the design of ligand structures, pharmacological properties, and preclinical and clinical evaluation. Furthermore, future directions are discussed from this perspective.

Design, preclinical evaluation, and first-in-human PET study of [68Ga]Ga-PSFA-01: a PSMA/FAP heterobivalent tracer

PURPOSE

Prostate cancer (PCa), characterized by tumor heterogeneity, may exhibit low or absent prostate-specific membrane antigen (PSMA) expression in cancerous lesions, limiting the detection sensitivity of monospecific probes. Given that fibroblast activation protein (FAP) is frequently overexpressed in the tumor microenvironment (TME), we developed a PSMA/FAP dual-targeting tracer to address this limitation.

METHODS

The precursor (PSFA-01) was synthesized by coupling a quinolone-based FAP-targeting scaffold and EuK with HBED-CC via amide bonds. The dual-receptor-binding affinity and cell uptake of PSFA-01 and [natGa]Ga-PSFA-01 was evaluated in vitro. Micro-PET/CT imaging was performed on 22Rv1 and U87MG tumor-bearing mice. The feasibility of [68Ga]Ga-PSFA-01 PET/CT in a clinical setting was evaluated in a metastatic prostate cancer patient, and the results were compared with those of [68Ga]Ga-FAPI-04 and [68Ga]Ga-PSMA-11 PET/CT.

RESULTS

PSFA-01 and [natGa]Ga-PSFA-01 showed high affinity for both FAP and PSMA proteins (Ki = 0.14-1.02 nM). On micro-PET/CT imaging, the 22Rv1 tumor uptake of [68Ga]Ga-PSFA-01 (SUVmax = 3.89 ± 0.47) was higher than that of [68Ga]Ga-PSMA-11 (SUVmax = 2.96 ± 0.48). The U87MG tumor uptake of [68Ga]Ga-PSFA-01 was significantly higher (SUVmax = 7.29 ± 1.13) than [68Ga]Ga-FAPI-04 (SUVmax = 0.28 ± 0.12), showing tumor to muscle ratio as 12.68 ± 1.93 at 1 h p.i. On clinical trial, the primary tumor and metastatic lesions were distinctly identified by [68Ga]Ga-PSFA-01 (21 lesions), demonstrating superior performance compared to [68Ga]Ga-FAPI-04 (3 lesions) and [68Ga]Ga-PSMA-11 (13 lesions) in terms of lesion count and specificity.

CONCLUSIONS

[68Ga]Ga-PSFA-01 exhibited satisfactory PSMA and FAP dual-receptor-targeting properties both in vitro and in vivo. This study highlights the clinical feasibility of [68Ga]Ga-PSFA-01 PET/CT for detecting metastatic tumors of prostate cancer more sensitively compared to monomeric [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04, which also suggests that a PSMA/FAP dual-targeted radionuclide therapy could potentially overcome challenges related to tumor heterogeneity and insufficient PSMA expression in PCa.

TRIAL REGISTRATION

Clinical trial registry NCT06387381, Registered 1 May 2024.

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.

PSMA and Sigma-1 receptor dual-targeted peptide mediates superior radionuclide imaging and therapy of prostate cancer

Radionuclide therapy, in particular peptide receptor radionuclide therapy (PRRT), has emerged as a valuable means to combat malignant tumors. The specific affinity of ACUPA peptide toward prostate-specific membrane antigen (PSMA) renders the successful development of PRRT for prostate cancer. The clinical outcome of PRRT is, however, generally challenged by moderate tumor uptake and off-target toxicity. Here, we report on a novel design of Sigma-1 receptor and PSMA dual-receptor targeted peptide (S1R/PSMA-P) for superior radionuclide imaging and therapy of prostate cancer. S1R/PSMA-P was acquired with good purity and could efficiently be labeled with 177Lu to yield 177Lu-S1R/PSMA-P with high specific activity and radiostability. Interestingly, 177Lu-S1R/PSMA-P revealed greatly enhanced affinity to LNCaP cells over single-targeted control 177Lu-PSMA-617. The single photon emission computed tomography (SPECT) imaging demonstrated exceptional uptake and retention of 177Lu-S1R/PSMA-P in LNCaP tumor, affording about 2-fold better tumor accumulation while largely reduced uptake by most normal tissues compared to 177Lu-PSMA-617. The selective uptake in LNCaP tumor was also visualized by positron emission tomography (PET) with 68Ga-S1R/PSMA-P. In accordance, a single and low dosage of 177Lu-S1R/PSMA-P at 11.1 MBq effectively suppressed tumor growth without causing apparent side effects. This dual-targeting strategy presents an appealing radionuclide therapy for malignant tumors.

Detection rate of gastrin-releasing peptide receptor (GRPr) targeted tracers for positron emission tomography (PET) imaging in primary prostate cancer: a systematic review and meta-analysis

The gastrin-releasing peptide receptor (GRPr) has gained recognition as a promising target for both diagnostic and therapeutic applications in a variety of human cancers. This study aims to explore the primary tumor detection capabilities of [68Ga] Ga-GRPr PET imaging, specifically in newly diagnosed intra-prostatic prostate cancer lesions (PCa). Following PRISMA-DTA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses of Diagnostic Test Accuracy Studies) guidelines, a systematic literature search was conducted using the Medline, Embase, Scopus, and Web of Science databases. Data regarding patient characteristics and imaging procedure details-including the type of radiotracer used, administered activity, image acquisition time, scanner modality, criteria, and detection rate of index test-were extracted from the included studies. The pooled patient-and lesion-based detection rates, along with their corresponding 95% confidence intervals (CI), were calculated using a random effects model. The final analysis included 9 studies involving 291 patients and 350 intra-prostatic lesions with [68Ga] Ga-GRPr PET imaging in primary PCa. In per-patient-based analysis of [68Ga] Ga-GRPr PET imaging, the pooled detection rates of overall and patients with Gleason score ≥ 7 were 87.09% (95% CI 74.98-93.82) and 89.01% (95% CI 68.17-96.84), respectively. In per-lesion-based analysis, the pooled detection rate [68Ga] Ga-GRPr PET imaging was 78.54% (95% CI 69.8-85.29). The pooled detection rate mpMRI (multiparametric magnetic resonance imaging) in patient-based analysis was 91.85% (95% CI 80.12-96.92). The difference between the detection rates of the mpMRI and [68Ga] Ga-GRPr PET imaging was not statistically significant (OR 0.90, 95% CI 0.23-3.51). Our findings suggest that [68Ga] Ga-GRPr PET imaging has the potential as a diagnostic target for primary PCa. Future research is needed to determine the effectiveness of [68Ga] Ga-GRPr PET in delivering additional imaging data and guiding therapeutic decisions.

Molecular Imaging of Fibroblast Activation in Rabbit Atherosclerotic Plaques: a Preclinical PET/CT Study

AIM

Atherosclerosis remains the pathological basis of myocardial infarction and ischemic stroke. Early and accurate identification of plauqes is crucial to improve clinical outcomes of atherosclerosis patients. Our study aims to evaluate the potential value of fibroblast activation protein inhibitor (FAPI)-04 PET/CT in identifying plaques via a preclinical rabbit model of atherosclerosis.

METHODS

New Zealand white rabbits were fed high-fat diet (HFD), and randomly divided into the model group injured by the balloon, and the sham group only with incisions. Ultrasound was performed to detect plaques, and FAPI-avid was determined through Al18F-NOTA-FAPI-04 PET/CT. Mean standardized uptake values (SUVmean) in lesions were compared, and biodistribution of Al18F-NOTA-FAPI-04 and target-to-background ratios (TBRs) were calculated. Histological staining was performed to display arterial plaques, and autoradiography (ARG) was employed to measure the in vitro intensity of Al18F-NOTA-FAPI-04. At last, the correlation among FAP levels, plaque area, SUVmean values and fibrous cap thickness was assessed.

RESULTS

The rabbit carotid and abdominal atherosclerosis model was established. Al18F-NOTA-FAPI-04 showed a higher uptake in carotid plaques (SUVmean 1.32 ± 0.11) and abdominal plaques (SUVmean 0.73 ± 0.13) compared to corresponding controls (SUVmean 1.07 ± 0.06; 0.46 ± 0.03) (P < 0.05). Biodistribution analysis of Al18F-NOTA-FAPI-04 revealed that the bigger plaques were delineated with higher TBRs. Pathological staining showed the formation of arterial plaques, and ARG staining exhibited a higher intensity of Al18F-NOTA-FAPI-04 in the bigger plaques. Lastly, plaque area was found to be positively correlated to FAP expression and SUVmean, while FAP expression was negatively correlated to fibrous cap thickness of plaques.

CONCLUSIONS

We successfully achieve molecular imaging of fibroblast activation in atherosclerotic lesions of rabbits, suggesting Al18F-NOTA-FAPI-04 PET/CT may be a potentially valuable tool to identify plaques.

The Potential Role of PSMA-Targeted PET in Salivary Gland Malignancies: An Updated Systematic Review

BACKGROUND

Recent studies have suggested using positron emission tomography/computed tomography (PET/CT) with prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals for the detection of salivary gland malignancies (SGM), particularly adenoid-cystic carcinoma (ACC).

METHODS

The authors conducted an extensive review of the scientific literature to examine the potential diagnostic role of PET/CT using PSMA-targeting radiopharmaceuticals in salivary gland malignancies (SGMs) and adenoid cystic carcinoma (ACC). This study included newly diagnosed SGM patients and those with disease recurrence in their imaging evaluation.

RESULTS

This updated systematic review included a total of six studies that examined the diagnostic performance of PSMA-targeted PET/CT in ACC. The articles provided evidence of a high detection rate of PSMA-targeting PET/CT in ACC across all clinical contexts examined. SGMs other than ACC exhibited poorer diagnostic performance.

CONCLUSIONS

PSMA-targeted PET/CT seems promising in detecting ACC lesions; moreover, PSMA appears to be a suitable potential target for radioligand therapy. Prospective multicentric studies are warranted to strengthen the role of PSMA-targeting radiopharmaceuticals in ACC, as both diagnostic and theragnostic agents.

Diagnostic yield of FAP-guided positron emission tomography in thyroid cancer: a systematic review

Background

Several recent studies have proposed the possible application of positron emission tomography/computed tomography (PET/CT) administering radiolabelled fibroblast-activation protein (FAP) inhibitors for various forms of thyroid cancer (TC), including differentiated TC (DTC), and medullary TC (MTC).

Methods

The authors conducted an extensive literature search of original studies examining the effectiveness of FAP-guided PET/CT in patients with TC. The papers included were original publications exploring the use of FAP-targeted molecular imaging in restaging metastatic DTC and MTC patients.

Results

A total of 6 studies concerning the diagnostic yield of FAP-targeted PET/CT in TC (274 patients, of which 247 DTC and 27 MTC) were included in this systematic review. The included articles reported high values of FAP-targeted PET/CT detection rates in TC, ranging from 81 to 100% in different anatomical sites and overall superior to the comparative imaging method.

Conclusion

Although there are promising results, the existing literature on the diagnostic accuracy of FAP-guided PET in this context is still quite limited. To thoroughly evaluate its potential significance in TC patients, it is needed to conduct prospective randomized multicentric trials.

Development, preclinical evaluation and preliminary dosimetry profiling of SB03178, a first-of-its-kind benzo[h]quinoline-based fibroblast activation protein-α-targeted radiotheranostic for cancer imaging and therapy

Fibroblast activation protein-α (FAP) is a marker of cancer-associated fibroblasts (CAFs) that constitute a significant portion of most carcinomas. Since it plays a critical role in tumor growth and metastasis, its timely detection to identify tumor lesions in early developmental stages using targeted radiopharmaceuticals has gained significant impetus. In the present work, two novel FAP-targeted precursors SB03178 and SB04033 comprising of an atypical benzo[h]quinoline construct were synthesized and either chelated to diagnostic radionuclide gallium-68 or therapeutic radionuclide lutetium-177, with ≥90% radiochemical purities and 22-76% decay-corrected radiochemical yields. natGa-labeled complexes displayed dose-dependent FAP inhibition, with binding potency of natGa-SB03178 being ∼17 times higher than natGa-SB04033. To evaluate their pharmacokinetic profiles, PET imaging and ex vivo biodistribution analyses were executed in FAP-overexpressing HEK293T:hFAP tumor-bearing mice. While both tracers displayed clear tumor visualization that was primarily FAP-arbitrated, with negligible uptake in most peripheral tissues, [68Ga]Ga-SB03178 demonstrated higher tumor uptake and superior tumor-to-background contrast ratios than [68Ga]Ga-SB04033. 177Lu-labeled SB03178 was subjected to tumor retention studies, mouse dosimetry profiling and mouse-to-human dose extrapolations also using the HEK293T:hFAP tumor model. [177Lu]Lu-SB03178 exhibited a combination of high and sustained tumor uptake, with excellent tumor-to-critical organ uptake ratios resulting in a high radiation absorbed dose to the tumor and a low estimated whole-body dose to humans. Our preliminary findings are considerably encouraging to support clinical development of [68Ga]Ga-/[177Lu]Lu-SB03178 theranostic pair for use in a vast majority of FAP-overexpressing neoplasms, particularly carcinomas.

Contribution of 68 Ga-DOTA-FAPI-04 PET/CT to Prostate Cancer Imaging : Complementary Role in PSMA-Negative Cases

PURPOSE

Prostate-specific membrane antigen (PSMA)-targeted PET/CT is a well-established imaging method in prostate cancer (PC) for both staging and restaging, and also for theranostic applications. An alternative imaging method is crucial for 15% PSMA-negative cases. We aimed to investigate the contribution of 68 Ga-DOTA-FAPI-04 PET/CT to PC imaging.

PATIENTS AND METHODS

Thirty-six patients diagnosed with PC were included. Patients underwent both 68 Ga-PSMA PET/CT and 68 Ga-DOTA-FAPI-04 PET/CT imaging within 1 week. In staging group, primary tumor uptake values were compared, and also correlations were done with histopathological findings, MRI findings, and total PSA levels. In biochemical recurrence group, the uptake values in prostatic region and metastases were evaluated to define the local recurrence or metastatic disease.

RESULTS

In staging group, PSMA PET showed increased uptake in the primary lesion area in 14/27 (52%) patients, whereas 20/27 (74%) patients were positive in FAPI-04 PET. FAPI-04 positivity was found to be quite high, such as 54%, in PSMA-negative patients. A significant difference was observed between ISUP grade 1-3 patients and ISUP grade 4-5 patients in FAPI-04 PET ( P = 0.03). Local recurrence was detected in 3 patients, pelvic lymph node metastasis in 1 patient, and sacrum metastasis in 1 patient in biochemical recurrence group, and all of the lesions had more intense uptake in PSMA PET than FAPI-04 PET.

CONCLUSIONS

FAPI PET imaging seems to have a potential to contribute PSMA PET imaging with FAPI positivity in more than half of PSMA-negative cases. Also, FAPI-targeted radionuclide therapy may be a promising method in patients resistant to PSMA-targeted therapy.

Synthesis and Preclinical Evaluation of Two Novel 68Ga-Labeled Bispecific PSMA/FAP-Targeted Tracers with 2-Nal-Containing PSMA-Targeted Pharmacophore and Pyridine-Based FAP-Targeted Pharmacophore

Some bispecific radiotracers have been developed to overcome the limitations of monospecific tracers and improve detection sensitivity for heterogeneous tumor lesions. Here, we aim to synthesize two bispecific tracers targeting prostate-specific membrane antigen (PSMA) and fibroblast activation protein (FAP), which are key markers expressed in prostate cancer. A pyridine-based FAP-targeted ligand was synthesized through multi-step organic synthesis and then connected to the 2-Nal-containing PSMA-targeted motif. The Ki(PSMA) values of Ga-complexed bispecific ligands, Ga-AV01084 and Ga-AV01088, were 11.6 ± 3.25 and 28.7 ± 6.05 nM, respectively, and the IC50(FAP) values of Ga-AV01084 and Ga-AV01088 were 10.9 ± 0.67 and 16.7 ± 1.53 nM, respectively. Both [68Ga]Ga-AV01084 and [68Ga]Ga-AV01088 enabled the visualization of PSMA-expressing LNCaP tumor xenografts and FAP-expressing HEK293T:hFAP tumor xenografts in PET images acquired at 1 h post-injection. However, the tumor uptake values from the bispecific tracers were still lower than those obtained from the monospecific tracers, PSMA-targeted [68Ga]Ga-PSMA-617 and FAP-targeted [68Ga]Ga-AV02070. Further investigations are needed to optimize the selection of linkers and targeted pharmacophores to improve the tumor uptake of bispecific PSMA/FAP tracers for prostate cancer imaging.

Diagnostic Accuracy of [68Ga]Ga Labeled Fibroblast-Activation Protein Inhibitors in Detecting Head and Neck Cancer Lesions Using Positron Emission Tomography: A Systematic Review and a Meta-Analysis

Several studies have examined the use of positron emission tomography (PET) using [68Ga]Ga-radiolabeled fibroblast-activation protein inhibitors (FAPi) across multiple subtypes of head and neck cancer (HNC). The purpose of the present study was to evaluate the diagnostic accuracy of a newly developed molecular imaging approach in the context of HNC through a comprehensive review and meta-analysis. A thorough literature review was conducted to identify scholarly articles about the diagnostic effectiveness of FAP-targeted PET imaging. The present study incorporates original publications assessing the efficacy of this innovative molecular imaging test in both newly diagnosed and previously treated HNC patients. This systematic review examined eleven investigations, of which nine were deemed suitable for inclusion in the subsequent meta-analysis. The quantitative synthesis yielded a pooled detection rate of 99% for primary HNC lesions. Additionally, on a per patient-based analysis, the pooled sensitivity and specificity for regional lymph node metastases were found to be 90% and 84%, respectively. The analysis revealed a statistical heterogeneity among the studies for the detection rate of primary HNC lesions. The quantitative findings presented in this study indicate a favorable diagnostic performance of FAP-targeted PET imaging in detecting primary HNC tumors. In contrast, discordant results concerning the diagnostic accuracy of lymph node metastases were found. However, further multicentric trials are required to validate the efficacy of FAP-targeted PET in this specific group of patients.

Recent Pre-Clinical Advancements in Nuclear Medicine: Pioneering the Path to a Limitless Future

The theranostic approach in oncology holds significant importance in personalized medicine and stands as an exciting field of molecular medicine. Significant achievements have been made in this field in recent decades, particularly in treating neuroendocrine tumors using 177-Lu-radiolabeled somatostatin analogs and, more recently, in addressing prostate cancer through prostate-specific-membrane-antigen targeted radionuclide therapy. The promising clinical results obtained in these indications paved the way for the further development of this approach. With the continuous discovery of new molecular players in tumorigenesis, the development of novel radiopharmaceuticals, and the potential combination of theranostics agents with immunotherapy, nuclear medicine is poised for significant advancements. The strategy of theranostics in oncology can be categorized into (1) repurposing nuclear medicine agents for other indications, (2) improving existing radiopharmaceuticals, and (3) developing new theranostics agents for tumor-specific antigens. In this review, we provide an overview of theranostic development and shed light on its potential integration into combined treatment strategies.

Lu-177-Labeled Hetero-Bivalent Agents Targeting PSMA and Bone Metastases for Radionuclide Therapy

Prostate-specific membrane antigen (PSMA) is an excellent target for imaging and radionuclide therapy of prostate cancer. Recently, [177Lu]Lu-PSMA-617 (Pluvicto) was approved by the FDA for radionuclide therapy. To develop hetero-bivalent agents targeting both PSMA and bone metastasis, [177Lu]Lu-P17-079 ([177Lu]Lu-1) and [177Lu]Lu-P17-081 ([177Lu]Lu-2) were prepared. In vivo biodistribution studies of [177Lu]Lu-PSMA-617, [177Lu]Lu-1, and [177Lu]Lu-2 in mice bearing PC3-PIP (PSMA positive) tumor showed high uptake in PSMA-positive tumor (14.5, 14.7, and 11.3% ID/g at 1 h, respectively) and distinctively different bone uptakes (0.52, 6.52, and 5.82% ID/g at 1 h, respectively). PET imaging using [68Ga]Ga-P17-079 ([68Ga]Ga-1) in the same mouse model displayed excellent images confirming the expected dual-targeting to PSMA-positive tumor and bone. Results suggest that [177Lu]Lu-P17-079 ([177Lu]Lu-1) is a promising candidate for further development as a hetero-bivalent radionuclide therapy agent targeting both PSMA expression and bone metastases for the treatment of prostate cancer.

Synthesis and Preclinical Evaluation of Novel 68Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Fibroblast activation protein (FAP) is a membrane-tethered serine protease overexpressed in the reactive stromal fibroblasts of >90% human carcinomas, which makes it a promising target for developing radiopharmaceuticals for the imaging and therapy of carcinomas. Here, we synthesized two novel (R)-pyrrolidin-2-yl-boronic acid-based FAP-targeted ligands: SB02055 (DOTA-conjugated (R)-(1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)glycyl)pyrrolidin-2-yl)boronic acid) and SB04028 (DOTA-conjugated ((R)-1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)-D-alanyl)pyrrolidin-2-yl)boronic acid). ^natGa- and ⁶⁸Ga-complexes of both ligands were evaluated in preclinical studies and compared to previously reported ^natGa/⁶⁸Ga-complexed PNT6555. Enzymatic assays showed that FAP binding affinities (IC₅₀) of ^natGa-SB02055, ^natGa-SB04028 and ^natGa-PNT6555 were 0.41 ± 0.06, 13.9 ± 1.29 and 78.1 ± 4.59 nM, respectively. PET imaging and biodistribution studies in HEK293T:hFAP tumor-bearing mice showed that while [⁶⁸Ga]Ga-SB02055 presented with a nominal tumor uptake (1.08 ± 0.37 %ID/g), [⁶⁸Ga]Ga-SB04028 demonstrated clear tumor visualization with ~1.5-fold higher tumor uptake (10.1 ± 0.42 %ID/g) compared to [⁶⁸Ga]Ga-PNT6555 (6.38 ± 0.45 %ID/g). High accumulation in the bladder indicated renal excretion of all three tracers. [⁶⁸Ga]Ga-SB04028 displayed a low background level uptake in most normal organs, and comparable to [⁶⁸Ga]Ga-PNT6555. However, since its tumor uptake was considerably higher than [⁶⁸Ga]Ga-PNT6555, the corresponding tumor-to-organ uptake ratios for [⁶⁸Ga]Ga-SB04028 were also significantly greater than [⁶⁸Ga]Ga-PNT6555. Our data demonstrate that (R)-(((quinoline-4-carbonyl)-d-alanyl)pyrrolidin-2-yl)boronic acid is a promising pharmacophore for the design of FAP-targeted radiopharmaceuticals for cancer imaging and radioligand therapy.

Synthesis and Evaluation of 68Ga-Labeled (2S,4S)-4-Fluoropyrrolidine-2-Carbonitrile and (4R)-Thiazolidine-4-Carbonitrile Derivatives as Novel Fibroblast Activation Protein-Targeted PET Tracers for Cancer Imaging

Fibroblast activation protein α (FAP-α) is a cell-surface protein overexpressed on cancer-associated fibroblasts that constitute a substantial component of tumor stroma and drive tumorigenesis. FAP is minimally expressed by most healthy tissues, including normal fibroblasts. This makes it a promising pan-cancer diagnostic and therapeutic target. In the present study, we synthesized two novel tracers, [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058, bearing a (2S,4S)-4-fluoropyrrolidine-2-carbonitrile or a (4R)-thiazolidine-4-carbonitrile pharmacophore, respectively. [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058 were evaluated for their FAP-targeting capabilities using substrate-based in vitro binding assays, and in PET/CT imaging and ex vivo biodistribution studies in an HEK293T:hFAP tumor xenograft mouse model. The IC₅₀ values of ^natGa-SB03045 (1.59 ± 0.45 nM) and ^natGa-SB03058 (0.68 ± 0.09 nM) were found to be lower than those of the clinically validated ^natGa-FAPI-04 (4.11 ± 1.42 nM). Contrary to the results obtained in the FAP-binding assay, [⁶⁸Ga]Ga-SB03058 demonstrated a ~1.5 fold lower tumor uptake than that of [⁶⁸Ga]Ga-FAPI-04 (7.93 ± 1.33 vs. 11.90 ± 2.17 %ID/g), whereas [⁶⁸Ga]Ga-SB03045 (11.8 ± 2.35 %ID/g) exhibited a tumor uptake comparable to that of [⁶⁸Ga]Ga-FAPI-04. Thus, our data suggest that the (2S,4S)-4-fluoropyrrolidine-2-carbonitrile scaffold holds potential as a promising pharmacophore for the design of FAP-targeted radioligands for cancer diagnosis and therapy.

Novel 68Ga-Labeled Pyridine-Based Fibroblast Activation Protein-Targeted Tracers with High Tumor-to-Background Contrast

Compared to quinoline-based fibroblast activation protein (FAP)-targeted radiotracers, pyridine-based FAP-targeted tracers are expected to have faster pharmacokinetics due to their smaller molecular size and higher hydrophilicity, which we hypothesize would improve the tumor-to-background image contrast. We aim to develop 68Ga-labeled pyridine-based FAP-targeted tracers for cancer imaging with positron emission tomography (PET), and compare their imaging potential with the clinically validated [68Ga]Ga-FAPI-04. Two DOTA-conjugated pyridine-based AV02053 and AV02070 were synthesized through multi-step organic synthesis. IC50(FAP) values of Ga-AV02053 and Ga-AV02070 were determined by an enzymatic assay to be 187 ± 52.0 and 17.1 ± 4.60 nM, respectively. PET imaging and biodistribution studies were conducted in HEK293T:hFAP tumor-bearing mice at 1 h post-injection. The HEK293T:hFAP tumor xenografts were clearly visualized with good contrast on PET images by [68Ga]Ga-AV02053 and [68Ga]Ga-AV02070, and both tracers were excreted mainly through the renal pathway. The tumor uptake values of [68Ga]Ga-AV02070 (7.93 ± 1.88%ID/g) and [68Ga]Ga-AV02053 (5.6 ± 1.12%ID/g) were lower than that of previously reported [68Ga]Ga-FAPI-04 (12.5 ± 2.00%ID/g). However, both [68Ga]Ga-AV02070 and [68Ga]Ga-AV02053 showed higher tumor-to-background (blood, muscle, and bone) uptake ratios than [68Ga]Ga-FAPI-04. Our data suggests that pyridine-based pharmacophores are promising for the design of FAP-targeted tracers. Future optimization on the selection of a linker will be explored to increase tumor uptake while maintaining or even further improving the high tumor-to-background contrast.