Synthesis and Preclinical Evaluation of Druglike ¹⁸F-Labeled Fibroblast Activation Protein (FAP) Inhibitors with Enhanced Tumor Retention

Karuna Adhikari ¹, Sergei Grintsevich ², Anke de Groot ³

⁰Molecular Imaging and Radiology, Faculty of Medicine and Health Sciences, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium.

Bioconjugate Chemistry +

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August 12, 2025

View abstract on PubMed

Summary

New druglike radiotracers, [18F]5a and [18F]5b, show improved tumor retention for fibroblast activation protein (FAP) imaging. These FAP inhibitors offer enhanced retention compared to current agents, aiding FAP-positive tumor diagnosis and therapy.

Area Of Science

Radiochemistry and Molecular Imaging
Oncology and Cancer Therapeutics
Biomedical Engineering

Background

Fibroblast activation protein (FAP) is a key biomarker overexpressed in various cancers and fibrotic diseases.
Current FAP-targeting radioligands often face challenges like poor tissue penetration and rapid clearance from tumors.
Developing novel FAP inhibitors with improved pharmacokinetic profiles is crucial for effective cancer imaging and therapy.

Purpose Of The Study

To design and synthesize novel, covalently 18F-labeled, druglike FAP inhibitors, specifically [18F]5a and [18F]5b.
To evaluate the in vitro and in vivo performance of these new radiotracers for FAP-targeted imaging.
To compare the tumor retention of [18F]5a and [18F]5b with a clinically relevant FAP tracer, [18F]AlF-NOTA-FAPI-74.

Main Methods

Development of two novel 18F-labeled FAP inhibitors, [18F]5a and [18F]5b, incorporating quaternary ammonium moieties and PEG linkers.
In vitro assays to assess binding affinity and specificity.
In vivo studies in relevant models to evaluate pharmacokinetics, biodistribution, and tumor uptake.
Comparative analysis of tumor retention at 6 hours post-injection against [18F]AlF-NOTA-FAPI-74.

Main Results

Both [18F]5a and [18F]5b demonstrated high affinity and specific FAP targeting in vitro and in vivo.
The novel radiotracers exhibited favorable pharmacokinetics and selectivity.
Significantly enhanced tumor retention was observed for [18F]5a (4.48 ± 0.34%IA/g) and [18F]5b (6.70 ± 0.22%IA/g) at 6 h p.i. compared to [18F]AlF-NOTA-FAPI-74 (0.54 ± 0.08%IA/g).

Conclusions

The incorporation of quaternary ammonium groups and polarity tuning effectively improves sustained tumor retention of FAP radioligands.
[18F]5a and [18F]5b represent promising next-generation radiotheranostic agents for FAP-positive tumors.
This study provides a valuable design strategy for developing novel FAP inhibitors with covalently bound radionuclides for enhanced imaging and therapy.