The tumor microenvironment (TME) plays a central role in cancer progression and therapeutic resistance. Among its non-malignant cellular components, cancer-associated fibroblasts (CAFs) have emerged as a highly promising therapeutic target. CAFs drive the formation of a pro-tumorigenic ecosystem by remodeling the extracellular matrix, promoting angiogenesis, and modulating immune responses [1], [2]. Fibroblast activation protein (FAP), a type II transmembrane serine protease specifically and highly overexpressed on CAFs in the vast majority of epithelial tumors [3], represents an ideal target for tumor-targeted theranostics due to its strict localization within the tumor stroma and its strong association with poor clinical outcomes.

The development of small-molecule radioligands based on fibroblast activation protein inhibitors (FAPIs), particularly the 2018 report of 68Ga-FAPI-04 [4], has revolutionized molecular imaging of the tumor stroma, enabling high-contrast FAPI-PET. However, first-generation probes represented by FAPI-04 face a critical translational bottleneck in targeted radioligand therapy (TRT): excessively rapid blood clearance (half-life ∼0.4 h), leading to insufficient intratumoral retention time and cumulative radiation dose, thereby limiting therapeutic efficacy [5], [6]. Consequently, current research is focused on rationally optimizing the pharmacokinetics of FAPI probes, for example, through the introduction of albumin-binding moieties and multivalent constructs, aimed at extending circulation half-life and enhancing tumor accumulation to fully unleash the potential of FAP-TRT [7], [8]. To visually compare the performance of these strategies in U87MG models, we summarize key pharmacokinetic parameters of representative monomeric, albumin-binding, and polymeric FAPI probes (Table 1). The data show that conventional non-albumin-binding monomers (e.g., FAPI-04, FAPI-02) have serum half-lives of only ∼0.4 h. While background blood activity is low, tumor uptake remains limited (SUVmean ∼0.35). In stark contrast, the albumin-binding strategy, exemplified by Evans Blue-modified EB-FAPI-B1/B2, exhibits biphasic blood clearance with a prolonged β-phase half-life up to 68 h, resulting in significantly enhanced tumor uptake (>10 %ID/g at 96 h). Multivalent designs increase tumor SUVmean to 1.0–1.2 within 0.5–4 h but, without extending blood circulation, lead to markedly increased non-target organ uptake (e.g., kidneys). In summary, the albumin-binding strategy enhances tumor uptake and retention by orders of magnitude through increased passive tumor penetration and entrapment [13], [14], [15], [16], [17], [18], [19], making it currently the most promising approach for improving the efficacy of FAP-TRT. However, this strategy may also elevate background uptake in the hepatobiliary system, underscoring the need to achieve a precise balance between optimized pharmacokinetics and controlled off-target toxicity.

Based on the above background, this study focuses on a novel albumin-binding FAPI probe—FAPI-X5. Its design aims to achieve an integrated balance between pharmacokinetic properties and target affinity. To this end, we retained the well-validated FAP inhibitor core pharmacophore to ensure high target affinity, and optimized the pharmacokinetic profile by conjugating an albumin-binding moiety—4-(p-iodophenyl) butyric acid—which has been extensively documented to extend circulation half-life [20], [21]. In molecular docking simulations, the core pharmacophore effectively embeds into the FAP catalytic domain (PDB: 1Z68), forming hydrogen bonds with key active residues (e.g., Asn704, His734) and engaging in π-π stacking with Trp623, while the linker further establishes a stable hydrogen-bond network with multiple surrounding residues, providing structural support for its high affinity. The entire molecule incorporates DOTA as the chelating unit, ensuring stable complexation with a variety of theranostic radionuclides, such as 68Ga, 177Lu, and 47Sc. This integrated design theoretically combines “prolonged circulation” with “precision targeting,” laying a molecular foundation for subsequent theranostic applications.

The selection of the therapeutic radionuclide constitutes another critical element for the success of FAP-TRT. 177Lu has emerged as a workhorse radionuclide in TRT for neuroendocrine tumors and prostate cancer owing to its moderate β-particle energy, imageable γ-emissions, and well-established supply chain and clinical experience, making it a natural benchmark for evaluating the efficacy of FAPI-X5 [22], [23]. Concurrently, 47Sc presents a theoretically attractive theranostic candidate, emitting therapeutic β-particles (Eβ-av = 162 keV) alongside γ-photons suitable for SPECT imaging (Eγ = 159 keV). Its shorter particle range may offer potential advantages for treating micrometastases [24]. Nevertheless, compared with 177Lu, the radiopharmaceutical chemistry of 47Sc remains less developed. Key aspects such as its complexation kinetics with DOTA, in vivo stability, and potential differences in final biodistribution have yet to be fully elucidated within FAP-targeted systems.

This study aimed to compare the performance of 177Lu- and 47Sc-labeled FAPI-X5 under identical experimental conditions. The central scientific question was to determine whether 47Sc, when coupled with the same pharmacokinetically optimized ligand platform (FAPI-X5), can achieve tumor targeting, therapeutic potential, and safety profiles comparable to those of 177Lu—thereby positioning itself as a feasible alternative therapeutic radionuclide—or whether its inherent physicochemical or chelation properties would lead to unfavorable biodistribution, imposing specific constraints on clinical translation. Accordingly, this work is positioned not as a translational proof-of-concept for 47Sc-FAPI-X5, but as a mechanistic comparative study designed to elucidate isotope-specific limitations.