Abstract View

Objectives:

Fibroblast activation protein (FAP), abundantly expressed in cancer-associated fibroblasts (CAFs), has emerged as a promising target for theranostic radiopharmaceutical development [1]. We previously developed an ¹⁸F-labeled FAP radioligand showing specific tumor visualization in U87MG glioblastoma and pancreatic ductal adenocarcinoma (PDAC) models. Here, we report the development of a [²¹¹At]CREANT-305 (Figure 1A), a therapeutic ²¹¹At-labeled FAP radioligand designed as a complementary counterpart to the diagnostic agent. Radiolabeling was performed using electrophilic astatodestannylation and copper-mediated aromatic astatination strategies. As in vivo deastatination remains a major challenge for the in vivo application of ²¹¹At-radiopharmaceuticals, strategies to improve astatine stability through ortho-functionalized aromatic stabilization motifs (Figure 1B) are currently additionally investigated.

Methods:

Aryl- boronic acid and stannane precursors, together with the corresponding iodinated reference compounds, were synthesized for electrophilic astatodestannylation and copper-mediated aromatic astatination approaches. Copper-mediated radioastatination (CMRA) was performed using [Cu(OTf)2(py)4], while electrophilic astatodestannylation employed N-chlorosuccinimide (NCS) in acetic acid. Radiolabeling conditions were optimized to obtain [²¹¹At]CREANT-305 and structurally related ortho-functionalized derivatives. Radiochemical conversion (RCC), and radiochemical purity (RCP), were evaluated using radio-TLC and radio-HPLC. 

[²¹¹At]CREANT-305 was characterized in vitro for lipophilicity, plasma, PBS stability, and FAP binding specificity. In vivo metabolic stability, tumor targeting, and therapeutic efficacy were assessed in U87MG tumor-bearing mice, including blocking studies for confirmation of FAP specificity. To improve astatine stability, aromatic stabilization motifs together with corresponding organoboron precursors for CMRA and iodine reference compounds were synthesized as model systems incorporating electronic and steric shielding effects.

Results:

Electrophilic astatodestannylation using arylstannane precursor 1 enabled reproducible synthesis of [²¹¹At]CREANT-305 (Figure 1A) with RCYs up to 90% and radiochemical purities >99%, whereas copper-mediated radioastatination using boronic acid precursor 2 afforded lower RCCs (~70%). [²¹¹At]CREANT-305 demonstrated high FAP specificity in vitro and in vivo. Although significant in vivo radiodeastatination was observed, with only ~8% intact radiotracer remaining 2 h post injection, the tracer still exhibited a clear dose-dependent therapeutic effect, resulting in delayed tumor growth and prolonged survival in treated animals compared with vehicle controls.

In addition, ortho-functionalized derivatives ([²¹¹At]3 to [²¹¹At]5) (Figure 1B) were successfully synthesized using CMRA from organoboron precursors with RCCs > 60% and are currently undergoing microsomal stability evaluation. 

Conclusions:

This work demonstrates the feasibility of a matched ¹⁸F/²¹¹At FAP-targeted radiotheranostic platform using complementary aromatic radiohalogenation strategies. [²¹¹At]CREANT-305 demonstrated FAP-specific targeting and a dose-dependent therapeutic effect in vivo despite partial radiodeastatination. Furthermore, ortho-functionalized stabilization motifs were developed as a strategy to improve astatine stability and are currently under evaluation. These findings provide an important framework for the future development of more stable ²¹¹At-labeled radiopharmaceuticals for targeted alpha therapy.

Figure 1. Development of the ²¹¹At labeled FAP-radioligand using electrophilic astatodestannylation and copper-mediated aromatic astatination strategies and evaluation of aromatic stabilization motifs for improved stability.

References:

[1] Fitzgerald AA, Weiner LM. The role of fibroblast activation protein in health and malignancy. Cancer and Metastasis Reviews. 2020;39(3):783-803.