Evaluation of Targeted Alpha Therapy Using [211At]FAPI1 in Triple-Negative Breast Cancer Xenograft Models

Abstract

Triple-negative breast cancer (TNBC) presents limited therapeutic options and is associated with poor prognosis. Early detection and the development of novel therapeutic agents are therefore imperative. Fibroblast activation protein (FAP) is a membrane protein expressed on cancer-associated fibroblasts (CAFs) that plays an essential role in TNBC proliferation, migration, and invasion. Consequently, it is hypothesized that the Astatine (²¹¹At)-labeled FAP inhibitor (FAPI) selectively exerts anti-tumor effects through alpha-particle emission. In this study, we aimed to assess its theranostic capabilities by integrating [¹⁸F]FAPI-74 PET imaging with targeted alpha therapy using [²¹¹At]FAPI1 in TNBC models. Mice xenografts were established by transplanting MDA-MB-231 and HT1080 cells (control). As a parallel diagnostic method, [¹⁸F]FAPI-74 was administered for PET imaging to validate FAP expression. A single dose of [²¹¹At]FAPI1 (1.04 ± 0.10 MBq) was administered to evaluate the therapeutic efficacy. [¹⁸F]FAPI-74 exhibited high accumulation in MDA-MB-231 xenografts, and FAP expression was pathologically confirmed via immunostaining. The group that received [²¹¹At]FAPI1 (n = 11) demonstrated a significantly enhanced anti-tumor effect compared with the control group (n = 7) (p = 0.002). In conclusion, [¹⁸F]FAPI-74 PET imaging was successfully used to diagnose FAP expression, and as [²¹¹At]FAPI1 showed promising therapeutic efficacy in TNBC models, it is expected to be a viable therapeutic option.

Keywords: Astatine (211At); CAF; FAP; FAPI-PET; [211At]FAPI1; theranostics; triple-negative breast cancer; α-emitting nuclides.

Figure 1

Representative [¹⁸F]FAPI-74 PET images of HT1080 and MDA-MB-231 xenograft models. The tumors are indicated by arrows.

Figure 2

PET-based biodistributions: ratios of tumor SUVmax to normal tissue SUVmean for HT1080 (n = 3) and MDA-MB-231 xenografts (n = 6), with p = 0.003.

Figure 3

Histological and immunohistochemical staining of xenografts. FAP expression observed in the MDA-MB-231 xenograft. FAP immunohistochemical staining was more intense at the tumor margins. (a) Tumor: H&E; (b) tumor: FAP immunohistochemical staining negative control; (c) tumor: FAP immunohistochemical staining; (d) tumor margin: FAP immunohistochemical staining; (e) border between the center (asterisk) and the margin (star) of the tumor. In MDA-MB-231, xenograft karyolysis, cytoplasmic swelling (arrow), apoptotic bodies, and chromatin aggregation (arrowhead) were observed; in the central legions: H&E was observed. NC: negative control; ×2 images; scale bars = 500 μm, ×20 images; scale bars = 50 μm.

Figure 4

Representative FAP staining of MDA-MB-231 xenograft (high magnification). FAP expression was noted in the (a) tumor cells and (b) stroma. Scale bars = 25 μm.

Figure 5

%ID/g and %ID of [²¹¹At]FAPI1 in various organs at 1 h and 3 h post-injection. * p < 0.05, ** p < 0.01, and *** p < 0.005.

Figure 6

Changes in tumor size and relative body weight in [²¹¹At]FAPI1 and control groups. (a) Relative tumor size and (b) relative body weight. * p < 0.05 and *** p < 0.005; ns = not significant.