Pharmacokinetic and dosimetric evaluation of [177Lu]Lu-FAPI for stromal-targeted radionuclide therapy in triple-negative breast cancer: a preclinical biodistribution and dosimetry study

Background Triple-negative breast cancer (TNBC) has limited targeted treatment options and is strongly influenced by stromal biology. This study evaluated [ 177 Lu]Lu-FAPI for fibroblast activation protein-alpha (FAP)-directed tumor microenvironment targeting in TNBC models. Methods [ 177 Lu]Lu-FAPI was assessed using TNBC stromal FAP analysis, radiochemical quality control, in vitro stability, cellular uptake and blocking assays, internalization and retention studies, serial biodistribution, Single-photon emission computed tomography/computed tomography (SPECT/CT), autoradiography, Medical Internal Radiation Dose (MIRD)-based dosimetry, and preliminary systemic and histologic safety evaluation. The DOTA-FAPI-04 precursor (FAPI-4; CAS 2374782-02-0; molecular formula C 40 H 54 F 2 N 10 O 10 ; molecular weight, 872.92 g/mol) was obtained from a commercial supplier (MedChemExpress, Shanghai, China, Cat. No. HY-130196) with confirmed purity ≥95% via high-performance liquid chromatography (HPLC). Results TNBC stroma showed prominent FAP expression with α-SMA co-localization. [ 177 Lu]Lu-FAPI showed radiochemical purity of 98.7% ± 0.8% via radio-HPLC and 96.1% ± 1.2% via radio-thin-layer chromatography (radio-TLC). At 72 h, intact tracer remained at 92.1% ± 2.7% in saline and 83.4% ± 2.9% in serum. Competitive binding assays revealed an IC 50 of 12.3 ± 2.1 nM and a K i of 8.7 ± 1.5 nM for FAP. FAP-positive cancer-associated fibroblast (CAF)-like cells showed blocking-sensitive uptake, progressive internalization, and prolonged retention. Tumor uptake persisted through 72 h, while blood activity declined to 0.02%ID/g. SPECT/CT and autoradiography confirmed FAP-associated tumor localization. Revised dosimetry estimated absorbed dose as 0.73 ± 0.15 mGy/MBq for tumor and 4.83 ± 0.52 mGy/MBq for the kidney. The tumor-to-kidney absorbed-dose ratio was 0.15, confirming the kidney as the primary organ-at-risk consideration. Conclusions [ 177 Lu]Lu-FAPI showed stromal-targeted TNBC localization and feasible preclinical dosimetry, with the kidney remaining the main organ-at-risk consideration. These findings establish the pharmacokinetic and dosimetric foundation for [ 177 Lu]Lu-FAPI in TNBC; however, therapeutic efficacy remains to be demonstrated in future studies.