Integrated [⁶⁸Ga]Ga-FAPI-7ND PET/CT and stromal profiling of immunocompetent syngeneic tumor models for FAP-targeted radionuclide therapy.

Abstract

BackgroundMost preclinical studies of fibroblast activation protein (FAP)-targeted radionuclide therapy (FAP-RNT) use immunodeficient xenografts, limiting evaluation within an intact immune microenvironment. This study aimed to characterized FAP-associated stromal and FAP inhibitor (FAPI) imaging phenotypes across immunocompetent syngeneic tumors and explored FAP-RNT effects in a representative model.MethodsFour syngeneic models (MC38, PAN02, LLC1, and 4 T1) were assessed using [⁶⁸Ga]Ga-FAPI-7ND PET/CT, Western blotting, immunohistochemistry, and multiplex immunofluorescence for tracer uptake, FAP and α-SMA expression, and co-expression. Based on these findings, 4 T1-bearing mice underwent an exploratory therapeutic evaluation with saline or a single intravenous injection of [¹⁷⁷Lu]Lu-FAPI-7ND. Tumor volume and body weight were measured every 2 days, and tumors were harvested on day 7 for histological and molecular analyses.ResultsAll models showed higher tumor uptake than contralateral muscle (all P +α-SMA+ stroma, whereas 4 T1 combined relatively high uptake with pronounced α-SMA-associated stromal activation. Because FAP-RNT response may reflect both tracer accumulation and stromal state, 4 T1 was selected for exploratory treatment. [¹⁷⁷Lu]Lu-FAPI-7ND inhibited tumor growth (P + T-cell (P + regulatory T-cell densities (P = 0.012), did not alter their ratio (P = 0.537), and increased CD206-associated macrophage features (P ConclusionSyngeneic models exhibit distinct FAP-associated stromal and imaging phenotypes. In 4 T1 tumors, [¹⁷⁷Lu]Lu-FAPI-7ND showed antitumor activity accompanied by immune microenvironmental alterations, supporting integrated model selection for immunocompetent FAP-RNT studies.