Abstract Background Renal biopsy plays a crucial role in diagnosing and assessing the severity of IgA nephropathy (IgAN), despite being an invasive procedure with potential risk of failure. Our study focused on evaluating the capability of [18F] AlF-NOTA-FAPI-04 PET/CT in identifying the extent of pathological alterations in IgAN. Methods Twenty patients (13 males and 7 females; mean age, 44 ± 16 years) with newly diagnosed primary IgAN and 10 patients (7 males and 3 females; mean age, 51 ± 4 years) without known renal disease underwent [18F] AlF-NOTA-FAPI-04 PET/CT imaging. Kidney tissues from biopsies were stained with various techniques and examined using immunofluorescence. The Oxford classification was used to evaluate pathological indicators. Immunohistochemical staining was conducted to assess α-smooth muscle actin (αSMA) and FAP expression. Renal FAPI uptake measured by PET/CT (SUVmax and SUVmean) was correlated with histological findings. Results The renal parenchymal FAPI uptake was significantly higher in IgAN patients compared to control patients (SUVmax = 3.9 ± 1.3 vs 1.9 ± 0.4, SUVmean = 3.6 ± 1.2 vs 1.5 ± 0.4; all P < 0.001). We identified a significant difference in renal parenchymal FAPI uptake among the various categories of the Oxford classification. Correlation analysis revealed a positive association between SUVmax and interstitial fibrosis and tubular atrophy (IF/TA), as well as tubulointerstitial inflammation (TII) scores in scarred cortex and non-scarred cortex (r = 0.637, 0.593, and 0.491, all P < 0.05), Similar associations were observed between SUVmean and these scores (r = 0.641, 0.592, and 0.479, all P < 0.05). Furthermore, significant positive correlations were observed between SUVmax or SUVmean and the staining scores for glomerular αSMA and FAP, as well as for tubulointerstitial αSMA and FAP (all P < 0.01). Conclusion [18F] AlF-NOTA-FAPI-04 PET/CT imaging offers IgAN patients a non-invasive and reproducible auxiliary modality to monitor disease progression.
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