Synergistic Methylene Blue-Driven FAPI Uptake Mechanism Advancing the Detection of Myocardial Fibrosis in...

Myocardial fibrosis, a key pathological feature of hypertensive heart disease (HHD), remains diagnostically challenging due to limited clinical tools. In this study, a FAPI-targeted uptake mechanism previously reported by our group, originally developed for tumor imaging, is extended to the detection of myocardial fibrosis in HHD using [¹⁸F]F-NOTA-FAPI-MB. The diagnostic performance of this tracer is compared with those of [¹⁸F]F-FDG, [¹⁸F]F-FAPI-42, and [¹⁸F]F-NOTA-FAP2286, and its potential for fluorescence imaging is also evaluated. Myocardial fibrosis associated with HHD was induced in Sprague-Dawley rats using isoproterenol. Evaluation of cardiac structure and function was conducted through echocardiography. Following each administration of a radioactive tracer via tail vein injection, static PET/CT imaging was performed at 1 and 2 h after injection. Additionally, cardiac excision allowed for ex vivo fluorescence imaging using NOTA-FAPI-MB. All three FAP-targeting probes enabled the precise localization and quantification of fibrotic activity within affected myocardial regions. Among them, [¹⁸F]F-NOTA-FAPI-MB demonstrated the most pronounced increase in TBR from 1 to 2 h in the ISO group and exhibited the highest 2 h/1 h ratio. Blocking experiments revealed limited competitive inhibition of the [¹⁸F]F-NOTA-FAPI-MB uptake by unlabeled FAPI-42. Ex Vivo cardiac fluorescence imaging further confirmed the significant accumulation of NOTA-FAPI-MB. [¹⁸F]F-FAPI-42 exhibited an inferior retention ability at the target site compared to [¹⁸F]F-NOTA-FAPI-MB. In contrast, [¹⁸F]F-NOTA-FAP2286 showed a lower uptake and the weakest retention in fibrotic regions among the three tracers. However, [¹⁸F]F-FDG imaging showed no significant changes in myocardial metabolic activity in HHD, suggesting that [¹⁸F]F-FDG imaging is not effective in detecting cardiac abnormalities associated with HHD. The successful translation of the MB-driven uptake mechanism from tumors to HHD is evidenced by the enhanced myocardial accumulation of [¹⁸F]F-NOTA-FAPI-MB via MB synergy, confirming its mechanism-based targeting. Although further clinical validation is required, this radiopharmaceutical holds strong potential as a dual-modality imaging agent for HHD with a reduced radiation burden.