Fibroblast activation protein (FAP) is highly expressed on the cancer-associated fibroblasts (CAF) of the tumor stroma. Recently, we reported the preclinical evaluation and clinical biokinetics of a novel ^99mTc-labeled FAP inhibitor radioligand ([^99mTc]Tc-iFAP). This research aimed to evaluate [^99mTc]Tc-iFAP for the tumor stroma imaging of six different cancerous entities and analyze them from the perspective of stromal heterogeneity. [^99mTc]Tc-iFAP was prepared from freeze-dried kits with a radiochemical purity of 98 ± 1%. The study included thirty-two patients diagnosed with glioma (n = 5); adrenal cortex neuroendocrine tumor (n = 1); and breast (n = 21), lung (n = 2), colorectal (n = 1) and cervical (n = 3) cancer. Patients with glioma had been evaluated with a previous cranial MRI scan and the rest of the patients had been involved in a [¹⁸F]FDG PET/CT study. All oncological diagnoses were corroborated histopathologically. The patients underwent SPECT/CT brain imaging (glioma) or thoracoabdominal imaging 1 h after [^99mTc]Tc-iFAP administration (i.v., 735 ± 63 MBq). The total lesions (n = 111) were divided into three categories: primary tumors (PT), lymph node metastases (LNm), and distant metastases (Dm). [^99mTc]Tc-iFAP brain imaging was positive in four high-grade WHO III–IV gliomas and negative in one treatment-naive low-grade glioma. Both [^99mTc]Tc-iFAP and [¹⁸F]FDG detected 26 (100%) PT, although the number of positive LNm and Dm was significantly higher with [¹⁸F]FDG [82 (96%)], in comparison to [^99mTc]Tc-iFAP imaging (35 (41%)). Peritoneal carcinomatosis lesions in a patient with recurrent colorectal cancer were only visualized with [^99mTc]Tc-iFAP. In patients with breast cancer, a significant positive correlation was demonstrated among [^99mTc]Tc-iFAP uptake values (Bq/cm³) of PT and the molecular subtype, being higher for subtypes HER2+ and Luminal B HER2-enriched. Four different CAF subpopulations have previously been described for LNm of breast cancer (from CAF-S1 to CAF-S4). The only subpopulation that expresses FAP is CAF-S1, which is preferentially detected in aggressive subtypes (HER2 and triple-negative), confirming that FAP+ is a marker for poor disease prognosis. The results of this pilot clinical research show that [^99mTc]Tc-iFAP SPECT imaging is a promising tool in the prognostic assessment of some solid tumors, particularly breast cancer.

Tumor microenvironment fibroblasts overexpress the fibroblast activation protein (FAP). We recently reported the preclinical evaluation of [^99mTc]Tc-iFAP as a new SPECT radioligand capable of detecting FAP. This research aimed to evaluate the kinetic and dosimetric profile of [^99mTc]Tc-iFAP in healthy volunteers, and to assess the radioligand uptake by different solid tumors in three cancer patients. [^99mTc]Tc-iFAP was obtained from lyophilized formulations prepared under GMP conditions with >98% radiochemical purity. Whole-body scans of six healthy subjects were obtained at 0.5, 2, 4, and 24 h after [^99mTc]Tc-iFAP (740 MBq) administration. A 2D-planar/3D-SPECT hybrid activity quantitation method was used to fit the biokinetic models of the source organs (volume of interest: VOI) as exponential functions (A(t)_VOI). The total nuclear transformations (N) that occurred in the source organs were calculated from the mathematical integration (0,∞) of A(t)_VOI. The OLINDA code was used to estimate the radiation doses. Three treatment-naive patients (breast, lung, and cervical cancer) with a prior [¹⁸F]FDG PET/CT scan underwent whole-body, chest, and abdominal SPECT/CT scanning after [^99mTc]Tc-iFAP (740 MBq) administration. Both imaging methods were compared visually and quantitatively. Oncological diagnoses were performed histopathologically. The results showed favorable [^99mTc]Tc-iFAP biodistribution and kinetics due to rapid blood activity removal (t_1/2α = 2.22 min and t_1/2β = 90 min) and mainly renal clearance. The mean radiation equivalent doses were 5.2 ± 0.8 mSv for the kidney and 1.7 ± 0.3 mSv for the liver after administration of 740 MBq. The effective dose was 2.3 ± 0.4 mSv/740 MBq. [^99mTc]Tc-iFAP demonstrated high and reliable uptake in the primary tumor lesions and lymph node metastases in patients with breast, cervical, and lung cancer, which correlated with that detected by [¹⁸F]FDG PET/CT. The tumor microenvironment molecular imaging from cancer patients obtained in this research validates the performance of additional clinical studies to determine the utility of [^99mTc]Tc-iFAP in the diagnosis and prognosis of different types of solid tumors.

Recently, we reported a new fibroblast activation protein (FAP) inhibitor radiopharmaceutical based on the ^99mTc-((R)-1-((6-hydrazinylnicotinoyl)-D-alanyl) pyrrolidin-2-yl) boronic acid (^99mTc-HYNIC-D-Alanine-BoroPro)(^99mTc-HYNIC-iFAP) structure for tumor microenvironment SPECT imaging. This research aimed to synthesize ⁶⁸Ga-[2,2′,2″,2‴-(2-(4-(2-(5-(((S)-1-((S)-2-boronopyrrolidin-1-yl)-1-oxopropan-2-yl)carbamoyl)pyridin-2-yl)hydrazine-1-carbothioamido)benzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid] (⁶⁸Ga-DOTA-D-Alanine-BoroPro)(⁶⁸Ga-iFAP) as a novel radiotracer for PET imaging and evaluate its usefulness for FAP expression in malignant and non-malignant tissues. The coupling of p-SCN-benzene DOTA with HYNIC-iFAP was used for the chemical synthesis and further labeling with ⁶⁸Ga. Radiochemical purity was verified by radio-HPLC. The specificity of ⁶⁸Ga-iFAP was evaluated in HCT116 cells, in which FAP expression was verified by immunofluorescence and Western blot. Biodistribution and biokinetic studies were performed in murine models. ⁶⁸Ga-iFAP uptake at the myocardial level was assessed in mice with induced infarction. First-in-human images of ⁶⁸Ga-iFAP in healthy subjects and patients with myocardial infarction, glioblastoma, prostate cancer, and breast cancer were also obtained. DOTA-D-Alanine BoroPro was prepared with a chemical purity of 98% and was characterized by UPLC mass spectroscopy, FT-IR, and UV-vis. The ⁶⁸Ga-iFAP was obtained with a radiochemical purity of >95%. In vitro and in vivo studies demonstrated ⁶⁸Ga-iFAP-specific recognition for FAP, rapid renal elimination, and adequate visualization of the glioblastoma, breast tumor, prostate cancer, and myocardial infarction sites. The results of this research justify further dosimetry and clinical trials to establish the specificity and sensitivity of ⁶⁸Ga-iFAP PET for FAP expression imaging.

Thyroid cancer is a common endocrinological malignancy worldwide, accounting for 3% of the global incidence of all cancers. Meta-analyses, systematic reviews, and descriptive reviews mention the use of positron emission tomography / computed tomography (PET/CT) as an alternative to morphological imaging such as computed tomography or magnetic resonance imaging to clarify the diagnosis. The aim: analysis of positron emission tomography/computed tomography tracers in the differential diagnosis of thyroid carcinomas. Materials and methods: Review about PET/CT tracers different than 2-[18F] fluorodeoxyglucose (FDG) in patients with diagnosed differentiated thyroid carcinoma. Evidence Synthesis: PET/CT is an alternative to morphological diagnosis imaging when is inconclusive or negative due to the suspicion of tumor persistence or recurrence, elevation of tumor markers, dedifferentiation thyroid carcinoma, non-conventional therapeutic options. 2-[18F] FDG is the most uses a tracer, but there are scenarios where can be negative or inconclusive, for this reason, in recent years other PET tracers have been used: [124I] NaI, [18F] Tetrafluoroborate, [68Ga] Ga-NOTE-PRGD2 or [18F] AIF-NOTE-PRGD2, [68Ga] Ga-DOTA-FAPI, [18F] Fluorocholine or [11C] C-Choline, [18F] or [68Ga] Ga-PSMA, [68Ga] Ga DOTA-TATE/TOC/NOC/LAN, [18F]-FAZA, L-[methyl-11C] Methionine and [89Zr] DFO-PAS200-Fab. Conclusions. There are multiple radiopharmaceuticals different than 2-[18F] FDG, which can be adequate in the context of differentiated thyroid carcinoma: 2-[18F] FDG PET/CT negative, TENIS syndrome, radioiodine-refractory thyroid cancer suspected, thyroid dedifferentiated carcinoma, and some cases theragnostic tools.