Automatic Production and Preliminary PET Imaging of a New Imaging Agent [18F]AlF-FAPT

A comparison of [18F]AlF- and 68Ga-labeled dual targeting heterodimer FAPI-RGD in malignant tumor: preclinical evaluation and pilot clinical PET/CT imaging

Due to the heterogeneity of tumors, strategies to improve the effectiveness of dual-targeting tracers in tumor diagnostics have been intensively practiced. In this study, the radiolabeled [18F]AlF-NOTA-FAPI-RGD (denoted as [18F]AlF-LNC1007), a dual-targeting heterodimer tracer targeting both fibroblast activation protein (FAP) and integrin αvβ3 to enhance specific tumor uptake and retention, was synthesized and evaluated. The tracer was compared with [68Ga]Ga-LNC1007 in preclinical and clinical settings.

METHODS

The preparation of [18F]AlF- and 68Ga-labeled FAPI-RGD was carried out with an optimized protocol. The stability was tested in PBS and fetal bovine serum (FBS). Cellular uptake and in vivo distribution of the two products were compared and carried out on the U87MG cell line and its xenograft model. The safety and dosimetry of [18F]AlF-LNC1007 PET/CT scan were evaluated in six patients with malignant tumors.

RESULTS

Two radiolabeling protocols of [18F]AlF-/[68Ga]Ga-LNC1007 were developed and optimized to give a high yield of tracers with good stability. In vivo microPET images showed that the two tracers exhibited comparable pharmacokinetic characteristics, with high tumor uptake and prolonged tumor retention. In vivo distribution data showed that the target-to-non-target ratios of [18F]AlF-LNC1007 were similar to[68Ga]Ga-LNC1007. A total of six patients underwent [18F]AlF-LNC1007 PET/CT evaluation while two had head-to-head [18F]FDG PET/CT scans. The total body effective dose was 9.94E-03 mSv/MBq. The biodistribution curve showed optimal normal organ uptake with high tumor uptake and long retention of up to 3h p.i., and notably, the tumor-to-background ratio increased over time.

CONCLUSION

We successfully prepared an [18F]AlF-LNC1007 dual-targeting PET probe with comparable performances as [68Ga]Ga-LNC1007. With prolonged tumor retention and tumor specificity, it produced good imaging quality in preclinical and clinical translational studies, indicating that [18F]AlF-LNC1007 is a promising non-invasive tracer for detecting tumors expressing FAP and/or integrin avβ3, with the prospect of clinical implementation.

Druglike, 18F-labeled PET Tracers Targeting Fibroblast Activation Protein

Fibroblast activation protein (FAP) is a very reliable biomarker for tissue remodeling. FAP has so far mainly been studied in oncology, but there is growing interest in the enzyme in other diseases like fibrosis. Recently, FAP-targeting diagnostics and therapeutics have emerged, of which the so-called FAPIs are among the most promising representatives. FAPIs typically have a relatively high molecular weight and contain very polar, multicharged chelator moieties. While this is not limiting the application of FAPIs in oncology, more druglike FAPIs could be required to optimally study diseases characterized by denser, less permeable tissue. In response, we designed the first druglike 18F-labeled FAPIs. We report target potencies, biodistribution, and pharmacokinetics and demonstrate FAP-dependent uptake in murine tumor xenografts. Finally, this paper puts forward compound 10 as a highly promising, druglike FAPI for 18F-PET imaging. This molecule is fit for additional studies in fibrosis and its preclinical profile warrants clinical investigation.

Formulation and Preclinical Testing of Tc-99m-Labeled HYNIC-Glc-FAPT as a FAP-Targeting Tumor Radiotracer

Molecular imaging and targeted radiotherapy with radiolabeled fibroblast activation protein inhibitor (FAPI) targeting peptide probes hold great potential for enhancing the clinical management of patients with FAP-expressing cancers. However, the high cost of PET probes has prompted us to search for new FAP-targeting single-photon imaging agents. In this study, HYNIC-Glc-FAPT is synthesized and radiolabeled with technetium-99m using tricine/EDDA or dimer tricine as coligands to produce [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT. Both [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT were effectively synthesized with an excellent radiochemistry yield (both >97%, n = 6) in a single-step technique, and their stability in PBS and human serum was satisfactory. Compared to [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT, [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT exhibited a more hydrophilic nature with a log P of -3.53 ± 0.12. In vitro cellular uptake and blocking assays, internalization, efflux experiments, and affinity experiments all suggested a mechanism with high FAP-specificity and affinity. SPECT imaging and biodistribution of [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT demonstrated sustained high tumor uptake in BALB/c nude mice bearing U87MG tumors for 6 h. It demonstrated a long-range retention characteristic and more rapid clearance ability from nontarget organs. Collectively, we successfully synthesized [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT, and the excellent targeting properties of [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT suggest a potential diagnostic value in future clinical studies for advanced-stage FAP-expressing malignancies, especially in prognostic evaluation of tumors for it low price and convenient source.

Development and Preclinical Evaluation of [68Ga]BMSH as a New Potent Positron Emission Tomography Tracer for Imaging Programmed Death-Ligand 1 Expression

Immunotherapy targeting the programmed death-ligand 1 (PD-L1)/programmed cell death protein 1 (PD-1) pathway has shown remarkable efficacy against various cancers, but the overall response rate (ORR) is still low. PD-L1 expression in tumors may predict treatment response to immunotherapy. Indeed, ongoing clinical studies utilize a few PD-L1 radiotracers to assess PD-L1 expression as a predictive biomarker for immunotherapy. Here, we present a novel positron emission tomography (PET) radiotracer called [68Ga]BMSH, which is derived from a small molecule inhibitor specifically targeting the binding site of PD-L1. The inhibitor was modified to optimize its in vivo pharmacokinetic properties and enable chelation of 68Ga. In vitro evaluation revealed [68Ga]BMSH possessed a strong binding affinity, high specificity, and rapid internalization in PD-L1 overexpressing cells. Biodistribution studies showed that PD-L1 overexpressing tumors had an uptake of [68Ga]BMSH at 4.22 ± 0.65%ID/g in mice, while the number was 2.23 ± 0.41%ID/g in PD-L1 low-expressing tumors. Micro-PET/CT imaging of tumor-bearing mice further confirmed that, compared to [18F]FDG, [68Ga]BMSH can specifically identify tumors with varying levels of PD-L1 expression. Our findings suggest that the [68Ga]BMSH is a PD-L1 radioligand with ideal imaging properties, and its further application in the clinical screening of PD-L1 overexpressing tumors may improve ORR for immunotherapy.

Noninvasive imaging of FAP expression using positron emission tomography: A comparative evaluation of a [18F]-labeled glycopeptide-containing FAPI with [18F]FAPI-42

PURPOSE

Research on fibroblast activating protein (FAP)-targeting inhibitor (FAPI) has become an important focus for cancer imaging and radiotherapy. Quinoline-based tracers [68 Ga]FAPI-04 and [18F]FAPI-42 have been widely used for positron emission tomography (PET) imaging of most tumors. However, there exist some limitations of these tracers with high uptake in biliary duct system and unstable uptake in pancreas, unsuitable for abdominal tumors PET imaging. Here we developed a [18F]-labeled glycopeptide-containing FAPI tracer (named [18F]FAPT) for PET imaging of FAP in cancers.

METHODS

[18F]FAPT was synthesized manually and automatically. The competitive binding to FAP, cellular internalization, and efflux characteristics were examined in vitro using A549-FAP cells. Dynamic MicroPET and biodistribution studies of [18F]FAPT were then conducted in A549-FAP and U87MG xenograft tumor mouse models compared with [18F]FAPI-42. Five healthy volunteers and three patients with cancer underwent [18F]FAPT PET/CT.

RESULTS

Preclinical and clinical studies showed specific binding of [18F]FAPT to FAP and favorable pharmacokinetic properties with better hydrophilicity, lower uptake in biliary duct system, higher tumor uptake and longer tumor retention compared with [18F]FAPI-42. The biodistribution of [18F]FAPT in healthy volunteers and patients with cancer displayed low uptake in most normal tissues except for pancreas, thyroid and salivary gland, which could contribute to high tumor-to-background ratios in most cancers.

CONCLUSION

[18F]FAPT is better PET tracer than [18F]FAPI-42 for imaging of biliary duct system cancer, potentially providing a tool to examine FAP expression in most cancers with high tumor-to-background ratios.

Recent Trends in Diagnostic Biomarkers of Tumor Microenvironment

The tumor microenvironment (TME) play critical roles in tumor survival, progression, and metastasis and can be considered potential targets for molecular imaging of cancer. The targeting agents for imaging of TME components (e.g., fibroblasts, mesenchymal stromal cells, immune cells, extracellular matrix, blood vessels) provide a promising strategy to target these biomarkers for the early diagnosis of cancers. Moreover, various cancer types have similar tumor immune microenvironment (TIME) features that targeting those biomarkers and offer clinically translatable molecular imaging of cancers. In this review, we categorize and summarize the components in TME which have been targeted for molecular imaging. Moreover, this review updated the recent progress in targeted imaging of TIME biological molecules by various modalities for the early detection of cancer.