Bifunctional octadentate pseudopeptides as Zirconium-89 chelators for immuno-PET applications.

BACKGROUND: Positron emission tomography (PET) is a highly sensitive method that provides fine resolution images, useful in the field of clinical diagnostics. In this context, Zirconium-89 (89Zr)-based imaging agents have represented a great challenge in molecular imaging with immuno-PET, which employs antibodies (mAbs) as biological vectors. Indeed, immuno-PET requires radionuclides that can be attached to the mAb to provide stable in vivo conjugates, and for this purpose, the radioactive element should have a decay half-life compatible with the time needed for the biodistribution of the immunoglobulin. In this regard, 89Zr is an ideal radioisotope for immuno-PET because its half-life perfectly matches the in vivo pharmacokinetics of mAbs. RESULTS: The main objective of this work was the design and synthesis of a series of bifunctional octadentate pseudopeptides able to generate stable 89Zr complexes. To achieve this, here we investigated hydroxamate, N-methylhydroxamate and catecholate chelating moieties in complexing radioactive zirconium. N-methylhydroxamate proved to be the most effective 89Zr-chelating group. Furthermore, the increased flexibility and hydrophilicity obtained by using polyoxyethylene groups spacing the hydroxamate units led to chelators capable of rapidly forming (15 min) stable and water-soluble complexes with 89Zr under mild reaction conditions (aqueous environment, room temperature, and physiological pH) that are mandatory for complexation reactions involving biomolecules. Additionally, we report challenge experiments with the competitor ligand EDTA and metal ions such as Fe3+, Zn2+ and Cu2+. In all examined conditions, the chelators demonstrated stability against transmetallation. Finally, a maleimide moiety was introduced to apply one of the most promising ligands in bioconjugation reactions through Thiol-Michael chemistry. CONCLUSION: Combining solid phase and solution synthesis techniques, we identified novel 89Zr-chelating molecules with a peptide scaffold. The adopted chemical design allowed modulation of molecular flexibility, hydrophilicity, as well as the decoration with different zirconium chelating groups. Best results in terms of 89Zr-chelating properties were achieved with the N-methyl hydroxamate moiety. The Zirconium complexes obtained with the most effective compounds were water-soluble, stable to transmetallation, and resistant to peptidases for at least 6 days. Further studies are needed to assess the potential of this novel class of molecules as Zirconium-chelating agents for in vivo applications.

PET/MR in recurrent glioblastoma patients treated with regorafenib: [18F]FET and DWI-ADC for response assessment and survival prediction.

OBJECTIVE: The use of regorafenib in recurrent glioblastoma patients has been recently approved by the Italian Medicines Agency (AIFA) and added to the National Comprehensive Cancer Network (NCCN) 2020 guidelines as a preferred regimen. Given its complex effects at the molecular level, the most appropriate imaging tools to assess early response to treatment is still a matter of debate. Diffusion-weighted imaging and O-(2-18F-fluoroethyl)-L-tyrosine positron emission tomography ([18F]FET PET) are promising methodologies providing additional information to the currently used RANO criteria. The aim of this study was to evaluate the variations in diffusion-weighted imaging/apparent diffusion coefficient (ADC) and [18F]FET PET-derived parameters in patients who underwent PET/MR at both baseline and after starting regorafenib. METHODS: We retrospectively reviewed 16 consecutive GBM patients who underwent [18F]FET PET/MR before and after two cycles of regorafenib. Patients were sorted into stable (SD) or progressive disease (PD) categories in accordance with RANO criteria. We were also able to analyze four SD patients who underwent a third PET/MR after another four cycles of regorafenib. [18F]FET uptake greater than 1.6 times the mean background activity was used to define an area to be superimposed on an ADC map at baseline and after treatment. Several metrics were then derived and compared. Log-rank test was applied for overall survival analysis. RESULTS: Percentage difference in FET volumes correlates with the corresponding percentage difference in ADC (R = 0.54). Patients with a twofold increase in FET after regorafenib showed a significantly higher increase in ADC pathological volume than the remaining subjects (p = 0.0023). Kaplan-Meier analysis, performed to compare the performance in overall survival prediction, revealed that the percentage variations of FET- and ADC-derived metrics performed at least as well as RANO criteria (p = 0.02, p = 0.024 and p = 0.04 respectively) and in some cases even better. TBR Max and TBR mean are not able to accurately predict overall survival. CONCLUSION: In recurrent glioblastoma patients treated with regorafenib, [18F]FET and ADC metrics, are able to predict overall survival and being obtained from completely different measures as compared to RANO, could serve as semi-quantitative independent biomarkers of response to treatment. ADVANCES IN KNOWLEDGE: Simultaneous evaluation of [18F]FET and ADC metrics using PET/MR allows an early and reliable identification of response to treatment and predict overall survival.

A Universal Cassette-Based System for the Dissolution of Solid Targets.

Cyclotron-based radionuclides production by using solid targets has become important in the last years due to the growing demand of radiometals, e.g., 68Ga, 89Zr, 43/47Sc, and 52/54Mn. This shifted the focus on solid target management, where the first fundamental step of the radiochemical processing is the target dissolution. Currently, this step is generally performed with commercial or home-made modules separated from the following purification/radiolabelling modules. The aim of this work is the realization of a flexible solid target dissolution system to be easily installed on commercial cassette-based synthesis modules. This would offer a complete target processing and radiopharmaceutical synthesis performable in a single module continuously. The presented solid target dissolution system concept relies on an open-bottomed vial positioned upon a target coin. In particular, the idea is to use the movement mechanism of a syringe pump to position the vial up and down on the target, and to exploit the heater/cooler reactor of the module as a target holder. All the steps can be remotely controlled and are incorporated in the cassette manifold together with the purification and radiolabelling steps. The performance of the device was tested by processing three different irradiated targets under different dissolution conditions.

[18F]F-PSMA-1007 Radiolabelling without an On-Site Cyclotron: A Quality Issue.

Radiopharmaceuticals targeting the prostate-specific membrane antigen (PSMA) has become the gold standard for PET imaging of prostate cancer. [68Ga]Ga-PSMA-11 has been the forerunner but a [18F]F-PSMA ligand has been developed because of the intrinsic advantages of Fluorine-18. Fluorine-18 labelled compounds are usually prepared in centers with an on-site cyclotron. Since our center has not an on-site cyclotron, we decided to verify the feasibility of producing the experimental 18F-labelled radiopharmaceutical [18F]F-PSMA-1007 with [18F]F- from different external suppliers. A quality agreement has been signed with two different suppliers, and a well-established and correctly implemented quality assurance protocol has been followed. The [18F]F- was produced with cyclotrons, on Nb target, but with different beam energy and current. Extensive validation of the [18F]F-PSMA-1007 synthesis process has been performed. The aim of this paper was the description of all the quality documentation which allowed the submission and approval of the Investigational Medicinal Product Dossier (IMPD) to the Competent Authority, addressing the quality problems due to different external suppliers. The result indicates that no significant differences have been found between the [18F]F- from the two suppliers in terms of radionuclidic and radiochemical purity and [18F]F- impacted neither the radiochemical yield of the labelling reaction nor the quality control parameters of the IMP [18F]F-PSMA-1007. These results prove how a correct quality assurance system can overcome some Regulatory Authorities issue that may represent an obstacle to the clinical use of F-18-labelled radiopharmaceuticals without an on-site cyclotron.

Accuracy of a new diagnostic tool in deep infiltrating endometriosis: Positron emission tomography-computed tomography with 16α-[18F]fluoro-17ß-estradiol.

AIM: Preoperative workup of deep infiltrating endometriosis is limited in the evaluation of extragenital and extrapelvic disease and in distinguishing between the previous surgical scar and active lesion. Histological verification remains the gold standard for diagnosis. The aim of this study was therefore to evaluate positron emission tomography-computed tomography (PET/CT) with an experimental estrogen receptor tracer (16α-[18F]fluoro-17ß-estradiol; [18F]FES) for accurate staging and non-invasive diagnosis of the disease. The primary endpoint was the feasibility of this tool on comparison with histology. The secondary endpoint was the accuracy of PET/CT in comparison with magnetic resonance imaging (MRI). METHODS: Four eligible subjects with extragenital endometriosis underwent MRI, PET/CT with [18F]FES, and laparoscopic excision of endometriosis in the same month. Region-by-region analysis was used to compare the findings of the two diagnostic tools with surgical histological specimens obtained during laparoscopy. RESULTS: A total of 40 anatomical regions were examined: seven were [18F]FES positive, four were positive on MRI and eight positive on histology. A total of nine regions were discordant. PET/CT agreed with histology in 9/9 of the discrepant findings. CONCLUSION: PET/CT with [18F]FES was feasible and had greater accuracy than MRI, particularly in patients with previous surgery. Further studies are needed, however, to investigate its role in bowel endometriosis in sites other than recto-sigmoid junction, nerve localization, and subcentimetric disease.

18F-NaF PET/CT Imaging of Brain Metastases.

F-NaF is a radiopharmaceutical widely used in PET imaging to detect bone metastases. Several cases of F-NaF uptake from brain metastases have been described, but a specific protocol for the evaluation of brain metastases with F-NaF has not been developed yet. Here we report images of F-NaF PET/CT, standard CT, and MRI of a brain metastasis in a patient with non-small lung cancer. Through a dynamic acquisition procedure, we have identified the first minutes after injection as the preferable time point of imaging acquisition for the study of brain metastases with F-NaF.

Radioguided video-assisted resection of non-palpable solitary pulmonary nodule/ground glass opacity: how to do it.

BACKGROUND: Detection of subcentimeter solitary pulmonary nodules (SPN) and ground glass opacities (GGO) is increased but their small size may make them difficult to be reached by computerized tomography (CT) guided fine needle agobiopsy or transbronchial biopsy. Surgical resection provides the gold standard for obtaining a specimen for histopathologic diagnosis, and video-assisted thoracic surgery (VATS) allows in many cases a minimally invasive technique of resections. The limit of VATS techniques is the need of nodule localization. Often-digital palpation is all needed to identify the appropriate area of resection, but sometimes it may be very difficult to identify and remove small, deep, non-palpable lesions. The criteria for nodule marking are unclear and variety of localization methods have been developed and they are effective but burdened by significant failure rate and complications. To increase the efficacy of thoracoscopic localization/ resection of small pulmonary nodules, we used the radioguided technique. METHODS: Under CT guidance, the nodule was identified and a needle was inserted to reach lesional or perilesional tissue. A solution of 99mtechnetium (99mTc) macro-aggregates albumin diluted with iodized contrast medium was injected. After injection, CT was performed to confirm precise staining. RESULTS: At VATS, a gamma detector probe allowed localization of nodules in all patients. Resection was performed, and suture margins were checked with the probe to search for residual hyperabsorption. All specimens underwent frozen section. Frozen section revealed diagnosis in all cases. CONCLUSIONS: Radioguided surgery is a cost-effective strategy for evaluating suspicious SPN/GGO with a success rate close to 100%, extremely low morbidity, and zero mortality. Radioguided VATS may be useful for preoperative localization of deep, small lung nodules that cannot be digitally localized or for GGO opacities that can be difficult to palpate even with the open technique.

Small-animal radionuclide luminescence imaging of thyroid and salivary glands with Tc99m-pertechnetate.

The in vitro and in vivo detection of visible photons from radioisotopes using optical techniques is a fast-growing field in molecular imaging. Tc99m-pertechnetate is used as an alternative to I123 in imaging of the thyroid and is generally imaged with gamma cameras or single photon emission tomography instruments. The uptake in the thyroid tissue is mediated by the sodium-iodide symporter (NIS), a glycoprotein that actively mediates iodide transport into the thyroid follicular cells and several extrathyroidal tissues. The luminescence of the gamma emitter Tc99m-pertechnetate in order to visualize its biodistribution in healthy small living animals by using a commercial optical imaging system is investigated. Here we show that in Nu/Nu mice, the uptake of Tc99m-pertechnetate in the thyroid gland and in salivary glands is very detectable by using radionuclide luminescence imaging. We also found light emission from the stomach in accordance with the literature. The localization of the light signals in the anatomical regions where the radiopharmaceutical is expected, confirmed by resections, shows that it is possible to image NIS-expressing tissues.