Clinical impact of using [18F]AlF-NOTA-octreotide PET/CT instead of [68Ga]Ga-DOTA-SSA PET/CT: Secondary endpoint analysis of a multicenter, prospective trial.

[18F]AlF-NOTA-octreotide ([18F]AlF-OC) is a promising alternative for [68Ga]Ga-DOTA-somatostatin analogs (SSAs) in positron emission tomography (PET) imaging of the somatostatin receptor (SSTR). Our aim is to assess changes in TNM staging and differences in patient management between [18F]AlF-OC PET/CT and [68Ga]Ga-DOTA-SSA PET/CT in the work-up of neuroendocrine tumor (NET) patients. Patients who underwent both [18F]AlF-OC and [68Ga]Ga-DOTA-TATE or [68Ga]Ga-DOTA-NOC PET/CT in our multicenter study (Pauwels et al., J Nucl Med.2023;63:632-638) with a NET were included for analysis. TNM staging was determined and compared for both tracers. For each patient, the blinded [68Ga]Ga-DOTA-SSA or [18F]AlF-OC PET/CT images were presented in random order at a multidisciplinary team board. The images were presented together with clinical information and compared with previous SSTR and [18F]FDG PET/CT imaging. After a consensus decision for patient management was recorded, the board was presented with the PET/CT images from the other SSTR tracer and a decision was made for the second tracer. Differences in management were classified as major if it entailed an intermodality change and minor if it led to an intramodality change. Compared with [68Ga]Ga-DOTA-SSA, the use of [18F]AlF-OC led to a change in 16/75 patients: TNM staging changes in 10/75 patients (13.3%; downstaging in 3/10, upstaging in 7/10) and differences in clinical management were seen in 10/75 patients (13.3%), leading to a major difference in 7/10 cases and a minor change in 3/10 cases. All 10 cases with a difference in patient management between both PET tracers were caused by additional lesion detection by [18F]AlF-OC. The use of [18F]AlF-OC did not impact TNM staging or clinical management in the large majority of the patients (86.7%), further validating the potential for routine clinical use of [18F]AlF-OC PET/CT as an alternative for [68Ga]Ga-DOTA-SSA PET/CT. The trial is registered under ClinicalTrials.gov identifier NCT04552847 and EudraCT 2020-000549-15.

Can current preclinical strategies for radiopharmaceutical development meet the needs of targeted alpha therapy?

Preclinical studies are essential for effectively evaluating TAT radiopharmaceuticals. Given the current suboptimal supply chain of these radionuclides, animal studies must be refined to produce the most translatable TAT agents with the greatest clinical potential. Vector design is pivotal, emphasizing harmonious physical and biological characteristics among the vector, target, and radionuclide. The scarcity of alpha-emitting radionuclides remains a significant consideration. Actinium-225 and lead-212 appear as the most readily available radionuclides at this stage. Available animal models for researchers encompass xenografts, allografts, and PDX (patient-derived xenograft) models. Emerging strategies for imaging alpha-emitters are also briefly explored. Ultimately, preclinical research must address two critical aspects: (1) offering valuable insights into balancing safety and efficacy, and (2) providing guidance on the optimal dosing of the TAT agent.

Second generation Al18F-labeled D-amino acid peptide for CXCR4 targeted molecular imaging.

BACKGROUND: The C-X-C chemokine receptor type 4 (CXCR4) is overexpressed in many cancers, e.g. multiple myeloma and acute leukemia, yet solely [68Ga]PentixaFor is used for clinical PET imaging. The aim of this study was to develop and assess a second generation Al18F-labeled D-amino acid peptide based on the viral macrophage inflammatory protein II for CXCR4 targeted molecular imaging. METHODS: We designed a library of monomer and multimer constructs and evaluated their binding affinity for human and mouse CXCR4. Based on these results, we selected the best vector molecule for development of an Al18F-labeled ligand, [18F]AlF-NOTA-2xDV1(c11sc12s), which was further evaluated in a cell-based binding assay to assess its binding properties and specificity for CXCR4. Next, pharmacokinetics and tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) were evaluated in naïve mice and mice with xenografts derived from U87.CXCR4 cells. Finally, we performed an imaging study in a non-human primate to assess the in vivo distribution of this novel radioligand in a species closely related to humans. RESULTS: The lead ligand AlF-NOTA-2xDV1(c11sc12s) showed six-fold higher affinity for human CXCR4 compared to Ga-Pentixafor. The corresponding radiotracer was obtained in a good radiochemical yield of 40.1 ± 13.5 % (n = 4) and apparent molar activity of 20.4 ± 3.3 MBq/nmol (n = 4) after optimization. In U87.CD4.CXCR4 cell binding assays, the total bound fraction of [18F]AlF-NOTA-(2×)DV1(c11sc12s) was 32.4 ± 1.8 %. This fraction could be reduced by 82.5 % in the presence of 75 µM AMD3100. In naïve mice, [18F]AlF-NOTA-2xDV1(c11sc12s) accumulated in organs expressing mouse CXCR4, e.g. the liver (SUVmean (mean standardized uptake value) 75 min p.i. 11.7 ± 0.6), which was blockable by co-injecting AMD3100 (5 mg/kg). In U87.CXCR4 xenografted tumor mice, the tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) remained low (SUVmean 0.5 ± 0.1), but was reduced by co-administration of AMD3100. Surprisingly, [18F]AlF-NOTA-2xDV1(c11sc12s) exhibited a similar biodistribution in a non-human primate as in mice indicating off-target binding of [18F]AlF-NOTA-2xDV1(c11sc12s) in liver tissue. We confirmed that [18F]AlF-NOTA-2xDV1(c11sc12s) is taken up by hepatocytes using in vitro studies and that the uptake can be blocked with AMD3100 and rifampicin, a potent organic anion-transporting-polypeptide (OATP)1B1 and OATP1B3 inhibitor. CONCLUSION: The second generation D-peptide AlF-NOTA-2xDV1(c11sc12s) showed high affinity for human CXCR4 and the corresponding radiotracer was produced in good radiochemical yields. However, [18F]AlF-NOTA-2xDV1(c11sc12s) is not specific for CXCR4 and is also a substrate for OATP1B1 and/or OATP1B3, known to mediate hepatic uptake. Therefore, D-amino acid peptides, based on the viral macrophage inflammatory protein II, are not the prefered vector molecule for the development of CXCR4 targeting molecular imaging tools.

Effectiveness of [67Cu]Cu-trastuzumab as a theranostic against HER2-positive breast cancer.

PURPOSE: To evaluate the imaging and therapeutic properties (theranostic) of 67Cu-labeled anti-human epidermal growth factor receptor II (HER2) monoclonal antibody trastuzumab against HER2-positive breast cancer (BC). METHODS: We conjugated trastuzumab with p-SCN-Bn-NOTA, 3p-C-NETA-NCS, or p-SCN-Bn-DOTA, and radiolabeled with [67Cu]CuCl2. Immunoconjugate internalization was evaluated in BT-474, JIMT-1 and MCF-7 BC cells. In vitro stability was studied in human serum (HS) and Phosphate Buffered Saline (PBS). Flow cytometry, radioligand binding and immunoreactive fraction assays were carried out. ImmunoSPECT imaging of [67Cu]Cu-NOTA-trastuzumab was done in mice bearing BT-474, JIMT-1 and MCF-7 xenografts. Pharmacokinetic was studied in healthy Balb/c mice while dosimetry was done in both healthy Balb/c and in athymic nude mice bearing JIMT-1 xenograft. The therapeutic effectiveness of [67Cu]Cu-NOTA-trastuzumab was evaluated in mice bearing BT-474 and JIMT-1 xenografts after a single intravenous (i.v.) injection of ~ 16.8 MBq. RESULTS: Pure immunoconjugates and radioimmunoconjugates (> 95%) were obtained. Internalization was HER2 density-dependent with highest internalization observed with NOTA-trastuzumab. After 5 days, in vitro stabilities were 97 ± 1.7%, 31 ± 6.2%, and 28 ± 4% in HS, and 79 ± 3.5%, 94 ± 1.2%, and 86 ± 2.3% in PBS for [67Cu]Cu-NOTA-trastuzumab, [67Cu]Cu-3p-C-NETA-trastuzumab and [67Cu]Cu-DOTA-trastuzumab, respectively. [67Cu]Cu-NOTA-trastuzumab was chosen for further evaluation. BT-474 flow cytometry showed low KD, 8.2 ± 0.2 nM for trastuzumab vs 26.5 ± 1.6 nM for NOTA-trastuzumab. There were 2.9 NOTA molecules per trastuzumab molecule. Radioligand binding assay showed a low KD of 2.1 ± 0.4 nM and immunoreactive fraction of 69.3 ± 0.9. Highest uptake of [67Cu]Cu-NOTA-trastuzumab was observed in JIMT-1 (33.9 ± 5.5% IA/g) and BT-474 (33.1 ± 10.6% IA/g) xenograft at 120 h post injection (p.i.). Effectiveness of the radioimmunoconjugate was also expressed as percent tumor growth inhibition (%TGI). [67Cu]Cu-NOTA-trastuzumab was more effective than trastuzumab against BT-474 xenografts (78% vs 54% TGI after 28 days), and JIMT-1 xenografts (90% vs 23% TGI after 19 days). Mean survival of [67Cu]Cu-NOTA-trastuzumab, trastuzumab and saline treated groups were > 90, 77 and 72 days for BT-474 xenografts, while that of JIMT-1 were 78, 24, and 20 days, respectively. CONCLUSION: [67Cu]Cu-NOTA-trastuzumab is a promising theranostic agent against HER2-positive BC.

Terbium radionuclides for theranostic applications in nuclear medicine: from atom to bedside.

Terbium features four clinically interesting radionuclides for application in nuclear medicine: terbium-149, terbium-152, terbium-155, and terbium-161. Their identical chemical properties enable the synthesis of radiopharmaceuticals with the same pharmacokinetic character, while their distinctive decay characteristics make them valuable for both imaging and therapeutic applications. In particular, terbium-152 and terbium-155 are useful candidates for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, respectively; whereas terbium-149 and terbium-161 find application in α- and ß--/Auger electron therapy, respectively. This unique characteristic makes the terbium family ideal for the "matched-pair" principle of theranostics. In this review, the advantages and challenges of terbium-based radiopharmaceuticals are discussed, covering the entire chain from radionuclide production to bedside administration. It elaborates on the fundamental properties of terbium, the production routes of the four interesting radionuclides and gives an overview of the available bifunctional chelators. Finally, we discuss the preclinical and clinical studies as well as the prospects of this promising development in nuclear medicine.

Synthesis and Preclinical Evaluation of PSMA-Targeted 111In-Radioconjugates Containing a Mitochondria-Tropic Triphenylphosphonium Carrier.

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted 111In-radioconjugates ([111In]In-TPP-DOTAGA-PSMA and [111In]In-TPP-DOTAGA-G3-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (111In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted 111In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [111In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [111In]In-TPP-DOTAGA-G3-PSMA to a higher extent than observed for the single-targeted congener [111In]In-PSMA-617. µSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the "golden standard" [111In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by 111In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., 161Tb or 165Er) and to further optimize the designed dual-targeting constructs.

Radiopharmaceuticals for Cancer Imaging and Therapy.

Nuclear medicine has emerged as a pivotal player in cancer patient care, revolutionizing the way cancer is detected, diagnosed, monitored, and treated [...].

Prospective comparison of [18F]AlF-NOTA-octreotide PET/MRI to [68Ga]Ga-DOTATATE PET/CT in neuroendocrine tumor patients.

BACKGROUND: Fluorine-18-labeled SSAs have the potential to become the next-generation tracer in SSTR-imaging in neuroendocrine tumor (NET) patients given their logistical advantages over the current gold standard gallium-68-labeled SSAs. In particular, [18F]AlF-OC has already shown excellent clinical performance. We demonstrated in our previous report from our prospective multicenter trial that [18F]AlF-OC PET/CT outperforms [68Ga]Ga-DOTA-SSA, but histological confirmation was lacking due to ethical and practical reasons. In this second arm, we therefore aimed to provide evidence that the vast majority of [18F]AlF-OC PET lesions are in fact true NET lesions by analyzing their MR characteristics on simultaneously acquired MRI. We had a special interest in lesions solely detected by [18F]AlF-OC ("incremental lesions"). METHODS: Ten patients with a histologically confirmed neuroendocrine tumor (NET) and a standard-of-care [68Ga]Ga-DOTATATE PET/CT, performed within 3 months, were prospectively included. Patients underwent a whole-body PET/MRI (TOF, 3 T, GE Signa), 2 hours after IV injection of 4 MBq/kg [18F]AlF-OC. Positive PET lesions were evaluated for a corresponding lesion on MRI. The diagnostic performance of both PET tracers was evaluated by determining the detection ratio (DR) for each scan and the differential detection ratio (DDR) per patient. RESULTS: In total, 195 unique lesions were detected: 167 with [68Ga]Ga-DOTATATE and 193 with [18F]AlF-OC. The DR for [18F]AlF-OC was 99.1% versus 91.4% for [68Ga]Ga-DOTATATE, significant for non-inferiority testing (p = 0.0001). Out of these 193 [18F]AlF-OC lesions, 96.2% were confirmed by MRI to be NET lesions. Thirty-three incremental lesions were identified by [18F]AlF-OC, of which 91% were confirmed by MRI and considered true positives. CONCLUSION: The DR of [18F]AlF-OC was numerically higher and non-inferior to the DR of [68Ga]Ga-DOTATATE. [18F]AlF-OC lesions and especially incremental lesions were confirmed as true positives by MRI in more than 90% of lesions. Taken together, these data further validate [18F]AlF-OC as a new alternative for SSTR PET in clinical practice. Trial registration ClinicalTrials.gov: NCT04552847. Registered 17 September 2020, https://beta. CLINICALTRIALS: gov/study/NCT04552847.

Radiochemical and Biological Evaluation of 3p-C-NETA-ePSMA-16, a Promising PSMA-Targeting Agent for Radiotheranostics.

Bifunctional chelators (BFCs) are a key element in the design of radiopharmaceuticals. By selecting a BFC that efficiently complexes diagnostic and therapeutic radionuclides, a theranostic pair possessing almost similar biodistribution and pharmacokinetic properties can be developed. We have previously reported 3p-C-NETA as a promising theranostic BFC, and the encouraging preclinical outcomes obtained with [18F]AlF-3p-C-NETA-TATE led us to conjugate this chelator to a PSMA-targeting vector for imaging and treatment of prostate cancer. In this study, we synthesized 3p-C-NETA-ePSMA-16 and radiolabeled it with different diagnostic (111In, 18F) and therapeutic (177Lu, 213Bi) radionuclides. 3p-C-NETA-ePSMA-16 showed high affinity to PSMA (IC50 = 4.61 ± 1.33 nM), and [111In]In-3p-C-NETA-ePSMA-16 showed specific cell uptake (1.41 ± 0.20% ID/106 cells) in PSMA expressing LS174T cells. Specific tumor uptake of [111In]In-3p-C-NETA-ePSMA-16 was observed up to 4 h p.i. (1.62 ± 0.55% ID/g at 1 h p.i.; 0.89 ± 0.58% ID/g at 4 h p.i.) in LS174T tumor-bearing mice. Only a faint signal could be seen at 1 h p.i. in the SPECT/CT scans, whereas dynamic PET/CT scans performed after administration of [18F]AlF-3p-C-NETA-ePSMA-16 in PC3-Pip tumor xenografted mice resulted in a better tumor visualization and imaging contrast. Therapy studies with short-lived radionuclides such as 213Bi could further elucidate the therapeutic potential of 3p-C-NETA-ePSMA-16 as a radiotheranostic.

18F-Labeled Somatostatin Analogs as PET Tracers for the Somatostatin Receptor: Ready for Clinical Use.

Molecular imaging of the somatostatin receptor plays a key role in the clinical management of neuroendocrine tumors. PET imaging with somatostatin analogs (SSAs) labeled with 68Ga or 64Cu is currently the gold standard in clinical practice. However, widespread implementation of 68Ga imaging is often hampered by practical and economic issues related to 68Ge/68Ga generators. 18F offers several advantages to tackle these issues. Recent developments in radiochemistry have allowed a shift from 68Ga toward 18F labeling, leading to promising clinical translations of 18F-labeled SSAs, such as Gluc-Lys-[18F]FP-TOCA, [18F]F-FET-ßAG-TOCA, [18F]AlF-NOTA-octreotide, [18F]SiTATE, and [18F]AlF-NOTA-JR11. This review gives an update of currently available clinical data regarding 18F-labeled SSA tracers and provides justification for the clinical application of this class of tracers.

Direct comparison of [18F]AlF-NOTA-JR11 and [18F]AlF-NOTA-octreotide for PET imaging of neuroendocrine tumors: Antagonist versus agonist.

BACKGROUND: [18F]AlF-NOTA-octreotide is an 18F-labeled somatostatin analogue which is a good clinical alternative for 68Ga-labeled somatostatin analogues. However, radiolabeled somatostatin receptor (SSTR) antagonists might outperform agonists regarding imaging sensitivity of neuroendocrine tumors (NETs). No direct comparison between the antagonist [18F]AlF-NOTA-JR11 and the agonist [18F]AlF-NOTA-octreotide as SSTR PET probes is available. Herein, we present the radiosynthesis of [18F]AlF-NOTA-JR11 and compare its NETs imaging properties directly with the established agonist radioligand [18F]AlF-NOTA-octreotide preclinically. METHODS: [18F]AlF-NOTA-JR11 was synthesized in an automated synthesis module. The in vitro binding characteristics (IC50) of [natF]AlF-NOTA-JR11 and [natF]AlF-NOTA-octreotide were evaluated and the in vitro stability of [18F]AlF-NOTA-JR11 was determined in human serum. In vitro cell binding and internalization was performed with [18F]AlF-NOTA-JR11 and [18F]AlF-NOTA-octreotide using SSTR2 expressing cells and the pharmacokinetics were evaluated using µPET/CT in mice bearing BON1.SSTR2 tumor xenografts. RESULTS: Excellent binding affinity for SSTR2 was found for [natF]AlF-NOTA-octreotide (IC50 of 25.7 ± 7.9 nM). However, the IC50 value for [natF]AlF-NOTA-JR11 (290.6 ± 71 nM) was 11-fold higher compared to [natF]AlF-NOTA-octreotide, indicating lower affinity for SSTR2. [18F]AlF-NOTA-JR11 was obtained in a good RCY (50 ± 6 %) but with moderate RCP of 94 ± 1 %. [18F]AlF-NOTA-JR11 demonstrated excellent stability in human serum (>95 % after 240 min). 2.7-fold higher cell binding was observed for [18F]AlF-NOTA-JR11 as compared to [18F]AlF-NOTA-octreotide after 60 min. µPET/CT images demonstrated comparable pharmacokinetics and tumor uptake between [18F]AlF-NOTA-JR11 (SUVmax: 3.7 ± 0.8) and [18F]AlF-NOTA-octreotide (SUVmax: 3.6 ± 0.4). CONCLUSIONS: [18F]AlF-NOTA-JR11 was obtained in good RCY, albeit with a moderate RCP. The cell binding study showed significant higher binding of [18F]AlF-NOTA-JR11 compared to [18F]AlF-NOTA-octreotide, despite the higher IC50 value of AlF-NOTA-JR11. However, pharmacokinetics and in vivo tumor uptake was comparable for both radiotracers. Novel Al18F-labeled derivatives of JR11 with higher SSTR2 affinity should be developed for increased tumor uptake and NET imaging sensitivity.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board.

BACKGROUND: The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biannual highlight commentary to update the readership on trends in the field of radiopharmaceutical development. MAIN BODY: This selection of highlights provides commentary on 21 different topics selected by each coauthoring Editorial Board member addressing a variety of aspects ranging from novel radiochemistry to first-in-human application of novel radiopharmaceuticals. CONCLUSION: Trends in radiochemistry and radiopharmacy are highlighted. Hot topics cover the entire scope of EJNMMI Radiopharmacy and Chemistry, demonstrating the progress in the research field, and include new PET-labelling methods for 11C and 18F, the importance of choosing the proper chelator for a given radioactive metal ion, implications of total body PET on use of radiopharmaceuticals, legislation issues and radionuclide therapy including the emerging role of 161Tb.

18F-AlF-NOTA-Octreotide Outperforms 68Ga-DOTATATE/NOC PET in Neuroendocrine Tumor Patients: Results from a Prospective, Multicenter Study.

18F-labeled somatostatin analogs (SSAs) could represent a valid alternative to the current gold standard, 68Ga-labeled SSAs, for somatostatin receptor imaging in patients with neuroendocrine tumors (NETs), given their logistic advantages. Recently, 18F-AlF-NOTA-octreotide (18F-AlF-OC) has emerged as a promising candidate, but a thorough comparison with 68Ga-DOTA-SSA in large patient groups is needed. This prospective, multicenter trial aims to demonstrate noninferiority of 18F-AlF-OC compared with 68Ga-DOTA-SSA PET in NET patients (ClinicalTrials.gov, NCT04552847). Methods: Seventy-five patients with histologically confirmed NET and routine clinical 68Ga-DOTATATE (n = 56) or 68Ga-DOTANOC (n = 19) PET, performed within a 3-mo interval of the study scan (median, 7 d; range, -30 to +32 d), were included. Patients underwent a whole-body PET 2 h after intravenous injection of 4 MBq/kg of 18F-AlF-OC. A randomized, masked consensus read was performed by 2 experienced readers to count tumor lesions. After unmasking, the detection ratio (DR) was determined for each scan, that is, the fraction of lesions detected on a scan compared with the union of lesions of both scans. The differential DR (DDR; difference in DR between 18F-AlF-OC and 68Ga-DOTATATE/NOC) per patient was calculated. Tracer uptake was evaluated by comparing SUVmax and tumor-to-background ratios in concordant lesions. Results: In total, 4,709 different tumor lesions were detected: 3,454 with 68Ga-DOTATATE/NOC and 4,278 with 18F-AlF-OC. The mean DR with 18F-AlF-OC was significantly higher than with 68Ga-DOTATATE/NOC (91.1% vs. 75.3%; P < 10-5). The resulting mean DDR was 15.8%, with a lower margin of the 95% CI (95% CI, 9.6%-22.0%) higher than -15%, which is the prespecified boundary for noninferiority. The mean DDRs for the 68Ga-DOTATATE and 68Ga-DOTANOC subgroups were 11.8% (95% CI, 4.3-19.3) and 27.5% (95% CI, 17.8-37.1), respectively. The mean DDR for most organs was higher than zero, except for bone lesions (mean DDR, -2.8%; 95% CI, -17.8 to 12.2). No significant differences in mean SUVmax were observed (P = 0.067), but mean tumor-to-background ratio was significantly higher with 18F-AlF-OC than with 68Ga-DOTATATE/NOC (31.7 ± 36.5 vs. 25.1 ± 32.7; P = 0.001). Conclusion: 18F-AlF-OC is noninferior and even superior to 68Ga-DOTATATE/NOC PET in NET patients. This validates 18F-AlF-OC as an option for clinical practice somatostatin receptor PET.

3p-C-NETA: A versatile and effective chelator for development of Al18F-labeled and therapeutic radiopharmaceuticals.

Background: Radiolabeled somatostatin analogues (e.g. [68Ga]Ga-DOTATATE and [177Lu]Lu-DOTATATE) have been used to diagnose, monitor, and treat neuroendocrine tumour (NET) patients with great success. [18F]AlF-NOTA-octreotide, a promising 18F-labeled somatostatin analogue and potential alternative for 68Ga-DOTA-peptides, is under clinical evaluation. However, ideally, the same precursor (combination of chelator-linker-vector) can be used for production of both diagnostic and therapeutic radiopharmaceuticals with very similar (e.g. Al18F-method in combination with therapeutic radiometals 213Bi/177Lu) or identical (e.g. complementary Tb-radionuclides) pharmacokinetic properties, allowing for accurate personalised dosimetry estimation and radionuclide therapy of NET patients. In this study we evaluated 3p-C-NETA, as potential theranostic Al18F-chelator and present first results of radiosynthesis and preclinical evaluation of [18F]AlF-3p-C-NETA-TATE. Methods: 3p-C-NETA was synthesized and radiolabeled with diagnostic (68Ga, Al18F) or therapeutic (177Lu, 161Tb, 213Bi, 225Ac and 67Cu) radionuclides at different temperatures (25-95 °C). The in vitro stability of the corresponding radiocomplexes was determined in phosphate-buffered saline (PBS) and human serum. 3p-C-NETA-TATE was synthesized using standard solid/liquid-phase peptide synthesis. [18F]AlF-3p-C-NETA-TATE was synthesized in an automated AllinOne® synthesis module and the in vitro stability of [18F]AlF-3p-C-NETA-TATE was evaluated in formulation buffer, PBS and human serum. [18F]AlF-3p-C-NETA-TATE pharmacokinetics were evaluated using µPET/MRI in healthy rats, with [18F]AlF-NOTA-Octreotide as benchmark. Results: 3p-C-NETA quantitatively sequestered 177Lu, 213Bi and 67Cu at 25 °C while heating was required to bind Al18F, 68Ga, 161Tb and 225Ac efficiently. The [18F]AlF-, [177Lu]Lu- and [161Tb]Tb-3p-C-NETA-complex showed excellent in vitro stability in both PBS and human serum over the study period. In contrast, [67Cu]Cu- and [225Ac]Ac-, [68Ga]Ga-3p-C-NETA were stable in PBS, but not in human serum. [18F]AlF-3p-C-NETA-TATE was obtained in good radiochemical yield and radiochemical purity. [18F]AlF-3p-C-NETA-TATE displayed good in vitro stability for 4 h in all tested conditions. Finally, [18F]AlF-3p-C-NETA-TATE showed excellent pharmacokinetic properties comparable with the results obtained for [18F]AlF-NOTA-Octreotide. Conclusions: 3p-C-NETA is a versatile chelator that can be used for both diagnostic applications (Al18F) and targeted radionuclide therapy (213Bi, 177Lu, 161Tb). It has the potential to be the new theranostic chelator of choice for clinical applications in nuclear medicine.

D-Peptide-Based Probe for CXCR4-Targeted Molecular Imaging and Radionuclide Therapy.

Positron emission tomography (PET) imaging of the C-X-C chemokine receptor 4 (CXCR4) with [68Ga]PentixaFor has intrinsic diagnostic value and is used to select patients for personalized CXCR4-targeted radionuclide therapy with its therapeutic radiopharmaceutical companion [177Lu]PentixaTher. However, a CXCR4-targeting radiopharmaceutical labeled with fluorine-18 is still of high value due to its favorable characteristics over gallium-68. Furthermore, clinical results with [177Lu]PentixaTher are promising, but there is still room for improvement regarding pharmacokinetics and dosimetry profile. Therefore, this study aimed to develop innovative CXCR4-targeting radiopharmaceuticals, both for diagnostic and therapeutic purposes, starting from a D-amino acid-based peptide probe (DV1-k-(DV3)) that conserves high CXCR4 binding affinity after radiolabeling. AlF-NOTA-DV1-k-(DV3) showed similar in vitro binding affinity to human CXCR4 (hCXCR4) compared to [natGa]PentixaFor (half-maximal inhibitory concentration (IC50): 5.3 ± 0.9 nM and 8.6 ± 1.1 nM, respectively) and also binds to murine CXCR4 (mCXCR4) (IC50: 33.4 ± 13.5 nM) while [natGa]PentixaFor is selective for hCXCR4 (IC50 > 1000 nM for mCXCR4). Both the diagnostic radiotracers based on the DV1-k-(DV3) vector platform, [18F]AlF-NOTA-DV1-k-(DV3) and [68Ga]Ga-DOTA-DV1-k-(DV3), and their therapeutic companion [177Lu]Lu-DOTA-DV1-k-(DV3) were successfully produced in high yield, demonstrated high in vitro and in vivo stability, and have the same favorable pharmacokinetic profile. Furthermore, in wild-type mice and a hCXCR4-expressing tumor model, [18F]AlF-NOTA-DV1-k-(DV3) shows CXCR4-specific targeting in mCXCR4-expressing organs such as liver (mean standardized uptake value (SUVmean) 8.2 ± 1.0 at 75 min post-injection (p.i.)), spleen (SUVmean 2.5 ± 1.0 at 75 min p.i.), and bone (SUVmean 0.4 ± 0.1 at 75 min p.i., femur harboring bone marrow) that can be blocked with the CXCR4 antagonist AMD3100. However, in a hCXCR4-expressing tumor model, tumor uptake of [18F]AlF-NOTA-DV1-k-(DV3) was significantly lower (SUVmean 0.6 ± 0.2) compared to [68Ga]PentixaFor (SUVmean 2.9). This might be explained by the high affinity of [18F]AlF-NOTA-DV1-k-(DV3) toward both mCXCR4 and hCXCR4. High mCXCR4 expression in mouse liver results in a large fraction of [18F]AlF-NOTA-DV1-k-(DV3) that is sequestered to the liver, resulting despite its similar in vitro affinity for hCXCR4, in lower tumor accumulation compared to [68Ga]PentixaFor. As CXCR4 is not expressed in healthy human liver, the findings in mice are not predictive for the potential clinical performance of this novel class of CXCR4-targeting radiotracers. In conclusion, the DV1-k-(DV3) scaffold is a promising vector platform for translational CXCR4-directed research.

Radiolabeling of Human Serum Albumin With Terbium-161 Using Mild Conditions and Evaluation of in vivo Stability.

Targeted radionuclide therapy (TRNT) is a promising approach for cancer therapy. Terbium has four medically interesting isotopes (149Tb, 152Tb, 155Tb and 161Tb) which span the entire radiopharmaceutical space (TRNT, PET and SPECT imaging). Since the same element is used, accessing the various diagnostic or therapeutic properties without changing radiochemical procedures and pharmacokinetic properties is advantageous. The use of (heat-sensitive) biomolecules as vector molecule with high affinity and selectivity for a certain molecular target is promising. However, mild radiolabeling conditions are required to prevent thermal degradation of the biomolecule. Herein, we report the evaluation of potential bifunctional chelators for Tb-labeling of heat-sensitive biomolecules using human serum albumin (HSA) to assess the in vivo stability of the constructs. p-SCN-Bn-CHX-A"-DTPA, p-SCN-Bn-DOTA, p-NCS-Bz-DOTA-GA and p-SCN-3p-C-NETA were conjugated to HSA via a lysine coupling method. All HSA-constructs were labeled with [161Tb]TbCl3 at 40°C with radiochemical yields higher than 98%. The radiolabeled constructs were stable in human serum up to 24 h at 37°C. 161Tb-HSA-constructs were injected in mice to evaluate their in vivo stability. Increasing bone accumulation as a function of time was observed for [161Tb]TbCl3 and [161Tb]Tb-DTPA-CHX-A"-Bn-HSA, while negligible bone uptake was observed with the DOTA, DOTA-GA and NETA variants over a 7-day period. The results indicate that the p-SCN-Bn-DOTA, p-NCS-Bz-DOTA-GA and p-SCN-3p-C-NETA are suitable bifunctional ligands for Tb-based radiopharmaceuticals, allowing for high yield radiolabeling in mild conditions.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board.

BACKGROUND: The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development. RESULTS: This commentary of highlights has resulted in 21 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals. Also the first contribution in relation to MRI-agents is included. CONCLUSIONS: Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Bismuth-213 for Targeted Radionuclide Therapy: From Atom to Bedside.

In contrast to external high energy photon or proton therapy, targeted radionuclide therapy (TRNT) is a systemic cancer treatment allowing targeted irradiation of a primary tumor and all its metastases, resulting in less collateral damage to normal tissues. The α-emitting radionuclide bismuth-213 (213Bi) has interesting properties and can be considered as a magic bullet for TRNT. The benefits and drawbacks of targeted alpha therapy with 213Bi are discussed in this review, covering the entire chain from radionuclide production to bedside. First, the radionuclide properties and production of 225Ac and its daughter 213Bi are discussed, followed by the fundamental chemical properties of bismuth. Next, an overview of available acyclic and macrocyclic bifunctional chelators for bismuth and general considerations for designing a 213Bi-radiopharmaceutical are provided. Finally, we provide an overview of preclinical and clinical studies involving 213Bi-radiopharmaceuticals, as well as the future perspectives of this promising cancer treatment option.

[18F]AlF-NOTA-octreotide PET imaging: biodistribution, dosimetry and first comparison with [68Ga]Ga-DOTATATE in neuroendocrine tumour patients.

PURPOSE: The widespread use of gallium-68-labelled somatostatin analogue (SSA) PET, the current standard for somatostatin receptor (SSTR) imaging, is limited by practical and economic challenges that could be overcome by a fluorine-18-labelled alternative, such as the recently introduced [18F]AlF-NOTA-octreotide ([18F]AlF-OC). This prospective trial aimed to evaluate safety, dosimetry, biodistribution, pharmacokinetics and lesion targeting of [18F]AlF-OC and perform the first comparison with [68Ga]Ga-DOTATATE in neuroendocrine tumour (NET) patients. METHODS: Six healthy volunteers and six NET patients with a previous clinical [68Ga]Ga-DOTATATE PET were injected with an IV bolus of 4 MBq/kg [18F]AlF-OC. Healthy volunteers underwent serial whole-body PET scans from time of tracer injection up to 90 min post-injection, with an additional PET/CT at 150 and 300 min post-injection. In patients, a 45-min dynamic PET was acquired and three whole-body PET scans at 60, 90 and 180 min post-injection. Absorbed organ doses and effective doses were calculated using OLINDA/EXM. Normal organ uptake (SUVmean) and tumour lesion uptake (SUVmax and tumour-to-background ratio (TBR)) were measured. A lesion-by-lesion analysis was performed and the detection ratio (DR), defined as the fraction of detected lesions was determined for each tracer. RESULTS: [18F]AlF-OC administration was safe and well tolerated. The highest dose was received by the spleen (0.159 ± 0.062 mGy/MBq), followed by the urinary bladder wall (0.135 ± 0.046 mGy/mBq) and the kidneys (0.070 ± 0.018 mGy/MBq), in accordance with the expected SSTR-specific uptake in the spleen and renal excretion of the tracer. The effective dose was 22.4 ± 4.4 µSv/MBq. The physiologic uptake pattern of [18F]AlF-OC was comparable to [68Ga]Ga-DOTATATE. Mean tumour SUVmax was lower for [18F]AlF-OC (12.3 ± 6.5 at 2 h post-injection vs. 18.3 ± 9.5; p = 0.03). However, no significant differences were found in TBR (9.8 ± 6.7 at 2 h post-injection vs. 13.6 ± 11.8; p = 0.35). DR was high and comparable for both tracers (86.0% for [68Ga]Ga-DOTATATE vs. 90.1% for [18F]AlF-OC at 2 h post-injection; p = 0.68). CONCLUSION: [18F]AlF-OC shows favourable kinetic and imaging characteristics in patients that warrant further head-to-head comparison to validate [18F]AlF-OC as a fluorine-18-labelled alternative for gallium-68-labelled SSA clinical PET. TRIAL REGISTRATION: Clinicaltrials.gov : NCT03883776, EudraCT: 2018-002827-40.

Simultaneous in vivo PET/MRI using fluorine-18 labeled Fe3O4@Al(OH)3 nanoparticles: comparison of nanoparticle and nanoparticle-labeled stem cell distribution.

BACKGROUND: Mesenchymal stem cells (MSCs) have shown potential for treatment of different diseases. However, their working mechanism is still unknown. To elucidate this, the non-invasive and longitudinal tracking of MSCs would be beneficial. Both iron oxide-based nanoparticles (Fe3O4 NPs) for magnetic resonance imaging (MRI) and radiotracers for positron emission tomography (PET) have shown potential as in vivo cell imaging agents. However, they are limited by their negative contrast and lack of spatial information as well as short half-life, respectively. In this proof-of-principle study, we evaluated the potential of Fe3O4@Al(OH)3 NPs as dual PET/MRI contrast agents, as they allow stable binding of [18F]F- ions to the NPs and thus, NP visualization and quantification with both imaging modalities. RESULTS: 18F-labeled Fe3O4@Al(OH)3 NPs (radiolabeled NPs) or mouse MSCs (mMSCs) labeled with these radiolabeled NPs were intravenously injected in healthy C57Bl/6 mice, and their biodistribution was studied using simultaneous PET/MRI acquisition. While liver uptake of radiolabeled NPs was seen with both PET and MRI, mMSCs uptake in the lungs could only be observed with PET. Even some initial loss of fluoride label did not impair NPs/mMSCs visualization. Furthermore, no negative effects on blood cell populations were seen after injection of either the NPs or mMSCs, indicating good biocompatibility. CONCLUSION: We present the application of novel 18F-labeled Fe3O4@Al(OH)3 NPs as safe cell tracking agents for simultaneous PET/MRI. Combining both modalities allows fast and easy NP and mMSC localization and quantification using PET at early time points, while MRI provides high-resolution, anatomic background information and long-term NP follow-up, hereby overcoming limitations of the individual imaging modalities.