PET/CT with Gluc-Lys-([(18)F]FP)-TOCA: correlation between uptake, size and arterial perfusion in somatostatin receptor positive lesions

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-βAG-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.

[18F]FET-βAG-TOCA: The Design, Evaluation and Clinical Translation of a Fluorinated Octreotide

The success of Lutathera™ ([¹⁷⁷Lu]Lu-DOTA-TATE) in the NETTER-1 clinical trial as a peptide receptor radionuclide therapy (PRRT) for somatostatin receptor expressing (SSTR) neuroendocrine tumours (NET) is likely to increase the demand for patient stratification by positron emission tomography (PET). The current gold standard of gallium-68 radiolabelled somatostatin analogues (e.g., [⁶⁸Ga]Ga-DOTA-TATE) works effectively, but access is constrained by the limited availability and scalability of gallium-68 radiopharmaceutical production. The aim of this review is three-fold: firstly, we discuss the peptide library design, biological evaluation and clinical translation of [¹⁸F]fluoroethyltriazole-βAG-TOCA ([¹⁸F]FET-βAG-TOCA), our fluorine-18 radiolabelled octreotide; secondly, to exemplify the potential of the 2-[¹⁸F]fluoroethylazide prosthetic group and copper-catalysed azide-alkyne cycloaddition (CuAAC) chemistry in accessing good manufacturing practice (GMP) compatible radiopharmaceuticals; thirdly, we aim to illustrate a framework for the translation of similarly radiolabelled peptides, in which in vivo pharmacokinetics drives candidate selection, supported by robust radiochemistry methodology and a route to GMP production. It is hoped that this review will continue to inspire the development and translation of fluorine-18 radiolabelled peptides into clinical studies for the benefit of patients.

The Search for an Alternative to [68Ga]Ga-DOTA-TATE in Neuroendocrine Tumor Theranostics: Current State of 18F-labeled Somatostatin Analog Development

The trend to inform personalized molecular radiotherapy with molecular imaging diagnostics, a concept referred to as theranostics, has transformed the field of nuclear medicine in recent years. The development of theranostic pairs comprising somatostatin receptor (SSTR)-targeting nuclear imaging probes and therapeutic agents for the treatment of patients with neuroendocrine tumors (NETs) has been a driving force behind this development. With the Neuroendocrine Tumor Therapy (NETTER-1) phase 3 trial reporting encouraging results in the treatment of well-differentiated, metastatic midgut NETs, peptide radioligand therapy (RLT) with the 177Lu-labeled somatostatin analog (SSA) [177Lu]Lu-DOTA-TATE is now anticipated to become the standard of care. On the diagnostics side, the field is currently dominated by 68Ga-labeled SSAs for the molecular imaging of NETs with positron emission tomography-computed tomography (PET/CT). PET/CT imaging with SSAs such as [68Ga]Ga-DOTA-TATE, [68Ga]Ga-DOTA-TOC, and [68Ga]Ga-DOTA-NOC allows for NET staging with high accuracy and is used to qualify patients for RLT. Driven by the demand for PET/CT imaging of NETs, a commercial kit for the production of [68Ga]Ga-DOTA-TATE (NETSPOT) was approved by the U.S. Food and Drug Administration (FDA). The synthesis of 68Ga-labeled SSAs from a 68Ge/68Ga-generator is straightforward and allows for a decentralized production, but there are economic and logistic difficulties associated with these approaches that warrant the search for a viable, generator-independent alternative. The clinical introduction of an 18F-labeled SSTR-imaging probe can help mitigate the shortcomings of the generator-based synthesis approach, but despite extensive research efforts, none of the proposed 18F-labeled SSAs has been translated past prospective first-in-humans studies so far. Here, we review the current state of probe-development from a translational viewpoint and make a case for a clinically viable, 18F-labeled alternative to the current standard [68Ga]Ga-DOTA-TATE.

Clinical Translation of a Click-Labeled 18F-Octreotate Radioligand for Imaging Neuroendocrine Tumors

We conducted the first-in-human study of (18)F-fluoroethyl triazole [Tyr(3)] octreotate ((18)F-FET-βAG-TOCA) in patients with neuroendocrine tumors (NETs) to evaluate biodistribution, dosimetry, and safety. Despite advances in clinical imaging, detection and quantification of NET activity remains a challenge, with no universally accepted imaging standard.

METHODS

Nine patients were enrolled. Eight patients had sporadic NETs, and 1 had multiple endocrine neoplasia type 1 syndrome. Patients received 137-163 MBq (mean ± SD, 155.7 ± 8 MBq) of (18)F-FET-βAG-TOCA. Safety data were obtained during and 24 h after radioligand administration. Patients underwent detailed whole-body PET/CT multibed scanning over 4 h with sampling of venous bloods for radioactivity and radioactive metabolite quantification. Regions of interest were defined to derive individual and mean organ residence times; effective dose was calculated with OLINDA 1.1.

RESULTS

All patients tolerated (18)F-FET-βAG-TOCA with no adverse events. Over 60% parent radioligand was present in plasma at 60 min. High tumor (primary and metastases)-to-background contrast images were observed. Physiologic distribution was seen in the pituitary, salivary glands, thyroid, and spleen, with low background distribution in the liver, an organ in which metastases commonly occur. The organs receiving highest absorbed dose were the gallbladder, spleen, stomach, liver, kidneys, and bladder. The calculated effective dose over all subjects (mean ± SD) was 0.029 ± 0.004 mSv/MBq.

CONCLUSION

The favorable safety, imaging, and dosimetric profile makes (18)F-FET-βAG-TOCA a promising candidate radioligand for staging and management of NETs. Clinical studies in an expanded cohort are ongoing to clinically qualify this agent.

Rerouting the metabolic pathway of (18)F-labeled peptides: the influence of prosthetic groups

Current translational cancer research is directed to the development of high affinity peptide ligands for targeting neuropeptide receptors overexpressed in different types of cancer. Besides their desired high binding affinity to the receptor, the suitability of radiolabeled peptides as targeting vectors for molecular imaging and therapy depends on additional aspects such as high tumor-to-background ratio, favorable clearance pattern from nontarget tissue, and sufficient metabolic stability in vivo. This study reports how a switch from the prosthetic group, N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB), to 2-deoxy-2-[(18)F]fluoro-d-glucose ([(18)F]FDG) effects the metabolic pathway of an (18)F-labeled bombesin derivative, QWAV-Sar-H-FA01010-Tle-NH2. (18)F-Labeled bombesin derivatives represent potent peptide ligands for selective targeting of gastrin-releasing peptide (GRP) receptor-expressing prostate cancer. Radiosynthesis of (18)F-labeled bombesin analogues [(18)F]FBz-Ava-BBN2 and [(18)F]FDG-AOAc-BBN2 was achieved in good radiochemical yields of ~50% at a specific activity exceeding 40 GBq/μmol. Both nonradioactive compounds FBz-Ava-BBN2 and FDG-AOAc-BBN2 inhibited binding of [(125)I]Tyr(4)-bombesin(1-14) in PC3 cells with IC50 values of 9 and 16 nM, respectively, indicating high inhibitory potency. Influence of each prosthetic group was further investigated in PC3 mouse xenografts using dynamic small animal PET imaging. In comparison to [(18)F]FBz-Ava-BBN2, total tumor uptake levels were doubled after injection of [(18)F]FDG-AOAc-BBN2 while renal elimination was increased. Blood clearance and in vivo metabolic stability were similar for both compounds. The switch from [(18)F]SFB to [(18)F]FDG as the prosthetic group led to a significant reduction in lipophilicity which resulted in more favorable renal clearance and increased tumor uptake. The presented single step radiolabeling-glycosylation approach represents an innovative strategy for site-directed peptide labeling with the short-lived positron emitter (18)F while providing a favorable pharmacokinetic profile of (18)F-labeled peptides.

Using 5-deoxy-5-[18F]fluororibose to glycosylate peptides for positron emission tomography

So far seven peptide-based (18)F-radiopharmaceuticals for diagnostic applications with positron emission tomography (PET) have entered into clinical trials. Three candidates out of these seven are glycosylated peptides, which may be explained by the beneficial influence of glycosylation on in vivo pharmacokinetics of peptide tracers. This protocol describes the method for labeling peptides with 5-deoxy-5-[(18)F]fluororibose ([(18)F]FDR) as a prosthetic group. The synthesis of [(18)F]FDR is effected by a nucleophilic fluorination step by using dried Kryptofix 2.2.2-K2CO3-K(18)F complex and a subsequent HCl-catalyzed hydrolysis. The conjugation of [(18)F]FDR to the N-terminus aminooxy (-ONH2)-functionalized peptides is carried out in anilinium buffer at pH 4.6 and at room temperature (RT, 21-23 °C), with the concentration of peptide precursors being 0.3 mM. The procedure takes about 120 min and includes two cartridge isolation steps and two reversed-phase (RP) HPLC purification steps. The quaternary methyl amine (QMA) anion exchange cartridge and the hydrophilic-lipophilic balanced (HLB) cartridge are used for the isolation of (18)F-fluoride and [(18)F]FDR-conjugated peptides, respectively. The first HPLC purification provides the (18)F-fluorinated precursor of [(18)F]FDR and the second HPLC purification is to separate labeled peptides from their unlabeled precursors. The final product is formulated in PBS ready for injection, with a radiochemical purity of >98% and a radiochemical yield (RCY) of 27-37% starting from the end of bombardment (EOB). The carbohydrate nature of [(18)F]FDR and the operational convenience of this protocol should facilitate its general use.

Quantification in (68)Ga-DOTA(0)-Phe(1)-Tyr(3)-octreotide positron emission tomography/computed tomography: can we be impartial about partial volume effects?

AIM

In combined positron emission tomography/computed tomography (PET/CT) of neuroendocrine neoplasms using (68)Ga-DOTA(0)-Phe(1)-Tyr(3)-octreotide ((68)Ga-DOTATOC), partial volume effects (PVEs) may occur in smaller lesions. This study determined the lesional cutoff size for the occurrence of PVEs in a clinical setting.

METHODS

Retrospective assessment of 51 PET/CT examinations (16-slice PET/CT device) for malignant PET foci was carried out. In all foci, the maximal standardized uptake value (SUVmax) and maximal lesion diameter on axial CT was documented. Determined SUVmax and lesional sizes were correlated via LOESS regression. In the resulting curve, the cutoff point for SUVmax size dependency was determined visually and mathematically using 2 approximating straight lines.

RESULTS

In 45 patients, 313 of 413 PET foci found were malignant, measurable on CT and had a roughly spherical geometry (SUVmax: 2.5-103.3, mean ± SD 20.5 ± 15.18; CT diameter: 5-103 mm, mean ± SD 21.8 ± 13.1 mm). The cutoff lesional size for the occurrence of PVEs was 20.4 mm by the mathematical approach and 25 mm by visual assessment.

CONCLUSION

In (68)Ga-DOTATOC imaging, the clinical lesional size threshold is far larger than expected from systemic resolution only. Thus, tracer uptake quantification is only acceptable in sufficiently large lesions.

Current molecular imaging positron emitting radiotracers in oncology

Molecular imaging is one of the fastest growing areas of medical imaging. Positron emission tomography (PET) has been widely used in the clinical management of patients with cancer. Nuclear imaging provides biological information at the cellular, subcellular, and molecular level in living subjects with non-invasive procedures. In particular, PET imaging takes advantage of traditional diagnostic imaging techniques and introduces positron-emitting probes to determine the expression of indicative molecular targets at different stages of cancer. (18)F-fluorodeoxyglucose ((18)F-FDG), the only FDA approved oncological PET tracer, has been widely utilized in cancer diagnosis, staging, restaging, and even monitoring response to therapy; however, (18)F-FDG is not a tumor-specific PET tracer. Over the last decade, many promising tumor-specific PET tracers have been developed and evaluated in preclinical and clinical studies. This review provides an overview of the current non-(18)F-FDG PET tracers in oncology that have been developed based on tumor characteristics such as increased metabolism, hyperproliferation, angiogenesis, hypoxia, apoptosis, and tumor-specific antigens and surface receptors.

Detection of unknown primary neuroendocrine tumours (CUP-NET) using (68)Ga-DOTA-NOC receptor PET/CT

PURPOSE

This bi-centric study aimed to determine the role of receptor PET/CT using (68)Ga-DOTA-NOC in the detection of undiagnosed primary sites of neuroendocrine tumours (NETs) and to understand the molecular behaviour of the primarily undiagnosed tumours.

METHODS

Overall 59 patients (33 men and 26 women, age: 65 + or - 9 years) with documented NET and unknown primary were enrolled. PET/CT was performed after injection of approximately 100 MBq (46-260 MBq) of (68)Ga-DOTA-NOC. The maximum standardised uptake values (SUV(max)) were calculated and compared with SUV(max) in known pancreatic NET (pNET) and ileum/jejunum/duodenum (SI-NET). The results of PET/CT were also correlated with CT alone.

RESULTS

In 35 of 59 patients (59%), (68)Ga-DOTA-NOC PET/CT localised the site of the primary: ileum/jejunum (14), pancreas (16), rectum/colon (2), lungs (2) and paraganglioma (1). CT alone (on retrospective analyses) confirmed the findings in 12 of 59 patients (20%). The mean SUV(max) of identified previously unknown pNET and SI-NET were 18.6 + or - 9.8 (range: 7.8-34.8) and 9.1 + or - 6.0 (range: 4.2-27.8), respectively. SUV(max) in patients with previously known pNET and SI-NET were 26.1 + or - 14.5 (range: 8.7-42.4) and 11.3 + or - 3.7 (range: 5.6-17.9). The SUV(max) of the unknown pNET and SI-NET were significantly lower (p < 0.05) as compared to the ones with known primary tumour sites; 19% of the patients had high-grade and 81% low-grade NET. Based on (68)Ga-DOTA-NOC receptor PET/CT, 6 of 59 patients were operated and the primary was removed (4 pancreatic, 1 ileal and 1 rectal tumour) resulting in a management change in approximately 10% of the patients. In the remaining 29 patients, because of the far advanced stage of the disease (due to distant metastases), the primary tumours were not operated. Additional histopathological sampling was available from one patient with bronchial carcinoid (through bronchoscopy).

CONCLUSION

Our data indicate that (68)Ga-DOTA-NOC PET/CT is highly superior to (111)In-OctreoScan (39% detection rate for CUP according to the literature) and can play a major role in the management of patients with CUP-NET.

The current role of PET-CT in the characterization of hepatobiliary malignancies

BACKGROUND

Surgery has become heavily dependent on accurate imaging in the assessment and treatment of suspected or confirmed intra-abdominal malignancy. Positron emission tomography-computed tomography (PET-CT) fuses uptake of a radiotracer combined with CT images to assess both functional tissue activity and anatomical detail. Since its introduction it has offered new ways of treating gastrointestinal cancers.

METHODS

The review analyses the present literature regarding the use of PET-CT in the assessment, diagnosis, staging and treatment of hepatobiliary malignancies.

RESULTS

PET-CT is widely used in pre-operative tumours staging for colorectal liver metastases. There is convincing data that it may also be applicable for neuroendocrine tumours, assessment of indeterminate pancreas lesions and clinical drug trials. PET-CT is of limited value in hepatocellular cancers, although new techniques in dual-tracer PET-CT may change this.

CONCLUSION

Knowledge of the strengths and limitations of PET-CT is important for all surgeons managing cancer of the hepatobiliary system. More clinical data are required on PET-CT, particularly its effect on long-term survival in PET-CT-staged patients undergoing resection.

Correlation of immunohistopathological expression of somatostatin receptor 2 with standardised uptake values in 68Ga-DOTATOC PET/CT

PURPOSE

In clinical routine somatostatin analogue positron emission tomography/computed tomography (PET/CT) such as (68)Ga-DOTA-Tyr-octreotide (DOTATOC)-PET/CT could substitute conventional (111)In-Octreotide scintigraphy. Immunohistochemistry (IHC) for somatostatin receptor 2 (SSTR2) might be a tool to predict positivity of (68)Ga-DOTATOC in patients where initial staging was not performed, e.g., in incidental findings. We therefore compared a score of SSTR2-IHC with the in vivo standard uptake value (SUV) of preoperative or prebiopsy (68)Ga-DOTATOC PET/CT.

MATERIALS AND METHODS

In 18 patients, (68)Ga-DOTATOC PET/CT scans were quantified with SUV calculations and correlated to a cell membrane-based SSTR2-IHC score (ranging from 0 to 3).

RESULTS

Negative IHC scores were consistent with SUV values below 10. Furthermore, all score 2 and 3 specimens corresponded with high SUV values (above 15).

CONCLUSION

SSTR2-IHC scores correlated well with SUV values and we propose to use SSTR2 immunohistochemistry in patients missing a preoperative PET scan to indicate (68)Ga-DOTATOC-PET/CT as method for restaging and follow-up in individual patients.