Are radiogallium-labelled DOTA-conjugated somatostatin analogues superior to those labelled with other radiometals?

68Ga labeled EphA2-targeted cyclic peptide: a novel positron imaging tracer for triple-negative breast cancer?

The absence of better biomarkers currently limits early diagnosis and treatment of triple-negative breast cancer (TNBC). Our previously published study reported that the cyclic-peptide SD01 exhibited specific binding to EphA2 (Ephrin type-A receptor 2) on TNBC. To develop a novel PET imaging agent, we prepared gallium-68 (68Ga) labeled-DOTA-SD01 and evaluated its specificity and effectiveness through micro PET/CT imaging in a TNBC-bearing mouse model. SD01 and a control linear peptide YSA were conjugated to DOTA and subsequently labeled with 68Ga, obtaining 68Ga-DOTA-SD01 and 68Ga-DOTA-YSA. Both showed high radiochemical purity, stability, good hydrophilicity, and high binding affinity to 4T1 cells. Micro PET/CT imaging showed high radioactivity accumulation in tumors; SUVmean (mean standardized uptake value) of tumors in the group of 68Ga-DOTA-SD01 was 3.34 ± 0.25 and 2.65 ± 0.32 in the group of 68Ga-DOTA-YSA; T/NT ratios (target to non-target, SUVmean ratios of tumor to muscle) were 3.12 ± 0.06 and 2.77 ± 0.11 at 30 min, respectively (p < 0.05). The biodistribution study showed that tumor uptake % ID per g (percentage of injected dose per gram of tissue) in the group of 68Ga-DOTA-SD01 was 2.73 ± 0.34, and 1.77 ± 0.38 in the group of 68Ga-DOTA-YSA; T/NT ratios (radioactivity of tumor to muscle) were 3.55 ± 0.12 and 3.05 ± 0.10 for both groups at 30 min, respectively (p < 0.05). All these suggest that 68Ga-DOTA-SD01 may act as a better novel PET imaging agent for EphA2 positive tumors, such as TNBC.

New horizon of radiopharmaceuticals in management of neuroendocrine tumors

Neuroendocrine neoplasms are rare and heterogenous group of tumors with varying degrees of clinical presentations and involvement of multiple organ systems in the body. In the modern clinical practice somatostatin receptor molecular imaging and targeted radioligand therapy plays a vital role in the diagnosis and management of the disease. Several new and promising radiotracers for NET imaging and theranostics, belonging to various groups and classes are being studied and investigated. This exponential growth of radiotracers poses concerns about the indication, clinical benefit, and safety profile of the agents. We discuss the basis behind these radiotracers clinical use, receptor targeting and intra and inter tumor heterogeneity. Furthermore, role of dual tracer imaging, combination therapy and potential applications of dosimetry in predicting treatment outcome and safety profile is reviewed. Individualized precision medicine with better tumor characterization, maximum therapeutic benefit and minimum toxicity is the way forward for future medicine.

Prognostic Implications of PET-Derived Tumor Volume and Uptake in Patients with Neuroendocrine Tumors

Historically, molecular imaging of somatostatin receptor (SSTR) expression in patients with neuroendocrine tumors (NET) was performed using SSTR scintigraphy (SRS). Sustained advances in medical imaging have led to its gradual replacement with SSTR positron-emission tomography (SSTR-PET). The higher sensitivity in comparison to SRS on the one hand and conventional cross-sectional imaging, on the other hand, enables more accurate staging and allows for image quantification. In addition, in recent years, a growing body of evidence has assessed the prognostic implications of SSTR-PET-derived prognostic biomarkers for NET patients, with the aim of risk stratification, outcome prognostication, and prediction of response to peptide receptor radionuclide therapy. In this narrative review, we give an overview of studies examining the prognostic value of advanced SSTR-PET-derived (semi-)quantitative metrics like tumor volume, uptake, and composite metrics. Complementing this analysis, a discussion of the current trends, clinical implications, and future directions is provided.

Gastroenteropancreatic Neuroendocrine Tumor Diagnosis: DOTATATE PET/CT

Gastroenteropancreatic neuroendocrine tumors(NETs), which arise from the small intestine, rectum, colon, appendix, or pancreas, have variable malignant potential with clinical behavior determined by proliferative activity according to the Ki-67 index and tumor differentiation. Somatostatin receptor (SSTR) expression by NETs allows SSTR imaging using 68Ga-DOTATATE PET/computed tomography (CT) and treatment with octreotide or SSTR-targeted peptide receptor radionuclide therapy (PRRT). 68Ga-DOTATATE PET/CT is indicated for localization of the primary tumor in select cases, staging patients with known NET, and selecting patients for PRRT. NCCN guidelines consider imaging with 68Ga-DOTATATE PET/CT appropriate for staging and receptor status assessment.

Correlations between [68Ga]Ga-DOTA-TOC Uptake and Absorbed Dose from [177Lu]Lu-DOTA-TATE

PURPOSE

The aim of this paper was to investigate correlations between pre- therapeutic [⁶⁸Ga]Ga-DOTA-TOC uptake and absorbed dose to tumours from therapy with [¹⁷⁷Lu]Lu-DOTA-TATE.

METHODS

This retrospective study included 301 tumours from 54 GEP-NET patients. The tumours were segmented on pre-therapeutic [⁶⁸Ga]Ga-DOTA-TOC PET/CT, and post-therapy [¹⁷⁷Lu]Lu-DOTA-TATE SPECT/CT images, using a fixed 40% threshold. The SPECT/CT images were used for absorbed dose calculations by assuming a linear build-up from time zero to day one, and mono-exponential wash-out after that. Both SUV_mean and SUV_max were measured from the PET images. A linear absorbed-dose prediction model was formed with SUV_mean as the independent variable, and the accuracy was tested with a split 70-30 training-test set.

RESULTS

Mean SUV_mean and SUV_max from [⁶⁸Ga]Ga-DOTA-TOC PET was 24.0 (3.6-84.4) and 41.0 (6.7-146.5), and the mean absorbed dose from [¹⁷⁷Lu]Lu-DOTA-TATE was 26.9 Gy (2.4-101.9). A linear relationship between SUV_mean and [¹⁷⁷Lu]Lu-DOTA-TATE activity concentration at 24 h post injection was found (R² = 0.44, p < 0.05). In the prediction model, a root mean squared error and a mean absolute error of 1.77 and 1.33 Gy/GBq, respectively, were found for the test set.

CONCLUSIONS

There was a high inter- and intra-patient variability in tumour measurements, both for [⁶⁸Ga]Ga-DOTA-TOC SUVs and absorbed doses from [¹⁷⁷Lu]Lu-DOTA-TATE. Depending on the required accuracy, [⁶⁸Ga]Ga-DOTA-TOC PET imaging may estimate the [¹⁷⁷Lu]Lu-DOTA-TATE uptake. However, there could be a high variance between predicted and actual absorbed doses.

A personal acquisition time regimen of 68Ga-DOTATATE total-body PET/CT in patients with neuroendocrine tumor (NET): a feasibility study

BACKGROUND

The injection activity of tracer, acquisition time, patient-specific photon attenuation, and large body mass, can influence on image quality. Fixed acquisition time and body mass related injection activity in clinical practice results in a large difference in image quality. Thus, this study proposes a patient-specific acquisition time regimen of ⁶⁸ Ga-DOTATATE total-body positron emission tomography-computed tomography (PET/CT) to counteract the influence of body mass (BM, kg) on image quality, and acquire an acceptable and constant image of patients with neuroendocrine tumors (NETs).

METHODS

The development cohort consisting of 19 consecutive patients with full activity (88.7-204.9 MBq, 2.0 ± 0.1 MBq/kg) was to establish the acquisition time regimen. The liver SNR (signal-to-noise ratio, SNR_L) was normalized (SNR_norm) by the product of injected activity (MBq) and acquisition time (min). Fitting of SNR_norm against body mass (BM, kg) in linear correlation was performed. Subjective assessment of image quality was performed using a 5-point Likert scale to determine the acceptable threshold of SNR_L, and an optimized acquisition regimen based on BM was proposed, and validated its feasibility through the validation cohort of 57 consecutive NET patients with half activity (66.9 ± 11.3 MBq, 1.0 ± 0.1 MBq/kg) and a fixed acquisition time regimen.

RESULTS

The linear correlation (R² = 0.63) between SNR_norm and BM (kg) was SNR_norm = -0.01*BM + 1.50. The threshold SNR_L of acceptable image quality was 11.2. The patient-specific variable acquisition time regimen was determined as: t (min) = 125.4/(injective activity)*(-0.01*BM + 1.50)². Based on that proposed regimen, the average acquisition time for acceptable image quality in the validation cohort was 2.99 ± 0.91 min, ranging from 2.18 to 6.35 min, which was reduced by 36.50% ~ 78.20% compared with the fixed acquisition time of 10 min. Subjective evaluation showed that acceptable image quality could be obtained at 3.00 min in the validation group, with an average subjective score of 3.44 ± 0.53 (kappa = 0.97, 95% CI: 0.96 ~ 0.98). Bland-Altman analysis revealed good agreement between the proposed regimen and the fixed acquisition time cohort.

CONCLUSION

A patient-specific acquisition time regimen was proposed in NET patients in development cohort and validated its feasibility in patients with NETs in validation cohort by ⁶⁸ Ga-DOTATATE total-body PET/CT imaging. Based on the proposed regimen, the homogenous image quality with optimal acquisition time was available independent of body mass.

[68Ga]Ga-DOTANOC Uptake at Pancreatic Head/Uncinate Process: Is It a Persistent Diagnostic Pitfall Over Time?

Simple Summary

The purpose of the present study is to describe the frequency of non-malignant [68Ga]Ga-DOTANOC uncinate process (UP) uptake and to evaluate its variations over time. Among the first 6 months of enrolment of a monocentric prospective observational electronic archive, analyses of a large number of PET/CT scans (n = 268) of NEN pts (n = 172) demonstrated that: UP uptake is a frequent finding (observed in almost half of the cases, slightly higher than previously reported), mostly presenting with a diffuse pattern and, interestingly, characterized by variations over time in almost one third of the cases. The consciousness of this diagnostic pitfall is of utmost importance for accurate [68Ga]Ga-DOTANOC PET/CT reporting, since the pancreas represents one of the most frequent sites of NEN.

Abstract

Purpose: [68Ga]Ga-DOTA-peptide uptake in the pancreatic head/uncinate process (UP) is a frequent PET/CT finding. Although mostly physiologic, it can represent a pitfall in PET/CT reading, especially when focal. An increased frequency of UP uptake has been reported in patients (pts) affected by diabetes mellitus (DM). The aim of the study is to describe the frequency of [68Ga]Ga-DOTANOC UP uptake to evaluate its variations over time and its possible correlation with DM. Methods: In September 2017, a monocentric prospective observational electronic archive was initiated at our center to collect clinical and imaging data of pts undergoing [68Ga]Ga-DOTANOC PET/CT. Among the pts enrolled in the first 6 months (Sept 2017 to Feb 2018), those presenting [68Ga]Ga-DOTANOC PET/CT uptake at UP level were included. Pts with UP lesions already documented on CT/MRI or those that underwent surgical excision of UP before PET/CT were excluded from the analysis. [68Ga]Ga-DOTANOC UP uptake was classified as diffuse or focal and compared with the pattern observed in previous PET/CT scans performed at our center. An increased frequency of UP uptake was also correlated with the presence of DM. Results: In the first 6 months, 253 pts were enrolled in the archive and 172 out of them were included in the analysis. UP increased uptake was frequently observed (77/172, 44.8%) and was mostly diffuse (62/77). In 75/172 pts (43.6%), previous [68Ga]Ga-DOTANOC PET/CT scans were available (overall 268 scans; number of previous PET per pt range: 1–20) and were retrospectively reviewed. Despite the fact that, in most pts, the uptake pattern was stable over time (54/75 pts, 72%), it changed in approximately one third of cases (21/75, 28%). Among DM pts (29/172), only 10/29 (34.4%) presented increased UP uptake. Conclusions: UP [68Ga]Ga-DOTANOC uptake is a frequent non-malignant finding (slightly higher than previously reported), mostly presenting with a diffuse pattern. However, contrary to previous reports, our data show that the pattern of uptake may vary over time in approximately one third of the cases and it is not more frequently observed in pts with DM.

An Appraisal and Update of Fluorodeoxyglucose and Non-Fluorodeoxyglucose-PET Tracers in Thyroid and Non-Thyroid Endocrine Neoplasms

Endocrine neoplasms and malignancies are a diverse group of tumors with varied clinical, histopathologic, and functional features. These tumors vary from sporadic to hereditary, isolated entities to multiple neoplastic syndromes, functioning and non functioning tumors, unifocal locally invasive, and advanced to multifocal tumors with disseminated distant metastases. The presence of various specific biomarkers and specific receptor targets serves as valuable tools for diagnosis, prognosis, and management. PET-CT with FDG and a multitude of novel and specific radiotracers towards specific therapeutic targets mandates personalization of their use, so as to ensure maximum clinical benefit in the management of these neoplasms.

Evaluation of a Radiolabelled Macrocyclic Peptide as Potential PET Imaging Probe for PD-L1

The interaction between the immune checkpoint PD‐1 and PD−L1 promotes T‐cell deactivation and cancer proliferation. Therefore, immune checkpoint inhibition therapy, which relies on prior assessment of the target, has been widely used for many cancers. As a non‐invasive molecular imaging tool, radiotracers bring novel information on the in vivo expression of biomarkers (e. g., PD−L1), enabling a personalized treatment of patients. Our work aimed at the development of a PD−L1‐specific, peptide‐based PET radiotracer. We synthesized and evaluated a radiolabeled macrocyclic peptide adapted from a patent by Bristol Myers Squibb. Synthesis of [⁶⁸Ga]Ga‐NJMP1 yielded a product with a radiochemical purity>95 % that was evaluated in vitro. However, experiments on CHO−K1 hPD−L1 cells showed very low cell binding and internalization rates of [⁶⁸Ga]Ga‐NJMP1 in comparison to a control radiopeptide (WL12). Non‐radioactive cellular assays using time‐resolved fluorescence energy transfer confirmed the low affinity of the reported parent peptide and the DOTA‐derivatives towards PD−L1. The results of our studies indicate that the macrocyclic peptide scaffold reported in the patent literature is not suitable for radiotracer development due to insufficient affinity towards PD−L1 and that C‐terminal modifications of the macrocyclic peptide interfere with important ligand/receptor interactions.

Radiolabeled Somatostatin Analogs-A Continuously Evolving Class of Radiopharmaceuticals

Somatostatin receptors (SSTs) are recognized as favorable molecular targets in neuroendocrine tumors (NETs) and neuroendocrine neoplasms (NENs), with subtype 2 (SST₂) being the predominantly and most frequently expressed. PET/CT imaging with ⁶⁸Ga-labeled SST agonists, e.g., ⁶⁸Ga-DOTA-TOC (SomaKit TOC^®) or ⁶⁸Ga-DOTA-TATE (NETSPOT^®), plays an important role in staging and restaging these tumors and can identify patients who qualify and would potentially benefit from peptide receptor radionuclide therapy (PRRT) with the therapeutic counterparts ¹⁷⁷Lu-DOTA-TOC or ¹⁷⁷Lu-DOTA-TATE (Lutathera^®). This is an important feature of SST targeting, as it allows a personalized treatment approach (theranostic approach). Today, new developments hold promise for enhancing diagnostic accuracy and therapeutic efficacy. Among them, the use of SST₂ antagonists, such as JR11 and LM3, has shown certain advantages in improving image sensitivity and tumor radiation dose, and there is evidence that they may find application in other oncological indications beyond NETs and NENs. In addition, PRRT performed with more cytotoxic α-emitters, such as ²²⁵Ac, or β^- and Auger electrons, such as ¹⁶¹Tb, presents higher efficacy. It remains to be seen if any of these new developments will overpower the established radiolabeled SST analogs and PRRT with β^--emitters.

Treatment of Neuroendocrine Neoplasms with Radiolabeled Peptides-Where Are We Now

Peptide receptor radionuclide therapy (PRRT) has been one of the most successful and exciting examples of theranostics in nuclear medicine in recent decades and is now firmly embedded in many treatment algorithms for unresectable or metastatic neuroendocrine neoplasms (NENs) worldwide. It is widely considered to be an effective treatment for well- or moderately differentiated neoplasms, which express high levels of somatostatin receptors that can be selectively targeted. This review article outlines the scientific basis of PRRT in treatment of NENs and describes its discovery dating back to the early 1990s. Early treatments utilizing Indium-111, a γ-emitter, showed promise in reduction in tumor size and improvement in biochemistry, but were also met with high radiation doses and myelotoxic and nephrotoxic effects. Subsequently, stable conjugation of DOTA-peptides with β-emitting radionuclides, such as Yttrium-90 and Lutetium-177, served as a breakthrough for PRRT and studies highlighted their potential in eliciting progression-free survival and quality of life benefits. This article will also elaborate on the key trials which paved the way for its approval and will discuss therapeutic considerations, such as patient selection and administration technique, to optimize its use.

Prognostic Value of Volume-Based Parameters Measured by SSTR PET/CT in Neuroendocrine Tumors: A Systematic Review and Meta-Analysis

Purpose: A meta-analysis was conducted to investigate the value of the volume parameters based on somatostatin receptor (SSTR)-positron emission tomography (PET) in predicting the prognosis in patients with neuroendocrine tumors (NETs). Material: PUBMED, EMBASE, Cochrane library, and Web of Knowledge were searched from January 1990 to May 2021 for studies evaluating prognostic value of volume-based parameters of SSTR PET/CT in NETs. The terms used were "volume," "positron emission tomography," "neuroendocrine tumors," and "somatostatin receptor." Pooled hazard ratio (HR) values were calculated to assess the correlations between volumetric parameters, including total tumor volume (TTV) and total-lesion SSTR expression (TL-SSTR), with progression-free survival (PFS) and overall survival (OS). Heterogeneity and subgroup analysis were performed. Funnel plots, Begg's and Egger's test were used to assess possible underlying publication bias. Results: Eight eligible studies involving 593 patients were included in the meta-analysis. In TTV, the pooled HRs of its prognostic value of PFS and OS were 2.24 (95% CI: 1.73-2.89; P < 0.00001) and 3.54 (95% CI, 1.77-7.09; P = 0.0004), respectively. In TL-SSTR, the pooled HR of the predictive value was 1.61 (95% CI, 0.48-5.44, P = 0.44) for PFS. Conclusion: High TTV was associated with a worse prognosis for PFS and OS in with patients NETs. The TTV of SSTR PET is a potential objective prognosis predictor.

Prognostic value of whole-body PET volumetric parameters extracted from 68Ga-DOTATOC-PET/CT in well-differentiated neuroendocrine tumors

Aim: To evaluate the prognostic value of somatostatin receptor tumor burden (SRTB) at ⁶⁸Ga-DOTATOC positron emission tomography/computed tomography (PET/CT) in patients with well-differentiated neuroendocrine tumors (WD-NETs). Methods: We retrospectively analyzed ⁶⁸Ga-DOTATOC-PET/CT of 84 patients with histologically confirmed WD-NETs (51 G1, 30 G2 and 3 G3). For each PET/CT, all DOTATOC-avid lesions were independently segmented by 2 operators using a customized threshold based on the healthy liver maximum standardized uptake value (SUV_max) using LIFEx 5.1. Somatostatin receptor expressing tumor volume (SRETV) and total lesion somatostatin receptor expression (TLSRE=SRETV*SUVmean) were extracted for each lesion and then whole-body SRETV and TLSRE (SRETVwb and TLSREwb) were defined as the sum of SRETV and TLSRE of all segmented lesions in each patient, respectively. Time to progression (TTP) was defined as the combination of disease-free-survival in patients undergoing curative surgery (n = 10) and progression-free survival for patients with unresectable/metastatic disease (n = 74). TTP and overall survival (OS) were calculated by Kaplan-Meier analysis, log-rank test, and Cox's proportional hazard model. Results: After a median follow-up period of 15.5 months disease progression was confirmed in 35 patients (41.7%) and 14 patients died. Higher SRETVwb (>39.1ml) and TLSREwb (>306.8g) were significantly correlated with shorter median TTP (TTP = 12months vs not reached; p<0.001). In multivariate analysis, SRETVwb (P = 0.005) was the only independent predictor of TTP regardless of histopathologic grade and TNM staging. Conclusion: According to our results, SRETVwb and TLSREwb extracted from ⁶⁸Ga-DOTATOC-PET/CT could predict TTP/OS and might have an important clinical utility in the management of in patients with WD-NETs.

SSTR2 in Nasopharyngeal Carcinoma: Relationship with Latent EBV Infection and Potential as a Therapeutic Target

Nasopharyngeal carcinoma (NPC) is a malignant epithelial tumor, most commonly located in the pharyngeal recess and endemic to parts of Asia. It is often detected at a late stage which is associated with poor prognosis (5-year survival rate of 63%). Treatment for this malignancy relies predominantly on radiotherapy and/or systemic chemotherapy, which can be associated with significant morbidity and impaired quality of life. In endemic regions NPC is associated with infection by Epstein-Barr virus (EBV) which was shown to upregulate the somatostatin receptor 2 (SSTR2) cell surface receptor. With recent advances in molecular techniques allowing for an improved understanding of the molecular aetiology of this disease and its relation to SSTR2 expression, we provide a comprehensive and up-to-date overview of this disease and highlight the emergence of SSTR2 as a key tumor biomarker and promising target for imaging and therapy.

Utility of 18F-AlF-NOTA-Octreotide PET/CT in the Localization of Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a paraneoplastic disorder, usually caused by benign mesenchymal tumors that produce high levels of fibroblast growth factor 23. The only curative therapy is resection of the causative tumors.

OBJECTIVE

This research was conducted to evaluate the efficacy of 18F-AlF-NOTA-octreotide (18F-OC) positron emission tomography/computed tomography (PET/CT) in detecting TIO and its impact on patient management.

METHODS

Retrospective analysis was conducted of 17 patients with hypophosphatemic osteomalacia suspected to be TIO. A  18F-OC PET/CT study was performed in all 17 patients to localize the tumor and 68Ga-DOTATATE PET/CT was performed in 4 out of 17 patients; both studies were performed within 1 week of each other. Both studies were interpreted blindly without the knowledge of other imaging findings. The image findings were compared with the results of histopathological examinations and clinical follow-ups.

RESULTS

The 18F-OC PET/CT scans were positive in 14 patients. Furthermore, 4 of 14 patients were scanned with both 18F-OC and 68Ga-DOTATATE PET/CT. Both studies were able to localize the tumor in all 4 patients. In total, 14 patients had surgery to remove the lesions. Postsurgical pathological examination confirmed causative tumors in these patients, whose symptoms diminished promptly. Serum phosphate levels normalized, confirming the diagnosis of TIO. 18F-OC PET/CT sensitivity, specificity, and accuracy were 87.5%, 100%, and 88.2% respectively. 18F-OC PET/CT findings affected patient management in 88.2% of cases.

CONCLUSION

18F-OC PET/CT scan is useful in the detection of tumors causing TIO. Further studies with larger patient populations are needed to validate the result.

Discordant Uptake Between Diagnostic 68Ga-HA-DOTATATE PET/CT and Posttherapy 177Lu-HA-DOTATATE SPECT/CT in Patients With Neuroendocrine Tumors

ABSTRACT

Patients with unresectable or metastasized neuroendocrine tumors are assumed eligible for PRRT (peptide receptor radionuclide therapy) with 177Lu-HA-DOTATATE if tumor uptake on somatostatin receptor imaging is higher than normal liver tissue. In our clinic, 2 patients presented with sufficient uptake of 68Ga-HA-DOTATATE in most metastases but with limited uptake in liver lesions. Posttherapy 177Lu imaging, however, showed good uptake in all neuroendocrine tumor lesions, including all liver metastases. Therefore, the presence of liver metastases in which the uptake of 68Ga-HA-DOTATATE is not or only slightly higher than in surrounding normal liver tissue should not be an absolute contraindication for PRRT.

PET Imaging for Gynecologic Malignancies

This review article summarizes the clinical applications of established and emerging PET tracers in the evaluation of the 5 most common gynecologic malignancies: endometrial, ovarian, cervical, vaginal, and vulvar cancers. Emphasis is given to 2-deoxy-2-[¹⁸F]fluoro-d-glucose as the most widely used and studied tracer, with additional clinical tracers also explored. The common imaging protocols are discussed, including standard dose ranges and uptake times, established roles, as well as the challenges and future directions of these imaging techniques. The key points are emphasized with images from selected cases.

Nuclear Imaging of Neuroendocrine Tumors

Consensus guidelines acknowledge the role of gallium Ga-68 (⁶⁸Ga) 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic (DOTA) somatostatin receptor (SSTR) positron emission tomography/computed tomography (PET/CT) in management of neuroendocrine tumor (NET) patients. ⁶⁸Ga-DOTA-SSTR PET/CT demonstrates superior performance to conventional imaging in initial detection, staging, detection of recurrent tumor, and detection of unknown primary in known metastatic disease. ⁶⁸Ga-DOTA-SSTR PET/CT is low yield for NET detection in the setting of symptoms or elevated biomarkers when conventional imaging is negative, but may still guide management. The role of ⁶⁸Ga-DOTA-SSTR PET/CT is not established in monitoring response to systemic therapy but may identify progression through detection of new metastases.

Imaging of Pheochromocytoma and Paraganglioma

Imaging plays a critical role in the management of pheochromocytomas and paragangliomas and often guides treatment. The discovery of susceptibility genes associated with these tumors has led to better understanding of clinical and imaging phenotypes. Functional imaging is of prime importance because of its sensitivity and specificity in subtypes of pheochromocytoma and paraganglioma. Several radiopharmaceuticals have been developed to target specific receptors and metabolic processes seen in pheochromocytomas and paragangliomas, including ¹³¹I/¹²³I-metaiodobenzylguanidine, 6-¹⁸F-fluoro-l-3,4-dihydroxyphenylalanine, ¹⁸F-FDG, and ⁶⁸Ga-DOTA-somatostatin analogs. Two of these have consequently been adapted for therapy. This educational review focuses on the current imaging approaches used in pheochromocytomas and paragangliomas, which vary among clinical and genotypic presentations.

Incidental Finding of Squamous Cell Carcinoma on a 68Ga-DOTATATE PET Scan

Neuroendocrine tumors (NETs) are a relatively rare entity; however, the incidence and prevalence of these tumors are increasing, likely attributed to improved diagnostic accuracy. The diagnosis of suspected NETs is facilitated by clinical symptoms, laboratory test abnormalities such as elevated chromogranin-A, and other diagnostic modalities such as the use of computed tomography scans, magnetic resonance imaging scans, positron emission tomography (PET) scans, and biopsy. The expression of high levels of somatostatin receptors in NETs enables the use of a specialized PET scan using the radiolabeled somatostatin analogues 68Ga-DOTATATE. The sensitivity and specificity of 68Ga-DOTATATE PET is very high for the diagnosis of NETs, but the specificity decreases especially with no clear symptoms and with only borderline elevated tumor markers. We present a case of a suspected NET, which was initially diagnosed as a metastatic NET by virtue of a positive 68Ga-DOTATATE PET scan; however, on biopsy it was revealed to be a squamous cell carcinoma originating from the head and neck.

Correlation of 68Ga-DOTATATE uptake on PET/CT with pathologic features of cellular proliferation in neuroendocrine neoplasms

OBJECTIVE

68Ga-DOTATATE positron emission tomography/computed tomography (PET/CT) is a useful tool for diagnosing and staging neuroendocrine neoplasms (NEN). Unlike other PET tracers like FDG, the meaningfulness and use of standardized uptake values (SUVs) of 68Ga-DOTATATE is not well-established. This study aimed to determine if a correlation exists between intensity of 68Ga-DOTATATE uptake and markers of cellular proliferation.

METHODS

This retrospective study included 79 patients with positive 68Ga-DOTATATE PET/CT and Ki-67 and/or mitotic index (MI) available on pathology report. SUV_max of the most intense lesion and the most intense organ-matched lesion were determined. Demographics and pathology results for Ki-67 and MI were collected from the electronic medical record. Correlations and trends for correlations of SUV_max to Ki-67 and MI were performed using Kruskal-Wallis and Cuzick trend tests.

RESULTS

A trend for an association between SUV_max and Ki-67 grade was found; median SUV_max of Ki-67 < 3%, 3-20%, and > 20% was 35.2, 31.8, and 12.8 (p = 0.077), respectively. There was also a trend between SUV_max and Ki-67 categories in organ-matched lesions (p = 0.08). The median organ-matched SUV_max of MI < 2, 2-20, and > 20 lesions was 34.2, 18, and 21.7, respectively, (Cuzick trend test p = 0.066). The median SUV_max for small bowel, pancreatic, and other primary locations was 27.6, 46.9, and 9.3 (p < 0.01), respectively.

CONCLUSIONS

The association between 68Ga-DOTATATE SUV_max, histologic grade, and primary site of NEN demonstrates its potential use for prognostication, or potentially as a surrogate for histologic grading when biopsy is not possible.

The value of [68Ga]Ga-DOTA-TATE PET/CT in diagnosis and management of suspected pituitary tumors

Background

Gallium 68-tetraazacyclododecane-tetraacetic acid-octreotate ([68Ga]Ga-DOTA-TATE) is a selective somatostatin analogue ligand, which shows increased affinity for somatostatin receptor subtype (SSTR) 2 and has been used routinely for imaging neuroendocrine tumors with PET/CT. We investigated the utility of [68Ga]Ga-DOTA-TATE positron emission tomography/computed tomography (PET/CT) in patients with suspected pituitary pathology. We reviewed imaging for twenty consecutive patients (8 men, 12 women, mean age of 48.2, range 14–78) with suspected pituitary pathology who were referred for [68Ga]Ga-DOTA-TATE PET/CT.

Results

Nine patients presented with recurrent Cushing’s syndrome following surgical resection of pituitary adenomas due to recurrent Cushing’s disease (seven patients) and ectopic ACTH secreting tumor (2 patients). All seven patients with recurrent Cushing’s disease showed positive pituitary [68Ga]Ga-DOTA-TATE uptake while both cases of ectopic hormonal secretion had absented pituitary uptake. In 1 of these 2 patients, [68Ga]Ga-DOTA-TATE was able to localize the source of ectopic ACTH tumor.

Six patients presented de novo with Cushing’s due to ectopic ACTH secretion; [68Ga]Ga-DOTA-TATE PET/CT was able to localize ectopic tumors in six of eight patients (3 lungs, 2 pancreases, 1 mid-gut)

There was high uptake [68Ga]Ga-DOTA-TATE in 3 cases of recurrent central hyperthyroidism (SUVmax 6.6–14.3) and 2 cases of prolactinoma (SUVmax 5.5 and 11.3).

Conclusion

Absent [68Ga]Ga-DOTA-TATE activity in the pituitary fossa is useful in excluding pituitary disease in recurrent Cushing’s. Recurrent pituitary thyrotropinomas and prolactinomas showed moderate to high pituitary activity. In addition, in Cushing’s syndrome, [68Ga]Ga-DOTA-TATE is useful for detection of ectopic sources of ACTH production, especially where anatomic imaging is negative.

SPECT Imaging of SST2-Expressing Tumors with 99mTc-Based Somatostatin Receptor Antagonists: The Role of Tetraamine, HYNIC, and Spacers

[^99mTc]Tc-HYNIC-TOC is the most widely used ^99mTc-labeled somatostatin receptor (SST) agonist for the SPECT imaging of SST-expressing tumors, such as neuroendocrine tumors. Recently, radiolabeled SST antagonists have shown improved diagnostic efficacy over agonists. ^99mTc-labeled SST antagonists are lacking in clinical practice. Surprisingly, when [^99mTc]Tc-HYNIC was conjugated to the SST2 antagonist SS01, SST2 imaging was not feasible. This was not the case when [^99mTc]Tc-N4 was conjugated to SS01. Here, we assessed the introduction of different spacers (X: β-Ala, Ahx, Aun and PEG₄) among HYNIC and SS01 with the aim of restoring the affinity of HYNIC conjugates. In addition, we used the alternative antagonist JR11 for determining the suitability of HYNIC with ^99mTc-labeled SST2 antagonists. We performed a head-to-head comparison of the N4 conjugates of SS01 and JR11. [^99mTc]Tc-HYNIC-TOC was used as a reference, and HEK-SST2 cells were used for in vitro and in vivo evaluation. EDDA was used as a co-ligand for all [^99mTc]Tc-HYNIC conjugates. The introduction of Ahx restored, to a great extent, the SST2-mediated cellular uptake of the [^99mTc]Tc-HYNIC-X conjugates (X: spacer), albeit lower than the corresponding [^99mTc]Tc-N4-conjugates. SPECT/CT images showed that all ^99mTc-labeled conjugates accumulated in the tumor and kidneys with [^99mTc]Tc-HYNIC-PEG₄-SS01, [^99mTc]Tc-N4-SS01 and [^99mTc]Tc-N4-JR11 having notably higher kidney uptake. Biodistribution studies showed similar or better tumor-to-non-tumor ratios for the [^99mTc]Tc-HYNIC-Ahx conjugates, compared to the [^99mTc]Tc-N4 counterparts. The [^99mTc]Tc-HYNIC-Ahx conjugates of SS01 and JR11 were comparable to [^99mTc]Tc-HYNIC-TOC as imaging agents. HYNIC is a suitable chelator for the development of ^99mTc-labeled SST2 antagonists when a spacer of appropriate length, such as Ahx, is used.

Comparison of 18 F-FDG PET/CT and 68 Ga-DOTATATE PET/CT in the Targeted Imaging of Culprit Tumors Causing Osteomalacia

Objective

To assess and compare the performance of fluorine‐18‐labeled fluorodeoxyglucose positron emission tomography (¹⁸F‐FDG‐PET/ CT) and gallium‐68‐labeled tetraazacyclododecanetetraacetic acid‐DPhe1‐Tyr3‐octreotate (⁶⁸Ga‐ DOTATATE) PET/CT in the targeted imaging of culprit tumors causing osteomalacia.

Methods

This was a clinical retrospective analysis. We analyzed 13 patients (five men, eight women; mean age, 49 years; range, 19–55 years) with suspicion of tumor‐induced osteomalacia (TIO) between March 2017 and October 2019. All patients underwent two functional imaging methods to locate the culprittumors. Studies were performed on a PET/CT scanner. The injection doses of ¹⁸F‐ FDG and ⁶⁸Ga‐DOTATATE were 0.5mCi/kg and approximately 5.0mCi, respectively. In the two scans, the whole body was captured from head to toe 45 to 60 min after intravenous tracer injection. ⁶⁸Ga‐DOTATATE PET/CT and ¹⁸F‐FDG PET/CT imaging results locate culprit tumors according to the following criteria: (i) abnormal foci uptake concentration was observed locally, and the uptake level was higher than the background level of the right lobe of the liver; (ii) combined CT showed or did not have obvious abnormal density changes; and (iii) non‐specific ingestion lesions due to fracture, arthritis, necrosis of femoral head are excluded. Compared with the results of pathological examination and clinical follow‐up, the sensitivity, specificity and accuracy of ⁶⁸Ga‐DOTATATE PET/CT imaging and ¹⁸F‐FDG PET/CT imaging for TIO were analyzed.

Results

All patients had symptoms of osteomalacia and hypophosphatemia. The lag time (symptoms to PET diagnosis) ranged from 2 to 12 years. There were eight cases of TIO patients and five cases of non‐TIO patients confirmed by surgery, pathology and follow‐up. Among the eight TIO patients, there were six cases (75.0%) of PMTs, one case (12.5%) of giant cell tumor, one case (12.5%) of hemangiopericutoma. Most (n = 6, 75.0%) of the confirmed tumors in our patient population were in the lower extremities, followed by craniofacial regions (n = 1, 12.5%), and torso (n = 1, 12.5%), respectively. Among the five non‐TIO patients, there were two cases of Fanconi syndrome, one case of rickets, and two cases of sporadic osteomalacia hypophosphorus. The culprit tumors could be located either in the bone (n = 5, 62.5%) or the soft tissue (n = 3, 37.5%). ¹⁸F‐FDG PET/CT was able to localize the tumor in six (6/13, 46.1%) patients. ⁶⁸Ga‐DOTATATE PET/CT detected tumor in 8 (83.3%) of 13 patients. The sensitivity of ⁶⁸Ga‐DOTATATE PET/CT imaging and ¹⁸F‐FDG PET/CT imaging in the evaluation of TIO in our patient population were 100% (8/8) vs 75% (6/8). The specificity of the two different methods was 80% (4/5). The overall accuracy was 92.3% (12/13) vs 76.9% (10/13).

Conclusions

⁶⁸Ga‐DOTATATE PET/CT is very effective in assessing hypophosphatemia patients with TIO typical symptoms compared with ¹⁸F‐FDG. Therefore, in clinically suspected cases of hypophosphatemic osteomalacia, ⁶⁸Ga‐DOTATATE PET/CT should be preferred as an imaging modality investigation to avoid delay in the treatment of this disease.

Somatostatin Receptor Imaging PET in Neuroendocrine Neoplasm

PET/computed tomography (CT) imaging increasingly is used in neuroendocrine neoplasms (NENs) for diagnosis, staging, monitoring, prognostication, and choosing treatment. Somatostatin PET analog tracers have added to the specificity by obtaining higher affinity to somatostatin receptors with ⁶⁸Ga-labeled or ⁶⁴Cu-labeled DOTA peptides compared with single-photon emission CT imaging isotopes. PET uptake correlates to tumor grade and is an essential part of theranostics with peptide receptor radionuclide treatment. This article focuses on the literature on head-to-head studies and meta-analyses of different combinations of peptide agonists and a few antagonists. Overall, the published data support the diagnostic capability of PET/CT imaging in NENs.

Management Impact of 68Ga-DOTATATE PET/CT in Neuroendocrine Tumors

Purpose

The goal of our retrospective single tertiary academic medical center investigation was to examine the added diagnostic value and clinical impact of ⁶⁸Ga-DOTATATE PET/CT in the therapeutic management of patients with neuroendocrine tumors (NETs).

Methods

Imaging database was queried for all "PET-DOTATATE" examinations performed at our tertiary care academic institution using MONTAGE™. The patient's clinical history and recent prior imaging were reviewed. The additional diagnostic value and clinical management impact of ⁶⁸Ga-DOTATATE were assessed through retrospective chart review.

Results

A total of 81 ⁶⁸Ga-DOTATATE PET/CT scans in 74 patients were found, and 11 patients were excluded from analysis as they had no prior imaging available for comparison, with resultant analysis cohort of 63 patients. Six patients had 2 or more ⁶⁸Ga-DOTATATE PET/CT examinations. The most common primary diagnosis was undifferentiated NET (63.5%), followed by carcinoid (27.0%), paraganglioma (4.8%), insulinoma (3.2%), and pheochromocytoma (1.6%). The primary sites of disease from the most to the least common were the pancreas (36.5%), small bowel (22.2%), unknown primary (15.9%), lung (6.3%), large bowel (6.3%), and mesentery (4.8%), and other locations accounted for 7.9%. In patients who had prior imaging available for comparison, there were new lesions identified on ⁶⁸Ga-DOTATATE PET/CT in 21 patients (33.3%) that were not identified on other prior imaging modalities. Of these patients, 5 underwent subsequent MRI and 1 had a repeat ⁶⁸Ga-DOTATATE PET/CT to further characterize new lesions seen. Moreover, 15 patients (23.8%) had a change in treatment plan, including altering medical therapy in 9 patients, change in planned extent of surgical management in 5 patients, and cancelation of a planned primary tumor resection in 1 patient with metastatic disease.

Conclusion

Our retrospective cohort demonstrated that ⁶⁸Ga-DOTATATE PET/CT improves lesion detection over conventional imaging in 33.3% and impacts the therapeutic management in 23.8% of patients with NET.

Primary and Secondary Optic Nerve Sheath Meningioma

Objective  This study was aimed to review issues relating to the recognition, radiographic diagnosis, monitoring, and management of primary and secondary optic nerve sheath meningioma (ONSM). Design  This study is a review of peer-reviewed literature combined with illustrative case studies. Participants and Methods  A literature search was conducted via the PubMed database using pertinent search terms. Selected articles were limited to those written or translated into English. Additional works cited within articles were also included. Individual cases were drawn from the experience of a tertiary academic neuroophthalmic and orbital practice. Tables summarize radiotherapeutic and surgical studies, excluding single case reports and studies focusing on meningioma of intracranial origin. Main Outcome Measurements  Review of reported surgical and radiotherapeutic series is the primary measurement. Results  The natural history of optic nerve sheath meningiomas is primarily characterized by progressive ipsilateral vision loss. Diagnosis is typically based on radiographic imaging findings, with biopsy remaining indicated in some patients. Management strategies may include observation, radiation, and/or surgical intervention, or a combination of these approaches. The role of surgery, especially with respect to primary ONSM (pONSM), remains controversial. Advancement of radiotherapy techniques has shifted modern treatment paradigms in pONSM toward radiation as primary treatment, as surgical outcomes are inferior in major studies. Although radiation remains the treatment of choice in many cases, selected patients may benefit from surgery, especially in the setting of secondary ONSM (sONSM). Conclusion  A wide variety of radiotherapeutic and surgical treatment modalities for ONSM exist. The specific indications for each management strategy continue to be redefined.

Somatostatin receptor-positive breast lesions on 68Ga-DOTATATE PET/CT

OBJECTIVE

This study sets out to evaluate patients with increased uptake in breast lesions on ⁶⁸Ga-DOTATATE PET/CT (DOTA PET) and determine the clinical significance of somatostatin receptor (SSTR) positive breast lesions.

METHODS

We retrospectively evaluated all patients with increased SSTR uptake in breast lesions on DOTA PET. Patients with physiological (e.g., lactation) or normal variant breast uptake (e.g., mild diffuse glandular uptake) were excluded. The maximum standard uptake value (SUVmax) was calculated using a manually drawn region of interest in the most intense uptake of breast lesions. All lesions were correlated with breast imaging, including mammography and ultrasonography. Histopathological correlation was performed if the lesion was suspicious for malignancy. Lesions were followed up radiologically (1-8 years).

RESULTS

Out of 1573 retrospectively analyzed DOTA PET scans, the incidence of SSTR + breast lesions was measured as 1.1% (n = 18); however, 4 of 18 patients were excluded due to the lack of final diagnosis of lesions. The median age was 35 (range 14-58 years), and all patients were female. The median SUVmax of SSTR + breast lesions was 5.2 (range 1.5-12.6) for a total of 14 patients. Twelve patients had a single SSTR + breast lesion, while 2 patients had multiple SSTR + lesions on bilateral breasts. In 6 patients, single SSTR + lesions were considered as fibroadenoma; in 2 patients, multiple SSTR + lesions were considered as metastases of NET, based on correlative breast imaging. In 6 patients, histopathological confirmation was needed for the final diagnosis. Histopathologic findings confirmed fibroadenoma in 4 patients by biopsy, in 1 patient with surgical removal of the lesion. The last patient who had a history of IDC was diagnosed with a recurrence of IDC with biopsy. The median SUVmax was 5.1 (range 1.5-9.4) for malignant breast lesions and 5.4 (range 2.2-12.6) for benign breast lesions.

CONCLUSION

SSTR + breast lesions on DOTA PET are rarely seen in clinical practice. Uptakes of breast lesions in our cases were variable and not useful for differential diagnosis of lesions. It seems that SSTR + breast lesions should be evaluated with clinical and radiological characteristics, and correlative breast imaging and/or histopathological verification should be performed for suspicious lesions to avoid misdiagnosis.

Recent Advances in the Management of Typical and Atypical Lung Carcinoids

Neuroendocrine neoplasms of the lung represent about 20% to 30% of all neuroendocrine tumors. On the basis of clinical and pathologic characteristics, 2 different categories of tumors may be defined: poorly differentiated neuroendocrine neoplasms, characterized by a high rate of recurrences and poor prognosis, and well-differentiated neuroendocrine neoplasms (typical carcinoids and atypical carcinoids), which generally display an indolent course. Lung carcinoids represent only 1% to 5% of all lung malignancies, but their incidence has significantly increased over the past 30 years. Surgery is the reference standard of treatment for lung carcinoids with locoregional disease. For advanced or unresectable lung carcinoids, several therapeutic options are available, but the choice should be shared within a multidisciplinary team to ensure optimal therapeutic outcomes. We describe the current management of these rare neoplasms.

68Ga-DOTATATE Uptake in an Endolymphatic Sac Tumor: Radiologic-Pathologic Correlation

After dedicated CT and MRI, Ga-DOTATATE PET/CT was performed in a patient with a temporal bone mass with primary diagnostic considerations of an endolymphatic sac tumor versus a glomus jugulotympanicum paraganglioma. The Ga-DOTATATE PET showed mild radiotracer uptake in the mass (SUVmax, 10.9). After surgical resection, pathology revealed an endolymphatic sac tumor. Immunohistochemical staining demonstrated somatostatin receptor type 2A expression in the vasculature of the mass, but not in the tumor cells.

Non-FDG PET/CT

The major applications for molecular imaging with PET in clinical practice concern cancer imaging. Undoubtedly, 18F-FDG represents the backbone of nuclear oncology as it remains so far the most widely employed positron emitter compound. The acquired knowledge on cancer features, however, allowed the recognition in the last decades of multiple metabolic or pathogenic pathways within the cancer cells, which stimulated the development of novel radiopharmaceuticals. An endless list of PET tracers, substantially covering all hallmarks of cancer, has entered clinical routine or is being investigated in diagnostic trials. Some of them guard significant clinical applications, whereas others mostly bear a huge potential. This chapter summarizes a selected list of non-FDG PET tracers, described based on their introduction into and impact on clinical practice.

Comparison of 68Ga-DOTA-NaI3-Octreotide/tyr3-octreotate positron emission tomography/computed tomography and contrast-enhanced computed tomography in localization of tumors in multiple endocrine neoplasia 1 syndrome

The optimum imaging modality for the screening of multiple endocrine neoplasia type 1 (MEN1)-associated tumors is not well established. Here, we compare the performance of contrast-enhanced CT (CECT) versus ⁶⁸Ga DOTA-NOC/TATE PET/CT in MEN1 patients. The retrospective case record study is conducted at a tertiary health-care center. Thirty-four patients, who have undergone both CECT and ⁶⁸Ga DOTA-NOC/ TATE PET, were included in the analysis. CECT had higher per-lesion sensitivity than ⁶⁸Ga DOTA-NOC/TATE PET/CT for the detection of parathyroid lesions, (82.6% vs. 24.6%, P < 0.001). ⁶⁸Ga DOTA-NOC/TATE PET/CT had higher per-lesion sensitivity than CECT for the detection of metastases (85% vs. 47.5%, P < 0.001) and gastrinomas (90% vs. 10%, P = 0.003). When combined use of the two imaging modalities is compared to CECT alone (63.7% vs. 93.1%, P = 0.00012) and ⁶⁸Ga-DOTA-NOC/TATE PET/CT alone (74.1% vs. 93.1%, P = 0.0057), it provided significantly higher per-lesion sensitivity for the detection of gastroenteropancreatic neuroendocrine tumors (GEP-NETs). ⁶⁸Ga-DOTA-NOC/ TATE PET was more sensitive for the detection of gastrinomas and metastases than CECT, whereas it was less sensitive for the detection of parathyroid lesions than CECT. The combined use of both the imaging modalities significantly increases the sensitivity for detection of GEP-NETs.

Evaluation of a New 177Lu-Labeled Somatostatin Analog for the Treatment of Tumors Expressing Somatostatin Receptor Subtypes 2 and 5

Targeted radionuclide therapy of somatostatin receptor (SST)-expressing tumors is only partially addressed by the established somatostatin analogs having an affinity for the SST subtype 2 (SST2). Aiming to target a broader spectrum of tumors, we evaluated the bis-iodo-substituted somatostatin analog ST8950 ((4-amino-3-iodo)-d-Phe-c[Cys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH₂), having subnanomolar affinity for SST2 and SST5, labeled with [¹⁷⁷Lu]Lu³⁺ via the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Human Embryonic Kidney (HEK) cells stably transfected with the human SST2 (HEK-SST2) and SST5 (HEK-SST5) were used for in vitro and in vivo evaluation on a dual SST2- and SST5-expressing xenografted mouse model. ^natLu-DOTA-ST8950 showed nanomolar affinity for both subtypes (IC₅₀ (95% confidence interval): 0.37 (0.22-0.65) nM for SST2 and 3.4 (2.3-5.2) for SST5). The biodistribution of [¹⁷⁷Lu]Lu-DOTA-ST8950 was influenced by the injected mass, with 100 pmol demonstrating lower background activity than 10 pmol. [¹⁷⁷Lu]Lu-DOTA-ST8950 reached its maximal uptake on SST2- and SST5-tumors at 1 h p.i. (14.17 ± 1.78 and 1.78 ± 0.35%IA/g, respectively), remaining unchanged 4 h p.i., with a mean residence time of 8.6 and 0.79 h, respectively. Overall, [¹⁷⁷Lu]Lu-DOTA-ST8950 targets SST2-, SST5-expressing tumors in vivo to a lower extent, and has an effective dose similar to clinically used radiolabeled somatostatin analogs. Its main drawbacks are the low uptake in SST5-tumors and the persistent kidney uptake.

Phase 0 Radiopharmaceutical-Agent Clinical Development

The evaluation of antibody-targeted or peptide-targeted radiopharmaceuticals as monotherapy or in oncological drug combinations requires programmatic collaboration within the National Cancer Institute (NCI) clinical trial enterprise. Phase 0 trials provide a flexible research platform for the study of radiopharmaceutical–drug pharmacokinetics, radiation dosimetry, biomarkers of DNA damage response modulation, and pharmacodynamic benchmarks predictive of therapeutic success. In this article, we discuss a phase 0 clinical development approach for human antibody-targeted or peptide-targeted radiopharmaceutical–agent combinations. We expect that early-phase radiopharmaceutical–agent combination trials will become a more tactical and more prevalent part of radiopharmaceutical clinical development in the near-term future for the NCI Cancer Therapy Evaluation Program.

A new 68Ga-labeled somatostatin analog containing two iodo-amino acids for dual somatostatin receptor subtype 2 and 5 targeting

Background

Somatostatin receptor (SST) targeting, specifically of the subtype 2 (SST2), with radiolabeled somatostatin analogs, is established for imaging and treatment of neuroendocrine tumors. Owing to the concomitant and heterogeneous expression of several subtypes on the same tumor, analogs targeting more subtypes than SST2 potentially target a broader spectrum of tumors and/or increase the uptake of a given tumor. The analog ST8950 ((4-amino-3-iodo)-d-Phe-c[Cys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH₂), bearing 2 iodo-amino acids, exhibits sub-nanomolar affinity to SST2 and SST5. We report herein the development and preclinical evaluation of DOTA-ST8950 labeled with ⁶⁸Ga, for imaging SST2- and SST5-expressing tumors. Comparative in vitro and in vivo studies were performed with the de-iodinated DOTA-ST8951 ((4-amino)-d-Phe-c[Cys-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH₂) and with the reference compounds DOTA-TATE (SST2 selective) and DOTA-NOC (for SST2 and SST5).

Results

Compared with ^natGa-DOTA-NOC, ^natGa-DOTA-ST8950 exhibited higher affinity to SST2 and SST5 (IC₅₀ (95%CI), nM = 0.32 (0.20–0.50) and 1.9 (1.1–3.1) vs 0.70 (0.50-0.96) and 3.4 (1.8-6.2), respectively), while ^natGa-DOTA-ST8951 lost affinity for both subtypes. ^natGa-DOTA-ST8950 had the same potency for inducing SST2-mediated cAMP accumulation as ^natGa-DOTA-TATE and slightly better than ^natGa-DOTA-NOC (EC₅₀, nM = 0.46 (0.23–0.92) vs 0.47 (0.15–1.5) vs 0.59 (0.18–1.9), respectively). [⁶⁷Ga]Ga-DOTA-ST8950 had a similar internalization rate as [⁶⁷Ga]Ga-DOTA-NOC in SST2-expressing cells (12.4 ± 1.6% vs 16.6 ± 2.2%, at 4 h, p = 0.0586). In vivo, [⁶⁸Ga]Ga-DOTA-ST8950 showed high and specific accumulation in SST2- and SST5-expressing tumors, comparable with [⁶⁸Ga]Ga-DOTA-NOC (26 ± 8 vs 30 ± 8 %IA/g, p = 0.4630 for SST2 and 15 ± 6 vs 12 ± 5 %IA/g, p = 0.3282, for SST5, 1 h p.i.) and accumulation in the SST-positive tissues, the kidneys and the liver. PET/CT images of [⁶⁸Ga]Ga-DOTA-ST8950, performed in a dual HEK-SST2 and HEK-SST5 tumor xenografted model, clearly visualized both tumors and illustrated high tumor-to-background contrast.

Conclusions

[⁶⁸Ga]Ga-DOTA-ST8950 reveals its potential for PET imaging SST2- and SST5-expressing tumors. It compares favorably with the clinically used [⁶⁸Ga]Ga-DOTA-NOC in terms of tumor uptake; however, its uptake in the liver remains a challenge for clinical translation. In addition, this study reveals the essential role of the iodo-substitutions in positions 1 and 3 of [⁶⁸Ga]Ga-DOTA-ST8950 for maintaining affinity to SST2 and SST5, as the de-iodinated [⁶⁸Ga]Ga-DOTA-ST8951 lost affinity for both receptor subtypes.

Ultrasonic-Assisted Solid-Phase Peptide Synthesis of DOTA-TATE and DOTA-linker-TATE Derivatives as a Simple and Low-Cost Method for the Facile Synthesis of Chelator-Peptide Conjugates

Peptides have been widely adopted as biological targeting vectors for applications in molecular imaging and peptide-receptor radionuclide therapy (PRRT). Somatostatin (SST) analogues such as octreotate (TATE) are exogenous ligands for somatostatin receptors (SSTRs), which are highly expressed on neuroendocrine tumors (NETs). Recently, both [⁶⁸Ga]Ga-DOTA-TATE (NETSPOT) and [¹⁷⁷Lu]Lu-DOTA-TATE (LUTATHERA) received U.S. Food and Drug Administration approval for positron emission tomography (PET) imaging and PRRT of NETs, respectively. However, to the best of our knowledge a well-described synthesis of DOTA-TATE has not been reported in the literature. Herein, we report a fully reoptimized DOTA-TATE synthesis, including the application of a simple ultrasonic bath to greatly improve yields, reduce coupling times, and decrease the amount of reagents required for each coupling step by a half. The most prevalently used cyclizing agents such as iodine, thallium(III) trifluoroacetate, hydrogen peroxide, and dimethyl sulfoxide were compared. On-resin cyclizations using mechanical agitation showed higher yields (23% and 25% using I₂ and Tl(III), respectively) than off-resin (1.3% and 11% using DMSO and H₂O₂, respectively), and the total synthesis time of DOTA-TATE was ∼540 min excluding the cyclization step, with a total synthesis yield of ∼23%. The same manual SPPS methods/reagents were reoptimized with ultrasonic (US) agitation, resulting in an immense reduction in the total synthesis time by ∼8-fold to ∼70 min for DOTA-TATE with a higher yield (∼29% yield), and ∼13-fold to 105 min for DOTA-PEG₄-TATE (∼29% yield). Also, the use of US agitation reduces the need for excess molar equivalents of the reagents to a half, which is particularly important when coupling expensive or custom-synthesized groups such as bifunctional chelators and linkers. Finally, the synthesized DOTA-TATE was successfully radiolabeled with [⁶⁸Ga]Ga³⁺ (t_1/2 = 68 min) with high radiochemical yields (30 min, 95 °C). We believe this work opens the door to the facile and low-cost synthesis of many new chelator-linker-peptide conjugates that were previously cumbersome or cost-prohibitive to produce with manual SPPS.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

Evolution of Neuroendocrine Tumor Therapy

To better understand developments in treatment of neuroendocrine tumors of the gastroenteropancreatic system, and the pivotal roles of native somatostatin and its long-acting analogues play in normal peptide regulation and neuropeptide excess associated with neuroendocrine tumors (NETs), this article delineates and defines distinct eras in the history and discovery of gastrointestinal endocrinology. We highlight the collaboration between academia and industry in basic science and the clinical research that advanced Lu-177-DOTATATE to approval as standard of care therapy for low-grade NETs. Examples of new radioisotopes and therapy compounds currently in development for diagnosis and therapy for high-grade NETs are also discussed.

68Ga-DOTATATE PET/CT Compared with 131I-MIBG SPECT/CT in the Evaluation of Neural Crest Tumors

Objective(s):

⁶⁸Ga-DOTATATE positron emission tomography (PET)/computed tomography (CT) has shown promising results in imaging of neural crest tumors (NCT). Herein, we compared the performance of ⁶⁸Ga-DOTATATE PET/CT and ¹³¹I-MIBG single photon emission computed tomography (SPECT)/CT in the initial diagnosis, staging and follow-up of patients with NCTs.

Methods:

Twenty-five patients (males:females=8:17; age range=2–71 years) with clinically proven or suspicious neuroblastoma, pheochromocytoma (PCC) or paraganglioma (PGL) were enrolled in this prospective study and underwent both ⁶⁸Ga-DOTATATE PET/CT and ¹³¹I-MIBG SPECT/CT. A composite reference standard derived from histopathological information, together with anatomical and functional imaging findings, was used to validate the results. Imaging findings were assessed on a per-patient and on a per-lesion basis. Sensitivity and accuracy were assessed using McNemar’s test.

Results:

Referring to radiological imaging and histopathological findings as reference standard, ⁶⁸Ga-DOTATATE and ¹³¹I-MIBG scans showed a sensitivity and accuracy of (100%, 96%) and (86.7%, 88%), respectively, on a per-patient basis. In PCC/PGL patients, on a per-patient basis, the sensitivity of ⁶⁸Ga-DOTATATE was 100% and that of ¹³¹I-MIBG was 77.8%. In neuroblastoma patients, on a per-patient basis, the sensitivities of both ⁶⁸Ga-DOTATATE and ¹³¹I-MIBG were 100%. Overall, in this patient cohort, ⁶⁸Ga-DOTATATE PET/CT identified 52 lesions and ¹³¹I-MIBG SPECT/CT identified only 30 lesions. On a per-lesion analysis, ⁶⁸Ga-DOTATATE was found to be superior to ¹³¹I-MIBG in detecting lesions in all anatomical locations, particularly osseous lesions. According to the McNemar test results, differences were not statistically significant.

Conclusion:

This relatively small patient cohort suggests ⁶⁸Ga-DOTATATE PET/CT be superior to ¹³¹I-MIBG SPECT/CT in providing particularly valuable information for both primary staging and follow-up in patients with NCT.

Evaluation of a 68Ga-Labeled DOTA-Tetrazine as a PET Alternative to 111In-SPECT Pretargeted Imaging

The bioorthogonal reaction between a tetrazine and strained trans-cyclooctene (TCO) has garnered success in pretargeted imaging. This reaction was first validated in nuclear imaging using an ¹¹¹In-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-linked bispyridyl tetrazine (Tz) ([¹¹¹In]In-DOTA-PEG₁₁-Tz) and a TCO functionalized CC49 antibody. Given the initial success of this Tz, it has been paired with TCO functionalized small molecules, diabodies, and affibodies for in vivo pretargeted studies. Furthermore, the single photon emission tomography (SPECT) radionuclide, ¹¹¹In, has been replaced with the β-emitter, ¹⁷⁷Lu and α-emitter, ²¹²Pb, both yielding the opportunity for targeted radiotherapy. Despite use of the ‘universal chelator’, DOTA, there is yet to be an analogue suitable for positron emission tomography (PET) using a widely available radionuclide. Here, a ⁶⁸Ga-labeled variant ([⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz) was developed and evaluated using two different in vivo pretargeting systems (Aln-TCO and TCO-CC49). Small animal imaging and ex vivo biodistribution studies were performed and revealed target specific uptake of [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz in the bone (3.7 %ID/g, knee) in mice pretreated with Aln-TCO and tumor specific uptake (5.8 %ID/g) with TCO-CC49 in mice bearing LS174 xenografts. Given the results of this study, [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz can serve as an alternative to [¹¹¹In]In-DOTA-PEG₁₁-Tz.

Diagnostic Approaches to Neuroendocrine Neoplasms of Unknown Primary Site

Neuroendocrine neoplasms (NENs) arise from cells of neuronal and endocrine differentiation. While they are a rare entity, an increasing proportion of patients with NEN present with metastatic disease and no evident primary site using routine imaging or histopathology. NENs of unknown primary site have a poorer prognosis, often due to the challenge of selecting appropriate evidence-based management. We review the available literature and guidelines for the management of NENs of unknown primary site including clinical features, biochemical tests, histopathology, imaging, surgical exploration and localised and systemic treatments. We also discuss novel molecular techniques currently under investigation to aid primary site identification.