68Gallium- and 90Yttrium-/ 177Lutetium: "theranostic twins" for diagnosis and treatment of NETs

Preparation of a Zirconium-89 Labeled Clickable DOTA Complex and Its Antibody Conjugate

Desferrioxamine B (DFO) is the clinical standard chelator for preparing zirconium-89 labeled antibodies. In the current study, the stabilities of a zirconium-89 labeled panitumumab (PAN; Vectibix®) with three different chelators (DFO, DFO*, and DOTA) were compared. PAN is an anti-HER1/EGFR monoclonal antibody approved by the FDA for the treatment of HER1-expressing colorectal cancers and was used as the model antibody for this study. DFO/DFO* conjugates of PAN were directly radiolabeled with zirconium-89 at room temperature to produce [89Zr]Zr-DFO/DFO*-PAN conjugates following a well-established procedure. A zirconium-89 labeled DOTA-PAN conjugate was prepared by an indirect radiolabeling method. A cyclooctyne-linked DOTA chelator (BCN-DOTA-GA) was first radiolabeled with zirconium-89 at 90 °C under a two-step basic pH adjustment method followed by conjugation with PAN-tetrazene at 37 °C to produce a labeled conjugate, BCN-[89Zr]Zr-DOTA-GA-PAN. High reproducibility of the radiolabeling was observed via this two-step basic pH adjustment. The overall radiochemical yield was 40-50% (n = 12, decay uncorrected) with a radiochemical purity of >95% in 2 h synthesis time. All three conjugates were stable in whole human serum for up to 7 days at 37 °C. The kinetic inertness of the conjugates was assessed against the EDTA challenge. BCN-[89Zr]Zr-DOTA-GA-PAN exhibited excellent inertness followed by [89Zr]Zr-DFO*-PAN. [89Zr]Zr-DFO-PAN displayed the lowest level of inertness.

Synthesis of 177Lu-Labeled, Somatostatin-2 Receptor-Targeted Metalla-Assemblies: Challenges in the Design of Supramolecular Radiotherapeutics

Self-assembled supramolecular coordination complexes (SCCs) hold promise for biomedical applications in cancer therapy, although their potential in the field of nuclear medicine is still substantially unexplored. Therefore, in this study an exo-functionalized cationic [Pd2L2]4+ metallacycle (L = 3,5-bis(3-ethynylpyridine)phenyl), targeted to the somatostatin-2 receptor (sst2R) and featuring the DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in order to bind the β-- and γ-emitter lutetium-177, was synthesized by self-assembly following ligand synthesis via standard solid-phase peptide synthesis (SPPS). This metallacycle was then characterized by reverse-phase high-performance liquid chromatography (RP-HPLC), electrospray ionization mass spectrometry (ESI-MS), and 1H and 1H-DOSY NMR (DOSY = diffusion-ordered spectroscopy). A procedure for the radiolabeling of the metallacycle with 177Lu was also optimized. The resulting [nat/177Lu]Lu-DOTA-metallacycle, termed [nat/177Lu]Lu-Cy, was evaluated concerning its stability and in vitro properties. The compound was more lipophilic compared to the reference [177Lu]Lu-DOTA-TATE (logPOct/H2O = -0.85 ± 0.10 versus -3.67 ± 0.04, respectively). While [natLu]Lu-Cy revealed low stability in a DMEM/F12 GlutaMax medium, it demonstrated good stability in other aqueous media as well as in DMSO. A high sst2R binding affinity (expressed as IC50) was determined in CHOsst2 cells (Chinese hamster ovary cells that were stably transfected with human sst2R). Moreover, the metallacycle exhibited high human serum albumin binding, as assessed by high-performance affinity chromatography (HPAC), and moderate stability in human serum compared to [177Lu]Lu-DOTA-TATE (TATE = (Tyr3)-octreotate). In order to improve stability, a heteroleptic approach was used to develop a less sterically hindered cage-like SCC that is potentially endowed with host-guest chemistry capability, which has been preliminarily characterized by RP-HPLC and ESI-MS. Overall, our initial results encourage future studies on sst2R-directed SCCs and have led to new insights into the chemistry of ss2R-directed SCCs for radiopharmaceutical applications.

Relevance of Volumetric Parameters Applied to [68Ga]Ga-DOTATOC PET/CT in NET Patients Treated with PRRT

BACKGROUND

this study aims to explore the prognostic and predictive role of volumetric parameters on [⁶⁸Ga]Ga-DOTATOC PET/CT in neuroendocrine tumors (NET) patients treated with peptide receptor radionuclide therapy (PRRT).

METHODS

We retrospectively evaluated 39 NET patients (21 male, 18 female; mean age 60.7 y) within the FENET-2016 trial (CTiD:NCT04790708). PRRT was proposed with [¹⁷⁷Lu]Lu-DOTATOC alone or combined with [⁹⁰Y]Y-DOTATOC. [⁶⁸Ga]Ga-DOTATOC PET/CT was performed at baseline and 3 months after PRRT. For each PET/CT, we calculated SUVmax, SUVmean, somatostatin receptor expressing tumor volume (SRETV), and total lesion somatostatin receptor expression (TLSRE), as well as their percentage of changes (Δ), both for liver (_L) and for total tumor burden (_WB). Early clinical response (3 months after PRRT) and PFS were evaluated according to RECIST 1.1 and institutional NET board.

RESULTS

Early clinical response identified 9 partial response (PR), 25 stable disease (SD), and 5 progressive disease (PD). Post-SRETV_WB and ΔSRETV_WB were progressively increased among response groups (p = 0.02 and p = 0.03, respectively). Likewise, median post-SRETV_L was significantly higher in PD patients (p = 0.03). SUVmax and TLSRE did not correlate with early clinical response. Median PFS was 31 months. Patients with ΔSRETV_WB lower than -4.17% as well as those with post-SRETV_WB lower than 34.8 cm³ showed a longer PFS (p = 0.006 and p = 0.06, respectively). Finally, multivariate analysis identified ΔSRETV_WB as an independent predictor for PFS.

CONCLUSIONS

our results could strengthen the importance of evaluating the burden of disease on [⁶⁸Ga]Ga-DOTATOC PET/CT in NET patients treated with PRRT.

Relevance of Palladium to Radiopharmaceutical Development Considering Enhanced Coordination Properties of TE1PA

The limited use of palladium‐103 and ‐109 radionuclides for molecular radiotherapy is surely due to the lack of appropriate ligands capable of fulfilling all criteria required for application in nuclear medicine. Furthermore, the thermodynamic properties of these complexes in solution remain difficult to establish. The challenge is compounded when considering that radiolabeling of compounds for translation to clinical trials requires fast complexation. Thus, the coordination of Pd(II) and ^103/109Pd‐nuclides is a huge challenge in terms of molecular design and physicochemical characterization. Herein, we report a comprehensive study highlighting TE1PA, a monopicolinate cyclam – already established in nuclear imaging with ⁶⁴Cu‐PET (positron emission tomography) imaging tracers – as a highly relevant chelator for natural Pd and subsequently ¹⁰⁹Pd‐nuclide. The structural, thermodynamic, kinetic and radiolabeling studies of Pd(II) with TE1PA, as well as the comparison of this complex with three structurally related derivatives, support palladium‐TE1PA radiopharmaceuticals as leading candidates for targeted nuclear medicine.

Immune Checkpoint Inhibitor-Mediated Cancer Theranostics with Radiolabeled Anti-Granzyme B Peptide

Although immune checkpoint inhibitors (ICI) have revolutionized cancer management, patient response can be heterogeneous, and the development of ICI resistance is increasingly reported. Novel treatment strategies are necessary not only to expand the use of ICI to previously unresponsive tumor types but also to overcome resistance. Targeted radionuclide therapy may synergize well with ICIs since it can promote a pro-inflammatory tumor microenvironment. We investigated the use of a granzyme B targeted peptide (GZP) as a cancer theranostic agent, radiolabeled with 68Ga (68Ga-GZP) as a PET imaging agent and radiolabeled with 90Y (90Y-GZP) as a targeted radionuclide therapy agent for combinational therapy with ICI in murine models of colon cancer. Our results demonstrate that GZP increasingly accumulates in tumor tissue after ICI and that the combination of ICI with 90Y-GZP promotes a dose-dependent response, achieving curative response in some settings and increased overall survival.

Radiolabeled Nanocarriers as Theranostics-Advancement from Peptides to Nanocarriers

Endogenous targeted radiotherapy is emerging as an integral modality to treat a variety of cancer entities. Nevertheless, despite the positive clinical outcome of the treatment using radiolabeled peptides, small molecules, antibodies, and nanobodies, a high degree of hepatotoxicity and nephrotoxicity still persist. This limits the amount of dose that can be injected. In an attempt to mitigate these side effects, the use of nanocarriers such as nanoparticles (NPs), dendrimers, micelles, liposomes, and nanogels (NGs) is currently being explored. Nanocarriers can prolong circulation time and tumor retention, maximize radiation dosage, and offer multifunctionality for different targeting strategies. In this review, the authors first provide a summary of radiation therapy and imaging and discuss the new radiotracers that are used preclinically and clinically. They then highlight and identify the advantages of radio-nanomedicine and its potential in overcoming the limitations of endogenous radiotherapy. Finally, the review points to the ongoing efforts to maximize the use of radio-nanomedicine for efficient clinical translation.

Theranostics bei gut bis mäßig differenzierten GEP-NEN

Neuroendokrine Neoplasien (NEN) sind seltene, heterogene und typischerweise langsam wachsende Tumoren. Die häufigsten Lokalisationen finden sich im gastro-entero-pankreatischen System (GEP-NEN). NENs werden nach proliferativer Aktivität (Ki-67-Index) eingeteilt (G1–3). Gut differenzierte Tumoren exprimieren dabei typischerweise Somatostatinrezeptoren (SSTR), die als Zielstruktur in der nuklearmedizinischen Theranostik dienen. Bei diesem Prinzip kann nach einer diagnostischen molekularen Bildgebung, meist mittels Positronenemissionstomografie/Computertomografie (PET/CT), eine individuell zugeschnittene Peptidradiorezeptortherapie (PRRT) mit einem β-Strahler-markierten Radiopharmakon erfolgen. In Metaanalysen zeigte die Diagnostik mittels SSTR-gerichteter PET/CT eine Sensitivität von 93% und eine Spezifität von 96%. Die SSTR-gerichtete Diagnostik kann auch zur radioaktiven Markierung von Tumoren verwendet werden, um eine zielgerichtete Chirurgie zu ermöglichen. Die Indikation zur Einleitung einer PRRT soll stets in einer interdisziplinären Tumorkonferenz getroffen werden. Ein Tumorprogress unter der vorangegangenen Therapie sollte dokumentiert sein. Die Therapie wird intravenös und insgesamt 4-mal in 8-wöchigem Abstand in spezialisierten nuklearmedizinischen Zentren verabreicht. Die Wirksamkeit der PRRT wurde in der NETTER-1-Studie prospektiv untersucht und konnte eine signifikante Verbesserung des progressionsfreien Überlebens (primärer Endpunkt) zeigen. Ausgehend von diesen Studienergebnissen steht mit Lutathera (177Lu-DOTATATE) inzwischen ein in Deutschland zugelassenes Radiopharmazeutikum zu Behandlung von nicht resektablen oder metastasierten bzw. progredienten, gut differenzierten (G1 und G2), SSTR-positiven GEP-NEN zur Verfügung.

Cold Kit Labeling: The Future of 68Ga Radiopharmaceuticals?

Over the last couple of decades, gallium-68 (68Ga) has gained a formidable interest for PET molecular imaging of various conditions, from cancer to infection, through cardiac pathologies or neuropathies. It has gained routine use, with successful radiopharmaceuticals such as somatostatin analogs ([68Ga]Ga-DOTATOC and [68Ga]GaDOTATATE) for neuroendocrine tumors, and PSMA ligands for prostate cancer. It represents a major clinical impact, particularly in the context of theranostics, coupled with their 177Lu-labeled counterparts. Beside those, a bunch of new 68Ga-labeled molecules are in the preclinical and clinical pipelines, with some of them showing great promise for patient care. Increasing clinical demand and regulatory issues have led to the development of automated procedures for the production of 68Ga radiopharmaceuticals. However, the widespread use of these radiopharmaceuticals may rely on simple and efficient radiolabeling methods, undemanding in terms of equipment and infrastructure. To make them technically and economically accessible to the medical community and its patients, it appears mandatory to develop a procedure similar to the well-established kit-based 99mTc chemistry. Already available commercial kits for the production of 68Ga radiopharmaceuticals have demonstrated the feasibility of using such an approach, thus paving the way for more kit-based 68Ga radiopharmaceuticals to be developed. This article discusses the development of 68Ga cold kit radiopharmacy, including technical issues, and regulatory aspects.

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.

The Protein Landscape of Mucinous Ovarian Cancer: Towards a Theranostic

MOC is a rare histotype of epithelial ovarian cancer, and current management options are inadequate for the treatment of late stage or recurrent disease. A shift towards personalised medicines in ovarian cancer is being observed, with trials targeting specific molecular pathways, however, MOC lags due to its rarity. Theranostics is a rapidly evolving category of personalised medicine, encompassing both a diagnostic and therapeutic approach by recognising targets that are expressed highly in tumour tissue in order to deliver a therapeutic payload. The present review evaluates the protein landscape of MOC in recent immunohistochemical- and proteomic-based research, aiming to identify potential candidates for theranostic application. Fourteen proteins were selected based on cell membrane localisation: HER2, EGFR, FOLR1, RAC1, GPR158, CEACAM6, MUC16, PD-L1, NHE1, CEACAM5, MUC1, ACE2, GP2, and PTPRH. Optimal proteins to target using theranostic agents must exhibit high membrane expression on cancerous tissue with low expression on healthy tissue to afford improved disease outcomes with minimal off-target effects and toxicities. We provide guidelines to consider in the selection of a theranostic target for MOC and suggest future directions in evaluating the results of this review.

90Y/177Lu-DOTATOC: From Preclinical Studies to Application in Humans

The PRRT (Peptide Receptor Radionuclide Therapy) is a promising modality treatment for patients with inoperable or metastatic neuroendocrine tumors (NETs). Progression-free survival (PFS) and overall survival (OS) of these patients are favorably comparable with standard therapies. The protagonist in this type of therapy is a somatostatin-modified peptide fragment ([Tyr3] octreotide), equipped with a specific chelating system (DOTA) capable of creating a stable bond with β-emitting radionuclides, such as yttrium-90 and lutetium-177. In this review, covering twenty five years of literature, we describe the characteristics and performances of the two most used therapeutic radiopharmaceuticals for the NETs radio-treatment: [⁹⁰Y]Y-DOTATOC and [¹⁷⁷Lu]Lu-DOTATOC taking this opportunity to retrace the most significant results that have determined their success, promoting them from preclinical studies to application in humans.

Complementary imaging of ultrasound and PET/CT: A new opportunity?

AIM

To evaluate the effectiveness of complementary imaging of high-resolution ultrasound including CEUS with PET/CT for tissue characterization and tumor detection.

MATERIAL AND METHODS

100 patients were examined with PET/CT and US/CEUS between January 2018 until February 2020. All patients underwent PET/CT followed by selective US/CEUS within 4 weeks. Comparison regarding concordant or diverging findings in PET/CT and US. Analysis of the differences concerning the lesions number of found by PET/CT and US/CEUS or the possibility of a secured diagnosis following ultrasound causing therapeutic changes.

RESULTS

Diverging findings regarding the number of liver lesions in PET/CT and CEUS were found in 35 out of 64 patients (54%). Regarding renal lesions, a more definite diagnosis following ultrasound, causing a change of therapeutic approach, was achieved in 89%. Concordant results in PET/CT and US were found in 83% of patients with splenic and nodal findings. In 78% of patients with increased musculoskeletal or soft tissue tracer uptake, US was able to make a secured diagnosis with therapeutic changes.

CONCLUSION

The present results indicate a strong benefit of complementary imaging of PET/CT and selective, high-resolution ultrasound especially in patients with liver, renal and musculoskeletal or soft tissue findings.

Recent Breakthrough in 68Ga-Radiopharmaceuticals Cold Kits for Convenient PET Radiopharmacy

⁶⁸Ga-PET has emerged as an important diagnostic tool for precise detection and monitoring of oncological situations. Availability, cost, and radiosynthesis procedure are determining steps for success of a radioisotope/radiopharmaceutical in nuclear medicine. Availability of ⁶⁸Ga from a ⁶⁸Ge/⁶⁸Ga generator containing a long-lived parent radioisotope (⁶⁸Ge: t_1/2 = 271 days) and an inexpensive, simplified production of ⁶⁸Ga-radiopharmaceuticals through kit methodology has allowed smooth accommodation of ⁶⁸Ga-PET in clinics. The uncomplicated formulation of ⁶⁸Ga-radiopharmaceuticals from a lyophilized, cold kit is an impending breakthrough in clinical PET. The huge success of ⁶⁸Ga in neuroendocrine tumor and prostate cancer imaging along with the regulatory approval of respective cold kits has opened a pathway for development of kits for other evolving radiotracers. There is a definite scope for increased participation of commercial manufacturers and distributors of cold kits to spread the potential of ⁶⁸Ga worldwide across all the geographical locations and satellite centers.

Nuclear model analysis of the 65Cu(α, n)68Ga reaction for the production of 68Ga up to 40 MeV

The charge particle (α) induced reactions on enriched copper (⁶⁵Cu) are investigated for the production of ⁶⁸Ga. The data sets of experimental cross sections are compiled, normalized and nuclear model analysis is done using calculational codes namely, ALICE-IPPE, TALYS 1.95 and EMPIRE 3.2. The theoretical production cross sections via alpha particle induced reactions are calculated to present a set of recommended cross sections. The calculated cross sections are utilized to deduce thick target yield (TTY) for the ⁶⁵Cu (α, n) ⁶⁸Ga reaction. The range of energy for production of ⁶⁸Ga is suggested up to 40 MeV having least contribution of radio-impurities.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

Post FDA approval analysis of 200 gallium-68 DOTATATE imaging: A retrospective analysis in neuroendocrine tumor patients

Gallium-68 DOTATATE provides physiologic imaging and assists in disease localization for somatostatin receptor (SSTR) positive neuroendocrine tumor (NET) patients. However, questions regarding usefulness of gallium- 68 DOTATATE imaging in identifying the primary site in neuroendocrine tumors (NETS) of unknown primary, correlation of NET grade with median Standardized Uptake Value (SUV) and effects of long acting somatostatin analog on gallium-68 DOTATATE imaging quality needs to be evaluated. A single institution retrospective review of the first 200 NET patients with gallium-68 DOTATATE imaging from Dec 2016 to Dec 2017 was conducted. Questions related to NETs of unknown primary, correlation of Standardized Uptake Value (SUV) to Ki-67 (which signifies proliferation rate), the effects of long-acting systemic somatostatin analog (SSA) on SUV were part of our data analysis. From these 200 patients, 59.5% (119) were females, 40.5% (81) were males; the median age was 62 years. The following primary tumor sites were identified: small bowel-37.5%; pancreas-18.5%; bronchial-14%; colon-3.5%; rectum-2%; appendix-1.5%; adrenal-0.5%; prostate-0.5%; others-3% and unknown primary-19%. Mean hepatic SUV of the lesion with the greatest radiolabeled uptake in 96 patients was similar irrespective to exposure to long acting SSA. Patients exposed to long acting SSA had mean SUV of 31.3 vs 27.8 for SSA naïve patients. The difference was not statistically significant. Gallium-68 DOTATATE imaging seems to distinguished G3 NET from G1/G2 based on mean SUV, and also identified the primary tumor site in 17 of 38 (45%) patients with unknown primary. Systemic exposure to long acting SSA does not appear to influence mean SUV of gallium-68 DOTATATE scan.

Impact of Tumor Burden on Quantitative [68Ga] DOTATOC Biodistribution

PURPOSE

As has been previously reported, the somatostatin receptor (SSTR) imaging agent [⁶⁸Ga]-labeled 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid-d-Phe(1)-Tyr(3)-octreotate ([⁶⁸Ga]DOTATATE) demonstrates lower uptake in normal organs in patients with a high neuroendocrine tumor (NET) burden. Given the higher SSTR affinity of [⁶⁸Ga] DOTATATE, we aimed to quantitatively investigate the biodistribution of [⁶⁸Ga]-labeled 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid-d-Phe(1)-Tyr(3)-octreotide ([⁶⁸Ga]DOTATOC) to determine a potential correlation between uptake in normal organs and NET burden.

PROCEDURES

Of the 44 included patients, 36/44 (82 %) patients demonstrated suspicious radiotracer uptake on [⁶⁸Ga] DOTATOC positron emission tomography (PET)/X-ray computed tomography (CT). Volumes of interest (VOIs) were defined for tumor lesions and normal organs (spleen, liver, kidneys, adrenals). Mean body weight corrected standardized uptake value (SUV_mean) for normal organs was assessed and was used to calculate the corresponding mean specific activity uptake (Upt: fraction of injected activity per kg of tissue). For the entire tumor burden, SUV_mean, maximum standardized uptake value (SUV_max), and the total mass (TBM) was calculated and the decay corrected tumor fractional uptake (TBU) was assessed. A Spearman's rank correlation coefficient was used to determine the correlations between normal organ uptake and tumor burden.

RESULTS

The median SUV_mean was 18.7 for the spleen (kidneys, 9.2; adrenals, 6.8; liver, 5.6). For tumor burden, the median values were SUV_mean 6.9, SUV_max 35.5, TBM 42.6 g, and TBU 1.2 %. With increasing volume of distribution, represented by lean body mass and body surface area (BSA), Upt decreased in kidneys, liver, and adrenal glands and SUV_mean increased in the spleen. Correlation improved only for both kidneys and adrenals when the influence of the tumor uptake on the activity available for organ uptake was taken into account by the factor 1/(1-TBU). TBU was neither predictive for SUV_mean nor for Upt in any of the organs. The distribution of organ Upt vs. BSA/(1-TBU) were not different for patients with minor TBU (<3 %) vs. higher TBU (>7 %), indicating that the correlations observed in the present study are explainable by the body size effect. High tumor mass and uptake mitigated against G1 NET.

CONCLUSIONS

There is no significant impact on normal organ biodistribution with increasing tumor burden on [⁶⁸Ga] DOTATOC PET/CT. Potential implications include increased normal organ dose with [¹⁷⁷Lu-DOTA]⁰-D-Phe¹-Tyr³-Octreotide and decreased absolute lesion detection with [⁶⁸Ga] DOTATOC in high NET burden.

The efficacy of 177Lu-DOTATATE peptide receptor radionuclide therapy (PRRT) in patients with metastatic neuroendocrine tumours: a systematic review and meta-analysis

PURPOSE

To evaluate the efficacy of ¹⁷⁷Lu-DOTA0-Tyr3-octreotate (¹⁷⁷Lu-DOTATATE) radionuclide therapy in patients with inoperable or metastatic neuroendocrine tumours (NETs), (PROSPERO ID CRD42019130755).

METHODS

All published clinical studies of NETs treated with ¹⁷⁷Lu-DOTATATE were identified based on systematic searches in the PubMed, EMBASE, Cochrane Library, Web of Science and ClinicalTrials.gov databases up to January 2019. Among these studies, only the reports evaluated with the "Response Evaluation Criteria in Solid Tumours (RECIST)" or "Southwest Oncology Group (SWOG)" criteria or both were included. We analysed the disease response rate (DRR) and disease control rate (DCR) of each group to evaluate the efficacy of ¹⁷⁷Lu-DOTATATE.

RESULTS

Fifteen studies were selected from 715 references. The pooled effect in the RECIST group (13 studies) was 27.58% (95% confidence interval (CI) 21.03-35.27%) for the DRR and 79.14% (95% CI 75.83-82.1%) for the DCR. In the SWOG criteria group (7 studies), the pooled effect was 20.59% (95% CI 10.89-35.51%) for the DRR and 78.28% (95% CI 74.39-81.72%) for the DCR. Therefore, the RECIST and SWOG groups showed similar DRRs and DCRs after¹⁷⁷Lu-DOTATATE treatment, indicating that ¹⁷⁷Lu-DOTATATE treatment has excellent efficacy with a control rate of approximately 78-79%. Moreover, adverse effects of ¹⁷⁷Lu-DOTATATE were minimal, including fatigue, nausea, vomiting and hormonal disorders.

CONCLUSIONS

For patients with inoperable or metastatic NETs, ¹⁷⁷Lu-DOTATATE is an effective treatment with minimal side effects.

Standardized image quality for 68Ga-DOTA-TATE PET/CT

BACKGROUND

Positron emission tomography (PET) imaging with 68Gallium labeled somatostatin analogues (68Ga-DOTA-SSA) plays a key role in neuroendocrine tumor management. The impact of patient size on PET image quality is not well known for PET imaging with 68Ga-DOTA-SSA. The aim of this study is to propose a dose regimen based on patient size that optimizes image quality and yields sufficient image quality for diagnosis.

METHODS

Twenty-one patients (12 males, 9 females) were prospectively included for 68Gallium-DOTA-Tyr3-Octreotate (68Ga-DOTA-TATE) PET/CT, which was acquired in whole body list mode using 6 min per bed position (mbp). The list-mode events were randomly sampled to obtain 1 to 6 mbp PET reconstructions. For semi-quantitative assessment of image quality, the signal-to-noise ratio (SNR) was measured in the liver. The SNR normalized (SNRnorm) for administered activity and mbp was correlated with body mass, length, body mass index, body mass/length, and lean body mass. Three experienced nuclear medicine physicians visually graded image quality using a 4-point scale, and categorically scored the number of somatostatin-receptor positive lesions for each reconstruction. To investigate the impact of image quality on lesion quantification, the mean, maximum, and peak standardized uptake values (SUVs) of one abdominal lesion were measured in the 1 to 6 mbp PET reconstructions.

RESULTS

Of all patient-dependent parameters, body mass showed the strongest correlation (R2 = 0.6) with SNRnorm. Lesion detectability analysis showed no significant difference for 3-5 mbp compared with the complete 6 mbp PET reconstruction. The SUV measurements showed no significant (p > 0.05) differences across the reconstructions. Visual assessment revealed that an SNR of 6.2 results in PET scans with moderate to good image quality. A non-linear expression was derived to calculate the required (dose × acquisition time) product (DTP) for the chosen SNR level of 6.2 that would yield a more constant image quality.

CONCLUSION

Body mass can be used to predict 68Ga-DOTA-TATE PET image quality. The proposed non-linear dose regimen based on body mass standardizes the image quality while maintaining sufficient image quality for diagnosis.

Comparison of biological properties of [177 Lu]Lu-ProBOMB1 and [177 Lu]Lu-NeoBOMB1 for GRPR targeting

The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer and other solid malignancies. Following up on our work on [68 Ga]Ga-ProBOMB1 that had better imaging characteristics than [68 Ga]Ga-NeoBOMB1, we investigated the effects of substituting 68 Ga for 177 Lu to determine if the resulting radiopharmaceuticals could be used with a therapeutic aim. We radiolabeled the bombesin antagonist ProBOMB1 (DOTA-pABzA-DIG-D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ-Pro-NH2 ) with lutetium-177 and compared it with [177 Lu]Lu-NeoBOMB1 (obtained in 54.2 ± 16.5% isolated radiochemical yield with >96% radiochemical purity and 440.8 ± 165.1 GBq/μmol molar activity) for GRPR targeting. Lu-NeoBOMB1 had better binding affinity for GRPR than Lu-ProBOMB1 (Ki values: 2.26 ± 0.24 and 30.2 ± 3.23nM). [177 Lu]Lu-ProBOMB1 was obtained in 53.7 ± 5.4% decay-corrected radiochemical yield with 444.2 ± 193.2 GBq/μmol molar activity and >95% radiochemical purity. In PC-3 prostate cancer xenograft mice, tumor uptake of [177 Lu]Lu-ProBOMB1 was 3.38 ± 1.00, 1.32 ± 0.24, and 0.31 ± 0.04%ID/g at 1, 4, and 24 hours pi. However, the uptake in tumor was lower than [177 Lu]Lu-NeoBOMB1 at all time points. [177 Lu]Lu-ProBOMB1 was inferior to [177 Lu]Lu-NeoBOMB1, which had better therapeutic index for the organs receiving the highest doses.

64Cu-DOTATOC PET-CT in Patients with Neuroendocrine Tumors

Introduction

Several radiolabeled somatostatin analogues have been developed for molecular imaging of neuroendocrine tumors (NETs) with single-photon emission computed tomography (SPECT) and positron-emission tomography (PET). The aim of the present study was to report our first results using ⁶⁴Cu-DOTATOC in patients with NETs.

Methods

Thirty-three patients with NETs (15 female, 18 male; mean age 64 ± 13 years) were included in this retrospective study. ⁶⁴Cu-DOTATOC PET–CT scans were performed on all patients.

Results

Five out of 33 patients with a history of NET after surgical removal of the primary lesion showed no pathological lesions on PET–CT imaging and 8/33 patients had enhanced uptake in the area of recurrent meningioma at the skull base. The remaining 20/33 patients had a history of neuroendocrine tumor in the gastrointestinal tract (GEP-NET) and were presented with at least one pathological lesion.

Conclusion

The high detection rate of suspected lesions in patients with NETs and the high target-to-background contrast found in this study hold promise for the safe application of ⁶⁴Cu-DOTATOC in patients with NET.

Preparation of [177Lu]Lu-DOTA-Ahx-Lys40-Exendin-4 for radiotherapy of insulinoma: a detailed insight into the radiochemical intricacies

INTRODUCTION

[¹⁷⁷Lu]Lu-DOTA-Ahx-Lys40-Exendin-4 ([¹⁷⁷Lu]Lu-DOTA-Exendin-4) is a potential agent for radiotherapy of insulinomas owing to its specificity towards GLP-1 (Glucagon like peptide-1) receptors over-expressed on such cancers. The objective of the present study is to optimize the various radiochemistry parameters for the consistent formulation of the agent with high radiolabeling yield using carrier added [¹⁷⁷Lu]LuCl₃ and also to evaluate its biological behaviour in small animal model.

METHODS

In order to optimize the radiolabeling parameters, DOTA-Exendin-4 was radiolabeled with [¹⁷⁷Lu]LuCl₃ in two different buffer systems (sodium acetate and HEPES) at three different temperatures (45, 65 and 95 °C) using three different ligand to metal ratios (3:1, 4:1 and 5:1). The radiolabeled peptide was characterized by both paper chromatography and HPLC. The effect of addition of three different radio-protectors on complexation yield was also studied. Bio-distribution studies were carried out in healthy Swiss mice to evaluate the pharmacokinetic behaviour of the radiolabeled peptide as well as to determine the in vivo specificity of the radiotracer towards GLP-1 receptors (blocking studies). Urine and kidney lysate of the animals were analyzed at various post-administration time-points in order to determine the in vivo stability of the radiolabeled peptide.

RESULTS

The [¹⁷⁷Lu]Lu-DOTA-Exendin-4 complex could be prepared consistently with >95% radiolabeling yield using the optimized reaction conditions. Bio-distribution studies revealed early accumulation of [¹⁷⁷Lu]Lu-DOTA-Exendin-4 in pancreas along with fast clearance via renal pathway. Significantly high accumulation of the radiotracer was observed in kidneys. Analyses of urine and kidney lysate of the animals revealed in vivo stability of [¹⁷⁷Lu]Lu-DOTA-Exendin-4. Blocking studies showed displacement of significant amount of radiotracer from GLP-1 receptor-positive organs such as, pancreas and lungs (p <0.05) in presence of unlabeled peptide, indicating the specificity of the radiolabeled preparation towards GLP-1 receptors.

CONCLUSIONS

Present study shows that [¹⁷⁷Lu]Lu-DOTA-Exendin-4 could be formulated for radiotherapeutic application with high radiochemical purity and adequate in vivo stability using [¹⁷⁷Lu]LuCl₃ produced via direct neutron irradiation.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Findings of the present study will be helpful in preparing the patient dose of [¹⁷⁷Lu]Lu-labeled Exendin for radiotherapy of insulinoma using carrier added [¹⁷⁷Lu]LuCl₃, produced in a medium flux reactor, without the requirement of post-labeling purification.

Recent Updates on Molecular Imaging Reporting and Data Systems (MI-RADS) for Theranostic Radiotracers-Navigating Pitfalls of SSTR- and PSMA-Targeted PET/CT

The theranostic concept represents a paradigmatic example of personalized treatment. It is based on the use of radiolabeled compounds which can be applied for both diagnostic molecular imaging and subsequent treatment, using different radionuclides for labelling. Clinically relevant examples include somatostatin receptor (SSTR)-targeted imaging and therapy for the treatment of neuroendocrine tumors (NET), as well as prostate-specific membrane antigen (PSMA)-targeted imaging and therapy for the treatment of prostate cancer (PC). As such, both classes of radiotracers can be used to triage patients for theranostic endoradiotherapy using positron emission tomography (PET). While interpreting PSMA- or SSTR-targeted PET/computed tomography scans, the reader has to navigate certain pitfalls, including (I.) varying normal biodistribution between different PSMA- and SSTR-targeting PET radiotracers, (II.) varying radiotracer uptake in numerous kinds of both benign and malignant lesions, and (III.) resulting false-positive and false-negative findings. Thus, two novel reporting and data system (RADS) classifications for PSMA- and SSTR-targeted PET imaging (PSMA- and SSTR-RADS) have been recently introduced under the umbrella term molecular imaging reporting and data systems (MI-RADS). Notably, PSMA- and SSTR-RADS are structured in a reciprocal fashion, i.e., if the reader is familiar with one system, the other system can readily be applied. Learning objectives of the present case-based review are as follows: (I.) the theranostic concept for the treatment of NET and PC will be briefly introduced, (II.) the most common pitfalls on PSMA- and SSTR-targeted PET/CT will be identified, (III.) the novel framework system for theranostic radiotracers (MI-RADS) will be explained, applied to complex clinical cases and recent studies in the field will be highlighted. Finally, current treatment strategies based on MI-RADS will be proposed, which will demonstrate how such a generalizable framework system truly paves the way for clinically meaningful molecular imaging-guided treatment of either PC or NET. Thus, beyond an introduction of MI-RADS, the present review aims to provide an update of recently published studies which have further validated the concept of structured reporting systems in the field of theranostics.

The role of multimodal imaging in guiding resectability and cytoreduction in pancreatic neuroendocrine tumors: focus on PET and MRI

Pancreatic neuroendocrine tumors (pNETs) are rare neoplasms that secrete peptides and neuro-amines. pNETs can be sporadic or hereditary, syndromic or non-syndromic with different clinical presentations and prognoses. The role of medical imaging includes locating the tumor, assessing its extent, and evaluating the feasibility of curative surgery or cytoreduction. Pancreatic NETs have very distinctive phenotypes on CT, MRI, and PET. PET have been demonstrated to be very sensitive to detect either well-differentiated pNETs using ⁶⁸Gallium somatostatin receptor (SSTR) radiotracers, or more aggressive undifferentiated pNETS using ¹⁸F-FDG. A comprehensive interpretation of multimodal imaging guides resectability and cytoreduction in pNETs. The imaging phenotype provides information on the differentiation and proliferation of pNETs, as well as the spatial and temporal heterogeneity of tumors with prognostic and therapeutic implications. This review provides a structured approach for standardized reading and reporting of medical imaging studies with a focus on PET and MR techniques. It explains which imaging approach should be used for different subtypes of pNET and what a radiologist should be looking for and reporting when interpreting these studies.

Cross-Isotopic Bioorthogonal Tools as Molecular Twins for Radiotheranostic Applications

Radiotheranostics are designed by labeling targeting (bio)molecules with radionuclides for diagnostic or therapeutic application. Because the pharmacokinetics of therapeutic compounds play a pivotal role, chemically closely related imaging agents are used to evaluate the overall feasibility of the therapeutic approach. "Theranostic relatives" that utilize different elements are frequently used in clinical practice. However, variations in pharmacokinetics, biodistribution, and target affinity due to different chemical properties of the radioisotopes remain as hurdles to the design of optimized clinical tools. Herein, the design and synthesis of structurally identical compounds, either for diagnostic (18 F and a stable metal isotope) or therapeutic application (radiometal and stable 19 F), are reported. Such "molecular twins" have been prepared by applying a modular strategy based on click chemistry that enables efficient radiolabeling of compounds containing a metal complex and a tetrazine moiety. This additional bioorthogonal functionality can be used for subsequent radiolabeling of (bio)molecules or pretargeting approaches, which is demonstrated in vitro.

Chitosan-TiO2 composite: A potential 68Ge/68Ga generator column material

A durable and ready to use ⁶⁸Ge-⁶⁸Ga generator column material is required for its routine use in radiopharmaceutical procedures. The present work comprises preliminary studies for development and evaluation of chitosan-TiO₂ based microsphere (C-TOM) composite towards its competence as a column material. The batch uptake studies showed higher distribution coefficients for ⁶⁸Ge vis-à-vis ⁶⁸Ga in the complete concentration range of HCl examined (0.01-1 mol.L^-1). Furthermore, C-TOM showed enduring physical and chemical stability in 0.01 mol.L^-1 HCl with persistent ⁶⁸Ga elution profiles (>95%) and negligible ⁶⁸Ge breakthrough (2 × 10^-4%) for the preliminary evaluation period of ∼2 months. Overall, the studies indicated that, ⁶⁸Ga with high radionuclidic purity (≥99.99%) can be eluted routinely in a small volume (∼1.5 mL) of 0.01 mol.L^-1 HCl proving its potentials as a novel solid phase extractant for ⁶⁸Ge/⁶⁸Ge generator system.

SEOM clinical guidelines for the diagnosis and treatment of gastroenteropancreatic and bronchial neuroendocrine neoplasms (NENs) (2018)

NENs are a heterogeneous family of tumors of challenging diagnosis and clinical management. Their incidence and prevalence continue to rise across all sites, stages and grades. Although improved diagnostic techniques have led to earlier detection and stage migration, the improved prognosis documented over time for advanced gastrointestinal and pancreatic neuroendocrine tumors also reflect improvements in therapy. The aim of this guideline is to update practical recommendations for the diagnosis and treatment of gastroenteropancreatic and lung NENs. Diagnostic procedures, histological classification and therapeutic options are briefly discussed, including surgery, liver-directed therapy, peptide receptor radionuclide therapy, and systemic hormonal, cytotoxic or targeted therapy, and treatment algorithms are provided.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

The theranostic promise for Neuroendocrine Tumors in the late 2010s - Where do we stand, where do we go?

More than 25 years after the first peptide receptor radionuclide therapy (PRRT), the concept of somatostatin receptor (SSTR)-directed imaging and therapy for neuroendocrine tumors (NET) is seeing rapidly increasing use. To maximize the full potential of its theranostic promise, efforts in recent years have expanded recommendations in current guidelines and included the evaluation of novel theranostic radiotracers for imaging and treatment of NET. Moreover, the introduction of standardized reporting framework systems may harmonize PET reading, address pitfalls in interpreting SSTR-PET/CT scans and guide the treating physician in selecting PRRT candidates. Notably, the concept of PRRT has also been applied beyond oncology, e.g. for treatment of inflammatory conditions like sarcoidosis. Future perspectives may include the efficacy evaluation of PRRT compared to other common treatment options for NET, novel strategies for closer monitoring of potential side effects, the introduction of novel radiotracers with beneficial pharmacodynamic and kinetic properties or the use of supervised machine learning approaches for outcome prediction. This article reviews how the SSTR-directed theranostic concept is currently applied and also reflects on recent developments that hold promise for the future of theranostics in this context.

Somatostatin receptor PET ligands - the next generation for clinical practice

Somatostatin receptors (SSTRs) are variably expressed by a variety of malignancies. Using radiolabeled somatostatin analogs (SSAs), the presence of SSTRs on tumor cells may be exploited for molecular imaging and for peptide receptor radionuclide therapy. ¹¹¹In-DTPA-octreotide has long been the standard in SSTR scintigraphy. A major leap forward was the introduction of gallium-68 labeled SSAs for positron emission tomography (PET) offering improved sensitivity. Tracers currently in clinical use are ⁶⁸Ga-DOTA-Tyr³-octreotide (⁶⁸Ga-DOTATOC), ⁶⁸Ga-DOTA-Tyr³-octreotate (⁶⁸Ga-DOTATATE) and ⁶⁸Ga-DOTA-1-NaI³-octreotide (⁶⁸Ga-DOTANOC), collectively referred to as ⁶⁸Ga-DOTA-peptides. ⁶⁸Ga-DOTA-peptide PET has superseded ¹¹¹In-DTPA-octreotide scintigraphy as the modality of choice for SSTR imaging. However, implementation of ⁶⁸Ga-DOTA-peptides in routine clinical practice is often limited by practical, economical and regulatory factors related to the use of the current generation of ⁶⁸Ge/⁶⁸Ga generators. Centralized production and distribution is challenging due to the low production yield and relatively short half-life of gallium-68. Furthermore, gallium-68 has a relatively long positron range, compromising spatial resolution on modern PET cameras. Therefore, possibilities of using other PET radionuclides are being explored. On the other hand, new developments in SSTR PET ligands are strongly driven by the need for improved lesion targeting, especially for tumors with low SSTR expression. This may be achieved by using peptide vectors having a higher affinity for the SSTR or a broader affinity profile for the different receptor subtypes or by using compounds recognizing more binding sites, such as SSTR antagonists. This review gives an overview of recent developments leading to the next generation of clinical PET tracers for SSTR imaging.

Moving into the next era of PET myocardial perfusion imaging: introduction of novel 18F-labeled tracers

The heart failure epidemic continues to rise with coronary artery disease as one of its main causes. Novel concepts for risk stratification to guide the referring cardiologist towards revascularization procedures are of significant value. Myocardial perfusion imaging using single-photon emission computed tomography (SPECT) agents has demonstrated high accuracy for the detection of clinically relevant stenoses. With positron emission tomography (PET) becoming more widely available, mainly due to its diagnostic performance in oncology, perfusion imaging with that modality is more practical than in the past and overcomes existing limitations of SPECT MPI. Advantages of PET include more reliable quantification of absolute myocardial blood flow, the routine use of computed tomography for attenuation correction, a higher spatiotemporal resolution and a higher count sensitivity. Current PET radiotracers such as rubidium-82 (half-life, 76 s), oxygen-15 water (2 min) or nitrogen-13 ammonia (10 min) are labeled with radionuclides with very short half-lives, necessitating that stress imaging is performed under pharmacological vasodilator stress instead of exercise testing. However, with the introduction of novel ¹⁸F-labeled MPI PET radiotracers (half-life, 110 min), the intrinsic advantages of PET can be combined with exercise testing. Additional advantages of those radiotracers include, but are not limited to: potentially improved cost-effectiveness due to the use of pre-existing delivery systems and superior imaging qualities, mainly due to the shortest positron range among available PET MPI probes. In the present review, widely used PET MPI radiotracers will be reviewed and potential novel ¹⁸F-labeled perfusion radiotracers will be discussed.

Molecular imaging reporting and data systems (MI-RADS): a generalizable framework for targeted radiotracers with theranostic implications

Both prostate-specific membrane antigen (PSMA)- and somatostatin receptor (SSTR)-targeted positron emission tomography (PET)-based imaging agents for prostate carcinoma and neuroendocrine tumors, respectively, are seeing rapidly expanding use. In addition to diagnostic applications, both classes of radiotracers can be used to triage patients for theranostic endoradiotherapy. While interpreting PSMA- or SSTR-targeted PET/computed tomography (CT) scans, the reader has to be aware of certain pitfalls. Adding to the complexity of the interpretation of those imaging agents, both normal biodistribution, and also false-positive and -negative findings differ between PSMA- and SSTR-targeted PET radiotracers. Herein summarized under the umbrella term molecular imaging reporting and data systems (MI-RADS), two novel RADS classifications for PSMA- and SSTR-targeted PET imaging are described (PSMA- and SSTR-RADS). Notably, PSMA- and SSTR-RADS are structured in a reciprocal fashion, i.e., if the reader is familiar with one system, the other system can readily be applied, as well. In the present review, we will discuss the most common pitfalls on PSMA- and SSTR-targeted PET/CT, briefly introduce PSMA- and SSTR-RADS, and define a potential future role of the umbrella framework MI-RADS compared to other classification systems.

Pretargeting in nuclear imaging and radionuclide therapy: Improving efficacy of theranostics and nanomedicines

Pretargeted nuclear imaging and radiotherapy have recently attracted increasing attention for diagnosis and treatment of cancer with nanomedicines. This is because it conceptually offers better imaging contrast and therapeutic efficiency while reducing the dose to radiosensitive tissues compared to conventional strategies. In conventional imaging and radiotherapy, a directly radiolabeled nano-sized vector is administered and allowed to accumulate in the tumor, typically on a timescale of several days. In contrast, pretargeting is based on a two-step approach. First, a tumor-accumulating vector carrying a tag is administered followed by injection of a fast clearing radiolabeled agent that rapidly recognizes the tag of the tumor-bound vector in vivo. Therefore, pretargeting circumvents the use of long-lived radionuclides that is a necessity for sufficient tumor accumulation and target-to-background ratios using conventional approaches. In this review, we give an overview of recent advances in pretargeted imaging strategies. We will critically reflect on the advantages and disadvantages of current state-of-the-art conventional imaging approaches and compare them to pretargeted strategies. We will discuss the pretargeted imaging concept and the involved chemistry. Finally, we will discuss the steps forward in respect to clinical translation, and how pretargeted strategies could be applied to improve state-of-the-art radiotherapeutic approaches.

Development of a radiolabeled caninized anti-EGFR antibody for comparative oncology trials

Due to large homology of human and canine EGFR, dogs suffering from spontaneous EGFR+ cancer can be considered as ideal translational models. Thereby, novel immunotherapeutic compounds can be developed for both human and veterinary patients. This study describes the radiolabeling of a canine anti-EGFR IgG antibody (can225IgG) with potential diagnostic and therapeutic value in comparative clinical settings. Can225IgG was functionalized with DTPA for subsequent chelation with the radionuclide ^99mTc. Successful coupling of 10 DTPA molecules per antibody on average was proven by significant mass increase in MALDI-TOF spectroscopy, gel electrophoresis and immunoblots. Following functionalization and radiolabeling, ^99mTc-DTPA-can225IgG fully retained its binding capacity towards human and canine EGFR in flow cytometry, immuno- and radioblots, and autoradiography. The affinity of radiolabeled can225IgG was determined to K_D 0.8 ±0.0031 nM in a real-time kinetics assay on canine carcinoma cells by a competition binding technique. Stability tests of the radiolabeled compound identified TRIS buffered saline as the ideal formulation for short-term storage with 87.11 ±6.04% intact compound being still detected 60 minutes post radiolabeling. High stability, specificity and EGFR binding affinity pinpoint towards ^99mTc-radiolabeled can225IgG antibody as an ideal lead compound for the first proof-of-concept diagnostic and therapeutic applications in canine cancer patients.

Current Status of Radiopharmaceuticals for the Theranostics of Neuroendocrine Neoplasms

Nuclear medicine plays a pivotal role in the management of patients affected by neuroendocrine neoplasms (NENs). Radiolabeled somatostatin receptor analogs are by far the most advanced radiopharmaceuticals for diagnosis and therapy (radiotheranostics) of NENs. Their clinical success emerged receptor-targeted radiolabeled peptides as an important class of radiopharmaceuticals and it paved the way for the investigation of other radioligand-receptor systems. Besides the somatostatin receptors (sstr), other receptors have also been linked to NENs and quite a number of potential radiolabeled peptides have been derived from them. The Glucagon-Like Peptide-1 Receptor (GLP-1R) is highly expressed in benign insulinomas, the Cholecystokinin 2 (CCK2)/Gastrin receptor is expressed in different NENs, in particular medullary thyroid cancer, and the Glucose-dependent Insulinotropic Polypeptide (GIP) receptor was found to be expressed in gastrointestinal and bronchial NENs, where interestingly, it is present in most of the sstr-negative and GLP-1R-negative NENs. Also in the field of sstr targeting new discoveries brought into light an alternative approach with the use of radiolabeled somatostatin receptor antagonists, instead of the clinically used agonists. The purpose of this review is to present the current status and the most innovative strategies for the diagnosis and treatment (theranostics) of neuroendocrine neoplasms using a cadre of radiolabeled regulatory peptides targeting their receptors.

Tunable and noncytotoxic PET/SPECT-MRI multimodality imaging probes using colloidally stable ligand-free superparamagnetic iron oxide nanoparticles

Physiologically stable multimodality imaging probes for positron emission tomography/single-photon emission computed tomography (PET/SPECT)-magnetic resonance imaging (MRI) were synthesized using the superparamagnetic maghemite iron oxide (γ-Fe2O3) nanoparticles (SPIONs). The SPIONs were sterically stabilized with a finely tuned mixture of diblock copolymers with either methoxypolyethylene glycol (MPEG) or primary amine NH2 end groups. The radioisotope for PET or SPECT imaging was incorporated with the SPIONs at high temperature. 57Co2+ ions with a long half-life of 270.9 days were used as a model for the radiotracer to study the kinetics of radiolabeling, characterization, and the stability of the radiolabeled SPIONs. Radioactive 67Ga3+ and Cu2+-labeled SPIONs were also produced successfully using the optimized conditions from the 57Co2+-labeling process. No free radioisotopes were detected in the aqueous phase for the radiolabeled SPIONs 1 week after dispersion in phosphate-buffered saline (PBS). All labeled SPIONs were not only well dispersed and stable under physiological conditions but also noncytotoxic in vitro. The ability to design and produce physiologically stable radiolabeled magnetic nanoparticles with a finely controlled number of functionalizable end groups on the SPIONs enables the generation of a desirable and biologically compatible multimodality PET/SPECT-MRI agent on a single T2 contrast MRI probe.

Thiourea derivatives as chelating agents for bioconjugation of rhenium and technetium

A^99mTc complex with a tetradentate thiocarbamoylbenzamidine group was used for the conjugation of angiotensin-II. The resulting bioconjugate is stablein vivoandin vitro.

Development of nitroxide-based theranostic compounds that act both as anti-inflammatory drugs and brain redox imaging probes in MRI

Theranostic probes provide both therapeutic and diagnostic imaging capabilities in one molecule and show significant promise for use in magnetic resonance imaging (MRI) examinations. The present study describes for the first time the synthesis and utility of nitroxide-based contrast agents exhibiting a nonsteroidal anti-inflammatory drug effect. The target theranostic probes were prepared by connecting the carboxyl group of ibuprofen or ketoprofen to the hydroxyl group of 3-hydroxymethyl-2,2,5,5-tetramethylprrolidine-1-oxyl by a condensation reaction in the presence of dicyclohexylcarbodiimide and 4-dimethylaminopyridine in dichloromethane. MRI of mouse heads after administration of either synthesized theranostic probe indicated that the probes enter the brain by passing through the blood-brain barrier (BBB), resulting in T1 contrast enhancement in mouse brain. This enhancement persisted for the duration of the half-life of about 40 min, which is longer than that obtained by most of pyrrolidine nitroxide molecules. The therapeutic capacities of these theranostic probes were examined using a lipopolysaccharide (LPS)-induced brain inflammation model. The production of nitric oxide, an inflammation marker in septic mouse brain induced by LPS, was remarkably inhibited by the addition of either synthesized probe, indicating that they also act as anti-inflammatory drugs. The present results indicate that nitroxide-based theranostic probes act as both BBB-permeable redox-sensitive contrast agents and as an anti-inflammatory drug in septic mouse brain. Copyright © 2016 John Wiley & Sons, Ltd.

Biodistribution and radiation dosimetry of (68)Ga-PSMA HBED CC-a PSMA specific probe for PET imaging of prostate cancer

PURPOSE

Positron emission tomography (PET) agents targeting the prostate-specific membrane antigen (PSMA) are currently under broad clinical and scientific investigation. (68)Ga-PSMA HBED-CC constitutes the first (68)Ga-labelled PSMA-inhibitor and has evolved as a promising agent for imaging PSMA expression in vivo. The aim of this study was to evaluate the whole-body distribution and radiation dosimetry of this new probe.

METHODS

Five patients with a history or high suspicion of prostate cancer were injected intravenously with a mean of 139.8 ± 13.7 MBq of (68)Ga-PSMA HBED-CC (range 120-158 MBq). Four static skull to mid-thigh scans using a whole-body fully integrated PET/MR-system were performed 10 min, 60 min, 130 min, and 175 min after the tracer injection. Time-dependent changes of the injected activity per organ were determined. Mean organ-absorbed doses and effective doses (ED) were calculated using OLINDA/EXM.

RESULTS

Injection of a standard activity of 150 MBq (68)Ga-PSMA HBED-CC resulted in a median effective dose of 2.37 mSv (Range 1.08E-02 - 2.46E-02 mSv/MBq). The urinary bladder wall (median absorbed dose 1.64E-01 mGv/MBq; range 8.76E-02 - 2.91E-01 mGv/MBq) was the critical organ, followed by the kidneys (median absorbed dose 1.21E-01 mGv/MBq; range 7.16E-02 - 1.75E-01), spleen (median absorbed dose 4.13E-02 mGv/MBq; range 1.57E-02 - 7.32E-02 mGv/MBq) and liver (median absorbed dose 2.07E-02 mGv/MBq; range 1.80E-02 - 2.57E-02 mGv/MBq). No drug-related pharmacological effects occurred.

CONCLUSION

The use of (68)Ga-PSMA HBED-CC results in a relatively low radiation exposure, delivering organ doses that are comparable to those of other (68)Ga-labelled PSMA-inhibitors used for PET-imaging. Total effective dose is lower than for other PET-agents used for prostate cancer imaging (e.g. (11)C- and (18)F-Choline).

Multi-technique hybrid imaging in PET/CT and PET/MR: what does the future hold?

Integrated positron-emission tomography and computed tomography (PET/CT) is one of the most important imaging techniques to have emerged in oncological practice in the last decade. Hybrid imaging, in general, remains a rapidly growing field, not only in developing countries, but also in western industrialised healthcare systems. A great deal of technological development and research is focused on improving hybrid imaging technology further and introducing new techniques, e.g., integrated PET and magnetic resonance imaging (PET/MRI). Additionally, there are several new PET tracers on the horizon, which have the potential to broaden clinical applications in hybrid imaging for diagnosis as well as therapy. This article aims to highlight some of the major technical and clinical advances that are currently taking place in PET/CT and PET/MRI that will potentially maintain the position of hybrid techniques at the forefront of medical imaging technologies.

18F-FDG PET/CT and PET/MRI Perform Equally Well in Cancer: Evidence from Studies on More Than 2,300 Patients

(18)F-FDG PET/CT has become the reference standard in oncologic imaging against which the performance of other imaging modalities is measured. The promise of PET/MRI includes multiparametric imaging to further improve diagnosis and phenotyping of cancer. Rather than focusing on these capabilities, many investigators have examined whether (18)F-FDG PET combined with mostly anatomic MRI improves cancer staging and restaging. After a description of PET/MRI scanner designs and a discussion of technical and operational issues, we review the available literature to determine whether cancer assessments are improved with PET/MRI. The available data show that PET/MRI is feasible and performs as well as PET/CT in most types of cancer. Diagnostic advantages may be achievable in prostate cancer and in bone metastases, whereas disadvantages exist in lung nodule assessments. We conclude that (18)F-FDG PET/MRI and PET/CT provide comparable diagnostic information when MRI is used simply to provide the anatomic framework. Thus, PET/MRI could be used in lieu of PET/CT if this approach becomes economically viable and if reasonable workflows can be established. Future studies should explore the multiparametric potential of MRI.

High Yield Production and Radiochemical Isolation of Isotopically Pure Arsenic-72 and Novel Radioarsenic Labeling Strategies for the Development of Theranostic Radiopharmaceuticals

Radioisotopes of arsenic are of considerable interest to the field of nuclear medicine with unique nuclear and chemical properties making them well-suited for use in novel theranostic radiopharmaceuticals. However, progress must still be made in the production of isotopically pure radioarsenic and in its stable conjugation to biological targeting vectors. This work presents the production and irradiation of isotopically enriched (72)Ge(m) discs in an irrigation-cooled target system allowing for the production of isotopically pure (72)As with capability on the order of 10 GBq. A radiochemical separation procedure isolated the reactive trivalent radioarsenic in a small volume buffered aqueous solution, while reclaiming (72)Ge target material. The direct thiol-labeling of a monoclonal antibody resulted in a conjugate exhibiting exceptionally poor in vivo stability in a mouse model. This prompted further investigations to alternative radioarsenic labeling strategies, including the labeling of the dithiol-containing chelator dihydrolipoic acid, and thiol-modified mesoporous silica nanoparticles (MSN-SH). Radioarsenic-labeled MSN-SH showed exceptional in vivo stability toward dearsenylation.

In vivo molecular imaging of chemokine receptor CXCR4 expression in patients with advanced multiple myeloma

CXCR4 is a G-protein-coupled receptor that mediates recruitment of blood cells toward its ligand SDF-1. In cancer, high CXCR4 expression is frequently associated with tumor dissemination and poor prognosis. We evaluated the novel CXCR4 probe [(68)Ga]Pentixafor for in vivo mapping of CXCR4 expression density in mice xenografted with human CXCR4-positive MM cell lines and patients with advanced MM by means of positron emission tomography (PET). [(68)Ga]Pentixafor PET provided images with excellent specificity and contrast. In 10 of 14 patients with advanced MM [(68)Ga]Pentixafor PET/CT scans revealed MM manifestations, whereas only nine of 14 standard [(18)F]fluorodeoxyglucose PET/CT scans were rated visually positive. Assessment of blood counts and standard CD34(+) flow cytometry did not reveal significant blood count changes associated with tracer application. Based on these highly encouraging data on clinical PET imaging of CXCR4 expression in a cohort of MM patients, we conclude that [(68)Ga]Pentixafor PET opens a broad field for clinical investigations on CXCR4 expression and for CXCR4-directed therapeutic approaches in MM and other diseases.