Design and in vitro characterization of highly sst2-selective somatostatin antagonists suitable for radiotargeting

Urocortin 3 contributes to paracrine inhibition of islet alpha cells in mice

Pancreatic alpha cell activity and glucagon secretion lower as glucose levels increase. While part of the decrease is regulated by glucose itself, paracrine signaling by their neighboring beta and delta cells also plays an important role. Somatostatin from delta cells is an important local inhibitor of alpha cells at high glucose. Additionally, urocortin 3 (UCN3) is a hormone that is co-released from beta cells with insulin and acts locally to potentiate somatostatin secretion from delta cells. UCN3 thus inhibits insulin secretion via a negative feedback loop with delta cells, but its role with respect to alpha cells and glucagon secretion is not understood. We hypothesize that the somatostatin-driven glucagon inhibition at high glucose is regulated in part by UCN3 from beta cells. Here, we use a combination of live functional Ca2+ and cAMP imaging as well as direct glucagon secretion measurement, all from alpha cells in intact mouse islets, to determine the contributions of UCN3 to alpha cell behavior. Exogenous UCN3 treatment decreased alpha cell Ca2+ and cAMP levels and inhibited glucagon release. Blocking endogenous UCN3 signaling increased alpha cell Ca2+ by 26.8 ± 7.6%, but this did not result in increased glucagon release at high glucose. Furthermore, constitutive deletion of Ucn3 did not increase Ca2+ activity or glucagon secretion relative to controls. UCN3 is thus capable of inhibiting mouse alpha cells, but, given the subtle effects of endogenous UCN3 signaling on alpha cells, we propose that UCN3-driven somatostatin may serve to regulate local paracrine glucagon levels in the islet instead of inhibiting gross systemic glucagon release.

Radiolabeled peptides and their expanding role in clinical imaging and targeted cancer therapy

There is an expanding body of evidence showing that synthetic peptides in combination with radioactive isotopes can be utilized for medical purposes. This area is of particular interest in oncology where applications in diagnosis and therapy are at different stages of development. We review the contributions in this area by the group originally founded by Carlo Pedone in Naples many years ago. We highlight the work of this group in the context of other developments in this area, focusing on three biologically relevant receptor systems: somatostatin, gastrin-releasing peptide, and cholecystokinin-2/gastrin receptors. We focus on key milestones, state of the art, and challenges in this area of research as well as the current and future outlook for expanding clinical applications.

Bis(Disulfide)-Bridged Somatostatin-14 Analogs and Their [111In]In-Radioligands: Synthesis and Preclinical Profile

The overexpression of one or more somatostatin receptors (SST1-5R) in human tumors has provided an opportunity for diagnosis and therapy with somatostatin-like radionuclide carriers. The application of "pansomatostatin" analogs is expected to broaden the clinical indications and upgrade the diagnostic/therapeutic efficacy of currently applied SST2R-prefering radioligands. In pursuit of this goal, we now introduce two bicyclic somatostatin-14 (SS14) analogs, AT5S (DOTA-Ala1-Gly2-c[Cys3-Lys4-Asn5-c[Cys6-Phe7-DTrp8-Lys9-Thr10-Cys11]-Thr12-Ser13-Cys14]) and AT6S (DOTA-Ala1-Gly2-c[Cys3-Lys4-c[Cys5-Phe6-Phe7-DTrp8-Lys9-Thr10-Phe11-Cys12]-Ser13-Cys14]), suitable for labeling with trivalent radiometals and designed to sustain in vivo degradation. Both AT5S and AT6S and the respective [111In]In-AT5S and [111In]In-AT6S were evaluated in a series of in vitro assays, while radioligand stability and biodistribution were studied in mice. The 8/12-mer bicyclic AT6S showed expanded affinity for all SST1-5R and agonistic properties at the SST2R, whereas AT5S lost all affinity to SST1-5R. Both [111In]In-AT5S and [111In]In-AT6S remained stable in the peripheral blood of mice, while [111In]In-AT6S displayed low, but specific uptake in AR4-2J tumors and higher uptake in HEK293-SST3R tumors in mice. In summary, high radioligand stability was acquired by the two disulfide bridges introduced into the SS14 motif, but only the 8/12-mer ring AT6S retained a pansomatostatin profile. In consequence, [111In]In-AT6S targeted SST2R-/SST3R-positive xenografts in mice. These results call for further research on pansomatostatin-like radioligands for cancer theranostics.

The usefulness of [68 Ga]Ga-DOTA-JR11 PET/CT in patients with meningioma: comparison with MRI

PURPOSE

Clinical studies of PET imaging using SSTR2 agonists have demonstrated high accuracy and correlation with SSTR2 expression in meningiomas. However, the usefulness of the SSTR2 antagonist with [68 Ga]Ga-DOTA-JR11 is uncertain. To evaluate the diagnostic performance of [68 Ga]Ga-DOTA-JR11 PET/CT and to clarify tumor characteristics in patients with suspected meningiomas.

MATERIALS AND METHODS

Patients with suspected de novo or recurrent meningioma in complex locations or atypical images were enrolled from August 2021 to October 2022 in prospective study. All patients underwent contrast-enhanced MRI (CE-MRI), [68 Ga]Ga-DOTA-JR11 PET/CT, and histopathological evaluation. Tumor uptake of [68 Ga]Ga-DOTA-JR11 was measured by SUVmax and tumor-endocranium ratio (TBR). Diagnostic performance was compared between PET and MRI.

RESULTS

Of 36 (50.0 ± 13.0 years of age, 20 women) patients, 32 were histopathologically confirmed meningiomas and four with other tumors. [68 Ga]Ga-DOTA-JR11 uptake was significantly higher in meningioma patients than in those with other tumors (SUVmax: 13.6 ± 7.7 vs. 5.2 ± 3.0, P < 0.001; TBR: 64.2 ± 27.7 vs. 25.0 ± 18.9, P = 0.001). [68 Ga]Ga-DOTA-JR11 PET/CT detected 31 meningiomas, while CE-MRI detected 17 meningiomas of 25 initial diagnosis and 11 recurrent tumors; [68 Ga]Ga-DOTA-JR11 PET had an incremental diagnostic value of 24% (6/25) over MRI in the group of initial diagnosis. There was no statistically significant difference in diagnostic efficacy between PET and MRI (P = 0.45) for all 36 patients. In skull base meningiomas, PET provided a more definitive diagnosis of pituitary involvement (in 12, not in12), compared to MRI (in eight, possible in six, possible not in six, not in four). PET revealed bone involvement in all 14 patients proven by pathology, while MRI identified only 11.

CONCLUSIONS

[68 Ga]Ga-DOTA-JR11 PET/CT provided high image quality and presented an ideal diagnostic performance in detecting meningioma and evaluating the involvement of the pituitary and bone. The study provides valuable evidence for the use of [68 Ga]Ga-DOTA-JR11 PET/CT as a complementary imaging modality to CE-MRI in the evaluation of meningiomas.

Comparative Saturation Binding Analysis of 64Cu-Labeled Somatostatin Analogues Using Cell Homogenates and Intact Cells

The development of novel ligands for G-protein-coupled receptors (GPCRs) typically entails the characterization of their binding affinity, which is often performed with radioligands in a competition or saturation binding assay format. Since GPCRs are transmembrane proteins, receptor samples for binding assays are prepared from tissue sections, cell membranes, cell homogenates, or intact cells. As part of our investigations on modulating the pharmacokinetics of radiolabeled peptides for improved theranostic targeting of neuroendocrine tumors with a high abundance of the somatostatin receptor sub-type 2 (SST₂), we characterized a series of ⁶⁴Cu-labeled [Tyr³]octreotate (TATE) derivatives in vitro in saturation binding assays. Herein, we report on the SST₂ binding parameters measured toward intact mouse pheochromocytoma cells and corresponding cell homogenates and discuss the observed differences taking the physiology of SST₂ and GPCRs in general into account. Furthermore, we point out method-specific advantages and limitations.

First preclinical evaluation of [225Ac]Ac-DOTA-JR11 and comparison with [177Lu]Lu-DOTA-JR11, alpha versus beta radionuclide therapy of NETs

BACKGROUND

The [¹⁷⁷Lu]Lu-DOTA-TATE mediated peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors (NETs) is sometimes leading to treatment resistance and disease recurrence. An interesting alternative could be the somatostatin antagonist, [¹⁷⁷Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [¹⁷⁷Lu]Lu-DOTA-TATE. Furthermore, treatment with alpha emitters showed improvement of the therapeutic index of PRRT due to the high LET offered by the alpha particles compared to beta emitters. Therefore, [²²⁵Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [²²⁵Ac]Ac(NO₃)₃ and [¹⁷⁷Lu]LuCl₃. Stability studies were performed in phosphate buffered saline (PBS) and mouse serum. In vitro competitive binding assay has been carried out in U2OS-SSTR2 + cells for ^natLa-DOTA-JR11, ^natLu-DOTA-JR11 and DOTA-JR11. Ex vivo biodistribution studies were performed in mice inoculated with H69 cells at 4, 24, 48 and 72 h after injection of [²²⁵Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [²²⁵Ac]Ac-DOTA-JR11 and [¹⁷⁷Lu]Lu-DOTA-JR11.

RESULTS

[²²⁵Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [²²⁵Ac]Ac-DOTA-JR11 showed reasonably good stability in PBS (77% intact radiopeptide at 24 h after incubation) and in mouse serum (~ 81% intact radiopeptide 24 h after incubation). [¹⁷⁷Lu]Lu-DOTA-JR11 demonstrated excellent stability in both media (> 93%) up to 24 h post incubation. Competitive binding assay revealed that complexation of DOTA-JR11 with ^natLa and ^natLu did not affect its binding affinity to SSTR2. Similar biodistribution profiles were observed for both radiopeptides, however, higher uptake was noticed in the kidneys, liver and bone for [²²⁵Ac]Ac-DOTA-JR11 than [¹⁷⁷Lu]Lu-DOTA-JR11.

CONCLUSION

[²²⁵Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [¹⁷⁷Lu]Lu-DOTA-JR11, which may limit further studies with this radiopeptide. However, several strategies can be explored to reduce nephrotoxicity and offer opportunities for future clinical investigations with [²²⁵Ac]Ac-DOTA-JR11.

Peptide Radioligands in Cancer Theranostics: Agonists and Antagonists

The clinical success of radiolabeled somatostatin analogs in the diagnosis and therapy-"theranostics"-of tumors expressing the somatostatin subtype 2 receptor (SST₂R) has paved the way for the development of a broader panel of peptide radioligands targeting different human tumors. This approach relies on the overexpression of other receptor-targets in different cancer types. In recent years, a shift in paradigm from internalizing agonists to antagonists has occurred. Thus, SST₂R-antagonist radioligands were first shown to accumulate more efficiently in tumor lesions and clear faster from the background in animal models and patients. The switch to receptor antagonists was soon adopted in the field of radiolabeled bombesin (BBN). Unlike the stable cyclic octapeptides used in the case of somatostatin, BBN-like peptides are linear, fast to biodegradable and elicit adverse effects in the body. Thus, the advent of BBN-like antagonists provided an elegant way to obtain effective and safe radiotheranostics. Likewise, the pursuit of gastrin and exendin antagonist-based radioligands is advancing with exciting new outcomes on the horizon. In the present review, we discuss these developments with a focus on clinical results, commenting on challenges and opportunities for personalized treatment of cancer patients by means of state-of-the-art antagonist-based radiopharmaceuticals.

[111In]In/[177Lu]Lu-AAZTA5-LM4 SST2R-Antagonists in Cancer Theranostics: From Preclinical Testing to First Patient Results

Aiming to expand the application of the SST2R-antagonist LM4 (DPhe-c[DCys-4Pal-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH2) beyond [68Ga]Ga-DATA5m-LM4 PET/CT (DATA5m, (6-pentanoic acid)-6-(amino)methy-1,4-diazepinetriacetate), we now introduce AAZTA5-LM4 (AAZTA5, 1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-[pentanoic-acid]perhydro-1,4-diazepine), allowing for the convenient coordination of trivalent radiometals of clinical interest, such as In-111 (for SPECT/CT) or Lu-177 (for radionuclide therapy). After labeling, the preclinical profiles of [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 were compared in HEK293-SST2R cells and double HEK293-SST2R/wtHEK293 tumor-bearing mice using [111In]In-DOTA-LM3 and [177Lu]Lu-DOTA-LM3 as references. The biodistribution of [177Lu]Lu-AAZTA5-LM4 was additionally studied for the first time in a NET patient. Both [111In]In-AAZTA5-LM4 and [177Lu]Lu-AAZTA5-LM4 displayed high and selective targeting of the HEK293-SST2R tumors in mice and fast background clearance via the kidneys and the urinary system. This pattern was reproduced for [177Lu]Lu-AAZTA5-LM4 in the patient according to SPECT/CT results in a monitoring time span of 4–72 h pi. In view of the above, we may conclude that [177Lu]Lu-AAZTA5-LM4 shows promise as a therapeutic radiopharmaceutical candidate for SST2R-expressing human NETs, based on previous [68Ga]Ga-DATA5m-LM4 PET/CT, but further studies are needed to fully assess its clinical value. Furthermore, [111In]In-AAZTA5-LM4 SPECT/CT may represent a legitimate alternative diagnostic option in cases where PET/CT is not available.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β^- emitters, particularly Yttrium-90 (⁹⁰Y) and Lutetium-177 (¹⁷⁷Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (²²⁵Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of ²²⁵Ac-PRRT in NETs, discuss the strengths and challenges of ²²⁵Ac complexes being used in PRRT; and envision the prospect of ²²⁵Ac-PRRT as a future alternative in the treatment of NETs.

[68Ga]Ga-DATA5m-LM4, a PET Radiotracer in the Diagnosis of SST2R-Positive Tumors: Preclinical and First Clinical Results

Radiolabeled somatostatin subtype 2 receptor (SST2R)-antagonists have shown advantageous profiles for cancer theranostics compared with agonists. On the other hand, the newly introduced hybrid chelator (6-pentanoic acid)-6-(amino)methyl-1,4-diazepinetriacetate (DATA^5m) rapidly binds Ga-68 (t_1/2: 67.7 min) at much lower temperature, thus allowing for quick access to "ready-for-injection" [⁶⁸Ga]Ga-tracers in hospitals. We herein introduce [⁶⁸Ga]Ga-DATA^5m-LM4 for PET/CT imaging of SST₂R-positive human tumors. LM4 was obtained by 4Pal³/Tyr³-substitution in the known SST₂R antagonist LM3 (H-DPhe-c[DCys-Tyr-DAph(Cbm)-Lys-Thr-Cys]-DTyr-NH₂) and DATA^5m was coupled at the N-terminus for labeling with radiogallium (Ga-67/68). [⁶⁷Ga]Ga-DATA^5m-LM4 was evaluated in HEK293-SST₂R cells and mice models in a head-to-head comparison with [⁶⁷Ga]Ga-DOTA-LM3. Clinical grade [⁶⁸Ga]Ga-DATA^5m-LM4 was prepared and injected in a neuroendocrine tumor (NET) patient for PET/CT imaging. DATA^5m-LM4 displayed high SST₂R binding affinity. [⁶⁷Ga]Ga-DATA^5m-LM4 showed markedly higher uptake in HEK293-SST₂R cells versus [⁶⁷Ga]Ga-DOTA-LM3 and was stable in vivo. In HEK293-SST₂R xenograft-bearing mice, it achieved longer tumor retention and less kidney uptake than [⁶⁷Ga]Ga-DOTA-LM3. [⁶⁸Ga]Ga-DATA^5m-LM4 accurately visualized tumor lesions with high contrast on PET/CT. In short, [⁶⁸Ga]Ga-DATA^5m-LM4 has shown excellent prospects for the PET/CT diagnosis of SST₂R-positive tumors, further highlighting the benefits of Ga-68 labeling in a hospital environment via the DATA^5m-chelator route.

Structural insights into ligand recognition and selectivity of somatostatin receptors

Somatostatin receptors (SSTRs) play versatile roles in inhibiting the secretion of multiple hormones such as growth hormone and thyroid-stimulating hormone, and thus are considered as targets for treating multiple tumors. Despite great progress made in therapeutic development against this diverse receptor family, drugs that target SSTRs still show limited efficacy with preferential binding affinity and conspicuous side-effects. Here, we report five structures of SSTR2 and SSTR4 in different states, including two crystal structures of SSTR2 in complex with a selective peptide antagonist and a non-peptide agonist, respectively, a cryo-electron microscopy (cryo-EM) structure of G_i1-bound SSTR2 in the presence of the endogenous ligand SST-14, as well as two cryo-EM structures of G_i1-bound SSTR4 in complex with SST-14 and a small-molecule agonist J-2156, respectively. By comparison of the SSTR structures in different states, molecular mechanisms of agonism and antagonism were illustrated. Together with computational and functional analyses, the key determinants responsible for ligand recognition and selectivity of different SSTR subtypes and multiform binding modes of peptide and non-peptide ligands were identified. Insights gained in this study will help uncover ligand selectivity of various SSTRs and accelerate the development of new molecules with better efficacy by targeting SSTRs.

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.

A prospective randomized, double-blind study to evaluate the diagnostic efficacy of 68Ga-NODAGA-LM3 and 68Ga-DOTA-LM3 in patients with well-differentiated neuroendocrine tumors: compared with 68Ga-DOTATATE

PURPOSE

The purpose of this study is to evaluate the diagnostic efficacy of ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 and compare them with ⁶⁸ Ga-DOTATATE in patients with well-differentiated neuroendocrine tumors.

METHODS

Patients were prospectively recruited and equally randomized into two arms: Arm A, patients would undergo a whole-body ⁶⁸ Ga-NODAGA-LM3 PET/CT scan on the 1st day and ⁶⁸ Ga-DOTATATE PET/CT scan on the 2nd day; Arm B, patients would undergo a whole-body ⁶⁸ Ga-DOTA-LM3 PET/CT scan on the 1st day and ⁶⁸ Ga-DOTATATE PET/CT scan on the 2nd day. Biodistribution in normal organs, lesion detection ability, and tumor uptake were compared between antagonist and agonist in each arm.

RESULTS

A total of 40 patients with well-differentiated NETs, 20 in each arm, were recruited in the study. ⁶⁸ Ga-NODAGA-LM3 showed a similar pattern as ⁶⁸ Ga-DOTATATE, while ⁶⁸ Ga-DOTA-LM3 demonstrated significantly lower uptake in almost all normal organs compared to ⁶⁸ Ga-DOTATATE. Both ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 showed superiority in lesion detection compared to ⁶⁸ Ga-DOTATATE on lesion-based and patient-based comparison. ⁶⁸ Ga-NODAGA-LM3 showed a significantly higher tumor uptake (median SUVmax 29.1 versus 21.6, P < 0.05) and tumor-to-background ratio (median tumor-to-liver ratio 5.0 versus 2.9, P < 0.05) compared to ⁶⁸ Ga-DOTATATE. ⁶⁸ Ga-DOTA-LM3 showed comparable uptake (median SUVmax 16.1 versus 17.8, P = 0.714) and higher tumor-to-background ratio (median tumor-to-liver ratio 5.2 versus 2.1, P < 0.05).

CONCLUSION

Both ⁶⁸ Ga-NODAGA-LM3 and ⁶⁸ Ga-DOTA-LM3 are promising SSTR2 antagonists for neuroendocrine tumors. They demonstrated superiority in diagnostic efficacy compared to agonist ⁶⁸ Ga-DOTATATE.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT04318561.

Emerging trends of receptor-mediated tumor targeting peptides: A review with perspective from molecular imaging modalities

Natural peptides extracted from natural components such are known to have a relatively short in-vivo half-life and can readily metabolize by endo- and exo-peptidases. Fortunately, synthetic peptides can be easily manipulated to increase in-vivo stability, membrane permeability and target specificity with some well-known natural families. Many natural as well as synthetic peptides target to their endogenous receptors for diagnosis and therapeutic applications. In order to detect these peptides externally, they must be modified with radionuclides compatible with single photon emission computed tomography (SPECT) or positron emission tomography (PET). Although, these techniques mainly rely on physiological changes and have profound diagnostic strength over anatomical modalities such as MRI and CT. However, both SPECT and PET observed to possess lack of anatomical reference frame which is a key weakness of these techniques, and unfortunately, cannot be available freely in most clinical centres especially in under-developing countries. Hence, it is need of the time to design and develop economic, patient friendly and versatile strategies to grapple with existing problems without any hazardous side effects. Optical molecular imaging (OMI) has emerged as a novel technique in field of medical science using fluorescent probes as imaging modality and has ability to couple with organic drugs, small molecules, chemotherapeutics, DNA, RNA, anticancer peptide and protein without adding chelators as necessary for radionuclides. Furthermore, this review focuses on difference in imaging modalities and provides ample knowledge about reliable, economic and patient friendly optical imaging technique rather radionuclide-based imaging techniques.

First-in-human study of novel SSTR antagonist 177Lu-DOTA-LM3 for peptide receptor radionuclide therapy in patients with metastatic neuroendocrine neoplasms: dosimetry, safety and efficacy

The objective of this study was to assess the safety, dosimetry, and efficacy of the ¹⁷⁷Lu-labeled somatostatin receptor (SSTR) antagonist DOTA-p-Cl-Phe-cyclo (D-Cys-Tyr-D-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)D-Tyr-NH2 (¹⁷⁷Lu-DOTA-LM3) in patients with metastatic neuroendocrine neoplasms (NENs). Methods: Fifty-one patients (age 27-76, mean 51.6±13.9 years) with metastatic NENs underwent peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-DOTA-LM3 between August 2017 and December 2019. The median administered activity per cycle was 6.1±0.88 GBq (range 2.8-7.4 GBq). ⁶⁸Ga-NODAGA-LM3 PET/CT was used for patient selection and follow-up after ¹⁷⁷Lu-DOTA-LM3 PRRT. Morphologic and molecular responses were evaluated in accordance with RECIST 1.1 and European Organization for Research and Treatment of Cancer (EORTC) criteria. Treatment-related adverse events were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 5.0. Dosimetry was performed in 11 patients and compared with the SSTR agonist ¹⁷⁷Lu-DOTATOC in 247 patients undergoing PRRT on the same dosimetry protocol. Results: Higher uptake and a longer effective half-life of ¹⁷⁷Lu-DOTA-LM3 was found for whole-body as well as kidneys, spleen, and metastases, resulting in higher mean absorbed organ and tumor doses as compared to the agonist ¹⁷⁷Lu-DOTA-TOC. All patients tolerated therapy without any serious acute adverse effects. Mild nausea without vomiting was observed in 5 (9.8%) patients; no other symptoms were reported. The most severe delayed adverse event was CTC-3 thrombocytopenia in 3 (5.9%) patients. Neither CTC-4 thrombocytopenia nor CTC-3-4 anemia or leukopenia was observed after treatment. No significant decline in renal function was observed, nor was hepatotoxicity. According to RECIST 1.1, disease control could be reached in 40 patients (disease control rate, 85.1%) of 47 patients monitored after ¹⁷⁷Lu-DOTA-LM3 PRRT, with a partial response in 17 (36.2%) and stable disease in 23 (48.9%), whereas 7 (14.9%) patients had progressive disease, and by EORTC criteria, complete remission in 2 (4.3%), partial remission in 21 (44.7%), stable disease in 18 (38.3%), and progressive disease in 6 (12.8%) patients. Conclusion: "Antagonist PRRT" with ¹⁷⁷Lu-DOTA-LM3 could be administered without severe adverse effects and was well tolerated by the majority of patients, with thrombocytopenia occurring only in a few patients. No other severe adverse effects were observed, particularly no nephrotoxicity. The SSTR antagonist ¹⁷⁷Lu-DOTA-LM3 appears to be very promising for PRRT, provides favorable biodistribution and higher tumor radiation doses than SSTR agonists, and was very effective in treating advanced metastatic NENs, especially in patients with low or no SSTR agonist binding, even achieving complete remission in some patients.

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.

Synthesis, preclinical evaluation, and a pilot clinical imaging study of [18F]AlF-NOTA-JR11 for neuroendocrine neoplasms compared with [68Ga]Ga-DOTA-TATE

PURPOSE

A [¹⁸F]AlF-labeled somatostatin receptor (SSTR) antagonist was developed for imaging of neuroendocrine neoplasms (NENs), evaluated and compared with [⁶⁸Ga]Ga-DOTA-TATE.

METHOD

[¹⁸F]AlF-NOTA-JR11 was synthesized manually and qualified with high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). The cellular uptake, internalization, and saturation binding were performed with HEK293-SSTR2 cells. Biodistribution and micro-PET imaging were carried out with HEK293-SSTR2 tumor-bearing mice. [¹⁸F]AlF-NOTA-JR11 PET/MR imaging and [⁶⁸Ga]Ga-DOTA-TATE PET/CT were performed with ten patients of NEN at 50~60 min post-injection (p.i.). Normal organ biodistribution and tumor detectability were evaluated.

RESULT

[¹⁸F]AlF-NOTA-JR11(24~36 GBq/μmol) was prepared within 30 min and 51.35 ± 3.30% (n > 10)of radiochemical yield. The radiochemical purity was 98.74 ± 1.24% (n > 10). Two stereoisomers were found and confirmed by LC-MS. The cellular uptake of [¹⁸F]AlF-NOTA-JR11 and [⁶⁸Ga]Ga-DOTA-TATE were 4.50 ± 0.31 and 4.50 ± 0.13 %AD/10⁵ cells at 30 min, and the internalization at 37 °C of [¹⁸F]AlF-NOTA-JR11 (5.47 ± 0.32% at 60 min) was significantly lower than [⁶⁸Ga]Ga-DOTA-TATE (66.89 ± 1.62% at 60 min). The affinity of [¹⁸F]AlF-NOTA-JR11 (K_d = 11.59 ± 1.31 nM) was slightly lower than [⁶⁸Ga]Ga-DOTA-TATE (K_d = 7.36 ± 1.02 nM); [¹⁸F]AlF-NOTA-JR11 showed high uptake in tumor (9.02 ± 0.92 %ID/g at 60 min p.i.) which can be blocked by 50 μg of NOTA-JR11 (3.40 ± 1.64 %ID/g at 60 min p.i.); the result was coincident with micro-PET imaging. Imaging study of NEN patients showed that more lesions were found only by [¹⁸F]AlF-NOTA-JR11 (n = 67 vs. 1 only by [⁶⁸Ga]Ga-DOTA-TATE), and the uptakes of [¹⁸F]AlF-NOTA-JR11 in majority normal organs were significantly lower than [⁶⁸Ga]Ga-DOTA-TATE. The target to nontarget of maximum of standard uptake value (SUVmax) of [¹⁸F]AlF-NOTA-JR11 in liver lesions were significantly higher than those of [⁶⁸Ga]Ga-DOTA-TATE.

CONCLUSION

Qualitied [¹⁸F]AlF-NOTA-JR11 is prepared conveniently with reasonable yield, and it can bind SSTR2 specifically with high affinity. Excellent imaging capability of [¹⁸F]AlF-NOTA-JR11 for NENs is superior to [⁶⁸Ga]Ga-DOTA-TATE, especially in digestive system. It has a great potential for imaging of NENs.

Successful Intra-arterial Peptide Receptor Radionuclide Therapy of DOTATOC-Negative High-Grade Liver Metastases of a Pancreatic Neuroendocrine Neoplasm Using 177Lu-DOTA-LM3: A Somatostatin Receptor Antagonist

Radiolabeled somatostatin receptor (SSTR) antagonists have shown promise for imaging neuroendocrine neoplasms and the superiority to SSTR agonists, with lower liver background especially for the sensitive detection of liver metastases, higher tumor-to-background ratio, and favorable pharmacokinetics. The clinical data of radiolabeled SSTR antagonists for therapy are still limited. We report our experience treating a young patient with DOTATOC-negative high-grade liver metastases of a pancreatic neuroendocrine neoplasm who underwent intra-arterial peptide receptor radionuclide therapy using SSTR antagonist Lu-DOTA-LM3, demonstrating an excellent response, nearly complete remission according to molecular imaging criteria and morphological partial remission, without any significant toxicity.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Design and development of the theranostic pair 177 Lu-OPS201/68 Ga-OPS202 for targeting somatostatin receptor expressing tumors

Radiolabeled somatostatin receptor (sstr) antagonists have shown superiority in different preclinical and clinical settings compared with the well-established and clinically used agonists for targeting sstr-expressing tumors, with regard to pharmacokinetics, tumor uptake, and retention. The theranostic pair ¹⁷⁷ Lu-OPS201/⁶⁸ Ga-OPS202, based on the sstr2 antagonist JR11 (Cpa-c[d-Cys-Aph(Hor)-d-Aph(Cbm)-Lys-Thr-Cys]-d-Tyr-NH₂ ), is the most advanced pair of the antagonist family in terms of preclinical development and is currently under clinical evaluation. OPS201 and OPS202 share the same amino acid sequence (JR11) but feature different conjugated chelators needed for radiolabeling, DOTA for OPS201 and NODAGA for OPS202. In this review, the design and development of the peptidic analog, JR11, and the selection of chelators and radiometals that led to ¹⁷⁷ Lu-OPS201/⁶⁸ Ga-OPS202 are discussed. Furthermore, the preclinical evaluation of both radiolabeled analogs from bench to bedside and the clinical trials involving the theranostic pair are presented.

Receptor radionuclide targeting for neuroendocrine tumors (NET) diagnostic and therapy

Neuroendocrine tumors (NET) represent a heterogeneous group of tumors originating from cells of neuroendocrine origin, which express somatostatin receptors (SSTR). This property allowed the successful development of radionuclides for diagnostic and peptide radionuclide radiation therapy (PRRT). This is the paradigm for the theragnostic concept in NET personalized medicine. The only phase III study to date (NETTER-1) clearly demonstrated the ability of ¹⁷⁷Lutetium-based PRRT to improve progression-free survival in advanced intestinal NETs. In clinical practice, the indications are limited to G1-G2 well-differentiated NETs with high expression of SSTR. NETs with a low tumor burden and slow progression are probably the optimal indication. This treatment is now available in France. However, its precise position in the treatment algorithm remains to be explored. We provide an overview of receptor radionuclide utilization and mechanism in diagnostic and pretherapeutic imaging and we focus on PRRT for endocrine tumors.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Synthesis and evaluation of Re/99mTc(I) complexes bearing a somatostatin receptor-targeting antagonist and labeled via a novel [N,S,O] clickable bifunctional chelating agent

The somatostatin receptor subtype 2 (SSTR2) is often highly expressed on neuroendocrine tumors (NETs), making it a popular in vivo target for diagnostic and therapeutic approaches aimed toward management of NETs. In this work, an antagonist peptide (sst₂-ANT) with high affinity for SSTR2 was modified at the N-terminus with a novel [N,S,O] bifunctional chelator (2) designed for tridentate chelation of rhenium(I) and technetium(I) tricarbonyl cores, [Re(CO)₃]⁺ and [^99mTc][Tc(CO)₃]⁺. The chelator-peptide conjugation was performed via a Cu(I)-assisted click reaction of the alkyne-bearing chelator (2) with an azide-functionalized sst₂-ANT peptide (3), to yield NSO-sst₂-ANT (4). Two synthetic methods were used to prepare Re-4 at the macroscopic scale, which differed based on the relative timing of the click conjugation to the [Re(CO)₃]⁺ complexation by 2. The resulting products demonstrated the expected molecular mass and nanomolar in vitro SSTR2 affinity (IC₅₀ values under 30 nM, AR42J cells, [¹²⁵I]iodo-Tyr¹¹-somatostatin-14 radioligand standard). However, a difference in their HPLC retention times suggested a difference in metal coordination modes, which was attributed to a competing N-triazole donor ligand formed during click conjugation. Surprisingly, the radiotracer scale reaction of [^99mTc][Tc(OH₂)₃(CO)₃]⁺ (^99mTc; t_½ = 6 h, 141 keV γ) with 4 formed a third product, distinct from the Re analogues, making this one of the unusual cases in which Re and Tc chemistries are not well matched. Nevertheless, the [^99mTc]Tc-4 product demonstrated excellent in vitro stability to challenges by cysteine and histidine (≥98% intact through 24 h), along with 75% stability in mouse serum through 4 h. In vivo biodistribution and microSPECT/CT imaging studies performed in AR42J tumor-bearing mice revealed improved clearance of this radiotracer in comparison to a similar [^99mTc][Tc(CO)₃]-labeled sst₂-ANT derivative previously studied. Yet despite having adequate tumor uptake at 1 h (4.9% ID/g), tumor uptake was not blocked by co-administration of a receptor-saturating dose of SS-14. Aimed toward realignment of the Re and Tc product structures, future efforts should include distancing the alkyne group from the intended donor atoms of the chelator, to reduce the coordination options available to the [M(CO)₃]⁺ core (M = Re, ^99mTc) by disfavoring involvement of the N-triazole.

The tetraamine chelator outperforms HYNIC in a new technetium-99m-labelled somatostatin receptor 2 antagonist

Background

Somatostatin receptor targeting radiopeptides are successfully being used to image, stage, and monitor patients with neuroendocrine tumours. They are exclusively agonists that internalise upon binding to the relevant receptor. According to recent reports, antagonists may be preferable to agonists. To date, ^99mTc-labelled somatostatin receptor antagonists have attracted little attention. Here, we report on a new somatostatin receptor subtype 2 (sst2) antagonist, SS-01 (p-Cl-Phe-cyclo(D-Cys-Tyr-D-Trp-Lys-Thr-Cys)D-Tyr-NH₂), with the aim of developing ^99mTc-labelled ligands for SPECT/CT imaging. SS-01 was prepared using Fmoc solid-phase synthesis and subsequently coupled to the chelators 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 6-carboxy-1,4,8,11-tetraazaundecane (N4), and 6-hydrazinonicotinic acid (HYNIC) to form the corresponding peptide-chelator conjugates SS-03, SS-04, and SS-05, respectively. SS-04 and SS-05 were radiolabelled with ^99mTc and SS-03 with ¹⁷⁷Lu. Binding affinity and antagonistic properties were determined using autoradiography and immunofluorescence microscopy. Biodistribution and small animal SPECT/CT studies were performed on mice bearing HEK293-rsst2 xenografts.

Results

The conjugates showed low nanomolar sst2 affinity and antagonistic properties. ¹⁷⁷Lu-DOTA-SS-01 (¹⁷⁷Lu-SS-03) and ^99mTc-N4-SS-01 (^99mTc-SS-04) demonstrated high cell binding and low internalisation, whereas ^99mTc-HYNIC/edda-SS-01 (^99mTc-SS-05) showed practically no cellular uptake in vitro. The ^99mTc-SS-04 demonstrated impressive tumour uptake at early time points, with 47% injected activity per gram tumour (%IA/g) at 1 h post-injection. The tumour uptake persisted after 4 h and was 32.5 %IA/g at 24 h. The uptake in all other organs decreased much more rapidly leading to high tumour-to-normal organ ratios, which was reflected in high-contrast SPECT/CT images.

Conclusions

These data indicate a very promising ^99mTc-labelled sst2-targeting antagonist. The results demonstrate high sensitivity of the ^99mTc-labelling strategy, which was shown to strongly influence the receptor affinity, contrary to corresponding agonists. ^99mTc-SS-04 exhibits excellent pharmacokinetics and imaging properties and appears to be a suitable candidate for SPECT/CT clinical translation.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0428-y) contains supplementary material, which is available to authorized users.

The somatostatin receptor 2 antagonist 64Cu-NODAGA-JR11 outperforms 64Cu-DOTA-TATE in a mouse xenograft model

Copper-64 is an attractive radionuclide for PET imaging and is frequently used in clinical applications. The aim of this study was to perform a side-by-side comparison of the in vitro and in vivo performance of 64Cu-NODAGA-JR11 (NODAGA = 1,4,7-triazacyclononane,1-glutaric acid,4,7-acetic acid, JR11 = p-Cl-Phe-cyclo(D-Cys-Aph(Hor)-D-Aph(cbm)-Lys-Thr-Cys)D-Tyr-NH2), a somatostatin receptor 2 antagonist, with the clinically used sst2 agonist 64Cu-DOTA-TATE ((TATE = D-Phe-cyclo(Cys-Tyr-D-Trp-Lys-Thr-Cys)Thr). In vitro studies demonstrated Kd values of 5.7±0.95 nM (Bmax = 4.1±0.18 nM) for the antagonist 64/natCu-NODAGA-JR11 and 20.1±4.4. nM (Bmax = 0.48±0.18 nM) for the agonist 64/natCu-DOTA-TATE. Cell uptake studies showed the expected differences between agonists and antagonists. Whereas 64Cu-DOTA-TATE (the agonist) showed very effective internalization in the cell culture assay (with 50% internalized at 4 hours post-peptide addition under the given experimental conditions), 64Cu-NODAGA-JR11 (the antagonist) showed little internalization but strong receptor-mediated uptake at the cell membrane. Biodistribution studies of 64Cu-NODAGA-JR11 showed rapid blood clearance and tumor uptake with increasing tumor-to-relevant organ ratios within the first 4 hours and in some cases, 24 hours, respectively. The tumor washout was slow or non-existent in the first 4 hours, whereas the kidney washout was very efficient, leading to high and increasing tumor-to-kidney ratios over time. Specificity of tumor uptake was proven by co-injection of high excess of non-radiolabeled peptide, which led to >80% tumor blocking. 64Cu-DOTA-TATE showed less favorable pharmacokinetics, with the exception of lower kidney uptake. Blood clearance was distinctly slower and persistent higher blood values were found at 24 hours. Uptake in the liver and lung was relatively high and also persistent. The tumor uptake was specific and similar to that of 64Cu-NODAGA-JR11 at 1 h, but release from the tumor was very fast, particularly between 4 and 24 hours. Tumor-to-normal organ ratios were distinctly lower after 1 hour. This is indicative of insufficient in vivo stability. PET studies of 64Cu-NODAGA-JR11 reflected the biodistribution data with nicely delineated tumor and low background. 64Cu-NODAGA-JR11 shows promising pharmacokinetic properties for further translation into the clinic.

Somatostatin Receptor Antagonists for Imaging and Therapy

Somatostatin receptor (sstr) scintigraphy for imaging and sstr analogs for treatment have been used for more than 20 y. An important improvement in recent years was the introduction of peptide receptor radionuclide therapy with radiolabeled sstr agonists, such as [⁹⁰Y-DOTA⁰,Tyr³]octreotide or [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotide (⁹⁰Y- or ¹⁷⁷Lu-DOTATOC, respectively) and [¹⁷⁷Lu-DOTA⁰,Tyr³]octreotate (¹⁷⁷Lu-DOTATATE). PET/CT with ⁶⁸Ga-labeled sstr agonists, such as ⁶⁸Ga-DOTATOC, ⁶⁸Ga-DOTATATE, and [⁶⁸Ga-DOTA,1-Nal³]octreotide (⁶⁸Ga-DOTANOC), plays an important role in staging and restaging neuroendocrine tumors. Most importantly, sstr scintigraphy and sstr PET/CT can distinguish patients who will qualify for and benefit from peptide receptor radionuclide therapy. This characteristic of sstr targeting is important because it allows a personalized treatment approach (theranostic approach). Until recently, it was thought that internalization of the radiolabeled agonist was mandatory for sstr-mediated imaging and therapy. It was Ginj et al. who proposed in 2006 the paradigm shift that radiolabeled sstr antagonists may perform better than agonists despite the lack of internalization. Despite the rather limited number of head-to-head comparisons of sstr antagonists and agonists, the superiority of sstr antagonists was demonstrated in several cases. From a small library of sstr antagonists, the analog JR11 (Cpa-c[d-Cys-Aph(Hor)-d-Aph(Cbm)-Lys-Thr-Cys]-d-Tyr-NH₂), an antagonist with selectivity for sstr subtype 2, showed the best overall characteristics for sstr subtype 2 targeting and was therefore selected for clinical translation. JR11 is under clinical development as a PET imaging agent when labeled with ⁶⁸Ga (⁶⁸Ga-NODAGA-JR11 or ⁶⁸Ga-OPS202) and as a therapeutic agent when labeled with ¹⁷⁷Lu (¹⁷⁷Lu-DOTA-JR11 or ¹⁷⁷Lu-OPS201). In this article, we discuss the development and current status of radiolabeled sstr antagonists. Evidence based on preclinical work, on quantitative in vivo autoradiography of human tumor slices, and on human data now supports a shift to sstr antagonists.

New Insights in the Design of Bioactive Peptides and Chelating Agents for Imaging and Therapy in Oncology

Many synthetic peptides have been developed for diagnosis and therapy of human cancers based on their ability to target specific receptors on cancer cell surface or to penetrate the cell membrane. Chemical modifications of amino acid chains have significantly improved the biological activity, the stability and efficacy of peptide analogues currently employed as anticancer drugs or as molecular imaging tracers. The stability of somatostatin, integrins and bombesin analogues in the human body have been significantly increased by cyclization and/or insertion of non-natural amino acids in the peptide sequences. Moreover, the overall pharmacokinetic properties of such analogues and others (including cholecystokinin, vasoactive intestinal peptide and neurotensin analogues) have been improved by PEGylation and glycosylation. Furthermore, conjugation of those peptide analogues to new linkers and bifunctional chelators (such as AAZTA, TETA, TRAP, NOPO etc.), produced radiolabeled moieties with increased half life and higher binding affinity to the cognate receptors. This review describes the most important and recent chemical modifications introduced in the amino acid sequences as well as linkers and new bifunctional chelators which have significantly improved the specificity and sensitivity of peptides used in oncologic diagnosis and therapy.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

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.

Characterization of Multisubstituted Corticotropin Releasing Factor (CRF) Peptide Antagonists (Astressins)

CRF mediates numerous stress-related endocrine, autonomic, metabolic, and behavioral responses. We present the synthesis and chemical and biological properties of astressin B analogues {cyclo(30-33)[D-Phe(12),Nle(21,38),C(α)MeLeu(27,40),Glu(30),Lys(33)]-acetyl-h/r-CRF(9-41)}. Out of 37 novel peptides, 17 (2, 4, 6-8, 10, 11, 16, 17, 27, 29, 30, 32-36) and 16 (3, 5, 9, 12-15, 18, 19, 22-26, 28, 31) had k(i) to CRF receptors in the high picomolar and low nanomole ranges, respectively. Peptides 1, 2, and 11 inhibited h/rCRF and urocortin 1-induced cAMP release from AtT20 and A7r5 cells. When Astressin C 2 was administered to adrenalectomized rats at 1.0 mg subcutaneously, it inhibited ACTH release for >7 d. Additional rat data based on the inhibitory effect of (2) on h/rCRF-induced stimulation of colonic secretory motor activity and urocortin 2-induced delayed gastric emptying also indicate a safe and long-lasting antagonistic effect. The overall properties of selected analogues may fulfill the criteria expected from clinical candidates.

Gastroenteropancreatic neuroendocrine tumours (GEP-NET) - Imaging and staging

Detection of gastroenteropancreatic neuroendocrine tumours (GEP-NETs) and monitoring of treatment response relies mainly on morphological imaging such as computed tomography (CT) and magnetic resonance imaging (MRI). Molecular imaging techniques also in combination with CT (hybrid imaging) greatly benefit patient management, including better localization of occult tumours and better staging. Somatostatin receptor scintigraphy (SRS) and somatostatin receptor (SSTR) positron emission tomography (PET) play a central role in the diagnostic work-up of patients with well-differentiated GEP-NETs. SSTR PET/CT is superior to SRS and should be used whenever available. (18)F-DOPA and (18)F-FDG PET/CT is inferior to SSTR PET/CT at least in patients with well-differentiated GEP-NETs. Both SSTR PET/CT and SRS have limitations, such as relatively low detection rate of benign insulinomas, poorly differentiated GEP-NETs and liver metastases. New innovations such as SSTR PET/MRI, radiolabelled SSTR antagonists and glucagon-like peptide-1 receptor (GLP-1R) agonists might further improve imaging of GEP-NETs.

The role of brain somatostatin receptor 2 in the regulation of feeding and drinking behavior

Somatostatin was discovered four decades ago as hypothalamic factor inhibiting growth hormone release. Subsequently, somatostatin was found to be widely distributed throughout the brain and to exert pleiotropic actions via interaction with five somatostatin receptors (sst1-5) that are also widely expressed throughout the brain. Interestingly, in contrast to the predominantly inhibitory actions of peripheral somatostatin, the activation of brain sst2 signaling by intracerebroventricular injection of stable somatostatin agonists potently stimulates food intake and independently, drinking behavior in rodents. The orexigenic response involves downstream orexin-1, neuropeptide Y1 and μ receptor signaling while the dipsogenic effect is mediated through the activation of the brain angiotensin 1 receptor. Brain sst2 activation is part of mechanisms underlying the stimulation of feeding and more prominently water intake in the dark phase and is able to counteract the anorexic response to visceral stressors.

Urocortin3 mediates somatostatin-dependent negative feedback control of insulin secretion

The peptide hormone urocortin3 (Ucn3) is abundantly expressed by mature beta cells, yet its physiological role is unknown. Here we demonstrate that Ucn3 is stored and co-released with insulin and potentiates glucose-stimulated somatostatin secretion via cognate receptors on delta cells. Further, we found that islets lacking endogenous Ucn3 have fewer delta cells, reduced somatostatin content, impaired somatostatin secretion, and exaggerated insulin release, and that these defects are rectified by treatment with synthetic Ucn3 in vitro. Our observations indicate that the paracrine actions of Ucn3 activate a negative feedback loop that promotes somatostatin release to ensure the timely reduction of insulin secretion upon normalization of plasma glucose. Moreover, Ucn3 is markedly depleted from beta cells in mouse and macaque models of diabetes and in human diabetic islets. This suggests that Ucn3 is a key contributor to stable glycemic control, whose reduction during diabetes aggravates glycemic volatility and contributes to the pathophysiology of this disease.

Selective agonists of somatostatin receptor subtype 1 or 2 injected peripherally induce antihyperalgesic effect in two models of visceral hypersensitivity in mice

Somatostatin interacts with five G-protein-coupled receptor (sst1-5). Octreotide, a stable sst2≫3≥5 agonist, exerts a visceral anti-hyperalgesic effect in experimental and clinical studies. Little is known on the receptor subtypes involved. We investigated the influence of the stable sst1-5 agonist, ODT8-SST and selective receptor subtype peptide agonists (3 or 10μg/mouse) injected intraperitoneally (ip) on visceral hypersensitivity in mice induced by repeated noxious colorectal distensions (four sets of three CRD, each at 55mmHg) or corticotropin-releasing factor receptor 1 agonist, cortagine given between two sets of graded CRD (15, 30, 45, and 60mmHg, three times each pressure). The mean visceromotor response (VMR) was assessed using a non-invasive manometry method and values were expressed as percentage of the VMR to the 1st set of CRD baseline or to the 60mmHg CRD, respectively. ODT8-SST (10μg) and the sst2 agonist, S-346-011 (3 and 10μg) prevented mechanically induced visceral hypersensitivity in the three sets of CRD, the sst1 agonist (10μg) blocked only the 2nd set and showed a trend at 3μg while the sst4 agonist had no effect. The selective sst2 antagonist, S-406-028 blocked the sst2 agonist but not the sst1 agonist effect. The sst1 agonist (3 and 10μg) prevented cortagine-induced hypersensitivity to CRD at each pressure while the sst2 agonist at 10μg reduced it. These data indicate that in addition to sst2, the sst1 agonist may provide a novel promising target to alleviate visceral hypersensitivity induced by mechanoreceptor sensitization and more prominently, stress-related visceral nociceptive sensitization.

Brain somatostatin receptor 2 mediates the dipsogenic effect of central somatostatin and cortistatin in rats: role in drinking behavior

Intracerebroventricular injection of stable somatostatin (SST) agonists stimulates food and water intake in rats. We investigated the receptor subtype(s) involved in the dipsogenic effect of intracerebroventricular injection of SST agonists, mechanisms of action, and role. In nonfasted and non-water-deprived male rats with chronic intracerebroventricular cannula, intake of water without food or food without water was monitored separately to avoid any interactions compared with intracerebroventricular vehicle. SST-14 and cortistatin (CST-14) (1 μg/rat icv) increased water intake by 3.1- and 2.7-fold, respectively, while both peptides did not alter food intake at 1 h postinjection in the light phase. By contrast, the stable pan-somatostatin agonist ODT8-SST (1 μg/rat icv) increased both water and food intake by 4.9- and 3.7-fold, respectively. S-346-011, a selective receptor 2 (sst2) agonist (1 μg/rat icv) induced water ingestion, while sst1 or sst4 agonist, injected under the same conditions, did not. The sst2 antagonist S-406-028 (1 μg/rat icv) prevented the 1-h water intake induced by intracerebroventricular ODT8-SST and CST-14. Losartan (100 μg/rat icv), an angiotensin receptor 1 (AT1) antagonist, completely blocked the water consumption induced by intracerebroventricular ODT8-SST, whereas intracerebroventricular injection of S-406-028 did not modify the intracerebroventricular ANG II-induced dipsogenic response. The sst2 antagonist reduced by 40% the increase of the 3-h water intake in the early dark phase. These data indicate that SST-14 and CST-14 interact with sst2 to exert a potent dipsogenic effect, which is mediated downstream by angiotensin-AT1 signaling. These data also indicate that sst2 activation by brain SST-14 and/or CST-14 may play an important role in the regulation of drinking behavior.

Orexin-1 receptor mediates the increased food and water intake induced by intracerebroventricular injection of the stable somatostatin pan-agonist, ODT8-SST in rats

Intracerebroventricular (icv) injection of the stable somatostatin pan-agonist, ODT8-SST induces a somatostatin 2 receptor (sst2) mediated robust feeding response that involves neuropeptide Y and opioid systems in rats. We investigated whether the orexigenic system driven by orexin also plays a role. Food and water intake after icv injection was measured concomitantly in non-fasted and non-water deprived rats during the light phase. In vehicle treated rats (100% DMSO, icv), ODT8-SST (1μg/rat, icv) significantly increased the 2-h food and water intake compared to icv vehicle plus saline (5.1±1.0g vs. 1.2±0.4g and 11.3±1.9mL vs. 2.5±1.2mL, respectively). The orexin-1 receptor antagonist, SB-334867 (16μg/rat, icv) completely inhibited the 2-h food and water intake induced by icv ODT8-SST. In contrast, the icv pretreatment with the selective somatostatin sst2 antagonist, S-406-028, established to block the orexigenic effect of icv ODT8-SST, did not modify the increased food and water intake induced by icv orexin-A (10.7μg/rat). These data indicate that orexin-1 receptor signaling system is part of the brain neurocircuitry contributing to the orexigenic and dipsogenic responses induced by icv ODT8-SST and that orexin-A stimulates food intake independently from brain sst2 activation.

Orexigenic response to tail pinch: role of brain NPY(1) and corticotropin releasing factor receptors

Tail pinch stimulates food intake in rats. We investigated brain mechanisms of this response and the influence of repeated exposure. Sprague-Dawley rats received acute (5 min) or repeated (5 min/day for 14 days) tail pinch using a padded clip. Acute tail pinch increased 5-min food intake compared with control (0.92 ± 0.2 vs. 0.03 ± 0.01 g, P < 0.01). This response was inhibited by 76% by intracerebroventricular injection of BIBP-3226, a neuropeptide Y1 (NPY1) receptor antagonist, increased by 48% by astressin-B, a corticotropin-releasing factor (CRF) receptor antagonist, and not modified by S-406-028, a somatostatin subtype 2 antagonist. After the 5-min tail pinch without food, blood glucose rose by 21% (P < 0.01) while changes in plasma acyl ghrelin (+41%) and adrenocorticotropic hormone (+37%) were not significant. Two tail pinches (45 min apart) activate pontine and hindbrain catecholaminergic and hypothalamic paraventricular CRF neurons. After 14 days of repeated tail pinch, the 5-min orexigenic response was not significantly different from days 2 to 11 but reduced by 50% thereafter (P < 0.001). Simultaneously, the 5-min fecal pellet output increased during the last 5 days compared with the first 5 days (+58%, P < 0.05). At day 14, the body weight gain was reduced by 22%, with a 99% inhibition of fat gain and a 25% reduction in lean mass (P < 0.05). The orexigenic response to acute 5-min tail pinch is likely to involve the activation of brain NPY1 signaling, whereas that of CRF tends to dampen the acute response and may contribute to increased defecation and decreased body weight gain induced by repeated tail pinch.

[DOTA]Somatostatin-14 analogs and their (111)In-radioligands: effects of decreasing ring-size on sst1-5 profile, stability and tumor targeting

Multiple somatostatin receptor (sst)-subtype expression has been manifested in several human tumors. Hence, the availability of radiopeptides retaining the full pansomatostatin profile of the native hormone (SS14) is expected to increase the sensitivity and broaden the clinical indications of currently applied sst2-preferring cyclic octapeptide radioligands, like OctreoScan(®) ([(111)In-DTPA]octreotide). On the other hand, SS14 has been excluded from clinical use due to its rapid in vivo degradation. We herein present a small library of seven novel cyclic SS14-mimics carrying at their N-terminus the universal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for stable binding of medically useful radiometals, like (111)In. By decreasing the number of amino acids composing the ring in their structure from 12 up to 6 AA, we induced important changes in key-biological parameters in vitro and in vivo. In particular, we observed unexpected changes and even total loss of sst1-5-affinity (6AA-ring), as well as weaker sst2-internalization efficacy as the ring size decreased. In contrast, in vivo stability increased with decreasing ring size, reaching its maximum in the 6AA-ring analogs. Interestingly, only the 12AA- and 9AA-ring members of this series showed sst2-specific uptake in AR4-2J tumors in mice revealing the prominent role of ring size on the biological response of tested SS14-derived radioligands.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Illuminating somatostatin analog action at neuroendocrine tumor receptors

Somatostatin analogs for the diagnosis and therapy of neuroendocrine tumors (NETs) have been used in clinical applications for more than two decades. Five somatostatin receptor subtypes have been identified and molecular mechanisms of somatostatin receptor signaling and regulation have been elucidated. These advances increased understanding of the biological role of each somatostatin receptor subtype, their distribution in NETs, as well as agonist-specific regulation of receptor signaling, internalization, and phosphorylation, particularly for the sst2 receptor subtype, which is the primary target of current somatostatin analog therapy for NETs. Various hypotheses exist to explain differences in patient responsiveness to somatostatin analog inhibition of tumor secretion and growth as well as differences in the development of tumor resistance to therapy. In addition, we now have a better understanding of the action of both first generation (octreotide, lanreotide, Octreoscan) and second generation (pasireotide) FDA-approved somatostatin analogs, including the biased agonistic character of some agonists. The increased understanding of somatostatin receptor pharmacology provides new opportunities to design more sophisticated assays to aid the future development of somatostatin analogs with increased efficacy.

Modulation of the adaptive response to stress by brain activation of selective somatostatin receptor subtypes

Somatostatin-14 was discovered in 1973 in the hypothalamus as a peptide inhibiting growth hormone release. Somatostatin interacts with five receptor subtypes (sst(1-5)) which are widely distributed in the brain with a distinct, but overlapping, expression pattern. During the last few years, the development of highly selective peptide agonists and antagonists provided new insight to characterize the role of somatostatin receptor subtypes in the pleiotropic actions of somatostatin. Recent evidence in rodents indicates that the activation of selective somatostatin receptor subtypes in the brain blunts stress-corticotropin-releasing factor (CRF) related ACTH release (sst2/5), sympathetic-adrenal activaton (sst5), stimulation of colonic motility (sst1), delayed gastric emptying (sst5), suppression of food intake (sst2) and the anxiogenic-like (sst2) response. These findings suggest that brain somatostatin signaling pathways may play an important role in dampening CRF-mediated endocrine, sympathetic, behavioral and visceral responses to stress.