Design, Synthesis, and Biological Evaluation of Somatostatin-Based Radiopeptides

Elevating theranostics: The emergence and promise of radiopharmaceutical cell-targeting heterodimers in human cancers

Optimal therapeutic and diagnostic efficacy is essential for healthcare's global mission of advancing oncologic drug development. Accurate diagnosis and detection are crucial prerequisites for effective risk stratification and personalized patient care in clinical oncology. A paradigm shift is emerging with the promise of multi-receptor-targeting compounds. While existing detection and staging methods have demonstrated some success, the traditional approach of monotherapy is being reevaluated to enhance therapeutic effectiveness. Heterodimeric site-specific agents are a versatile solution by targeting two distinct biomarkers with a single theranostic agent. This review describes the innovation of dual-targeting compounds, examining their design strategies, therapeutic implications, and the promising path they present for addressing complex diseases.

Advances in MRI: Peptide and peptidomimetic-based contrast agents

Magnetic resonance imaging (MRI) is a common medical imaging technique that provides three-dimensional body images. MRI contrast agents improve image contrast by raising the rate of water proton relaxation in specific tissues. Peptides and peptidomimetics act as scaffolds for MRI imaging agents because of their increased size and offer the possibility to engine a higher hydration value within the design. The design of a new Gd-based contrast agent must take into account high stability constants to avoid free Gd(III), with the subsequent nephrotoxicity, and high relaxivity values. This review analyzes various synthetic approaches, reports studies of relaxometric parameters, and focuses on the description and application of Gd(III)-chelates based on peptide and peptidomimetic scaffolds. In addition, the X-ray molecular structures of three DOTA complexes will be reported to emphasize the necessity of using the X-ray diffraction analysis to identify the coordination sphere of the metals and the mechanism of action of the compounds.

Preclinical Evaluation of [155/161Tb]Tb-Crown-TATE—A Novel SPECT Imaging Theranostic Agent Targeting Neuroendocrine Tumours

Terbium radioisotopes (149Tb, 152Tb, 155Tb, 161Tb) offer a unique class of radionuclides which encompass all four medicinally relevant nuclear decay modalities (α, β+, γ, β−/e−), and show high potential for the development of element-matched theranostic radiopharmaceuticals. The goal of this study was to design, synthesise, and evaluate the suitability of crown-TATE as a new peptide-conjugate for radiolabelling of [155Tb]Tb3+ and [161Tb]Tb3+, and to assess the imaging and pharmacokinetic properties of each radiotracer in tumour-bearing mice. [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE were prepared efficiently under mild conditions, and exhibited excellent stability in human serum (>99.5% RCP over 7 days). Longitudinal SPECT/CT images were acquired for 155Tb- and 161Tb- labelled crown-TATE in male NRG mice bearing AR42J tumours. The radiotracers, [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE, showed high tumour targeting (32.6 and 30.0 %ID/g, respectively) and minimal retention in non-target organs at 2.5 h post-administration. Biodistribution studies confirmed the SPECT/CT results, showing high tumour uptake (38.7 ± 8.0 %ID/g and 38.5 ± 3.5 %ID/g, respectively) and favourable tumour-to-background ratios. Blocking studies further confirmed SSTR2-specific tumour accumulation. Overall, these findings suggest that crown-TATE has great potential for element-matched molecular imaging and radionuclide therapy using 155Tb and 161Tb.

Epigenetic-Like Stimulation of Receptor Expression in SSTR2 Transfected HEK293 Cells as a New Therapeutic Strategy

Simple Summary

Neuroendocrine tumors (NETs) expressing the somatostatin receptor subtype 2 (SSTR2) are promising targets for peptide receptor radionuclide therapy (PRRT) using the somatostatin analogue Lu-177-DOTATATE. Patients expressing low levels of SSTR2 do not benefit from PRRT. Therefore, an approach to increase the efficacy of PRRT utilizing the effects of 5-aza-2′-deoxycytidine (5-aza-dC) and valproic acid (VPA) on the SSTR2 expression levels is investigated. The cell lines HEKsst₂ and PC3 are incubated with 5-aza-dC and VPA in different combinations. The drug pretreatment of HEKsst₂ cells leads to increased Lu-177-DOTATATE uptake values (factor 28) and lower cell survival (factor 4) in comparison to unstimulated cells; in PC3 cells, the effects are negligible. Further, for the stimulated cell types, the maintenance of the intrinsic radiosensitivity in each cell type is confirmed by X-ray irradiation. The increased SSTR2 expression induced by VPA and 5-aza-dC stimulation in HEKsst₂ cells might improve treatment strategies for patients with NETs.

Abstract

The aim of the study was to increase the uptake of the SSTR2-targeted radioligand Lu-177-DOTATATE using the DNA methyltransferase inhibitor (DNMTi) 5-aza-2′-deoxycytidine (5-aza-dC) and the histone deacetylase inhibitor (HDACi) valproic acid (VPA). The HEKsst₂ and PC3 cells were incubated with variable concentrations of 5-aza-dC and VPA to investigate the uptake of Lu-177-DOTATATE. Cell survival, subsequent to external X-rays (0.6 or 1.2 Gy) and a 24 h incubation with 57.5 or 136 kBq/mL Lu-177-DOTATATE, was investigated via colony formation assay to examine the effect of the epidrugs. In the case of stimulated HEKsst₂ cells, the uptake of Lu-177-DOTATATE increased by a factor of 28 in comparison to the unstimulated cells. Further, stimulated HEKsst₂ cells demonstrated lower survival fractions (factor 4). The survival fractions of the PC3 cells remained almost unchanged. VPA and 5-aza-dC did not induce changes to the intrinsic radiosensitivity of the cells after X-ray irradiation. Clear stimulatory effects on HEKsst₂ cells were demonstrated by increased cell uptake of the radioligand and enhanced SST2 receptor quantity. In conclusion, the investigated approach is suitable to stimulate the somatostatin receptor expression and thus the uptake of Lu-177-DOTATATE, enabling a more efficient treatment for patients with poor response to peptide radionuclide therapy (PRRT).

Diagnostic Value of Radiolabelled Somatostatin Analogues for Neuroendocrine Tumour Diagnosis: The Benefits and Drawbacks of [64Cu]Cu-DOTA-TOC

Simple Summary

One of the most incredible advances in nuclear medicine is early detection of neuroendocrine tumors, which leads to appropriate and expedient treatment pathways. Advances made with somatostatin analogue derivatives radiolabeled with Gallium-68 clarified the paths of diagnosis and treatment properly. Despite the significant improvements, widespread efforts are in progress to attain the most specific radiopharmaceutical for this purpose. In this literature review, we will provide a short overview on the role of nuclear medicine in the diagnosis of neuroendocrine tumors focusing on [⁶⁴Cu]Cu-DOTA-TOC as a new radiopharmaceutical with promising clinical results.

Abstract

Neuroendocrine tumours (NETs) arise from secondary epithelial cell lines in the gastrointestinal or respiratory system organs. The rate of development of these tumours varies from an indolent to an aggressive course, typically being initially asymptomatic. The identification of these tumours is difficult, particularly because the primary tumour is often small and undetectable by conventional anatomical imaging. Consequently, diagnosis of NETs is complicated and has been a significant challenge until recently. In the last 30 years, the advent of novel nuclear medicine diagnostic procedures has led to a substantial increase in NET detection. Great varieties of exclusive single photon emission computed tomography (SPECT) and positron emission tomography (PET) radiopharmaceuticals for detecting NETs are being applied successfully in clinical settings, including [¹¹¹In]In-pentetreotide, [^99mTc]Tc-HYNIC-TOC/TATE, [⁶⁸Ga]Ga-DOTA-TATE, and [⁶⁴Cu]Cu-DOTA-TOC/TATE. Among these tracers for functional imaging, PET radiopharmaceuticals are clearly and substantially superior to planar or SPECT imaging radiopharmaceuticals. The main advantages include higher resolution, better sensitivity and increased lesion-to-background uptake. An advantage of diagnosis with a radiopharmaceutical is the capacity of theranostics to provide concomitant diagnosis and treatment with particulate radionuclides, such as beta and alpha emitters including Lutetium-177 (¹⁷⁷Lu) and Actinium-225 (²²⁵Ac). Due to these unique challenges involved with diagnosing NETs, various PET tracers have been developed. This review compares the clinical characteristics of radiolabelled somatostatin analogues for NET diagnosis, focusing on the most recently FDA-approved [⁶⁴Cu]Cu-DOTA-TATE as a state-of-the art NET-PET/CT radiopharmaceutical.

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.

Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, 99mTc and 186Re metal centers: Radiochemistry and in vitro evaluation

INTRODUCTION

With the long-term goal of developing a diagnostic (^99mTc) and therapeutic (¹⁸⁶Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂ (1) with Re (in Re-1), ^99mTc (in [^99mTc]Tc-1) and ¹⁸⁶Re (in [¹⁸⁶Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from ^99mTc-labeled glucoheptonate or [¹⁸⁶Re]ReOCl₃(PPh₃)₂, yielding [^99mTc]Tc-1 or [¹⁸⁶Re]Re-1, respectively. In vitro stability of [^99mTc]Tc-1/[¹⁸⁶Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [^99mTc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC₅₀ = 43 ± 6 nM). Optimization of the ^99mTc labeling through varying reaction parameters such as pH, reaction time, and Sn²⁺ and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [¹⁸⁶Re]Re-1 was prepared in reasonable RCY (≥50%). Both ^99mTc/¹⁸⁶Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [^99mTc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [¹⁸⁶Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel ^99mTc/¹⁸⁶Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [^99mTc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The ^99mTc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.

Peptide-Based Therapeutics for Oncology

The development of peptide-based drugs, which are usually synthetic analogues of endogenous peptides, is currently one of the most topical directions in drug development. Among them, antitumor peptide-based drugs are of great interest. Anticancer peptides can be classified into three main groups based on their mechanism of action: inhibitory, necrosis-inducing and pro-apoptotic peptides. As an antitumor therapy, peptides are considered to have at least the same efficacy as chemotherapy or surgical treatment, but offer advantages in terms of safety and tolerability, given that chemotherapy is usually characterized by severe adverse effects, and surgery carries additional risks for patients. Short peptides have a number of benefits over other molecules. First, compared with full-length proteins and antibodies, short peptides are less immunogenic, more stable ex-vivo (prolonged storage at room temperature), and have better tumor or organ permeability. Moreover, the production of such short peptide-based drugs is more cost effective. Second, in comparison with small organic molecules, peptides have higher efficacy and specificity. Finally, due to the fact that the main products of peptide metabolism are amino acids, these drugs are usually characterized by lower toxicity. Short peptides have a highly selective mechanism of action, thereby demonstrating low toxicity. Furthermore, with the addition of different stabilizing structural modifications, as well as novel drug delivery systems, the peptide-based drugs are proving to be promising therapeutics for cancer mono- or polytherapy. However, challenges remain including that endogenous and synthetic peptide molecules can be oncogenic. Therefore, it is important to investigate whether peptides contribute to tumor growth. In order to answer such questions, numerous preclinical and clinical studies of peptide-based therapeutics are currently being conducted.

TargetAntiAngio: A Sequence-Based Tool for the Prediction and Analysis of Anti-Angiogenic Peptides

Cancer remains one of the major causes of death worldwide. Angiogenesis is crucial for the pathogenesis of various human diseases, especially solid tumors. The discovery of anti-angiogenic peptides is a promising therapeutic route for cancer treatment. Thus, reliably identifying anti-angiogenic peptides is extremely important for understanding their biophysical and biochemical properties that serve as the basis for the discovery of new anti-cancer drugs. This study aims to develop an efficient and interpretable computational model called TargetAntiAngio for predicting and characterizing anti-angiogenic peptides. TargetAntiAngio was developed using the random forest classifier in conjunction with various classes of peptide features. It was observed via an independent validation test that TargetAntiAngio can identify anti-angiogenic peptides with an average accuracy of 77.50% on an objective benchmark dataset. Comparisons demonstrated that TargetAntiAngio is superior to other existing methods. In addition, results revealed the following important characteristics of anti-angiogenic peptides: (i) disulfide bond forming Cys residues play an important role for inhibiting blood vessel proliferation; (ii) Cys located at the C-terminal domain can decrease endothelial formatting activity and suppress tumor growth; and (iii) Cyclic disulfide-rich peptides contribute to the inhibition of angiogenesis and cell migration, selectivity and stability. Finally, for the convenience of experimental scientists, the TargetAntiAngio web server was established and made freely available online.

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.

Synergistic Catalysis by "Polymeric Microzymes and Inorganic Nanozymes": The 1+1>2 Effect for Intramolecular Cyclization of Peptides

In this work, we developed an efficient "molecularly imprinted polymer microzymes and inorganic magnetic nanozymes" synergistic catalysis strategy for the formation of disulfide bonds in peptides. The polymeric microzymes showed excellent selectivity toward the template peptide as well as the main reactant (linear peptide), and the Fe₃O₄ magnetic nanoparticle (MNP) nanozymes inhibited the intermolecular reaction during the formation of disulfide bonds in peptides. As a result, the integration of the two different artificial enzymes in one process facilitates the intramolecular cyclization in high product yields (59.3%) with excellent selectivity. Mechanism study indicates the synergistic effect was occurred by using a "reversed solid phase synthesis" strategy with an enhanced shift of reaction balance to product generation. We believe the synergistic catalysis by "polymeric microzymes and inorganic nanozymes" presented in the present work may open new opportunities in creation of multifunctional enzyme mimics for sensing, imaging, and drug delivery.

DOTA-Derivatives of Octreotide Dicarba-Analogs with High Affinity for Somatostatin sst2,5 Receptors

In vivo somatostatin receptor scintigraphy is a valuable method for the visualization of human endocrine tumors and their metastases. In fact, peptide ligands of somatostatin receptors (sst's) conjugated with chelating agents are in clinical use. We have recently developed octreotide dicarba-analogs, which show interesting binding profiles at sst's. In this context, it was mandatory to explore the possibility that our analogs could maintain their activity also upon conjugation with DOTA. In this paper, we report and discuss the synthesis, binding affinity and conformational preferences of three DOTA-conjugated dicarba-analogs of octreotide. Interestingly, two conjugated analogs exhibited nanomolar affinities on sst₂ and sst₅ somatostatin receptor subtypes.

In vivo enzyme inhibition improves the targeting of [177Lu]DOTA-GRP(13-27) in GRPR-positive tumors in mice

INTRODUCTION

Gastrin-releasing peptide receptors (GRPR) and GRP-derived analogs have attracted attention due to high receptor expression in frequently occurring human neoplasia. The authors recently synthesized a series of GRPR-affine peptide analogs based on the 27-mer GRP and derivatized with the DOTA chelator at the N-terminus for (111)In-labeling. In this study, the authors evaluated the most promising from these series, DOTA-GRP(13-27), after radiolabeling with (177)Lu for future therapeutic applications. In addition, to improve in vivo stability of the peptide against in vivo degradation by the protease neutral endopeptidase (NEP), the authors coinjected [(177)Lu]DOTA-GRP(13-27) with the potent NEP inhibitor phosphoramidon (PA). The authors also aimed at reducing renal uptake by coadministration of lysine.

METHODS

In vivo stability studies were performed in Swiss albino mice. Biodistribution studies were conducted in NMRI nu/nu mice bearing prostate cancer (PC)-3 xenografts. Ex vivo autoradiography was performed using frozen sections from PC-3 xenografts and kidneys.

RESULTS AND DISCUSSION

Coadministration of PA significantly increased the percentage of intact radiopeptide in the mouse circulation. From biodistribution and ex vivo autoradiography studies, coadministration of both lysine and PA with [(177)Lu]DOTA-GRP(13-27) appeared to induce a clear improvement of tumor uptake as well as lower levels of renal radioactivity, causing a promising ninefold increase in tumor/kidney ratios.

Twins in spirit part II: DOTATATE and high-affinity DOTATATE--the clinical experience

PURPOSE

Over recent decades interest in diagnosis and treatment of neuroendocrine tumours (NET) has steadily grown. The basis for diagnosis and therapy of NET with radiolabelled somatostatin (hsst) analogues is the variable overexpression of hsst receptors (hsst1-5 receptors). We hypothesized that radiometal derivatives of DOTA-iodo-Tyr(3)-octreotide analogues might be excellent candidates for somatostatin receptor imaging. We therefore explored the diagnostic potential of (68)Ga-DOTA-iodo-Tyr(3)-octreotate [(68)Ga-DOTA,3-iodo-Tyr(3),Thr(8)]octreotide ((68)Ga-HA-DOTATATE; HA, high-affinity) compared to the established (68)Ga-DOTA-Tyr(3)-octreotate ((68)Ga-DOTATATE) in vivo.

METHODS

The study included 23 patients with known somatostatin receptor-positive metastases from NETs, thyroid cancer or glomus tumours who were investigated with both (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE. A patient-based and a lesion-based comparative analysis was carried out of normal tissue distribution and lesion detectability in a qualitative and a semiquantitative manner.

RESULTS

(68)Ga-HA-DOTATATE and (68)Ga-DOTATATE showed comparable uptake in the liver (SUVmean 8.9 ± 2.2 vs. 9.3 ± 2.5, n.s.), renal cortex (SUVmean 13.3 ± 3.9 vs. 14.5 ± 3.7, n.s.) and spleen (SUVmean 24.0 ± 6.7 vs. 22.9 ± 7.3, n.s.). A somewhat higher pituitary uptake was found with (68)Ga-HA-DOTATATE (SUVmean 6.3 ± 1.8 vs. 5.4 ± 2.1, p < 0.05). On a lesion-by-lesion basis a total of 344 lesions were detected. (68)Ga-HA-DOTATATE demonstrated 328 lesions (95.3% of total lesions seen), and (68)Ga-DOTATATE demonstrated 332 lesions (96.4%). The mean SUVmax of all lesions was not significantly different between (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE (17.8 ± 11.4 vs. 16.7 ± 10.7, n.s.).

CONCLUSION

Our analysis demonstrated very good concordance between (68)Ga-HA-DOTATATE and (68)Ga-DOTATATE PET data. As the availability and use of (68)Ga-HA-DOTATATE is not governed by patent restrictions it may be an attractive alternative to other (68)Ga-labelled hsst analogues.

Radiopharmaceutical development of radiolabelled peptides

Receptor targeting with radiolabelled peptides has become very important in nuclear medicine and oncology in the past few years. The overexpression of many peptide receptors in numerous cancers, compared to their relatively low density in physiological organs, represents the molecular basis for in vivo imaging and targeted radionuclide therapy with radiolabelled peptide-based probes. The prototypes are analogs of somatostatin which are routinely used in the clinic. More recent developments include somatostatin analogs with a broader receptor subtype profile or with antagonistic properties. Many other peptide families such as bombesin, cholecystokinin/gastrin, glucagon-like peptide-1 (GLP-1)/exendin, arginine-glycine-aspartic acid (RGD) etc. have been explored during the last few years and quite a number of potential radiolabelled probes have been derived from them. On the other hand, a variety of strategies and optimized protocols for efficient labelling of peptides with clinically relevant radionuclides such as (99m)Tc, M(3+) radiometals ((111)In, (86/90)Y, (177)Lu, (67/68)Ga), (64/67)Cu, (18)F or radioisotopes of iodine have been developed. The labelling approaches include direct labelling, the use of bifunctional chelators or prosthetic groups. The choice of the labelling approach is driven by the nature and the chemical properties of the radionuclide. Additionally, chemical strategies, including modification of the amino acid sequence and introduction of linkers/spacers with different characteristics, have been explored for the improvement of the overall performance of the radiopeptides, e.g. metabolic stability and pharmacokinetics. Herein, we discuss the development of peptides as radiopharmaceuticals starting from the choice of the labelling method and the conditions to the design and optimization of the peptide probe, as well as some recent developments, focusing on a selected list of peptide families, including somatostatin, bombesin, cholecystokinin/gastrin, GLP-1/exendin and RGD.

(68)Ga-labeled DOTA-peptides and (68)Ga-labeled radiopharmaceuticals for positron emission tomography: current status of research, clinical applications, and future perspectives

In this review we give an overview of current knowledge of (68)Ga-labeled pharmaceuticals, with focus on imaging receptor-mediated processes. A major advantage of a (68)Ge/(68)Ga generator is its continuous source of (68)Ga, independently from an on-site cyclotron. The increase in knowledge of purification and concentration of the eluate and the complex ligand chemistry has led to (68)Ga-labeled pharmaceuticals with major clinical impact. (68)Ga-labeled pharmaceuticals have the potential to cover all today's clinical options with (99m)Tc, with the concordant higher resolution of positron emission tomography (PET) in comparison with single photon emission computed tomography. (68)Ga-labeled analogs of octreotide, such as DOTATOC, DOTANOC, and DOTA-TATE, are in clinical application in nuclear medicine, and these analogs are now the most frequently applied of all (68)Ga-labeled pharmaceuticals. All the above-mentioned items in favor of successful application of (68)Ga-labeled radiopharmaceuticals for imaging in patients are strong arguments for the development of a (68)Ge/(68)Ga generator with Marketing Authorization and thus to provide pharmaceutical grade eluate. Moreover, now not one United States Food and Drug Administration-approved or European Medicines Agency-approved (68)Ga-radiopharmaceutical is available. As soon as these are achieved, a whole new radiopharmacy providing PET radiopharmaceuticals might develop.

Preclinical and clinical studies of peptide receptor radionuclide therapy

In the 1980s, the (111)In-labeled somatostatin analog OctreoScan (Covidien, Hazelwood, MO) was developed for imaging of somatostatin receptor subtype 2 (sst(2)) overexpressing tumors. On the basis of this success, peptide receptor radionuclide therapy (PRRT) was developed using similar somatostatin analogs with different therapeutic radionuclides. Clinical application of PRRT demonstrated impressive results on tumor response, overall survival, and quality of life in patients with gastroenteropancreatic neuroendocrine tumors. The peptides 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), Tyr(3)-octreotate (DOTATATE) and DOTA, Tyr(3)-octreotide (DOTATOC) (brand name Onalta), predominantly targeting sst(2), have been granted Orphan Drug status by the European Medicines Agency and the US Food and Drug Administration for application in PRRT. Besides somatostatin receptor-targeting peptides, multiple other radiopeptide analogs were developed targeting several other receptors overexpressed on various tumors. Some of these peptide analogs, including cholecystokinin, gastrin, gastrin-releasing peptide, arginine-glycine-aspartate (RGD)-peptides, and glucagon-like peptide 1 analogs appeared very promising in preclinical and clinical imaging and PRRT studies. Although the success of PRRT with radiolabeled somatostatin analogs has been established, there is still room for improvement. The therapeutic window of PRRT could be enlarged by the use of new and improved targeting compounds, of which new antagonists with excellent tumor to background ratios are very promising. Furthermore, locoregional administration, improved healthy tissue protection, and combination treatment can be applied to increase the effectiveness of PRRT. Combination treatment might include cocktails of different peptide analogs of different therapeutic radionuclides and of radiolabeled peptides with chemotherapeutic or radiosensitizing agents. This review summarizes results of PRRT and describes clinical and preclinical studies regarding PRRT optimizing strategies.

Tumor imaging and therapy using radiolabeled somatostatin analogues

Molecular imaging plays an essential role in balancing the clinical benefits and risks of radionuclide-based cancer therapy. To effectively treat individual patients, careful assessment of biodistribution, dosimetry, and toxicity is essential. In this Account, we describe advances that combine features of molecular imaging and radionuclide therapy to provide new avenues toward individualized cancer treatment. Selective receptor-targeting radiopeptides have emerged as an important class of radiopharmaceuticals for molecular imaging and therapy of tumors that overexpress peptide receptors on the cell membrane. After such peptides labeled with gamma-emitting radionuclides bind to their receptors, they allow clinicians to visualize receptor-expressing tumors non-invasively. Peptides labeled with beta-particle emitters could also eradicate receptor-expressing tumors. The somatostatin receptors, which are overexpressed in a majority of neuroendocrine tumors, represent the first and best example of targets for radiopeptide-based imaging and radionuclide therapy. The somatostatin analogue (111)In-octreotide permits the localization and staging of neuroendocrine tumors that express the appropriate somatostatin receptors. Newer modified somatostatin analogues, including Tyr(3)-octreotide and Tyr(3)-octreotate, are successfully being used for tumor imaging and radionuclide therapy. Because there are few effective therapies for patients with inoperable or metastasized neuroendocrine tumors, this therapy is a promising novel treatment option for these patients. Peptide receptor imaging and radionuclide therapy can be combined in a single probe, called a "theranostic". To select patients who are likely to benefit from this type of intervention, we first use a peptide analogue labeled with a diagnostic radionuclide to obtain a scan. Selected patients will be treated using the same or a similar peptide analogue labeled with a therapeutic radionuclide. The development of such theranostics could greatly advance the development of personalized treatments. Apart from patient selection for radionuclide therapy, other imaging applications of targeted radiopeptides include localization of primary tumors, detection of metastatic disease (staging/restaging), dosimetry (prediction of response and radiotoxicity), monitoring effects of surgery, radio(nuclide)therapy or chemotherapy, and detection of progression of disease or relapse (follow up). For further evaluation of tumor receptor expression and to increase the value of cancer targeting using radiopeptides, researchers have introduced and evaluated different radiolabeled analogues of other peptide families, such as cholecystokinin (CCK), gastrin, bombesin, substance P, vasoactive intestinal peptide (VIP), and neuropeptide (NP)-Y analogues. We expect improvements in the development of new peptide analogues: such advances could reduce side effects and allow for the use of combination therapy (for example, combining radiopeptide analogues with chemotherapeutics).

Clinical indications for Gallium-68 positron emission tomography imaging

BACKGROUND

(68)Ga-PET imaging is showing slow but steady progress when compared to (18)F-FDG PET. The advantage of in-house preparation of (68)Ga without necessity of a cyclotron, and the new generator configuration with future possibility of freeze-dried kits would make it a promising PET agent for the future.

METHODS

An exhaustive literature exploration was performed using the search engines High-Wire Press, Pubmed, Embase and library databases. Recent reviews on the subject and up-to-date studies on the topic were found that described the role of (68)Ga-PET imaging. Clinical experiences, including our own are described.

RESULTS

Recent resurgence in development of peptides labelled with radiometals, for diagnostic and therapeutic purposes, resulted in a new beginning for (68)Ga-PET imaging. Pre-clinical experience employing animal models and investigation of tracer kinetics/tumour uptake measurements using dynamic (68)Ga-PET have provided data regarding identification of Somatostatin receptors subtypes on many tumours. Present published experiences including our own support these and highlight current clinical utility of (68)Ga-PET imaging. (68)Ga-DOTATOC and (68)Ga-DOTANOC are the most prominent radiopharmaceuticals used nowadays.

CONCLUSION

(68)Ga-PET is employed in the management of neuroendocrine tumours and neural crest tumours (phaeochromocytoma and paraganglioma) with diagnostic and therapeutic implications where it compliments present radiologic and scintigraphic procedures. Diagnosis and radiotherapy treatment planning for meningiomas in pertinent clinical setting is another potential use of (68)Ga-PET. Limited studies have shown its utility in prostate cancer but further studies are contemplated. Therefore, current experience tends to open a new horizon for the clinical utility of (68)Ga-PET imaging in future.

Differential effects of octreotide and pasireotide on somatostatin receptor internalization and trafficking in vitro

OBJECTIVE

The clinically used somatostatin analogs, octreotide and lanreotide, act primarily by binding to somatostatin receptor 2 (sst2). In contrast, the novel multireceptor ligand pasireotide (SOM230) binds with high affinity to somatostatin receptor subtypes sst1, sst2, sst3, and sst5. SOM230 is currently under clinical evaluation for treatment of acromegaly, Cushing's disease, and octreotide-resistant carcinoid tumors. However, the effects of SOM230 on internalization and postendosomal sorting of individual human somatostatin receptor subtypes have not been determined so far.

RESULTS

Here we show that SOM230 was less potent than octreotide in inducing internalization and signaling of sst2 receptors expressed in human embryonic kidney cells. In contrast, SOM230 was more potent than octreotide in inducing internalization and signaling of sst3 and sst5 receptors. Both SOM230 and octreotide stimulated a rapid down-regulation of sst3 but not of sst2 or sst5 receptors. SOM230 and octreotide profoundly differed in their patterns of sst2-stimulated beta-arrestin mobilization. Whereas octreotide-mediated receptor activation led to the formation of stable complexes facilitating the internalization of sst2 and beta-arrestin-2 into the same endocytic vesicles, SOM230-mediated receptor activation led to the formation of unstable complexes that dissociated at or near the plasma membrane. Consequently, sst2 receptors recycled rapidly to the plasma membrane after endocytosis in SOM230-treated cells, but not in octreotide-treated cells.

CONCLUSION

We show that SOM230 modulates somatostatin receptor trafficking in a manner clearly distinct from octreotide and somatostatin. These findings may provide an explanation for the differential regulation of somatostatin receptor responsiveness during long-term administration of stable somatostatin analogs.

PET Imaging of Pheochromocytoma

Pheochromocytomas are tumors derived from chromaffin cells of the adrenal medulla that synthesize, store, metabolize, and usually, but not always, secrete catecholamines. Although pheochromocytomas are the cause of hypertension in only a small number of patients, they can precipitate life-threatening hypertension or cardiac arrhythmias caused by excessive and episodic catecholamine secretion. This article reviews the genetics, clinical presentation, and imaging of pheochromocytoma, with special emphasis on new positron emission tomographic radiopharmaceutical agents.