In vivo monitoring of the antiangiogenic effect of neurotensin receptor-mediated radiotherapy by small-animal positron emission tomography: a pilot study

Theranostic cobalt-55/58m for neurotensin receptor-mediated radiotherapy in vivo: A pilot study with dosimetry

Neurotensin receptor 1 (NTSR1) can stimulate tumor proliferation through neurotensin (NTS) activation and are overexpressed by a variety of cancers. The high binding affinity of NTS/NTSR1 makes radiolabeled NTS derivatives interesting for cancer diagnosis and staging. Internalization of NTS/NTSR1 also suggests therapeutic application with high LET alpha particles and low energy electrons. We investigated the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo using murine models xenografted with NTSR1-positive HT29 human colorectal adenocarcinoma cells, and utilized [55Co]Co-NOTA-NT-20.3 for dosimetry.

METHODS

Targeting properties and cytotoxicity of [55/58mCo]Co-NOTA-NT-20.3 were assessed with HT29 cells. Female nude mice were xenografted with HT29 tumors and administered [55Co or 58mCo]Co-NOTA-NT-20.3 to evaluate pharmacokinetics or for therapy, respectively. Dosimetry calculations followed the Medical Internal Radiation Dose (MIRD) formalism and human absorbed dose rate per unit activity were obtained from OpenDose. The pilot therapy study consisted of two groups (each N = 3) receiving 110 ± 15 MBq and 26 ± 6 MBq [58mCo]Co-NOTA-NT-20.3 one week after tumor inoculation, and control (N = 3). Tumor sizes and masses were measured twice a week after therapy. Complete blood count and kidney histology were also performed to assess toxicity.

RESULTS

HPLC measured radiochemical purity of [55,58mCo]Co-NOTA-NT-20.3 > 99 %. Labeled compounds retained NTS targeting properties. [58mCo]Co-NOTA-NT-20.3 exhibited cytotoxicity for HT29 cells and was >15× more potent than [58mCo]CoCl2. Xenografted tumors responded modestly to administered doses, but mice showed no signs of radiotoxicity. Absorbed dose to tumor and kidney with 110 MBq [58mCo]Co-NOTA-NT-20.3 were 0.6 Gy and 0.8 Gy, respectively, and other organs received less than half of the absorbed dose to tumor. Off-target radiation dose from cobalt-58g was small but reduces the therapeutic window.

CONCLUSION

The enhanced in vitro cytotoxicity and high tumor-to-background led us to investigate the therapeutic efficacy of [58mCo]Co-NOTA-NT-20.3 in vivo. Although we were unable to induce tumor response commensurate with [177Lu]Lu-NT127 (NLys-Lys-Pro-Tyr-Tle-Leu) studies involving similar time-integrated activity, the absence of observed toxicity may constitute an opportunity for targeting vectors with improved uptake and/or retention to avoid the aftereffects of other high-LET radioactive emissions. Future studies with higher uptake, activity and/or multiple dosing regimens are warranted. The theranostic approach employed in this work was crucial for dosimetry analysis.

PET Imaging of the Neurotensin Targeting Peptide NOTA-NT-20.3 Using Cobalt-55, Copper-64 and Gallium-68

Introduction: Neurotensin receptor 1 (NTSR1) is an emerging target for imaging and therapy of many types of cancer. Nuclear imaging of NTSR1 allows for noninvasive assessment of the receptor levels of NTSR1 on the primary tumor, as well as potential metastases. This work focuses on a the neurotensin peptide analogue NT-20.3 conjugated to the chelator NOTA for radiolabeling for use in noninvasive positron emission tomography (PET). NOTA-NT-20.3 was radiolabeled with gallium-68, copper-64, and cobalt-55 to determine the effect that modification of the radiometal has on imaging and potential therapeutic properties of NOTA-NT-20.3. Methods: In vitro assays investigating cell uptake and subcellular localization of the radiolabeled peptides were performed using human colorectal adenocarcinoma HT29 cells. In vivo PET/CT imaging was used to determine the distribution and clearance of the peptide in mice bearing NTSR1 expressing HT29 tumors. Results: Cell uptake studies showed that the highest uptake was obtained with [55Co] Co-NOTA-NT-20.3 (18.70 ± 1.30%ID/mg), followed by [64Cu] Cu-NOTA-NT-20.3 (15.46 ± 0.91%ID/mg), and lastly [68Ga] Ga-NOTA-NT-20.3 (10.94 ± 0.46%ID/mg) (p < 0.001). Subcellular distribution was similar across the three constructs, with the membranous fraction containing the highest amount of radioactivity. In vivo PET/CT imaging of the three constructs revealed similar distribution and tumor uptake at the 1 h imaging timepoint. Tumor uptake was receptor-specific and blockable by co-injection of non-radiolabeled NOTA-NT-20.3. SUV ratios of tumor to heart at the 24 h imaging timepoint show that [55Co] Co-NOTA-NT-20.3 (20.28 ± 3.04) outperformed [64Cu] Cu-NOTA-NT-20.3 (6.52 ± 1.97). In conclusion, our studies show that enhanced cell uptake and increasing tumor to blood ratios over time displayed the superiority of [55Co] Co-NOTA-NT-20.3 over [68Ga] Ga-NOTA-NT-20.3 and [64Cu] Cu-NOTA-NT-20.3 for the targeting of NTSR1.

Bivalent ligands promote endosomal trafficking of the dopamine D3 receptor-neurotensin receptor 1 heterodimer

Bivalent ligands are composed of two pharmacophores connected by a spacer of variable size. These ligands are able to simultaneously recognize two binding sites, for example in a G protein-coupled receptor heterodimer, resulting in enhanced binding affinity. Taking advantage of previously described heterobivalent dopamine-neurotensin receptor ligands, we demonstrate specific interactions between dopamine D3 (D₃R) and neurotensin receptor 1 (NTSR1), two receptors with expression in overlapping brain areas that are associated with neuropsychiatric diseases and addiction. Bivalent ligand binding to D₃R-NTSR1 dimers results in picomolar binding affinity and high selectivity compared to the binding to monomeric receptors. Specificity of the ligands for the D₃R-NTSR1 receptor pair over D₂R-NTSR1 dimers can be achieved by a careful choice of the linker length. Bivalent ligands enhance and stabilize the receptor-receptor interaction leading to NTSR1-controlled internalization of D₃R into endosomes via recruitment of β-arrestin, highlighting a potential mechanism for dimer-specific receptor trafficking and signalling.

Potential use of radiolabelled neurotensin in PET imaging and therapy of patients with pancreatic cancer

Pancreatic cancer is the fourth leading cause of cancer-related death in both men and women. Neurotensin receptors are overexpressed in different malignancies, above all pancreatic cancer. On the other hand, neurotensin receptor expression in inflammation is quite low. This fact can probably solve the most important problem of F-FDG PET imaging - distinguishing malignant and inflammatory processes. The first therapeutic injection of radiolabelled neurotensin in human with pancreatic cancer has been successfully performed. Animal experiments are also very close to the first in human injection of radiolabelled neurotensin for diagnostic purposes. The purpose of this article is to provide an overview of radiolabelled neurotensin analogues that can be used in imaging and therapy in patients with pancreatic ductal adenocarcinoma.

Therapeutic approaches targeting the neurotensin receptors

Introduction: Neurotensin is a gut-brain peptide hormone, a 13 amino acid neuropeptide found in the central nervous system and in the GI tract. The neurotensinergic system is implicated in various physiological and pathological processes related to neuropsychiatric and metabolic machineries, cancer growth, food, and drug intake. NT mediates its functions through its two G protein-coupled receptors: neurotensin receptor 1 (NTS1/NTSR1) and neurotensin receptor 2 (NTS2/NTSR2). Over the past decade, the role of NTS3/NTSR3/sortilin has also gained importance in human pathologies. Several approaches have appeared dealing with the discovery of compounds able to modulate the functions of this neuropeptide through its receptors for therapeutic gain.Areas covered: The article provides an overview of over four decades of research and details the drug discovery approaches and patented strategies targeting NTSR in the past decade.Expert opinion: Neurotensin is an important neurotransmitter that enables crosstalk with various neurotransmitter and neuroendocrine systems. While significant efforts have been made that have led to selective agonists and antagonists with promising in vitro and in vivo activities, the therapeutic potential of compounds targeting the neurotensinergic system is still to be fully harnessed for successful clinical translation of compounds for the treatment of several pathologies.

Design of Bimodal Ligands of Neurotensin Receptor 1 for Positron Emission Tomography Imaging and Fluorescence-Guided Surgery of Pancreatic Cancer

Neurotensin receptor 1 (NTSR1) is overexpressed in most human pancreatic ductal adenocarcinomas. It makes it an attractive target for the development of pancreatic cancer imaging agents. In this study, we sought to develop a bimodal positron emission tomography (PET)/fluorescent imaging agent capable of specifically targeting these receptors. Starting from the structure of a known NTSR1 agonist, a series of tracers were synthesized, radiometalated with gallium-68, and evaluated in vitro and in vivo, in mice bearing an AsPC-1 xenograft. PET imaging allowed us to identify the compound [⁶⁸Ga]Ga-NODAGA-Lys(Cy5**)-AEEAc-[Me-Arg⁸,Tle¹²]-NT(7-13) as the one with the most promising biodistribution profile, characterized by high tumor uptake (2.56 ± 0.97%ID/g, 1 h post-injection) and rapid elimination from nontargeted organs, through urinary excretion. Fluorescence imaging gave similar results. On this basis, fluorescence-guided resection of tumor masses was successfully carried out on a preclinical model.

Modifications at Arg and Ile Give Neurotensin(8-13) Derivatives with High Stability and Retained NTS1 Receptor Affinity

Due to its expression in various malignant tumors, the neurotensin receptor 1 (NTS₁R) has been suggested and explored as a target for tumor diagnosis and therapy. Animal model-based investigations of various radiolabeled NTS₁R ligands derived from the hexapeptide neurotensin(8-13) (NT(8-13)), e.g. ⁶⁸Ga- and ¹⁸F-labeled compounds for PET diagnostics, give rise to optimize such radiotracers for clinical use. As NT(8-13) is rapidly degraded in vivo; structural modifications are required in terms of increased metabolic stability. In this study, the stabilization of the peptide backbone of NT(8-13) against enzymatic degradation was systematically explored by performing an N-methyl scan, replacing Ile¹² by tert-butylglycine¹² (Tle¹²) and N-terminal acylation. N-Methylation of either arginine, Arg⁸, or Arg⁹, combined with the Ile¹²/Tle¹² exchange, proved to be most favorable with respect to NTS₁R affinity (K _i < 2 nM) and stability in human plasma (t _1/2 > 48 h), a valuable result regarding the development of radiopharmaceuticals derived from NT(8-13).

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.

Theranostic Value of Multimers: Lessons Learned from Trimerization of Neurotensin Receptor Ligands and Other Targeting Vectors

Neurotensin receptor 1 (NTS1) is overexpressed on a variety of cancer entities; for example, prostate cancer, ductal pancreatic adenocarcinoma, and breast cancer. Therefore, it represents an interesting target for the diagnosis of these cancers types by positron emission tomography (PET). The metabolically-stabilized neurotensin (NT) derivative peptide Nlys⁸-Lys⁹-Pro¹⁰-Tyr¹¹-Tle¹²-Leu¹³-OH was elongated at the N-terminus with 6-azido norleucine and coupled with the 1,4,7-triazacyclononane-1,4,7-tris[(2-carboxyethyl)methylenephosphinic acid] (TRAP) chelator TRAP(alkyne)₃ in order to synthesize a NT trimer with subnanomolar affinity and high stability. The ⁶⁸Ga-labeled peptide [⁶⁸Ga]Ga-TRAP(NT4)₃ was characterized in vitro using the NTS1-expressing human colorectal adenocarcinoma cell line HT29. It displayed fast and high internalization rates of >90%, but also fast efflux rates of 50% over 15 min. In vivo, [⁶⁸Ga]Ga-TRAP(NT4)₃ showed moderate HT29 tumor uptake values of 1.7 %ID/g at 60 min post-injection (p.i.), but also high uptake and retention in the kidneys and liver. A comparison of data for trimer/monomer pairs of NT ligands and other targeting vectors (peptides and peptoids targeting integrins α_vβ₃, α₅β₁, and α_vβ₆, the PSMA-ligand DUPA (2-[3-(1,3-dicarboxypropyl)-ureido]pentanedioic acid), and nitroimidazoles targeting hypoxia) revealed that multimers always exhibit higher target affinities and tumor uptake, but not necessarily improved tumor-to-tissue ratios. Thus, although in vitro data are not suitable for prediction of in vivo performance, multimers are potentially superior to monomers, particularly for applications where high tumor accumulation is crucial.

Proof of Therapeutic Efficacy of a 177Lu-Labeled Neurotensin Receptor 1 Antagonist in a Colon Carcinoma Xenograft Model

Increased expression of neurotensin receptor 1 (NTR1) has been shown in a large number of tumor entities such as pancreatic or colon carcinoma. Hence, this receptor is a promising target for diagnostic imaging and radioligand therapy. Using the favorable biodistribution data of the NTR1-targeting agent ¹¹¹In-3BP-227, we investigated the therapeutic effect of its ¹⁷⁷Lu-labeled analog on the tumor growth of NTR1-positive HT29 colon carcinoma xenografts. Methods: 3BP-227 was labeled with ¹⁷⁷Lu. To assess its biodistribution properties, SPECT and CT scans of HT29-xenografted nude mice injected with ¹⁷⁷Lu-3BP-227 were acquired, and ex vivo tissue activity was determined. To evaluate therapeutic efficacy, 2 groups of mice received the radiopharmaceutical in a median dose of either 165 MBq (129-232 MBq, n = 10) or 110 MBq (82-116 MBq, n = 10), whereas control mice were injected with vehicle (n = 10). Tumor sizes and body weights were monitored for up to 49 d. Renal function and histologic morphology were evaluated. Results: Whole-body SPECT/CT images allowed clear tumor visualization with low background activity and high tumor-to-kidney and -liver ratios. Ex vivo biodistribution data confirmed high and persistent uptake of ¹⁷⁷Lu-3BP-227 in HT29 tumors (19.0 ± 3.6 vs. 2.7 ± 1.6 percentage injected dose per gram at 3 and 69 h after injection, respectively). The application of ¹⁷⁷Lu-3BP-227 resulted in a distinct delay of tumor growth. Median tumor doubling time for controls was 5.5 d (interquartile range [IQR], 2.8-7.0), compared with 17.5 d (IQR, 5.5-22.5 d) for the 110-MBq and 41.0 d (IQR, 27.5-55.0) for the 165-MBg group. Compared with controls, median relative tumor volume at day 23 after injection was reduced by 55% (P = 0.034) in the 110-MBq and by 88% (P < 0.01) in the 165-MBq group. Renal histology and clinical chemistry results did not differ between radiotherapy groups and controls, suggesting absence of therapy-induced acute renal damage. Conclusion: These data demonstrate that the novel NTR1-targeting theranostic agent 3BP-227 is an effective and promising candidate for radioligand therapy, with a favorable preliminary safety profile and high potential for clinical translation.

Development of Covalent Ligand-Receptor Pairs to Study the Binding Properties of Nonpeptidic Neurotensin Receptor 1 Antagonists

The neurotensin receptor NTS1 has been suggested to be of pharmaceutical relevance, as it was found to exert modulatory effects on dopaminergic signal transduction and to be involved in tumor progression. Rational drug design of NTS1 receptor ligands requires molecular insights into the binding behavior of a particular lead compound. Although crystal structures of NTS1 have revealed the molecular determinants of peptide-agonist interactions, the binding mode of small-molecule antagonists remains largely unknown. Employing a disulfide-based tethering approach, we developed covalently binding molecular probes. The ligands 1 and 2 are based on the pharmacophore of the nonpeptidic NTS1 antagonist SR142948A, allowing the formation of a disulfide bond to an engineered cysteine residue of NTS1. The position of the covalent bond between Cys127(2.65) and the ligand was used to predict the binding mode of the covalent antagonist 1 and its parent compound SR142948A by molecular dynamics simulations.

Targeting neuropeptide receptors for cancer imaging and therapy: perspectives with bombesin, neurotensin, and neuropeptide-Y receptors

Receptors for some regulatory peptides are highly expressed in tumors. Selective radiolabeled peptides can bind with high affinity and specificity to these receptors and exhibit favorable pharmacologic and pharmacokinetic properties, making them suitable agents for imaging or targeted therapy. The success encountered with radiolabeled somatostatin analogs is probably the first of a long list, as multiple peptide receptors are now recognized as potential targets. This review focuses on 3 neuropeptide receptor systems (bombesin, neurotensin, and neuropeptide-Y) that offer high potential in the field of nuclear oncology. The underlying biology of these peptide/receptor systems, their physiologic and pathologic roles, and their differential distribution in normal and tumoral tissues are described with emphasis on breast, prostate, and lung cancers. Radiolabeled analogs that selectively target these receptors are highlighted.