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

N-Terminal Stabilization of Radiolabeled Neurotensin Analogues for Improved Tumor Uptake

Peptide-based radiopharmaceuticals targeting neurotensin-receptor-1 (NTS1) are mainly stabilized using chemical modifications at the NT[8-13] sequence, thus increasing the stability and the uptake of the corresponding radionuclide-macrocycle-linker-bioconjugate. We postulate that the introduction of the linker at the N-term part induces additional cleavage sites that can be further stabilized to achieve a prolonged uptake. Double (JMV 7259 and JMV 7222) and triple-stabilized neurotensin analogues (JMV 7258 and JMV 7490) were synthesized, radiolabeled, and evaluated on HT-29 cells (NTS1+). Nanomolar NTS1-affinity and high internalization rates were observed for all of the radiopharmaceuticals. Efflux was lower for radiolabeled JMV 7490. Consequently, [111In]In-JMV 7490 showed uptake of 5.86 ± 0.86 and 3.65 ± 0.29% ID/g of tissue in HT-29 xenografts at 1 and 4 h, respectively. We have successfully shown that high and persistent uptake of NTS1-positive tumor cells is achievable by stabilization of the N-term part. Efflux also appears to be a critical parameter for the successful targeting of NTS1 using radiopharmaceuticals.

Development of Macrocyclic Neurotensin Receptor Type 2 (NTS2) Opioid-Free Analgesics

The opioid crisis has highlighted the urgent need to develop non-opioid alternatives for managing pain, with an effective, safe, and non-addictive pharmacotherapeutic profile. Using an extensive structure-activity relationship approach, here we have identified a new series of highly selective neurotensin receptor type 2 (NTS2) macrocyclic compounds that exert potent, opioid-independent analgesia in various experimental pain models. To our knowledge, the constrained macrocycle in which the Ile12 residue of NT(7-12) was substituted by cyclopentylalanine, Pro7 and Pro10 were replaced by allyl-glycine followed by side-chain to side-chain cyclization is the most selective analog targeting NTS2 identified to date (Ki 2.9 nM), showing 30,000-fold selectivity over NTS1. Of particular importance, this macrocyclic analog is also able to potentiate the analgesic effects of morphine in a dose- and time-dependent manner. Exerting complementary analgesic actions via distinct mechanisms of nociceptive transmission, NTS2-selective macrocycles can therefore be exploited as opioid-free analgesics or as opioid-sparing therapeutics, offering superior pain relief with reduced adverse effects to pain patients.

Dually Labeled Neurotensin NTS1R Ligands for Probing Radiochemical and Fluorescence-Based Binding Assays

The determination of ligand-receptor binding affinities plays a key role in the development process of pharmaceuticals. While the classical radiochemical binding assay uses radioligands, fluorescence-based binding assays require fluorescent probes. Usually, radio- and fluorescence-labeled ligands are dissimilar in terms of structure and bioactivity, and can be used in either radiochemical or fluorescence-based assays. Aiming for a close comparison of both assay types, we synthesized tritiated fluorescent neurotensin receptor ligands ([3H]13, [3H]18) and their nontritiated analogues (13, 18). The labeled probes were studied in radiochemical and fluorescence-based (high-content imaging, flow cytometry, fluorescence anisotropy) binding assays. Equilibrium saturation binding yielded well-comparable ligand-receptor affinities, indicating that all these setups can be used for the screening of new drugs. In contrast, discrepancies were found in the kinetic behavior of the probes, which can be attributed to technical differences of the methods and require further studies with respect to the elucidation of the underlying mechanisms.

Extended Metadynamics Protocol for Binding/Unbinding Free Energies of Peptide Ligands to Class A G-Protein-Coupled Receptors

A metadynamics protocol is presented to characterize the binding and unbinding of peptide ligands to class A G-protein-coupled receptors (GPCRs). The protocol expands on the one previously presented for binding and unbinding small-molecule ligands to class A GPCRs and accounts for the more demanding nature of the peptide binding-unbinding process. It applies to almost all class A GPCRs. Exemplary simulations are described for subtypes Y1R, Y2R, and Y4R of the neuropeptide Y receptor family, vasopressin binding to the vasopressin V2 receptor (V2R), and oxytocin binding to the oxytocin receptor (OTR). Binding free energies and the positions of alternative binding sites are presented and, where possible, compared with the experiment.

Toward Stability Enhancement of NTS1R-Targeted Radioligands: Structural Interventions on [99mTc]Tc-DT1

The neurotensin subtype 1 receptor (NTS1R) is overexpressed in a number of human tumors, thereby representing a valid target for cancer theranostics with radiolabeled neurotensin (NT) analogs like [99mTc]Tc-DT1 (DT1, N4-Gly7-NT(8-13)). Thus far, the fast degradation of intravenously injected NT-radioligands by neprilysin (NEP) and angiotensin-converting enzyme (ACE) has compromised their clinical applicability. Aiming at metabolic stability enhancements, we herein introduce (i) DT7 ([DAsn14]DT1) and (ii) DT8 ([β-Homoleucine13]DT1), modified at the C-terminus, along with (iii) DT9 ([(palmitoyl)Lys7]DT1), carrying an albumin-binding domain (ABD) at Lys7. The biological profiles of the new [99mTc]Tc-radioligands were compared with [99mTc]Tc-DT1, using NTS1R-expressing AsPC-1 cells and mice models without or during NEP/ACE inhibition. The radioligands showed enhanced in vivo stability vs. [99mTc]Tc-DT1, with [99mTc]Tc-DT9 displaying full resistance to both peptidases. Furthermore, [99mTc]Tc-DT9 achieved the highest cell internalization and tumor uptake even without NEP/ACE-inhibition but with unfavorably high background radioactivity levels. Hence, unlike C-terminal modification, the introduction of a pendant ABD group in the linker turned out to be the most promising strategy toward metabolic stability, cell uptake, and tumor accumulation of [99mTc]Tc-DT1 mimics. To improve the observed suboptimal pharmacokinetics of [99mTc]Tc-DT9, the replacement of palmitoyl on Lys7 by other ABD groups is currently being pursued.

Neurotensin and Alcohol Use Disorders: Towards a Pharmacological Treatment

Harmful alcohol use is responsible for a group of disorders collectively named alcohol use disorders (AUDs), according to the DSM-5 classification. The damage induced by alcohol depends on the amount, time, and consumption patterns (continuous and heavy episodic drinking). It affects individual global well-being and social and familial environments with variable impact. Alcohol addiction manifests with different degrees of organ and mental health detriment for the individual, exhibiting two main traits: compulsive drinking and negative emotional states occurring at withdrawal, frequently causing relapse episodes. Numerous individual and living conditions, including the concomitant use of other psychoactive substances, lie in the complexity of AUD. Ethanol and its metabolites directly impact the tissues and may cause local damage or alter the homeostasis of brain neurotransmission, immunity scaffolding, or cell repair biochemical pathways. Brain modulator and neurotransmitter-assembled neurocircuitries govern reward, reinforcement, social interaction, and consumption of alcohol behaviors in an intertwined manner. Experimental evidence supports the participation of neurotensin (NT) in preclinical models of alcohol addiction. For example, NT neurons in the central nucleus of the amygdala projecting to the parabrachial nucleus strengthen alcohol consumption and preference. In addition, the levels of NT in the frontal cortex were found to be lower in rats bred to prefer alcohol to water in a free alcohol-water choice compared to wild-type animals. NT receptors 1 and 2 seem to be involved in alcohol consumption and alcohol effects in several models of knockout mice. This review aims to present an updated picture of the role of NT systems in alcohol addiction and the possible use of nonpeptide ligands modulating the activity of the NT system, applied to experimental animal models of harmful drinking behavior mimicking alcohol addiction leading to health ruin in humans.

Neurotensin(8-13) analogs as dual NTS1 and NTS2 receptor ligands with enhanced effects on a mouse model of Parkinson's disease

The modulatory interactions between neurotensin (NT) and the dopaminergic neurotransmitter system in the brain suggest that NT may be associated with the progression of Parkinson's disease (PD). NT exerts its neurophysiological effects by interactions with the human NT receptors type 1 (hNTS1) and 2 (hNTS2). Therefore, both receptor subtypes are promising targets for the development of novel NT-based analogs for the treatment of PD. In this study, we used a virtually guided molecular modeling approach to predict the activity of NT(8-13) analogs by investigating the docking models of ligands designed for binding to the human NTS1 and NTS2 receptors. The importance of the residues at positions 8 and/or 9 for hNTS1 and hNTS2 receptor binding affinity was experimentally confirmed by radioligand binding assays. Further in vitro ADME profiling and in vivo studies revealed that, compared to the parent peptide NT(8-13), compound 10 exhibited improved stability and BBB permeability combined with a significant enhancement of the motor function and memory in a mouse model of PD. The herein reported NTS1/NTS2 dual-specific NT(8-13) analogs represent an attractive tool for the development of therapeutic strategies against PD and potentially other CNS disorders.

Insertion of Nanoluc into the Extracellular Loops as a Complementary Method To Establish BRET-Based Binding Assays for GPCRs

Luminescence-based techniques play an increasingly important role in all areas of biochemical research, including investigations on G protein-coupled receptors (GPCRs). One quite recent and popular addition has been made by introducing bioluminescence resonance energy transfer (BRET)-based binding assays for GPCRs, which are based on the fusion of nanoluciferase (Nluc) to the N-terminus of the receptor and the occurring energy transfer via BRET to a bound fluorescent ligand. However, being based on BRET, the technique is strongly dependent on the distance/orientation between the luciferase and the fluorescent ligand. Here we describe an alternative strategy to establish BRET-based binding assays for GPCRs, where the N-terminal fusion of Nluc did not result in functioning test systems with our fluorescent ligands (e.g., for the neuropeptide Y Y1 receptor (Y1R) and the neurotensin receptor type 1 (NTS1R)). Instead, we introduced Nluc into their second extracellular loop and we obtained binding data for the fluorescent ligands and reported standard ligands (in saturation and competition binding experiments, respectively) comparable to data from the literature. The strategy was transferred to the angiotensin II receptor type 1 (AT1R) and the M1 muscarinic acetylcholine receptor (M1R), which led to affinity estimates comparable to data from radioligand binding experiments. Additionally, an analysis of the binding kinetics of all fluorescent ligands at their respective target was performed using the newly described receptor/Nluc-constructs.

Neurotensin analogs by fluoroglycosylation at Nω-carbamoylated arginines for PET imaging of NTS1-positive tumors

Since neurotensin (NT) receptors of subtype-1 (NTS1) are expressed by different types of malignant tumors, such as pancreatic adenocarcinoma, colorectal and prostate carcinoma, they represent an interesting target for tumor imaging by positron emission tomography (PET) and endoradiotherapy. Previously reported neurotensin-derived NTS1 ligands for PET were radiolabeled by modification and prelongation of the N-terminus of NT(8-13) peptide analogs. In this study, we demonstrate that modifying Arg8 or Arg9 by Nω-carbamoylation and subsequent fluoroglycosylation provides a suitable approach for the development of NT(8-13) analogs as PET imaging agents. The Nω-carbamoylated and fluoroglycosylated NT(8-13) analogs retained high NTS1 affinity in the one-digit nanomolar range as well as high metabolic stability in vitro. In vivo, the radioligand [18F]21 demonstrated favorable biokinetics in HT-29 tumor-bearing mice with high tumor uptake and high retention, predominantly renal clearance, and fast wash-out from blood and other non-target tissues. Therefore, [18F]21 has the potential to be used as molecular probe for the imaging of NTS1-expressing tumors by PET.

Optimized Opioid-Neurotensin Multitarget Peptides: From Design to Structure-Activity Relationship Studies

Fusion of nonopioid pharmacophores, such as neurotensin, with opioid ligands represents an attractive approach for pain treatment. Herein, the μ-/δ-opioid agonist tetrapeptide H-Dmt-d-Arg-Aba-β-Ala-NH2 (KGOP01) was fused to NT(8-13) analogues. Since the NTS1 receptor has been linked to adverse effects, selective MOR-NTS2 ligands are preferred. Modifications were introduced within the native NT sequence, particularly a β3-homo amino acid in position 8 and Tyr11 substitutions. Combination of β3hArg and Dmt led to peptide 7, a MOR agonist, showing the highest NTS2 affinity described to date (Ki = 3 pM) and good NTS1 affinity (Ki = 4 nM), providing a >1300-fold NTS2 selectivity. The (6-OH)Tic-containing analogue 9 also exhibited high NTS2 affinity (Ki = 1.7 nM), with low NTS1 affinity (Ki = 4.7 μM), resulting in an excellent NTS2 selectivity (>2700). In mice, hybrid 7 produced significant and prolonged antinociception (up to 8 h), as compared to the KGOP01 opioid parent compound.

Optimizing the Profile of [99mTc]Tc-NT(7-13) Tracers in Pancreatic Cancer Models by Means of Protease Inhibitors

Background: The overexpression of neurotensin subtype 1 receptors (NTS1Rs) in human tumors may be elegantly exploited for directing neurotensin (NT)-based radionuclide carriers specifically to cancer sites for theranostic purposes. We have recently shown that [^99mTc]Tc–DT1 ([^99mTc]Tc–[N₄–Gly⁷]NT(7–13)) and [^99mTc]Tc–DT5 ([^99mTc]Tc–[N₄–βAla⁷,Dab⁹]NT(7–13)) show notably improved uptake in human colon adenocarcinoma WiDr xenografts in mice treated with neprilysin (NEP) inhibitors and/or angiotensin-converting enzyme (ACE) inhibitors compared with untreated controls. Aiming toward translation of this promising approach in NTS1R-positive pancreatic ductal adenocarcinoma (PDAC) patients, we now report on the impact of registered NEP/ACE inhibitors on the performance of [^99mTc]Tc–DT1 and [^99mTc]Tc–DT5 in pancreatic cancer models. Methods: The cellular uptake of [^99mTc]Tc–DT1 and [^99mTc]Tc–DT5 was tested in a panel of pancreatic cell lines, and their stability was assessed in mice treated or not treated with Entresto, lisinopril, or their combinations. Biodistribution was conducted in severe combined immunodeficiency (SCID) mice bearing pancreatic AsPC-1 xenografts. Results: The Entresto + lisinopril combination maximized the metabolic stability of the fast-internalizing [^99mTc]Tc–DT1 in mice, resulting in notably enhanced tumor uptake (7.05 ± 0.80% injected activity (IA)/g vs. 1.25 ± 0.80% IA/g in non-treated controls at 4 h post-injection; p < 0.0001). Conclusions: This study has shown the feasibility of optimizing the uptake of [^99mTc]Tc–DT1 in pancreatic cancer models with the aid of clinically established NEP/ACE inhibitors, in favor of clinical translation prospects.

Neurotensins and their therapeutic potential: research field study

The natural tridecapeptide neurotensin has been emerged as a promising therapeutic scaffold for the treatment of neurological diseases and cancer. In this work, we aimed to identify the top 100 most cited original research papers as well as recent key studies related to neurotensins. The Web of Science Core Collection database was searched and the retrieved research articles were analyzed by using the VOSviewer software. The most cited original articles were published between 1973 and 2013. The top-cited article was by Carraway and Leeman reporting the discovery of neurotensin in 1973. The highly cited terms were associated with hypotension and angiotensin-converting-enzyme. The conducted analysis reveals the therapeutic potentials of neurotensin, and further impactful research toward its clinical development is warrantied.

Key-Protease Inhibition Regimens Promote Tumor Targeting of Neurotensin Radioligands

Neurotensin subtype 1 receptors (NTS1R) represent attractive molecular targets for directing radiolabeled neurotensin (NT) analogs to tumor lesions for diagnostic and therapeutic purposes. This approach has been largely undermined by the rapid in vivo degradation of linear NT-based radioligands. Herein, we aim to increase the tumor targeting of three ^99mTc-labeled NT analogs by the in-situ inhibition of two key proteases involved in their catabolism. DT1 ([N₄-Gly⁷]NT(7-13)), DT5 ([N₄-βAla⁷,Dab⁹]NT(7-13)), and DT6 ([N₄-βAla⁷,Dab⁹,Tle¹²]]NT(7-13)) were labeled with ^99mTc. Their profiles were investigated in NTS1R-positive colon adenocarcinoma WiDr cells and mice treated or not with the neprilysin (NEP)-inhibitor phosphoramidon (PA) and/or the angiotensin converting enzyme (ACE)-inhibitor lisinopril (Lis). Structural modifications led to the partial stabilization of ^99mTc-DT6 in peripheral mice blood (55.1 ± 3.9% intact), whereas ^99mTc-DT1 and ^99mTc-DT5 were totally degraded within 5 min. Coinjection of PA and/or Lis significantly stabilized all three analogs, leading to a remarkable enhancement of tumor uptake for ^99mTc-DT1 and ^99mTc-DT5, but was less effective in the case of poorly internalizing ^99mTc-DT6. In conclusion, NEP and/or ACE inhibition represents a powerful tool to improve tumor targeting and the overall pharmacokinetics of NT-based radioligands, and warrants further validation in the field of NTS1R-targeted tumor imaging and therapy.

An Alkyne-functionalized Arginine for Solid-Phase Synthesis Enabling "Bioorthogonal" Peptide Conjugation

Lately, amino-functionalized N ^ω-carbamoylated arginines were introduced as arginine surrogates enabling peptide labeling. However, this approach is hardly compatible with peptides also containing lysine or cysteine. Here, we present the synthesis of an alkyne-functionalized, N ^ω-carbamoylated arginine building block (7), which is compatible with Fmoc-strategy solid-phase peptide synthesis. The alkynylated arginine was incorporated into three biologically active linear hexapeptides and into a cyclic pentapeptide. Peptide conjugation to an azido-functionalized fluorescent dye via "click" chemistry was successfully demonstrated. In the case of a peptide also containing lysine besides the alkyne-functionalized arginine, this was feasible in a "bioorthogonal" manner.

Fluorescence Labeling of Neurotensin(8-13) via Arginine Residues Gives Molecular Tools with High Receptor Affinity

Fluorescence-labeled receptor ligands have emerged as valuable molecular tools, being indispensable for studying receptor-ligand interactions by fluorescence-based techniques such as high-content imaging, fluorescence microscopy, and fluorescence polarization. Through application of a new labeling strategy for peptides, a series of fluorescent neurotensin(8-13) derivatives was synthesized by attaching red-emitting fluorophores (indolinium- and pyridinium-type cyanine dyes) to carbamoylated arginine residues in neurotensin(8-13) analogues, yielding fluorescent probes with high NTS₁R affinity (pK _i values: 8.15-9.12) and potency (pEC₅₀ values (Ca²⁺ mobilization): 8.23-9.43). Selected fluorescent ligands were investigated by flow cytometry and high-content imaging (saturation binding, kinetic studies, and competition binding) as well as by confocal microscopy using intact CHO-hNTS₁R cells. The study demonstrates the applicability of the fluorescent probes as molecular tools to obtain, for example, information about the localization of receptors in cells and to determine binding affinities of nonlabeled ligands.