Evaluation of Lactam-Bridged Neurotensin Analogues Adjusting ψ(Pro10) Close to the Experimentally Derived Bioactive Conformation of NT(8−13)

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.

Base-Mediated Divergent Synthesis of Spiro-heterocycles Using Pronucleophiles and Ethylene via Thianthrenation

Spirocyclic compounds are abundant in biologically active products. However, the divergent synthesis of spirocyclic compounds using low-cost and abundant available starting materials remains a challenge. Herein, we report an effective method for producing spirocyclic motifs using a cyclic β-carbonyl ester or amide and ethylene via thianthrenation. This strategy highlights the exciting possibility of utilizing abundant ethylene as a C-2 synthon and allows regulating the core structure of the spirocyclic compound by simply altering the base type.

Encounter Complexes Between the N-terminal of Neurotensin with the Extracellular Loop 2 of the Neurotensin Receptor 1 Steer Neurotensin to the Orthosteric Binding Pocket

Neurotensin (NT) is a linear disordered peptide that activates two different class A GPCRs, neurotensin receptor 1 (NTS1) and NTS2. Resolved structures of the complex of the C-terminal fragment of NT, NT8-13, with NTS1 shows the peptide takes a well-defined structure in the bound state. However, the mechanisms underlying NT recognition of NTS1, and the conformational transition of NT upon binding NTS1 is an open question that if answered may aid discovery of highly selective drugs and reveal potential secondary binding sites on the surface of the receptor. Herein we investigated the interactions guiding NT to the orthosteric binding pocket of NTS1 by combining NMR experiments with kinetic analysis of the binding pathway using stopped-flow fluorescence and mutagenesis on both NT and NTS1. We show the presence of transient structures in the middle part of NT that kinetically regulate the binding of NT to NTS1. Moreover, our results indicate that the binding pathway of NT onto NTS1 is mediated via electrostatic interactions between the N-terminal region of NT with the extracellular loop 2 of NTS1. These interactions induce backbone conformational changes in neurotensin similar to the bound-state neurotensin, suggesting that the N-terminal region of NT and these interactions should be considered for development of selective drugs against NTS1.

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).

In Search of the Optimal Macrocyclization Site for Neurotensin

Neurotensin exerts potent analgesic effects following activation of its cognate GPCRs. In this study, we describe a systematic exploration, using structure-based design, of conformationally constraining neurotensin (8-13) with the help of macrocyclization and the resulting impacts on binding affinity, signaling, and proteolytic stability. This exploratory study led to a macrocyclic scaffold with submicromolar binding affinity, agonist activity, and greatly improved plasma stability.

Syntheses, receptor bindings, in vitro and in vivo stabilities and biodistributions of DOTA-neurotensin(8-13) derivatives containing β-amino acid residues - a lesson about the importance of animal experiments

Neurotensin(8-13) (NTS(8-13)) analogs with C- and/or N-terminal β-amino acid residues and three DOTA derivatives thereof have been synthesized (i.e., 1-6). A virtual docking experiment showed almost perfect fit of one of the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) derivatives, 6a, into a crystallographically identified receptor NTSR1 (Fig.1). The affinities for the receptors of the NTS analogs and derivatives are low, when determined with cell-membrane homogenates, while, with NTSR1-exhibiting cancer tissues, affinities in the single-digit nanomolar range can be observed (Table 2). Most of the β-amino acid-containing NTS(8-13) analogs (Table 1 and Fig.2), including the (68) Ga complexes of the DOTA-substituted ones (6; Figs.2 and 5), are stable for ca. 1 h in human serum and plasma, and in murine plasma. The biodistributions of two (68) Ga complexes (of 6a and 6b) in HT29 tumor-bearing nude mice, in the absence and in the presence of a blocking compound, after 10, 30, and 60 min (Figs. 3 and 4) lead to the conclusion that the amount of specifically bound radioligand is rather low. This was confirmed by PET-imaging experiments with the tumor-bearing mice (Fig.6). Comparison of the in vitro plasma stability (after 1 h) with the ex vivo blood content (after 10-15 min) of the two (68) Ga complexes shows that they are rapidly cleaved in the animals (Fig.5).

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

The neurotensin receptor (NTS1) has emerged as an interesting target for molecular imaging and radiotherapy of NTS-positive tumors due to the overexpression in a range of tumors. The aim of this study was to develop a ¹⁷⁷Lu-labeled NTS1 radioligand, its application for radiotherapy in a preclinical model and the imaging of therapy success by small-animal positron emission tomography (µPET) using [⁶⁸Ga]DOTA-RGD as a specific tracer for imaging angiogenesis. The ¹⁷⁷Lu-labeled peptide was subjected to studies on HT29-tumor-bearing nude mice in vivo, defining four groups of animals (single dose, two fractionated doses, four fractionated doses and sham-treated animals). Body weight and tumor diameters were determined three times per week. Up to day 28 after treatment, µPET studies were performed with [⁶⁸Ga]DOTA-RGD. At days 7–10 after treatment with four fractionated doses of 11–14 MBq (each at days 0, 3, 6 and 10), the tumor growth was slightly decreased in comparison with untreated animals. Using a single high dose of 51 MBq, a significantly decreased tumor diameter of about 50% was observed with the beginning of treatment. Our preliminary PET imaging data suggested decreased tumor uptake values of [⁶⁸Ga]DOTA-RGD in treated animals compared to controls at day 7 after treatment. This pilot study suggests that early PET imaging with [⁶⁸Ga]DOTA-RGD in radiotherapy studies to monitor integrin expression could be a promising tool to predict therapy success in vivo. Further successive PET experiments are needed to confirm the significance and predictive value of RGD-PET for NTS-mediated radiotherapy.

Glycosylated neurotensin analogues exhibit sub-picomolar anticonvulsant potency in a pharmacoresistant model of epilepsy

Neurotensin (NT) is an endogenous neuropeptide involved in a variety of central and peripheral neuromodulatory effects. Herein we show the effects of site-specific glycosylation on the in vitro and in vivo pharmacological properties of this neuropeptide. NT analogues containing O-linked disaccharides (beta-melibiose and alpha-TF antigen) or beta-lactose units linked by a PEG(3) spacer were designed and chemically synthesized using Fmoc chemistry. For the latter analogue, Fmoc-Glu-(beta-Lac-PEG(3)-amide) was prepared. Our results indicate that the addition of the disaccharides does not negatively affect the sub-nanomolar affinity or the low-nanomolar agonist potency for the neurotensin receptor subtype 1 (NTS1). Interestingly, three glycosylated analogues exhibited sub-picomolar potency in the 6 Hz limbic seizure mouse model of pharmacoresistant epilepsy following intracerebroventricular administration. Our results suggest for the first time that chemically modified NT analogues may lead to novel antiepileptic therapies.

Synthesis and applications of polyamine amino acid residues: improving the bioactivity of an analgesic neuropeptide, neurotensin

Conjugated polyamines are potential carriers for biotherapeutics targeting the central nervous system. We describe an efficient synthesis of a polyamine-based amino acid, lysine-trimethylene(diNosyl)-spermine(triBoc) with Dde or Fmoc orthogonal protecting groups. This nonnatural amino acid was incorporated into a neurotensin analogue using standard Fmoc-based protocols. The analogue maintained high affinity and agonist potency for neurotensin receptors and exhibited dramatically improved analgesia in mice. Our work provides a basis for use of polyamine amino acids in polypeptides.

Proline derived spirobarbiturates as highly effective beta-turn mimetics incorporating polar and functionalizable constraint elements

A practical and efficient synthesis of spirobarbiturates of type III is reported when NH acidity of the imide function of the hydrophilic linker element allowed the introduction of different substituents. Structural characterization, which was based on both X-ray crystallography and spectroscopic investigations, indicated type II beta-turn formation. Introduction of the molecular scaffold into solid phase peptide synthesis gave rise to spirocyclic neuropeptide analogs.

Molecular building kit of fused-proline-derived peptide mimetics allowing specific adjustment of the dihedral Psi angle

Proline-derived peptide mimetics have become an area of paramount importance in peptide and protein chemistry. Since protein crystal structures frequently display Psi angles of 140-170 degrees for prolyl moieties, our intention was to design a completely novel series of 2,3-fused-proline-derived lactams covering this particular conformational space. Extending our recently described toolset of spirocyclic reverse-turn mimetics, we synthesized pyrrolidinyl-fused seven-, eight-, and nine-membered unsaturated lactam model peptides taking advantage of Grubbs' ring-closing metathesis. Investigating the seven-membered lactam 3a by means of IR and NMR spectroscopy and semiempirical molecular dynamics simulations, we could not observe a U-turn conformation; however, increasing the ring size to give eight- and nine-membered congeners revealed moderate and high type IotaIota beta-turn inducing properties. Interestingly, the conformational properties of our model systems depend on both the ring size of the fused dehydro-Freidinger lactam and the position of the endocyclic double bond. Superior reverse-turn inducing properties could be observed for the fused azacyclononenone 3e. According to diagnostic transanular NOEs, a discrete folding principle of the lactam ring strongly deviating from the regioisomeric lactams 3c,f explains the conformational behavior. Hence, we were able to establish a molecular building kit that allows adjustments of a wide range of naturally occurring proline Psi angles and thus can be exploited to probe molecular recognition and functional properties of biological systems.

Structural diversity of bicyclic amino acids

Over the years biomedical research has been constantly oriented towards the development of new therapeutics based on bioactive peptides and their analogues. In particular, the generation of compounds having structures and functions similar to bioactive peptides, named "peptidomimetics", raised much interest among organic and medicinal chemists due to the possibility by using such compounds to improve both potency and stability of peptidic lead compounds. In the context of this research area, unnatural amino acids are of great interest in drug discovery, and their use as new building blocks for the development of peptidomimetics with high diversity level and possessing high-ordered structures is of special interest. In particular, medicinal chemistry has taken advantage of the use of amino acid homologues and of cyclic and polycyclic templates to introduce elements of diversity for the generation of new molecules as drug candidates. Bicyclic amino acids have been developed as reverse turn mimetics and dipeptide isosteres, and the constraint imposed by their structures has been reported as a tool for controlling the conformational preferences of modified peptides. Moreover, synthetic efforts have been driven to the generation of diverse structures based on the modulation of ring size and scaffold decoration by suitable functional groups. Herein is reported an overview of different classes of bicyclic amino acids, taking into account the strategies to achieve structurally diverse templates, and some implications in medicinal chemistry are also disclosed.

Concise, asymmetric, stereocontrolled total synthesis of stephacidins A, B and notoamide B

Concise asymmetric total syntheses of the fungal metabolites (-)-stephacidin A, (+)-stephacidin B, and (+)-notoamide B are described. Key features of these total syntheses include (1) a facile synthesis of (R)-allyl proline methyl ester, (2) a revised route toward the pyranoindole ring system, (3) a novel cross-metathesis strategy for the introduction of important functional groups, and (4) an SN2' cyclization to form the [2.2.2] bridged bicyclic ring system. Furthermore, our synthesis has taken advantage of microwave heating to shorten reaction times as well as increase yields for the preparation of vital intermediates.

Chirospecific Synthesis of Spirocyclic β-Lactams and Their Characterization as Potent Type II β-Turn Inducing Peptide Mimetics

[reaction: see text] Starting from natural proline, a practical chirospecific synthesis of spirocyclic beta-lactams of type 2 is described when a methylene moiety showing minimal steric demand is employed as a constraint element for adjusting the dihedral angle psi(i + 1). Employing the concept of self-reproduction of chirality, C-formylation of the oxazolidinone 5 afforded the key intermediate 7 taking advantage of an intermediate protection of the bridging element as a vinyl moiety. NMR- and IR-based conformational studies clearly indicated that spiro-beta-lactams of type 2 can serve as efficient beta-turn nucleators.

Design, synthesis, and evaluation of the antipsychotic potential of orally bioavailable neurotensin (8-13) analogues containing non-natural arginine and lysine residues

Neurotensin (NT) and its active fragment NT(8-13) elicit behavioral responses typical of clinically used antipsychotic drugs when administered directly to the brain. However, limited peptide stability and oral bioavailability have prevented these compounds from being developed as relevant pharmaceuticals. Recently, our laboratory designed and studied a first-generation NT(8-13) derivative, KK13, that elicited key pharmacokinetic and behavioral responses typical of clinically used antipsychotic drugs when administered to rats parenterally. This compound was the basis for the rational design of a series of second-generation NT(8-13) analogues (KH1-KH30) studied in this paper. Initial screening of these analogues for CNS activity by monitoring hypothermia induction after peripheral administration defined several compounds (KH11, KH24, KH26, and KH28-KH30) that warranted further investigation. Each compound maintained binding affinity for NTR(1), however, only KH24, KH26, and KH28 (as well as KK13) elicited significant hypothermic responses after oral administration. Of these, KH28 demonstrated an oral activity 3-fold greater than any other analogue; hence it was further characterized in a series of rat behavioral assays. KH28 attenuated d-amphetamine induced hyperlocomotion, a hallmark of current clinically effective antipsychotic drugs, after both IP and oral administration. In addition, tolerance to the compound did not develop after repeated daily dosing, as measured by hypothermic induction as well as attenuation of d-amphetamine induced hyperlocomotion. Finally, KH28 did not produce catalepsy, a deleterious side-effect elicited by classical antipsychotic drugs. KH28 is considered to be an ideal compound for further development as a potential novel antipsychotic.

Investigation of Ligand-Receptor Systems by High-Resolution Solid-State NMR: Recent Progress and Perspectives

Solid-state Nuclear Magnetic Resonance (NMR) provides a general method to study molecular structure and dynamics in a non-crystalline and insoluble environment. We discuss the latest methodological progress to construct 3D molecular structures from solid-state NMR data obtained under magic-angle-spinning conditions. As shown for the neurotensin/NTS-1 system, these methods can be readily applied to the investigation of ligand-binding to G-protein coupled receptors.