Synthesis and human neurotensin receptor binding activities of neurotensin(8-13) analogues containing position 8 alpha-azido-N-alkylated derivatives of ornithine, lysine, and homolysine

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.

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).

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.

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.

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

The neurotensin C-terminal hexapeptide, NT(8-13), which has been found to adopt a beta-strand-like conformation while bound to the NT1 receptor, was modified by the introduction of conformational constraints. Synthesis of the four stereoisomeric 4.4-spirolactams 1-4 and subsequent NT1 receptor binding studies showed that the restriction of psi(Pro10) to approximately 130 degrees leads to a more than 1000-fold increase of binding affinity for 1 (Ki = 12 nM) when compared to the more flexible analogue [NMeTyr11]NT(8-13).

Novel bioactive and stable neurotensin peptide analogues capable of delivering radiopharmaceuticals and molecular beacons to tumors

The prevalence of neurotensin receptor (NTR) in several human tumors makes it an attractive target for the delivery of cytotoxic drugs and imaging agents. Native neurotensin (NT) is a tridecapeptide that binds to NTR and induces tumor growth. Unfortunately, NT has a short plasma half-life, which hinders its use for in vivo biomedical applications. Numerous reports suggest that Arg(8)-Arg(9) and Tyr(11)-Ile(12) amide bonds are particularly susceptible to degradation by proteolytic enzymes. Predicated on this observation, we substituted Arg(8), Arg(9), and Ile(12) amino acids with the corresponding commercially available mimics. These surrogate amino acids are amenable to standard Fmoc peptide synthesis strategy, and the resulting compounds are stable in biological media for >4 h and bind to NTR with high affinity. Furthermore, conjugating DTPA to the new peptides and subsequent labeling with (111)In-DTPA for nuclear imaging or fluorescein for optical imaging did not diminish the NTR binding affinities of the peptides. In vivo biodistribution of a representative (111)In-DTPA-NT peptide analogue in SCID mice bearing NTR-positive human adenocarcinoma (HT29) xenograft shows that the compound was primarily retained in tumor tissue (2.2% ID/g) and the kidneys (4.8% ID/g) at 4 h postinjection. Coinjection of cold NT and the radiolabeled NT peptide analogue inhibited the tumor but not the kidney uptake, demonstrating that retention of the radiolabeled compound in tumor tissue was mediated by NTR specific uptake while it accumulates in the kidneys by a nonspecific mechanism. These findings show that the new NT peptide analogues are robust and can deliver imaging agents to NTR-positive tumors such as pancreatic cancer.

Monitoring neurotensin[8-13] degradation in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A method was developed to quantify neurotensin (NT) fragment [8-13] and a novel NT[8-13] derivative, KK1, in human and rat serum utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The method allows for simultaneous quantification of the major NT[8-13] metabolite, NT[9-13] (according to molecular mass), and detection of the major KK1 metabolite, KK1M (according to molecular mass). The degradation rates of NT[8-13] and KK1 were calculated to be 24.1+/-1.0 and 193+/-8min in human serum and 5.90+/-0.22 and 153+/-4min in rat serum, respectively. The method utilizes a novel sample drying technique and spectrum acquisition protocol. In addition, an internal standard dissimilar in structure to the analytes was used. This method may be broadly applicable to the quantification of NT[8-13] and other peptide analogues of varying structure.

Chiral tricyclic iminolactone derived from (1R)-(+)-camphor as a glycine equivalent for the asymmetric synthesis of alpha-amino acids

The development of a highly efficient and stereoselective methodology for the preparation of alpha-amino acids is described. The chiral template, tricyclic iminolactone 7, was synthesized from (1R)-(+)-camphor in five steps in 50% overall yield. Alkylation of iminolactone 7 afforded the alpha-monosubstituted products in good yields (74-96%) and excellent diastereoselectivities (>98%). Hydrolysis of the alkylated iminolactones furnished the desired alpha-amino acids in good yields and enantioselectivities with nearly quantitative recovery of the chiral auxiliary 4.

Topography of the neurotensin (NT)(8-9) binding site of human NT receptor-1 probed with NT(8-13) analogs

A series of neurotensin (NT)(8-13) analogs featuring substitution of the Arg8 and/or Arg9 residues with non-natural cationic amino acids was synthesized and evaluated for binding to the human NT receptor-1 (hNTR-1). The modifications were designed to probe specific steric and electrostatic requirements in the N-terminal cationic region of NT(8-13) for receptor binding as a general evaluation of the feasibility of incorporating minor structural changes into a peptide at a crucial polar receptor binding site. Many of the non-natural amino acids are more or less isosteric to Arg but more lipophilic as a result of addition of alkyl groups or through removal or replacement of NH character with methylene or methyl substituents, whereas others vary the distance between the cation and the alpha-amino acid carbon. Substitution of Arg8 with N(G)-alkylated Arg derivatives or homolysine (Hlys) maintained the subnanomolar affinity of NT(8-13) to the hNTR-1. Position 8 incorporation of Hlys produced the most favorable primary amine side-chain substitution to date. Moderate losses in affinity observed with position 9 substitutions were attributed to adverse steric effects. Doubly substituted [Hlys8, DAB9]NT(8-13), in which DAB is 2,4-diaminobutyric acid, was also prepared and tested as the shorter side-chain of DAB is known to be favored in position 9 of NT(8-13). This analog maintained 60% of NT(8-13) binding affinity making it the most favored des-guanidinium-containing analog known. These results demonstrate that adequate receptor binding affinity can be maintained over a structural range of Arg analogs, thus providing a range of peptides expected to exhibit altered pharmacokinetic properties. From the standpoint of the hNTR-1 cationic binding sites, these results help to map out the structural stringency inherent in the formation of a tight binding complex with NT(8-13) and related analogs.

Selectivity enhancement induced by substitution of non-natural analogues of arginine and lysine in arginine-based thrombin inhibitors

Seven non-natural analogues of arginine and lysine have been substituted in an established arginine-based thrombin inhibitor. Four of the new compounds exhibited significant thrombin inhibition (K(i)'s 0.53-3.95 microM) and were subsequently tested for selectivity against trypsin. The two best compounds gave selectivity ratios of 962 and 525 (trypsin/thrombin), improving upon the parent compound.

Synthesis of neurotensin(9-13) analogues exhibiting enhanced human neurotensin receptor binding affinities

Recent evidence is consistent with neurotensin (NT)(8-13) adopting a Type I beta-turn conformation while binding the NT receptor, which would place the cationic side-chains of Arg(8) and Arg(9) in close proximity. This was the basis for the design, synthesis and analysis of truncated NT(9-13) analogues 1-5 with dicationic position 9 side-chains to emulate the functions of the 8 and 9 side-chains of NT(8-13).