Guanidine-acylguanidine bioisosteric approach in the design of radioligands: synthesis of a tritium-labeled N(G)-propionylargininamide ([3H]-UR-MK114) as a highly potent and selective neuropeptide Y Y1 receptor antagonist

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.

Guanidine-to-piperidine switch affords high affinity small molecule NPFF ligands with preference for NPFF1-R and NPFF2-R subtypes

The Neuropeptide FF (NPFF) receptor system is known to modulate opioid actions and has been shown to mediate opioid-induced hyperalgesia and tolerance. The lack of subtype selective small molecule compounds has hampered further exploration of the pharmacology of this receptor system. The vast majority of available NPFF ligands possess a highly basic guanidine group, including our lead small molecule, MES304. Despite providing strong receptor binding, the guanidine group presents a potential pharmacokinetic liability for in vivo pharmacological tool development. Through structure-activity relationship exploration, we were able to modify our lead molecule MES304 to arrive at guanidine-free NPFF ligands. The novel piperidine analogues 8b and 16a are among the few non-guanidine based NPFF ligands known in literature. Both compounds displayed nanomolar NPFF-R binding affinity approaching that of the parent molecule. Moreover, while MES304 was non-subtype selective, these two analogues presented new starting points for subtype selective scaffolds, whereby 8b displayed a 15-fold preference for NPFF1-R, and 16a demonstrated an 8-fold preference for NPFF2-R. Both analogues showed no agonist activity on either receptor subtype in the in vitro functional activity assay, while 8b displayed antagonistic properties at NPFF1-R. The calculated physicochemical properties of 8b and 16a were also shown to be more favorable for in vivo tool design. These results indicate the possibility of developing potent, subtype selective NPFF ligands devoid of a guanidine functionality.

Discovery of a Tritiated Radioligand with High Affinity and Selectivity for the Histamine H3 Receptor

Radioligands used previously for histamine H3 receptor (H3R) are accompanied by a number of disadvantages. In this study, we report the synthesis of the new H3R radioligand [3H]UR-MN259 ([3H]11) with high (radio)chemical purity and stability. The radioligand exhibits sub-nanomolar affinity for the target receptor (pKi (H3R) = 9.56) and displays an outstanding selectivity profile within the histamine receptor family (>100,000-fold selective). [3H]UR-MN259 is ideally suitable for the characterization of H3R ligands in competition binding and shows one-site binding to the H3R in saturation binding experiments. The radiotracer shows fast association to the receptor (τassoc = 6.11 min), as well as full dissociation from the receptor (τdissoc = 14.48 min) in kinetic binding studies. The distinguished profile of [3H]UR-MN259 makes it a highly promising pharmacological tool to further investigate the role of the H3R in the CNS.

Structure-Based Design of High-Affinity Fluorescent Probes for the Neuropeptide Y Y1 Receptor

The recent crystallization of the neuropeptide Y Y₁ receptor (Y₁R) in complex with the argininamide-type Y₁R selective antagonist UR-MK299 (2) opened up a new approach toward structure-based design of nonpeptidic Y₁R ligands. We designed novel fluorescent probes showing excellent Y₁R selectivity and, in contrast to previously described fluorescent Y₁R ligands, considerably higher (∼100-fold) binding affinity. This was achieved through the attachment of different fluorescent dyes to the diphenylacetyl moiety in 2 via an amine-functionalized linker. The fluorescent ligands exhibited picomolar Y₁R binding affinities (pK_i values of 9.36-9.95) and proved to be Y₁R antagonists, as validated in a Fura-2 calcium assay. The versatile applicability of the probes as tool compounds was demonstrated by flow cytometry- and fluorescence anisotropy-based Y₁R binding studies (saturation and competition binding and association and dissociation kinetics) as well as by widefield and total internal reflection fluorescence (TIRF) microscopy of live tumor cells, revealing that fluorescence was mainly localized at the plasma membrane.

Synthesis of 2-[2-(tert-butoxycarbonyl)-3-(acyl)guanidino]ethylamine salts for convergent introduction of acyl guanidines

A series of acylguanidines with pre-installed ethylamino linkers are described that can be incorporated into larger structures through amide coupling.

Biased agonists at the human Y1 receptor lead to prolonged membrane residency and extended receptor G protein interaction

Functionally selective ligands to address specific cellular responses downstream of G protein-coupled receptors (GPCR) open up new possibilities for therapeutics. We designed and characterized novel subtype- and pathway-selective ligands. Substitution of position Q34 of neuropeptide Y to glycine (G34-NPY) results in unprecedented selectivity over all other YR subtypes. Moreover, this ligand displays a significant bias towards activation of the Gi/o pathway over recruitment of arrestin-3. Notably, no bias is observed for an established Y1R versus Y2R selective ligand carrying a proline at position 34 (F7,P34-NPY). Next, we investigated the spatio-temporal signaling at the Y1R and demonstrated that G protein-biased ligands promote a prolonged localization at the cell membrane, which leads to enhanced G protein signaling, while endosomal receptors do not contribute to cAMP signaling. Thus, spatial components are critical for the signaling of the Y1R that can be modulated by tailored ligands and represent a novel mode for biased pathways.

Determination of neuropeptide Y Y1 receptor antagonist BIBP 3226 and evaluation of receptor expression based on liquid chromatography coupled with tandem mass spectrometry

Neuropeptide Y (NPY) is a peptide widely distributed throughout the body that is involved in various physiological processes, including the regulation of feeding behavior and energy homeostasis. 5-Carbamimidamido-2-(2,2-diphenylacetamido)-N-[(4-hydroxyphenyl)methyl]pentanamide (BIBP 3226) is a selective NPY Y1 receptor antagonist with recognized application in bone regeneration studies, requiring quantification at picogram levels. Hence, BIBP 3226 determination is proposed here by a validated HPLC-MS/MS method, based on a reversed-phase Kinetex® core-shell C8 column (2.6 μm, 150 × 2.1 mm) at 30 °C, elution in isocratic mode using a mixture of acetonitrile and water (30:70, v/v), containing 0.1% (v/v) formic acid, at 0.25 mL min^-1, detection in positive ionization mode, and data acquisition in selected reaction monitoring mode. Calibration curves were linear for concentrations ranging from 0.25 to 30 ng mL^-1 with LOD and LOQ values as low as 0.1 and 0.3 pg in cell extracts and 16 and 48 pg in supernatant culture media, respectively. BIBP 3226 was successfully determined in cell extracts and supernatants obtained from internalization assays. Using similar exposure conditions, the amount of BIBP 3226 found in breast cancer cells (MCF7) was 72 to 657 times higher than that found in bone marrow cells (Wt C57BL/6 mice), providing an indirect indicator of NPY Y1 receptor expression.

Argininamide-type neuropeptide Y Y1 receptor antagonists: the nature of N ω-carbamoyl substituents determines Y1R binding mode and affinity

The recently resolved crystal structure of the neuropeptide Y Y1 receptor (Y1R), co-crystallized with the high-affinity (pK i: 10.11), argininamide-type Y1R antagonist UR-MK299 (2), revealed that the N ω-carbamoyl substituent (van der Waals volume: 139 Å3) is deeply buried in the receptor, occupying a hydrophobic pocket. We synthesized and characterized a series of argininamides, structurally related to 2. Y1R affinity decreased with increasing size of the carbamoyl residue (minimal pK i: 5.67). Exceeding a critical size of the substituent (van der Waals volume: 212 Å3), the ligands bound in an inverted mode with the carbamoyl side chain located at the surface of the receptor, as suggested by induced-fit docking and MD simulations.

18F-labelled triazolyl-linked argininamides targeting the neuropeptide Y Y1R for PET imaging of mammary carcinoma

Neuropeptide Y Y₁ receptors (Y₁R) have been found to be overexpressed in a number of different tumours, such as breast, ovarian or renal cell cancer. In mammary carcinoma the high Y₁R density together with its high incidence of 85% in primary human breast cancers and 100% in breast cancer derived lymph node metastases attracted special attention. Therefore, the aim of this study was the development of radioligands for Y₁R imaging by positron emission tomography (PET) with a special emphasis on imaging agents with reduced lipophilicity to provide a PET ligand with improved biodistribution in comparison with previously published tracers targeting the Y₁R. Three new radioligands based on BIBP3226, bearing an ¹⁸F-fluoroethoxy linker (12), an ¹⁸F-PEG-linker (13) or an ¹⁸F-fluoroglycosyl moiety (11) were radiosynthesised in high radioactivity yields. The new radioligands displayed Y₁R affinities of 2.8 nM (12), 29 nM (13) and 208 nM (11) and were characterised in vitro regarding binding to human breast cancer MCF-7-Y1 cells and slices of tumour xenografts. In vivo, small animal PET studies were conducted in nude mice bearing MCF-7-Y1 tumours. The binding to tumours, solid tumour slices and tumour cells correlated well with the Y₁R affinities. Although 12 and 13 showed displaceable and specific binding to Y₁R in vitro and in vivo, the radioligands still need to be optimised to achieve higher tumour-to-background ratios for Y₁R imaging by PET. Yet the present study is another step towards an optimized PET radioligand for imaging of Y₁R in vivo.

Discovery of 4,6-bis(2-((E)-benzylidene)hydrazinyl)pyrimidin-2-Amine with Antibiotic Activity

Robenidine (E)-N'-((E)-1-(4-chlorophenyl)ethylidene)-2-(1-(4-chlorophenyl)ethylidene)hydrazine-1-carboximidhydrazide displays methicillin-resistant Staphyoccoccus aureus (MRSA) and vancomycin-resistant Enterococci (VRE) MICs of 2 μg mL-1. Herein we describe the structure-activity relationship development of a novel series of guanidine to 2-aminopyrimidine isosteres that ameliorate the low levels of mammalian cytotoxicity in the lead compound while retaining good antibiotic activity. Removal of the 2-NH2 pyrimidine moiety renders these analogues inactive. Introduction of a central 2-NH2 triazine moiety saw a 10-fold activity reduction. Phenyl to cyclohexyl isosteres were inactive. The 4-BrPh and 4-CH3Ph with MIC values of 2 and 4 μg mL-1, against MRSA and VRE respectively, are promising candidates for future development.

Initial Characterization of Transgenic Mice Overexpressing Human Histamine H2 Receptors

In an integrative approach, we studied the role of histamine H₂ receptors in the mouse heart. We noted that histamine, added cumulatively to the organ bath, failed to affect the force of contraction in left atrial preparations and did not change spontaneous heart rate in right atrial preparations from wild-type mice. By contrast, in the same preparations from mice that overexpressed the human H₂ receptor in a cardiac-specific way, histamine exerted concentration- and time-dependent positive inotropic and positive chronotropic effects. Messenger RNA of the human H₂ receptor was only detected in transgenic mice. Likewise, immunohistology and autoradiography only gave signals in transgenic but not in wild-type cardiac preparations. Similarly, a positive inotropic and positive chronotropic effect was observed with histamine in echocardiography of living transgenic mice and isolated perfused hearts (Langendorff preparation). Phosphorylation of phospholamban was increased in atrial and ventricular preparations from transgenic mice, but not in wild-type animals. The effects of histamine were mimicked by dimaprit and amthamine and antagonized by cimetidine. In summary, we generated a new model to study the physiologic and pathophysiologic cardiac role of the human H₂ receptor.

Guanidine cyclic diimides and their polymers

We report the formation and degradation of a unique guanidine cyclic diimide (GCDI) structure under mild conditions. Furthermore, the GCDI-based polymers can be readily synthesized from guanidine and dianhydride monomers.

Photo- and dioxygen-enabled radical C(sp3)–N(sp2) cross-coupling between guanidines and perfluoroalkyl iodides

The utilization of visible light and dioxygen (air) as green and sustainable mediators in photocatalyzed synthetic chemistry is demonstrated.

Structural basis of ligand binding modes at the neuropeptide Y Y1 receptor

Neuropeptide Y (NPY) receptors belong to the G-protein-coupled receptor superfamily and have important roles in food intake, anxiety and cancer biology 1,2 . The NPY-Y receptor system has emerged as one of the most complex networks with three peptide ligands (NPY, peptide YY and pancreatic polypeptide) binding to four receptors in most mammals, namely the Y1, Y2, Y4 and Y5 receptors, with different affinity and selectivity 3 . NPY is the most powerful stimulant of food intake and this effect is primarily mediated by the Y1 receptor (Y1R) 4 . A number of peptides and small-molecule compounds have been characterized as Y1R antagonists and have shown clinical potential in the treatment of obesity 4 , tumour 1 and bone loss 5 . However, their clinical usage has been hampered by low potency and selectivity, poor brain penetration ability or lack of oral bioavailability 6 . Here we report crystal structures of the human Y1R bound to the two selective antagonists UR-MK299 and BMS-193885 at 2.7 and 3.0 Å resolution, respectively. The structures combined with mutagenesis studies reveal the binding modes of Y1R to several structurally diverse antagonists and the determinants of ligand selectivity. The Y1R structure and molecular docking of the endogenous agonist NPY, together with nuclear magnetic resonance, photo-crosslinking and functional studies, provide insights into the binding behaviour of the agonist and for the first time, to our knowledge, determine the interaction of its N terminus with the receptor. These insights into Y1R can enable structure-based drug discovery that targets NPY receptors.

Heterodimerization of Dibenzodiazepinone-Type Muscarinic Acetylcholine Receptor Ligands Leads to Increased M2R Affinity and Selectivity

In search for selective ligands for the muscarinic acetylcholine receptor (MR) subtype M₂, the dimeric ligand approach, that is combining two pharmacophores in one and the same molecule, was pursued. Different types (agonists, antagonists, orthosteric, and allosteric) of monomeric MR ligands were combined by various linkers with a dibenzodiazepinone-type MR antagonist, affording five types of heterodimeric compounds ("DIBA-xanomeline," "DIBA-TBPB," "DIBA-77-LH-28-1," "DIBA-propantheline," and "DIBA-4-DAMP"), which showed high M₂R affinities (pK_i > 8.3). The heterodimeric ligand UR-SK75 (46) exhibited the highest M₂R affinity and selectivity [pK_i (M₁R-M₅R): 8.84, 10.14, 7.88, 8.59, and 7.47]. Two tritium-labeled dimeric derivatives ("DIBA-xanomeline"-type: [³H]UR-SK71 ([³H]44) and "DIBA-TBPB"-type: [³H]UR-SK59 ([³H]64)) were prepared to investigate their binding modes at hM₂R. Saturation-binding experiments showed that these compounds address the orthosteric binding site of the M₂R. The investigation of the effect of various allosteric MR modulators [gallamine (13), W84 (14), and LY2119620 (15)] on the equilibrium (13-15) or saturation (14) binding of [³H]64 suggested a competitive mechanism between [³H]64 and the investigated allosteric ligands, and consequently a dualsteric binding mode of 64 at the M₂R.

Marine Terpenoid Diacylguanidines: Structure, Synthesis, and Biological Evaluation of Naturally Occurring Actinofide and Synthetic Analogues

A new diacylguanidine, actinofide (1), has been isolated from the marine mollusk Actinocyclus papillatus. The structure, exhibiting a guanidine moiety acylated by two terpenoid acid units, has been established by spectroscopic methods and secured by synthesis. Following this, a series of structural analogues have been synthesized using the same procedure. All of the compounds have been evaluated in vitro for the growth inhibitory activity against a variety of cancer cell lines.

Prototypic 18F-Labeled Argininamide-Type Neuropeptide Y Y1R Antagonists as Tracers for PET Imaging of Mammary Carcinoma

The neuropeptide Y (NPY) Y₁ receptor (Y₁R) selective radioligand (R)-N^α-(2,2-diphenylacetyl)-N^ω-[4-(2-[¹⁸F]fluoropropanoylamino)butyl]aminocarbonyl-N-(4-hydroxybenzyl)argininamide ([¹⁸F]23), derived from the high-affinity Y₁R antagonist BIBP3226, was developed for imaging studies of Y₁R-positive tumors. Starting from the argininamide core bearing amine-functionalized spacer moieties, a series of fluoropropanoylated and fluorobenzoylated derivatives was synthesized and studied for Y₁R affinity. The fluoropropanoylated derivative 23 displayed high affinity (K_i = 1.3 nM) and selectivity toward Y₁R. Radiosynthesis was accomplished via ¹⁸F-fluoropropanoylation, yielding [¹⁸F]23 with excellent stability in mice; however, the biodistribution study revealed pronounced hepatobiliary clearance with high accumulation in the gall bladder (>100 %ID/g). Despite the unfavorable biodistribution, [¹⁸F]23 was successfully used for imaging of Y₁R positive MCF-7 tumors in nude mice. Therefore, we suggest [¹⁸F]23 as a lead for the design of PET ligands with optimized physicochemical properties resulting in more favorable biodistribution and higher Y₁R-dependent enrichment in mammary carcinoma.

Mimicking of Arginine by Functionalized N(ω)-Carbamoylated Arginine As a New Broadly Applicable Approach to Labeled Bioactive Peptides: High Affinity Angiotensin, Neuropeptide Y, Neuropeptide FF, and Neurotensin Receptor Ligands As Examples

Derivatization of biologically active peptides by conjugation with fluorophores or radionuclide-bearing moieties is an effective and commonly used approach to prepare molecular tools and diagnostic agents. Whereas lysine, cysteine, and N-terminal amino acids have been mostly used for peptide conjugation, we describe a new, widely applicable approach to peptide conjugation based on the nonclassical bioisosteric replacement of the guanidine group in arginine by a functionalized carbamoylguanidine moiety. Four arginine-containing peptide receptor ligands (angiotensin II, neurotensin(8-13), an analogue of the C-terminal pentapeptide of neuropeptide Y, and a neuropeptide FF analogue) were subject of this proof-of-concept study. The N(ω)-carbamoylated arginines, bearing spacers with a terminal amino group, were incorporated into the peptides by standard Fmoc solid phase peptide synthesis. The synthesized chemically stable peptide derivatives showed high receptor affinities with Ki values in the low nanomolar range, even when bulky fluorophores had been attached. Two new tritiated tracers for angiotensin and neurotensin receptors are described.

N(ω)-Carbamoylation of the Argininamide Moiety: An Avenue to Insurmountable NPY Y1 Receptor Antagonists and a Radiolabeled Selective High-Affinity Molecular Tool ([(3)H]UR-MK299) with Extended Residence Time

Analogues of the argininamide-type NPY Y1 receptor (Y1R) antagonist BIBP3226, bearing carbamoyl moieties at the guanidine group, revealed subnanomolar Ki values and caused depression of the maximal response to NPY (calcium assay) by up to 90% in a concentration- and time-dependent manner, suggesting insurmountable antagonism. To gain insight into the mechanism of binding of the synthesized compounds, a tritiated antagonist, (R)-N(α)-diphenylacetyl-N(ω)-[2-([2,3-(3)H]propionylamino)ethyl]aminocarbonyl-(4-hydroxybenzyl)arginin-amide ([(3)H]UR-MK299, [(3)H]38), was prepared. [(3)H]38 revealed a dissociation constant in the picomolar range (Kd 0.044 nM, SK-N-MC cells) and very high Y1R selectivity. Apart from superior affinity, a considerably lower target off-rate (t1/2 95 min) was characteristic of [(3)H]38 compared to that of the higher homologue containing a tetramethylene instead of an ethylene spacer (t1/2 3 min, Kd 2.0 nM). Y1R binding of [(3)H]38 was fully reversible and fully displaceable by nonpeptide antagonists and the agonist pNPY. Therefore, the insurmountable antagonism observed in the functional assay has to be attributed to the extended target-residence time, a phenomenon of relevance in drug research beyond the NPY receptor field.

Synthesis and in vitro and in vivo evaluation of an (18)F-labeled neuropeptide Y analogue for imaging of breast cancer by PET

Imaging of Y1R expression in breast cancer is still a challenging task. Herein, we report a suitable (18)F-labeled high-molecular-weight glycopeptide for imaging of peripheral neuropeptide Y (NPY) Y1 receptor (Y1R)-positive tumors by preclinical small-animal positron emission tomography (PET). The Y1R-preferring NPY [F(7),P(34)]NPY analogue was functionalized with an alkyne-bearing propargylglycine (Pra) in position 4. The corresponding fluoroglycosylated (FGlc) peptide analogue [Pra(4)(FGlc),F(7),P(34)]NPY and its (18)F-labeled analogue were synthesized by click chemistry-based fluoroglycosylation. The radiosynthesis was performed by (18)F-fluoroglycosylation starting from the 2-triflate of the β-mannosylazide and the alkyne peptide [Pra(4),F(7),P(34)]NPY. The radiosynthesis of the(18)F-labeled analogue was optimized using a minimum amount of peptide precursor (40 nmol), proceeding with an overall radiochemical yield of 20-25% (nondecay corrected) in a total synthesis time of 75 min with specific activities of 40-70 GBq/μmol. In comparison to NPY and [F(7),P(34)]NPY, in vitro Y1R and Y2R activation studies with the cold [Pra(4)(FGlc),F(7),P(34)]NPY on stably transfected COS-7 cells displayed a high potency for the induction of Y1R-specific inositol accumulation (pEC50 = 8.5 ± 0.1), whereas the potency at Y2R was significantly decreased. Internalization studies on stably transfected HEK293 cells confirmed a strong glycopeptide-mediated Y1R internalization and a substantial Y1R subtype selectivity over Y2R. In vitro autoradiography with Y1R-positive MCF-7 tumor tissue slices indicated high specific binding of the (18)F-labeled glycopeptide, when binding was reduced by 95% ([Pra(4),F(7),P(34)]NPY) and by 86% (BIBP3226 Y1R antagonist) in competition studies. Biodistribution and small-animal PET studies on MCF-7 breast tumor-bearing nude mice revealed radiotracer uptake in the MCF-7 tumor of 1.8%ID/g at 20 min p.i. and 0.7%ID/g at 120 min p.i. (n = 3-4), increasing tumor-to-blood ratios from 1.2 to 2.4, and a tumor retention of 76 ± 4% (n = 4; 45-90 min p.i.). PET imaging studies with MCF-7 tumor-bearing nude mice demonstrated uptake of the (18)F-labeled glycopeptide in the tumor region at 60 min p.i., whereas only negligible tumor uptake was observed in animals injected with a nonbinding (18)F-labeled glycopeptide pendant as a measure of nonspecific binding. In conclusion, PET imaging experiments with the (18)F-labeled NPY glycopeptide revealed Y1R-specific binding uptake in MCF-7 tumors in vivo together with decreased kidney uptake compared to DOTA-derivatives of this peptide. We consider this glycopeptide to be a potent lead peptide for the design of improved (18)F-glycopeptides with shorter amino acid sequences that would further facilitate PET imaging studies of Y1R-positive breast tumors.

Unprecedented one-pot chemocontrolled entry to thioxoimidazolidinones and aminoimidazolones: synthesis of kinase inhibitor leucettamine B

A novel and highly chemoselective protocol for the construction of thioxoimidazolidinone and aminoimidazolone frameworks was explored, and the influence of the reaction conditions on product formation was studied to establish two distinct approaches for their selective formation. In this one-pot reaction, ambient temperature generally resulted in the formation of thioxoimidazolidinones, whereas microwave irradiation provided aminoimidazolones exclusively. An attempt to elucidate the observed chemoselectivity is described, and the products were confirmed by X-ray studies. One-pot synthesis toward Leucettamine B, a marine alkaloid, was achieved on the basis of this protocol.

Direct arginine modification in native peptides and application to chemical probe development

An efficient method for the direct labeling of the Arg guanidinium group in native peptides is reported. This straightforward procedure allows modifying the arginine moiety in peptides with various reporter groups, such as fluorophores, biotin, etc., under mild conditions in an operationally simple procedure. The scope of this method tolerates various functionalized amino acids such as His, Ser, Trp, Tyr, Glu, etc., while the only limitations uncovered so far are restricted to cysteine and free amine residues. The utility of this late-stage diversification method was demonstrated in direct labeling of leuprolide, a clinically used drug, for distribution monitoring in Daphnia, and in labeling of microcystin, a cyanobacterial toxin.

[(3) H]UR-DE257: development of a tritium-labeled squaramide-type selective histamine H2 receptor antagonist

A series of new piperidinomethylphenoxypropylamine-type histamine H2 receptor (H2 R) antagonists with different substituted "urea equivalents" was synthesized and characterized in functional in vitro assays. Based on these data as selection criteria, radiosynthesis of N-[6-(3,4-dioxo-2-{3-[3-(piperidin-1-ylmethyl)phenoxy]propylamino}cyclobut-1-enylamino)hexyl]-(2,3-(3) H2 )propionic amide ([(3) H]UR-DE257) was performed. The radioligand (specific activity: 63 Ci mmol(-1) ) had high affinity for human, rat, and guinea pig H2 R (hH2 R, Sf9 cells: Kd , saturation binding: 31 nM, kinetic studies: 20 nM). UR-DE257 revealed high H2 R selectivity on membranes of Sf9 cells, expressing the respective hHx R subtype (Ki values: hH1 R: >10000 nM, hH2 R: 28 nM, hH3 R: 3800 nM, hH4 R: >10000 nM). In spite of insurmountable antagonism, probably due to rebinding of [(3) H]UR-DE257 to the H2 R (extended residence time), the title compound proved to be a valuable pharmacological tool for the determination of H2 R affinities in competition binding assays.

Novel acylureidoindolin-2-one derivatives as dual Aurora B/FLT3 inhibitors for the treatment of acute myeloid leukemia

A series of 6-acylureido derivatives containing a 3-(pyrrol-2-ylmethylidene)indolin-2-one scaffold were synthesized as potential dual Aurora B/FLT3 inhibitors by replacing the 6-arylureido moiety in 6-arylureidoindolin-2-one-based multi-kinase inhibitors. (Z)-N-(2-(pyrrolidin-1-yl)ethyl)-5-((6-(3-(2-fluoro-4-methoxybenzoyl)ureido)-2-oxoindolin-3-ylidene)methyl)-2,4-dimethyl-1H-pyrrole-3-carboxamide (54) was identified as a dual Aurora B/FLT3 inhibitor (IC50 = 0.4 nM and 0.5 nM, respectively). Compound 54 also exhibited potent cytotoxicity with single-digit nanomolar IC50 values against the FLT3 mutant-associated human acute myeloid leukemia (AML) cell lines MV4-11 (FLT3-ITD) and MOLM-13 (FLT3-ITD). Compound 54 also specifically induced extrinsic apoptosis by inhibiting the phosphorylation of the Aurora B and FLT3 pathways in MOLM-13 cells. Compound 54 had a moderate pharmacokinetic profile. The mesylate salt of 54 efficiently inhibited tumor growth and reduced the mortality of BALB/c nude mice (subcutaneous xenograft model) that had been implanted with AML MOLM-13 cells. Compound 54 is more potent than sunitinib not only against FLT3-WT AML cells but also active against sunitinib-resistant FLT3-ITD AML cells. This study demonstrates the significance of dual Aurora B/FLT3 inhibitors for the development of potential agents to treat AML.

A mild method for the synthesis of carbamate-protected guanidines using the Burgess reagent

A simple method for the synthesis of carbamate-protected guanidines from primary amines is described. A variety of thioureas derived from primary amines and isothiocyanates react with the Burgess reagent to give the corresponding guanidines via either a stepwise or one-pot procedure. By tuning the carbamoyl units of isothiocyanates and the Burgess reagent, differentially N,N'-diprotected guanidines can be obtained. Selective deprotection of the products affords N-monoprotected guanidines.

Novel approaches to screening guanidine derivatives

INTRODUCTION

Compounds containing guanidine moiety, originating both from natural and synthetic sources, have found potential applications in both synthetic and medicinal chemistry. Indeed, guanidine functionality can be found in many natural and pharmaceutical products as well as in cosmetic ingredients produced by synthetic methods.

AREAS COVERED

This review covers the latest developments in the research undertaken for the therapeutic application of newly synthesized guanidine derivatives including: small peptides and peptidomimetics. This article encompasses the selected literature published in the last three decades with a focus on the novel approaches for screening of lead drug candidates with their pharmacological action.

EXPERT OPINION

Guanidines, as they are both organically based and also hydrophilic in nature, have undergone a mammoth amount of screening and testing to discover promising lead structures with a CN3 core, appropriate for potential future drug development. The compounds have the potential to be neurodegenerative therapeutic options, as well as: anti-inflammatory, anti-protozoal, anti-HIV, chemotherapeutic, anti-diabetic agents and so on. It is true that guanidine-based compounds of natural sources also, like synthetic and virtually designed drugs, have been of significant interest and have the potential to be useful therapeutic options in the future. As for now, however, there is not sufficient data to support their use in a number of the suggested areas, and further studies are required.

The neuropeptide y y(1) receptor: a diagnostic marker? Expression in mcf-7 breast cancer cells is down-regulated by antiestrogens in vitro and in xenografts

The neuropeptide Y (NPY) Y(1) receptor (Y(1)R) has been suggested as a tumor marker for in vivo imaging and as a therapeutic target. In view of the assumed link between estrogen receptor (ER) and Y(1)R in mammary carcinoma and with respect to the development of new diagnostic tools, we investigated the Y(1)R protein expression in human MCF-7 cell variants differing in ER content and sensitivity against antiestrogens. ER and Y(1)R expression were quantified by radioligand binding using [(3)H]-17β-estradiol and the Y(1)R selective antagonist [(3)H]-UR-MK114, respectively. The latter was used for cellular binding studies and for autoradiography of MCF-7 xenografts. The fluorescent ligands Cy5-pNPY (universal Y(1)R, Y(2)R and Y(5)R agonist) and UR-MK22 (selective Y(1)R antagonist), as well as the selective antagonists BIBP3226 (Y(1)R), BIIE0246 (Y(2)R) and CGP71683 (Y(5)R) were used to identify the NPY receptor subtype(s) by confocal microscopy. Y(1)R functionality was determined by mobilization of intracellular Ca(2+). Sensitivity of MCF-7 cells against antiestrogen 4-hydroxytamoxifen correlated directly with the ER content. The exclusive expression of Y(1)Rs was confirmed by confocal microscopy. The Y(1)R protein was up-regulated (100%) by 17β-estradiol (EC(50) 20 pM) and the predominant role of ERα was demonstrated by using the ERα-selective agonist "propylpyrazole triol". 17β-Estradiol-induced over-expression of functional Y(1)R protein was reverted by the antiestrogen fulvestrant (IC(50) 5 nM) in vitro. Furthermore, tamoxifen treatment of nude mice resulted in an almost total loss of Y(1)Rs in MCF-7 xenografts. In conclusion, the value of the Y(1)R as a target for therapy and imaging in breast cancer patients may be compromised due to Y(1)R down-regulation induced by hormonal (antiestrogen) treatment.

Synthesis and characterization of DMAP-modified NPY Y1 receptor antagonists as acyl-transfer catalysts

Starting from the working hypothesis that specific chemical labelling may be an attractive approach to detect and study G protein-coupled receptors (GPCRs) we synthesized catalytically active antagonists of the neuropeptide Y1 receptor (Y1R). An argininamide-type Y1R antagonist scaffold was combined with a DMAP moiety via spacers of different length and chemical nature. These hybrid compounds have Y1R affinities in the two-digit nanomolar range and are capable of catalysing acyl-transfer reaction to surrogates of bionucleophiles, as demonstrated in the absence of cells by using esters of fluorescent dyes as substrates in buffer. By contrast, selective staining of Y1Rs on living MCF-7 cells was not achieved due to significant non-catalysed (Y1R ligand independent) reaction with biomolecules and the limited density of Y1R on the cell surface. Although this may also depend on insufficient selectivity of the staining reagents, the results of this study suggest that the general applicability of catalytic staining to GPCRs has to be reconsidered, as this approach is hampered by a very low portion of receptor of interest compared to the total amount of membrane proteins.

Chemical shift assignment and conformational analysis of monoalkylated acylguanidines

Monoalkylated acylguanidines are important functional groups in many biologically active compounds and additionally applied in coordination chemistry. Yet a straightforward assignment of the individual NH chemical shifts and the acylguanidine conformations is still missing. Therefore, in this study, NMR spectroscopic approaches for the chemical and especially the conformational assignment of protonated monoalkylated acylguanidines are presented. While NOESY and (3)J(H, H) scalar couplings cannot be applied successfully for the assignment of acylguanidines, (4)J(H, H) scalar couplings in (1)H,(1)H COSY spectra allow for an unambiguous chemical shift and conformational assignment. It is shown that these (4)J(H, H) long-range couplings between individual acylguanidinium NH resonances are observed solely across all-trans (w) pathways. Already one cis orientation in the magnetisation transfer pathway leads to signal intensities below the actual detection limit and significantly lower than cross-peaks from (2)J(NH, NH) couplings or chemical exchange. However, it should be noted that also in the case of conformational exchange being fast on the NMR time scale, averaged cross-peaks from all-trans (4)J(H, H) scalar couplings are detected, which may lead at first glance to an incomplete or even wrong conformational analysis.

G protein-coupled receptors function as logic gates for nanoparticle binding and cell uptake

More selective interactions of nanoparticles with cells would substantially increase their potential for diagnostic and therapeutic applications. Thus, it would not only be highly desirable that nanoparticles can be addressed to any cell with high target specificity and affinity, but that we could unequivocally define whether they rest immobilized on the cell surface as a diagnostic tag, or if they are internalized to serve as a delivery vehicle for drugs. To date no class of targets is known that would allow direction of nanoparticle interactions with cells alternatively into one of these mutually exclusive events. Using MCF-7 breast cancer cells expressing the human Y(1)-receptor, we demonstrate that G protein-coupled receptors provide us with this option. We show that quantum dots carrying a surface-immobilized antagonist remain with nanomolar affinity on the cell surface, and particles carrying an agonist are internalized upon receptor binding. The receptor functions like a logic "and-gate" that grants cell access only to those particles that carry a receptor ligand "and" where the ligand is an agonist. We found that agonist- and antagonist-modified nanoparticles bind to several receptor molecules at a time. This multiligand binding leads to five orders of magnitude increased-receptor affinities, compared with free ligand, in displacement studies. More than 800 G protein-coupled receptors in humans provide us with the paramount advantage that targeting of a plethora of cells is possible, and that switching from cell recognition to cell uptake is simply a matter of nanoparticle surface modification with the appropriate choice of ligand type.

Bivalent argininamide-type neuropeptide y y(1) antagonists do not support the hypothesis of receptor dimerisation

Bivalent ligands are potential tools to investigate the dimerisation of G-protein-coupled receptors. Based on the (R)-argininamide BIBP 3226, a potent and selective neuropeptide Y Y(1) receptor (Y(1)R) antagonist, we prepared a series of bivalent Y(1)R ligands with a wide range of linker lengths (8-36 atoms). Exploiting the high eudismic ratio (>1000) of the parent compound, we synthesised sets of R,R-, R,S- and S,S-configured bivalent ligands to gain insight into the "bridging" of two Y(1)Rs by simultaneous interaction with both binding sites of a putative receptor dimer. Except for the S,S isomers, the bivalent ligands are high-affinity Y(1)R antagonists, as determined by Ca(2+) assays on HEL cells and radioligand competition assays on human Y(1)R-expressing SK-N-MC and MCF-7 cells. Whereas the R,R enantiomers are most potent, no marked differences were observed relative to the corresponding meso forms. The difference between R,R and R,S diastereomers was most pronounced (about sixfold) in the case of the Y(1)R antagonist containing a spacer of 20 atoms in length. Among the R,R enantiomers, linker length and structural diversity had little effect on Y(1)R affinity. Although the bivalent ligands preferentially bind to the Y(1)R, the selectivity toward human Y(2), Y(4), and Y(5) receptors was markedly lower than that of the monovalent argininamides. The results of this study neither support the presence of Y(1)R dimers nor the simultaneous occupation of both binding pockets by the twin compounds. However, as the interaction with Y(1)R dimers cannot be unequivocally ruled out, the preparation of a bivalent radioligand is suggested to determine the ligand-receptor stoichiometry. Aiming at such radiolabelled pharmacological tools, prototype twin compounds were synthesised, containing an N-propionylated amino-functionalised branched linker (K(i)> or =18 nM), a tritiated form of which can be easily prepared.