Binding profile of benextramine at neuropeptide Y receptor subtypes in rat brain areas

Recent advances in the multitarget-directed ligands approach for the treatment of Alzheimer's disease

With 27 million cases worldwide documented in 2006, Alzheimer's disease (AD) constitutes an overwhelming health, social, economic, and political problem to nations. Unless a new medicine capable to delay disease progression is found, the number of cases will reach 107 million in 2050. So far, the therapeutic paradigm one-compound-one-target has failed. This could be due to the multiple pathogenic mechanisms involved in AD including amyloid β (Aβ) aggregation to form plaques, τ hyperphosphorylation to disrupt microtubule to form neurofibrillary tangles, calcium imbalance, enhanced oxidative stress, impaired mitochondrial function, apoptotic neuronal death, and deterioration of synaptic transmission, particularly at cholinergic neurons. Approximately 100 compounds are presently been investigated directed to single targets, namely inhibitors of β and γ secretase, vaccines or antibodies that clear Aβ, metal chelators to inhibit Aβ aggregation, blockers of glycogen synthase kinase 3β, enhancers of mitochondrial function, antioxidants, modulators of calcium-permeable channels such as voltage-dependent calcium channels, N-methyl-D-aspartate receptors for glutamate, or enhancers of cholinergic neurotransmission such as inhibitors of acetylcholinesterase or butyrylcholinesterase. In view of this complex pathogenic mechanisms, and the successful treatment of chronic diseases such as HIV or cancer, with multiple drugs having complementary mechanisms of action, the concern is growing that AD could better be treated with a single compound targeting two or more of the pathogenic mechanisms leading to neuronal death. This review summarizes the current therapeutic strategies based on the paradigm one-compound-various targets to treat AD. A treatment that delays disease onset and/or progression by 5 years could halve the number of people requiring institutionalization and/or dying from AD. 

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.

Phenoxybenzamine and benextramine, but not 4-diphenylacetoxy-N-[2-chloroethyl]piperidine hydrochloride, display irreversible noncompetitive antagonism at G protein-coupled receptors

Many irreversible antagonists have been shown to inactivate G protein-coupled receptors (GPCRs) and used to study agonists and spare receptors. Presumably, they bind to primary (agonist) binding sites on the GPCR, although noncompetitive mechanisms of antagonism have been demonstrated but not thoroughly investigated. We studied noncompetitive antagonism by phenoxybenzamine and benextramine at alpha(2A)-adrenoceptors in stably transfected Chinese hamster ovary cells, benextramine and 4-diphenylacetoxy-N-[2-chloroethyl]piperidine hydrochloride (4-DAMP mustard) at endogenous muscarinic acetylcholine (mACh) receptors in human neuroblastoma SH-SY5Y cells, and benextramine at serotonin 5-HT(2A) receptors in stably transfected SH-SY5Y cells. Primary binding sites were protected by reversible competitive antagonists during pretreatment with irreversible antagonists. We conducted appropriate radioligand binding assays by measuring remaining primary binding sites and agonist affinity, functional assays to evaluate agonist-induced responses, and constitutive guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS)-Galpha(o) binding assays to determine remaining G protein activity. Phenoxybenzamine (100 microM; 20 min) and benextramine (10 or 100 microM; 20 min) at alpha(2A)-adrenoceptors, but not 4-DAMP mustard (100 nM; 120 min) at mACh receptors, displayed irreversible noncompetitive antagonism in addition to their known irreversible competitive antagonism. Although agonist binding affinity is not influenced, signal transduction is modulated in a G protein-dependent manner via allotopic interactions. Benextramine noncompetitively inhibits agonist-induced responses at three different GPCR types (alpha(2A), mACh, and 5-HT(2A) receptors) that signal via three families of G proteins (G(i/o), G(s), and G(q/11)). We conclude that, where irreversible antagonists are utilized to study drug-receptor interaction mechanisms, the presence of significant irreversible noncompetitive antagonism may influence the interpretation of results under the experimental conditions used.

Structure-activity relationships of methoctramine-related polyamines as muscular nicotinic receptor noncompetitive antagonists. 3. Effect of inserting the tetraamine backbone into a macrocyclic structure

The present article expands on the study of another aspect of structure-activity relationships of the polymethylene tetraamines, namely, the effect of inserting the tetraamine backbone into a macrocyclic structure. To this end, compounds 8-12 were designed by linking the two terminal nitrogen atoms of prototype methoctramine 2 to an aryl moiety. Alternatively, 2 was first modified to achieve compounds 6 and 7, which in turn were cyclized by linking the two terminal primary amine functions to a polyphenyl spacer, affording 13-20. All the compounds were tested on muscle-type nAChRs and most of them as well on AChE. Furthermore, selected compounds were tested also on peripheral M(2) and M(3) mAChRs. All these cyclic derivatives, like prototypes, were potent noncompetitive antagonists at both frog and Torpedo nAChRs, suggesting that polyamines do not need to be linear or in extended conformation to optimally interact with the nicotinic channel; rather, they may bind in a U-shaped conformation. Relative to muscarinic activity, macrocyclic compounds 10, 13, 14, and 20, in contrast with the profile displayed by 2, were almost devoid of affinity. It is derived that an aryl spacer is detrimental to the interaction of polyamines with mAChRs. Finally, all the diamine diamides investigated in this study were much less potent in inhibiting AChE activity than prototype 3, suggesting that a macrocyclic structure may not be suitable for AChE inhibition.

Characterization of Y1, Y2 and Y5 subtypes of the neuropeptide Y (NPY) receptor in rabbit kidney. Sensitivity of ligand binding to guanine nucleotides and phospholipase C inhibitors

The binding of two peptide YY/neuropeptide Y analogues selective for major subtypes of neuropeptide Y (NPY) receptors was compared in particulates from rabbit kidney cortex employing modulators of activity of G-proteins, phospholipase enzymes, and ion channels. The binding of (Leu31,Pro34)human peptide YY resembled the patterns observed previously for the brain tissue Y1 receptor, exhibiting a high sensitivity to monovalent cations, disulfide disruptors, guanosine polyphosphates and phospholipase C inhibitors. However, this binding was bimodal in response to human pancreatic polypeptide and to peptides selective for the Y2 subtype of the NPY receptor, displaying a large component pharmacologically similar to the brain Y5 receptor. This kidney Y5-like binding largely shared the sensitivity to monovalent cations, guanine nucleotides and phospholipase C inhibitors found for either the kidney or the brain Y1 receptor, and also was activated by Ca2+ ion. Both Y1- and Y5-like binding in the kidney displayed a uniformly low reactivity to a nonpeptidic Y1 antagonist, BIBP-3226, and to a receptor peptide mimetic, mastoparan analogue MAS-7. The kidney Y2 binding shared the low sensitivity to ionic environment observed for the brain Y2 subtype, and was only partially sensitive to guanine nucleotides or to MAS-7. The Y2 liganding had a sensitivity to phospholipase C inhibitors similar to the Y1/Y5 binding. This reactivity was retained in the fraction of the Y2 receptor persisting detergent solubilization in a high-affinity form, which, however, was activated rather than inhibited by G-protein agonists.

Synthesis and characterization of a selective peptide antagonist of neuropeptide Y vascular postsynaptic receptors

1. A cyclic dimeric nonapeptide neuropeptide Y (NPY) receptor antagonist, 1229U91, was synthesized by Fmoc chemistry and dimerised in solution. Its effects were assayed in mesenteric arteries from rats and mice, and in rat vas deferens. 2. Mesenteric arteries were cannulated and pressurised to 55 mmHg and the external diameters continuously measured. NPY, PYY, Leu31Pro34NPY and NPY(13-36) each caused concentration-related contractions with the order of potency PYY > or = Leu31Pro34NPY = NPY > NPY (13-36), consistent with the Y1 receptor subtype. 3. 1229U91 had no agonist activity in the arteries but caused a concentration-related rightward shift of NPY (mouse arteries) or Leu31Pro34NPY (rat) concentration-response curves. The antagonism was competitive with pKBS of 7.69 +/- 0.15 and 7.47 +/- 0.13 in the mouse and rat arteries, respectively. 4. Sympathetic nerves in the vas deferens were stimulated with a single electrical field pulse every 20 s and the twitch responses recorded. NPY, PYY, Leu31Pro34NPY and NPY(13-36) inhibited the twitches with the order of potency PYY > NPY > NPY(13-36) >> Leu31Pro34NPY, consistent with the Y2 receptor subtype. 5. 1229U91 inhibited the vas deferens twitch with a shallow concentration-response curve and a time-course of inhibition distinct from that of NPY. 1229U91 (30 microM) did not cause a rightward shift of the NPY concentration-response curve. 1229U91 is at least 5 orders of magnitude less potent in the vas deferens than in rat brain Y2 binding assays reported by others, suggesting that the brain and vas deferens Y2 receptors are different. 6. It is concluded that 1229U91 is a competitive antagonist of NPY Y1 vascular receptors and has additional properties that inhibit the electrically evoked twitch of the rat vas deferens.

Failure of the putative neuropeptide Y antagonists, benextramine and PYX-2, to inhibit Y2 receptors in rat isolated prostatic vas deferens

1. The pharmacological activity of neuropeptide Y (NPY) and some analogues in inhibiting the twitch contractions induced by electrical stimulation (single pulses at 25 V, 0.15 Hz, 1 ms) in the prostatic portion of the rat isolated vas deferens was investigated. The rank order of agonist potency was: PYY > NPY2-36 > NPY >> NPY13-36 >> NPY18-36 >> [Leu31,Pro34]NPY = hPP, which is consistent with the activation of a Y2 receptor. 2. The putative Y1 and Y2 antagonist, benextramine (BXT), incubated at 100 microM for 10 or 60 min, was ineffective against PYY-induced inhibition of the twitch response, suggesting that the prejunctional Y2 receptor in this tissue is different from the postjunctional one reported in the literature to be sensitive to BXT blockade. 3. The putative NPY antagonist, PYX-2, incubated at 1 microM for 20 min, was completely ineffective in antagonizing PYY-induced inhibition of twitches. 4. The twitch response was totally inhibited by suramin (100 microM) but was little affected by prazosin (1 microM). Furthermore, NPY was without effect on the dose-response curve to ATP in resting conditions. Taken together, these results suggest that in our paradigm, NPY inhibits the release of a purinergic neurotransmitter which mediates contraction of the prostatic portion of the rat vas deferens.