Privileged structure based ligands for melanocortin receptors--substituted benzylic piperazine derivatives

Replacement of the aryl piperazine moiety in compound 1 with a variety of substituted benzylic piperazines (6) yields compounds that afford melanocortin receptor 4 (MCR4) activity. Analogs with ortho substitution on the aromatic ring afforded the highest affinity. Resolution of the stereocenter of the benzylic piperazine based privileged structure revealed that the R-enantiomer was more active.     

Privileged structure based ligands for melanocortin receptors--4,4-disubstituted piperidine derivatives

Homologation and cyclization back to the chiral methine of compound 3 yields achiral 4,4-disubstituted piperidine privileged structures (e.g., 8a) useful in the construction of melanocortin 4 receptor (MC4R) ligands. The piperidine nitrogen was replaced with carbon, oxygen, sulfur, and sulfone with minor erosion of binding. The methyl cyclohexane substituent was the most potent while significant affinity was still seen for smaller lipophilic groups such as ethyl.     

Beta-turn secondary structure and melanocortin ligands

The melanocortin pathway has emerged during this past decade as an important target area for the discovery and development of therapeutic agents related to obesity and type 2 diabetes. This peptide-G-protein coupled receptor (GPCR) pathway has evolved from peptide-based ligands to small molecules possessing a variety of different molecular scaffolds. Herein, we summarize the originating hypothesis of the importance of the reverse beta-turn secondary structure for agonist ligand potency at the melanocortin receptors and how that information was utilized for the discovery of small molecules based upon this type of turn structure.     

Design, synthesis, and evaluation of proline based melanocortin receptor ligands

A series of proline based melanocortin ligands has been developed on the basis of initial piperazine leads by using a more conformationally rigid scaffold. A number of these novel ligands showed significant binding affinity for MC3 and MC4 receptors.     

Privileged structure based ligands for melanocortin-4 receptors--aliphatic piperazine derivatives

Aliphatic carbocyclic replacement of the benzyl group of compound 1 yielded compounds with high affinity for the melanocortin-4 receptor (MC4R). Compounds with a cyclohexyl group showed a consistent high affinity, while different polar groups with less basicity were good replacements for the original diethyl amines. Substitution of the polar group found in these privileged structures with an aliphatic moiety produced compounds with high affinity for MC4R.     

Tethered indoles as functionalizable ligands for the estrogen receptor

To create ligands for the estrogen receptor that contain pendant groups for tethering to a poly(amido)amine (PAMAM) dendrimer, we have explored a class of N-substituted 2-phenyl indoles. Attachment of tethers of different length and chemical nature to this non-steroidal indole scaffold gave high affinity ligand-tether conjugates that can be easily functionalized. To further explore the utility of this system, an indole-conjugated dendrimer was prepared and evaluated as an estrogen receptor ligand.     

Streptavidin-binding and -dimerizing ligands discovered by phage display, topochemistry, and structure-based design

Structural and mechanistic determinants of affinity of streptavidin-binding peptide ligands discovered by phage display are reviewed along with the use of streptavidin as a paradigm for structure-based design. A novel way of producing protein-dimerizing ligands in the streptavidin model system is discussed, in which crystal packing topochemically mediates or even catalyzes dimerization of adjacent bound ligands whose reactive ligating groups are presented toward one another in productive orientations in the crystal lattice. Finally, through crystallography on a set of streptavidin complexes with small molecule and peptide ligands at multiple pHs in two space groups, the mechanism by which ligands enhance intersubunit stabilization of the streptavidin tetramer is probed.     

Privileged structure-based quinazolinone natural product-templated libraries: identification of novel tubulin polymerization inhibitors

A focused quinazolinone natural product-templated library was designed and synthesized. Compounds from this privileged structure-based library were identified as antimitotic agents acting through destabilization of tubulin polymerization. The results suggested that 2 could be a privileged substructure.     

A review of melanocortin receptor small molecule ligands

The melanocortin system (MC) is implicated in the regulation of a variety of physiological pathways including pigmentation, steroid function, energy homeostasis, food intake, obesity, cardiovascular, sexual function, and normal gland regulation. The melanocortin system consists of five receptors identified to date (MC1-5R), melanocortin agonists derived from the pro-opiomelanocortin prohormone (POMC) and two naturally existing antagonists. Melanocortin receptor ligand structure-activity studies have been performed since the 1960s, primarily focused on the pigmentation aspect of physiology. During the 1990s, the melanocortin-4 receptor was identified to play a significant physiological role in the regulation of both food intake and obesity. Subsequently, a concerted drug design effort has focused on the design and discovery of melanocortin receptor small molecules. Herein, we present an overview of melanocortin receptor heterocyclic small molecules.     

Design and synthesis of trivalent ligands targeting opioid,cholecystokinin, and melanocortin receptors for the treatment of pain

It has been known that co-administration of morphine with either cholecystokinin (CCK) receptor or melanocortin (MC) receptor antagonists enhance morphine’s analgesic efficacy by reducing serious side effects such as tolerance and addiction.1, 2, 3, 4 Considering these synergistic effects, we have designed trivalent ligands in which all three different pharmacophores for opioid, CCK, and MC receptors are combined in such a way as to conserve their own topographical pharmacophore structures. These ligands, excluding the cyclic compound, were synthesized by solid phase synthesis using Rink-amide resin under microwave assistance in very high yields. These trivalent ligands bind to their respective receptors well demonstrating that the topographical pharmacophore structures for the three receptors were retained for receptor binding. Ligand 10 was a lead compound to show the best biological activities at all three receptors.     

Optimization of privileged structures for selective and potent melanocortin subtype-4 receptor ligands

Design, syntheses and structure–activity relationships of N-acetylated piperazine privileged structures containing MC4R agonist compounds were described. The most potent derivatives were low nM MC4R selective full agonists. Several compounds from the series had modest pharmacokinetic properties.     

Estrogen receptor ligands. Part 16: 2-Aryl indoles as highly subtype selective ligands for ERalpha

A novel class of indole ligands for estrogen receptor alpha have been discovered which exhibit potent affinity and high selectivity. Substitution of the bazedoxifene skeleton to the linker present in the HTS lead 1a provided 22b which was found to be 130-fold alpha-selective and acted as an antagonist of estradiol activity in uterine tissue and MCF-7 cancer cells.     

Synthesis and structure-activity relationships of novel dipeptides and reduced dipeptides as ligands for melanocortin subtype-4 receptor

A series of benzylic piperazines (e.g., 4 and 5) attached to an 'address element', the dipeptide H-D-Tic-D-p-Cl-Phe-OH, 3 has been identified as ligands for the melanocortin subtype-4 receptor (MC4R). We describe herein the structure-activity relationship (SAR) studies on the N-terminal residue of the 'address element'. Several novel dipeptides and reduced dipeptides with high MC4R binding affinities and selectivity emerged from this SAR study.     

Feeding, body weight, and sensitivity to non-ingestive reward stimuli during and after 12-day continuous central infusions of melanocortin receptor ligands

The brain melanocortin system mediates downstream effects of hypothalamic leptin and insulin signaling. Yet, there have been few studies of chronic intracerebroventricular (i.c.v.) melanocortin receptor (MCR) agonist or antagonist infusion. Although there is evidence of interaction between melanocortin and dopamine (DA) systems, effects of chronic MCR ligand infusion on behavioral sensitivity to non-ingestive reward stimuli have not been investigated. The objective of this study was to investigate effects of chronic i.c.v. infusion of the MCR agonist, MTII, and the MCR antagonist, SHU9119, on food intake, body weight, and sensitivity to rewarding lateral hypothalamic electrical stimulation (LHSS) and the reward-potentiating (i.e., threshold-lowering) effect of D-amphetamine. The MCR antagonist, SHU9119 (0.02 microg/h) produced sustained hyperphagia and weight gain during the 12-day infusion period, followed by compensatory hypophagia and an arrest of body weight gain during the 24-day post-infusion period. At no point during the experiment was sensitivity to LHSS or D-amphetamine (0.25mg/kg, i.p.) altered. The MCR agonist, MTII (0.02 microg/h) produced a brief hypophagia (3 days) followed by a return to control levels of daily intake, but with body weight remaining at a reduced level throughout the 12-day infusion period. This was followed by compensatory hyperphagia and weight gain during the 24-day post-infusion period. There was no change in sensitivity to non-ingestive reward stimuli during the infusion of MTII. However, sensitivity to D-amphetamine was increased during the 24-day post-infusion period. It therefore seems that changes in ingestive behavior that occur during chronic MCR ligand infusion may not affect the response to non-ingestive reward stimuli. However, it is possible that the drive to re-feed and restore body weight following MCR agonist treatment includes neuroadaptations that enhance the incentive effects of drug stimuli.     

Dissecting physiological roles of estrogen receptor alpha and beta with potent selective ligands from structure-based design

The distinct roles of the two estrogen receptor (ER) isotypes, ERalpha and ERbeta, in mediating the physiological responses to estrogens are not completely understood. Although knockout animal experiments have been aiding to gain insight into estrogen signaling, additional information on the function of ERalpha and ERbeta will be provided by the application of isotype-selective ER agonists. Based on the crystal structure of the ERalpha ligand binding domain and a homology model of the ERbeta-ligand binding domain, we have designed steroidal ligands that exploit the differences in size and flexibility of the two ligand binding cavities. Compounds predicted to bind preferentially to either ERalpha or ERbeta were synthesized and tested in vitro using radio-ligand competition and transactivation assays. This approach directly led to highly ER isotype-selective (approximately 200-fold) and potent ligands. To unravel physiological roles of the two receptors, in vivo experiments with rats were conducted using the ERalpha- and ERbeta-selective agonists in comparison to 17beta-estradiol. The ERalpha agonist induced uterine growth, caused bone-protective effects, reduced LH and FSH plasma levels, and increased angiotensin I, whereas the ERbeta agonist did not at all or only at high doses lead to such effects, despite high plasma levels. It can thus be concluded that estrogen effects on the uterus, pituitary, bone, and liver are primarily mediated via ERalpha. Simultaneous administration of the ERalpha and ERbeta ligand did not lead to an attenuation of ERalpha-mediated effects on the uterus, pituitary, and liver parameters.     

Optimization of a privileged structure leading to potent and selective human melanocortin subtype-4 receptor ligands

Design and synthesis of potent MC4 selective agonists based on cyclohexylpiperidine derived cyclic urea, oxazolidinones, and sulfonamide based privileged structures are disclosed.     

Design, synthesis and pharmacological evaluation of conformationally restricted N-arylsulfonyl-3-aminoalkoxy indoles as a potential 5-HT6 receptor ligands

A series of novel conformationally restricted N(1)-arylsulfonyl-3-aminoalkoxy indoles were designed and synthesized as 5-HT(6) receptor (5-HT(6)R) ligands. Many of the synthesized compounds have moderate in vitro-binding affinities at 5-HT(6)R. The lead compound 8b (% inhibition?=?97.2 at 1 μM) from this series has good pharmacokinetic profile in male Wister rats and is active in animal model of cognition like Morris water maze. The details of chemistry, SAR, pharmacokinetics and pharmacological data constitute the subject matter of this report.     

Trypsin mutants for structure-based drug design: expression,refolding and crystallisation

New techniques in drug discovery are essential for the fast and efficient development of novel innovative drugs to deal with the challenges of the future. Structure determinations of various members of serine proteinases have provided a basis for computer-based drug design within this class of enzymes. In many proteins of interest, however, this course is blocked through a lack of suitable crystals. As a strategy for circumventing such problems, we have investigated the use of surrogate proteins for studying protein-ligand interactions. To test the feasibility of this approach, we have chosen bovine trypsin as a scaffold to reconstruct the ligand binding site of factor Xa. The simple modular design of trypsin, its readiness to crystallise and straightforward handling lends itself to such drug design by proxy. The expression, folding, purification, crystallographic and kinetic characterisation of bovine trypsin forms with factor Xa phenotype are presented.     

Regulation of eumelanin/pheomelanin synthesis and visible pigmentation in melanocytes by ligands of the melanocortin 1 receptor

The production of melanin in the hair and skin is tightly regulated by the melanocortin 1 receptor (MC1R) whose activation is controlled by two secreted ligands, alpha-melanocyte stimulating hormone (alphaMSH) and agouti signal protein (ASP). As melanin is extremely stable, lasting years in biological tissues, the mechanism underlying the relatively rapid decrease in visible pigmentation elicited by ASP is of obvious interest. In this study, the effects of ASP and alphaMSH on the regulation of melanin synthesis and on visible pigmentation were assessed in normal murine melanocytes and were compared with the quick depigmenting effect of the tyrosinase inhibitor, phenylthiourea (PTU). alphaMSH increased pheomelanin levels prior to increasing eumelanin content over 4 days of treatment. Conversely, ASP switched off the pigment synthesis pathway, reducing eu- and pheo-melanin synthesis within 1 day of treatment that was proportional to the decrease in tyrosinase protein level and activity. These results demonstrate that the visible depigmentation of melanocytes induced by ASP does not require the degradation of existing melanin but rather is due to the dilution of existing melanin by melanocyte turnover, which emphasizes the importance of pigment distribution to visible color.