Design, synthesis and biological evaluation of novel beta-substituted indol-3-yl ethylamido melatoninergic analogues

Melatonin receptor ligands: A pharmaco-chemical perspective

Melatonin MT₁ and MT₂ receptor ligands have been vigorously explored for the last 4 decades. Inspection of approximately 80 publications in the field revealed that most melatonergic ligands were structural analogues of melatonin combining three essential features of the parent compound: an aromatic ring bearing a methoxy group and an amide side chain in a relative arrangement similar to that present in melatonin. While several series of MT₂ -selective agents-agonists, antagonists, or partial agonists-were reported, the field was lacking MT₁ -selective agents. Herein, we describe various approaches toward the development of melatonergic ligands, keeping in mind that most of the molecules/pharmacophores obtained were essentially melatonin copies, even though diverse tri- or tetra-cyclic compounds were explored. In addition to lack of structural diversity, only few studies examined the activity of the reported melatonergic ligands in vivo. Moreover, an extensive pharmacological characterization including biopharmaceutical stability, pharmacokinetic properties, specificity toward other major receptors to name a few remained scarce. For example, many of the antagonists described were not stable in vivo, were not selective for the melatonin receptor subtype of interest, and were not fully characterized from a pharmacological standpoint. Indeed, virtual screening of large compound libraries has led to the recent discovery of potent and selective melatonin receptor agonists and partial agonists of new chemotypes. Having said this, the melatonergic field is still lacking subtype-selective melatonin receptor antagonists "active" in vivo, which are critical to our understanding of melatonin and melatonin receptors' role in basic physiology and disease.

Asenjonamides A-C, antibacterial metabolites isolated from Streptomyces asenjonii strain KNN 42.f from an extreme-hyper arid Atacama Desert soil

Bio-guided fractionation of the culture broth extract of Streptomyces asenjonii strain KNN 42.f recovered from an extreme hyper-arid Atacama Desert soil in northern Chile led to the isolation of three new bioactive β-diketones; asenjonamides A-C (1-3) in addition to the known N-(2-(1H-indol-3-yl)-2-oxoethyl)acetamide (4), a series of bioactive acylated 4-aminoheptosyl-β-N-glycosides; spicamycins A-E (5-9), and seven known diketopiperazines (10-16). All isolated compounds were characterized by HRESIMS and NMR analyses and tested for their antibacterial effect against a panel of bacteria.

Design, synthesis and melatoninergic activity of new unsubstituted and β,β′-difunctionalised 2,3-dihydro-1H-pyrrolo[3,2,1-ij]quinolin-6-alkanamides

A series of new 2,3-dihydro-1H-pyrrolo[3,2,1-ij]quinolin-6-alkanamides, with and without alkyl and cycloalkyl moieties in the beta-position of the alkanamido side chain, have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores and bind to the recombinant human MT(1) and MT(2) melatonin receptor subtypes expressed in NIH 3T3 cells. An increase of the spacer's length in the side chain by a methylene unit (from 17d to 21d) leads to a six-fold decrease in antagonistic activity. On the other hand, the introduction of two methyl groups in the beta-position of the side chain of 17a induces agonist potency (compound 24), implying thus that the two beta-methyl groups are not only tolerated by the receptor, but constitute functional probes in its dynamic agonist-antagonist conformational equilibrium. The presence of more bulky beta-substituents, regardless of the size of the R group, compounds 24a,b, seems to lead to antagonism and to a noteworthy MT(2) subtype selectivity. Last, the new N1-C7 annulated derivatives presented herein are substantially more potent than their respective N1-C2 annulated counterparts, previously reported.

Design, synthesis and melatoninergic potency of new N-acyl 8,9-dihydro-4-methoxy-7H-2-benzo[de]quinolinalkanamines

A series of new N-acyl 8,9-dihydro-4-methoxy-7H-2-benzo[de]quinolinalkanamines have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores and bind to the recombinant human MT(1) and MT(2) melatonin receptor subtypes expressed in NIH 3T3 cells. Compounds with a single methylene spacer in the side chain (7) have no agonist activity, but are weak antagonists in the Xenopus melanophore assay, irrespectively of the size or shape of the R substituent (R=CH(3) to c-C(4)H(7)). In contrast, compounds with two (8) or three (9) methylene spacers show partial agonist activity, though this does vary with the nature of the R substituent. Interestingly, the cyclopropane and cyclobutane R substituents, which are usually linked with antagonism, render the cyclopropanecarboxamido analog 9d and its cyclobutanecarboxamido congener 9e weak agonists. It seems, therefore, that in these compounds the R substituent constitutes a functional probe in the dynamic agonist-antagonist conformational equilibrium. One of the new molecules, antagonist 8c, exhibits a noteworthy MT(2) subtype selectivity (13-fold), whereas the acetamido analog 9a (with a three methylene units spacer) also acts as an antagonist and is the only analog exhibiting MT(1) selectivity (>10-fold). In contrast to the analogous N1-C7 annulated indole derivatives, recently reported, the new C1-C8 condensed isoquinolines are not all pure antagonists. Despite their modest receptor affinity at the binding site these compounds demonstrate that the nature of the response (agonist or antagonist activity) is dependent, in this case, on both the side chain spacer's length and the size and shape of the R group.

Synthesis and biological evaluation of new beta,beta'-disubstituted 6,7,8,9-tetrahydropyrido[1,2-a]indol-10-yl ethylamido melatoninergic ligands

Tricyclic analogs of melatonin with alkyl and cycloalkyl moieties in the beta position of the ethylamido chain have been prepared and tested for their ability to activate pigment granule aggregation in Xenopus laevis melanophores. The introduction of two methyl groups in the beta position of the side-chain of the methoxyl-substituted ligands induces a synergistic effect in agonist potency, which, importantly, is maintained after the methoxyl substituent is removed. The presence of more bulky beta-substituents, regardless of the size of the R group, seems to lead to antagonism.