Melatonin-vorinostat hybrid ligands show higher histone deacetylase and cancer cell growth inhibition than vorinostat.

Anticancer drug conjugates are an emerging approach for future cancer treatment. Here, we report a series of hybrid ligands merging the neurohormone melatonin with the approved histone deacetylase (HDAC) inhibitor vorinostat, using melatonin's amide side chain (3a-e), its indolic nitrogen (5a-d), and its ether oxygen (7a-d) as attachment points. Several hybrid ligands showed higher potency thanvorinostat in both HDAC inhibition and cellular assays on different cultured cancer cell lines. In the most potent HDAC1 and HDAC6 inhibitors, 3e, 5c, and 7c, the hydroxamic acid moiety of vorinostat is linked to melatonin through a hexamethylene spacer. Hybrid ligands 5c and 7c were also found to be potent growth inhibitors of MCF-7, PC-3M-Luc, and HL-60 cancer cell lines. As these compounds showed only weak agonist activity at melatonin MT1 receptors, the findings indicate that their anticancer actions are driven by HDAC inhibition.

O-linked melatonin dimers as bivalent ligands targeting dimeric melatonin receptors.

A series of dimeric melatonin analogues 3a-e obtained by connecting two melatonin molecules through the methoxy oxygen atoms with spacers spanning 16-24 atoms and the agomelatine dimer 7 were synthesized and characterized in 2-[125-I]-iodomelatonin binding assays, bioluminescence resonance energy transfer (BRET) experiments, and in functional cAMP and ß-arrestin recruitment assays at MT1 and MT2 receptors. The binding affinity of 3a-e generally increased with increasing linker length. Bivalent ligands 3a-e increased BRET signals of MT1 dimers up to 3-fold compared to the monomeric control ligand indicating the simultaneous binding of the two pharmacophores to dimeric receptors. Bivalent ligands 3c and 7 exhibited important changes in functional properties on the Gi/cAMP pathway but not on the ß-arrestin pathway compared to their monomeric counterparts. Interestingly, 3c (20 atoms spacer) shows inverse agonistic properties at MT2 on the Gi/cAMP pathway. In conclusion, these findings indicate that O-linked melatonin dimers are promising tools to develop signaling pathway-based bivalent melatonin receptor ligands.

Compared pharmacology of human histamine H3 and H4 receptors: structure-activity relationships of histamine derivatives.

Various histamine derivatives were investigated at the human H3 receptor (H3R) and H4 receptor (H4R) stably expressed in human embryonic kidney (HEK)-293 cells using [125I]iodoproxyfan and [3H]histamine binding, respectively. In Tris buffer, [3H]histamine binding to membranes of HEK(hH4R) cells was monophasic (K(D) of 3.8+/-0.8 nM). In phosphate buffer, the Hill coefficient was decreased (n(H) = 0.5+/-0.1) and a large fraction of the binding was converted into a low-affinity component (K(D) = 67+/-27 nM). The inhibition of [3H]histamine binding by two agonists, a protean agonist and five antagonists/inverse agonists confirms that the potency of many H3R ligands is retained or only slightly reduced at the H4R. Histamine derivatives substituted with methyl groups in alpha, beta or N(alpha) position of the side chain retained a nanomolar potency at the H3R, but their affinity was dramatically decreased at the H4R. With relative potencies to histamine of 282 and 0.13% at the H3R and H4R, respectively, (+/-)-alpha,beta-dimethylhistamine is a potent and selective H3R agonist. Chiral alpha-branched analogues exhibited a marked stereoselectivity at the H3R and H4R, the enantiomers with a configuration equivalent to L-histidine being preferred at both receptors. The methylsubstitution of the imidazole ring was also studied. The relative potency to histamine of 4-methylhistamine (4-MeHA) at the H4R (67%) was similar to that reported at H2 receptors but, owing to its high affinity at the H4R (Ki = 7.0+/-1.2 nM) and very low potency at H1- and H3-receptors, it can be considered as a potent and selective H4R agonist. On inhibition of forskolin-induced cAMP formation, all the compounds tested, including 4-MeHA, behaved as full agonists at both receptors. However, the maximal inhibition achieved at the H4R (approximately -30%) was much lower than at the H3R (approximately -80%). Thioperamide behaved as an inverse agonist at both receptors and increased cAMP formation with the same maximal effect (approximately +25%). In conclusion, although the pharmacological profiles of the human H3R and H4R overlap, the structure-activity relationships of histamine derivatives at both receptors strongly differ and lead to the identification of selective compounds.