Synthesis, Biological Evaluation and Receptor Docking Simulations of 2-[(Acylamino)ethyl]-1,4-benzodiazepines as k-Opioid Receptor Agonists Endowed with Antinociceptive and Antiamnestic Activity

The synthesis and biological evaluation of a series of new derivatives of 2-substituted 5-phenyl-1,4-benzodiazepines, structurally related to tifluadom (5), are reported. Chemical and pharmacological studies on compounds 6 have been pursued with the aim of expanding the SAR data and validating the previously proposed model of interaction of this class of compounds with the κ-opioid receptor. The synthesis of the previously described compounds 6 has been reinvestigated in order to obtain a more direct synthetic procedure. To study the relationship between the stereochemistry and the receptor binding affinity, compounds 6e and 6k were selected on the basis of their evident structural resemblance to tifluadom. Since a different specificity of action could be expected for the enantiomers of 6e and 6k, owing to the results shown by (S)- and (R)-tifluadom, their racemic mixtures have been resolved by means of liquid chromatography with chiral stationary phases (CSP), and the absolute configuration of the enantiomers has been studied by circular dichroism (CD) and 1H NMR techniques. Moreover, some new 2-[(acylamino)ethyl]-1,4-benzodiazepine derivatives, 6a-d, f,g,j, have been synthesized, while the whole series (6a-o) has been tested for its potential affinity toward human cloned κ-opioid receptor. The most impressive result obtained from the binding studies lies in the fact that this series of 2-[2-(acylamino)ethyl]-1,4-benzodiazepine derivatives binds the human cloned κ-opioid receptor subtype very tightly. Indeed, almost all the ligands within this class show subnanomolar Ki values, and the least potent compound 6o shows, in any case, an affinity in the nanomolar range. A comparison of the affinities obtained in human cloned κ-receptor with the correspondent one obtained in native guinea pig κ-receptor suggests that the human cloned κ-receptor is less effective in discriminating the substitution pattern than the native guinea pig κ-receptor. Furthermore, the results obtained are discussed with respect to the interaction with the homology model of the human κ-opioid receptor, built on the recently solved crystal structure of rhodopsin. Finally, the potential antinociceptive and antiamnesic properties of compounds 6e and 6i have been investigated by means of the hot-plate and passive avoidance test in mice, respectively.