Novel imidazoline ligands potentially useful in managing pain, opiate withdrawal syndrome and psychiatric comorbidity

My research has been addressed to discover novel molecules useful in managing severe pathologies such as pain, opiate withdrawal syndrome, and psychiatric comorbidity. At present, my production consists of four published articles and of nine participations to scientific meetings, four as oral presentations and five as posters. The research group where I am preparing my PhD thesis has been engaged for several years in the study of biologically active ligands bearing the imidazoline structural motif. The privileged nature of this nucleus has been repeatedly confirmed. Indeed, depending on the peculiar nature of the substituent in position 2 of the imidazoline backbone and/or on the NH function, it has been possible to modulate the ligand biological profile both versus different systems such as α2-adrenergic receptors (α2-ARs), nicotinic and dopaminergic receptors, imidazoline binding sites (IBS) and MAO enzymes, and within the same system with resultant enhanced subtype selectivity. Previous studies performed in our laboratories had highlighted the interesting α2C-AR agonism/α2AAR antagonism of allyphenyline (1). In in vivo studies, 1 enhanced morphine analgesia (due to its α2C-AR agonism), was devoid of sedative effects (due to its α2A-AR antagonism), and contrasted and prevented morphine tolerance and dependence at very low dose (0.05 mg/Kg). Therefore, a wide development of such observations has represented a goal of my research. Novel allyphenyline analogues have been prepared and sounder structure-activity relationships were obtained. Among them, cyclomethyline (2) exhibited a very potent α2C-AR agonism/α2A-AR antagonism. From a molecular superposition carried out fitting energetically allowed conformations of the (S)-enantiomers of 1, 2 and their phenyl analogue biphenyline (3) to the crystallographic data of the (S)-lofexidine taken as α2-agonist reference compound, it emerged that dual α2C-AR agonism/α2A-AR antagonism is associated with a preferred extended conformation of the ligand. On the contrary, a preferred folded conformation, as observed with 3, induces simultaneous α2C-/α2A-AR agonism. This latter conformation is probably induced by the electron-rich ortho phenyl substituent, able to provide a stable p-cation like charge transfer complex with the imidazoline moiety. Such considerations are also supported by NOESY experiments. With the aim to confirm the aforementioned results and discover novel tools useful in the management of opioid withdrawal symptoms, that have a major role in relapse to drug-taking behavior after detoxification, I have extended the study to 2. Similarly to what was previously performed with 1, the enantiomers of 2 have been prepared and studied. Therefore, besides the usual contribution to the synthesis, purification and characterization of the final compounds, in this work I had a share in the achievement of the enantiomers by resolution of the corresponding racemates and in attribution of the absolute configuration by stereospecific synthesis. Analogously to 1 and its enantiomers, 2 and its enantiomers, devoid of sedative effect due to their α2A-AR antagonism, were able at low dose (0.05 mg/Kg) to significantly reduce the naloxone-precipitate withdrawal syndrome. To learn more about the pharmacological properties of this interesting class of compounds, especially in light of the strong comorbidity between opioid addiction and depressive disorders seen in several clinical studies, 1 and 2 and their enantiomers were evaluated in behavioral model of depression in mice (Forced Swimming Test). The obtained results indicated that 1 or the single (S)-(+)-1, 2 or both its enantiomers, all behaving as α2C-AR agonists/α2A-AR antagonists/5-HT1A-R agonists, exerted at the same low dose of 0.05 mg/Kg a potent antidepressant effect. Experiments carried out in the presence of the α2-AR antagonist yohimbine and the 5-HT1A-R antagonist WAY100135 suggested that dual α2C-AR/5-HT1A-R activation was required for the antidepressant-like effect induced by low doses of the aforementioned compounds. Since prolonged abstinence remains a major challenge, I explored other strategies addressed to discover further multifunctional tools, similarly able to ameliorate withdrawal symptoms and relieve depressive disorders. In this case the multitarget approach regarded the combination of α2C-AR agonism/α2A-AR antagonism/I2-IBS interaction. Our interest for I2-IBS was stimulated by the observation that also these imidazoline binding proteins are involved in depression and modulation of morphine analgesia as well as tolerance and opioid addiction The prepared compounds (e.g. 4), though characterized by the same pharmacophore of the aforementioned ligands, bore a bridge OCH2, that previous studies of the same research group indicated to be compatible with α2-ARs and I2-IBS. As expected, the novel compounds displayed significant α2C-AR agonism/α2A-AR antagonism and a general high I2-IBS affinity. The in vivo studies of 4 demonstrated that such a multitarget combination can provide ligands beneficial to opioid withdrawal syndrome and associated depression. Moreover, I carried out a study on ligands inspired to idazoxan, an α2-AR subtype unselective antagonist. In this case the bridge X was represented by the 1,4-dioxane nucleus. In particular, in contrast to idazoxan, compound 5, showing a trans stereochemical relationship between 5-phenyl and 2-imidazoline group, due to favourable synergism between its ability to antagonize the sole α2A-AR subtype and to interact with I2-IBS, potently enhanced morphine analgesia. All aforementioned agents, lacking in sedative effects due to their α2A-AR antagonism, might afford an improvement over current therapies with clonidine-like drugs. Suitable decorations of imidazoline scaffold allowed me to synthesize also molecules interacting with D2-dopaminergic receptors. This study is in progress. Nevertheless, the prelimary screening performed at National Institute on Drug Abuse of Baltimore, where I stayed for 6 months, have highlighted for compounds 6 and 7, respectively, an interesting D3 or D4 selectivity. The research is going on also in collaboration with Dr. Amy H. Newman, expert on dopaminergic function and responsible of the Medicinal Chemistry Section at NIH. I have orally presented at XXXVIII edition of the summer course of Organic Synthesis ''A. Corbella'' (Gargnano, June 17-21, 2013) the first results of this scientific collaboration. My research allowed us to maintain the already active national and international scientific collaborations (Scheinin, University of Turku; Carrieri, Universita'  di Bari; Hudson, University of Toronto; Marchioro, Aptuit, Verona; Varrone, Siena Biotech; Poggesi, Recordati, Milano) and to begin new ones (Lanza, Rottapharm, Monza; Newman, NIDA NIH, Baltimore).