NEW PLAYERS IN AN OLD GAME: Pharmacological Evaluation of the Benzomorphan-Based Compound LP1. Design and Synthesis of Conformationally Constrained Compounds as New Tramadol-like Candidates.

For the clinical management of acute and chronic pain, a possible therapeutic approach is the use of associations between two or more drugs, which produce their biological effects on two or more different sites of action, in order to modulate directly or indirectly profile analgesic and adverse effects (Argoff 2011). Given the advantages of polypharmacology, in the drug discovery process has been established the strategy "one-molecule multiple targets" (Morphy and Rankovic, 2009). The multitarget ligands provided better analgesic activity and fewer side effects - already observed in the association of drugs coupled with favourable pharmacokinetic and pharmacodynamic characteristics. It is known that endogenous opioid peptides are the key mediators in the modulation of the pain transmission in descending pathways. Similarly, monoamine neurotransmitters - according to the location and type of receptor involved - can positively or negatively modulate the transmission of pain sensation. Considering that the various mediators involved in the circuit of pain represent potential targets for different pharmacological interventions, multitarget ligands possessing opioid-opioid or non-opioid-opioid mechanisms of action are potential drug candidates for the management of various pain conditions. Multitarget ligands, able to act simultaneously on multiple opioid receptors subtypes, showed low propensity to induce side effects. In particular, it was found an improved analgesic profile associated with a reduced tendency to induce tolerance in mixed MOR (mu-opioid receptor)-DOR (delta-opioid receptor) ligands (Schiller 2010). In previous studies was demonstrated that the benzomorphan-based compound LP1 (Pasquinucci et al. 2010) exhibited high and good affinities versus MOR and DOR, respectively, and an analgesic potency comparable to morphine completely NX-reversed. Given this background, the present thesis focused on the study of the functional profile of LP1 through [35S]GTPãS binding assay and tail flick test using selective MOR, DOR and KOR antagonists. Moreover, to further delineate its pharmacological profile, NX-M was administered either s.c. or i.v. to investigate if the LP1 action is centrally- or peripherally-mediated and it was also measured the LP1 ability to induce tolerance in regimen of repeated administration. Finally, LP1 s behavioural effects antihyperalgesia and antiallodynia in animal models of persistent pain were studied. Collected data indicated LP1 as a central-acting MOR agonist-DOR antagonist (Parenti et al., 2012b) with low capability to induce tolerance (Pasquinucci et al., 2012). Moreover, antihyperalgesic and antiallodynic effects of LP1 in animal model of persistent pain suggested this multitarget compound as a possible useful tool for chronic pain treatment. In the multitarget ligand context, another strategy widely investigated is the combination of the MOR agonism with the monoamines reuptake inhibition (Bannister et al. 2009). A compound corresponding to this pharmacological profile is Tramadol (Leppert 2009), which has been selected as lead compound. The chemistry program was engaged with the aim to obtain compounds that maintained the mechanism of action of the parental ligand and showed an improved analgesic efficacy. First, pharmacophoric features and their critical distances have been highlighted to identify a model that represented the interaction with MOR of Tramadol and its derivatives. Then, it has been designed and synthesised a series of conformationally constrained compounds as new Tramadol-like candidates, in which two pharmacophoric elements of tramadol - the lateral chain and the basic nitrogen - are constrained in a cyclised structures represented by the trans-decahydroisoquinoline and octahydro-1H-cyclopenta[c]pyridine nuclei.