Design, synthesis and pharmacological investigation of novel bifunctional ligands targeting nicotine and dopamine receptor subtypes

Background - An increasing number of receptor interactions depends on their physical association, and receptor hetero(di)mers usually show pharmacological and functional properties different from those of their constituent partners, thus behaving as new receptor entities.1 In particular, striatal dopaminergic transmission appears to be under the control of receptor heteromers containing D2 autoreceptors and non-alpha7 nicotinic acetylcholine receptors (nAChRs).2 This evidence stimulated our interest in investigating the biochemical and functional interactions between dopaminergic autoreceptors and nAChRs containing the beta2 subunit. Aims - The deliberate, rational design of bifunctional (multiple) ligands has gained a relevant interest in the medicinal chemistry research. These compounds may serve as suitable molecular probes to study in depth receptor architectures and, in some instances, represent innovative approaches for therapeutic intervention.3,4 The results of the present research project are directed to explore new potential pharmacological approaches to the therapy of nicotinic addiction on one hand and of Parkinson’s disease on the other. Methods - A common strategy was applied to the synthesis of two classes of designed bifunctional derivatives, that is we linked the chosen monovalent fragments with spacers of different length. Since the spacer should in principle not interfere with the ligand-receptor interactions involving the monomeric moieties, polymethylene chains were used to connect the pharmacophoric component parts. We synthesized bivalent derivatives formed by a) a D2/D3 agonist moiety and a nicotinic alpha4beta2 antagonist fragment and b) a D2/D3 antagonist moiety and a nicotinic alpha4beta2 agonist fragment. The structures of the parent ligands were selected after a critical inspection of the literature. Results - We initially prepared bivalent derivatives characterized by a D2/D3 agonist moiety and a nicotinic alpha4beta2 antagonist fragment. The structural features of the chosen selective alpha4beta2 antagonists were those of N-n-alkylnicotinium analogues (NONI and NDNI), whereas we chose 2-(aminomethyl)chromane (2-AMC) was the molecular portion endowed with the required D2/D3 receptor agonist properties. Bitopic compounds with the above cited molecular features should ideally inhibit dopamine release, a condition that could be exploited in therapeutic protocols for nicotine addiction. On the other hand, the synthesis of bivalent derivatives incorporating a D2/D3 antagonist moiety and a nicotinic alpha4beta2 agonist fragment was achieved using the D2/D3 antagonist Raclopride and the selective highly potent alpha4beta2 agonist A-84543 as building blocks. Bitopic compounds with such a molecular skeleton, able to recognize presynaptic heterodimeric receptor populations, should functionally enhance dopamine release, thus representing potential drug candidates for the treatment as Parkinson’s disease. The synthetic routes to target compounds will be presented and commented. Both classes of hybrid derivatives and their individual precursor ligands were assayed a) for their affinity and specificity at the nAChRs in striatal tissues and at the D2 receptor subtypes transfected into HEK cells, and b) for their functional activity, that is the capacity to favor or inhibit the release of [3H]dopamine (DA) from striatal slices stimulated by nicotinic agonists and/or the depolarizing agent potassium chloride. As an example, in the first group of derivatives, one of the bivalent compounds showed an interesting functional profile, since it totally inhibited the release of [3H]DA, being more potent then its parent nicotinic alpha4beta2antagonist. Conclusion - The PhD experimental activity was devoted to the design, synthesis, and structure-activity studies of two distinct groups of bifunctional ligands targeting specific nicotinic and dopaminergic receptor subtypes. The preliminary results obtained for some of the studied derivatives encourage a further deepening of their functional profile (i.e., with electrophysiological experiments) coupled with the proof of concept of a truly bivalent mode of action. References 1. Fiorentini, C.; Busi, C.; Gorruso, E.; Gotti, C.; Spano, P.; Missale, C. Reciprocal regulation of dopamine D1 and D3 receptor function and trafficking by heterodimerization. Mol. Pharmacol. 2008, 74, 59-69. 2. Quarta, D.; Ciruela, F.; Patkar, K.; Borycz, J.; Solinas, M.; Lluis, C.; Franco, R.; Wise, R. A.; Goldberg, S. R.; Hope, B. T.; Woods, A.; Ferré, S. Heteromeric nicotinic acetylcholine-dopamine autoreceptor complexes modulate striatal dopamine release. Neuropsychopharmacol. 2007, 32, 35-42. 3. Morphy, R.; Rankovic, Z. Designed multiple ligands. An emerging drug discovery paradigm. J. Med. Chem. 2005, 48, 6523-6543. 4. Rozenfeld, R.; Devi, L. A. Receptor heteromerization and drug discovery. Trends Pharmacol. Sci. 2010, 31, 124-130.