77-LH-28-1 as a model for the rational design of selective dopamine D4 receptor ligands

M1 muscarinic acetylcholine receptor (M1 mAChR) represents an attractive target for the treatment of cognitive deficits associated with several pathologies, including Alzheimer's disease and schizophrenia. However, the discovery of subtype-selective agonists is hampered by the high degree of homology among the M1-M5 mAChR subtypes at the orthosteric binding site. The advent of functional screening assays allowed the identification of ligands, such as 77-LH-28-1, which bound to an allosteric site and selectively activated the M1 mAChR (1). Initially described as an allosteric agonist by GlaxoSmithKline, at present 77-LH-28-1 is considered a bitopic agonist (2). It displayed antipsychotic and cognition-enhancing efficacy in pre-clinical models of schizophrenia and Alzheimer’s disease (1). Unfortunately, its efficacy was confounded by nonselective effects on other receptors (3). Among these receptors, 77-LH-28-1 has been reported to bind the short isoform of the dopamine D2 receptor (D2SR) (4). Dopamine D2-like subfamily includes D2R, D3R and D4R subtypes. The wide expression of D2-like receptors in the central nervous system and the modulation of various neurological processes, including gratification, cognition, learning and memory, make them attractive therapeutic targets (5). To get more information about the pharmacological dopaminergic properties of 77-LH-28-1, this compound was evaluated for its affinity at dopamine D2-like receptor subtypes by radioligand binding assays. Surprisingly, 77-LH-28-1 showed high affinity and selectivity for D4R over D2R and D3R. To better understand the structural features required for the selective interaction with D4R, the aliphatic butyl chain of 77-LH-28-1 was modified and the novel compounds 1-6 were prepared. Moreover, the piperidine ring of 77-LH-28-1 was replaced by a piperazine nucleus, to give the novel derivatives 7-13 (Figure 1). All the compounds were evaluated for their affinity at dopamine D2R, D3R and D4R subtypes, as well as at the five mAChR subtypes. Compounds showing the highest affinities at D4R were also evaluated for their functional activity considering both G-protein activation and β-arrestin recruitment. The most interesting derivatives can be emphasized as biased D4R compounds, behaving as potent partial agonists for G-protein activation and potent antagonists in β-arrestin recruitment. The detailed results of the biological assays performed to the new derivatives will be reported.