Design and Synthesis of New Potential Therapeutic Tools for the Treatment of CNS Diseases

Most of dopaminergic agonist structures bear the 2-(3-hydroxyphenyl)ethylamine (1) pharmocophoric moiety or its bioisosteres (chapter 2). However, D2-like receptor ligands endowed with good affinity, although lacking of such a scaffold are known. In the past years, this research group developed new potential D2-like ligands based on the template structure of phenoxyethylamine (2, 3, 4, 5; chapter 2), which is known to be a good lead to build up dopaminergic ligands. Many SAR studies have reported phenolic and non-phenolic structures endowed with high dopaminergic affinity, based on the N-benzyl-2-(3-hydroxyphenoxy)ethylamine (6; chapter 2). Dopaminergic ligands bearing the same fragment embedded in more conformationally restricted skeletons are known. For instance, compound 7 (chapter 2) is reported to bind D2 receptors with high affinity and good selectivity towards alpha1-adrenergic receptors and 5-HT1A. Moreover, fragment 6 (chapter 2) can be identified in the doxantrine structure (8; chapter 2), a full D1-like agonist endowed with high affinity. Furthermore, WB-4101 (9; chapter 2), a well known adrenergic drug, bears a benzodioxanmethylamine fragment also present in compound 7 (chapter 2). Quaglia et al., reported an interesting value of D2-like receptors affinity for such a compound (pKi = 6.91), even though with a lower affinity compared to both alpha-adrenergic and 5-HT1A serotoninergic receptors. It can be observed that WB-4101 embeds the molecular doubling of phenoxyethylamine. Such a fragment might be responsible for the affinity towards dopaminergic receptors. In a deep SARs study on muscarinc, adrenergic and serotoninergic receptors, the same authors demonstrated that the 1,4-benzodioxane scaffold may positively affect the ligands affinity towards GPCRs. In the same work, the 1,4-benzodioxane nucleus of WB-4101 was substituted with the 6-pheny-1,4-dioxane scaffold leading to adrenergic and 5-HT1A ligands endowed with lower affinity values. However, the introduction of a second aromatic ring in the 6 position led to an improvement of such affinity values. Compounds 10 and 11 might also be seen as phenoxyethylamine derivatives (2; chapter) and they even share structural similarities with A68390, a selective D1-like agonist. Thus, we decided to evaluate the effect of the 6,6-diphenyl-1,4-dioxan-2-methylene scaffold as nitrogen substituent of the phenoxyethylamines. Moreover various substitution on the aromatic moiety of the phenoxyethylamine were also investigated in order to evaluate the influence on the dopamine receptors affinity of a potential hydrogen-bond acceptor. For such an aim the methoxy group was considered the most suitable. Several studies demonstrated the versatility of the imidazoline ring (chapter 3). In fact, depending on the particular kind of substituent inserted in position 2 of imidazoline nucleus, it was possible to modulate the ligand profile, both with regard to different systems (alpha2-Adrenergic Receptors, Imidazoline Binding Sites, Nicotinic and Muscarinic Receptors, MAO-A inhibitors) and inside the same system, with resultant enhanced subtype selectivity. Imidazoline compounds have not been yet explored as DA receptor ligands, but the well known prototype of I2 receptor ligands 2-(benzofuran-2-yl)-4,5-dihydro-1H-imidazole (2-BFI) behaved also as DA indirect agonist and displayed a 47 mM binding to the DA D2 receptors. Therefore, with the aim to improve the DA D2-like receptor binding affinity, novel molecules based on the 2-BFI scaffold were designed and synthesized (chapter 3). The 5 and/or 6 substituents R1-R3 were chosen among those reported to be beneficial for the binding of DA D2-like receptor agonists. Competitive binding of the new compounds at DA D2-like receptors has been evaluated on porcine striatal membranes. The affinity at I2-IBS and alpha2-ARs has also been evaluated on rat brain membranes. All values are expressed as Ki (mM) values. The most interesting compounds have also been tested for D2-like potency and intrinsic activity. Interestingly, it has been observed that the 2-BFI is able to induce a partial activation of D2-like receptor (EC50 = 37.7mM; i.a.= 0.57). The modest and conservative modifications performed on the basic structure of the lead modulated its biological profile. The introduction of one hydroxyl group in position 6 of the aromatic ring (compound 3f; chapter 3 ) slightly enhances, the D2-like profile of the lead. In addition, 3f (chapter 3) behaves as partial agonist (EC50=5.7mM, i.a.=0.48) displays a potency comparable to that of DA. Moreover, it maintains significant I2-IBS affinity (Ki = 0.076 mM). On the contrary, the isomer 5-hydroxy substituted 3a (chapter 3), still endowed with good I2-IBS, was unable to bind at the D2-like receptors. It appears that only the maintenance of a meta relationship between the OH group with respect to the benzofuranic oxygen atom, proved to be endowed with D2 affinity. The negative influence of the 5-hydroxy substitution is also observed in the catechol derivative 3i (chapter 3) that showed strongly reduced I2 character compared to the lead, and was not able to interact with D2-like receptors. Interestingly, the N-substitution seems to impart a more decisive modulation of the biological profile to the lead. In fact, such a modification caused general decrease of the I2-affinity, with, in some cases, accompanying significant D2-like intrinsic activity enhancement. In particular, the N-benzyl derivatives behaved as a nearly D2 full agonists (3e: i.a.=0.85; 3h: i.a. = 0.82), with affinity (3e: Ki = 5.00 mM; 3h: Ki = 5.66 mM), and potency (3e: 6.3 mM; 3h: 5.8 mM; chapter 3) comparable to those of DA (Ki = 3.9 mM; EC50 = 4.8 mM). By our results it can be argued that the degree of receptor activation, is favored by the N-substitution, and might be affected by peculiar features of the pendant group (i.e. steric hindrance) and, sometimes, by the structural characteristics of the parent compound. Till today a large number of SAR studies have been developed to understand the interaction between DA receptors and their ligands (chapter 4). Such studies have led to the identification of several DA receptor ligands embedding the DA moiety. It is well known that the catechol ring plays a crucial role, especially on D1-like agonism. Nevertheless, DA agonists, bearing catechol or phenol rings, are endowed with low oral bioavailability and short effect duration. Moreover, the potential toxicity of catecholic compounds is a big concern. Thus, approaches addressed to circumvent catecholic potential toxicity are of great significance. The possibility to develop new non-catecholic DA receptor ligands represents an attractive perspective and the replacement of catechol or phenol rings with heterocycles proved to be a successfully approach. In view of the reported involvement of 4-aryl-1,2,3,4-tetrahydroisoquinolines as a core structure of drugs endowed with different useful pharmacological applications, we designed and synthesized the novel compounds 5a-g (chaper 4), characterized by the catechol replacement on the base structure of 1 (chapter 4) with a decorated pyridyl or phenyl group. The binding affinities for DA receptors of the new compounds were evaluated. Although all the tested compounds showed D1-like and D2-like affinities lower than those of DA, observing the biological profile of compounds 5a, 5b and 5d (chapter 4), a preferential D2-like affinity turned out. Interesting suggestions also emerged from the study of the phenyl derivatives 5f-g (chapter 4). Indeed, these two non-phenolic compounds, bearing the same kind of substituent in different positions, highlighted that the proper para substitution might represent a structural feature to modulate D1-/D2-like selectivity. In conclusion, such a study suggested that properly substituted pyridine or phenyl groups might replace the catecholic group and represent useful moieties for building novel ligands endowed with preferential D1- or D2-like affinity. Thus the new chemical entities 5a-g (chapter 4) might represent starting points to develop new interesting DA ligands. The presence of Aβ-peptide containing plaques in the brain, is a hallmark of Alzheimer‘s disease (AD) (chapter 5). Several studies have suggested a pathogenic role in AD for Aβ42, the longer form of the two Aβ-peptides, which has a strongest tendency to deposit insoluble plaques in the AD brain. Aβ peptides are produced from β-Amyloid Precursor Protein (APP) through the sequential cleavage by two proteases, namely β-secretase and γ-secretase. Firstly, β-secretase produces a C-terminal fragment of APP (CTFβ), then γ-secretase cleaves the CTFβ to produce Aβ peptides of different lengths. Preventing the formation or intracellular deposition of Aβ42-peptide represents a strategy to stop or slow down the progression of the AD. At this purpose, inhibition of γ-secretase has proven to be an interesting target for drug discovery. One of the first compounds which efficaciously inhibited Aβ production in animal models of AD blocking γ-secretase activity was LY411575 (chapter 5). It has been demonstrated that inhibition of γ-secretase reduce Aβ in a murine model of AD, but has potentially undesirable biological effects, most likely due to of the inhibition of Notch cleavage, a transmembrane receptor involved in regulating cell-fate decision. Thus, selective inhibition of Aβ42 production seems to be a promising alternative to the complete inhibition of γ-secretase activity. The pursuit of such a result was made possible by the surprising finding that certain non-steroidal anti-inflammatory drugs (NSAIDs), including flurbiprofen, display preferential Aβ42-lowering activity without affecting Notch cleavage. The proposed mechanism for this activity is an allosteric modulation of γ-secretase activity, the enzyme responsible for the formation of Aβ. Starting from these data we proposed the synthesis of new LY411575 analogues (11a-f; chapter 5) that were designed to function as potential γ-secretase modulators able to penetrate the blood-brain barrier.