Recent Developments in New Therapeutic Agents against Alzheimer and Parkinson Diseases: In-Silico Approaches.

Neurodegenerative diseases (ND), including Alzheimer's (AD) and Parkinson's Disease (PD), are becoming increasingly more common and are recognized as a social problem in modern societies. These disorders are characterized by a progressive neurodegeneration and are considered one of the main causes of disability and mortality worldwide. Currently, there is no existing cure for AD nor PD and the clinically used drugs aim only at symptomatic relief, and are not capable of stopping neurodegeneration. Over the last years, several drug candidates reached clinical trials phases, but they were suspended, mainly because of the unsatisfactory pharmacological benefits. Recently, the number of compounds developed using in silico approaches has been increasing at a promising rate, mainly evaluating the affinity for several macromolecular targets and applying filters to exclude compounds with potentially unfavorable pharmacokinetics. Thus, in this review, an overview of the current therapeutics in use for these two ND, the main targets in drug development, and the primary studies published in the last five years that used in silico approaches to design novel drug candidates for AD and PD treatment will be presented. In addition, future perspectives for the treatment of these ND will also be briefly discussed.

Synthesis and dopaminergic activity of a series of new 1-aryl tetrahydroisoquinolines and 2-substituted 1-aryl-3-tetrahydrobenzazepines.

A series of new 1-aryl-6,7-dihydroxy tetrahydroisoquinolines with several substitution patterns in the 1-aryl group at C-1 were prepared in good yields. The influence of each substituent on the affinity and selectivity for D1 and D2 dopaminergic receptors was studied. Moreover, N-alkyl salts of these tetrahydroisoquinolines were used as starting material to synthesize a series of new 1-aryl-7,8-dihydroxy 3-tetrahydrobenzazepines derivatives with electron-withdrawing substituents at C-2 position by the diastereoselective Stevens rearrangement. The structure-activity relationship of these compounds was explored to evaluate the effect of the functional group at C-2 in benzazepines and the modification in the aryl group at the isoquinoline C-1 position towards the affinity and selectivity for the mentioned receptors. The 1-aryl-6,7-dihydroxy tetrahydroisoquinoline 4c shows significant affinity towards D2 receptor, with Ki value of 31 nM. This significant affinity can be attributed to the presence of a thiomethyl group, and it is the most active 1-aryl-6,7-dihydroxy tetrahydroisoquinoline derivative reported to date.

Synthesis, Biological, and Computational Evaluation of Substituted 1-(2-Methoxyphenyl)-4-(1-phenethylpiperidin-4-yl)piperazines and 1-(2-Methoxyphenyl)-4-[(1-phenethylpiperidin-4-yl)methyl]piperazines as Dopaminergic Ligands.

Sixteen new 1-(2-methoxyphenyl)-4-(1-phenethylpiperidin-4-yl)piperazines and 1-(2-methoxyphenyl)-4-[(1-phenethylpiperidin-4-yl)methyl]piperazines were synthesized to be used as probes for mapping the dopamine D2 receptor (D2 DAR) arylpiperazine binding site. All compounds were evaluated for their affinity toward D2 DAR in an in vitro competitive displacement assay. The most active one was 1-(2-methoxyphenyl)-4-{[1-(3-nitrophenethyl)piperidin-4-yl]methyl}piperazine (25) with an affinity of Ki = 54 nM. Docking analysis was conducted on all herein described compounds, whereas molecular dynamic simulation was performed on ligand 25 to establish its mode of interaction with D2 DAR. Two possible docking orientations are proposed; the one with a salt bridge between the piperidine moiety and Asp114 of D2 DAR is more stable.

Optimization and synthesis of an (18) F-labeled dopamine D3 receptor ligand using [(18) F]fluorophenylazocarboxylic tert-butylester.

There is still no efficient fluorine-18-labeled dopamine D3 subtype selective receptor ligand for studies with positron emission tomography. We aim at improving the D3 selectivity and hydrophilicity of a candidate ligand by changing the substitution pattern to a 2,3-dichlorophenylpiperazine and hydroxylation of the butyl chain. The compound [(18) F]3 exhibited D3 affinity of Ki = 3.6 nM, increased subtype selectivity (Ki (D2 /D3 ) = 60), and low affinity to 5-HT1A and α1 receptors (Ki (5-HT1A /D3 ) = 34; Ki (α1 /D3 ) = 100). The two-step radiosynthesis was optimized for analog [(18) F]4 by reducing the necessary concentration of the precursor amine (57 mM), which reacted with [(18) F]fluorophenylazocarboxylic tert-butylester under basic conditions. The optimization of the base (Cs 2 CO3 , 23 mM) and the adjustment of reaction temperature led to the radiochemical yield of 63% after 5 min at 35°C. The optimized reaction conditions were transferred on to the synthesis of [(18) F]3 with an overall non-decay corrected yield of 8-12% in a specific activity of 32-102 GBq/µmol after a total synthesis time of 30-35 min. This provides a D 3 radioligand candidate with improved attributes concerning selectivity and radiosynthesis for further preclinical studies.

Labeling of benzodioxin piperazines with fluorine-18 as prospective radioligands for selective imaging of dopamine D4 receptors.

The D(4) receptor is of high interest for research and clinical application but puts high demands on appropriate radioligands to be useful tools for investigation. Search for adequate radioligands suitable for in vivo imaging is therefore still in progress. The potential neuroleptic drug 6-(4-[4-fluorobenzyl]piperazin-1-yl)benzodioxin shows high affinity and selectivity to the D(4) receptor. Derivatization of this lead structure by adding hydrophilic moieties was carried out in order to lower its lipophilicity what led to three new putative dopamine receptor D(4) ligands. A comprehensive description of the syntheses of standard compounds and corresponding labeling precursors is given which were obtained in satisfactory yields. Furthermore, the radiosyntheses by direct (18) F-labeling and build-up synthesis were compared. All derivatives of 6-(4-[4-fluorobenzyl]-piperazin-1-yl)benzodioxin were successfully synthesized in (18) F-labeled form with radiochemical yields of 9-35% and molar activities of 30-60 GBq/µmol using one-pot procedures.

Further SAR study on 11-O-substituted aporphine analogues: identification of highly potent dopamine D3 receptor ligands.

A series of new aporphine analogues (aporlogues) were prepared from appropriate aporphine precursors and arylpiperazines using the Click reaction protocol. These compounds displayed good to high affinity at the D(3) receptor, low or no affinity at the D(1) and D(2) receptors. Compounds 7f and 11c stood out as the most potent at the D(3) receptor among our newly synthesized aporlogues with K(i) values of 2.67 and 1.14 nM, respectively. Further assay at the 5-HT(1A) receptor revealed that aporlogues 7f and 11c also showed high affinity at this receptor with K(i) values of 9.68 and 7.59 nM, respectively. They were 3.6- and 6.6-fold more potent at the D(3) over 5-HT(1A) receptors. Such D(3)/5-HT(1A) dual property of these compounds may be useful in the treatment of several brain disorders.

Synthesis of benzopyrano[3,4-c]pyridin-5-ones as dopamine D4 receptor ligands.

The dopamine D4 receptor is highly expressed in prefrontal cortex, hippocampus, amygdala, hypothalamus and is hypothesized to relate with the pathophysiology and pharmacotherapy of schizophrenia while its level in brain regions is much lower. To date, no specific ligand is available for the study of D4 receptor in vivo. In this study, we report the synthesis and in vitro receptor binding assay of three benzopyrano[3,4-c]pyridin-5-ones as potential dopamine D4 receptor ligands. These new compounds have higher affinity and selectivity toward dopamine D4 receptor and their K(i) values for D4 receptor are in the nanomolar (nM) range.

Isoquinolinone derivatives as potent CNS multi-receptor D2/5-HT1A/5-HT2A/5-HT6/5-HT7 agents: Synthesis and pharmacological evaluation.

In this study, a series of novel Isoquinolinone derivatives were synthesized as potential multi-target antipsychotics. Among these, compound 13 showed high affinity for dopamine D2 and serotonin 5-HT1A, 5-HT2A, 5-HT6, and 5-HT7 receptors, showed low affinity for off-target receptors (5-HT2C, H1, and α1), and negligible effects on ether-a-gogo-related gene (hERG; i.e., reduced QT interval prolongation). An animal behavioral study revealed that compound 13 reversed APO-induced hyperlocomotion, MK-801-induced hyperactivity, and DOI-induced head twitch. Moreover, compound 13 exhibited a high threshold for acute toxicity, a lack of tendency to induce catalepsy, and did not cause prolactin secretion or weight gain when compared to risperidone. Furthermore, in the forced swim test, tail suspension test, and novel object recognition test, treatment with compound 13 resulted in improvements in depression and cognitive impairment. In addition, compound 13 had a favorable pharmacokinetic profile in rats. Thus, the antipsychotic drug-like effects of compound 13 indicate that it may be useful for developing a novel class of drugs for the treatment of schizophrenia.

The chemistry toolbox of multitarget-directed ligands for Alzheimer's disease.

The discovery and development of multitarget-directed ligands (MTDLs) is a promising strategy to find new therapeutic solutions for neurodegenerative diseases (NDs), in particular for Alzheimer's disease (AD). Currently approved drugs for the clinical management of AD are based on a single-target strategy and focus on restoring neurotransmitter homeostasis. Finding disease-modifying therapies AD and other NDs remains an urgent unmet clinical need. The growing consensus that AD is a multifactorial disease, with several interconnected and deregulated pathological pathways, boosted an intensive research in the design of MTDLs. Due to this scientific boom, the knowledge behind the development of MTDLs remains diffuse and lacks balanced guidelines. To rationalize the large amount of data obtained in this field, we herein revise the progress made over the last 5 years on the development of MTDLs inspired by drugs approved for AD. Due to their putative therapeutic benefit in AD, MTDLs based on MAO-B inhibitors will also be discussed in this review.

Synthesis of chiral 3-methyl- and 3-methyl-N-propargyl-1,2,3,4-tetrahydroisoquinoline and prevention of MPP+ -induced cytotoxicity.

The chemical structure of selegiline, a commercially available drug for Parkinson's disease (PD), resembles that of 1,2,3,4-tetrahydroisoquinoline (TIQ), an endogenous parkinsonism-inducing compound. In the present study, we evaluated the direct cytotoxicity of (R)- and (S)-3-methyl-TIQ (3-MeTIQ) and (R)- and (S)-3-methyl-N-propargyl-TIQ (3-Me-N-propargyl-TIQ), as selegiline-mimetic TIQ derivatives, and their ability to prevent 1-methyl-4-phenylpyridinium iodide (MPP(+))-induced cell death. Synthesis of optically-pure 3-MeTIQs was achieved via the super acid-induced cyclization of chiral N-benzyl-N-[1-methyl-2-(phenylsulfinyl)ethyl]formamide using a Pummerer-type cyclization reaction as the key step in producing excellent yields. Subsequent N-propargylation of chiral 3-MeTIQs using propynylbromide gave the corresponding 3-Me-N-propargyl-TIQs. In our in vitro experiments, the direct cytotoxicity of chiral 3-MeTIQs and 3-Me-N-propargyl-TIQs was almost identical, with no relationship to optical chirality except for (S)-3-Me-N-propargyl-TIQ, which had significantly weaker direct cytotoxicity than the other 3-MeTIQ derivatives. However, the decreased viability of PC12 cells induced by treatment with MPP(+) was accelerated by the coexistence of 3-MeTIQs and inhibited by 3-Me-N-propargyl-TIQs without any participation of the stereochemistry at the 3-position. These results suggest that the N-propargyl group is necessary for protection of cells against the toxicity of MPP(+). Furthermore, the stereochemistry of the 3-position appears to partially participate in the direct cytotoxicity of 3-Me-N-propargyl-TIQs.

Design and synthesis of photoaffinity-labeling ligands of the L-prolyl-L-leucylglycinamide binding site involved in the allosteric modulation of the dopamine receptor.

Pro-Leu-Gly-NH(2) (PLG), in addition to its endocrine effects, possesses the ability to modulate dopamine D(2) receptors within the central nervous system. However, the precise binding site of PLG is unknown. Potential photoaffinity-labeling ligands of the PLG binding site were designed as tools to be used in the identification of the macromolecule that possesses this binding site. Six different photoaffinity-labeling ligands were designed and synthesized on the basis of the gamma-lactam PLG peptidomimetic 1. The 4-azidobenzoyl and 4-azido-2-hydroxybenzoyl photoaffinity-labeling moieties were placed at opposite ends of PLG peptidomimetic 1 to generate a series of ligands that potentially could be used to map the PLG binding site. All of the compounds that were synthesized possessed activity comparable to or better than PLG in enhancing [(3)H]-N-propylnorapomorphine agonist binding to dopamine receptors. Photoaffinity ligands that were cross-linked to the receptor preparation produced a modulatory effect that was either comparable to or greater than the increase in agonist binding produced by the respective ligands that were not cross-linked to the dopamine receptor. The results indicate that these photoaffinity-labeling agents are binding at the same allosteric site as PLG and PLG peptidomimetic 1.

Design, synthesis and preliminary evaluation of dopamine-amino acid conjugates as potential D1 dopaminergic modulators.

The dopamine-amino acid conjugate DA-Phen was firstly designed to obtain a useful prodrug for the therapy of Parkinson's disease, but experimental evidence shows that it effectively interacts with D1 dopamine receptors (D1DRs), leading to an enhancement in cognitive flexibility and to the development of adaptive strategies in aversive mazes, together with a decrease in despair-like behavior. In this paper, homology modelling, molecular dynamics, and site mapping of D1 receptor were carried out with the aim of further performing docking studies on other dopamine conjugates compared with D1 agonists, in the attempt to identify new compounds with potential dopaminergic activity. Two new conjugates (DA-Trp 2C, and DA-Leu 3C) have been identified as the most promising candidates, and consequently synthesized. Preliminary evaluation in terms of distribution coefficient (DpH7.4), stability in rat brain homogenate, and in human plasma confirmed that DA-Trp (2C), and DA-Leu (3C) could be considered as very valuable candidates for further in vivo studies as new dopaminergic drugs.

Synthesis, dopamine D2 receptor binding studies and docking analysis of 5-[3-(4-arylpiperazin-1-yl)propyl]-1H-benzimidazole, 5-[2-(4-arylpiperazin-1-yl)ethoxy]-1H-benzimidazole and their analogs.

5-[3-(4-Arylpiperazin-1-yl)propyl]-1H-benzimidazoles and 5-[2-(4-arylpiperazin-1-yl)ethoxy]-1H-benzimidazoles were synthesized and their affinity for the D1, D2 and 5-HT1A receptors examined. They expressed a rather high affinity for the D2 dopamine receptor. The main features of ligand-D2 receptor interactions revealed by docking analyses were: salt bridge between piperazine ring protonated N1 and Asp 86, hydrogen bonds of ligand bezimidazole part with Ser 141, Ser 122 and His 189, edge-to-face interactions of arylpiperazine aromatic ring with Phe 178, Tyr 216 and Trp 182 and hydrogen bond between ethereal oxygen in ethylenoxy ligands and hydrogen of Phe 185 or Trp 115. The most active 5-[2-[4-(2-methoxyphenyl)-piperazin-1-yl]ethoxy]-1,3-dihydro-2H-benzimidazole-2-thione (27) has a maximal number of attractive interactions. A satisfactory correlation between docking of the compounds into the D2 receptor and competition binding results was observed.

Synthesis of hexahydrocyclopenta[ij]isoquinolines as a new class of dopaminergic agents.

In this study, we have described the synthesis of the tricyclic 1,2,3,7,8,8a-hexahydrocyclopenta [ij]isoquinoline (HCPIQ). Herein, six differently substituted 5,6-dioxygenated-7-phenyl-HCPIQs have been synthesized using a new methodology via (E)-1-styryl-THIQ by Friedel-Crafts cyclization with Eaton's reagent. Results showed that HCPIQs (3, 3a-e) displayed a moderate affinity for D1 dopamine receptors (DR) in the micromolar range, furthermore the catecholic HCPIQs 3a (NH), 3c (NCH3) and 3e (NCH2CHCH2) exhibited outstanding affinity and high selectivity towards D2 DR. Indeed, 3a, 3c and 3e showed Ki values of 29 nM, 13 nM and 18 nM, respectively, and HCPIQs 3a (NH) and 3c (NCH3) displayed a remarkable selectivity (Ki D1/D2 ratio ∼ 1000-2500). In addition, none of the catecholic compounds showed any cytotoxicity in freshly isolated human neutrophils. Although further studies are needed, these compounds and particularly catecholic HCPIQs, show high potential in the treatment of Parkinson's disease, psychosis or depression.

Discovery of a Novel Class of Negative Allosteric Modulator of the Dopamine D2 Receptor Through Fragmentation of a Bitopic Ligand.

Recently, we have demonstrated that N-((trans)-4-(2-(7-cyano-3,4-dihydroisoquinolin-2(1H)-yl)ethyl)cyclohexyl)-1H-indole-2-carboxamide (SB269652) (1) adopts a bitopic pose at one protomer of a dopamine D2 receptor (D2R) dimer to negatively modulate the binding of dopamine at the other protomer. The 1H-indole-2-carboxamide moiety of 1 extends into a secondary pocket between the extracellular ends of TM2 and TM7 within the D2R protomer. To target this putative allosteric site, we generated and characterized fragments that include and extend from the 1H-indole-2-carboxamide moiety of 1. N-Isopropyl-1H-indole-2-carboxamide (3) displayed allosteric pharmacology and sensitivity to mutations of the same residues at the top of TM2 as was observed for 1. Using 3 as an "allosteric lead", we designed and synthesized an extensive fragment library to generate novel SAR and identify N-butyl-1H-indole-2-carboxamide (11d), which displayed both increased negative cooperativity and affinity for the D2R. These data illustrate that fragmentation of extended compounds can expose fragments with purely allosteric pharmacology.

Synthesis and dopaminergic activity of 2-substituted octahydrobenzo[f]quinolines.

A series of 2-substituted octahydrobenzo[f]quinolines has been synthesized and assayed for dopamine agonist activity. Only the compounds corresponding to the beta-rotameric conformation of dopamine showed biphasic activity in competition binding studies with the radioligand [3H]spiroperidol. These findings suggest that the congeners possessing the beta-rotamer conformation show receptor-binding characteristics that resemble those of the ergolines more closely than do those of the corresponding alpha-rotamer congeners.

R and S enantiomers of 11-hydroxy- and 10,11-dihydroxy-N-allylnoraporphine: synthesis and affinity for dopamine receptors in rat brain tissue.

The R-(-)- and S-(+)-enantiomers of 11-hydroxy-N-allyl (4), and 10,11-dihydroxy-N-allyl (3) congeners of 11-hydroxy-N-n-propylnorapomorphine (11-OH-NPa, 2) or N-n-propylnorapomorphine (NPA, 1) were synthesized. Binding affinity of these compounds at dopamine (DA) receptor sites was evaluated with a membrane preparation of corpus striatum from rat brain. The R/S enantiomeric receptor affinity ratio was enhanced by allylic substitution of 3 and 4 and their R isomers had high DA receptor affinity similar to that of the N-n-propyl congeners. These N-allylnoraporphines are proposed as useful precursors to the preparation of their tritiated N-n-propyl enantiomers.

Synthesis and dopamine receptor affinities of enantiomers of 2-substituted apomorphines and their N-n-propyl analogues.

Syntheses of (R)-(-)-2-methoxyapomorphine (R-8), its antipode S-8, and its (R)-(-)-N-n-propyl R-9 derivatives are described. The dopaminergic receptor affinities of these compounds and their 2-unsubstituted counterparts (R)-(-)-apomorphine (R(-)-APO, R-1), (S)-(+)-apomorphine (S(+)-APO, S-1), and (R)-(-)-N-n-propylnorapomorphine (R(-)-NPA, R-2), as well as those of (R)-(-)-2-chloroapomorphine (R(-)-2-Cl-APO, R-6), (R)-(-)-2-bromoapomorphine (R(-)-2-Br-APO, R-6), were determined with tissue membrane preparations of corpus striatum from rat brain. Contribution of both an N-n-propyl and a 2-hydroxy in (R)-(-)-2-hydroxy-N-n-propylnorapomorphine (R(-)-2-OH-NPA, R-7) or a methoxy group in (R)-(-)-2-methoxy-N-n-propylnorapomorphine (R(-)-2-OCH3-NPA, R-9) produced the highest D2 affinity (0.053 and 0.17 nM) and D2 over D1 selectivity (17,300 and 10,500 times) of the compounds evaluated. The structure-affinity relationships of these 2-substituted aporphines suggest that secondary binding sites of D2 receptors interact with 2-substituents on the A ring of aporphines through H-bonding.

Synthesis and dopamine receptor modulating activity of lactam conformationally constrained analogues of Pro-Leu-Gly-NH2.

A series of analogues of the potent analogue of Pro-Leu-Gly-NH2 (PLG), 2-oxo-3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-1-pyrrolidineacet amide (2) were synthesized in which the (R)-gamma-lactam residue of 2 was replaced with a (R)-beta-lactam, (R)-aminosuccinimide, (R)-cycloseryl, (R)-delta-lactam, (R)-epsilon-lactam, or (S)-epsilon-lactam residue to give analogues 3-8, respectively. These substitutions were made so as to vary the psi 2 torsion angle. The analogues were tested for their ability to enhance the binding of the dopamine receptor agonist ADTN to the dopamine receptor. Analogues 3-6 and 8 exhibited dose-response curves that were bell-shaped in nature with the maximum effect occurring at a concentration of 1 microM. Analogue 7 was inactive. Analogues 3 and 4 were found to be as effective as PLG, while analogues 5, 6, and 8 appeared to be more effective than PLG in terms of enhancing the binding of ADTN to dopamine receptors. The activity of analogues 3-6 and 8 with their psi 2 angles in the vicinity of that observed in a type II beta-turn is consistent with the hypothesis that this type of turn is the bioactive conformation of PLG.

Synthesis and evaluation of N,N-di-n-propyltetrahydrobenz[f]indol-7-amine and related congeners as dopaminergic agonists.

An evaluation of 6-[2-(di-n-propylamino)ethyl]indole (4), its rigid analogue N,N-di-n-propyl-5,6,7,8-tetrahydrobenz[f]indol-7-amine (5), and some related congeners, for ability to suppress serum prolactin in reserpinized rats, revealed modest biological activity in this in vivo model of dopaminergic activity. Although the indole N-H in these compounds can be considered to be oriented "meta" with respect to the ethylamine side chain, compounds with the indole N-H located in the other "meta" position (i.e. 4-[2-(di-n-propylamino)ethyl]indole (2) or its rigid benz[e]indole analogue 3) were much more potent dopamine agonists. The results argue for a particular orientation of the indole N-H vector. In addition, relatively potent dopamine agonists also resulted when the pyrrole portion of the indole ring was replaced by a methanesulfonamido function, supporting the idea that the indole N-H serves as a hydrogen-bond donor.