Elsevier

Bioorganic & Medicinal Chemistry

Volume 24, Issue 20, 15 October 2016, Pages 4835-4854
Bioorganic & Medicinal Chemistry

Design, synthesis and biological evaluation of N-methyl-N-[(1,2,3-triazol-4-yl)alkyl]propargylamines as novel monoamine oxidase B inhibitors

https://doi.org/10.1016/j.bmc.2016.06.045Get rights and content

Abstract

Different azides and alkynes have been coupled via Cu-catalyzed 1,3-dipolar Huisgen cycloaddition to afford a novel family of N1- and C5-substituted 1,2,3-triazole derivatives that feature the propargylamine group typical of irreversible MAO-B inhibitors at the C4-side chain of the triazole ring. All the synthesized compounds were evaluated against human MAO-A and MAO-B. Structure–activity relationships and molecular modeling were utilized to gain insight into the structural and chemical features that enhance the binding affinity and selectivity between the two enzyme isoforms. Several lead compounds, in terms of potency (submicromolar to low micromolar range), MAO-B selective recognition, and brain permeability, were identified. One of these leads (MAO-B IC50 of 3.54 μM, selectivity MAO-A/MAO-B index of 27.7) was further subjected to reversibility and time-dependence inhibition studies, which disclosed a slow and irreversible inhibition of human MAO-B. Overall, the results support the suitability of the 4-triazolylalkyl propargylamine scaffold for exploring the design of multipotent anti-Alzheimer compounds endowed with irreversible MAO-B inhibitory activity.

Introduction

Alzheimer’s disease (AD) is one of the most relevant age-related neurodegenerative disorders currently representing the fourth leading cause of death and afflicting over 46.8 million people worldwide.1, 2 Its clinical manifestation is mainly reflected in a progressive loss of memory and cognitive functions, often in association with behavioral disturbances and depression.3 Its complex and multifaceted etiopathology, which involves massive loss of cholinergic neurons,4 oxidative stress,5, 6, 7 metal dyshomeostasis,8 excitotoxicity,9 neurofibrillary tangles and β-amyloid aggregate deposition,10, 11 has precluded so far the discovery of effective disease-modifying drugs. Currently approved drugs, namely, three acetylcholinesterase (AChE) inhibitors (rivastigmine, galantamine, and donepezil),12, 13, 14 and an N-methyl-d-aspartate receptor antagonist (memantine),15 primarily exert palliative effects. On the other hand, the failure in clinical trials of a number of drug candidates designed against targets mainly involved in β-amyloid biology has shifted drug discovery efforts toward the compounds hitting less explored biological targets alone or in combination with other key targets, i.e., the so-called multi-target-directed ligands (MTDLs).16, 17, 18

In this context, monoamine oxidase (MAO, E.C.1.4.3.4) has emerged as a promising target because of the neuroprotective properties exerted by their inhibitors.2, 19, 20, 21 MAO is a flavin adenine dinucleotide (FAD)-containing enzyme that catalyzes the degradation of biogenic and xenobiotic amines. Two isoforms, namely MAO-A and MAO-B, have been characterized by their amino acid sequence, tissue distribution, substrate specificity and inhibitor sensitivity.22, 23, 24 MAO-A, preferentially degrading serotonin, adrenaline and noradrenaline, is irreversibly inhibited by clorgyline, whereas MAO-B, specifically responsible for the oxidative deamination of phenylethylamine and benzylamine, is irreversibly inhibited by (R)-(−)-deprenyl (selegiline) (Fig. 1). These trends reflect structural differences in the binding sites as revealed by high-resolution X-ray structures.25, 26, 27, 28, 29 In particular, a key structural feature in shaping the substrate cavity is the replacement of the pair Phe208/Ile335 in MAO-A by Ile199/Tyr326 in MAO-B, leading to the distinction between ‘substrate’ and ‘entrance’ sites in MAO-B. The replacement of Ile180/Asn181 and Val210 in MAO-A by Leu17/Cys172 and Thr201 in MAO-B are additional differences in the binding sites, which may modulate the selective inhibition by certain MAO inhibitors.30

The neuroprotection exerted by MAO inhibitors may stem not only from the increased amine neurotransmission, but also from preventing the formation of neurotoxic species, which may lead to neuronal damage,31, 32 and from the anti-apoptotic properties of the propargylamine group present in some MAO inhibitors.33, 34 Interestingly, the levels of MAO-B increase with age and its activity is elevated in AD patients, which results in increased brain levels of neurotoxic free radicals.20 In this context, the development of MAO-B-inhibitor-based MTDLs emerges as a promising strategy for the design of neuroprotective agents with potential disease-modifying activity towards AD and other neurodegenerative disorders, such as Parkinson disease.35, 36

Most efforts have been addressed toward the design of MTDLs targeting AChE and/or butyrylcholinesterase (BuChE) and MAO. A successful example is ladostigil (Fig. 1), a dual inhibitor of MAO-B and AChE that combines the carbamate moiety of the AChE inhibitor rivastigmine with the indolamine moiety of the selective MAO-B inhibitor rasagiline, and shows neuroprotective and anti-apoptotic activities.37, 38 A novel series of MAO/ChE inhibitors that combine the N-benzylpiperidine moiety of the AChE inhibitor donepezil with the indolyl propargylamine of the potent MAO-B inhibitor PF9601N has been reported.39 Within this series, the most promising MDTL compound (ASS234, 6 in Fig. 1) also showed anti-aggregating activity on β-amyloid, antioxidant and antiapoptotic behavior, and crossed easily the blood–brain barrier (BBB).40 Other strategies have relied on the hybridization of coumarins with either N-benzyl-N-alkyloxy groups41 or tacrine,42 leading to multipotent inhibitors of both MAO and ChEs, or alternatively have pursued the development of MTDLs targeting both MAO inhibition and additional activities, such as metal chelation.43, 44 Also, natural products endowed with MAO and ChEs inhibitory activities have been recently reported.45

Although the development of MAO-inhibitor-based MTDLs is very attractive, it is challenged by the need to keep a good balance among potencies against multiple targets and optimal ADME-T properties.46, 47, 48, 49, 50 Furthermore, the success of this strategy depends on the suitability of an efficient synthetic approach, which should afford the fusion or linkage of chemical scaffolds while minimizing drastic alterations in the activity against the multiple targets. In this context, the Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) enables the synthesis of a virtually unlimited source of ligands containing the 1,4-disubstituted 1,2,3-triazole core.51, 52, 53, 54 The CuAAC reaction between libraries of azides and alkynes featuring different pharmacophoric moieties has been used for the synthesis of triazole-linked hybrid compounds as MTDLs.55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 Azide–alkyne cycloaddition reactions have also been used for the synthesis of high-affinity multisite enzyme inhibitors inside the biological target, in the absence of copper catalysis, i.e., the so-called in situ click chemistry.66, 67, 68, 69, 70 In these compounds the triazole ring is used not only as a non-hydrolyzable, non-oxidizable, and non-reducible robust linker between the pharmacophoric moieties, but also provides favorable physicochemical properties and potential interactions with the biological target.71

In this context, as the first step of a program directed to the synthesis of MAO-B-inhibitor-based anti-Alzheimer MTDLs, here we have explored the CuAAC-mediated synthesis of a series of 1,2,3-triazole derivatives, featuring the propargylamine group of typical irreversible MAO-B inhibitors in the side chain at position 4, while the overall hydrophobicity has been modulated through different lipophilic substituents at positions 1 and 5 (Scheme 1). To determine the therapeutic potential of the target compounds and their usefulness as the MAO-B pharmacophoric moiety of novel families of MTDLs, we have assessed the in vitro inhibitory activity of the novel compounds against human MAO-B and MAO-A. Molecular modeling studies have been performed to rationalize the differences in inhibitory potency and selectivity between the MAO isoforms, and the mechanism of action of these compounds has been studied by reversibility and time-dependence inhibition studies. Finally, the brain penetration of the novel compounds has been examined through the widely used parallel artificial membrane permeability assay (PAMPA-BBB).

Section snippets

Design and synthesis of the target N-methyl-N-[(1,2,3-triazol-4-yl)alkyl]propargylamines

We initially planned the synthesis of the 1-ethyltriazolylmethyl and 1-ethyltriazolylethyl propargylamines 31 and 32 (Scheme 2), unsubstituted at position 5 of the triazole ring, and their 5-methyl-substituted analogues 51 and 52, as the early simple prototypes to validate the suitability of the CuAAC-assisted synthetic methodology to deliver 1,4-disubstituted and 1,4,5-trisubstituted 1,2,3-triazole scaffolds featuring the propargylamine group of irreversible MAO inhibitors. These compounds are

Conclusion

The results reported in this study support the feasibility of the CuAAC-mediated synthesis of 1,2,3-triazole derivatives featuring the propargylamine group typical of irreversible MAO-A and MAO-B inhibitors, leading to suitable templates for the design of MAO-B inhibitor-based MTDLs. The synthetic procedures of the 1,4-disubstituted or 1,4,5-trisubstituted 1,2,3-triazole scaffolds involve 4–6-step sequences from the CuAAC of alkyl or phenyl azides with an alkyne bearing an N-Boc-protected

Chemistry. General methods

Melting points were determined in open capillary tubes with a MFB 595010M Gallenkamp melting point apparatus. 400 MHz 1H/100.6 MHz 13C NMR spectra were recorded on a Varian Mercury 400 spectrometer at the Centres Científics i Tecnològics of the University of Barcelona (CCiTUB). The chemical shifts are reported in ppm (δ scale) relative to solvent signals (CD3OD at 3.31 and 49.0 ppm in the 1H and 13C NMR spectra, respectively; CDCl3 at 7.26 and 77.16 ppm in the 1H and 13C NMR spectra, respectively),

Acknowledgements

We thank the financial support from Ministerio de Economía y Competitividad (SAF2014-57094-R) and the Generalitat de Catalunya (GC; 2014SGR52 and 2014SGR1189). We are grateful to the Consorci de Serveis Universitaris de Catalunya for computational resources. F.J.L. acknowledges the support from ICREA Academia. Fellowships from GC to O.D.P., N.A., E.V., J.J.-J. and I.S. are gratefully acknowledged.

References and notes (85)

  • E.K. Perry et al.

    Lancet

    (1977)
  • G. Perry et al.

    Free Radical Biol. Med.

    (2000)
  • A.J. Mishizen-Eberz et al.

    Neurobiol. Dis.

    (2004)
  • M. Goedert et al.

    Neuron

    (1989)
  • G.G. Glenner et al.

    J. Neurol. Sci.

    (1989)
  • T. Thomas

    Neurobiol. Aging

    (2000)
  • P. Riederer et al.

    Neurotoxicology

    (2004)
  • B.S. Kristal et al.

    Free Radical Biol. Med.

    (2001)
  • K. Chen et al.

    J. Biol. Chem.

    (2004)
  • J. Juárez-Jiménez et al.

    Biochim. Biophys. Acta

    (2014)
  • M.S. Song et al.

    Prog. Neuropsychopharmacol. Biol. Psychiatry

    (2013)
  • W.J. Burke et al.

    Neurotoxicology

    (2004)
  • S.S. Xie et al.

    Eur. J. Med. Chem.

    (2015)
  • C.S. Passos et al.

    Phytochemistry

    (2013)
  • J. Ramprasad et al.

    Bioorg. Med. Chem. Lett.

    (2015)
  • N.M. Mishra et al.

    Org. Biomol. Chem.

    (2015)
  • L.-Y. Ma et al.

    Eur. J. Med. Chem.

    (2014)
  • P. Fabbrizzi et al.

    Tetrahedron

    (2014)
  • H.C. Kolb et al.

    Drug Discovery Today

    (2003)
  • Z.-J. Shi et al.

    Compr. Inorg. Chem. II

    (2013)
  • A. Orsini et al.

    Tetrahedron Lett.

    (2005)
  • F. Hubálek et al.

    J. Biol. Chem.

    (2005)
  • G. Esteban et al.

    Biochim. Biophys. Acta

    (2014)
  • L. Di et al.

    Eur. J. Med. Chem.

    (2003)
  • M. Zhou et al.

    Anal. Biochem.

    (1997)
  • M. Prince et al.

    World Alzheimer Report 2015. The Global Impact of Dementia. An Analysis of Prevalence, Incidence, Cost & Trends

    (2015)
  • I. Bolea et al.

    J. Neural Transm.

    (2013)
  • N. Guzior et al.

    Curr. Med. Chem.

    (2015)
  • J.T. Coyle et al.

    Science

    (1993)
  • A. Gella et al.

    Cell Adhes. Migr.

    (2009)
  • X. Huang et al.

    Ann. N.Y. Acad. Sci.

    (2004)
  • J. Birks et al.

    Cochrane Database Syst. Rev.

    (2006)
  • J. Birks et al.

    Cochrane Database Syst. Rev.

    (2000)
  • C. Loy et al.

    Cochrane Database Syst. Rev.

    (2004)
  • S.A. Areosa et al.

    Cochrane Database Syst. Rev.

    (2005)
  • X. Chen et al.

    Curr. Med. Chem.

    (2013)
  • C. Galdeano et al.

    Curr. Pharm. Des.

    (2010)
  • A. Matinez

    Emerging Drugs and Targets for Alzheimer’s Disease. Vol. 1. Beta-Amyloid, Tau Protein and Glucose Metabolism

    (2010)
  • D.E. Edmondson et al.

    Biochemistry

    (2009)
  • K.F. Tripton et al.

    Curr. Med. Chem.

    (2004)
  • C. Binda et al.

    Nat. Struct. Biol.

    (2002)
  • C. Binda et al.

    Proc. Natl. Acad. Sci. U.S.A.

    (2003)
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