J Med Chem. 2014 Aug 14;57(15):6679-703.

Indazole- and indole-5-carboxamides: selective and reversible monoamine oxidase B inhibitors with subnanomolar potency.

Nikolay T. Tzvetkov,a* Sonja Hinz,b Petra Küppers,b Marcus Gastreich,c Christa E. Müllerb

aUniversity of Bonn, Pharmaceutical Institute, Pharmaceutical Chemistry II, Bonn, Germany

aPharmaCenter Bonn, University of Bonn, Pharmaceutical Institute, Pharmaceutical Chemistry I, Bonn, Germany

bBioSolveIT GmbH, An der Ziegelei 79, 53757 St. Augustin, Germany



Indazole- and indole-carboxamides were discovered as highly potent, selective, competitive and reversible inhibitors of monoamine oxidase B (MAO-B). The compounds are easily accessible by standard synthetic procedures with high overall yields. The most potent derivatives were N-(3,4-dichlorophenyl)-1-methyl-1H-indazole-5-carboxamide (NTZ-1091; former PSB-1491, IC50 human MAO-B 0.386 nM, >25000-fold selective versus MAO-A) and N-(3,4-dichlorophenyl)-1H-indole-5-carboxamide (NTZ-1010, IC50 human MAO-B 0.227 nM, >5700-fold selective versus MAO-A). Replacement of the carboxamide linker with a methanimine spacer led a further novel class of highly potent and selective MAO-B inhibitors. In N-(3,4-difluorophenyl-1H-indazole-5-carboxamide (NTZ-1034, IC50 human MAO-B 1.59 nM, selectivity versus MAO-A >6000-fold) high potency and selectivity are optimally combined with superior physicochemical properties. Computational docking studies provided insights into the inhibitors’ interaction with the enzyme binding site and a rationale for their high potency despite their small molecular size.

PMID: 24955776



Monoamine oxidases (МАО, EC are flavin adenine dinucleotide (FAD) containing enzymes localized on the mitochondrial outer membrane, which catalyze the oxidative deamination of biogenic amines and monoamine neurotransmitters.Two isoforms of MAO are present in most mammalian tissues, MAO-A and MAO-B, distinguished by their substrate and inhibitor selectivity [1;2]. The reaction catalyzed by MAO results in the production of hydrogen peroxide (H2O2) and other reactive oxygen species (ROS), which may contribute to oxidative stress and cell damage [3]. The activity and the expression levels of MAO-B in the human brain, but not those of MAO-A, increase with aging and may be associated with the loss of dopaminergic neurons in the substantia nigra, where MAO-B is the main form in glial cells [4]. The relationship between oxidative stress and progressive neuronal impairment indicates that inhibition of MAO-B activity may have neuroprotective effects [4;5]. Therefore, MAO-B inhibitors may be useful for the treatment of other neurodegenerative diseases as well including Alzheimer´s disease (AD) [6].

Reversible inhibition is expected to have considerable advantages compared to irreversible inhibition [7]. In the present study, novel classes of most potent reversible MAO-B inhibitors were discovered, namely indazole-5-carboxamides (designated class I), indole-5-carboxamides (class II) and (indazol-5-yl)methanimine derivatives (class III). The new compounds were evaluated at rat and human MAO A and B, and optimized in order to improve their MAO-B affinity and selectivity, as well as their physicochemical and drug-like properties. Computational studies were performed to understand their binding modes and to explain their exceedingly high affinities.

The indazol-5-carboxamide scaffold served as a starting point for the design, synthesis and biological evaluation of a number of compounds, many of them with MAO-B inhibitory activity in the subnanomolar range and significantly improved physicochemical properties. We also prepared two additional classes of MAO-B inhibitors either by replacement of the 5-substituted indazole by a 5-substituted indole (class II), or by exchange of the amide connection by a methanimine linker (class III).

As the most promising MAO-B inhibitors of the present series with drug-like properties, two compounds have to be highlighted (Figure 1).


Figure 1. Structure optimization process of lead structure NTZ-1006 towards derivative NTZ-1034 with improved properties by replacement of the 3-Cl (step a), or the 3,4-di-Cl atoms (step b) with one (NTZ-1036) or two fluorine atoms (NTZ-1034). The binding efficiency index (BEI) is defined as BEI = pIC50/MR (in 1/kDa).


Structural optimization and SAR analyses led to the discovery of remarkably potent competitive and reversible MAO-B inhibitors with subnanomolar potency. Furthermore, the binding mode of selected C5- versus C6-substituted indazole-carboxamide derivatives within the binding pocket of the human MAO-B enzyme was investigated. The molecular modeling studies provided insights into the main interactions and structural requirements of enzyme-inhibitor binding and broadened our understanding of the compounds’ requirements for achieving high MAO-B affinity and selectivity. Thus, several indazole-5- and indole-5-carboxamide analogues with different substituents at the phenyl 3- and 4-positions and those with an N1-methylated indole or indazole moiety were identified that may serve as promising lead structures or even drug candidates e.g., for the treatment of PD and AD. Moreover, they will be highly useful as pharmacological tools for in vitro and in vivo studies, and may be suitable for the development of radioligands, including diagnostics for positron emission tomography (PET). As an example, compound NTZ-1034 can be highlighted because of its remarkable in vitro MAO-B inhibitory activity and its well-balanced physicochemical profile, which is predictive of CNS bioavailability. Future efforts will be directed towards further improving the compounds’ drug-like properties with regard to water-solubility, bioavailability, metabolism and toxicity, and to evaluate the new MAO-B inhibitors in relevant animal models.



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[7]   Tzvetkov NT, Pajeva IK. Binding and interactions of a novel potent indole-5-carboxamide MAO-B inhibitor. Comp. Rend. Acad. Bulg. Sci. 67(7): 937–942.(2014).



Dr. Nikolay T. Tzvetkov

University of Bonn

Pharmaceutical Institute

Pharmaceutical Chemistry II

An der Immneburg 4

53121 Bonn, Germany




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