**2.2** *N***-Myristoyltransferase (glycylpeptide N-tetradecanoyltransferase, NMT; EC 2.3.1.97)**

NMT catalyzes the co- and post-translational addition of myristic acid (saturated, 14-carbon fatty acid) onto the N-terminal glycine of specific proteins in

eukaryotes (**Figure 2**). This physiological pathway, *N*-myristoylation, plays an important role in the correct cellular localization and biological functions. NMT enzyme was purified and characterized from yeasts for the first time and it is thought to be a target for development of a new class of antifungal drugs [26]. The presence of NMT in *L. major* was verified in 1997 [27]. Later, NMT enzyme activity was proven essential for viability in *Leishmania sp*. then, it attracted attention as a potential drug target in kinetoplastid parasites [28]. The validation of this enzyme as a target for antitrypanosomal and antileishmanial drug discovery was not until 2010 (**Figure 2**) [29, 30].

A group of antifungal agents was tested to identify the first NMT inhibitors by Panethymitaki et al. in 2006 [31]. Although some of the tested compounds were found to be NMP inhibitors in a low μM concentration range, their antileishmanial activity has not been reported [31].

In an HTS campaign led by Pfizer, around 150.000 compounds from the Pfizer Global Diverse Representative Set were screened against protozoan NMTs. Four different scaffolds, namely aminoacylpyrrolidine (PF-03402623 IC50 of 0.093 μM), piperidinylindole (PF-03393842 IC50 of 0.102 μM), thienopyrimidine (PF-00349412 (IC50 of 0.482 μM), and biphenyl (PF-00075634 (IC50 of 0.158 μM) derivatives were identified as novel inhibitors of *Labrus donovani* NMP (**Figure 3**) [32].

Following the previous study, the crystal structures of PF-03393842 and PF-03402623 with the enzyme, the initial hits selected in the HTS campaign, were elucidated. Based on this data, a fused hybrid compound **43** was developed as a highly potent *L*. *donovani* NMT inhibitor (Ki of 1.6 nM) with good selectivity over the human isoform of the enzyme (Ki 27 nM) (**Figure 3**) [33]. Although the lack of cell activity of 43 attributed to its poor uptake, the HTS campaign, and hybridization of the hit compounds have resulted in the discovery of a new scaffold [33].

Another HTS assay dedicated to identifying novel *Leishmania sp.* NMT inhibitors was focused on a set of 1600 pyrazolyl sulfonamide compounds [34]. Interestingly, no correlation between the enzyme potency of these inhibitors and their cellular activity against *L. donovani* axenic amastigotes was observed. This might be rationalized by the fact that poor cellular uptake considering the basicity of the compounds. The most potent inhibitor of *Lm*NMT (compound 2, Ki of 0.34 nM) exhibited modest activity against *L. donovani* intracellular amastigotes

**Figure 2.** *Myristoylated proteins with NMT.*

*Toward New Antileishmanial Compounds: Molecular Targets for Leishmaniasis Treatment DOI: http://dx.doi.org/10.5772/intechopen.101132*

**Figure 3.** *Examples of NMT inhibitor structures with antileishmanial activity.*

(EC50 of 2.4 μM). Yet, advanced studies on compound 2 confirmed the on-target mechanism. Moreover, oral use of compound 2 resulted in a 52% reduction in parasite burden in the mouse model of VL (**Figure 3**) [34].

Other NMT inhibitors as potential antileishmanial compounds were reported in a few publications and patents. In these studies, pyrrolidines, piperidinylindoles, azetidinopyrimidines, aminomethylindazoles, benzimidazoles, thienopyrimidines, biphenyl derivatives, benzofuranes, benzothiophenes, oxadiazoles, (pyrazolomethyl)-1,3,4-oxadiazoles and thienopyrimidine scaffolds, and peptidomimetic inhibitors were reported with their NMT inhibitory properties [35–38].
