**2. Potential molecular targets for the treatment of leishmaniasis**

### **2.1 Pteridine reductase (PTR1, Pteridine reductase 1, EC 1.5.1.33)**

PTR1 enzyme is an NADPH-dependent, short-chained reductase enzyme family member [4]. It is broadly active and can reduce a variety of unconjugated pteridines, as well as folates [5]. This enzyme has been investigated in studies of resistance to the dihydrofolate reductase inhibitor methotrexate (MTX) [6, 7]. After finding the missing link of resistance, researchers have suggested that inhibition of PTR1 may be a rational target for chemotherapy [4]. Since trypanosomatids are auxotrophic for folates and pterins, the inhibition of the PTR1 enzyme may also lead to selectivity. Therefore, PTR1 appears to be a rational target for antileishmanial drug development.

The first reported PTR1 inhibitors are pteridine analogs (diaminopteridines and quinazolines) and their activity was tested against purified *Leishmania major* pteridine reductase (*Lm*PTR1) [8]. The structure of *Lm*PTR1 in complex with NADPH and the inhibitor 2,4,6-triaminoquinazoline (TAQ ) were reported in 2004 [9]. Based on its crystal structure, Cavazzutti *et al.* analyzed a library of 440 synthetic folate-like compounds and tested selected compounds on *Lm*PTR1 among other enzymes such as DHFR [10]. In this study compound, 6b was found to be the most promising compound with a Ki value of 37 nM toward LmPTR1. Then, the crystal structure of the *Lm*PTR1:NADPH:6b ternary complex revealed a substrate-like binding mode (**Figure 1**) [10].

It was reported that pteridine, pyrrolopyrimidine, and 2,4-diaminopyrimidine scaffold as PTR1 inhibitors with a structure-based approach by Tulloch et al. [11]. Among the tested compounds, compounds 11 and 13 bearings pyrrolopyrimidine core were reported with a modest ED50 value and a good lethality to the parasites. Additionally, a combination of MTX and compound 13 resulted in an improvement in efficacy [11]. Based on these hit molecules, TbPTR1 inhibitors were developed for the treatment of human African trypanosomiasis (**Figure 1**) [12].

Also, nonfolate scaffolds with *Lm*PTR1 inhibition activity were reported. After three rounds of election considering computational and experimental results, 18 compounds were selected, and among them, compound 28b and compound 5c known CNS active drug, showed promising activity with their IC50 values of 93 μM and 50 μM, Ki values of 7 μM and 4 μM, respectively (**Figure 1**) [13]. Moreover, 5c in combination with pyrimethamine showed antileishmanial activity on promastigotes with no hDHFR inhibition [14]. Another nonfolate scaffold, hexahydro pyrimido pyrimidinone, was introduced with potential antileishmanial activity in a virtual screening study. Compound 7 was reported as a potent *Ld*PTR1 enzyme inhibitor (Ki of 0.72 μM) and showed promising *Leishmania donovani* amastigote and *Labrus donovani promastigote* activity with the IC50 value of 3 μM and 29 μM, respectively [15].

Apart from the compounds summed up so far, thianthrene [16], dihydropyrimidines [17], benzothiazoles [18], thiazolidinedione [19, 20], thienopyrimidine [21], thiazolopyrimidine [22], and natural products such as flavanone derivatives *Toward New Antileishmanial Compounds: Molecular Targets for Leishmaniasis Treatment DOI: http://dx.doi.org/10.5772/intechopen.101132*

**Figure 1.** *Examples of PTR1 inhibitor structures with antileishmanial activity.*

[23], 2,3-dehydrosilybin A, and sophoraflavanone G [24], kaurane-type diterpenes [25] were reported as PTR1 inhibitors with antileishmanial properties in the literature (**Figure 1**).
