tricyclic heteroaromatic ring systems were synthesized and tested against
PfDHODH and HsDHODH. It was observed that tricyclic derivatives, i.e.,
diethyl-2-((dibenzo[b,d]thiophen-2-ylamino)methylene)malonate(Fig. 13,
XIII)
and ethyl-2-cyano-3-((9-ethyl-9H-carbazol-3-yl)amino)acrylate (Fig. 13, XIV),
show the most promising results with PfDHODHIC 50 in lower micromolar range
(0.16 and 0.44 µM, respectively) and relatively high selectivity toward PfDHODH
(182- and 1208-fold selectivity, respectively, against HsDHODH). Heteroaromatic
bicyclic compounds (indazole and benzimidazole derivatives), phenyl derivatives,
and m-biphenyl derivatives have PfDHODHIC 50 values in higher micromolar range
and low selectivity (1–10 folds). The p-biphenyl derivatives were found to be
inactive in both PfDHODH and HsDHODH. The molecular docking results of the
tricyclic compounds (in the crystal structure with PDB ID 1TV5) demonstrated the
importance of planar aromatic hydrophobic groups for p-stacking interaction with
Phe188 (the selectivity is due to the presence of Ala59 in HsDHODH in place of
Phe188 in PfDHODH). The non-planar biphenyl rings are not accommodated into
the hydrophobic site, and hence, biphenyl derivatives are not suitable. The polar
groups of the active compounds showed hydrogen bonding interactions with
His185, Arg265, and Tyr528 amino acids at the end of the tunnel [79].
3.6.3 Triazolopyrimidine
Phillips et al. (2008) first reported triazolopyrimidine derivative obtained through
high-throughput screening studies on PfDHODH [78]. A total of 220,000 molecules
were screened through colorometric enzyme assay from which DSM1 (Fig. 14b)
was identified (PfDHODHIC 50 value of 0.047 ± 0.022 µM). This molecule showed
an EC 50 value of 0.079 ± 0.048 µM and 0.14 ± 0.05 µM in whole-cell assay
against non-resistant strain (3D7) and multidrug-resistant strain (Dd2), respectively.
The hit also showed >5000-fold selectivity against HsDHODH.
It was observed that primary amine is essential for the activity. Methyl group
substitution is suitable for R and R 1 position (Fig. 14a). Naphthyl group at R 3
(a)
(b)
Fig. 14 a General structure of triazolopyrimidine class of compounds. b Structure of DSM1,
DSM2, and DSM74
Structure-Based Design of PfDHODH Inhibitors …
201
PfDHODH and HsDHODH. It was observed that tricyclic derivatives, i.e.,
diethyl-2-((dibenzo[b,d]thiophen-2-ylamino)methylene)malonate(Fig. 13,
XIII)
and ethyl-2-cyano-3-((9-ethyl-9H-carbazol-3-yl)amino)acrylate (Fig. 13, XIV),
show the most promising results with PfDHODHIC 50 in lower micromolar range
(0.16 and 0.44 µM, respectively) and relatively high selectivity toward PfDHODH
(182- and 1208-fold selectivity, respectively, against HsDHODH). Heteroaromatic
bicyclic compounds (indazole and benzimidazole derivatives), phenyl derivatives,
and m-biphenyl derivatives have PfDHODHIC 50 values in higher micromolar range
and low selectivity (1–10 folds). The p-biphenyl derivatives were found to be
inactive in both PfDHODH and HsDHODH. The molecular docking results of the
tricyclic compounds (in the crystal structure with PDB ID 1TV5) demonstrated the
importance of planar aromatic hydrophobic groups for p-stacking interaction with
Phe188 (the selectivity is due to the presence of Ala59 in HsDHODH in place of
Phe188 in PfDHODH). The non-planar biphenyl rings are not accommodated into
the hydrophobic site, and hence, biphenyl derivatives are not suitable. The polar
groups of the active compounds showed hydrogen bonding interactions with
His185, Arg265, and Tyr528 amino acids at the end of the tunnel [79].
3.6.3 Triazolopyrimidine
Phillips et al. (2008) first reported triazolopyrimidine derivative obtained through
high-throughput screening studies on PfDHODH [78]. A total of 220,000 molecules
were screened through colorometric enzyme assay from which DSM1 (Fig. 14b)
was identified (PfDHODHIC 50 value of 0.047 ± 0.022 µM). This molecule showed
an EC 50 value of 0.079 ± 0.048 µM and 0.14 ± 0.05 µM in whole-cell assay
against non-resistant strain (3D7) and multidrug-resistant strain (Dd2), respectively.
The hit also showed >5000-fold selectivity against HsDHODH.
It was observed that primary amine is essential for the activity. Methyl group
substitution is suitable for R and R 1 position (Fig. 14a). Naphthyl group at R 3
(a)
(b)
Fig. 14 a General structure of triazolopyrimidine class of compounds. b Structure of DSM1,
DSM2, and DSM74
Structure-Based Design of PfDHODH Inhibitors …
201