observed that the N 1 –C 1 (Fig. 14a) bond length of the inhibitor was between single
and double bond (1.313 Å in DSM1) and partial positive charge was observed at
N 1 , indicated by the presence of chloride ion adjacent to the N 1 in the crystal
structure. It was suggested that the triazolopyrimidine undergoes electron delocalization between N 1 and N 5 , giving a low-range positive dipole at the N 1 center
which enhances its interaction with His185 and N 5 acquires a slight negative dipole
which allows it to form an ion pair with Arg265 [64]. This delocalization of charges
might be the reason for inactivity of compounds with O and S as bridging atoms.
Further modifications at the m- and p-positions led to the identification of DSM161
(Fig. 14a; R = CH 3 , R 1 /R 2 /R 4 = H, R 3 = 4-SF 5 -Ph) and DSM190 (Fig. 14a;
R = CH 3 , R 1 /R 2 /R 4 = H, R 3 = 3,5-diF-4-CF 3 -Ph) with PfDHODH IC 50 to be 0.13
and 0.19 µM, respectively. These compounds showed better plasma exposure and
improved efficacy in mouse model [88].
This activity was further improved by Coteron et al. (2011) with the design of
DSM265 (Fig. 14a; R = CH 3 , R 1 /R 2 = H, R 3 = 4-SF 5 -Ph, R 4 = CF 2 CH 3 ) which
was found to be active against both sensitive and resistant strains of P. falciparum
[66]. As discussed above [64], the crystal structure of PfDHODH with triazolopyrimidines showed a narrow channel existing between the FMN and inhibitor.
In order to improve the pharmacokinetics along with the activity, this information
was utilized and modifications were done at the R 4 position. Small hydrophobic
electron-withdrawing groups were found to fit in the narrow space, out of which
CF 2 CH 3 was found to be most suitable. It showed potency similar to chloroquine in
humanized SCID mouse Pfmodel. The compound also showed excellent oral
bioavailability, long half-life, and low clearance in humanized SCID mouse
Pfmodel. DSM265 was found to possess excellent in vivo efficacy with once a day
dose in mice. Further extended studies gave very promising results in order to
consider DSM265 as a drug candidate [69]. The Pf and Pb IC 50 were found to be
0.033 and 2.5 µM, respectively, with the Pf 3D7 cells EC 50 to be 0.046 µM. It
showed a high selectivity, >100 µM, against HsDHODH. The compound was
analyzed to act on both liver and blood stage of the parasite and active against
isolated resistant strains. 200–400 mg dose for eight days is well tolerated in
repeated dose with cardiovascular safety in mice and dogs. DSM265 thus shows an
excellent safety profile, blood–liver stage activity and a predicted long half-life in
humans [69]. The crystal structure (PDB ID 4RX0, entry 11, Table 1) showed that
the –CF 2 CH 3 group shows van der Waals interactions with amino acid residues
Ile263, Ile272, the hydrophobic potion of Arg265 side chain and Tyr528. Also, the
electron-withdrawing effect of fluorine reduces the electron density on triazole ring
nitrogens, which may be responsible for increased potency. Recently, Kokkonda
et al. proposed tetrahydro-2-naphthyl and 2-indanyl substituted triazolopyrimidines
with improved potency and selectivity over DSM265 [71]. However, these compounds have high metabolic clearance and are proposed to be tolerated only in
multi-dose regime.
In 2012, Bedingfield et al. proposed selectivity factors responsible which can be
exploited to design HsDHODH and PfDHODH selective triazolopyrimidine class
of inhibitors [75]. It was observed that His Hs 56 and His Pf 185 play important role in
Structure-Based Design of PfDHODH Inhibitors …
203
and double bond (1.313 Å in DSM1) and partial positive charge was observed at
N 1 , indicated by the presence of chloride ion adjacent to the N 1 in the crystal
structure. It was suggested that the triazolopyrimidine undergoes electron delocalization between N 1 and N 5 , giving a low-range positive dipole at the N 1 center
which enhances its interaction with His185 and N 5 acquires a slight negative dipole
which allows it to form an ion pair with Arg265 [64]. This delocalization of charges
might be the reason for inactivity of compounds with O and S as bridging atoms.
Further modifications at the m- and p-positions led to the identification of DSM161
(Fig. 14a; R = CH 3 , R 1 /R 2 /R 4 = H, R 3 = 4-SF 5 -Ph) and DSM190 (Fig. 14a;
R = CH 3 , R 1 /R 2 /R 4 = H, R 3 = 3,5-diF-4-CF 3 -Ph) with PfDHODH IC 50 to be 0.13
and 0.19 µM, respectively. These compounds showed better plasma exposure and
improved efficacy in mouse model [88].
This activity was further improved by Coteron et al. (2011) with the design of
DSM265 (Fig. 14a; R = CH 3 , R 1 /R 2 = H, R 3 = 4-SF 5 -Ph, R 4 = CF 2 CH 3 ) which
was found to be active against both sensitive and resistant strains of P. falciparum
[66]. As discussed above [64], the crystal structure of PfDHODH with triazolopyrimidines showed a narrow channel existing between the FMN and inhibitor.
In order to improve the pharmacokinetics along with the activity, this information
was utilized and modifications were done at the R 4 position. Small hydrophobic
electron-withdrawing groups were found to fit in the narrow space, out of which
CF 2 CH 3 was found to be most suitable. It showed potency similar to chloroquine in
humanized SCID mouse Pfmodel. The compound also showed excellent oral
bioavailability, long half-life, and low clearance in humanized SCID mouse
Pfmodel. DSM265 was found to possess excellent in vivo efficacy with once a day
dose in mice. Further extended studies gave very promising results in order to
consider DSM265 as a drug candidate [69]. The Pf and Pb IC 50 were found to be
0.033 and 2.5 µM, respectively, with the Pf 3D7 cells EC 50 to be 0.046 µM. It
showed a high selectivity, >100 µM, against HsDHODH. The compound was
analyzed to act on both liver and blood stage of the parasite and active against
isolated resistant strains. 200–400 mg dose for eight days is well tolerated in
repeated dose with cardiovascular safety in mice and dogs. DSM265 thus shows an
excellent safety profile, blood–liver stage activity and a predicted long half-life in
humans [69]. The crystal structure (PDB ID 4RX0, entry 11, Table 1) showed that
the –CF 2 CH 3 group shows van der Waals interactions with amino acid residues
Ile263, Ile272, the hydrophobic potion of Arg265 side chain and Tyr528. Also, the
electron-withdrawing effect of fluorine reduces the electron density on triazole ring
nitrogens, which may be responsible for increased potency. Recently, Kokkonda
et al. proposed tetrahydro-2-naphthyl and 2-indanyl substituted triazolopyrimidines
with improved potency and selectivity over DSM265 [71]. However, these compounds have high metabolic clearance and are proposed to be tolerated only in
multi-dose regime.
In 2012, Bedingfield et al. proposed selectivity factors responsible which can be
exploited to design HsDHODH and PfDHODH selective triazolopyrimidine class
of inhibitors [75]. It was observed that His Hs 56 and His Pf 185 play important role in
Structure-Based Design of PfDHODH Inhibitors …
203