Biomedical Engineering Reference
In-Depth Information
bond that was cleaved with the -CH
2
-NH- or 'methyleneamino' peptidomi-
metic, a strong substrate was converted to a strong inhibitor. The resulting
peptides would not be drug candidates because they have a peptidic character
and would be subject to rapid proteolysis by host enzymes as well as possibly by
parasite enzymes of various classes. However, they serve one important func-
tion: they are nearly ideal active-site ligands for co-crystallization with the
plasmepsins due to the interaction of multiple positions with the active site
cleft. This was seen very clearly in the structure determined with compound A
bound to plasmepsin II reported in Liu et al.
25
When adequate amounts of
purified plasmepsins are obtained, this strategy will be continued to determine
details of the binding interactions so that optimal compounds may be designed
using this structure-based process.
11.5.1 HIV PR Inhibitors and Malaria
Reports have appeared in recent literature suggesting that HIV-1 protease
inhibitors, in use as antiviral agents, have a significant antimalarial activity. In
2005, Parikh et al. studied the binding of seven of the FDA-approved HIV
protease inhibitors against malaria parasites in culture.
115
They used four
strains of the parasite, HB3, D6, Dd2, and W2. While all seven compounds
inhibited the development of the parasite in culture, lopinavir was the most
potent compound with an apparent IC
50
in the range of 0.9-2.1 mM. These
concentrations are above the levels of the inhibitors found in patients when on
anti-retroviral therapy, especially when ritonavir is used to 'boost' the activity
of other HIV protease inhibitors. Lopinavir also inhibited plasmepsin II at low
micromolar concentrations; however, the effect on other plasmepsins was not
determined in this report. In a follow-up paper a year later, Parikh et al. showed
that HIV protease inhibitors were not synergistic with a cysteine protease
inhibitor and were not more active in a parasite where falcipain, an important
cysteine protease, was knocked out.
116
Furthermore, they showed that the
effect of the HIV inhibitors was the same against the wild-type parasites as
against parasites where the digestive vacuole enzymes were knocked out. This
seems to show clearly that plasmepsins I, II, IV, and HAP were not the targets
of the inhibitors used for HIV therapy. Thus, the effects of those compounds on
the parasites must be derived from their inhibition of plasmepsins V, IX, or X.
Andrews et al.
117
studied the effects of saquinavir, ritonavir, or lopinavir on
the growth of the 3D7 strain of the parasite in culture and found similar results
to those reported by Parikh. In addition, the Andrews report demonstrated
that, in a murine malaria model using P. chabaudi, the level of parasitemia is
greatly reduced over a 25-day period with various combinations of HIV pro-
tease inhibitors. In direct enzyme assays, they showed that saquinavir, lopi-
navir, and ritonavir were roughly micromolar inhibitors of plasmepsins II and
IV. Furthermore, they used molecular modeling to show that the HIV protease
inhibitors could fit within the active sites of plasmepsins II and IV from
P. falciparum and the enzyme from P. chadaubi, which we now know to have
identity with plasmepsin IV from P. falciparum. By analyzing known structures
