New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 378

Bumped kinase inhibitors—ineffective against mammalian kinases—
inhibit T. gondii calcium-dependent kinase I (TgCDPKI) at low nano-
molecular levels and interfere with the infection of cells at early stages
( Ojo et al., 2010 ). Based on these findings, a panel of pyrazolopyrimidine
compounds acting as ATP competitive inhibitors has been developed and
successfully tested against T. gondii ( Johnson et al., 2012 ) and C. parvum
( Larson et al., 2012 ). In Plasmodium sp., this class of inhibitors does not affect
intra- and extraerythrocytic stages in humans, but inhibits the sexual stages,
namely microgametocyte exflagellation, oocyst formation and sporozoite
production, necessary for transmission to mammals, in mosquitoes ( Ojo
et al., 2012 ). In combination with drugs active against blood and/or liver
stages of Plasmodium in humans, these inhibitors may provide an important
tool for malaria control and eradication by stopping genetic recombination
and the transmission from infected to healthy persons.
What is the role of these kinases? In T. gondii , KT5926, a specific inhibi-
tor of CDPKs, blocks the motility of tachyzoites and their attachment to
host cells by inhibiting only a single KT5926-sensitive protein kinase activity,
TgCDPK1, a kinase with three major substrates ( Kieschnick et al., 2001 ).
Based on these observations, a central role is suggested for TgCDPK1 in regu-
lating Toxoplasma motility and host cell invasion with the myosin XIV motor
complex as a probable target ( Gilk et al., 2009 ). In Plasmodium sp., CDPK1
phosphorylates two components of the motor complex in schizonts, thus in
asexual stages ( Green et al., 2008 ). A better candidate for the inhibitors men-
tioned above is CDPK4, or a related kinase. Genetic studies show that this
kinase is a molecular switch essential for the sexual reproduction and mosquito
transmission of Plasmodium berghei ( Billker et al., 2004 ). These examples sug-
gest that inhibition of CDPK1 in apicomplexan parasites interferes with host
cell invasion. Conversely, intracellular stages would not be affected. Therefore,
additional compounds would be needed in order to cure these stages.
3.4. Membrane Integrity as a Drug Target
In the previous sections, we have provided information on the identifica-
tion of proteins or nucleic acids as drug targets. Some very powerful anti-
microbial compounds do, however, not target these macromolecules, but
destroy the membrane integrity, and thereby cause cell death. These com-
pounds are, in general, weak acids that, in the protonated state, freely dif-
fuse through membranes. As a consequence, within minutes after addition
of such a compound to a cell, the transmembrane proton or ion gradients
are destroyed and essential cellular functions such as ATP biosynthesis are