New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 370

designed, and assessed in similar studies, at least in theory. This approach is
not “hypothesis-free” in the sense that the class of target molecules is known.
The great advantage is that—once the genome information is known and
accessible—this approach is very straightforward. By employing modern
high-yield expression of recombinant proteins, high-throughput screen-
ing systems can be developed and compound libraries can be analyzed on
several isoforms of target proteins. If the structure of the target protein is
resolved, quantitative structure activity relationship (QSAR) studies can be
performed predicting the interaction of potential effectors with the binding
site. Since these studies are based on isolated, recombinant target proteins, a
careful validation of screened compounds by in vitro and in vivo methods
(“physiological screening”) is paramount.
2.6. Validation of Drug Targets
In a next step, the targets identified by any of the strategies mentioned above
have to be validated ( Egner et al., 2005 ; Wang, 1997 ). If the drug target is a
protein, this validation includes the expression of the protein in a suitable
organism, followed by binding and/or inhibition studies in vitro and struc-
tural analysis of protein-ligand complexes. A bona fide drug target should
have binding affinities to the drug in the same order of magnitude or lower
as the inhibition constant of the drug on the parasite. If the parasites allow
overexpression or silencing of genes, the drug target protein should further
be validated by knockdown and overexpression studies of the corresponding
gene. In particular, this approach allows distinguishing between target pro-
teins that are directly involved in the mode of action of the drug and proteins
that are only indirectly linked to the mode of action, e.g. drug metabolizing
enzymes. In the first case, overexpression would enhance drug susceptibility,
whereas in the latter case, overexpression would enhance drug resistance.
Selected examples of studies on specific drug targets are given in the follow-
ing chapters. Most studies combine the strategies mentioned above.
3. KINDS OF DRUG TARGETS
3.1. Drug Targets in the Gene-Expression Machinery
3.1.1. DNA Intercalating Agents
Among the first antiparasitic drugs are compounds that directly intercalate
into DNA. The acridine derivative quinacrine, an early antimalarial and the
first antigiardial drug, interacts with DNA, and thereby inhibits replication