New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 386

et al., 2009 ). Thus, for mefloquine, there must be other targets besides
heme degradation. Mefloquine and other quinolines inhibit various protein
kinases but at relatively high concentrations ( Wang et al., 1994 ). Mefloquine
inhibits the proliferation of N. caninum and T. gondii tachyzoites cultured in
human foreskin fibroblasts, however, the therapeutic window is very small
( Müller and Hemphill, 2011a ). Therefore, a target in human cells should
exist. Affinity chromatography employing epoxy-sepharose-bound meflo-
quin and extracts of HFF resulted in the pull-down of nicotinamide phos-
phoribosyltransferase (accession no NP_005737 ) as a mefloquine-binding
protein (authors, unpublished data). Further studies are required to elucidate
the role of this enzyme in the mode of action of mefloquine. If human
nicotinamide phosphoribosyltransferase, a key enzyme for the biosynthesis
of NAD, is a true target for mefloquine, this may explain the side effects of
this drug, especially in the central nervous system, as shown experimentally
in rats ( Dow et al., 2006 ) and in a survey study in humans ( Barrett et al.,
1996 ). In S. mansonii , the same approach has yielded enolase as a mefloquine
binding protein. Mefloquine inhibits enolase activity in crude extracts, but
has no effect on the recombinant enzyme expressed in E. coli ( Manneck
et al., 2012 ).
Taken together, both case studies show that broad-spectrum antipro-
tozoal drugs may interact with multiple proteins or even other macro-
molecules including such from host cells. Therefore, care must be taken
concerning the assertion of drug target functions to macromolecules iden-
tified by a single approach. Only via a multidimensional approach includ-
ing the analysis of resistant strains, overexpression and downregulation of
potential target proteins in suitable cellular contexts, such an assertion bona
fide can be made.
5. CONCLUSIONS AND OUTLOOK
In this chapter, we have provided an overview on the identification
of antiprotozoal drug targets. Despite an increasing effort on target-based
drug discovery strategies, all antiprotozoal drugs currently available on the
market, as well as novel promising drugs ( Rottmann et al., 2010 ), have been
derived from whole organism screenings. Therefore, many drug targets and
modes of action described in this chapter have been discovered after the
corresponding compound has been launched. Target-based drug discovery
has not been successful in terms of introducing novel antiparasitic drugs.
One possible explanation for this is that target-based drug development is