New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 384

compounds thus represent a new class of compounds with potential activity
against human malarias ( Donaldson and Kim, 2010 ). Related compounds,
acyclic immucillin phosphonates, inhibit HGXPRT ( Hazleton et al., 2012 ).
As a result, imucillin derivatives may inhibit the purine salvage pathway of
Plasmodium at two different steps and thus, constitute valuable compounds
against malaria.
4. CASE STUDIES
In the following sections, we will illustrate the problems concerning
drug target identification with two case studies on antiprotozoal drugs with
a broad host spectrum, namely the thiazolides with the mother compound
nitazoxanide, and mefloquine, originally developed as an antimalarial.
4.1. Thiazolides, a Novel Drug Family
Since reports on novel antiparasitic drugs are rare, the release of a class of
compounds, thiazolides, has been followed with great interest since the
1990s ( Hemphill et al., 2006 , 2007 ). Nitazoxanide, the mother compound
of this class, has been approved in the USA for the treatment of persis-
tent diarrhea due to cryptosporidiosis and giardiasis in children and adults,
and there are a couple of clinical studies with unclear readouts ( Hemphill
et al., 2007 ). Upon oral uptake, nitazoxanide is rapidly deacetylated to
tizoxanide and further metabolized to tizoxanide-glucuronide ( Broekhuy-
sen et al., 2000 ). Tizoxanide has been reported to display antimicrobial
activity similar to nitazoxanide, while tizoxanide-glucuronide is largely
inactive against a number of pathogens ( Adagu et al., 2002 ). In anaerobic
or microaerophilic pathogens like G. lamblia or T. vaginalis , the presence
of a nitro group is a prerequisite for efficacy ( Adagu et al., 2002 ; Müller
et al., 2006 ), and the mode of action is claimed as being similar to other
nitro compounds like metronidazole with PFOR as a potential target. By
affinity chromatography using tizoxanide as a ligand, we have pulled down
an NR as a tizoxanide-binding protein from G. lamblia extracts. Activity
of the recombinant enzyme is inhibited by nitazoxanide ( Müller et al.,
2007b ). Overexpression of the corresponding mRNA in G. lamblia and
E. coli is correlated with an increased susceptibility to nitazoxanide and
metronidazole ( Nillius et al., 2011 ), lower expression levels in a G. lamblia
clone are correlated with resistance ( Müller et al., 2008a, 2007a ; Nillius
et al., 2011 ). Thus, this NR behaves as a true target of nitazoxanide and
metronidazole.