New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 371

and RNA and protein biosynthesis ( Ciak and Hahn, 1967 ). Like other acri-
dine derivatives, quinacrine is an intercalating agent binding to DNA with
a preference for (A+T)-rich regions and thereby blocking DNA replication
and RNA biosynthesis. One of the most commonly known intercalating
agents, ethidium bromide, has been used since the 1950s as antitrypano-
somal drug in African cattle ( Roy Chowdhury et al., 2010 ).
The binding affinities of pentamidine derivatives ( Bell et al., 1991 ) and
bis-benzimidazoles ( Bell et al., 1993 ) to calf thymus DNA are strongly cor-
related to their efficacies against G. lamblia . Pentamidine-derivatives such as
diamidines and arylimidamides are effective not only against G. lamblia but
also against a variety of other parasites ( Torres-Gómez et al., 2008 ) includ-
ing trypanosomatids ( Shapiro and Englund, 1990 ; Soeiro et al., 2008 , 2005 )
and apicomplexa, namely N. caninum in vitro ( Leepin et al., 2008 ; Schorer
et al., 2012 ) and in vivo ( Debache et al., 2011 ), and T. gondii in vitro ( Kropf
et al., 2012 ). Since DNA as a target is ubiquitous, the relative specificity of
DNA-intercalating agents as antiparasitic drugs must reside in other proper-
ties such as differential uptake/metabolism in parasite vs host cells. Analy-
sis of a G. lamblia strain resistant against quinacrine has shown differential
uptake to be a key factor of resistance formation against this intercalating
agent ( Upcroft et al., 1996 ). Moreover, the presence of circular DNA in
organelles essential for the parasite such as the kinetoplast in trypanosoma-
tids ( Shapiro and Englund, 1990 ) or the apicoplast in apicomplexa ( Dahl
and Rosenthal, 2008 ; Fichera and Roos, 1997 ) may render those organisms
more susceptible to these drugs than the host cells.
3.1.2. DNA Modifying Enzymes
Some intercalating agents interact not only with DNA but also with DNA
modifying enzymes. This may enhance their specificity. DNA modify-
ing enzymes may thus constitute more selective targets for antimicrobial
agents. For instance, pentamidines and a variety of other DNA binding
compounds, such as etoposide, induce the cleavage of trypanosome DNA
minicircles in a pattern that resembles the action of topoisomerase II inhibi-
tors ( Shapiro and Englund, 1990 ). Fluoroquinolones, such as ciprofloxacine,
inhibit DNA-gyrases and topoisomerases from prokaryotes ( Kidwai et al.,
1998 ) and are highly effective against G. lamblia as shown in a descriptive
study ( Sousa and Poiares-da-Silva, 2001 ), which, however, does not clarify
which type of enzyme is inhibited. A phylogenetic analysis of type II topoi-
somerases reveals that the gene coding for the G. lamblia topoisomerase 2
(GlTop2) is eukaryote-like ( He et al., 2005 ) and thus not closely related to