**8. Molecules which inhibit the activities of redox balancing enzymes**

The GSH-system plays an important role in the maintenance of the redox status in *Plasmodi‐ um* (Kehr et al., 2011). It is involved in detoxifying free heme (ferriprotoporphyrin IX) (Atamna et al., 1995; Müller, 2003) and in the termination of radical-based chain reactions (Frey, 1997). Therefore, enzymatic reactions within this system are highly druggable. The GSR is one of the key enzymes of the GSH-system and consequently several compounds have been synthesized to successfully interfere with its catalysis *in vitro* and *in vivo* (Biot et al., 2004; Gallo et al., 2009; Grellier et al., 2010; Muller et al., 2011). Inhibitory compounds comprise for example isoalloxazines, quinacrines, tertiary amides that reveal antimalarial ac‐ tivity at low doses against the chloroquine sensitive *P. falciparum* strain 3D7 (Sarma et al., 2003; Friebolin et al., 2008; Chibale et al., 2001). Methylene blue (Fig. 2), a noncompetitive inhibitor of the *P. falciparum* GR, shows antiplasmodial activity against all blood stage forms, whereas only a marginal cytotoxic effect against mammalian cells has been reported (Biot et al., 2004; Buchholz et al., 2008; Atamna et al., 1996; Akoachere et al., 2005; Badyopadhyay et al., 2004; Krauth-Siegel et al., 2005; Garavito et al., 2007).

**7. Approaches to increase oxidative stress within the malaria parasite**

tial for the intracellular proliferation of the plasmodial pathogen.

like hemozoin.

232 Drug Discovery

Malaria is a devastating and quite often a deadly parasitic disease, which causes important public health problems in the tropics. The population in more than 90 countries, with more than 2000 million citizens, is exposed to the infection. Malaria infection is responsible for an estimated 500 million clinical cases per annum, causing more than one million deaths; most of these are children in Africa. The malaria parasite *Plasmodium falciparum,* the causative agent of Malaria tropica, is proliferating within human red blood cells, thereby exploiting host's nutrient sources and hiding from the human immune response. A vaccine is not avail‐ able and the control of the disease depends solely on the administration of a small number of drugs. Due to mutational modification of the genome of the malaria parasite, an ongoing rapid adaptation to environmental changes and drug resistance is occurring (Greenwood et al., 2008). At the moment – which is just a question of time - solely artemesinin is still effec‐ tive against the malaria parasite. However, first reports already demonstrated drug resist‐ ance against artemisinin (Wangroongsarb et al., 2011). Therefore, continuous discovery and development of new drugs are urgently needed. A variety of the current anti-malaria drugs are targeting the redox balance of the parasite. As outlined above, redox systems are essen‐

In general, *P. falciparum* uses the two interacting systems, GSH- and TRX-system, to protect against reactive ROS (Kanzok et al., 2002; Kanzok et at., 2000; Kawazu et al., 2001; Kehr et al., 2011; Krnajski et al., 2001; Krnajski et al., 2002; Kumar et al., 2008; Liebau et al., 2002). Both systems can be link by the redox protein plasmoredoxin (Becker et al., 2003). Active in‐ terference by employing redox-active antiparasitic drugs, however, harms the parasite and results in its death. Compounds which disturb the redox balance can be categorised into three different groups: (i) molecules that are responsible for the *de novo* synthesis of ROS and thus lead to parasite death, (ii) molecules which inhibit the activities of redox balancing en‐ zymes and (iii) molecules that interfere in the scavenging of pro-oxidant metabolic products

**8. Molecules which inhibit the activities of redox balancing enzymes**

The GSH-system plays an important role in the maintenance of the redox status in *Plasmodi‐ um* (Kehr et al., 2011). It is involved in detoxifying free heme (ferriprotoporphyrin IX) (Atamna et al., 1995; Müller, 2003) and in the termination of radical-based chain reactions (Frey, 1997). Therefore, enzymatic reactions within this system are highly druggable. The GSR is one of the key enzymes of the GSH-system and consequently several compounds have been synthesized to successfully interfere with its catalysis *in vitro* and *in vivo* (Biot et al., 2004; Gallo et al., 2009; Grellier et al., 2010; Muller et al., 2011). Inhibitory compounds comprise for example isoalloxazines, quinacrines, tertiary amides that reveal antimalarial ac‐ tivity at low doses against the chloroquine sensitive *P. falciparum* strain 3D7 (Sarma et al., 2003; Friebolin et al., 2008; Chibale et al., 2001). Methylene blue (Fig. 2), a noncompetitive The GST is one of the most abundant proteins expressed in *P. falciparum*. Additionally to its detoxifying role, it efficiently binds parasitotoxic heme not only in the presence of GSH, but also when GSSG is present, thereby protecting the parasite from hemin even under severe oxidative stress conditions. Here, a peculiar loop region, that is both crucial for the gluta‐ thione-dependent tetramerization/inactivation process and for hemin-binding, represents an ideal drug target (Liebau et al., 2005; 2009). Recently chemical synthesis to design effective compounds to target GST has been performed which show some promising antiplasmodial activity (Ahmad et al., 2007; Sturm et al., 2009). Furthermore, the development of drugs that overcome resistance to available antimalarial drugs also are of great interest. The action of multidrug resistance protein (MRP)-like transporters is associated with the efflux of xenobi‐ otics in both unaltered and GSH-conjugated form and it is conceivable that they are in‐ volved in the development of drug resistance in malarial parasites (Koenderink et al., 2010). Since coordinated expression and synergistic interactions between GST and efflux pumps have been observed (Sau et al., 2010), a promising new intervening strategy might be the in‐ hibition of GST and/or the development of GST-activated pro-drugs that overcome drug re‐ sistance by blocking the drug binding sites of the transporters.

Another promising antimalarial drug target is the *P. falciparum* TrxR (Banerjee et al., 2009). Chalcone derivatives and Eosin B exhibit antiplasmodial activity by inhibiting the plasmo‐ dial TrxR (Li et al., 1995; Massimine et al., 2006).

For many years it was thought that the malaria parasite had no need for an endogenous SOD and simply adopted the host´s enzyme for its purpose. However, in 2002, an iron-de‐ pendent SOD was described in *P. falciparum* (Boucher et al., 2006). Being quite distinct from the human tetrameric Mn and Cu/Zn SOD, it is exploited as anti-malaria drug target (Sou‐ lere et al., 2003).
