Inhibiting Plasmodium cytochrome bc1: a complex issue

doi:10.1016/j.cbpa.2010.05.005

Current Opinion in Chemical Biology

Volume 14, Issue 4, August 2010, Pages 440-446

https://doi.org/10.1016/j.cbpa.2010.05.005 Get rights and content

The cytochrome bc₁ complex is a key mitochondrial enzyme that catalyses transfer of electrons maintaining the membrane potential of mitochondria. Currently, atovaquone is the only drug in clinical use targeting the Plasmodium falciparum bc₁ complex. The rapid emergence of resistance to atovaquone resulted in a costly combination with proguanil (Malarone™), limiting its widespread use in resource-poor disease-endemic areas. Cheaper alternatives that can overcome resistance are desperately required. Here we describe recent advances of bc₁-targeted inhibitors that include hydroxynaphthoquinones (atovaquone analogues), pyridones (clodipol analogues), acridine related compounds (acridinediones and acridones) and quinolones. Significantly, many of these developmental compounds demonstrate little cross resistance with atovaquone-resistant parasite strains, and selected classes have excellent oral activity profiles in rodent models of malaria.

Section snippets

Cytochrome bc₁ – the target

The cytochrome bc₁ complex (ubiquinol: cytochrome c oxidoreductase, respiratory Complex III) is a key enzyme of the mitochondrial electron-transfer chain in all metazoa and many fungi and protozoa, catalysing the transfer of electrons from ubiquinol to cytochrome c [1, 2]. This electron transfer is coupled to the vectorial translocation of protons across the inner mitochondrial membrane, with the resulting electrochemical gradient utilised for ATP production by the F_oF₁ ATP synthase (Complex V).

Inhibition of P. falciparum cytochrome b by atovaquone

Inhibition of the cytochrome bc₁ complex (such as by the anti-malarial compound atovaquone) collapses the mitochondrial membrane potential and is lethal to P. falciparum. A crystal structure for atovaquone-inhibited bc₁ is currently not available, but EPR spectroscopy of the Rieske [2Fe2S] cluster, site-directed mutagenesis of model organism cytochrome b and gene sequencing of atovaquone-resistant Plasmodium species have demonstrated that this compound is a competitive inhibitor of the Q_o site [

Atovaquone resistance mutations in P. falciparum cytochrome b

Resistance mutations associated with atovaquone are summarised in Table 1. The most common mutation observed in atovaquone-resistant isolates of P. falciparum associated with Malarone™ failure in infected patients is at position 268 in cytochrome b, exchanging tyrosine for serine (Y268S) or, less frequently, asparagine (Y268N) [14, 15, 16, 17]. Typically, this mutation increases the atovaquone IC₅₀ several hundred fold compared to sensitive parasite strains. For example, the IC₅₀ for atovaquone

Inhibitors of P. falciparum bc₁ complex

The mitochondrion of malaria parasites offers a unique target containing electron transport components with alternate complexes [20]. The best studied inhibitor target is the ubiquinol oxidation (Q_o) site of the cytochrome bc₁ complex. Currently there is an urgent need for bc₁ inhibitors with improved pharmaceutical properties and scope to ‘design-out’ potential resistance mechanisms.

Hydroxynapthoquinones

As discussed earlier, atovaquone, 1 (a hydroxynapthoquinone) acts as a competitive inhibitor of CoQ. Despite its excellent anti-malarial activity, atovaquone exhibits poor pharmaceutical properties such as low bioavailability and high plasma protein binding [21].

In an attempt to improve the bioavailability of atovaquone, El Hage et al. [22] have designed several alternatives that substitute the 3-hydroxyl function for more lipophilic ester and ether groups. All of the compounds had potent

Pyridones

The anti-malarial properties of pyridones have been known since the late 1960s when clopidol was shown to have activity against chloroquine resistant strains of P. falciparum. Clopidol has also been shown to potentiate the action of hydroxynaphthoquinones and to sustain activity against atovaquone-resistant strains [24]. This may suggest that pyridone derivatives bind at a different site than that of atovaquone in the Q_o pocket of the bc₁ complex.

In 2006 GlaxoSmithKline (GSK) reported the

Acridinediones

Acridines are known to be potent anti-malarial compounds. Mechanistically they have been shown to bind to heme and prevent crystallisation to hemozoin. Two hydroacridinediones, floxacrine and WR249685 (Figure 3) were investigated for their anti-malarial mode of action. Since acridine compounds are known to inhibit the formation of hemozoin, the heme binding properties of each compound were also assessed [27^•].

Floxacrine was shown to bind heme, not as efficiently as chloroquine or amodiaquine,

Concluding remarks

A number of drug discovery programs have been initiated to find specific and potent inhibitors of Plasmodium bc₁. Although many inhibitors have been identified, few candidates have all the desired pharmaceutical properties and selectivity to be considered for clinical trials. To our knowledge a number of the most promising candidates, the pyridones, have been suspended at clinical trials owing to toxicity. The fact that these potent pyridones were advanced to clinical trials, however, does

Conflicts of interest

The authors declare there are no conflicts of interest.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest
•• of outstanding interest

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Inhibiting Plasmodium cytochrome bc1: a complex issue

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Cytochrome bc1 – the target