Acridinediones: Selective and Potent Inhibitors of the Malaria Parasite Mitochondrialbc₁Complex

Abstract

The development of drug resistance to affordable drugs has contributed to a global increase in the number of deaths from malaria. This unacceptable situation has stimulated research for new drugs active against multidrug-resistant Plasmodium falciparum parasites. In this regard, we show here that deshydroxy-1-imino derivatives of acridine (i.e., dihydroacridinediones) are selective antimalarial drugs acting as potent (nanomolar K i ) inhibitors of parasite mitochondrial bc 1 complex. Inhibition of the bc 1 complex led to a collapse of the mitochondrial membrane potential, resulting in cell death (IC 50 ϳ15 nM). The selectivity of one of the dihydroacridinediones against the parasite enzyme was some 5000-fold higher than for the human bc 1 complex, significantly higher (ϳ200 fold) than that observed with atovaquone, a licensed bc 1 -specific antimalarial drug. Experiments performed with yeast manifesting mutations in the bc 1 complex reveal that binding is directed to the quinol oxidation site (Q o ) of the bc 1 complex. This is supported by favorable binding energies for in silico docking of dihydroacridinediones to P. falciparum bc 1 Q o . Dihydroacridinediones represent an entirely new class of bc 1 inhibitors and the potential of these compounds as novel antimalarial drugs is discussed.Death and morbidity caused by malaria are on the increase, largely as a result of parasite drug resistance (Snow et al., 2001). Consequently, there are more people dying of malaria now than there were 20 years ago. Recognition of this problem by the international community and the engagement of the pharmaceutical industry and other key stakeholders has catalyzed the concerted search for new antimalarial drugs with novel targets (Biagini et al

Structural analysis of atovaquone-inhibited cytochrome bc1 complex reveals the molecular basis of antimalarial drug action

“…Ile299 is also present in T. gondii, B. microti and P. jirovecci, whereas the other two residues vary among species. The comprehensive sequence alignment confirmed that the four amino acid deletion in the cd2 helix 26 is exclusively found in apicomplexa (Fig. 6c).…”

“…27)) depending on the study and cell type used as compared with P. falciparum (IC 50 ¼ 3.0 nM (ref. 26)). Even more natural diversity is seen for a ligand of the cyclohexyl ring, Leu275, which is present in S. cerevisiae and also in P. jirovecii and B. microti, whereas a phenylalanine is present at this position in Plasmodium species and T. gondii (Fig.…”

“…The phenylalanine side chain is in contact with Ile299 and its alanine substitution could destabilize binding interactions to the chlorophenyl ring and consequently lower the potency of atovaquone for inhibition of human cyt bc 1 (IC 50 between 70 nM (ref. 26) and 460 nM (ref. 27)) depending on the study and cell type used as compared with P. falciparum (IC 50 ¼ 3.0 nM (ref.…”

“…Atovaquone inhibits P. falciparum cyt bc 1 with high affinity in nanomolar range 26 . Similar potencies are described for other parasitic and fungal targets, but the drug also inhibits human cyt bc 1 , although with lower submicromolar affinity 27 .…”

Antimalarial 4(1H)-pyridones bind to the Q i site of cytochrome bc 1

“…A recent X-ray structure of cytochrome bc 1 from Saccharomyces cerevisiae has shown atovaquone bound in the catalytic Q o site, offering explanation for the cross-species resistance (20). Many different classes of compound have been investigated as potential drugs and much work has focused on inhibition of the Q o site (23,(29)(30)(31)(32). The 4(1H)-pyridones have been researched for their antimalarial properties since the 1960s, based on the compound clopidol (33,34) (Fig.…”