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The antimalarial activity of several antiretroviral protease inhibitor combinations was investigated. Data demonstrate that ritonavir and saquinavir behave synergistically with chloroquine and mefloquine. These data, and interactions with pepstatin-A, E-64, and bestatin, suggest that human immunodeficiency virus protease inhibitors do not target digestive-vacuole plasmepsins.
Although the antimalarial activity of a number of antiretroviral protease inhibitors (PIs; e.g., human immunodeficiency virus [HIV] PIs) in clinical use has been established (1, 15, 20), their mechanism of action against malaria parasites is not known. Understanding the action of these drugs may allow the design of a novel group of potent antimalarials. Our original hypothesis was that, in keeping with their action as aspartyl protease (AP) inhibitors against the HIV virus (21), these agents interfere with a parasite AP (plasmepsin). More specifically, we hypothesized that these drugs interfered with parasite hemoglobin digestion by inhibiting one or more of the digestive-vacuole plasmepsins. Hemoglobin digestion is essential to Plasmodium parasites by supplying amino acids for protein synthesis and development, providing space for the growing parasites, reducing the colloid-osmotic pressure of the erythrocyte, and preventing premature cell rupture (9). Known inhibitors of the hemoglobin digestion process with antimalarial activity have been identified (2, 8) and include pepstatin A, an AP inhibitor; E-64 [N-(trans-epoxysuccinyl)-l-leucine-4-guanidinobutylamide], a cysteine protease inhibitor; and bestatin, an aminopeptidase inhibitor. While the mode of chloroquine action is still debated, it is thought to inhibit parasite growth by preventing heme detoxification (19). In combination studies, vacuole plasmepsin inhibitors have been shown to behave antagonistically with chloroquine and mefloquine (12), while combinations of AP inhibitors with other inhibitors of the hemoglobin digestion process have been shown to behave synergistically (18).
Previous studies have also suggested that HIV PIs, particularly ritonavir, can reduce the efflux of a drug from cells. The synergy of mefloquine when used with HIV PIs against HIV has been proposed to be due, at least in part, to the ability of the HIV PIs to inhibit mefloquine efflux (14). It is unknown whether the HIV PIs may be able to alter drug efflux in Plasmodium falciparum, and if they can, whether this will have any ramifications for the treatment of multidrug-resistant parasites. The aim of the present study was to investigate the antimalarial activity of the HIV PI drugs saquinavir and ritonavir by examining their ability to inhibit the growth of P. falciparum when used in combination with chloroquine, mefloquine, pepstatin A, bestatin, or E-64.
P. falciparum clones Dd2, FAC8, and D10 were maintained in modified candle jars as previously described (22). Antimalarial drug combinations were assessed by isobolographic analysis (4). Assays were performed in triplicate in 96-well microtiter plates containing 100 μl of culture (2% hematocrit and 1% parasitemia) and 100 μl of drug dilutions or a control. Parasite growth was determined based on tritiated hypoxanthine incorporation. Media and vehicle controls were included on each plate. Experiments were repeated at least twice. Fifty percent effective concentrations (EC50) (Table (Table1)1) were determined and isobolograms constructed using data from all experiments. Isoboles were fitted to data using a standard hyperbolic function defined by the parameter I (5). Positive values of I indicate synergism, and negative values indicate antagonism; addition occurs when I equals 0. The significance of the difference between I and 0 was assessed with Student's t test.
Chloroquine behaved synergistically with saquinavir and ritonavir against the chloroquine-resistant line Dd2. I values of 1.3 and 2.2 for saquinavir and ritonavir, respectively (Fig. (Fig.1),1), were demonstrated. As was found with chloroquine, combinations of mefloquine with saquinavir and ritonavir were synergistic, with I values of 1.3 and 2.2, respectively (Fig. (Fig.1).1). The synergistic interaction of mefloquine with ritonavir was independent of the parasite line used. Although demonstrating the smaller I value of 0.9, mefloquine and ritonavir also behaved synergistically against D10 and FAC8 (Fig. (Fig.2).2). Combinations of saquinavir and pepstatin were suggestive of synergy (I = 0.6), whereas combinations of saquinavir and E-64 (I = −4.5) or saquinavir and bestatin (I = −2.9) were antagonistic (Fig. (Fig.3).3). As Dd2 was resistant to pepstatin with an EC50 of >20 μM (Table (Table1),1), all combinations with this agent were tested with D10. EC50 for saquinavir and ritonavir were within the range normally achieved in humans (20). EC50 values for all drugs are shown in Table Table11.
Although a full understanding of the action of chloroquine is lacking, evidence suggests that chloroquine kills sensitive malaria parasites by preventing heme polymerization (19). Other mechanisms of action have been suggested but are considered less important (10). The synergistic interaction of chloroquine with ritonavir and saquinavir suggests that these HIV PIs do not inhibit a digestive-vacuole plasmepsin. Digestive-vacuole plasmepsins have been shown to carry out the initial cleavage of heme from hemoglobin (3). If this process is inhibited, chloroquine would be prevented from accessing heme, resulting in antagonism, as described with other plasmepsin inhibitors (12). The antagonistic action of saquinavir with E-64 and bestatin also supports this theory. Combinations of PIs, especially those of the aspartyl and cysteine class, have been shown to behave synergistically (18). Similar to the way that other drug combinations inhibit sequential steps in a metabolic pathway, it is believed that the inhibition of sequential steps in the hemoglobin digestion pathway maximizes antimalarial activity. Combinations of E-64 with pepstatin, a known AP inhibitor, are reported to be synergistic (2, 18). The antagonistic nature of the E-64 and saquinavir combination has also been supported by a recent study demonstrating that E-64 and the HIV PI lopinavir behave antagonistically against P. falciparum (16). The mildly synergistic nature of the pepstatin and saquinavir combination provides additional evidence to suggest that the HIV PIs are not targeting vacuole plasmepsins, or if they are, that other factors are contributing to the action of these antimalarials. These data also support the observations of others who have shown that when food vacuole plasmepsins are “knocked out,” a lethal phenotype is not observed (11, 13). Further studies will be necessary to ascertain the primary target of these drugs.
Although extrapolation of our in vitro data to the in vivo setting should be undertaken with caution, data describing the synergistic interactions of saquinavir and ritonavir with mefloquine and chloroquine are encouraging and suggest that if chosen correctly, antimalarial and antiretroviral combinations may be useful in the field. The results with combinations of mefloquine with the HIV PIs suggest that these drugs may be more effective against mefloquine-resistant parasites (Table (Table1;1; Fig. Fig.11 and and2),2), and although the use of mefloquine in areas such as sub-Saharan Africa is likely to be restricted by cost, this finding may hold true for other antimalarial drugs. As is true of mefloquine, the development of malaria parasite resistance to quinine, the artemisinin drugs, and chloroquine has been associated with drug efflux (6, 7). Although additional studies are required to determine whether HIV PIs alter drug transport, our data suggest that Pfmdr1 copy number is not associated with the synergistic action of the HIV PIs with mefloquine. The HIV PI-mefloquine combinations were more effective against Dd2 (2 copies of Pfmdr1) than against D10 (1 copy) or FAC8 (3 copies) (17). Unless they can be shown to be effective against chloroquine-resistant parasites, HIV PI and chloroquine combinations may be of limited clinical value.
Tina Skinner-Adams is a recipient of a University of Queensland Women in Science Fellowship and a Ramaciotti Development Grant. Donald Gardiner is supported by NHMRC program grant 290208.
We thank Hoffmann-La Roche Ltd. and Abbott for the supply of saquinavir and ritonavir, respectively. We also thank the Australian Red Cross for the supply of blood products for tissue culture, the Tudor Foundation for their generous donation, and Ric Price for assessing the Pfmdr1 copy number of our P. falciparum clones.
Published ahead of print on 6 November 2006.