Jane Xu Kelly

Simple and Inexpensive Fluorescence-Based Technique for High-Throughput Antimalarial Drug Screening

ABSTRACT Radioisotopic assays involve expense, multistep protocols, equipment, and radioactivity safety requirements which are problematic in high-throughput drug testing. This study reports an alternative, simple, robust, inexpensive, one-step fluorescence assay for use in antimalarial drug screening. Parasite growth is determined by using SYBR Green I, a dye with marked fluorescence enhancement upon contact with Plasmodium DNA. A side-by-side comparison of this fluorescence assay and a standard radioisotopic method was performed by testing known antimalarial agents against Plasmodium falciparum strain D6. Both assay methods were used to determine the effective concentration of drug that resulted in a 50% reduction in the observed counts (EC50) after 48 h of parasite growth in the presence of each drug. The EC50s of chloroquine, quinine, mefloquine, artemisinin, and 3,6-bis-ε-(N,N-diethylamino)-amyloxyxanthone were similar or identical by both techniques. The results obtained with this new fluorescence assay suggest that it may be an ideal method for high-throughput antimalarial drug screening.

Quinolone-3-Diarylethers: A New Class of Antimalarial Drug

ELQ-300, an investigational drug for treating and preventing malaria, shows potent transmission-blocking activity in rodent models of malaria. Taking the Bite Out of Malaria Malaria is spread from person to person by mosquitoes that inject 8 to 10 sporozoite forms of the parasite in a single bite. The sporozoites reproduce in the liver to produce 10,000 to 30,000 merozoites before the liver schizont ruptures and parasites flood into the bloodstream where the absolute parasite burden may increase to a thousand billion (1012) circulating parasites. Some of these parasites develop into gametocytes that may be ingested by another mosquito where they progress through ookinete, oocyst, and sporozoite stages to complete the cycle. Like quinine, most antimalarial drugs in use today target only the symptomatic blood stage. The efficacy of these drugs has been compromised by resistance, and so there is a pressing need for new drugs that target multiple stages of the parasite life cycle for use in malaria treatment and prevention. Clearly, it is advantageous to strike at the liver stage where parasite numbers are low, to diminish the likelihood of selecting for a resistant mutant and before the infection has a chance to weaken the defenses of the human host. In a new study, Nilsen and colleagues describe ELQ-300, a 4(1H)-quinolone-3-diarylether, which targets the liver and blood stages, including the forms that are crucial to disease transmission (gametocytes, zygotes, and ookinetes). In mouse models of malaria, a single oral dose of 0.03 mg/kg prevented sporozoite-induced infections, whereas four daily doses of 1 mg/kg achieved complete cures of patent infections. ELQ-300 is a preclinical candidate that may be coformulated with other antimalarials to prevent and treat malaria, with the potential to aid in eradication of the disease. The goal for developing new antimalarial drugs is to find a molecule that can target multiple stages of the parasite’s life cycle, thus impacting prevention, treatment, and transmission of the disease. The 4(1H)-quinolone-3-diarylethers are selective potent inhibitors of the parasite’s mitochondrial cytochrome bc1 complex. These compounds are highly active against the human malaria parasites Plasmodium falciparum and Plasmodium vivax. They target both the liver and blood stages of the parasite as well as the forms that are crucial for disease transmission, that is, the gametocytes, the zygote, the ookinete, and the oocyst. Selected as a preclinical candidate, ELQ-300 has good oral bioavailability at efficacious doses in mice, is metabolically stable, and is highly active in blocking transmission in rodent models of malaria. Given its predicted low dose in patients and its predicted long half-life, ELQ-300 has potential as a new drug for the treatment, prevention, and, ultimately, eradication of human malaria.

Discovery of dual function acridones as a new antimalarial chemotype

Preventing and delaying the emergence of drug resistance is an essential goal of antimalarial drug development. Monotherapy and highly mutable drug targets have each facilitated resistance, and both are undesirable in effective long-term strategies against multi-drug-resistant malaria. Haem remains an immutable and vulnerable target, because it is not parasite-encoded and its detoxification during haemoglobin degradation, critical to parasite survival, can be subverted by drug–haem interaction as in the case of quinolines and many other drugs. Here we describe a new antimalarial chemotype that combines the haem-targeting character of acridones, together with a chemosensitizing component that counteracts resistance to quinoline antimalarial drugs. Beyond the essential intrinsic characteristics common to deserving candidate antimalarials (high potency in vitro against pan-sensitive and multi-drug-resistant Plasmodium falciparum, efficacy and safety in vivo after oral administration, inexpensive synthesis and favourable physicochemical properties), our initial lead, T3.5 (3-chloro-6-(2-diethylamino-ethoxy)-10-(2-diethylamino-ethyl)-acridone), demonstrates unique synergistic properties. In addition to ‘verapamil-like’ chemosensitization to chloroquine and amodiaquine against quinoline-resistant parasites, T3.5 also results in an apparently mechanistically distinct synergism with quinine and with piperaquine. This synergy, evident in both quinoline-sensitive and quinoline-resistant parasites, has been demonstrated both in vitro and in vivo. In summary, this innovative acridone design merges intrinsic potency and resistance-counteracting functions in one molecule, and represents a new strategy to expand, enhance and sustain effective antimalarial drug combinations.

Antimalarial Activity of Natural and Synthetic Prodiginines

Prodiginines are a family of linear and cyclic oligopyrrole red-pigmented compounds. Herein we describe the in vitro antimalarial activity of four natural (IC(50) = 1.7-8.0 nM) and three sets of synthetic prodiginines against Plasmodium falciparum. Set 1 compounds replaced the terminal nonalkylated pyrrole ring of natural prodiginines and had diminished activity (IC(50) > 2920 nM). Set 2 and set 3 prodiginines were monosubstituted or disubstituted at either the 3 or 5 position of the right-hand terminal pyrrole, respectively. Potent in vitro activity (IC(50) = 0.9-16.0 nM) was observed using alkyl or aryl substituents. Metacycloprodiginine and more potent synthetic analogues were evaluated in a P. yoelii murine patent infection using oral administration. Each analogue reduced parasitemia by more than 90% after 25 (mg/kg)/day dosing and in some cases provided a cure. The most favorable profile was 92% parasite reduction at 5 (mg/kg)/day, and 100% reduction at 25 (mg/kg)/day without any evident weight loses or clinical overt toxicity.

Xanthones as Antimalarial Agents: Discovery, Mode of Action, and Optimization

It is believed that at no time in the history of the human race malaria has been absent. This disease, which is caused by protozoa of the genus Plasmodium, in all likelihood has been responsible for the death of about half of all people who ever lived. Even today, after attempts at intervention on a worldwide scale, malaria remains the most significant parasitic disease in the tropics and sub-tropics, where it causes at least 500 million clinical episodes and claims 1.5 million lives each year, mostly young children and pregnant women. Widespread resistance to the best and least expensive antimalarials, chloroquine and S/P (i.e., a combination of sulfadoxine and pyrimethamine), combined with an increasing tolerance to insecticides in the mosquito vector, threaten a global malaria tragedy unless new countermeasures are developed. For malaria therapy, the great panacea would be the development of a long-lasting vaccine, but until this becomes a reality, people living in and traveling to endemic regions must rely on a dwindling cache of more expensive drugs; many beyond the economic reach of impoverished people living in malarious regions of the world. Our course to recognition of xanthones as potential antimalarial agents took a rather circuitous route, involving both serendipity and empiricism, and is described together with mechanistic details of drug action. From a chance encounter with a sea urchin collected near the city of Cannon Beach on the Oregon coast to naturally occurring and functionalized xanthones, it is revealed how these compounds target the Plasmodium parasites most vulnerable feature--the digestive vacuole.

Optimization of Xanthones for Antimalarial Activity: the 3,6-Bis-ω-Diethylaminoalkoxyxanthone Series

ABSTRACT Hydroxyxanthones have been identified as novel antimalarial agents. The compounds are believed to exert their activity by complexation to heme and inhibition of hemozoin formation. Modification of the xanthone structure was pursued to improve their antimalarial activity. Attachment of R-groups bearing protonatable nitrogen atoms was conducted to enhance heme affinity through ionic interactions with the propionate side chains of the metalloporphyrin and to facilitate drug accumulation in the parasite food vacuole. A series of 3,6-bis-ω-diethylaminoalkoxyxanthones with side chains ranging from 2 to 8 carbon atoms were prepared and evaluated. Measurement of heme affinity for each of the derivatives revealed a strong correlation (R2 = 0.97) between affinity and antimalarial potency. The two most active compounds in the series contained 5- and 6-carbon side chains and exhibited low nanomolar 50% inhibitory concentration (IC50) values against strains of chloroquine-susceptible and multidrug-resistant Plasmodium falciparum in vitro. Both of these xanthones exhibit stronger heme affinity (8.26 × 105 and 9.02 × 105 M−1, respectively) than either chloroquine or quinine under similar conditions and appear to complex heme in a unique manner.

Optimization of endochin-like quinolones for antimalarial activity

Our prior work on tricyclic acridones combined with a desire to minimize the tricyclic system led to an interest in antimalarial quinolones and a reexamination of endochin, an experimental antimalarial from the 1940s. In the present article, we show that endochin is unstable in the presence of murine, rat, and human microsomes which may explain its relatively poor antimalarial activity in mammalian systems. We also profile the structure-activity relationships of ≈ 30 endochin-like quinolone (ELQ) analogs and highlight features that are associated with enhanced metabolic stability, potent antiplasmodial activity against multidrug resistant strains of Plasmodium falciparum, and equal activity against an atovaquone-resistant clinical isolate. Our work also features an ELQ construct containing a polyethylene glycol carbonate pro-moiety that is highly efficacious by oral administration in a murine malaria model. These findings provide compelling evidence that development of ELQ therapeutics is feasible.

Design, Synthesis, and Evaluation of 10-N-Substituted Acridones as Novel Chemosensitizers in Plasmodium falciparum

ABSTRACT A series of novel 10-N-substituted acridones, bearing alkyl side chains with tertiary amine groups at the terminal position, were designed, synthesized, and evaluated for the ability to enhance the potency of quinoline drugs against multidrug-resistant (MDR) Plasmodium falciparum malaria parasites. A number of acridone derivatives, with side chains bridged three or more carbon atoms apart between the ring nitrogen and terminal nitrogen, demonstrated chloroquine (CQ)-chemosensitizing activity against the MDR strain of P. falciparum (Dd2). Isobologram analysis revealed that selected candidates demonstrated significant synergy with CQ in the CQ-resistant (CQR) parasite Dd2 but only additive (or indifferent) interaction in the CQ-sensitive (CQS) D6. These acridone derivatives also enhanced the sensitivity of other quinoline antimalarials, such as desethylchloroquine (DCQ) and quinine (QN), in Dd2. The patterns of chemosensitizing effects of selected acridones on CQ and QN were similar to those of verapamil against various parasite lines with mutations encoding amino acid 76 of the P. falciparum CQ resistance transporter (PfCRT). Unlike other known chemosensitizers with recognized psychotropic effects (e.g., desipramine, imipramine, and chlorpheniramine), these novel acridone derivatives exhibited no demonstrable effect on the uptake or binding of important biogenic amine neurotransmitters. The combined results indicate that 10-N-substituted acridones present novel pharmacophores for the development of chemosensitizers against P. falciparum.

A spectroscopic investigation of the binding interactions between 4,5-dihydroxyxanthone and heme

In order to investigate one possible mechanism by which xanthones inhibit growth of malaria-causing Plasmodium parasites, optical and NMR spectroscopic studies were performed on a prototypic xanthone, 4,5-dihydroxyxanthone (45X2), upon its complexation to heme. The 45X2 x heme complex stoichiometry in aqueous solution was found to be 1:2; this interaction was non-cooperative, and exhibited a very similar heme complex dissociation constant (K(d)=5.1 x 10(-6)) as observed for the common antimalarial agents, chloroquine and quinine. The 45X2 x heme(2) complex formation was found to be both pH- and solvent-dependent, with clear evidence of the xanthone carbonyl moiety coordinating with the iron of heme. Hydrogen bonding between the hydroxyl groups of 45X2 and the propionate side chains of heme, as well as pi-pi stacking between both aromatic systems appeared to contribute to the overall stability of the 45X2 x heme(2) complex, as judged by 1H NMR. It was concluded that 45X2 forms a complex with a heme dimer in aqueous solution, and that this interaction can be generalized to account for its in vivo detrimental effect of parasite growth through an effective inhibition of hemozoin aggregate formation.

Stereospecific Synthesis of 23-Hydroxyundecylprodiginines and Analogues and Conversion to Antimalarial Premarineosins via a Rieske Oxygenase Catalyzed Bicyclization

Facile and highly efficient synthetic routes for the synthesis of (S)- and (R)-23-hydroxyundecylprodiginines ((23S)-2, and (23R)-2), 23-ketoundecylprodiginine (3), and deuterium-labeled 23-hydroxyundecylprodiginine ([23-d]-2) have been developed. We demonstrated a novel Rieske oxygenase MarG catalyzed stereoselective bicyclization of (23S)-2 to premarineosin A (4), a key step in the tailoring process of the biosynthesis of marineosins, using a marG heterologous expression system. The synthesis of various A–C-ring functionalized prodiginines 32–41 was achieved to investigate the substrate promiscuity of MarG. The two analogues 32 and 33 exhibit antimalarial and cytotoxic activities stronger than those of the marineosin intermediate 2, against Plasmodium falciparum strains (CQS-D6, CQR-Dd2, and 7G8) and hepatocellular HepG2 cancer cell line, respectively. Feeding of 34–36 to Streptomyces venezuelae expressing marG led to production of novel premarineosins, paving a way for the production of marineosin analogues via a combinatorial synthetic/biosynthetic approach. This study presents the first example of oxidative bicyclization mediated by a Rieske oxygenase.