Frans J. Smit

Recent progress in the development of anti-malarial quinolones

Available anti-malarial tools have over the ten-year period prior to 2012 dramatically reduced the number of fatalities due to malaria from one million to less than six-hundred and thirty thousand. Although fewer people now die from malaria, emerging resistance to the first-line anti-malarial drugs, namely artemisinins in combination with quinolines and arylmethanols, necessitates the urgent development of new anti-malarial drugs to curb the disease. The quinolones are a promising class of compounds, with some demonstrating potent in vitro activity against the malaria parasite. This review summarizes the progress made in the development of potential anti-malarial quinolones since 2008. The efficacy of these compounds against both asexual blood stages and other stages of the malaria parasite, the nature of putative targets, and a comparison of these properties with anti-malarial drugs currently in clinical use, are discussed.

Synthesis and in vitro biological evaluation of aminoacridines and artemisinin–acridine hybrids

During this study, 9-aminoacridine and artemisinin-acridine hybrid compounds were synthesized and the in vitro for antimalarial activity against both the chloroquine sensitive but also gametocytocidal strain (NF54), and chloroquine resistant (Dd2) strains of Plasmodium falciparum was determined. In vitro cytotoxicity against CHO cells, apoptosis of HepG2 and SH-SY5Y as well as anticancer activity against HeLa cell lines were assessed. The hybrids were synthesized, using a microwave-assisted radiation method by covalently linking artemisinin and acridine pharmacophores by means of a liable, aminoethyl ether linker. The synthesized compounds were found active against both the Plasmodium strains and displayed superior selective toxicity towards the parasitic cells. Hybrid 7, however, containing ethylenediamine linker, proved the most active of all of the synthesized compounds. It had seven-fold higher antigametocytocidal activity compared to chloroquine and was also found to be seven-fold more potent than chloroquine against the Dd2 strain, with highly selective action towards the parasitic cells. This hybrid also showed favourable anti-cancer activity against the HeLa cells, three- and eight-fold higher than those of chloroquine and melphalan, respectively. This hybrid may therefore stand as drug candidate for further investigation in the search for new and effective drugs against malaria and cervical cancer.

Synthesis and in vitro biological evaluation of dihydroartemisinyl-chalcone esters.

A series of dihydroartemisinyl-chalcone esters were synthesized through esterification of chalcones with dihydroartemisinin (DHA). The hybrids were screened against chloroquine (CQ) sensitive (3D7) and CQ resistant (W2) strains of intraerythrocytic Plasmodium falciparum parasites, and were all found to be active, with IC50 values ranging between 1.5 and 11 nM against both strains, with SI values over 5800. The esters featuring oxygenated aryl rings (7, 10 and 11), were found to be equipotent to DHA, but were 2-3 times more active than artesunate against the 3D7 and W2 strains of the malaria parasites. They were also screened in vitro against a panel of three cancer cell lines consisting of TK-10, UACC-62 and MCF-7. Compound 7, bearing a furan ring, displayed the most potent overall antitumor activity against all three cancer cell lines. TGA revealed that the targeted hybrids were all thermally more stable than DHA, which may be beneficial to the high temperature storage conditions that prevail in malaria endemic countries. During this study, ester 7 was identified as the best candidate for further investigation as a potential drug in search for new, safe and effective antimalarial drugs.

The Case for Development of 11-Aza-artemisinins for Malaria.

The current treatment regimens for uncomplicated malaria comprise an artemisinin in combination with another drug (ACT). However, the recent emergence of resistance to ACTs in South East Asia dramatically emphasizes the need for new artemisinins. The current artemisinins have been in use since the late 1970s and have relatively poor thermal, chemical and metabolic stabilities - all are metabolized or hydrolyzed in vivo to dihydroartemisinin (DHA) that itself undergoes facile decomposition in vivo. The current artemisinins possess neurotoxicity as demonstrated in animal models, an issue that mandates increased vigilance in view of trends to use of protracted treatment regimens involving sequential administration of different ACTs against the resistant disease. As artemisinins induce the most rapid reduction in parasitaemia of any drug, common sense dictates that any new artemisinin derivative, selected on the bases of more robust chemical and thermal stability, metabolic stability with respect to the generation of DHA in vivo, and relatively benign neurotoxicity should be used in any new ACT whose components are rationally chosen in order to counter resistant malaria and inhibit transmission. 11-Azaartemisinin and its N-substituted derivatives attract because of overall ease of preparation from artemisinin. Some derivatives also possess notable thermal stabilities and although metabolic pathways of the derivatives are as yet unknown, none can provide DHA. The azaartemisinins synthesized over the past 20 years are critically discussed on the basis of their synthetic accessibility and biological activities with the view to assessing suitability to serve as new artemisinin derivatives for treatment of malaria.

Straightforward conversion of decoquinate into inexpensive tractable new derivatives with significant antimalarial activities

As part of a programme aimed at identifying rational new triple drug combinations for treatment of malaria, tuberculosis and toxoplasmosis, we have selected quinolones as one component, given that selected examples exhibit exceptionally good activities against the causative pathogens of the foregoing diseases. The quinolone decoquinate (DQ), an old and inexpensive coccidiostat, displays anti-malarial activity in vitro against Plasmodium falciparum (Pf). However, because of its exceedingly poor solubility in water or organic solvents, development of DQ as a drug is problematical. We have therefore converted DQ in straightforward fashion into tractable new derivatives that display good activities in vitro against chloroquine-sensitive NF54 and multidrug-resistant K1 and W2 Pf, and relatively low toxicities against human fibroblast cells. The most active compound, the N-acetyl derivative 30, is 5-fold more active than DQ against NF54 and K1 and equipotent with DQ against W2. It possesses an activity profile against all strains comparable with that of the artemisinin derivative artesunate. Overall, this compound and the other accessible and active derivatives serve as an attractive template for development of new and economic lead quinolones.

Activities of 11‐azaartemisinin and N‐sulfonyl derivatives against asexual and transmissible malaria parasites

Dihydroartemisinin (DHA), either used in its own right or as the active drug generated in vivo from the other artemisinins in current clinical use—artemether and artesunate—induces quiescence in ring‐stage parasites of Plasmodium falciparum (Pf). This induction of quiescence is linked to artemisinin resistance. Thus, we have turned to structurally disparate artemisinins that are incapable of providing DHA on metabolism. Accordingly, 11‐azaartemisinin 5 and selected N‐sulfonyl derivatives were screened against intraerythrocytic asexual stages of drug‐sensitive Pf NF54 and drug‐resistant K1 and W2 parasites. Most displayed appreciable activities against all three strains, with IC50 values <10.5 nm. The p‐trifluoromethylbenzenesulfonyl‐11‐azaartemisinin derivative 11 [(4′‐trifluoromethyl)benzenesulfonylazaartemisinin] was the most active, with IC50 values between 2 and 3 nm. The compounds were screened against Pf NF54 early and transmissible late intraerythrocytic‐stage gametocytes using luciferase and parasite lactate dehydrogenase (pLDH) assays. The 2′‐thienylsulfonyl derivative 16 (2′‐thiophenesulfonylazaartemisinin) was notably active against late‐stage (IV–V) gametocytes with an IC50 value of 8.7 nm. All compounds were relatively nontoxic to human fetal lung WI‐38 fibroblasts, showing selectivity indices of >2000 toward asexual parasites. Overall, the readily accessible 11‐azaartemisinin 5 and the sulfonyl derivatives 11 and 16 represent potential candidates for further development, in particular for transmission blocking of artemisinin‐resistant parasites.

Synthesis, in vitro antimalarial activities and cytotoxicities of amino-artemisinin-ferrocene derivatives

Novel derivatives bearing a ferrocene attached via a piperazine linker to C-10 of the artemisinin nucleus were prepared from dihydroartemisinin and screened against chloroquine (CQ) sensitive NF54 and CQ resistant K1 and W2 strains of Plasmodium falciparum (Pf) parasites. The overall aim is to imprint oxidant (from the artemisinin) and redox (from the ferrocene) activities. In a preliminary assessment, these compounds were shown to possess activities in the low nM range with the most active being compound 6 with IC50 values of 2.79 nM against Pf K1 and 3.2 nM against Pf W2. Overall the resistance indices indicate that the compounds have a low potential for cross resistance. Cytotoxicities were determined with Hek293 human embryonic kidney cells and activities against proliferating cells were assessed against A375 human malignant melanoma cells. The selectivity indices of the amino-artemisinin ferrocene derivatives indicate there is overall an appreciably higher selectivity towards the malaria parasite than mammalian cells.

Preliminary evaluation of artemisinin-cholesterol conjugates as potential drugs for the treatment of intractable forms of malaria and tuberculosis

To evaluate the feasibility of developing drugs that may be active against both malaria and tuberculosis (TB) by using in part putative cholesterol transporters in the causative pathogens and through enhancement of passive diffusion in granulomatous TB, artemisinin–cholesterol conjugates were synthesized by connecting the component molecules through various linkers. The compounds were screened in vitro against Plasmodium falciparum (Pf) and Mycobacterium tuberculosis (Mtb). Antimalarial activities (IC50) against Pf drug‐sensitive NF54, and drug‐resistant K1 and W2 strains ranged from 0.03–2.6, 0.03–1.9, and 0.02–1.7 μm. Although the compounds are less active than the precursor artemisinin derivatives, the cholesterol moiety renders the compounds relatively insoluble in the culture medium, and variation in solubilities among the different compounds may reflect in the range of efficacies observed. Activities against Mtb H37Rv were assessed using a standardized colony‐forming unit (CFU) assay after 24 h pretreatment of cultures with each of the compounds. Percentage inhibition ranged from 3–38 % and 18–52 % at 10 and 80 μm, respectively. Thus, in contrast to the comparator drug artemether, the conjugates display enhanced activities. The immediate aims include the preparation of conjugates with enhanced aqueous solubilities, assays against malaria and TB in vivo, and for TB, assays using an infected macrophage model and assessment of granuloma influx.

Activities of 11-Azaartemisinin and N-Sulfonyl Derivatives against Neospora caninum and Comparative Cytotoxicities

Neosporosis caused by the apicomplexan parasite Neospora caninum is an economically important disease that induces abortion in dairy and beef cattle. There are no vaccines or drugs available on the market for control or treatment of the disease in bovines. The peroxide artemisinin and its derivatives used clinically for treatment of malaria are active against N. caninum and other apicomplexan parasites. We have now evaluated the activities of the readily accessible and chemically robust 11‐azaartemisinin 5 and selected N‐sulfonyl derivatives prepared as described in the accompanying paper against N. caninum tachyzoites grown in infected human foreskin fibroblasts. Azaartemisinin elicited an IC50 value of 150 nm, and the 2′,5′‐dichloro‐3′‐thienylsulfonyl‐11‐azaartemisinin 17 was found to be the most active, with an IC50 value of 40 nm. Comparison with normal human fetal lung fibroblasts HFLF WI‐38 revealed relatively benign cytotoxicity. The compounds were also screened in vitro against TK‐10 (renal), UACC‐62 (melanoma) and MCF‐7 (breast) cancer cell lines; overall, in line with activities against HFLF cells, most compounds in the series were found to be inactive.

Accessible and distinct decoquinate derivatives active against Mycobacterium tuberculosis and apicomplexan parasites

The quinolone decoquinate is coadministered with feed for treatment of parasites which cause coccidiosis in poultry. However, from a drug-development perspective, the biological activity is often not adequately exploited due to poor physicochemical properties. Here we convert decoquinate into N-alkyl quinolone amides that, in contrast to decoquinate, are active against the tuberculosis bacterium with MIC90 values ranging from 1.4 to 3.64 µM, and quinoline O-carbamates active against apicomplexan parasites that cause malaria, toxoplasmosis, and neosporosis with IC50 values of 0.32–1.5 nM for the best derivative. Uniquely for the TB-active amides, disruption of cell wall homoeostasis is identified as one target. With IC50 values against fetal lung fibroblast cells of 40 to >100 μM, the derivatives are selective for the pathogens. Structures of the most active derivatives are determined by NMR spectroscopy and X-ray crystallography. Analogues lacking the decyl side chain of decoquinate are inactive.Decoquinate is a drug used in veterinary practice, which displays antimalarial activity in vitro but has poor bioavailability. Here, the authors convert decoquinate into more soluble amide and carbamate derivatives and assess their efficacy against tuberculosis bacteria and apicomplexan parasites.