The preparation of artemisinin-cholesterol conjugates as potential new drugs for treatment of intractable forms of tuberculosis and malaria
Morake, Mokhitli Zacharia
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Malaria and tuberculosis (TB) are two lethal infectious diseases that continue to plague mankind and claim many lives. These diseases are more prevalent in developing countries especially in Africa and Asia. Malaria alone is estimated to have infected 214 million and killed 438,000 people in 2014. The majority of the cases were in Africa (88%) which also accounted for most deaths (90%). However, it appears that the mortality due to malaria is decreasing given that in 2013 584,000 deaths are estimated to have occurred. Chemotherapy remains the most effective malaria control strategy. A number of drugs used for treatment of malaria have become ineffective because of resistance, mainly from the most virulent malaria parasite, Plasmodium falciparum (Pf). Artemisinin and its derivatives (collectively called artemisinins) are currently the most active drugs, but because of their short half-lives are used in combination with longer acting partner drugs in artemisinin combination therapies (ACTs). One aim associated with introduction of ACTs was to inhibit development of resistance. Nevertheless, reports of increased parasite clearance times associated with ACTs are now widely reported and it is clear that incipient development of resistance to artemisinins is taking place. This is a daunting development since there are currently no alternative drugs to artemisinins. More devastating are the 1.4 million deaths and 10.4 million new TB cases reported to have occurred in 2015. The majority of these cases were also in Africa (26%) and Asia (61%). Socio-economic factors hamper TB eradication in endemic regions. Moreover, the development of Mycobacterium tuberculosis (Mtb) strains, the causative agent of TB, resistant to current drugs vastly complicates TB control. Multidrug-resistant TB (MDR-TB), extensively drug-resistant TB (XDR-TB) and sporadic totally drug-resistant TB (TDR-TB) have emerged. These Mtb strains are resistant to first-line and second-line anti-TB drugs. Although bedaquiline and delamanid have recently been approved conditionally for use in treatment of MDR-TB, these drugs are still undergoing advanced clinical trials and their complete safety profiles still need to be established. Despite obvious differences in the life cycles of the pathogens of malaria and TB, cholesterol is vital during their development. The malaria parasite constantly diverts and salvages cholesterol during its liver stages. Cholesterol appears to be significant in the membrane architecture and forging nutrient passages into the parasite. In Mtb cholesterol is a carbon source and is metabolised by the bacterium. A putative cholesterol transporter, Mce4, actively shuttle this molecule into the bacterium. Artemisinins are oxidant drugs that interrupt electron transfer in the redox systems of the malaria parasite. Interference of these drugs with glutathione reductase and related reductases in the malaria parasite leads to greatly impaired redox homeostasis. In Mtb, mycothiol reductase and ergothionine are involved in redox homeostasis, and it is likely that artemisinins will act against these systems as well. Therefore, a single drug that targets these pathogens is in principle attainable. We herein report the synthesis of artemisinin-cholesterol conjugates with varied linkers. The compounds were screened in vitro against Pf, Mtb and the normal mammalian HEK293 embryonic kidney cell line. Antimalarial activities (IC50) against Pf chloroquine (CQ) CQ-sensitive NF54, and CQ-resistant K1 and W2 strains ranged from 0.03 – 2.6, 0.03 – 1.9 and 0.02 – 1.7 M, respectively. Most of the compounds were relatively insoluble that may have contributed to the low activities relative to comparator artemisinins. The most active were compounds 14 and 15 against all strains. All the compounds showed no cross resistance and were not cytotoxic, with selectivity indices between the mammalian cells and the parasites ranging from 28.9 – 3903. Activities against Mtb H37Rv cultures were assessed by counting the colony forming units (CFU/ml) and then noting percentage inhibition. Cultures were treated with compounds at 10 and 80 μM concentrations resulting in growth inhibition ranging from 3 – 38% and 18 – 52%, respectively. Compounds 15 and 23 were the most active in displaying 38 and 31% inhibition at 10 μM and 52 and 47% inhibition at 80 μM, respectively. Although the antimalarial activities of the artemisinin-cholesterol conjugates herein are less than the artemisinin comparator drugs, the appreciable antimalarial and especially antimycobacterial activities noted here will help in the development of conjugates exploiting putative transporters in each of Pf, and other malaria parasites such as P. vivax, and Mtb. The immediate aims are therefore to improve aqueous solubilities of the compounds and to perform in vivo antimalarial and antimycobacterial assays. Activities of compounds 15 and 23 will be assessed in infected macrophage models. Subsequent studies will be carried out to assess the influx of these compounds into granulomas, and their activities against dormant forms of Mtb.
- Health Sciences