Synthesis and biological evaluation of novel artemisone derivatives

Abstract

Despite intensive international efforts to eradicate malaria, the disease, continues to inflict an enormous toll on human lives, especially in Africa. An estimated 198 million malaria cases and 584, 000 deaths were reported in 2013. The burden is heaviest in the African region accounting for 90% of all malaria deaths in 2013. The most affected are pregnant women and children aged less than 5 years, the latter group alone accounting for 78% of all deaths. Artemisinins continue to remain the mainstay in the treatment of malaria more than 40 years after their discovery in the 1970s. They form the most important drug component in the so called "artemisinin combination therapies‟ (ACTs) used in the management of uncomplicated malaria in endemic countries today. Delayed clearance times of parasites treated with artemisinins were first reported in Cambodia and Thailand in Southeast Asia, and now that this phenomenon is definitively associated with resistance, a significant threat has emerged to the global initiative to control the disease. In 2011, the WHO issued the so called "Guideline for the global initiative for artemisinin resistance containment‟ which nevertheless has not been able to contain the threat. Furthermore, toxicity of the currently used artemisinins to laboratory animals continues to raise concerns on their safety to humans. Initiatives driven by the Medicines for Malaria Venture (MMV) in Geneva emphasized the urgent need for novel synthetic or semi-synthetic drugs safe and effective enough to replace the existing ones. Following its preparation, ertemisone – a second generation semi-synthetic derivative of artemisinin - did not result into any measurable neurotoxicity in both in vitro and in vivo assays. The drug has a longer half-life (3.1h) compared to all other derivatives, namely artesunate (~50 min), artemether (1.3 h) and DHA (~ 45 min) and shows superior activity to artesunate against both CQ-resistant and -sensitive P. falciparum strains, making it a drug-like compound. However, with a measured Log P value of 2.49 which is lower than those of artesunate (2.77), artemether (3.98), and DHA (~2.6) and an aqueous solubility of 89 mg/L, artemisone is rated as a polar compound, and the question therefore arises as to whether enhancing the lipophilicity of artemisone may result in a better drug. To address the polarity issue of artemisone, a series of novel lipophilic artemisone derivatives (6 – 22) were synthesized in low to moderate yields (11 - 38%) via acylation or alkylation of the a-sulfonyl carbanion dereived from treatment of artemisone with the strong non-nucleophilic base lithium N,N-diisoproplyamide in anhydrous tetrahydrofuran in an inert environment. The structures of the products were confirmed by means of nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR) and mass spectrometry (MS). The compounds were screened together with artemisone, artemether, and artesunate against NF54, K1 and W2 asexual stages of P. falciparum parasites as well as early stage gametocytes of the NF54 strain. Additionally, cytotoxicity was evaluated against the normal human fetal lung fibroblast WI-38 cell lines. All the compounds except the derivatives 13 and 14 with long aliphatic chains were highly active with IC50 values in the range of 0.42-5.9, 0.50-3.26 and 1-6.7 nM against NF54, K1 and W2 strains of P. falciprum respectively. Compounds 6 - 11, 17, 19 and 21 were the most active against the NF54 strain, while compounds 6, 7, 9 and 20 were the most active against the K1 strain, with IC50 values less than 1 nM. Compounds 7 - 9 were equipotent with artemisone, 2-4 and 3-6 times more potent than artesunate and artemether respectively against both the NF54 and K1 parasite strains. Compounds 13 and 14 were the least active in the series with IC50 values of 22 and >500 nM against the NF54 strain, 34.54 and >500 nM against K1 and 32.1 and >500 nM against the W2 strain, respectively. Artemisone derivatives 6 - 10 and 21 were again the most potent against the W2 strain in the series with IC50 values <1.8 nM, with potency comparable to that of of artemether, while being 4 – 6 times more potent than artesunate. The compounds were also very active against the early gametocyte stages of the NF54 strain. Compounds 6 - 12, 17 - 19, 21 and 22 were almost twice as active as artesunate and 5 - 6 times more active than artemether when tested against early stage gametocytes. Most compounds had impressive resistance indices (RIs) (RI = IC50 K1/ IC50 NF54) of less than 1.5, making them almost equally active towards the CQ sensitive and resistant parasites. In particular compounds 6 and 16 had RIs of less than one suggesting greater activity against the resistant K1 than the sensitive NF54 strain. Compound 6 was the only one with an RI (IC50 W2/IC50 NF54) of less than 1.5 and therefore the only non-cross resistant derivative against W2, making it the most active in the series with impressive RIs against all strains tested. Additionally most of the target compounds had excellent selectivity indices of more than 70,000 indicative of their selective antiparasitic effects resulting from intrinsic activity and not cytotoxicity. Compounds 6 and 10 were identified as the best candidates for further investigation as potential drugs in the search for new, effective and safe antimalarial drugs.