The Artemiside-Artemisox-Artemisone-M1 Tetrad: Efficacies against Blood Stage P. falciparum Parasites, DMPK Properties, and the Case for Artemiside

Efficacies and ADME properties of redox active methylene blue and phenoxazine analogues for use in new antimalarial triple drug combinations with amino-artemisinins

Efforts to develop new artemisinin triple combination therapies effective against artemisinin-tolerant strains of Plasmodium falciparum based on rational combinations comprising artemisone or other amino-artemisinins, a redox active drug and a third drug with a different mode of action have now been extended to evaluation of three potential redox partners. These are the diethyl analogue AD01 of methylene blue (MB), the benzo [α]phenoxazine PhX6, and the thiosemicarbazone DpNEt. IC50 values in vitro against CQ-sensitive and resistant P. falciparum strains ranged from 11.9 nM for AD01-41.8 nM for PhX6. PhX6 possessed the most favourable pharmacokinetic (PK) profile: intrinsic clearance rate CLint was 21.47 ± 1.76 mL/min/kg, bioavailability was 60% and half-life was 7.96 h. AD01 presented weaker, but manageable pharmacokinetic properties with a rapid CLint of 74.41 ± 6.68 mL/min/kg leading to a half-life of 2.51 ± 0.07 h and bioavailability of 15%. DpNEt exhibited a half-life of 1.12 h and bioavailability of 8%, data which discourage its further examination, despite a low CLint of 10.20 mL/min/kg and a high Cmax of 6.32 µM. Efficacies of AD01 and PhX6 were enhanced synergistically when each was paired with artemisone against asexual blood stages of P. falciparum NF54 in vitro. The favourable pharmacokinetics of PhX6 indicate this is the best partner among the compounds examined thus far for artemisone. Future work will focus on extending the drug combination studies to artemiside in vitro, and conducting efficacy studies in vivo for artemisone with each of PhX6 and the related benzo[α]phenoxazine SSJ-183.

Single and combination treatment of Toxoplasma gondii infections with a bumped kinase inhibitor and artemisone in vitro and with artemiside in experimentally infected mice

In previous studies, the artemisinin derivatives artemisone, its pro-drug artemiside and the bumped-kinase inhibitor BKI-1748 were effective against T. gondii via different modes of action. This suggests that they may act synergistically resulting in improved efficacies in vitro and in vivo. To test this hypothesis, the compounds were applied alone and in combination to T. gondii infected human fibroblast host cells in order to determine their inhibition constants and effects on cellular ultrastructure. In addition, the efficacy of either single- or combined treatments were assessed in an acute TgShSp1-oocyst infection model based on CD1 outbred mice. Whereas the IC50 of the compounds in combination (42 nM) was close to the IC50 of BKI-1748 alone (46 nM) and half of the IC50 of artemisone alone (92 nM), the IC90 of the combination was half of the values found with the single compounds (138 nM vs. ca. 270 nM). Another indication for synergistic effects in vitro were distinct alterations of the cellular ultrastructure of tachyzoites observed in combination, but not with the single compounds. These promising results could not be reproduced in vivo. There was no decrease in number of T. gondii positive brains by either treatment. However, the levels of infection in these brains, i. e. the number of tachyzoites, was significantly decreased upon BKI-1748 treatment alone, and the combination with artemiside did not produce any further decrease. The treatment with artemiside alone had no significant effects. A vertical transmission model could not be established since artemiside strongly interfered with pregnancy and caused abortion. These results show that is difficult to extrapolate from promising in vitro results to the situation in vivo.

Proteomic characterization of Toxoplasma gondii ME49 derived strains resistant to the artemisinin derivatives artemiside and artemisone implies potential mode of action independent of ROS formation

The sesquiterpene lactone artemisinin and its amino-artemisinin derivatives artemiside (GC008) and artemisone (GC003) are potent antimalarials. The mode of action of artemisinins against Plasmodium sp is popularly ascribed to 'activation' of the peroxide group by heme-Fe(II) or labile Fe(II) to generate C-radicals that alkylate parasite proteins. An alternative postulate is that artemisinins elicit formation of reactive oxygen species by interfering with flavin disulfide reductases resposible for maintaining intraparasitic redox homeostasis. However, in contradistinction to the heme-activation mechanism, the amino-artemisinins are effective in vitro against non-heme-degrading apicomplexan parasites including T. gondii, with IC ₅₀ values of 50-70 nM, and induce distinct ultrastructural alterations. However, T. gondii strains readily adapted to increased concentrations (2.5 μM) of these two compounds within few days. Thus, T. gondii strains that were resistant against artemisone and artemiside were generated by treating the T. gondii reference strain ME49 with stepwise increasing amounts of these compounds, yielding the artemisone resistant strain GC003^R and the artemiside resistant strain GC008^R. Differential analyses of the proteomes of these resistant strains compared to the wildtype ME49 revealed that 215 proteins were significantly downregulated in artemisone resistant tachyzoites and only 8 proteins in artemiside resistant tachyzoites as compared to their wildtype. Two proteins, namely a hypothetical protein encoded by ORF TGME49_236950, and the rhoptry neck protein RON2 encoded by ORF TGME49_300100 were downregulated in both resistant strains. Interestingly, eight proteins involved in ROS scavenging including catalase and superoxide dismutase were amongst the differentially downregulated proteins in the artemisone-resistant strain. In parallel, ROS formation was significantly enhanced in isolated tachyzoites from the artemisone resistant strain and - to a lesser extent - in tachyzoites from the artemiside resistant strain as compared to wildtype tachyzoites. These findings suggest that amino-artemisinin derivatives display a mechanism of action in T. gondii distinct from Plasmodium.

Antimalarial and antitumour activities of the steroidal quinone-methide celastrol and its combinations with artemiside, artemisone and methylene blue

Artemisinin, isolated from the traditional Chinese medicinal plant qīng hāo 青蒿 (Artemisia annua) and its derivatives are used for treatment of malaria. With treatment failures now being recorded for the derivatives and companion drugs used in artemisinin combination therapies new drug combinations are urgently required. The amino-artemisinins artemiside and artemisone display optimal efficacies in vitro against asexual and sexual blood stages of the malaria parasite Plasmodium falciparum and are active against tumour cell lines. In continuing the evolution of combinations of the amino-artemisinins with new drugs, we examine the triterpenoid quinone methide celastrol isolated from the traditional Chinese medicinal plant léi gōng téng 雷公藤 (Tripterygium wilfordii). This compound is redox active, and has attracted considerable attention because of potent biological activities against manifold targets. We report that celastrol displays good IC₅₀ activities ranging from 0.50–0.82 µM against drug-sensitive and resistant asexual blood stage Pf, and 1.16 and 0.28 µM respectively against immature and late stage Pf NF54 gametocytes. The combinations of celastrol with each of artemisone and methylene blue against asexual blood stage Pf are additive. Given that celastrol displays promising antitumour properties, we examined its activities alone and in combinations with amino-artemisinins against human liver HepG2 and other cell lines. IC₅₀ values of the amino-artemisinins and celastrol against HepG2 cancer cells ranged from 0.55–0.94 µM. Whereas the amino-artemisinins displayed notable selectivities (SI > 171) with respect to normal human hepatocytes, in contrast, celastrol displayed no selectivity (SI < 1). The combinations of celastrol with artemiside or artemisone against HepG2 cells are synergistic. Given the promise of celastrol, judiciously designed formulations or structural modifications are recommended for mitigating its toxicity.