Drug development against kinetoplastid parasites

Alkhaldi, Abdulsalam Abdulhadi (2012) Drug development against kinetoplastid parasites. PhD thesis, University of Glasgow.

Abstract

Human African trypanosomiasis and leishmaniasis are caused by parasites belonging to the genera Trypanosoma and Leishmania, respectively. Significant numbers of people are affected by these diseases worldwide, which are fatal if untreated. Animals can also be infected, posing agricultural and economic hindrances, especially in poor countries. Although chemotherapy can be used for treatment, many problems are associated with it, including drug toxicity, resistance, lack of guaranteed supply, and high treatment cost. Therefore, there is an urgent need for new treatment approaches. Here, we aim to examine the in vitro efficacy of curcumin and phosphonium compounds against these parasites, assay their toxicity to human kidney cells in vitro, and investigate the mechanism of antiparasite activity of curcumin.
The Alamar blue assay was used to test 158 curcumin analogues against Leishmania major promastigotes and Leishmania mexicana promastigotes and axenic amastigotes to obtain in vitro EC50 values. Many curcumin compounds such as AS-HK122 and AS-HK126 exhibited anti-leishmanial activities similar to or better than the current clinical drug pentamidine. Similarly, EC50 values of 83 phosphonium compounds against Trypanosoma brucei brucei bloodstream forms were determined. More than 20% of the tested compounds were found to be more active than the standard veterinary drug diminazene aceturate. Multi-drug resistant strains were used to determine that there is no cross-resistance between the tested compounds and the diamidine or melaminophenyl arsenical classes of trypanocides.
Structure activity relationship (SAR) analysis revealed that mono-O-demethylated curcumin compounds showed 10-fold higher activity against the parasites than curcumin. The addition of one or two pentyl pyridinium (C10H15N) groups on specific positions of the aromatic ring also increased the activity of these compounds. Furthermore, curcumin compounds with an isoxazole ring instead of the diketo motif showed higher activity and the lowest EC50 values. Similarly, pentyl bromide (OC5H10Br) substitutions on the phenyl rings improved the antiparasitic activity. Curcuminoids with trienone linkers showed increased antiparasitic activity against all parasites tested.
Eighty-three phosphonium analogues were tested against T. brucei brucei. SAR analysis indicated that the bulky substituents surrounding the bisphosphonium cations led to strong antiparasitic activity while the nature of the linker had less effect on the activity. Some monophosphonium analogues registered the lowest EC50 values of all the phosphonium compounds.
The toxicity of the curcumin and phosphonium analogues to HEK cells was analysed in vitro. All curcumin and phosphonium compounds demonstrated lower toxicity to HEK cells than to the parasites. Of the 83 phosphonium compounds, 60 displayed >200-fold in vitro selectivity index (SI).
We also investigated the mode of antiparasitic activity of curcumin compounds. Preliminary toxicity tests had revealed that AS-HK014 caused rapid depletion of glutathione content in rat hepatocytes. Therefore, we tested AS-HK014 activity in the presence of different concentrations of L-glutathione, and AS-HK014 activity was found to decrease with increased L-glutathione concentrations, strongly suggesting that glutathione reacted with the active compound. Indeed, a chemical adduct was observed between the two compounds and identified through mass spectrometry.
A trypanosome cell line (TA014) adapted to AS-HK014 was produced. TA014 and wild-type T. brucei brucei were treated with AS-HK014 and compared with each other and with untreated controls. The glutathione and trypanothione levels were lower in the treated WT cells than in the untreated cells. However, there was no change in the glutamate, ornithine, or spermidine levels, providing no evidence for the inhibition of trypanothione synthesis, suggesting that the effect is probably not metabolic but chemical.
AS-HK014 did not significantly affect thiol levels in TA014; this might reflect a higher level of trypanothione synthesis through increased glutathione synthetase (GS) and/or γ-glutamylcysteine synthetase (γ-GCS) expression. Therefore, we analysed the protein levels using western blotting, and sequenced the encoding genes in both WT and TA014 to identify any mutations in the open reading frames (ORFs). However, we found no changes in the GS and γ-GCS protein levels in resistant trypanosomes and no mutations were found in the GS and γ-GCS ORFs. It is clear that the resistance is to the reactive enone motif of AS-HK014 rather than to curcumin and curcuminoids in general, since TA014 only displayed resistance to AS-HK014 analogues bearing the enone motif while sensitivity to curcumin remained unchanged, confirming that this motif is responsible for the higher activity of AS-HK014 compared to curcumin.
The effects of bisphosphonium analogues on T. brucei brucei bloodstream forms were investigated to identify the target. All tested analogues rapidly reduced the T. brucei brucei mitochondrial membrane potential Ψm and decreased the intracellular ATP level after one hour of incubation, suggesting that the compounds may be targeting the mitochondria. The intracellular Ca2+ levels increased gradually after eight hours, suggesting that the damaged mitochondria are unable to retain the stored Ca2+ as their membrane potential dissipates.
We also studied the trypanosome cell cycle after incubating the parasites with bisphosphonium compounds. The cell cycle defects became apparent after eight hours of incubation: DNA synthesis could not be initiated, leading to a dramatic reduction of cells in the S phase. This result was also confirmed by fluorescence microscopic assessment of DNA configuration. After eight hours of incubation with the bisphosphonium compound CD38, the number of 2K1N cells significantly decreased as compared with the control. There may be a causal relationship between mitochondrial damage and cell cycle defects. Transmission electron microscopy images of the cells obtained after 12 h of exposure to CD38 also revealed the presence of mitochondrial damage.
We tested whether bisphosphonium compounds can induce programmed cell death in trypanosomes. A TUNEL assay was used to detecting DNA fragmentation; the results showed increased DNA fragmentation after 24-h treatment with two different bisphosphonium compounds, CD38 and EFpI7. This result indicates is consistent with apoptosis occurring in treated cells but there was no evidence suggesting that bisphosophonium-induced cell death in trypanosomes is dependent on new protein synthesis.
In conclusion, curcumin and phosphonium analogues exhibit promising antiparasitic activity, and some analogues could be optimised for in vivo evaluation. Further investigations on the site of action of phosphonium compounds in the mitochondrion are in progress.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Drug development, Trypanosoma, Leishmania, Human African trypanosomiasis, leishmaniasis, curcumin, phosphonium, Kinetoplastid
• Subjects: R Medicine > RB Pathology; R Medicine > RM Therapeutics. Pharmacology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Infection & Immunity
• Supervisor's Name: de Koning, Dr. Harry P.
• Date of Award: 2012
• Depositing User: Mr Abdulsalam Alkhaldi
• Unique ID: glathesis:2012-3637
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 16 Oct 2012
• Last Modified: 21 Oct 2015 14:26
• URI: https://theses.gla.ac.uk/id/eprint/3637