Malaria is a major health problem, mainly in developing countries, and causes an estimated 1 million deaths per year. Plasmodium falciparum is the major type of human malaria parasite, and causes the most infections and deaths. Malaria drugs, like any other drugs, suffer from possible side effects and the potential for emergence of resistance. Chloroquine, which was a very effective drug, has been used since about 1945, but its use is severely limited by resistance, even though it has mild side effects, and is otherwise very efficacious. Research has shown that there are chloroquine reversal agents, molecules that can reinstate antimalarial activity of chloroquine and chloroquine-like drugs; many such reversal agents are composed of two aromatic groups linked to a hydrogen bond acceptor several bonds away. By linking a chloroquine-like molecule to a reversal agent-like molecule, it was hoped that a hybrid molecule could be made with both antimalarial and reversal agent properties. In the Peyton Lab, such hybrid "Reversed Chloroquine" molecules have been synthesized and shown to have better antimalarial activity than chloroquine against the P. falciparum chloroquine-sensitive strain D6, as well as the P. falciparum chloroquine-resistant strains Dd2 and 7G8. The work reported in this manuscript involves simplifying the reversal agent head group of the Reversed Chloroquine molecules, to a single aromatic ring instead of the two rings groups described by others; this modification retained, or even enhanced, the antimalarial activity of the parent Reversed Chloroquine molecules. Of note was compound PL154, which had IC50 values of 0.3 nM and 0.5 nM against chloroquine-sensitive D6 and chloroquine-resistant Dd2. Compound PL106 was made to increase water solubility (a requirement for bioavailability) of the simplified Reversed Chloroquine molecules. Molecular modifications inherent to PL106 were not very detrimental to the antimalarial activity, and PL106 was found to be orally available in mice infected with P. yoelli, with an ED50 value of about 5.5 mg/kg/d. Varying the linker length between the quinoline ring and the protonatable nitrogen, or between the head group and the protonatable nitrogen, did not have adverse effects on the antimalarial activities of the simplified Reversed Chloroquine molecules, in accord with the trends observed for the original design of Reversed Chloroquine molecules as found from previous studies in the Peyton Lab. The simplified Reversed Chloroquine molecules even tolerated aliphatic head groups (rather than the original design which specified aromatic rings), showing that major modifications could be made on the Reversed Chloroquine molecules without major loss in activity. A bisquinoline compound, PL192, was made that contained secondary nitrogens at position 4 of the quinoline ring (PL192 is a modification of piperaquine, a known antimalarial drug that contains tertiary nitrogens at position 4 of the quinoline ring); this compound was more potent than piperaquine which had an IC50 value of 0.7 nM against CQS D6 and an IC50 of 1.5 nM against CQR Dd2, PL192 had IC50 values of 0.63 nM against chloroquine sensitive D6 and 0.02 nM against chloroquine resistant Dd2. Finally, the mechanism of action of these simplified "Reversed Chloroquines" was evaluated; it was found that the simplified "Reversed Chloroquines" behaved like chloroquine in inhibiting β-hematin formation and in heme binding. However, the simplified "Reversed Chloroquines" were found to inhibit chloroquine transport for chloroquine resistant P. falciparum chloroquine resistance transporter expressed in Xenopus oocytes to a lesser extant than the classical reversal agent verapamil. From these studies it was noted that the simplified "Reversed Chloroquines" may not behave as well as classical reversal agents would in restoring chloroquine efficacy, but they are very potent, and so could be a major step in developing drug candidates against malaria.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-1399 |
| Date | 01 January 2010 |
| Creators | Gunsaru, Bornface |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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