Malarial Dihydrofolate Reductase as Drug Target

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Revision as of 18:39, 29 November 2012

Introduction

There are currently antimalarial drugs that target the malarial dihydrofolate reductase (DHFR) such as pyrimethamine and cycloguanil. However, the effectiveness of these drugs has decreased because of mutations in the enzyme that have led to drug resistance. Since these mutations are becoming much more prevalent in malaria cases, new research in drug development must now incorporates both the wild-type as well as the quadruple mutant DHFR from the Plasmodium falciparum malarial strain, the most common and lethal of the malaria species.^[1]

spinqualitylabelsall models Crystal structure of Wild-type PfDHFR-TS COMPLEXED WITH NADPH, dUMP AND PYRIMETHAMINE (PDB entry 3QGT) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image Mutant, Drug Resistant PfDHFR The of PfDHFR has four within the active site: N51I, C59R, S108N, and I164L.[2] These mutations cause preferential decreased binding affinities of inhibitors, a huge factor being steric clashing, while having less of an effect on biological substrates. For pyrimethamine and cycloguanil specifically, the S108N mutation greatly effects their ability to bind the enzyme due to the drugs' rigid chlorophenyl substituents. The S108N mutation is also recognized as the the first mutation to have come about in the enzyme. The other mutations further decrease the binding affinities of inhibitors by resulting in conformational and other changes. In an effort to mitigate the effect of the mutations, the commonly occurring mutations and quadruple mutant must be addressed in the research of new PfDHFR targeting drugs. [3] A first step in this direction was to experiment with the drug , a 4,6-diamino-1,2-dihydro-1,3,5-triazine that, with its (2,4,5-trichlorophenoxy)propoxy side chain, addressed the steric clash that was subjected to with a Ser108Asn mutation. However further research with this drug was stopped because of its gastrointestinal toxicity in humans and low bioavailbility.[4] Wr99210 was significantly more basic than pyrimethamine, a pyrimidine and slightly acidic substance to match that of the gastrointestinal track, and thus was not readily absorbed in the intestines. When it was compared via in vitro and in vivo experiments to P65, a less basic compound with a 2,4-diaminopyrimidine scaffold, P65 was shown to have significantly higher absorption in vivo. Thus, P65 was chosen as the basis for more research and the later discussed drug, P218. Though Wr99210 was effective at getting deep into the mutant PfDHFR active site, its ineffectiveness as an oral drug would have made it unsuccessful. [5] P218 It was resolved that a structure containing 2,4-diaminopyrimidine anchor on a new drug would provide a solution for the steric hindrance present with pyrimethamine because of its rigid chlorophenyl group. In addition to this anchor allowing deep binding into the active site of PfDHFR, the other end of the molecule connected by a flexible linker region, a carboxylate group, would form strong hydrogen bonds with the conserved residue. These ideas culminated into the formation of P218. There are three regions of the DHFR active site that differ between and Plasmodium falciparum that allow a drug to target the PfDHFR specifically while not harming that of humans. One important such difference is the arginine at position 122 in PfDHFR and 70 in humans. The importance in devising a compound, such as P218, that would interact with that conserved Arg122 residue mentioned above is that the compound would interact with the PfDHFR Arg122 residue while leaving the and thus human DHFR alone. This design and intent for P218 was shown to be successful. In the quadruple mutant of PfDHFR, the P218 carboxylate side chain makes two hydrogen bonds with the Arg122 residue and has no interaction with the human Arg70. [6] During in vitro studies, P218 was shown to be very active against the quadruple mutant, pyrimethamine resistant PfDHFR.[7] In vivo, it was also shown to be very active against quadruple mutant PfDHFR in SCID mice.[8] This high level of inhibitor activity is also coupled with its success of being species selective and not causing any host toxicity at reasonable levels. However, there were experiments performed and it was found that threshold of adverse effects was about 100 mg/kg/d.[9] Conclusions P218 has been determined to be an effective inhibitor of both wild-type and mutant, drug resistant PfDHFR, minimize host toxicity, and have reasonable bioavailability (46%).[10] The key structural characteristics of the compound that allow for this are its pyrimidine side-chain flexibility and carboxylate group that forms hydrogen bonds with the conserved arginine. This ability to be of this inhibitor to overcome mutant forms of the enzyme and still be highly effective is really important to the future of antimalarial drugs. The species specific design is also greatly important to the drug in order to avoid host toxicity. P218 is thus a good candidate for further research as a malarial drug because of its shown promise to help to solve the current problem of drug resistant malaria effectively.

References

1. ↑ Somsak V, Uthaipibull C, Prommana P, Srichairatanakool S, Yuthavong Y, Kamchonwongpaisan S. Transgenic Plasmodium parasites stably expressing Plasmodium vivax dihydrofolate reductase-thymidylate synthase as in vitro and in vivo models for antifolate screening. Malar J. 2011 Oct 7;10:291. PMID: 21981896
2. ↑ Huang F, Tang L, Yang H, Zhou S, Liu H, Li J, Guo S. Molecular epidemiology of drug resistance markers of Plasmodium falciparum in Yunnan Province, China. Malar J. 2012 Jul 28;11:243. PMID: 22839209
3. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
4. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
5. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
6. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
7. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
8. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
9. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.
10. ↑ Yuthavong Y, Tarnchompoo B, Vilaivan T, Chitnumsub P, Kamchonwongpaisan S, Charman SA, McLennan DN, White KL, Vivas L, Bongard E, Thongphanchang C, Taweechai S, Vanichtanankul J, Rattanajak R, Arwon U, Fantauzzi P, Yuvaniyama J, Charman WN, Matthews D. Malarial dihydrofolate reductase as a paradigm for drug development against a resistance-compromised target. Proc Natl Acad Sci U S A. 2012 Oct 16;109(42):16823-8. Epub 2012 Oct 3. PMID:23035243. doi: 10.1073/pnas.1204556109.