Pharmacotherapeutic approaches against Plasmodium falciparum
by Vella Nikolova
Date of Examination:2025-05-28
Date of issue:2025-06-23
Advisor:Prof. Dr. Matthias Dobbelstein
Referee:Prof. Dr. Uwe Groß
Referee:Prof. Dr. Dirk Görlich
Persistent Address:
http://dx.doi.org/10.53846/goediss-11329
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Abstract
English
Malaria tropica persists as one of the most impactful infectious diseases, threatening the health of nearly half of the world’s population every year. The disease is caused by Plasmodium falciparum (P. falciparum), a protozoan parasite transmitted through the bite of female Anopheles mosquitoes. Currently available vaccines do not provide sufficient long-lasting immunity, prevention and eradication measures are only partially effective, and therapeutic options are limited. Furthermore, the use of artemisinin-based combination therapies (ACTs), which represent the most successful antimalarial strategies to date, has been threatened by rapid emergence of drug resistance, necessitating the need to develop and implement novel approaches against malaria. The following thesis comprises two independent projects, which present findings on antimalarial strategies that warrant further exploration. As a model system, P. falciparum-infected human red blood cells (RBCs) were used. First, we investigated membrane-permeable derivatives of 5-fluorodeoxyuridine triphosphate (FdUTP), which exhibited strong anti-parasitic activity against P. falciparum in vitro. Due to the lack of nucleoside kinases in this species, the use of nucleoside analogues has been precluded so far. Furthermore, phosphorylated nucleosides are too polar to permeate lipid membranes, which diminishes their efficacy. To overcome this challenge, we used cell-permeable analogues of FdUTP (cpFdUTP) with lipid-like modifications, which facilitated their entry into the red blood cell and the parasite. The membrane-permeable FdUTP derivatives demonstrated rapid and potent anti-parasitic effects, interfering with DNA synthesis and arresting parasite development predominantly at the trophozoite-to-schizont transition. Moreover, the toxicity of cpFdUTP against human cells was ameliorated by the addition of thymidine or cell-permeable deoxythymidine triphosphate (cpdTTP) without restoring parasite proliferation. Thus, this opens a therapeutic window and highlights the potential application of cpFdUTP in combination with thymidine as a novel approach to treat malaria. In addition to that, the combination of the commonly used antimalarial Pyrimethamine and the ferroptosis-inducing agent RAS-selective lethal 3 (RSL3) proved highly efficient in eliminating P. falciparum within human RBCs. Pyrimethamine is a well-known inhibitor of Plasmodium dihydrofolate reductase (DHFR), whereas RSL3 triggers lipid peroxidation-mediated cell death, and the combination of both exhibited strong synergy against the parasites. Initially, our hypothesis on the mechanism underlying this synergy was based on the ferroptosis-inducing activity of RSL3. Surprisingly, various combinations of DHFR inhibitors and alternative ferroptosis inducers did not achieve a comparable degree of synergy. Furthermore, Ferrostatin-1, an antagonist of lipid peroxidation, failed to rescue parasites from the deleterious impact of RSL3, pointing to a mechanism that is largely independent of oxidative stress and lipid peroxidation. In any case, the combination of RSL3 and Pyrimethamine holds promise as a novel antimalarial strategy that is worth investigating further. Taken together, the current studies present two different approaches for elimination of P. falciparum within human red blood cells, which have the potential to be translated into therapeutic strategies against malaria and warrant further exploration and development.
Keywords: Malaria; Plasmodium falciparum; antimalarial therapies; nucleoside analogues; nucleotides; fluorouridine; 5-fluorodeoxyuridine; ferroptosis; RSL3; Pyrimethamine
