Quinoline is a substance that has been used for centuries in antimalarial drugs, although its mechanism of action was unknown. Now an international team of researchers discovered how this organic compound acts in malaria-infected red blood cells under almost native conditions. The advance has been achieved using the light of three synchrotrons: ALBA in Spain, ESRF in France and BESSY in Germany.
Plasmodium falciparum, transmitted through mosquito bites, is the protozoan parasite that causes malaria. It reaches the blood and infects the red blood cells of its victim. During the last two decades, the parasite has developed resistance to drugs.
“Recently, the growing geographic dispersion of the species, as well as the emergence of resistant strains, are of concern to the scientific community, and to improve malaria drugs we have to know how they work precisely,” explains Sergey Kapishnikov, a researcher at the University from Copenhagen (Denmark) and the Weizmann Institute (Israel).
Kapishnikov leads an international study published in the PNAS that details the mechanism of action of one of the compounds traditionally used in antimalarial drugs: quinoline. Specifically, the researchers analyzed how it acts in red blood cells infected with malaria under conditions very similar to natural ones.
Once inside the red blood cells, Plasmodium takes hemoglobin as its nutrient (the protein responsible for transporting oxygen through the blood). After digestion, heme molecules that contain iron are released and are toxic to the parasite. However, these molecules crystallize in hemozoin, a waste product of this parasite’s digestion that causes the molecules to remain inert.
As the authors published in previous studies, for Plasmodium to survive, the rate at which heme molecules are released has to be slower or equal to the rate of crystallization of hemozoin. Otherwise, there would be an accumulation of toxic heme within the parasite and it would die.
The drugs of the quinoline family, including malaria pills based on quinine, effectively damage the parasite. The scientific community suspects that the reason for its success is the inability to crystallize heme groups. Until now, all studies on the action of the drug on heme crystals had been done in model systems or in dried parasite samples, a fact that provided limited data and opened the door to speculation.
Kapishnikov’s team, which includes scientists from Denmark, Spain, Germany, Israel and France, decided to know the action model of established drugs, such as chloroquine (although they use their bromoquine analogue) in red blood cells infected with fully hydrated Plasmodium falciparum and frozen. Rapid freezing is a method that creates snapshots of the different stages of cell life, so that the chemical distribution is not altered by sample preparation treatments.
Complementary analyzes between synchrotrons
In this case, the cryogenized cells traveled throughout Europe. The research team led them to analyze in different synchrotron light sources. First in the MISTRAL light line of the ALBA Synchrotron and BESSY-II in Berlin, so that its structure can be mapped in three dimensions by X-ray cryotomography. This technique, only available in four synchrotrons in the world (Diamond in the United Kingdom, ALS in the USA and the two mentioned), is the only way to visualize the whole cells and in their natural state without being sectioned or altered by any chemical treatment.
Finally, the cells were taken to the ESRF in Grenoble to map the distribution of bromine and iron using an X-ray fluorescence technique. The data obtained in all the synchrotrons were analyzed in Denmark where the scientific team determined the correlation between different imaging modalities and bromoquine concentrations on the surface of hemozoin crystals, on the membrane and within the lumen of the digestive vacuole of the parasite —the point of action of the drugs were calculated and interpreted.
The results of the infected red blood cells have shown that bromoquine captures hemozoin crystals, thereby inhibiting their growth and, therefore, sabotaging the detoxification of heme. Surprisingly, they have also seen that bromoquine accumulates in the digestive system of the parasite, a fact that improves the efficiency of the drug in depriving the coupling of heme molecules on the surface of the hemozoin crystal.
“These results show a model that can be generalized in all quinoline drugs , such as quinine. Our approach can also be extended to other families of antimalarial drugs, such as artemisinins,” explains Kapishnikov.
Malaria continues to be one of the leading causes of death in the poorest countries. Estimated deaths vary between 450,000 and 720,000, mostly in Africa. “We hope that this new discovery will allow a step towards the design of new, more effective drugs against resistant strains of malaria,” the researcher concludes.