Background
Malaria is an infectious disease generated by
Plasmodium spp., which continues to be a public health problem in Thailand. The 2018 Thai guidelines for the treatment of malaria recommend artemisinin-based combination therapy as the first-line regimen [
1]. Currently, the first-generation artemisinin derivatives, including artemisinin, artemether, arteether, artesunate, and dihydroartemisinin, are still widely used [
2]. Each derivative penetrates the red blood cells differently and has a distinctive ability to kill malaria parasites [
3,
4]. This study aims to determine the factors that confer a different capability to enter the red blood cells. Therefore, we selected 15 anti-malarial drugs according to the World Health Organization (WHO) malaria treatment guidelines for this study [
5]. These include artemisinin, dihydroartemisinin, artemether, arteether, artesunate, chloroquine, mefloquine, primaquine, amodiaquine, piperaquine, quinine, sulfadoxine, pyrimethamine, doxycycline, and proguanil. The screening procedures here identified the three most influential physicochemical parameters that could affect erythrocyte penetration. Information obtained from this study would be beneficial for the development of new anti-malarial drugs that are more effective in penetrating red blood cells.
Discussion
Machine learning is a powerful approach that widely used in many fields in the sciences for finding valuable information from data. The aims of a machine learning model development can be both to build a robust predictive model and to explain a relationship of features to outcomes. To create a predictive model needs a vast dataset to be learned by the model. While the anti-malarial drug is orphan, so the data of the drug is limited. Thus, the objective of this analysis was to investigate features that could involve drug-red blood cell partition, not to build a robust predictive model due to a limitation of data.
From the extreme gradient boosting regression of anti-malarial drugs and their abilities in red blood cell penetration, the R-squared was 0.27. Also, the multiple linear regression of anti-malarial drugs, and their abilities in protein binding Adjusted R-squared was 0.521. These could illustrate that the predictive power of the model is incompetent. However, we can find the essential feature protein binding and some statistically significant chemical descriptors from the model, which demonstrate the relationship of them to the drug-red blood cell partition. This conclusion might lead to new potential substances that can protect against malaria in the future.
According to the analysis of factors affecting penetration of 15 anti-malarial drugs into red blood cells, we found that protein binding dominantly affects the penetration. Low protein binding causes an increased level of free drug in plasma, allowing the drug to distribute and penetrate into red blood cells. This finding is consistent with the hypotheses in previous studies of different drugs. A study of cyclosporin A revealed that the level of free drug was directly related to the concentration of the drug in red blood cells, in similar manner to another study of phenytoin [
39‐
41]. Moreover, analysis of molecular descriptors affecting protein-binding property showed that the number of rotatable bonds, hydrogen bond acceptors, aliphatic hydrocarbons and aromatic hydrocarbons was significantly related to the protein-binding property of the drug. This property decreased with a lack of hydrogen bond acceptors and aromatic hydrocarbons; on the other hand, it increased with a lack of rotatable bonds and aliphatic hydrocarbons.
Approximately 50% of the protein in plasma is albumin. This protein plays an important role in binding to unbound drugs in plasma. There are two major binding sites in the albumin structure. The first site tends to fit with large drug molecules, while the other one is less flexible and stereo specifically bound to the drug [
41]. It is implied that the drug with large size and less flexibility has higher ability to bind to a protein. In this study, a molecule containing a higher number of rotatable bonds had less ability to bind to plasma proteins, as the molecule was flexible. The number of hydrogen bond acceptors is directly related to protein-binding property; thus, the fewer hydrogen bond acceptors, the higher the red blood cell penetration. The study of Samari et al. found that Van der Waals forces and hydrogen bonds were dominant in the binding between amodiaquine and albumin in plasma [
42]. The results presented here are also consistent with a previous study which found that drugs with a low tendency to create hydrogen bonds had increased penetration into red blood cells [
43]. As for the number of aliphatic and aromatic hydrocarbons, molecules with a high number of aliphatic hydrocarbons and a low number of aromatic hydrocarbons would have decreased protein-binding property, facilitating penetration into red blood cells. This concept was mentioned in a previous study; a drug containing not more than two aromatic hydrocarbons will have more unbound drug in plasma than a drug containing more than two aromatic hydrocarbons. It will also tend to bypass metabolism in the liver, leading to high concentration of the drug in plasma [
44].
In terms of pharmacokinetics and pharmacodynamics, the efficacy of an antimicrobial drug generally depends on its concentration and duration of exposure. Likewise, the efficacy of artemisinin derivatives was most related to its maximum concentration in plasma [
23]. More unbound drug in the plasma would be a factor that could lead to a higher concentration of the drug at the targeted site, which for an anti-malarial drug is the red blood cell. Accumulation of the drug in red blood cells increased its half-life and consequently increased the efficacy of the drug actions. In a practical aspect, anti-malarial drugs containing higher numbers of rotatable bonds and aliphatic hydrocarbons, and lower numbers of hydrogen bond acceptors and aromatic hydrocarbons, would have less protein-binding property. Therefore, more drug will penetrate through the red blood cells, facilitating its pharmacodynamic activities.
Conclusions
The most influential physicochemical factor for the penetration of anti-malarial drugs into red blood cells is protein binding. The less a drug is bound to protein, the more it is available in free form, which can penetrate into the red blood cell. For molecular descriptors affecting protein binding, drugs with a small number of hydrogen bond acceptors and aromatic hydrocarbons, together with a high number of rotatable bonds and aliphatic hydrocarbons, may have a higher amount of free drug in the plasma available to penetrate into the red blood cell.
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