QSAR Studies of Nitrobenzothiazole Derivatives as Antimalarial Agents

Quantitative Structure and Activity Relationship (QSAR) analyses were carried out for a series of 13 nitrobenzothiazole derivatives as antimalarial compounds to find out the structural relationship of their antimalarial activities against the W2 Plasmodium falciparum strain. The electronic descriptors have been determined using the atomic net charges (q), dipole moment (μ), ELUMO, EHOMO, polarizability (α) and Log P. In addition, the descriptors were calculated through HyperChem for Windows 8.0 using the PM3 semi-empirical method. The antimalarial activities (IC50) were taken from literature [1]. Furthermore, the QSAR model was determined by multiple linear regression (MLR) approach, giving equation model of QSAR: Log IC50 = 41.483 + 54.812 (qC2) – 50.058 (qS3) + 416.766 (qC4) + 440.734 (qC5) – 754.213 (qC7) – 73.721 (qC8) + 246.715 (qC9) + 0.551 (μ) – 13.269 (EHOMO) – 3.404 (ELUMO) + 0.042 (α) + 0.107 (Log P). The most statistically significant QSAR model with correlation coefficients n = 13, (r) = 1.00, (r) = 1.00, SE = 0, and PRESS = 3.40 were developed by MLR. Based on the model of the above QSAR equation 43 new nitrobenzothiazole derivatives were modeled and 24 of these compounds showed high antimalarial activity. It is recommended that these are synthesized for further investigation 4 new compounds (45, 49, 52 and 55) show equivalent activity to that achieved with chloroquine antimalarial drugs.


Introduction
Malaria is still one of the most dangerous diseases in the world and is a major cause of morbidity and mortality in people living in tropical countries. Globally, there were approximately 214 million clinical cases of malaria in 2015 [2,3]. Malaria remains one of the major public health problems and a major cause of illness, hospitalizations, and deaths in Nigeria. Facts prove that Nigeria is a malaria endemic area where 97% of the Nigerian population at risk of contracting malaria [4][5][6][7]. According to statistics, there are about 100 million cases of malaria per year in Nigeria. Each year, approximately 50% of adults Nigeria suffer from malaria, while children under the age of 5 will have malarial arracks 2-4 times each year [8]. Elsewhere, malaria remains a heavy burden across sub-Saharan Africa where transmission is maintained by some of the world's most efficient vectors. The primary malaria vectors in Africa are Anopheles gambiae, Anopheles funestus, and Anopheles arabiensis. Anopheles gambiae is a member of the Anopheles gambiae complex and is considered to be the most efficient malaria vector in existence [9].
More than 90 million of the Indonesians population are living in malaria endemic areas. In 2013, as many as 4.8 million people in Indonesian were infected with malaria with the majority of cases in eastern Indonesia [10,11]. Malaria attacks are common in Indonesia, especially in the Province of Maluku and Papua. Malaria remains a cause of health problems and is one of the largest causes of death of infants and children. In Maluku, malaria attacked the residents in the village of Wawasa Amarsekaru, Gorom, East Seram District, where 18 people were killed. The number of malaria patients reached 756 people, or half of the population of Wawasa. This malaria outbreak was described as an Extraordinary Circumstance [10].
As such, malaria remains a major global health problem in developing countries, and in Indonesian as particular. Efforts to eradicate malaria have not been successful. These have been hampered by an absence of malaria drugs coinciding with emerging resistance to antimalarials [12][13][14]. A study of rural communities of Nigeria reported that the cost of malaria treatment by households accounted for 49.9% of health care costs incurred by each household [15,16].
One attempt to develop potential malaria drugs was carried out using molecular modeling of drugs through computational chemistry techniques. This aimed to obtain the best equation model of the relationship between chemical structure and activity. Quantitative Structure and Activity Relationship (QSAR) analysis, through the application of computational chemistry concepts, will be able to provide many benefits that can save time and cost as it is relatively inexpensive when compared to experimental lab research. In the pharmaceutical field, computational chemistry methods have been used to perform molecular drug modeling to study the relationship between molecular structureactivity and drug interactions with receptors [17].
The development of molecular modeling with computational chemistry methods is strongly supported by the development of computer technology. Improved computer technology to provide support for the creation of software, is able to model compounds composed of hundreds or even thousands of atoms. A technique that is widely used in medicinal chemistry is the QSAR study. QSAR studies predict the theoretical chemical properties and the relationship between both electronic and geometric structures with the activity of a drug molecule that is modeled by computer. Based on these calculations can be predicted side electronically or other parameters that influence the activity of a drug effect. Therefore, it is essential to find new anti-malarial drugs that have a higher pharmacological activity than antimalarial drugs that are currently available. Here, QSAR analysis plays an important role to minimize trial and error when designing new antimalarial drugs [17]. The QSAR approach helps to correlate the specific biological activities or physical properties of a series of compounds with the measured or computed molecular properties of the compounds, in terms of descriptors [18]. QSAR methodologies save resources and expedite the process of the development of new molecular drugs. There have been many QSAR studies related to the design of anti-malarial drugs so far but a systematic QSAR study is yet to be carried out for series of new nitrobenzothiazole derivatives compounds, and the central task is to find a regression function that predicts the activity of the molecule in high accuracy.
Hadanu [17] has conducted molecular modeling of benzothiazole derivatives compounds through QSAR methods to result in 14 new benzothiazole derivative compounds. The all new benzathiazole derivative compounds were modeled through the best QSAR equation model to determine their antimalarial activity (IC 50 ) theoretically. The results of modeling benzothiazole derivative compounds show that higher antimalarial activity than the activity of chloroquine, but still activity is equivalent to halofantrine, anantimalarial drug that is currently on the market. To obtain more potent antimalarial drugs, it is necessary research the new nitrobenzothiazole derivative compounds through modeling. This study used descriptors: net charge of atomic, the Highest Occupied Molecular Orbital Energy (E LUMO ), the Low Unoccupied Molecular Orbital Energy (E LUMO ), and other parameters, namely the dipole moment, polarizability, and octanolwater partition coefficient (Log P) calculated by the semi-empirical PM3 method. The calculations were done in order to perform a quantitative analysis of the structure-activity relationship of the series of nitrobenzathiazole derivative antimalarial drugs. The nitrobenzathiazole derivative compounds used in this study have been synthesized and tested for antimalarial activity by Hout [1] as presented in Table 1.
The semi-empirical PM3 method was used in the calculation of descriptors in this QSAR analysis. Generally, for compounds having a large molecular weight are analyzed using the semi-empirical method of AM1. Because the nitrobenzothiazole derivative compound is an organic compound with a moderate molecular weight, the descriptor analysis does not need to use the semi-empirical method of AM1. The nitrobenzothiazole molecule can reach the most stable optimum condition using the semi-empirical method of PM3. The semi empirical PM3 method has a high accuracy when compared to the semi empirical AM1 method. Hadanu (2015) used the semi-empirical AM1 method to calculate the descriptors of aminobenzatiozole derivatives because the aminobenzotiazole derivative compounds have larger molecular weight and are more complex than nitrobenzoatiazole derivatives.

Experiment
Materials. The materials used in this study were nitrobenzothiazole derivative compounds synthesized by Hout [1]. The Inhibition Concentration (IC 50 ) was the dependent variable in Table 1.

Instrumentation.
In this research, a Sony Vaio Laptop equipped with Intel ® Dual Core Processor 2.20 GHz; RAM 1 GHz, and HDD 250 GB was used. All the compounds (Table 1) were calculated using package HyperChem ® Program Version 8.0 for Windows and complete geometry optimization with the semi-empirical PM3 method and there were analyzed using statistical program IBM ® SPSS ® version 16 for windows.
Calculating the descriptors. All the calculations were performed on Intel ® Dual Core Processor 2.20 GHz, Sony Vaio Laptop Computer with 894 MB of memory and 250 GB of scratch disk space. The descriptors were calculated for each of the compounds in Table 1 using the QSAR module of the semi-empirical PM3 method and this is reported in Table 2. The QSAR model is evaluated using sets of nitrobenzothiazole derivative compounds whose molecular structure and antiplasmodial activity is known ( Table 1). The antimalarial activity of these compounds was taken as the activity against chloroquine resistant P. falciparum W2 strain and is presented as the value of Log IC 50 , where IC 50 is an effective concentration inhibiting 50% growth of the parasite [1]. The 3D-structures of all compounds (Table 1) were sketched using the HyperChem ® Program, Version 8.0 for Windows. The descriptors of all compounds were calculated with the semi-empirical PM3 method. The final geometry optimization was optimized to a Root Mean Square (RMS) gradient of 0.001 kcal/(Å mol) in vacuo (Polak-Ribière method). The quantum chemical descriptors: net atomic charges, dipole moment, E HOMO , E LUMO , polarizability, and Log P, were calculated. The structural properties yielded from the single point calculation where atomic net charges and dipole moment, whereas descriptors of polarizability and Log P were obtained from the "menu compute" of QSAR properties. The E HOMO and E LUMO descriptors can be obtained from the menu compute, vibrations then click of orbital in the sub menu. All of the descriptors are given in Table 2.
From the descriptors mentioned, it can be considered that some give valuable information about the influence of electronic and coefficient partition features on the biological activity of molecular drugs. In this work, the molecular descriptors were selected so that they represent the features necessary to quantify such activity.

Development of the QSAR model and Statistical method.
The development of QSAR model has five steps. The first step is to determine the series of nitrobenzothiazole compounds to be analyzed along with the value of IC 50 yielded through laboratory experiment. The second is to select a set of descriptors, which are likely to be related to the biological activity of interest and to optimize for the most stable skeleton structure of the series nitrobenzothiazole compounds. The third is to calculate descriptors through the optimized structure. The fourth is to formulate a mathematical equation model that reflects the relationship between the biological activity and the chosen descriptors through statistic analysis using SPSS 16 for Windows to get the equation of QSAR, and the final step is to validate the QSAR models.

Results and Discussion
All descriptors resulting from QSAR analysis of the 13 compounds are presented in Table 1. QSAR analysis enabled the investigation of models with up to fourteen variables. The correlation between various descriptors [19,20] with biological activity is the most important outcome of the QSAR study. The descriptor is a parameter or property of a molecule used as an independent variable when calculating predicted activity (theoretical IC 50 ).
The descriptors used in this research are the atomic net charge, dipole moment, E HOMO , E LUMO , polarizability, and log P. The descriptors were obtained from the structural properties of each compound after the process of geometry optimization. The descriptors of the series of nitrobenzothiazole derivatives were calculated using the semi-empirical PM3 method. This method can be used for the analysis of a series of nitrobenzothiazole derivatives because they are organic compounds containing atoms: C, H, N, and S. In the statistical analysis, a regression method was used to find the QSAR models and their statistical properties. The best QSAR model built using the multiple linear regression (MLR) method is represented by the following equation: The R and R 2 in the QSAR model have high values indicating that the correlation independent variable between dependent variables is perfect and significant [21,22]. The statistical parameters commonly use R 2 value because it can be more correct than the R value. The value of r 2 has a larger interval than r so that small differences which are not perceived for the R value are perceived clearly with r 2 . The values of r and r 2 as statistical parameters which show only significant relationship between IC 50 with descriptors on the model of QSAR equation, then to determine the validity of the QSAR equation model is required to test other statistical parameters such as SE and PRESS.
The other statistical parameter, of interest is SE. The value of SE = 0 and the PRESS value = 3.40 (low) indicates that the QSAR model has a good ability to predict theoretical antimalarial activity (IC 50 ). The correlation of the experimental activities with the MLR calculated activities is illustrated in Figure 1. The result of the evaluation antimalarial activity [predicted] Log IC 50 and correlation with antimalarial activity [experiment Log IC 50 ] for the QSAR model using the semi-empirical PM3 method has linearity R 2 = 0.9875 and slope value of −0.3544 that can be seen in Figure 1. Based on the value of the variable dipole moment, the polarizability, E LUMO ; E HOMO , were obtained by the variation of atoms in MLR analysis (see model of QSAR), atomic net charges: qC 2 , qS 3 , qC 4 , qC 5 , qC 7 , qC 8 , qC 9 , dipole moment, E LUMO ; E HOMO, polarizability, and Log P seem the most responsible for the pharmacological activity.
This study of structure-antimalarial activity relationships interestingly reveal that change of the structure of substituent group at C 5 , C 7 , C 8 , and -R1 (see Table 1 and Table 3) commonly results in a change of bioactivity. Among those 2-substituent benzothiazole derivatives, other substituted molecules have already received considerable attention due to their potential activity. The addition of R2, R3 and R4 groups to the basic skeleton of the nitrobenzothiazole structure is intended to find the nitrobenzothiazole derivative compound with higher antimalarial activity. In designing the structure of the new antimalarial compounds, -OH, -CH 3,, -F, -OCH 3 , -SH, NH 2 , -NHR, NHCOR and -NR 2 substituents attached to the main structure of nitrobenzothiazoles were modified so that the higher antimalarial activity of the newly designed molecule compared to that of the previously synthesized compounds was achieved.  Table 3). These twenty-four candidates could guide the synthesis procedures of new candidates for nitrobenzothiazole derivatives.
The new nitrobenzothiazole derivatives modeling results have a high antimalarial activity with -NHCOPh, -NHCOPh-p-OH, -NHCOPh-p-OCH 3 , and -SH functional groups in the R1 position of the skeleton of the nitrobenzothiazole derivative compounds. In other nitrobenzothiazole derivatives, modeling results that have high antimalarial activity have -NH 2 , -SH in the R1, and -NO 2 functional groups in the R4 position of the skeleton of the nitrobenzothiazole derivative compounds. The newly suggested nitrobenzothiazole derivatives that are predicted to have high antimalarial activity are 45, 49, 52, and 55 compounds which have -Ph-p-NH 2 , Ph-p-NH 2 -m-CH 3 , Ph-p-OH functional groups in the R1 position, -F and -OH functional groups in the R2, -CH 3 , -F, -OH functional groups in the R3, and -F, -NO 2 functional groups in the R4 of the skeleton nitrobenzothiazole compounds.

Conclusions
In this study have used a PM3 semi-empirical molecular calculation to study the correlation of antimalarial activity of a series of nitrobenzothiazole derivative drugs with the chloroquine-resistant W2 strain. The QSAR model has predicted on the 95% level with statistical parameters. The overall correlation is given by the computed molecular properties of qN2, qS3, qC4, qC5, qC7, qC8, qC9, dipole moment (μ), E LUMO , E HOMO , polarizability (α) and Log P. A significant regression model was obtained by the multiple linear regression method for the structural properties of nitrobenzothiazole derivatives versus antimalarial activity against P. falciparum. In this research, it has been found that the descriptors: polarizability, E LUMO , E HOMO , Log P, and atomic net charges: qC5, qC7, qC8, and groups of -NHR and NH 2 (R1) as hypothetically active regions of the molecular nitrobenzothiazole derivatives, seem to be responsible for the pharmacological activity. Based on the best QSAR model obtained, new antimalarial compounds have been designed which have higher predicted antimalarial activities than those of the existing compounds. The new suggested compounds are the 45, 49, 52, and 55 and it is recommended that these are synthesized and then tested for antimalarial activity in the laboratory.