RELATIONSHIP BETWEEN THE BIOAVAILABILITY AND MOLECULAR PROPERTIES OF ANGIOTENSIN II RECEPTOR ANTAGONISTS

In the present study, we investigated the relationships between several molecular properties and bioavailability data for seven of the most commonly prescribed angiotensin II receptor antagonists (also known as angiotensin II receptor blockers (ARBs) or sartans), candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan and valsartan. The molecular descriptors of ARBs are:, aqueous solubility (logS values), polar surface area (PSA), molecular weight (Mw), volume value (Vol), lipophilicity (logP values) and the acidity descriptor (pKa1). The respective descriptors were calculated using four different software packages. The relevant bioavailability data were obtained from literature. Among calculated molecular descriptors, simple linear regression analysis showed the best correlation between bioavailability data and the lipophilicity descriptor, logP (R2=0.568). Multiple linear regression established good correlations between bioavailability and the lipophilicity descriptor, logP, using the molecular weight, Mw, or the acidity descriptor, pKa1, as an additional, independent variable (with R2=0.661 and 0.682, respectively). Finally, excluding candesartan from the calculations resulted in a very good correlation (R2=0.852) between the remaining ARB bioavailability and molecular descriptors MlogP and Mw as independent variables, determined by multiple linear regression.


INTRODUCTION
The ARB were introduced into clinical practice three decades ago and today are commonly prescribed for the treatment of hypertension, congestive heart failure and diabetic nephropathy [1].ARBs are acidic drugs that are mostly ionized at physiological conditions [1].In contrast to other ARBs, candesartan cilexetil and olmesartan medoxomil are rapidly and completely hydrolyzed in the intestinal wall into their active metabolites, candesartan and olmesartan, respectively.Approximately 14% of ingested losartan is oxidized to a more potent metabolite, while other ARBs are not converted to active metabolites.Although all ARBs have the same indications for usage, they demonstrate certain differences in pharmacological, pharmacokinetic and pharmacodynamic properties, which may affect their clinical efficacy.They have relatively high plasma protein binding (PPB) values (95-100%).With the exception of irbesartan with an oral bioavailability (BA) of 60-80% and telmisartan with BA of 42-58%, all other ARBs have lower (15-33%), but adequate values of oral bioavailability.Most of these compounds are excreted unchanged.They have a dual route of elimination, renal and fecal, which may be of importance for patients with renal failure [2][3][4].
In our recently published paper we assessed the correlation of the degree of PPB and the elimination of a selected ARBs with their molecular properties, molecular weight and volume using multiple regression (MLR) analysis [17,18].In our previous studies, the correlation between ACE inhibitor lipophilicity and PPB data [19,20] or absorption [21] were studied and suitable models were presented.Following on from this work, the aim of our present study was to investigate the relationship between the different molecular properties of seven ARBs, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan, and their BA.

Software
The seven most frequently prescribed ARB were investigated: candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan and valsartan.The software package Molinspiration Depiction Software (Molinspiration Cheminfirmatics) was used for the calculation of the "electronic" descriptors, polar surface area (PSA), constitutional parameter, molecular weight (Mw), and the geometric descriptor, volume value (Vol).The ARB lipophilicity descriptors, different logP values (AlogPs, AClogP, AB/logP, MilogP, AlogP, MlogP, KOWWINlogP, XLOGP2, XLOGP3), as well as their aqueous solubility data (logS) were calculated using the software package Virtual Computational Chemistry Laboratory [http://www.vcclab.org].Chemdraw ultra 12.0 was used for the calculation of another lipophilicity parameter, ClogP, while the software package DrugBank [http: //www.drugbank.ca]was used for the calculation of the acidity descriptor pK a of the selected ARB.Microsoft Excel 2003 and Origin 7.0 PRO (Origin Lab Corporation, USA) were used to perform statistical analysis of regression.The selected molecular descriptors are presented in Table 1; the provided oral bioavailability data were obtained from the relevant literature [2].

RESULTS
According to data from the relevant literature, the degrees of ARB BA vary from 15% to about 70% (Table 2) and [3].Eprosartan has the lowest oral BA (15%), while the highest were recorded for telmisartan and irbesartan (about 50% and 70%, respectively).The ARB molecular descriptors PSA, Mw, Vol, logS, logP and pK a values, were calculated using four different software packages.
The correlations between ARB oral BA data obtained from the literature and all calculated descrip-  In the next stage, the relationship between BA data and two different ARB molecular descriptors as independent variables were investigated with MLR.MlogP was chosen as the first independent variable since it showed the best correlation with ARB oral BA data in simple linear regression.The solubility data could not be used as the second independent variable since logS provided good correlation with MlogP values (R 2 =0.805).Therefore, PSA, Mw, Vol and pK a1 were chosen as possible, secondary independent variables, besides MlogP.The following correlations were obtained: The best correlation was established between ARB BA data obtained from the literature and the MlogP lipophilicity descriptor, using Mw or the acidity descriptor pK a1 as additional, independent variables with R 2 =0.661 and 0.682, respectively.The obtained results are presented in Table 2.It can be noticed that among the investigated ARB candesartan's predicted BA values were twice as high as those found in the available BA data for all obtained calculations.Therefore, it was decided to repeat calculations after excluding candesartan, and a significantly higher correlation was found in the first stage in simple linear regression between ARB BA data and MlogP values with R 2 =0.702.Subsequently, MLR analysis also provided a much better relationship between available BA data for ARB in the literature and all the calculated molecular descriptors, PSA, Mw, Vol and pK a1 (R 2 >0.70).The best MLR analysis was established with the application of MlogP and Mw as independent variables: Eq.5: BA pred (%)=29.42(±7.44)MlogP-0.30(±0.17The established correlation is presented in Fig. 1. Fig. 1.The relationship between ARB oral bioavailability data collected from the literature [3] (Series 1) and predicted in MLR, using MlogP and Mw, after candesartan exclusion (Series 2).The numbers denote ARB as presented in Table 1.

DISCUSSION
Oral BA is one of the most important pharmacologic properties in drug design and development.It is a subcategory of absorption and represents the fraction of administered drug dose that reaches systemic circulation.High oral BA reduces the amount of an administered drug needed to achieve the desired pharmacological effect, thereby reducing the risk of side effects and toxicity, while poor oral BA can result in low efficacy and lead to unpredictable response to a drug.BA for intravenously administered drugs is 100%.However, for orally administered drugs, BA usually decreases due to incomplete absorption and first-pass metabolism as well a high degree of plasma protein binding.Various physiological factors may reduce the bioavailability of drugs prior to their entry into the systemic circulation.Furthermore, drug administration with or without food also affects absorption.Concurrent intake of other drugs may alter absorption and first-pass metabolism, while intestinal motility alters the dissolution and may affect the degree of chemical degradation of the drug by intestinal microflora.Diseases affecting liver metabolism or gastrointestinal function will also have an effect on BA.A drug's physical properties (hydrophobicity, acidity, solubility, molecular mass, volume and polar surface area) or its formulation as well as the age and gender of the patients and the dosing scheme also exert important influences on its BA.
Previous studies evaluated the acidity, lipophilicity, solubility and absorption of different drugs based on their molecular structure with the application of computer programs [7][8][9]; they also evaluated the influence of molecular properties on drug BA [22,23], but ARB were not included in these studies.According to the available literature, the ARB pharmacokinetics and pharmacodynamics, including their effects and duration of action, were investigated by several authors [24][25][26][27][28][29].However, most of these methods had certain limitations and a new approach for a fast, easy and reliable evaluation of ARB oral BA was necessary.
The central aim of the study was to establish a high throughput approach for evaluating the oral BA data of selected ARB using simple or multiple linear regression analyses.In the first stage of the study, the relationship between all calculated logP values (ClogP, AlogPs, AClogP, AB/logP, milogP, AlogP, MlogP, KOWWINlogP, XLOGP2, XLOGP3) and ARB oral BA data were investigated.Of all logP values, only MlogP provided relatively good correlation (R 2 =0.567) with ARB oral BA.The methods used for lipophilicity descriptor calculation provided ten different logP values.They can be divided into substructure-based and property-based methods, and the substructurebased methods can be additionally subclassified into fragmental and atom-based [30].The lipophilicity descriptor MlogP was calculated with the propertybased method based on topological descriptors [30].
Secondly, the relationship between ARB and BA data and calculated molecular descriptors was investigated using MLR analysis with the application of two independent variables, MlogP and one of the following: PSA, Mw, Vol or pKa.Although the best correlation was established between ARB BA data and the MlogP lipophilicity descriptor, using the molecular weight (R 2 =0.661) or acidity (R 2 =0.682) as independent variables it was observed that the calculated BA values for candesartan differed notably from its literature available BA values (Table 2).Candesartan is actually an active metabolite and its prodrug form is candesartan cilexetil.Candesartan cilexetil is absorbed after oral administration and hydrolyzed during absorption to form candesartan.An absolute bioavailability for candesartan is about 15% following its oral administration.Peak plasma concentration occurs after 3 to 4 h.Candesartan has very high degree of plasma protein binding (PPB), higher than 99%.The high values of candesartan's PPB, together with incomplete absorption and first-pass metabolism may be some of the reasons for its low BA values.Following this observation, candesartan was excluded from further investigation resulting in much higher correlation (R 2 =0.702) between ARB BA data and MlogP values calculated using simple linear regression.MLR analysis of ARB BA data and the lipophilicity descriptor MlogP, with the application of different molecular descriptors established satisfactory correlations (R 2 >0.70), the best (R 2 >0.852) being Mw as an independent variable, confirming the important influence of ARB molecular properties on their oral BA.The results obtained can be considered as good considering the limited number of compounds and since values of R 2 higher than 0.81 are proposed as very high correlation by Asuero et al. [31].

CONCLUSION
The results of the present paper have confirmed that the molecular properties, especially lipophilicity, acidity and molecular weight of angiotensin II receptor blockers, are of considerable importance and that their calculation can be considered as a high-throughput screening for evaluation of the oral bioavailability of the selected compounds.For the selected group of angiotensin II receptor blockers, simple linear regression analysis provided a correlation (R 2 =0.568) between the calculated lipophilicity descriptor and bioavailability data.However, in multiple linear regression, better correlations were established between the bioavailability and lipophilicity descriptor, using the molecular weight or the acidity descriptor as additional, independent variables (with R 2 =0.661 and 0.682, respectively).Candesartan exclusion from multiple linear regression analysis resulted in very good correlation (R 2 =0.852) between the remaining angiotensin II receptor blockers' bioavailability, lipophilicity and molecular weight descriptors as independent variables.Since bioavailability affects drug action and activity and is considerably influenced by molecular properties, the examination of its relationship with computed molecular descriptors can be of great importance, especially in the development of newly synthesized drugs.

Table 1 .
Data of ARB oral bioavailability collected from relevant literature and calculated molecular descriptors. ARBBA%*

Table 2 .
Data of ARB oral bioavailability collected from relevant literature (MlogP and pK a1 values. *) and predicted from (A) MlogP and Vol; (B) MlogP and PSA; (C) MlogP and Mw; (D)