A Concise Review on Analytical Profile of Valsartan

Valsartan (VAL) is an orally active angiotensin-II receptor type-I antagonist. VAL is available alone in dosage form as well as multicomponent formulation with various antihypertensive drugs like nifedipine, hydrochlorothiazide, ramipril, amlodipine and nebivolol hydrochloride, for the management of hypertension. The present investigation assesses the various approaches for analysis of VAL in bulk drug as well as formulated products. A concise review represents the compilation and discussion of about more than 90 analytical methods which includes HPLC, HPTLC, capillary electrophoresis, electrochemical methods and UV-Spectrophotometry methods implemented for investigation of VAL in biological matrices, bulk samples and in different dosage formulations. The review describes the percentage utilization of the various approaches for analysis of VAL. The statistical data regarding the utility of these methods for estimation of VAL published during 2001 to 2016 have been included.


INTRODUCTION Valsartan (VAL) is chemically N-(1-oxopentyl)-N-[[2'-(1H-tetrazol-5-yl) [1, 1'-biphenyl]-4-yl]
methyl]-L-valine (Figure 1).It is crystalline in nature with melting point in the range of 116-117⁰C; It is soluble in water [1].VAL is an angiotensin-II receptor antagonist used in the management of hypertension.It may be used in patients with heart failure who are unable to tolerate ACE inhibitors [2].VAL lowers blood pressure by antagonizing the Renin-Angiotensin-Aldosterone System (RAAS); it competes with angiotensin-II for binding to the type-1 angiotensin-II receptor (AT1) subtype and prevents the blood pressure increasing effects of angiotensin II [3].VAL may be used to treat hypertension, isolated systolic hypertension, left ventricular hypertrophy and diabetic nephropathy.It may also be used as an alternative agent for the treatment of heart failure, systolic dysfunction, myocardial infarction and coronary artery diseases [4].The schematic diagram shows the mode of action of VAL, Figure 2. Antagonism of angiotensin II receptor leads to blood pressure (BP) reduction, as well as decreases vascular smooth muscle contraction [6].Pharmacokinetics reported that VAL is normally bound to serum protein -primarily serum albumin (94 -97 %.).VAL is excreted largely as unchanged drug (80 %) and is minimally metabolized in humans.Metabolism of VAL gives valeryl 4hydroxy metabolite is shown in Figure 3 [7].VAL is available in various doses, i.e. 10, 20,30,40,80,160 and 320 mg.It has also been reported that all these doses of VAL have been found to be safe and tolerable [8].VAL is also available in combination with other antihypertensive agents such as nifidipine, hydrochlorothiazide, ramipril, amlodipine, nebivolol hydrochloride and antihyperlipidic agent vice ezetimibe.The information regarding dosage forms, route of administration and recommended dose of VAL is summarized in Table 1.VAL contraindicated in a person suffering from the renal artery, abnormally low blood pressure, liver problem, serious kidney problem and during pregnancy.The most common side effects of VAL are dizziness, low blood pressure, diarrhea, joint and back pain, hypotension, impaired renal function, hyperkalemia-Some patients with heart failure have developed increases in potassium [9].

Analytical accounts on VAL
The extensive literature survey revealed, several analytical techniques viz UV/Visible-Spectrophotometry, Spectrofluorimetry, HPLC, HPTLC and LC-MS for the determination of VAL in bulk and pharmaceutical formulations.The reported methods describe the estimation of VAL in various dosage forms as single constituent and in combination with amlodipine, hydrochlorothiazide, propranolol, nifedipine, ezetimibe, aliskiren, losartan, irbesartan, ramipril, nebivolol hydrochloride, atorvastatin, fluvastatin, simvastatin acid, ketoprofen, pentaprazole, chlorthalidone and with cilnidipine.Figure 4 shows different analytical methods implemented for estimation of VAL.
USP reported HPLC assay method using C18 (12.5 cm × 3 mm, 5μm) column as a stationary phase and a mobile phase consisted of acetonitrile, water and glacial acetic acid (500:500:1v/v/v) with a flow rate of 0.4 mL/min.Column effluent was monitored at 273 nm [10].
IP depicted HPLC assay method using C18 (25 cm × 4.6 mm, 5μm) column as a stationary phase and mobile phase consisted of acetonitrile, water and glacial acetic acid (50:50:0.1v/v/v)with a flow rate of 1 mL/min.Column effluent was monitored at 273 nm [11].
BP describe the potentiometric procedure in which 0.170 g of VAL is dissolved in 70 ml of 2-propranol, titrate with 0.1M tetrabutylammonium hydroxide and end point determine by potentiometrically [12].

Accounts on Bio-analytical Method for Determination of VAL
Bio-analysis is a sub-discipline of analytical chemistry covering the quantitative measurement of xenobiotics (drugs and their metabolites, and biological molecules in unnatural locations or concentrations) and biotic (macromolecules, proteins, DNA, large molecule drugs, metabolites) in biological systems.[13] Literature survey revealed that HPLC is predominantly used for the bio-analysis of VAL.Piaov et al. (2008) established a validated simple and sensitive method for determining VAL concentration in human plasma, the given sample was extracted by simple protein precipitation using methanol as a solvent.The analyst was separated using acetonitrile with 15 mm potassium dihydrogen phosphate in water (42:58 v/v) (pH 2.0; adjusted with phosphoric acid) with the flow rate of 1.2 mL/min [14].

Zong-Zhu
Oskar Gonzalez et al. (2009) investigated the geometry optimization and the validation of a quantitative high-performance liquid-chromatography-photodiode array-fluorescence (HPLC-PDA-Fluo) method for the simultaneous analysis of combined drugs used in the treatment of cardiovascular diseases from human plasma.Separation of chlorthalidone (CLTD), VAL (VAL), VAL-M1 (VAL-M1) and fluvastatin (FLUV), using the mobile phase consisted of a mixture of acetonitrile and water containing 0.01% of formic acid and 10 mm of ammonium formate at (pH 4.1) [15].

D. R. Brunetto et al. (2009) studied
Column-switching HPLC method and validation of it for quantification of losartan, telmisartan, and VAL in human urine.While analysts were extracted from the matrix using an on-line solid-phase extraction using solution 2% methanol in 5mM phosphate buffer (pH 3.8) at a flow-rate of 0.8 mL/min [16].
Gorka Iriarte et al. (2007) reported a simple and fast method for the simultaneous determination of the antihypertensive drug VAL and its metabolite in human plasma.The proposed method deals with SPE, followed by an HPLC separation coupled with fluorimetric and photometric detection, the separation was performed on an RP C18 Atlantis 100 mm 63.9 mm column.The mobile phase consisted of a mixture of ACN 0.025% TFA and phosphate buffer (5 mm, pH = 2.5) 0.025% TFA and was delivered in gradient mode at a flow rate of 1.30 mL/min [17].
Nozomu Koseki et al. (2007) established a validated sensitive liquid chromatographytandem mass spectrometry (LC-MS/MS) method for the determination of VAL in human plasma; analysts were extracted by solid-phase extraction using MeOH/H2O (50:50 v/v) [18].Bioanalytical methods for determination of VAL are summarized in Table 2.

HPLC
Apart from pharmacopeial methods many HPLC methods were reported for determination for VAL in pharmaceutical formulations.The summary of the reported HPLC methods particularizing the mobile phase used for determination, sample matrix,  max and linearity is shown in Table 3. Instrumentation of HPLC methods for determination of VAL is summarized in Table 4.

HPTLC
Six simple HPTLC methods have been studied for simultaneous estimation of VAL in combined dosage form with CLN, RMP, HCTZ and NBH.The summary of the reported HPTLC methods is shown in Table 5.
Ritesh P. Bhole et al. (2015) developed and validated a simple method for VAL and CLN in combined dosage form, standard solution of VAL and CLN were applied to precoated silica gel 60F 254, and mobile phase used for development toluene: methanol: ethyl acetate: glacial acetic acid in the ratio of (8:1:1:0.1 v/v/v) and Rf value was found to be 0.29 and 0.56, respectively.Accuracy and precision of the proposed method were evaluated by recovery studied and % recovery for VAL and CLN was 99.03 % and 99.86 % [43].
Della Grace Thomas Parambi et al. (2011) investigated a simple, accurate and precise method for quantitative estimation of VAL in tablet matrix.Standard solution of VAL was applied to pre-coated silica gel 60F 254, mobile phase used for development chloroform: acetonitrile: toluene: glacial acetic acid, in the ratio (1:8:1:0.1 v/v/v/v), and Rf value was found to be 0.65.The method showed good repeatability and recovery with relative standard deviation less than 2 [44].

Stability-Indicating Methods (SIM) for Determination of VAL
About seven stability-indicating methods have been studied so for determination of VAL in bulk substances and pharmaceutical formulations implementing different analytical techniques.Amongst these, three methods are for estimation of VAL alone and four of them described in stability studies of VAL in its combined dosage form with other drugs.The reported stability-indicating methods for VAL, illustrating sample matrix,  max , linearity range and retention time/factor presented in Table 6.

Spectrophotometry Methods
Till the date, twenty-two UV-Spectrophotometry methods have been established for determination of VAL alone and in combined dosage forms.Also, two Spectrofluorimetry methods have been investigated analysis of VAL in tablets.The details Spectrophotometry and Spectrofluorimetry designating the basic principle, sample matrix,  max and solvent and linearity range is summarized in Table 7.

Approaches for Analysis of VAL as a Single Component
Gupta et al. (2010) has described zero order and second order UV-spectrophotometry method for determination of VAL in tablets using methanol as solvent.The zero order and second order derivative method involve the calculation of absorbance at 250 nm and 241 nm respectively [56].
Tarkase et al. (2012) also performed similar UV-spectrophotometry methods for VAL tablets using phosphate buffer for dissolution of VAL and absorbance was recorded at 220 nm in second order derivative method [57].Tatar et al. (2002) has employed ethanol for the solubilisation of VAL.The article described second order UV-spectrophotometry in which the distance between two extremum values peak-to-peak amplitudes 221.6 nm and 231.2 nm were measured for determination of VAL in capsules and calibration curves were constructed by plotting d 2 A/dλ 2 against concentrations of VAL solutions [61].

Kalaimagal et al.
(2012) has reported standard absorbance method, Area under Curve method, first order derivative and Q-absorbance method using 0.1 N NaOH as solvent for VAL with good recovery in the range of 98.6% to 102.26% [75].

Approaches for analysis of VAL in combined dosage form with other drugs
VAL is available in combination with many antihypertensive, diuretics and antihyperlipidic agents.Few UV-Spectrophotometry methods have been stated for simultaneous determination of VAL in dosage forms and simple, rapid, accurate and economical methods have been developed for the assessment of VAL and HCTZ in tablet dosage form.Abdallah O.M. et al. (2011) validated a first order derivative method for measurements of the amplitudes of 234.5 nm and 247 nm for AML using 30 μg/mL of VAL as a divisor and at 282 nm and 292 nm for VAL [65].
Chaudhary A.B. et al. (2010) described two UV-Spectrophotometry methods have been developed and validated for simultaneous estimation of VAL and HCTZ in a tablet dosage form.The first method employed solving of simultaneous equations based on the measurement of absorbance at two wavelengths, 249.4 nm and 272.6 nm,  max for VAL and hydrochlorothiazide, respectively.The second method was the absorbance ratio method, which involves the studies of Q-absorbance equation at 258.4 nm (isoabsorptive point) and also at 272.6 nm  max of hydrochlorothiazide [63].
Meyyanathan S. N. et al. (2010) reported two simple, precise and reproducible UVspectrophotometry methods, simultaneous equation method and Q-value analysis method, have been developed and validated for the simultaneous estimation of NEB, HCTZ and VAL 246.6 nm, 280.2 nm and 275 nm, respectively [76].cm RP C18 nano-flow column.Undesired polar substances in human plasma were washed out by using ACN-1% FA= (20: 80 v/v) as the loading mobile phase for drug analysis [84].Selvan et al. (2007) reported a rapid, sensitive and accurate liquid chromatographic-tandem mass spectrometry method for the simultaneous determination of NEB and VAL in human plasma.NEB and VAL were extracted from plasma using acetonitrile and separated on a C18 Column; the mobile phase consisting of a mixture of acetonitrile and 0.05mM formic acid (50:50 v/v, pH 3.5) was delivered at a flow rate of 0.25 mL/min [85].
Surbhi Mehta et al. (2010) demonstrated the applicability of a strategy involving use of liquid chromatography (LC) and liquid-chromatography mass spectrometry (LC-MS) techniques for identification and characterization of minute quantities of degradation products, without their isolation from the reaction matrix in pure form.In that, they used a VAL as a model drug, three small degradation products were formed, which were separated on a C-18 column using a gradient method [87].

Capillary Electrophoresis (CE) Method
S. Hillaert et al. (2003) implemented the capability of the capillary zone electrophoretic (CZE) and micellar electro kinetic capillary chromatographic (MEKC) methods to simultaneously separate hydrochlorothiazide and six angiotensin-II-receptor antagonists (ARA-IIs): candesartan, eprosartan, mesylate, irbesartan, losartan potassium, telmisartan, and VAL.Experiment were performed on thermo capillary electrophoresis, a fused silica capillary was used 85 cm in length and 50 mm.Absorbance was detected at 214 nm, two different internal standard were involved sulfanilamide and eprosartan mesylate for study [88].

Potentiometric Methods
Nazife Aslan et al. (2010) developed and validated a potentiometrically titration method for determination of VAL in pharmaceutical dosage forms.From the titration data, stochiometric protonation constants are calculated and these constants are found to be 4.57 and 5.47, titration were carried out in ethanol solutions using NaOH as titrant, at constant temperature of 25 ± 0.1°C and ionic strength of 0.10 M NaCl [89].
Shrikant H. Patil et al. (2012) developed and validated a novel and simple titrimetric method for determination of commonly used angiotensin-II-receptor antagonists (ARA-IIs).The direct acid base titration of four ARA-IIs, namely eprosartan mesylate, irbesartan, telmisartan and VAL, was carried out in the mixture of ethanol: water (1:1 v/v) as solvent using standardized sodium hydroxide aqueous solution as titrant, either visually using phenolphthalein as an indicator or potentiometrically using combined pH electrode [90].
Nesrin K. Ramadanv et al. (2012) reported a potentiometrically method for VAL and AML in that they concluded two poly (vinyl chloride) matrix membrane electrodes responsive to some drugs affecting the cardiovascular system [91].Potentiometric method offers a simple system and cost effective method than that of other methods which are having high cost, multiple steps and time consuming.

I. H. I. Habib et al. (2008) reported stripping voltammetry determination of VAL using a
Hanging Mercury Drop Electrode (HMDE), was based on adsorptive accumulation of the species at HMDE followed by first harmonic alternating current AC stripping sweep at pH 6, the response was linear over the concentration range of 0.08-0.64mg/mL with regression coefficient 0.999 [92].
Pinar Esra Erden et al. (2014) reported anodic behavior of binary mixture of AML and VAL on glassy carbon electrode based on the irreversible oxidation signal of AML at 0.95 and that of VAL at 1.15 V versus Ag/AgCl at pH 5.0 in Britton-Robinson buffer.Differential pulse voltammetry method was proposed to direct determination of AML and VAL in pharmaceuticals and spiked human serum.Linearity for AML was in the range from 1.0 μM to 35.0 μM and that for VAL was in the range from 1.5 μM to 32.0 μM [93].
Jinlong Yan et al. (2008) reported an electrochemical behavior of VAL in Britton-Robinson buffer solution at pH 7.0 at the Mercury Film Electrode (MFE) by cyclic, linear sweep, differential-pulse and square-wave voltammetry.The property of VAL adsorption at the MFE using accumulation potential of (+0.10V) was observed [94].Voltammetry method is simple and there is no need of expensive grade solutions, which are needed for other analytical methods such as HPLC.The voltammetry method may possibly a good substitute for simultaneous estimation of bulk drugs.

CONCLUSION
The present review illustrates various analytical approaches exercised for the estimation of VAL.A numerous investigation had perform including, Bio-analytical, HPLC, HPTLC, UV/Vis-Spectroscopy, Spectrofluorimetry, capillary electrophoresis, electrochemical method, LC-MS, LC-ESI-MS etc. for estimation of VAL in bulk and in its combined pharmaceutical formulations and in plasma.Liquid chromatography with UV detection has been found to be most studied for estimation of VAL in bulk as well as pharmaceutical dosage forms, while hyphenated LS-MS, LS-MS/MS methods are reported for determination of VAL and its metabolite in plasma and other biological fluids.Further, methods were reported for its pharmacokinetic as well as bioequivalence studies.Few chromatography approaches like HPTLC and Stability-indicating HPLC and HPTLC are also reported in literature.Certain Spectrophometric methods in UV-Visible along with fluorimetric are most often used for assessment for VAL.

Figure 3 .Figure 4 .
Figure 3. Metabolism of VAL to valeryl-4-hydroxy VAL Satana E et al. (2001)  developed a simple first order derivative method for analysis of VAL and HCTZ at wavelength 270.6 nm and 335 nm[59].

Figure 5 .
Figure 5. Statistics of research paper for estimation of VAL published during 2001 to 2016

Figure 6 .
Figure 6.Percentage Utility of Analytical Approaches used for estimation of VAL

VAL Dosage forms Route of Administration Indication/Dose
. Dosage forms, route of administration and recommended dose of

Table 2 .
Bioanalytical determination of VAL

Table 3 . HPLC methods for VAL Sr. No Name of drug/ Formulation Mobile phase composition Detecti on (nm) Discussion Ref
Acetonitrile: Phosphate buffer (70:30 v/v)273VAL exhibited linearity in the range of 10-50 μg/mL, Coefficient correlation was found to be 0.9993 and retention time was 3.5 min.

Table 3 .
HPLC methods for VAL (continued)

Table 4 .
HPLC chromatographic columns and optimized analytical parameters

Table 4 .
HPLC chromatographic columns and optimized analytical parameters (continued)

Table 5 .
HPTLC methods for determination of VAL

Table 6 .
Stability-indicating HPLC and HPTLC methods for determination of VAL

Table 7 .
Spectrophotometric and Spectrofluorimetric methods used for determination of VAL alone and in combined dosage form

Table 7 .
Spectrophotometric and Spectrofluorimetric methods used for determination of VAL alone and in combined dosage form (continued)