A simple HPLC method for quantitation of enalaprilat

Optimized and validated spectrophotometric methods for the determination of enalapril maleate in commercial dosage forms

Four simple, rapid and sensitive spectrophotometric methods have been proposed for the determination of enalapril maleate in pharmaceutical formulations. The first method is based on the reaction of carboxylic acid group of enalapril maleate with a mixture of potassium iodate (KIO₃) and iodide (KI) to form yellow colored product in aqueous medium at 25 ± 1°C. The reaction is followed spectrophotometrically by measuring the absorbance at 352 nm. The second, third and fourth methods are based on the charge transfer complexation reaction of the drug with p-chloranilic acid (pCA) in 1, 4-dioxan-methanol medium, 2, 3-dichloro 5, 6-dicyano 1, 4-benzoquinone (DDQ) in acetonitrile-1,4 dioxane medium and iodine in acetonitrile-dichloromethane medium. Under optimized experimental conditions, Beer’s law is obeyed in the concentration ranges of 2.5–50, 20–560, 5–75 and 10–200 μg mL⁻¹, respectively. All the methods have been applied to the determination of enalapril maleate in pharmaceutical dosage forms. Results of analysis are validated statistically.

High-performance liquid chromatographic determination of enalapril in human plasma by enzyme kinetic analytical method

A high-performance liquid chromatographic method for indirect determination of enalapril in human plasma, was developed and validated. An exogenous angiotensin converting enzyme after drug inhibition was determined by reacting with hippuryl-histidyl-leucine to produce hippuric acid (HA) which was inversely proportional to the amount of enalaprilat in plasma. The HPLC was carried out on a Lichrosphere 60RP-select B, C18, 5 microm (125 mm x 4.0 mm i.d.) column at flow rate of 1.0 ml/min. The analysis time per injection was within 6.5 min. The lowest concentration of enalaprilat to be quantitated was 3.0 ng/ml with the acceptable accuracy and precision. This successfully developed method was practically and accurately used for pharmacokinetics and bioequivalent study of enalapril.

RP-HPLC and NMR study of cis-trans isomerization of enalaprilat

The angiotensin converting enzyme inhibitor, enalaprilat can exist in solution as cis and trans conformers which interconvert around the amide bond at room temperature. A HPLC with UV detection was performed to study the influence of various chromatographic operational conditions on both rotamers separation and elution of enalaprilat as a single peak. In addition nuclear overhauser enhancement difference was used for the identification of the conformers. The isomer ratio integrated from the obtained 1H NMR result were 71.5:28.5 and 76:24 at 298 and 279 K, respectively where the trans was the major form.

Simultaneous quantitation of enalapril and enalaprilat in human plasma by 96-well solid-phase extraction and liquid chromatography/tandem mass spectrometry

A sensitive and rapid method based on liquid chromatography/tandem mass spectrometry (LC/MS/MS) combined with rapid solid-phase extraction (SPE) has been developed and validated for the quantitative determination of enalapril and its active metabolite enalaprilat in human plasma. After addition of internal standard to human plasma, samples were extracted by 96-well SPE cartridge. The extracts were analyzed by HPLC with the detection of the analyte in the multiple reaction monitoring (MRM) mode. This method for the simultaneous determination of enalapril and enalaprilat was accurate and reproducible, with respective limits of quantitation of 0.2 and 1.0 ng/mL in plasma. The standard calibration curves for both enalapril and enalaprilat were linear (r(2) = 0.9978 and 0.9998) over the concentration ranges 0.2-200 and 1.0-100 ng/mL in human plasma, respectively. The intra- and inter-day precision over the concentration range for enalapril and enalaprilat were lower than 13.3 and 15.4% (relative standard deviation, %RSD), and accuracy was between 89.2-105.0 and 91.9-104.7%, respectively.

Determination of enalapril and enalaprilat by enzyme linked immunosorbent assays: application to pharmacokinetic and pharmacodynamic analysis

We have developed two enzyme linked immunosorbent assay (ELISA) methods for determining enalapril and enalaprilat in plasma. In this study, 48 healthy subjects received an oral dose of either 10 or 20 mg of enalapril and plasma concentrations of enalapril and enalaprilat were determined by their specific ELISA methods. These plasma concentrations and blood pressure measurements were applied to evaluate the pharmacokinetic (PK) and pharmacodynamic (PD) parameters of both enalapril and enalaprilat. The enalapril values for the area under the curve (AUC(0)--> infinity ) were 480 +/- 216 and 832 +/- 325 ngh/mL, maximum plasma concentrations (C(max)) were 310 +/- 187 and 481 +/- 185 ng/mL, and times required to reach the maximum concentration t(max) were 1.13 +/- 0.22 and 1.09 +/- 0.33 h for 10 and 20 mg doses, respectively. The enalaprilat values for AUC(0)--> infinity were 256 +/- 122 and 383 +/- 158 ngh/mL, C(max) values were 57 +/- 29 and 72.9 +/- 33.6 ng/mL and t(max) values were 4.28 +/- 1.45 and 4.05 +/- 01.22 h for 10 and 20 mg doses, respectively. The C(max) values of enalapril were approximately 10 times higher than those in the literature, which were determined by angiotensin converting enzyme (ACE) inhibition assays following alkaline hydrolysis, but similar to those of enalaprilat. The PD profiles revealed a significant correlation between enalaprilat concentrations in plasma and the decrease in systolic and diastolic blood pressures (r = -0.95 with P < 0.001 and r = -0.95 with P < 0.001), respectively, following a single oral dose of enalapril. These ELISA methods have the advantage of being simple, accurate, sensitive, and do not depend on enalaprilat binding to ACE. Such methods can be used for analysis and kinetic testing of enalapril and enalaprilat in biological fluids.