Simultaneous determination of enalapril maleate and nitrendipine in tablets using spectrophotometric methods manipulating ratio spectra.

Abstract

A fixed dose combination of enalapril maleate (EN) and nitrendipine (NT) achieved satisfactory blood pressure control rather than monotherapy. Four accurate spectrophotometric methods utilizing ratio spectra were developed and validated for the simultaneous determination of EN and NT in combined pharmaceutical dosage form (Eneas® tablets). The first method was first derivative ratio spectrophotometric method (1DD) where the amplitudes maxima were measured at 219.2nm and 233.4nm for the determination of EN and NT, respectively. The second method was ratio difference spectrophotometric method where EN and NT were estimated by measuring the amplitudes difference between 213nm and 225nm on the ratio spectrum of EN and between 241nm and 227nm on the ratio spectrum of NT. The third method was ratio subtraction followed by extended ratio subtraction, EN was determined at 210nm by subtraction of the constant values from the ratio spectrum then multiplication with the divisor (NT spectrum). An extended ratio subtraction method was then applied in order to determine NT at 235nm by using the zero-order spectrum of EN (10μg/ml) as a divisor. The fourth method was ratio subtraction coupled with constant multiplication methods. The zero- order absorption spectrum of the laboratory prepared mixture was divided by NT (12μg/ml) spectrum, subsequently, the value of the constant was multiplied by the divisor to obtain the original spectrum of NT, followed by its subtraction from the laboratory prepared mixture to get the spectrum of EN. EN and NT were determined at 210nm and 235nm, respectively. Linearity was ascertained over the concentration ranges of (1-11μg/ml) for EN and (2-14μg/ml) for NT, for the first and second methods and (2-13μg/ml) for EN and (3-20μg/ml) of NT, for the third and fourth methods. Application of the methods to the determination of the cited drugs in Eneas® tablets gave satisfactory results. Methods validation confirmed their accuracy and selectivity. Statistical comparison of the results with the reported one showed no significant difference, regarding precision and accuracy. Routine analysis of the cited drugs in quality control laboratories could be performed using the developed methods.