Interference in drug assay by phytochemicals: An experience with colorimetric assay of amlodipine in physiological fluids

Purpose: To investigate the potential interactions between methanol extract of Aframomum melegueta seeds (AMSE) and amlodipine (AML).Methods: Amlodipine concentrations 2.5, 5.0, 7.5, 10, 12.5 and 15 mu g/mL were assayed in vitro with or without AMSE using potassium ferricyanide/FeCl3 (FeCl3/K4(Fe(CN)6)) method. The resulting solution was wavelength-scanned in the range 380 - 950 nm and absorbance read at 393.1, 455.6 and 774.8 nm.Results: Aframomum melegueta seeds (AMSE), when present in biological fluids and solutions of AML, interfered significantly with FeCl3/K4(Fe(CN))6. The highest interference occurred at 774.8 nm, and 50 mu g/mL AMSE gave as much as 1.5 absolute unit increase in absorbance of 2.5 mu g/mL when compared to its absence. Spectral scan of the sample revealed two additional peaks at 393.1 and 455.6 nm with only 0.07 and 0.16 nm unit increases in the presence of AMSE. Concentration-absorbance relationship at these two wavelengths satisfied Beer-Lambert's law. Beyond 15 mu g/mL AML at 774.8 nm, BeerLambert's law was not followed.Conclusion: This study highlights the potential interactions between components of the methanol extract of Aframomum melegueta seeds on assay methods of AML. Furthermore, reliable concentration measurements have been made at 393.1 or 455.6 nm. This should be kept in mind when carrying out drug measurements in populations where concurrent use of herbal remedies is highly probable.