Three simple, sensitive spectrophotometric methods were proposed for the quantification of two phosphodiesterase type 5-inhibitors; vardenafil HCl (VARD) and tadalafil (TDF) in pure forms as well as in tablets dosage forms. The methods use N-bromosuccinimide (NBS) as an analytical reagent and three dyes, amaranth methylene blue, and indigocarmine or orange G, as auxiliary reagents. The three methods are based on oxidation reaction of VARD or TDF with a known excess of N-bromosuccinimide (NBS) in acid medium, followed by determination of unreacted NBS by the reaction with a fixed amount of three dyes, amaranth, methylene blue and indigocarmineor orange G followed by the measurement of the absorbance at 520, 664 and 610 or 478 nm, respectively. Under the optimum conditions, the three methods are applicable over the concentration ranges of 1.0-16, 1.0-12 and 1.0-10 μ gmL-1 for VARD using amaranth methylene blue and indigocarmine methods, respectively and 2.0-12, 2.0-15 and 1.0-10 μ gmL-1 for TDF using amaranth methylene blue and orange G, respectively. The molar absorptivities, Sandell's sensitivity values, correlation coefficients, limits of detection and quantification are reported. Intra-day and inter-day accuracy and precision of the methods have been evaluated. No interference was observed from the common tablet excipients. The methods were successfully applied to the assay of VARD and TDF in tablets preparations and the results were statistically compared with those of the reference methods by applying Student's t-test and F-test. The reliability of the methods was further ascertained by performing recovery studies using the standard addition method.
Spectrophotometry, VardenafilHCl, Tadalafil, N-bromosuccinimide, Tablets