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Rapid, simple and sensitive and validated spectrophotometric methods have been developed for the assay of two phosphodiesterase type 5-inhibitors; vardenafil HCl (VARD) and tadalafil (TDF) in pure and dosage forms. The proposed methods were based on the oxidation of both drugs by a known excess of potassium permanganate (KMnO4) in acidic medium and estimating the unreacted permanganate by the reaction with a fixed amount of three dyes, amaranth, indigo carmine and methylene blue, in the same acid medium followed by measuring the absorbance at λmax=520, 610 and 664 nm, respectively. Different variables affecting the reaction were studied and optimized. The beer’s law is obeyed in the concentration ranges of 2.0-12, 2.0-15 and 2.0-12 μg mL for VARD and 2.0-15, 2.0-20 and 2.0-12 μg mL for TDF using amaranth, indigo carmine and methylene blue methods, respectively with a correlation coefficient ≥ 0.9992. The apparent molar absorptivity values are in the range 2.0956×10, 1.2138×10 and 1.7502×10 L mol cm for VARD and 1.0769×10, 0.7922×10 and 1.0918×10L mol cm for TDF, using amaranth, indigo carmine and methylene blue methods, respectively. The 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 tablet preparations and the results were statistically compared with those of the reported 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.

Sensitive Spectrophotometric Assay of Two Phosphodiesterase Type 5-Inhibitors in Pure and Dosage forms Using Potassium Permanganate