A simple, sensitive and accurate spectrophotometric methods have been developed for the determination of two phosphodiesterase type 5-inhibitors; vardenafil HCl (VARD) and tadalafil (TDF) in bulk drugs and pharmaceutical preparations. The methods are based on the oxidation of the studied drugs by a known excess of ceric(IV) ammonium sulphate (CAS) in acid medium followed by determination of unreacted oxidant by adding a fixed amount of orange G (OG), rhodamine B (RB), methylene blue (MB) and methyle orange (MO) dyes followed by measuring the absorbance at 478, 550, 664 and 510 nm, respectively. The experimental conditions affecting the reaction were studied and optimized. The beer’s law was obeyed in the concentration ranges of 1.0-8.0, 1.0-10, 1.0- 12 and 1.0-12 μg mL-1 for VARD using OG, RB, MB and MO methods, respectively and 2.0-12, 1.0-12 and 1.0-15 μg mL-1 for TDF using OG, RB and MB methods, respectively with a correlation coefficient ≥ 0.9990. The calculated molar absorptivity values are 4.5874 × 104, 3.4207 × 104, 2.1705 × 104 and 4.3091 × 104 L mol-1 cm-1 for VARD using OG, RB, MB and MO methods, respectively and 3.3086 × 104, 5.2058 × 104 and 3.3342 × 104 L mol-1 cm-1 for TDF, using OG, RB and MB methods, respectively. The limits of detection and quantification were reported. Intraday and inter-day accuracy and precision of the methods have been evaluated. No interference was observed from the additives and the applicability of the method was tested by analyzing the pharmaceutical preparations containing the investigated drugs. 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 ttest and F-test. The reliability of the methods was further ascertained by performing recovery studies using the standard addition method.
