UV Spectrophotometric Method for the Estimation of Tadalafil in Bulk and Tablet Dosage form

A simple, sensitive, precise and highly accurate UV spectrophotometric method has been developed for the determination of tadalafil in bulk and tablet dosage form. Solution of tadalafil in methanol shows maximum absorbance at 284 nm. Beer’s law was obeyed in the concentration range of 2-20 mcg mL with 1.65x10 mol cm, the slope, intercept, correlation coefficient, detection and quantitation limits were also calculated. The proposed method has been applied successfully for the analysis of the drug in pure and in its tablets dosage forms. Result of percentage recovery and placebo interference shows that the method was not affected by the presence of common excipients. The method was validated by determining its sensitivity, accuracy and precision which proves suitability of the developed method for the routine estimation of tadalafil in bulk and solid dosage form.


Introduction
Tadalafil, chemically pyrazino [1′,2′:1,6] pyrido [3,4-b] indole-1,4-dione,6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12 a-hexahydro-2-methyl-, ( 6R,12aR )-( Figure1 ), is an impotence agent.It is indicated for the treatment of erectile dysfunction [1][2] .It is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5) 3 .In previous studies, only one assay has been reported for the determination of tadalafil in human serum and urine by LC-MS-MS 4 .Tadalafil has also been quantified in pharmaceutical preparations, human serum and biological fluids by HPLC 5 with UV detection.Determination of tadalafil by densitometric (TLC) 6 and colorimetric 7 methods has also been reported.Although the ultraviolet spectrophotometric method is the instrumental method of choice commonly used in industrial laboratories because of their simplicity, selectivity and sensitivity.As of our knowledge no report has been mentioned in the literature for the determination of tadalafil by Wavelength, nm UV method.The aim of the present work was to develop simple, rapid, accurate and sensitive UV spectrophotometric method for the determination of tadalafil in bulk and tablet formulation.UV analysis of tadalafil was performed in methanol.The spectrum was recorded from 200 nm to 400 nm.The quantitative analysis was carried out at 284 nm.The method was validated and applied for the determination of tadalafil in tablet dosage form.

Experimental
Pharmaceutical grade of tadalafil was procured from Orchid chemicals and Pharmaceuticals Ltd., India.All the chemicals were of analytical reagent grade of Merck (Germany) unless otherwise specified.Methanol was used to prepare all solutions.Freshly prepared solutions were always employed.Different brands of tablets of tadalafil were supplied from local pharmacy.

Instrumentation
The spectrophotometric measurements were carried out using Elico UV/Visible double beam spectrophotometer SL-164 with 1 cm matched quartz cells.

Tadalafil stock solution
Standard stock solution was prepared by dissolving 50 mg of tadalafil in 50 mL of methanol to get concentration of 1000 µg/mL solution.It was further diluted to get working standard solution of 100 µg/mL.

Method development
Aliquot of working standard solution was further diluted with methanol to get concentration of 10 µg mL -1 and it was scanned between 200-400 nm which shows the maximum absorbance at 284 nm (Figure 2).The same λ max was used for the further measurement of the drug. .

Procedure for calibration curve
Aliquots of working standard solution were further diluted with methanol to get concentration of 2, 4, 8, 12, 16 and 20 µg mL -1 .Finally, the prepared standards were measured after standing for 5.0 min at λ max as recorded in Table 1, in each case against a solvent methanol as blank.A calibration graph of the absorbance versus the concentration of the drug was plotted (Figure 3).

Procedure for dosage forms
For analysis of commercial formulations, twenty tablets containing tadalafil were taken and powdered.Tablet powder equivalent to 50 mg of tadalafil was transferred to 50 mL volumetric flask and dissolved in methanol.Then the solution was sonicated for 15 min and filtered through Whatman filter paper No. 41 and it was further diluted to get the required concentration.The absorbance of the prepared sample solution was measure against methanol as a blank at 284 nm.A standard additions technique was also used to confirm the accuracy and precisions

Results and Discussion
The optical characteristics such as Beer's law limit, molar absorptivity, Sandell's sensitivity, correlation coefficient, slope and intercept, % Relative Standard Deviation (RSD), % range of error (0.05 and 0.01 confidence limits) were calculated and are summarized in Table 1.To ensure the reproducibility and accuracy of the method, recovery studies were carried out by adding a known quantity of pure drug with preanalyzed sample and contents were reanalyzed by the proposed method.From the amount of drug found, percentage recovery was calculated.The results of analysis and recovery studies are given in Table 2

Conclusion
In this study a simple, rapid, sensitive, accurate and precise UV spectrophotometric method for the determination of tadalafil in bulk and pharmaceutical formulation has been developed and validated.It was found that the common excepients present in the formulation did not interfere with the proposed method and can be used for the routine quality control analysis of tadalafil in bulk as well as in tablet formulations.

Table 1 .
. Optical characteristics and precision of the proposed method.
* Average of eight determinations.

Table 2 .
Assay of tadalafil in tablet dosage form.