Indirect spectrophotometric methods for the determination of Tadalafil

Two spectrophotometric methods were developed for the determination Tadalafil in pharmaceutical preparations. The methods are based on the oxidation reaction with known excess amount of Ce(IV) and estimation of the unreacted amount using Indigo carmine dye (Method A) and in Methylene blue dye (Method B). the factors affecting the reaction conditions were studied and the absorbance of absorbance of the oxidation reaction products were monitored at 610 and 600 nm for methods A and B respectively. Beer's law is obeyed in the concentration ranges 11–50 and 10–55 ppm, the limits of detection and quantification are reported. The proposed method was applied to the determination of the drug in pharmaceutical formulations and the results demonstrated that the method is equally accurate, precise and reproducible as the official methods. The validity of method was established by recovery studies with satisfactory results.


INTRODUCTION
Tadalafil (TDF) is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5), used in the management of erectile dysfunction.
In    Method B: The same method above was performed as in method A except 2.0 ml of (MB) (100 ppm) dye solutions were used, the absorbance of the final solutions were measured at (600 nm).

C. Procedures for drug formulations
An amount of finely ground tablets equivalent to 2.0 mg of (TD) was accurately weighed, dissolved in appropriate amount of distilled water and transferred to a 100-ml volumetric flask, the ANALYSIS flask was sonicated for about 20 minutes, finally the volume was made up to the mark.The content was kept aside for 5 min, and filtered using 0.45µm GHP filter paper.The first 10-ml portion of the filtrate was discarded and a suitable aliquot was used for the assay as described under General analytical procedure for the proposed method or using HPLC method.

RESULTS AND DISCUSSION
Tadalafil For both methods the Absorbance was monitored at 25°C with time which showed that the oxidation reaction is fast and complete in five minutes, and contact times up to 8 minutes had no effect on the absorbance of dyes.

A. Optimum reaction conditions
The optimum conditions for color development in each method were established by varying the parameters one at a time, keeping the others fixed and observing the effect produced on the absorbance of the colored species for methods A and B. In a series of experiments, the volumes of both dyes were varied using the constant concentrations of both (TD) and the selected oxidants, the results which are shown in (Fig. 2 & Fig. 3) revealed that the optimum volumes of both Indigo carmine and Methylene Blue dyes were 2.5 and 2 ml of the given concentrations respectively.For Methods A and B the effect of time on the absorbance values were investigated, the obtained results show no appreciable change after eight minutes, so it decided to measure the absorbance after 10 minutes from preparing the mixtures as shown in (Fig. 4).  1) below.

B. Method validation
Application to drug formulation: The suggested method were applied successfully for the determination of Tadalafil in commercial tablets, Statistical comparison of the accuracy and precision of the proposed methods with an HPLC method was performed using Student's t-tests at a 95% confidence level.The t-values did not exceed the theoretical values; there is no significant difference in accuracy or precision between the proposed and the official method as shown in (Table 2).

CONCLUSION
Tadalafil was determined in a two simple spectrophotometric methods that based on oxidation the drug by Ce(IV) oxidant in the presence of Indigo.
the present investigation, we report the development of accurate, reproducible, less time consuming and adequately sensitive validated spectrophotometric methods for the determination of TDF based on its oxidation with excess amount of Ce(IV) in sulfuric acid, the residual amount of Ce(IV) bleaches Indigo carmine and Methylene blue dyes, then the residual amount of these dyes correlate with the amount of (TDF) in the original solution.Similar work was done by the author [21].The proposed methods were applied to the determination of TDF in tablets dosage form.No interference was observed in the assay of TDF from common excipients in levels found in dosage form.These methods are validated by statistical data and can be adopted by the pharmaceutical laboratories for industrial quality control.

Figure 1
Figure 1 Chemical structure of Tadalafil

Figure 2
Figure 2 Effect of TD concentration on absorption spectrum using Method A.
Method A: Different volumes of standard (TD) (250 ppm) Solutions were pipette into 10 ml volumetric flasks, then 0.5 ml of Ce(IV) (100.0 ppm ) solution was added the mixture was then shacked and kept for 10 minutes, after that 2.5 ml of the (IN) (200 ppm) then the solution was made up to the 10.0 ml with distilled water.The absorbance was then measured at (610 nm) after 10 minutes.

Figure 3
Figure 3 Effect of TD concentration on absorption spectrum using Method B.

Figure 4
Figure 4 Effect of variation of volume of 1.0 mg/mL of Ce(IV) solutions on the Absorbance measured at 600 nm.(Method B).

Figure 5
Figure 5 Effect of variation the volume of 100 µg/mL (MB) on Absorbance measured at 600 nm.(Method B).
undergoes fast oxidation reaction with strong oxidizing agents.It is also shows no absorption band in the visible region which makes it difficult to be determined directly using simple spectrophotometric methods.So we suggest two simple and inexpensive procedures for the determination of (TD) in pure and pharmaceutical preparations based on treating (TD) solutions with an excess amount of oxidizing agent, then the residual amount of oxidant bleaches certain dye, so that the remaining amount of dye can be determined spectrophotometrically.As a result, a proportional increase in the absorbance for the two dyes is observed with increasing concentration of (TD).In method A. Ce(IV) in sulfuric acid was used as oxidizing agent and (IN) was used as a dye the absorbance was measured at 610 nm.In method B. Ce(IV) in sulfuric acid was used as oxidizing agent and (MB) was used as the dye which has a maximum absorption at 600 nm.Preliminary experiments were performed to fix the upper concentrations of the oxidants that could be used to maintain excess amounts.A Ce(IV) concentration of 8.0 ppm was found to bleach the color due to 5 ppm (IN) and (MB).
Analytical parameters:Calibration curves for (TD) determination using the proposed methods A and B were constructed by plotting absorbance vs. concentration using the optimized amounts of oxidants and dyes.The intercepts, slopes, and correlation coefficients were calculated using the method of least squares.Beer's law is obeyed over concentration ranges of 11-50 ppm for (Method A) and 10-55 ppm for (Method B).The mean molar absorptivity (ε), limit of detection (LOD = 3s/k) and limit of quantitation (LOQ = 10s/k) were calculated, where s is the standard deviation of replicate determinations in the absence of analyte under the same conditions as sample analysis and k is the slope.The LOD were 1.5 and 2.3 ppm using methods A and B respectively, these statistical results are shown in (Table

Table 1
Statistical analysis of calibration graphs and analytical data in the determination of Tadalafil

Table 2
Comparison between the proposed method and the