Synthesis of Sildenafil Citrate Validated HPLC Method and Its Pharmaceutical Dosage Forms

Sildenafil citrate is a selective inhibitor of cyclic Guanosine Monophosphate (GMP) or Specific phosphodiesterase type 5 (PDE 5), commercially developed by Pfizer, Inc._as Viagra®. Sildenafil citrate is designated chemically as 1-[[3-(6,7-dihydro1-methyl-7-oxo-3-propyl-1H–pyrazolo[4,3-d]pyrimidin-5-yl)-4ethoxyphenyl]sulphonyl]-4-methypiperazine citrate (Figure 1) [1]. The compound has the following structure:

With respect to polymorphic forms of sildenafil citrate, while there are no patents reported, but in a publication describe three polymorphic forms. The process for the preparation of sildenafil citrate of polymorphic form I as designated ( Figure 2). The process is from the penultimate intermediate namely 5-(5-chlorosulphonyl-2-ethoxy phenyl) -1-methyl-3-N-propyl-1,6-dihydro-7H-pyrasolo-(4,3-d)pyrimidin-7-one, which is herein will be referred to as chlorosulphonyl intermediate  The resulting product of condensation namely sildenafil base is reacted with citric acid in an aqueous medium to give sildenafil citrate (3). The crystallization conditions are well established to give crystalline form I [3]. The powder X-ray diffraction pattern of the sildenafil citrate polymorphic form I as given in Figure 1 and the 2θ values are given in Table 1.

Organic and Medicinal Chemistry International Journal
The Differential scanning calorimeter graph of the Sildenafil citrate polymorph I under specific conditions shows the melting point around 197.56_°C. Figure 3 depicts a comparison of DSC scanned at 5_˚C/min over a temperature range of 30_˚C to 350_˚C for sildenafil citrate polymorphic form 1 [4].

Materials and Methods
HPLC analysis was performed using a Schimadzu LC-2010 chromatographic system (Schimadzu, Kyoto, Japan) consisting of a LC-20AT Prominence liquid chromatography pump with DGU-20A5 Prominence degasser, a SPD-M20A Prominence Diode Array Detector, RF 10AXI fluorescence detector and a SIL-20 AC Prominence auto sampler. Data analyses were done using Class VP 7.3 Software [5,6]. The elution was carried out on a column Hypersil BDS-C18 (125 x 4 mm i.d., 5 mm), mobile phase consisted of phosphate buffer (20 mM, pH 2.8)-acetonitrile (71:29, V/V), flow rate 1.5 mL min-1, at controlled temperature (25_ o C) and auto sampler temperature at 4_ o C. Detection of sildenafil was carried out at 285 nm. Commercially available, film-coated tablets, containing 50 mg sildenafil as sildenafil citrate, were used in this study [7,8].

Results and Discussion
The method was fully validated according to the ICH (International Conference on Harmonization) guidelines by determination of linearity, precision, accuracy, limit of detection and limit of quantification. Linearity of the method was tested in the range of: 2 -100 mg mL-1 sildenafil [9]. Experimental data showed high level of linearity which was proved with the value for the correlation coefficient (R2 =0.9994).
Limit of Detection (LOD) and Quantification (LOQ) of the method were tested in the range of: 20 -200 ng µL-1 sildenafil. The results were: 0.23 ng and 0.68 ng for LOD and LOQ, respectively (9.2 ng µL-1 and 27.2 ng mL-1 for LOD and LOQ, respectively, obtained with 25 µL injected) [10,11]. Selectivity of the method was proved with the chromatographic peak resolution obtained between sildenafil and tadanafil (Rs = 10, 5) (Figures 4-5) and the characteristic UV-spectrum. Mean recovery for sildenafil was between 99,74_% and 100,88_% indicating that the developed method was accurate for determination of sildenafil in pharmaceutical formulation [12,13]. The proposed method was successfully applied for determination of sildenafil in film-coated tablets, containing 50 mg sildenafil as sildenafil citrate ( Figure

Example 4: Synthesis of Sildenafil Citrate (Form I)
In a 50-Litre glass assembly, deionised water (21 Litre) was charged and sildenafil base (840_gm; 1.769×10 3 mmoles) was added to it at 25-30_˚C. The reaction mixture was heated to 60-65_˚C for 1 h. Citric acid (370_gm; 1.76×10 3 mmoles) was added to the pre heated reaction mixture. The resultant mixture was further heated up and maintained at 80-85_˚C, for 1h and then charcoal treatment given at same temperature. Filter the reaction mass. Filtrate was allowed to cooled to 10-15_˚C, resultant product obtained was filtered and washed with deionised water (0.84 Litre). The product was dried in vacuum (about 10 mm Hg) at 75_oC as a polymorphic form I of sildenafil citrate salt 1.0_ kg. (HPLC purity-99.9_% and molar yield of 85_%).

Figure 7:
The powder X-Ray diffraction is given in Figure 7.
In a 1200 Litre SS reactor, sildenafil base (30kg) was mixed with water (750Litre.) at 25-30˚C and the reaction mixture heated to 60-65_˚C. Citric acid (13.2kg) was added to the pre heated reaction mixture and the resultant mixture was further heated to 80-85_˚C for 1h.The reaction mixture was treated with carbon charcoal and then filtered. The filtrate obtained was cooled to 10-15˚C, resultant product obtained was filtered and washed with deionised water. The product was dried in vacuum (10mm Hg) at 75_ o C as a polymorphic form I of sildenafil citrate Organic and Medicinal Chemistry International Journal salt 35.5-36_kg. (HPLC purity-99.9_% and molar yield of 85_%) ( Table 1) (Figures 7,8).

Conclusion
The present process, which describes the manufacturing process of sildenafil citrate, which e is a selective inhibitor of Cyclicguanosine Monophosphate (cGMP) specific Phosphodiesterase Type 5 (PDE 5), has the advantage of scaling up to the industrial level of production. The results of the validation demonstrated that the proposed analytical procedure is accurate, precise and reproducible for sildenafil analysis in pharmaceutical dosage forms. Furthermore, this procedure is relatively inexpensive and simple and is particularly suitable for routine analyses when tandem mass spectrometric detection is not available.
Additionally, it is important to mention that decreased consumption of organic solvent considerably reduces the laboratory expenses. The process uses safe reagents in the process which makes it better for industrial scale operations. The yields in the process are high which makes it a cost effective process. Residual solvents play a very important role in the impurity profile of APIs as per the ICH Guidelines ICH Q3C_(R4). In this process by carrying out the final step of condensation of Sildenafil base and citric acid in the aqueous medium followed by water crystallization.