INTRODUCTION:

Silodosin (SLD) Structure²¹shown in figure 1. Silodosin is Alpha 1 Antagonist used in treatment of BPH (Benign prostatic hyperplasia)²³Tadalafil (TLD) is Structure²² shown in figure 2. Tadalafil is Phosphodiesterase-5 Inhibitor used in treatment of ED (Erectile Dysfunction).

The goal of stability and related substance analysis is to illustrate how various environmental conditions affect the quality of a drug substance or drug product over time.^1,2. Pharmaceutical quality control relies significantly on HPLC, UV Spectrophotometric, and LC-MS/MS techniques, as the stability and integrity of substances are crucial for their efficacy and safety^3-13. The stability and pharmacological efficacy of APIs and are impacted by the presence of contaminants.^14-18.

CCD and AQbD, support regulatory compliance. For the development of RP-HPLC procedures, stress testing, or stability-indicating techniques ^19-20.

Fig. 1: Chemical structure of silodosin Molecular Formula: C₂₅H₃₂F₃N₃O₄ Molecular Weight: 495.534 g/mol IUPAC Name: 1-(3-hydroxypropy)-5-[2R)-2-[2-(2,2,2-trifluoroethoxy) phenoxyl] ethylaminol] propyl]-2,3-dihydroindole-7-carboxamide

Fig. 2: Chemical structure of Tadalafil Molecular Formula: C₂₂H₁₉/N₃₀₄ Molecular Weight: 389.41g/mol IUPAC Name: (6R-12aR)-6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hrxahydro-2-methyl-pyrazino(1’,2’:1,6.) pyrido (3,4-b) indole 1,4-dione

A search of the literature indicated that while there is an HPLC technique available for combination²⁴, there is no single approach published for stability showing simultaneous measurement of Silodosin and Tadalafil. There were few analytical methods reported for Silodosin such as UV²⁵ spectroscopy, UPLC²⁶, LC-MS^, RP-HPLC²⁷,ESI-LC-MS²⁸ and RP-HPLC (stability indication method)²⁹method reported for Silodosin in combination with others^30-31There were few analytical methods reported for Tadalafil such as UV spectroscopy³², RP-HPLC^33-36, UPLC, LC-MS-MS³⁷ and RP-HPLC^38-39 (stability indicating method) reported for Tadalafil in combination with others.

EXPERIMENTAL:

Materials and Instrumentation:

A std. sample of Silodosin was a gracious gift by Alembic Pharmaceuticals, Vadodara and Tadalafil was kindly gifted by Century Pharmaceuticals, Vadodara and Sidesaddle Pharmaceuticals, Ahmedabad. The method was developed using HPLC.

Optimized chromatographic condition:

Isocratic elution mode was utilized for chromatographic separation with Thermo BDS C18 (250mm × 4.6mm, 5 µ) column and M.Phase used the composition of 20mM Dipotassium hydrogen Phosphate buffer: Acetonitrile (60:40v/v). A 20µl injection volume was used. 40ºC was the temperature chosen for the column oven. A 1.5 ml/min flow rate was used. 220nm was chosen as the UV detector's detecting wavelength.

PREPARATION OF SOLUTIONS:

METHOD VALIDATION:

The proposed method's analytical validation parameters were established in accordance with the ICH Q2(R1) recommendation.⁴⁰

RESULTS AND DISCUSSION:

For chromatographic separation, an isocratic elution method was implemented with the following parameters: a Thermo BDS C18 column (250mm × 4.6 mm, 5µm), a mobile phase of 20mM dipotassium hydrogen phosphate buffer and acetonitrile (60:40 v/v), an injection volume of 20µl, a column oven temperature of 40°C, a flow rate of 1.5ml/min, and a UV detection wavelength of 220nm.

Fig. 3. Optimized Chromatogram of Standard Silodosin and Tadalafil

System Suitability Parameter:

They are acceptable as shown in Table no. 1

Table 1. System Suitability Parameter of Optimized Condition

Peak	Retention time (min)	Theoretical plate	Tailing factor	Resolution
SLD	2.905	10400	1.36	15.60
TLD	5.082	15161	0.94	15.60

FORCED DEGRADATION STUDIES:

Degradation study is acceptable as % degradation is less than 30 and all degradation peaks were separated from other peaks. The order of degradation was Acid hydrolysis > Oxidative degradation > Thermal degradation > Photolytic degradation > Base hydrolysis. Figure 4 and results are shown in Table 2.

Table 2. Forced degradation summary

Degradation Condition	Procedure	(R.T) (min) (SLD)	% Degradation (TLD)	Peak Angle (SLD)	Peak Threshold (TLD)
Acid Degradation	80 mg of Silodosin and 50 mg of Tadalafil in 100 ml flask with 5 ml of 1 N HCl for 8 hours. Neutralized with 5 ml of 1 N NaOH, diluted with methanol, and filtered.	3.199	5.273	20.88	28.25
Base Degradation	Similar to acid degradation, but with 5 ml of 1 N NaOH for 8 hours, then neutralized with 5 ml of 1 N HCl, diluted with methanol, and filtered.	2.879	4.963	9.08	5.14
Oxidative Degradation	Silodosin and Tadalafil treated with 3% H₂O₂ for 1.5 hours, then diluted with methanol.	2.879	4.963	7.81	26.06
Thermal Degradation	Standard powders of Silodosin and Tadalafil placed in hot air oven at 80°C for 24 hours.	2.879	4.963	8.96	18.38
Photolytic Degradation	Powders exposed to UV light achieving 1.2 million LUX.hr in a photostability chamber for 24 hours.	2.879	4.963	3.51	15.6

Fig. 4: Forced degradation chromatogram of (A) Acid degradation (B) Base degradation (c) Oxidative degradation (d) Photo degradation (E) Thermal degradation (F) Standard

Fig. 5. Calibration curve of Silodosin and Tadalafil

Accuracy:

The findings shown in Table 3 indicate that the % recovery for Tadalafil and Silodosin.

Precision:

The method is precise as RSD and was found to be less than 2. Results are shown in Table 3.

LOD and LOQ:

For SLD and TLD, the LOD was determined to be 0.40µg/mL and 0.20µg/mL, respectively. For SLD and TLD, the LOQ was determined to be 1.21µg/mL and 0.63µg/mL, respectively.

Robustness:

When various parameters including flow rate, mobile phase, and temperature were purposefully altered, it was discovered that the approach was reliable. When the settings were intentionally altered, the peak area's relative standard deviation was less than 2%.

Assay of synthetic mixture:

Using the established approach, a synthetic combination of 50mg Tadalafil and 80mg Silodosin was evaluated. The findings showed that the concentration of Tadalafil was 98.59% and the concentration of Silodosin was 98.67%.

CONCLUSION:

The creation and successful validation of a stability-indicating RP-HPLC technique for the detection of Tadalafil and Silodosin in mixtures are described in the current study. In accordance with ICHQ2(R1) requirements, validation has been completed for Tadalafil and Silodosin³⁴. By using RP-HPLC, it was possible to determine that the precision of Tadalafil and Silodosin was less than 2% RSD, and the accuracy of both drugs was found to be between 98.16 and 101.26 % and 98.00 and 100.14%, respectively. All other parameters were found to be within the ICH standards³⁴. The order of stability was Base hydrolysis >Photolytic degradation >Thermal degradation >Oxidative degradation >Acid hydrolysis. Because of this, the method was precise and could be applied to ascertain the amounts of Tadalafil and Silodosin in a synthetic blend.

ACKNOWLEDGEMENT:

The authors are thankful to Alembic Pharmaceuticals for providing standard sample of Silodosin and also thankful to Century Pharmaceuticals, Vadodara and Zydus cadila Pharmaceuticals, Ahmedabad for providing standard sample of Tadalafil.

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Received on 03.03.2024 Revised on 20.11.2024

Accepted on 23.04.2025 Published on 08.11.2025

Available online from November 13, 2025

Research J. Pharmacy and Technology. 2025;18(11):5288-5292.

DOI: 10.52711/0974-360X.2025.00762

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Creative Commons License.

Level %	Accuracy		Precision
	% Drug recovery		RSD
	% Drug recovery		Repeatability		Intra-day		Inter-day
	SLD	TLD	SLD	TLD	SLD	TLD	SLD	TLD
50	98.40	98.33	0.052	0.056	0.631	0.878	0.424	0.630
100	99.46	99.28			0.911	1.506	0.434	1.721
150	98.00	100.08			0.176	0.184	0.176	1.062