INTRODUCTION:^1-5

Clindamycin phosphate is a semisynthetic derivative of lincomycin, belonging to the lincosamide class of antibiotics. It is widely used in the treatment of various bacterial infections, particularly those caused by susceptible Gram-positive cocci and anaerobic bacteria. Topical formulations of Clindamycin are commonly prescribed for acne vulgaris due to its anti-inflammatory and antibacterial effects.

Clindamycin acts by binding to the 50S ribosomal subunit of the bacterial ribosome, inhibiting protein synthesis and thus impeding bacterial growth. It is available in multiple dosage forms including oral, intravenous, intramuscular, and topical preparations.

Chemically, Clindamycin phosphate is known as methyl 7-chloro-6,7,8-trideoxy-6-[[[(2S,4R)-1-methyl-4-propyl-L-prolyl]amino]thio] -1-thio-L-threo-α-D-galacto-octopyranoside 2-(dihydrogen phosphate). It has a molecular formula of C18H34ClN2O8PS and a molecular weight of 504.96 g/mol. The compound is white to off-white, odorless, and crystalline in appearance. It is freely soluble in water and soluble in methanol

Due to its high solubility in water, Clindamycin phosphate is suitable for aqueous preparations and analytical work involving UV-visible detection. Clindamycin exhibits a maximum absorbance at 210 nm, making it ideal for detection using reverse-phase high-performance liquid chromatography (RP-HPLC).

Although various methods have been reported for Clindamycin estimation using HPLC with UV detection, including single and simultaneous assays, there is still a need for a simple, accurate, and validated RP-HPLC method that can be applied to both bulk drug and topical formulations. The aim of this study is to develop and validate such a method according to ICH Q2(R1) guidelines.

Fig No. 1 Structure of Clindamycin⁶

Literature review revealed that although various RP-HPLC methods for the estimation of Clindamycin have been reported, most of them are focused on biological matrices such as human plasma or serum. Very few methods are available for the determination of Clindamycin phosphate as a single active ingredient in topical gel formulations. Hence, there is a need for a simple, specific, and validated RP-HPLC method for the quantitative estimation of Clindamycin phosphate in pharmaceutical dosage form.

HPLC (High Performance Liquid Chromatography):⁷

High Performance Liquid Chromatography (HPLC) is a highly precise and reproducible analytical technique widely employed for the separation, identification, and quantification of components within a mixture. It is an advanced form of column chromatography where the mobile phase is pumped through a tightly packed column under high pressure, often up to 400 atmospheres. This high-pressure mechanism ensures rapid analysis and higher resolution compared to conventional methods

In reverse-phase HPLC (RP-HPLC), the stationary phase is non-polar (commonly C18), while the mobile phase is relatively polar. The analytes separate based on their hydrophobic interactions with the stationary phase, resulting in distinct retention times. RP-HPLC has become the method of choice for pharmaceutical analysis because of its accuracy, robustness, and ability to separate analytes even in complex matrices

In the present study, RP-HPLC was employed for the estimation of Clindamycin phosphate in a topical gel formulation using UV detection at 210nm.

EXPERIMENTAL WORK:

· Reagents and Materials:

Clindamycin phosphate (API) was used as the reference standard. The marketed formulation Cleargel (1% w/w) was selected for analysis. Analytical grade reagents and solvents were used throughout the study. These included Acetonitrile (HPLC grade), Ortho Phosphoric Acid (OPA), and double distilled water. Membrane filters (PVDF and Nylon, 0.45μm) were used for sample filtration.

· Preparation of Stock Solution:

Accurately weighed quantity of Clindamycin phosphate equivalent to 20mg of Clindamycin was transferred into a 100mL volumetric flask. About 50mL of mobile phase (Acetonitrile: 0.05% OPA in water, 30:70v/v) was added and the mixture was sonicated to dissolve the drug. The volume was made up to the mark with the same mobile phase to yield a stock solution of 200μg/mL.

A working standard solution of 50μg/mL was prepared by transferring 2.5mL of the above stock solution into a 10mL volumetric flask and diluting to volume with mobile phase.

· Preparation of Mobile Phase:

The mobile phase was prepared by mixing Acetonitrile and 0.05% Ortho Phosphoric Acid in Water in the ratio of 30:70 v/v. The solution was filtered through a 0.45 μm membrane filter and degassed by sonication prior to use.

· The final optimized mobile phase is given below:

· Mode: Isocratic

· Column: Phenomenex C18, 250mm × 4.6mm, 5.0 µm

· Mobile Phase: Acetonitrile: 0.05% Ortho Phosphoric Acid in Water (30:70 v/v)

· Flow Rate: 1.0mL/min

· Detection Wavelength: 210nm

· Injection Volume: 20µL

· Column Oven Temperature: 40°C

· Run Time: 8 minutes

· Diluent: Mobile phase

METOD DEVELOPMENT⁸:

When no official or compendial method is available for a specific pharmaceutical formulation, a new analytical method must be developed to ensure accuracy, reliability, and cost-effectiveness. In the present study, a reverse-phase high-performance liquid chromatography (RP-HPLC) method was developed for the estimation of Clindamycin phosphate in bulk and in a topical gel formulation.

The method was optimized by evaluating different mobile phase compositions, flow rates, and detection wavelengths to achieve ideal separation and peak symmetry. The final chromatographic conditions were selected based on acceptable retention time, sharp peak shape, appropriate theoretical plate count, and satisfactory asymmetry. The chosen method utilizes a C18 column, UV detection at 210 nm, and a mobile phase composed of Acetonitrile and 0.05% Ortho Phosphoric Acid in Water (30:70 v/v), offering good reproducibility and resolution.

Analytical method development is grounded in the principles of analytical chemistry, which involve identifying, isolating, and quantifying the chemical constituents of pharmaceutical substances. These methods help ensure the drug’s purity, potency, identity, and quality throughout its shelf life.

RESULT AND DISCUSSION:

Method Validation:^9-12

Analytical method validation is performed to ensure that the procedure produces reliable and reproducible results under specified conditions. It includes evaluating several performance characteristics such as specificity, linearity, accuracy, precision, detection limit, quantification limit, robustness, and solution stability.

The validation of the developed RP-HPLC method was carried out in accordance with ICH guidelines. The method was confirmed to be suitable for its intended application and applicable for routine quality control analysis of Clindamycin phosphate in pharmaceutical dosage forms.

1. Linearity:

The linearity of the developed RP-HPLC method was evaluated by analyzing standard solutions of Clindamycin phosphate at five different concentrations: 5, 25, 50, 62.5, and 75µg/mL. A calibration curve was plotted using peak area versus concentration, and the method demonstrated a strong linear relationship across the selected range.

Table No. 1 Results of HPLC Linearity Data for Clindamycin

Level	Conc (µg/mL)	Area	Mean	% RSD
10%	5.0	718663	719186	0.072
		719203
		719691
50%	25.0	3623980	3623689	0.109
		3627481
		3619605
100%	50.0	7236069	7236965	0.081
		7231628
		7243197
125%	62.5	9027068	9031042	0.050
		9035971
		9030086
150%	75.0	10750367	10742770	0.070
		10735384
		10742560

Fig No. 2 Calibration curve of Clindamycin

A calibration curve was constructed by plotting the mean peak area versus concentration of Clindamycin phosphate for five standard solutions (5–75 µg/mL). The curve showed a strong linear correlation across the selected range. The regression equation was found to be:

Y=143589.51×X+25367.61

Where:

· Y = Mean Peak Area

· X = Concentration of Clindamycin (µg/mL)

· Slope (m) = 143589.51

· Intercept (c) = 25367.61

This equation reflects a strong linear relationship between concentration and peak area, confirmed by a correlation coefficient (R²) of 0.99996.

2. Repeatability:

Repeatability was evaluated by analyzing six replicates of Clindamycin phosphate standard solution at a concentration of 50 µg/mL. The %RSD of the peak area was calculated to assess the precision of the method. The %RSD was found to be 1.196%, which is within the acceptable limit of ±2%, indicating that the developed RP-HPLC method is precise and repeatable.

Intra-day and Inter-day:

The precision of the developed RP-HPLC method was evaluated by assessing both intra-day (repeatability) and inter-day (intermediate precision). For intra-day precision, six replicate injections of a standard solution of Clindamycin phosphate were analyzed on the same day. For inter-day precision, the same procedure was repeated over three consecutive days.

Table No. 2 Result of Intra- day and Inter- Day Precision for Clindamycin

Repeatability	Sample	Test Sample (mg)	Area	% Assay
	Sample 1	1001.6	7296002	100.25
	Sample 2	999.3	7172513	98.78
	Sample 3	1001.2	7150714	98.29
	Sample 4	1000.4	7035036	96.78
	Sample 5	1000.9	7207461	99.10
	Sample 6	999.7	7107602	97.84
	Mean			98.51
	STD DEV			1.178027
	% RSD			1.196
Intermediate precision (Inter-Day)	Sample 1	999.7	7161100	98.58
	Sample 2	1000.5	7116250	97.88
	Sample 3	999.8	7148917	98.40
	Sample 4	1001.3	7256073	99.73
	Sample 5	1000.2	7207586	99.17
	Sample 6	1000.8	7051025	96.96
	Mean			98.45
	STD DEV			0.971713
	% RSD			0.987
Repeatability Plus Inter-day	Mean			98.480
	STD DEV			1.02993
	% RSD			1.046

The %RSD was found to be 1.196% for intra-day and 1.046% for inter-day precision. As both values were within the acceptable limit of ±2%, the method is considered to be precise and reproducible under the tested conditions.

3. Accuracy (Recovery Studies):

The accuracy of the developed method was evaluated by performing recovery studies at three different concentration levels: 50%, 100%, and 150% of the target concentration. Known amounts of Clindamycin phosphate were added to the sample matrix and analyzed using the proposed method.

The percentage recovery was found to be in the range of 98.79% to 101.18%, which falls within the acceptable limits of 98%–102%, indicating that the method is accurate and free from interference by excipients.

Table No. 3 Result and statistical data of Accuracy of Clindamycin

Level	Area	Recovered conc (µg/mL)	Added conc (µg/mL)	% Recovery	Mean Recovery	% RSD
50	3687205	25.37	25.68	98.79	99.04	0.359
	3651506	25.12	25.26	99.45
	3750953	25.81	26.10	98.89
100	7289520	50.15	50.10	100.10	100.08	1.114
	7266014	49.99	50.52	98.95
	7366747	50.69	50.10	101.18
150	10960214	75.41	75.78	99.51	100.10	0.864
	11071823	76.18	75.36	101.09
	11040540	75.96	76.20	99.69

4. Robustness:

The robustness of the developed RP-HPLC method was evaluated by introducing small deliberate changes in method parameters such as wavelength (±3 nm), flow rate (±10%), and column oven temperature (±2°C). The retention time, peak area, asymmetry, and theoretical plates remained within acceptable limits under all altered conditions. This confirms that the method is robust and reliable under normal analytical variations.

Table No. 4 Result of Robustness study

Change in Parameter	R.T.	Standard area	Asymmetry	Theoretical plates
Wavelength by +3 NM (213 NM)	4.32	6723057	1.20	6975
Wavelength by -3 NM (207 NM)	4.33	9068251	1.19	6848
Flow rate by +10% (1.1mL/min)	3.94	6487946	1.18	7389
Flow rate by -10% (0.9mL/min)	4.81	8369048	1.15	6703
Column oven temp by +2ºC (42 ºC)	4.33	7220832	1.25	7271
Column oven temp by -2ºC (38 ºC)	4.34	7245683	1.24	7210

5. Limit of Detection (LOD):

The Limit of Detection (LOD) for Clindamycin phosphate was determined based on the standard deviation of the Y-intercept (σ) and the slope (S) of the calibration curve using the formula:

LOD=3.3×σ/S
=3.3×36570.34128/143589.5071
LOD = 0.840µg/mL

Where:

σ = 36570.34128 (standard deviation of the Y-intercept)

S = 143589.5071 (slope of the calibration curve)

6. Limit of Quantitation (LOQ):

The Limit of Quantitation (LOQ) was calculated using the formula:

LOQ=10×σ/S
=10×36570.34128/143589.5071
LOQ = 2.547 µg/mL

These results indicate that the developed RP-HPLC method is highly sensitive and suitable for detecting and quantifying low concentrations of Clindamycin phosphate in pharmaceutical formulations.

Fig No.3 Typical chromatogram of Blank solution.

Fig No. 4 Typical chromatogram of Placebo solution

7. Specificity:

The specificity of the developed RP-HPLC method was evaluated by analyzing the Clindamycin phosphate standard and sample solution in the presence of formulation excipients. The chromatogram showed no interfering peaks at the retention time of Clindamycin, confirming that the excipients in the topical gel formulation did not interfere with the detection of the drug. Thus, the method is considered specific for the estimation of Clindamycin phosphate.

CONCLUSION:

In conclusion, ensuring the accuracy, precision, and robustness of an analytical method is crucial when estimating Clindamycin in bulk and pharmaceutical dosage form. A reliable and reproducible RP-HPLC method was developed through systematic method optimization, including selection of the appropriate mobile phase, stationary phase, and chromatographic parameters. The optimized conditions produced a sharp, symmetrical peak with satisfactory retention time, theoretical plates, and asymmetry values.

The method was validated according to ICH Q2(R1) guidelines and found to be specific, accurate, linear, precise, and robust. Validation parameters, including system suitability, filtration study, solution stability, specificity, linearity (R² = 0.99996), accuracy (mean recovery = 99.74%), precision (intra- and inter-day %RSD < 2%), LOD (0.840 µg/mL), and LOQ (2.547 µg/mL), all met regulatory acceptance criteria.

This validated RP-HPLC method is suitable for the routine quality control, assay, and stability evaluation of Clindamycin in pharmaceutical formulations. It serves as a dependable analytical tool, supporting quality assurance practices to ensure the safety and efficacy of Clindamycin-based drug products

ACKNOWLEDGEMENT:

We would like to express our sincere gratitude to our Management and Principal, P.S.G.V.P. Mandal’s College of Pharmacy, Shahada for providing necessary facilities needed for this study.

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Received on 28.06.2025 Revised on 18.08.2025

Accepted on 01.10.2025 Published on 02.01.2026

Available online from January 05, 2026

Asian J. Res. Pharm. Sci. 2026; 16(1):15-19.

DOI: 10.52711/2231-5659.2026.00003

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