Anthralin Ointment

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Anthralin Ointment is Anthralin in a petrolatum or other oleaginous vehicle. Ointment labeled to contain more than 0.1% of anthralin contains NLT 90.0% and NMT 115.0% of the labeled amount of anthralin (C₁₄H₁₀O₃), and Ointment labeled to contain 0.1% or less of anthralin contains NLT 90.0% and NMT 130.0% of the labeled amount of anthralin (C₁₄H₁₀O₃).

2 ASSAY

PROCEDURE

[NOTE-Use low-actinic glassware.]

Mobile phase: n-Hexane, dichloromethane, and glacial acetic acid (82:12:6)

Internal standard solution: 0.5 mg/mL of o-nitroaniline in n-hexane prepared as follows. First dissolve o-nitroaniline in a small quantity of dichloromethane, and then dilute with n-hexane.

System suitability stock solution: 0.1 mg/mL of USP Anthralin RS and 0.2 mg/mL of danthron in dichloromethane

System suitability solution: Transfer 5 mL of the System suitability stock solution into a 25-mL volumetric flask, add 5 mL of n-hexane, and dilute with Mobile phase to volume.

Solvent blank solution: Mobile phase, n-hexane, and dichloromethane (3:1:1)

Standard stock solution: 0.25 mg/mL of USP Anthralin RS in dichloromethane

Standard solution: Transfer 2 mL each of Standard stock solution and Internal standard solution into a 25-mL volumetric flask, and dilute with Mobile phase to volume.

Sample stock solution: Weigh 5 g of Ointment into a 100-mL beaker. Add 20 mL of dichloromethane and 10 mL of glacial acetic acid, and stir to disperse the Ointment. Transfer the contents of the beaker to a filter paper (Whatman No. 4, or equivalent) with the aid of dichloromethane, and filter into a 100-mL volumetric flask. Thoroughly wash the precipitate with dichloromethane, and allow the washings to drain into the flask. Dilute with dichloromethane to volume.

Sample solution: Transfer a volume of Sample stock solution equivalent to 0.5 mg of anthralin and 2 mL of Internal standard solution into a 25-mL volumetric flask, and dilute with Mobile phase to volume. OFFI

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: UV 354 nm

Column: 4.6-mm x 25-cm; packing L3

Flow rate: 2 mL/min

Injection volume: 10 µL

2.1 System suitability

Samples: System suitability solution, Solvent blank solution, and Standard solution

[NOTE-The relative retention times for anthralin, danthron, dianthrone, and o-nitroaniline are 1.0, 1.2, 1.7, and 2.3, respectively.]

Suitability requirements

Resolution: NLT 1.3 between anthralin and danthron, System suitability solution

Tailing factor: NMT 1.5, System suitability solution

Relative standard deviation: NMT 2.0% of the ratio of the peak responses, Standard solution

Interference: No discernible signal is observed at the retention time of anthralin, Solvent blank solution

2.2 Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of anthralin (C₁₄H₁₀O₃) in the portion of Ointment taken:

Result = (R_u /R_s ) × (C_s /C_u ) × 100

R_u = peak response ratio of anthralin to o-nitroaniline from the Sample solution

R_s = peak response ratio of anthralin to o-nitroaniline from the Standard solution

C_s = concentration of USP Anthralin RS in the Standard solution (µg/mL)

C_u = nominal concentration of anthralin in the Sample solution (µg/mL)

Acceptance criteria: 90.0%–115.0% for Ointment labeled to contain more than 0.1% of anthralin; 90.0%–130.0% for Ointment labeled to contain 0.1% or less of anthralin

3 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers, in a cool place. Protect from light.

USP Reference Standards {11}

USP Anthralin RS