Minoxidil Topical Solution

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

Minoxidil Topical Solution contains NLT 90.0% and NMT 110.0% of the labeled amount of minoxidil (C₉H₁₅N₅O).

2 IDENTIFICATION

Change to read:

A. SPECTROSCOPIC IDENTIFICATION TESTS (197), Infrared Spectroscopy: 197M _{(CN 1-MAY-2020)}

Sample: Evaporate 1 mL of the Topical Solution under a stream of nitrogen while heating at 50^o.

Acceptance criteria: Meets the requirements

B. The retention time of the major peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

3 ASSAY

PROCEDURE

Solution A: Add 0.65 mL of heptafluorobutyric acid to a 1000-ml volumetric flask. Dilute with water to volume.

Solution B: Acetonitrile

Mobile phase: See Table 1.

Table 1

Diluent: Methanol and water (50:50)

System suitability solution: 0.4 mg/mL of USP Minoxidil RS and 0.001 mg/mL of USP Minoxidil Related Compound E RS in Diluent

Standard solution: 0.05 mg/mL of USP Minoxidil RS in Diluent

Sample solution: Nominally 0.05 mg/mL of minoxidil in Diluent

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 280 nm

Column: 2.1-mm × 10-cm; 1.7-µm packing L1

Column temperature: 35°

Flow rate: 0.4 mL/min

Injection volume: 1 µL

System suitability

Samples: System suitability solution and Standard solution

Suitability requirements

Resolution: NLT 1.5 between minoxidil and minoxidil related compound E, System suitability solution

Relative standard deviation: NMT 1.0%, Standard solution

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of minoxidil (C₉H₁₅N₅O) in the portion of Topical Solution taken:

Result = (r_u /r_s ) × (C_s /C_u ) × 100

r_u = peak response from the Sample solution

r_s = peak response from the Standard solution

C_s = concentration of USP Minoxidil RS in the Standard solution (mg/mL)

C_u = nominal concentration of minoxidil in the Sample solution (mg/mL)

Acceptance criteria: 90.0%–110.0%

4 IMPURITIES

Organic Impurities

Solution A, Solution B, Mobile phase, Diluent, System suitability solution, and Chromatographic system: Proceed as directed in the Assay.

Standard solution: 0.4 µg/mL of USP Minoxidil RS in Diluent

Sample solution: Nominally 0.4 mg/mL of minoxidil in Diluent. Pass through a suitable lter of 0.2-µm pore size.

System suitability

Samples: System suitability solution and Standard solution

Suitability requirements

Resolution: NLT 1.5 between minoxidil and minoxidil related compound E, System suitability solution

Relative standard deviation: NMT 2.8%, Standard solution

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of each unspecied impurity in the portion of Topical Solution taken:

Result = (r_u /r_s ) × (C_s /C_u ) × 100

r_u = peak response of each unspecied impurity from the Sample solution

r_s = peak response of minoxidil from the Standard solution

C_s = concentration of USP Minoxidil RS in the Standard solution (mg/mL)

C_u = nominal concentration of minoxidil in the Sample solution (mg/mL)

Acceptance criteria: See Table 2. Disregard any impurity peaks less than 0.05%.

Table 2

^a 2,6-Diamino-4-chloropyrimidine 1-oxide.

^b Process impurity included in the table for identication only. Process impurities are controlled in the drug substance, and are not to be reported or included in the total impurities for the drug product.

^c 6-Chloropyrimidine-2,4-diamine.

^d 6-(Piperidin-1-yl)pyrimidine-2,4-diamine.

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

USP Reference Standards 〈11〉

USP Minoxidil RS

USP Minoxidil Related Compound E RS

6-(Piperidin-1-yl)pyrimidine-2,4-diamine.

C₉H₁₅N₅ 193.25