Pyrimethamine Tablets

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

Pyrimethamine Tablets contain NLT 93.0% and NMT 107.0% of the labeled amount of pyrimethamine (C₁₂H₁₃ClN₄).

2 IDENTIFICATION

A. The UV absorption spectrum of the Sample solution exhibits maxima at the same wavelengths as that of a similar solution of USP Pyrimethamine RS, as obtained in the Assay.

B. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K

Sample: Nominally equivalent to about 250 mg of pyrimethamine from powdered Tablets

Analysis: Add 25 mL of acetone to the Sample, boil for 2 min, and filter through a sintered-glass crucible. Repeat this treatment three times with 25-mL portions of acetone. Evaporate the combined filtrates carefully on a steam bath with the aid of a current of air to dryness.

Acceptance criteria: The residue meets the requirements in 197K, and melts at 237°–242° (see Melting Range or Temperature 〈741〉).

3 ASSAY

Procedure

Standard stock solution: Prepare as directed for the Standard preparation in Salts of Organic Nitrogenous Bases 〈501〉.

Standard solution: Transfer 5.0 mL of the Standard stock solution to a 200-mL volumetric flask and dilute with 0.5 N sulfuric acid to volume.

Sample stock solution: Prepare as directed for the Assay preparation in Salts of Organic Nitrogenous Bases 〈501〉.

Sample solution: Transfer 5.0 mL of the Sample stock solution to a 200-mL volumetric flask and dilute with 0.5 N sulfuric acid to volume.

Instrumental conditions

Mode: UV

Analytical wavelength: Maximum absorbance at about 273 nm

Cell: 1 cm

Blank: 0.5 N sulfuric acid

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of pyrimethamine (C₁₂H₁₃ClN₄) in the portion of Tablets taken:

Result = (Aᵤ / Aₛ) × (Cₛ / Cᵤ) × 100

Aᵤ = absorbance of the Sample solution

Aₛ = absorbance of the Standard solution

Cₛ = concentration of USP Pyrimethamine RS in the Standard solution (mg/mL)

Cᵤ = nominal concentration of pyrimethamine in the Sample solution (mg/mL)

Acceptance criteria: 93.0%–107.0%

4 PERFORMANCE TESTS

Dissolution 〈711〉

Medium: 0.1 N hydrochloric acid; 900 mL

Apparatus 2: 50 rpm

Time: 45 min

Standard solution: A known concentration of USP Pyrimethamine RS in Medium

Sample solution: Filter portions of the solution under test, and suitably dilute with Medium if necessary.

Instrumental conditions

Mode: UV

Analytical wavelength: Maximum absorbance at about 273 nm

Analysis

Samples: Standard solution and Sample solution

Calculate the labeled amount of pyrimethamine (C₁₂H₁₃ClN₄) dissolved.

Tolerances: NLT 75% (Q) of the labeled amount of pyrimethamine (C₁₂H₁₃ClN₄) is dissolved.

Uniformity of Dosage Units 〈905〉

Procedure for content uniformity

Standard solution: 10 µg/mL of USP Pyrimethamine RS in 0.1 N hydrochloric acid

Sample solution: Nominally 10 µg/mL of pyrimethamine, prepared as follows. Transfer 1 Tablet to a 100-mL volumetric flask, add 25 mL of 0.1 N hydrochloric acid, warm the flask on a steam bath for 5 min, cool, dilute with 0.1 N hydrochloric acid to volume, mix, and filter, discarding the first few milliliters of the filtrate. Pipet a portion of the clear filtrate, equivalent to about 2.5 mg of pyrimethamine, into a 250-mL volumetric flask, and dilute with 0.1 N hydrochloric acid to volume.

Instrumental conditions

Mode: UV

Analytical wavelength: Maximum absorbance at about 273 nm

Cell: 1 cm

Blank: 0.1 N hydrochloric acid

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of pyrimethamine (C₁₂H₁₃ClN₄) in the Tablet taken:

Result = (Aᵤ / Aₛ) × (Cₛ / Cᵤ) × 100

Aᵤ = absorbance of the Sample solution

Aₛ = absorbance of the Standard solution

Cₛ = concentration of USP Pyrimethamine RS in the Standard solution (µg/mL)

Cᵤ = nominal concentration of pyrimethamine in the Sample solution (µg/mL)

Acceptance criteria: Meet the requirements

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight, light-resistant containers.

USP Reference Standards 〈11〉

USP Pyrimethamine RS