Pyrimethamine

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 contains ▲NLT 98.0% and NMT 102.0%▲ (USP 1-May-2024) of pyrimethamine (C₁₂H₁₃ClN₄) calculated on the dried basis.

2 IDENTIFICATION

Change to read:

A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: ▲197A or▲ (USP 1-May-2024) 197K

B. Identification Tests—General 〈191〉, Chemical Identification Tests, Chloride

Sample solution: Mix about 100 mg of Pyrimethamine with 500 mg of anhydrous sodium carbonate, and ignite the mixture. Cool, add 5 mL of hot water, heat for 5 min on a steam bath, filter, and neutralize the filtrate with nitric acid.

Acceptance criteria: Meets the requirements

C. 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

Change to read:

Procedure

Solution A: Add 1 mL of phosphoric acid to water and dilute with water to 1000 mL.

Mobile phase: Acetonitrile and Solution A (17:83)

Standard solution: 0.02 mg/mL of USP Pyrimethamine RS in Solution A

Sample solution: 0.02 mg/mL of Pyrimethamine in Solution A

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 210 nm

Column: 2.0-mm × 10-cm; 3-µm packing L11

Flow rate: 0.3 mL/min

Injection volume: 5 µL

System suitability

Sample: Standard solution

Suitability requirements

Tailing factor: NMT 1.8

Relative standard deviation: NMT ▲0.73%▲ (USP 1-May-2024)

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of pyrimethamine (C₁₂H₁₃ClN₄) in the portion of Pyrimethamine taken:

Result = (rᵤ / rₛ) × (Cₛ / Cᵤ) × 100

rᵤ = peak response of pyrimethamine from the Sample solution

rₛ = peak response of pyrimethamine from the Standard solution

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

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

Acceptance criteria: ▲98.0%–102.0%▲ (USP 1-May-2024) on the dried basis

4 IMPURITIES

Residue on Ignition 〈281〉: NMT 0.1%

Delete the following:

▲Ordinary impurities 〈466〉▲ (USP 1-May-2024)

Add the following:

Organic Impurities

Solution A: 2.72 g/L of potassium phosphate monobasic in water. Adjust with 25% ammonium hydroxide to a pH of 8.0.

Solution B: Acetonitrile

Mobile phase: See Table 1.

Diluent: Solution A and Solution B (50:50)

Sensitivity solution: 0.0005 mg/mL of USP Pyrimethamine RS in Diluent

Standard solution: 0.001 mg/mL of USP Pyrimethamine RS in Diluent. Sonicate to dissolve.

Sample solution: 1 mg/mL of Pyrimethamine in Diluent. Sonicate to dissolve.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 210 nm

Column: 4.6-mm × 25-cm; 5-µm packing L1

Column temperature: 30°

Flow rate: 1 mL/min

Injection volume: 10 µL

System suitability

Samples: Sensitivity solution and Standard solution

[Note—The relative retention times in Table 2 are provided as information that could aid in peak assignment.]

Table 2

ᵃ 2-(4-Chlorophenyl)-3-oxopentanenitrile.

ᵇ 5-(4-Chlorophenyl)-6-methylpyrimidine-2,4-diamine.

ᶜ 2-(4-Chlorophenyl)acetonitrile.

ᵈ 2-(4-Chlorophenyl)-2-(2-ethyl-1,3-dioxolan-2-yl)acetonitrile.

Suitability requirements

Relative standard deviation: NMT 5.0%, Standard solution

Signal-to-noise ratio: NLT 10, Sensitivity solution

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of each impurity in the portion of Pyrimethamine taken:

Result = (rᵤ / rₛ) × (Cₛ / Cᵤ) × (1/F) × 100

rᵤ = peak response of each impurity from the Sample solution

rₛ = peak response of pyrimethamine from the Standard solution

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

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

F = relative response factor (see Table 3)

Acceptance criteria: See Table 3. The reporting threshold is 0.05%.

Table 3

5 SPECIFIC TESTS

Delete the following:

▲Melting Range or Temperature 〈741〉▲ (USP 1-May-2024)

Loss on Drying 〈731〉

Analysis: Dry it at 105° for 4 h.

Acceptance criteria: NMT 0.5%

6 ADDITIONAL REQUIREMENTS

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

USP Reference Standards 〈11〉

USP Pyrimethamine RS