Sulfamethoxazole and Trimethoprim 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

Sulfamethoxazole and Trimethoprim Tablets contain NLT 93.0% and NMT 107.0% of the labeled amounts of sulfamethoxazole (C₁₀H₁₁N₃O₃S) and trimethoprim (C₁₄H₁₈N₄O₃).

2 IDENTIFICATION

A.

Standard solution A: 0.4 mg/mL of USP Trimethoprim RS in methanol

Standard solution B: 2 mg/mL of USP Sulfamethoxazole RS in methanol

Sample solution: Transfer an amount of finely ground Tablets, equivalent to 4 mg of trimethoprim, to a 10-mL volumetric flask, add 8 mL of methanol, and warm for several min on a steam bath with frequent shaking. Cool, dilute with methanol to volume, and centrifuge briefry.

Chromatographic system

Mode: TLC

Application volume: 5 μL

Developing solvent system: Chloroform, isopropyl alcohol, and diethylamine (6:5:1)

Analysis

Samples: Standard solution A, Standard solution B, and Sample solution

Apply the Samples to separate points about 3 cm from one end of the plate. Dry the spots in a current of warm air, and develop the plate with Developing solvent system in a chamber that is lined with filter paper. Remove the plate, dry, and examine under short-wavelength UV light.

Acceptance criteria: The trimethoprim and sulfamethoxazole spots from the solution under test have the same R values as the spots from the corresponding Standard solutions.

3 ASSAY

Procedure

Mobile phase: Mix 1400 mL of water, 400 mL of acetonitrile, and 2.0 mL of triethylamine in a 2000-mL volumetric flask. Allow to equilibrate to room temperature, and adjust with 0.2 N sodium hydroxide or dilute glacial acetic acid (1 in 100) to a pH of 5.9 ± 0.1. Dilute with water to volume, and pass through a filter of 0.45-μm pore size.

Standard stock solution: 0.32 mg/mL of USP Trimethoprim RS and 0.32J mg/mL of USP Sulfamethoxazole RS in methanol, where J is the ratio of the labeled amount, in mg, of sulfamethoxazole to the labeled amount, in mg, of trimethoprim in the dosage form

Standard solution: 0.032 mg/mL of USP Trimethoprim RS per mL and 0.032J mg/mL of USP Sulfamethoxazole RS per mL in Mobile phase from Standard stock solution

Sample stock solution: Transfer from finely powdered Tablets (NLT 20), an equivalent to 160 mg of sulfamethoxazole, to a 100-mL volumetric flask. Add 50 mL of methanol and sonicate, with intermittent shaking, for 5 min. Allow to equilibrate to room temperature, dilute with methanol to volume, and filter. Use the filtrate in the preparation of the Sample solution.

Sample solution: Nominally 0.16 mg/mL of sulfamethoxazole in Mobile phase from Sample stock solution

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 254 nm

Column: 3.9-mm × 30-cm; packing L1

Flow rate: 2 mL/min

Injection volume: 20 μL

System suitability

Sample: Standard solution

[Note—The relative retention times for trimethoprim and sulfamethoxazole are 1.0 and 1.8, respectively.]

Suitability requirements

Resolution: NLT 5.0 between sulfamethoxazole and trimethoprim

Tailing factor: NMT 2.0 for sulfamethoxazole and trimethoprim

Relative standard deviation: NMT 2.0% for replicate injections

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of trimethoprim (C₁₄H₁₈N₄O₃) and sulfamethoxazole (C₁₀H₁₁N₃O₃S) in the portion of

Tablets taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response of trimethoprim or sulfamethoxazole from the Sample solution

r_S = peak response of trimethoprim or sulfamethoxazole from the Standard solution

C_S = concentration of the USP Trimethoprim RS or USP Sulfamethoxazole RS in the Standard solution (mg/mL)

C_U = nominal concentration of trimethoprim or sulfamethoxazole in the Sample solution (mg/mL)

Acceptance criteria: 93.0%–107.0% each of sulfamethoxazole and trimethoprim 

4 PERFORMANCE TESTS

Dissolution 〈711〉

Medium: 0.1 N hydrochloric acid; 900 mL

Apparatus 2: 75 rpm

Time: 60 min

Standard solution: USP Sulfamethoxazole RS and USP Trimethoprim RS at a known concentration

Sample solution: Pass a portion of the solution under test through a suitable filter.

Mobile phase, Chromatographic system, and System suitability: Proceed as directed in the Assay.

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amounts of sulfamethoxazole and trimethoprim dissolved by comparison of the peak responses obtained from a filtered aliquot of the solution under test with the peak responses from the corresponding component obtained from the Standard solution.

Calculate the percentage of each active component dissolved

Tolerances: NLT 70% (Q) of the labeled amount of sulfamethoxazole  and trimethoprim  is dissolved.

Uniformity of Dosage Units 〈905〉: Meet the requirements

5 ADDITIONAL REQUIREMENTS

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

USP Reference Standards 〈11〉

USP Sulfamethoxazole RS

USP Trimethoprim RS