Fenbendazole

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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C₁₅H₁₃N₃O₂S 299.35

Carbamic acid, [5-(phenylthio)-1H-benzimidazol-2-yl]-, methyl ester;

Methyl 5-(phenylthio)-2-benzimidazolecarbamate CAS RN^®: 43210-67-9; UNII: 621BVT9M36.

1 DEFINITION

Fenbendazole contains NLT 98.0% and NMT 101.0% of fenbendazole (C₁₅H₁₃N₃O₂S), calculated on the dried basis.

2 IDENTIFICATION

Change to read:

A. SPECTROSCOPIC IDENTIFICATION TESTS (197), Infrared Spectroscopy: 1974, 197K, or 197M (CN 1-May-2020)

3 ASSAY

3.1 PROCEDURE

Sample: 200 mg

Analysis: Dissolve the Sample in 30 mL of glacial acetic acid, warming if to effect solution. Allow to cool, and titrate with 0.1 N perchloric acid VS, determining the endpoint potentiometrically. Each mL of 0.1 N perchloric acid is equivalent to 29.94 mg of fenbendazole (C₁₅H₁₃N₃O₂S).

Acceptance criteria: NLT 98.0%-101.0% on the dried basis

4 IMPURITIES

4.1 RESIDUE ON IGNITION (281)

NMT 0.3%

4.2 ORGANIC IMPURITIES

Solution A: Methanol, acetic acid, and water ( 30:1:70)

Solution B: Methanol, acetic acid, and water (70:1:30)

Mobile phase: See Table 1.

Table 1

Diluent: Methanol and hydrochloric acid (99:1)

System suitability stock solution: 0.1 mg/mL each of USP Fenbendazole RS and USP Mebendazole RS in methanol

System suitability solution: 0.01 mg/mL each of USP Fenbendazole RS and USP Mebendazole RS from the System suitability stock solution in Diluent

Standard stock solution A: 0.025 mg/mL of USP Fenbendazole RS prepared as follows. Dissolve a weighed quantity of USP Fenbendazole RS in Diluent to obtain a solution with a known concentration of 5 mg/mL. Dilute 1.0 mL of this solution with methanol to 200.0 mL.

Standard solution A: 0.0125 mg/mL of USP Fenbendazole RS from Standard stock solution A in Diluent

Standard stock solution B: 0.1 mg/mL of USP Fenbendazole Related Compound A RS in methanol

Standard solution B: 0.01 mg/mL of USP Fenbendazole Related Compound A RS from Standard stock solution B in Diluent

Standard stock solution C: 0.1 mg/mL of USP Fenbendazole Related Compound B RS in methanol

Standard solution C: 0.01 mg/mL of USP Fenbendazole Related Compound B RS from Standard stock solution C in Diluent

Sample solution: Dissolve 5.0 mg/mL of Fenbendazole in Diluent.

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: UV 280 nm

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

Flow rate: 1 mL/min

Injection volume: 10 µL

System suitability

Sample: System suitability solution

[NOTE-The relative retention times for mebendazole and fenbendazole are about 0.85 and 1.0, respectively.]

Suitability requirements

Resolution: NLT 1.5 between mebendazole and fenbendazole

Analysis

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

Calculate the percentages of fenbendazole related compound A and fenbendazole related compound B in the portion of Fenbendazole taken:

Result = (r_U/r_S) x (C_S/C_U) x 100

r_U = peak response of each related compound from the Sample solution

r_S = peak response of the corresponding related compound from Standard solution B or Standard solution C

C_S = concentration of the corresponding related compound in Standard solution B or Standard solution C (mg/mL)

C_U = concentration of Fenbendazole in the Sample solution (mg/mL)

Calculate the percentage of any other impurity in the portion of Fenbendazole taken:

Result = (r_U/r_S) x (C_S/C_U) x 100

r_U = peak response of any impurity from the Sample solution

r_S = peak response of USP Fenbendazole RS from Standard solution A

C_S = concentration of USP Fenbendazole RS in Standard solution A (mg/mL)

C_U = concentration of Fenbendazole in the Sample solution (mg/mL)

Acceptance criteria: See Table 2. The reporting level for impurities is 0.1%.

Table 2

5 SPECIFIC TESTS

LOSS ON DRYING (731)

Analysis: Dry at 100°-105° for 3 h.

Acceptance criteria: NMT 1.0%

6 ADDITIONAL REQUIREMENTS

6.1 PACKAGING AND STORAGE

Preserve in well-closed, light-resistant containers, and store at room temperature.

6.2 LABELING

Label to indicate that it is for veterinary use only.

6.3 USP REFERENCE STANDARDS (11).\

USP Fenbendazole RS

USP Fenbendazole Related Compound A RS

Methyl (1H-benzimidazole-2-yl)carbamate.

C₉H₉N₃O₂ 191.19

USP Fenbendazole Related Compound B RS

Methyl [5(6)-chlorobenzimidazole-2-yl]carbamate.

C₉H₈ClN₃O₂ 225.63

USP Mebendazole RS