Levodopa

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

Levodopa contains NLT 98.0% and NMT 102.0% of levodopa (C₉H₁₁NO₄), calculated on the dried basis.

2 IDENTIFICATION

Change to read:

A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197M (CN 1-May-2020)

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

Protect all solutions from light, and maintain them at 10° until they are injected into the chromatograph.

Diluent: 0.1% trifluoroacetic acid in water

Mobile phase: Tetrahydrofuran and Diluent (3:97)

System suitability solution: 10 μg/mL each of USP Levodopa RS, USP Levodopa Related Compound B RS, and USP l-Tyrosine RS in Diluent

Standard solution: 0.4 mg/mL of USP Levodopa RS in Diluent

Sample solution: 0.4 mg/mL of Levodopa in Diluent

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 280 nm

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

Flow rate: 1 mL/min

Injection volume: 20 μL

System suitability

Sample: System suitability solution

[Note—Refer to Table 1 in the test for Organic Impurities for the relative retention times.]

Suitability requirements

Resolution: NLT 3.0 between levodopa and l-tyrosine

Tailing factor: NMT 2.0 for levodopa

Relative standard deviation: NMT 2.0% for levodopa

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of levodopa (C₉H₁₁NO₄) in the portion of the sample taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak response of the Sample solution

r_S = peak response of the Standard solution

C_S = concentration of USP Levodopa RS in the Standard solution (mg/mL)

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

Acceptance criteria: 98.0%–102.0% on the dried basis

4 IMPURITIES

Residue on Ignition 〈281〉: NMT 0.1%

Change to read:

Organic Impurities

Protect all solutions from light, and maintain them at 10° until they are injected into the chromatograph.

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

Analysis

Samples: Standard solution and Sample solution

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

Result = (r_U/r_S) × (C_S/C_U) × (1/F) × 100

r_U = peak area for any impurity in the Sample solution

r_S = peak area for Levodopa in the Standard solution

C_S = concentration of USP Levodopa RS in the Standard solution (mg/mL)

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

F = relative response factor of each impurity (see Table 1)

Acceptance criteria: See Table 1.

Table 1

^a 3-(3,4,6-Trihydroxyphenyl)alanine▲; also known as 3-(2,4,5-Trihydroxyphenyl)-l-alanine.▲ (ERR 1-Nov-2018)

^b 3-(3,4-Dimethoxyphenyl)-l-alanine.

5 SPECIFIC TESTS

Optical Rotation, Specific Rotation 〈781S〉

Sample solution: 500 mg of Levodopa in a 25-mL volumetric flask. Add 10 mL of 1 N hydrochloric acid to dissolve the solid, add 5 g of methenamine, swirl the contents to dissolve the methenamine, and add 1 N hydrochloric acid to volume.

Analysis: Allow the Sample solution to stand in the dark at 25° for 3 h, and measure the rotation.

Acceptance criteria: −160° to −167°

Loss on Drying 〈731〉

Analysis: Dry at 105° for 4 h.

Acceptance criteria: NMT 0.5%

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight, light-resistant containers, store in a dry place, and prevent exposure to excessive heat.

USP Reference Standards 〈11〉

USP Levodopa RS

USP Levodopa Related Compound B RS

3-Methoxytyrosine.

C₁₀H₁₃NO₄ 211.22

USP l-Tyrosine RS