Erythromycin Ophthalmic Ointment

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

Erythromycin Ophthalmic Ointment is a sterile preparation of Erythromycin in a suitable ointment base. It contains NLT 90.0% and NMT 120.0% of the labeled amount of erythromycin (C₃₇H₆₇NO₁₃)

2 IDENTIFICATION

A. THIN-LAYER CHROMATOGRAPHY

Standard solution: 2.5 mg/mL, of USP Erythromycin RS in methanol

Sample solution: 2.5 mg/ml of erythromycin from Ophthalmic Ointment in methanol prepared as follows. Transfer an amount of Ophthalmic Ointment containing nominally 5 mg of erythromycin to a separator containing 50 mL of solvent hexane. Shake until dissolved. Extract with three separate 20-mL portions of methanol. Combine the methanol extracts in a beaker, and evaporate to dryness. Dissolve the residue in 2 mL of methanol.

Chromatographic system

(See Chromatography (621). Thin-Layer Chromatography.)

Adsorbent: 0.25-mm layer of chromatographic silica gel

Application volume: 10 µL

Developing solvent system: Methanol and chloroform (85:15)

Spray reagent: Alcohol, p-methoxybenzaldehyde, and sulfuric acid (90:5:5)

Analysis

Samples: Standard solution and Sample solution

Apply the Standard solution and the Sample solution to the plate. Place the plate in an unlined chromatographic chamber, and develop the chromatogram using the Developing solvent system until the solvent front has moved about 7 cm. Remove the plate from the chamber, mark the solvent front, and allow the solvent to evaporate. Spray the plate with Spray reagent. Heat the plate at 100° for 10 min, and examine the chromatogram, in which erythromycin appears as a black-to-purple spot.

Acceptance criteria: The R_F value of the principal spot of the Sample solution corresponds to that of the Standard solution.

3 ASSAY

PROCEDURE

Solution A: Acetonitrile and water (90:10). Store in a reservoir protected from air by sparging with helium.

Solution B: 0.04 mg/mL of sodium hydroxide in water

Mobile phase: Solution A and Solution B (56:44)

Diluent: Methanol and water (50:50)

Standard solution 1: 0.66 mg/mL of USP Erythromycin RS in Diluent

Standard solution 2: 0.034 mg/mL of USP Erythromycin B RS and USP Erythromycin C RS in Diluent

System suitability solution: Transfer 2 mg of USP Erythromycin Related Compound N. RS to a 10-mL volumetric flask, add 0.4 mL of Standard solution 1 and 6 mL of Standard solution 2, and mix. Dilute with Standard solution 2 to volume.

Sample solution: Nominally 0.6 mg/mL of erythromycin from Ophthalmic Ointment in Diluent prepared as follows. Transfer an amount of Ophthalmic Ointment containing nominally 60 mg of erythromycin to a 125-ml separator. Add 50 mL of solvent hexane, and shake until dissolved. Extract with four separate 20-mL portions of Diluent, collecting the extracts in a 100-ml volumetric flask. Dilute the combined extracts with Diluent to volume, and pass a portion of the solution through a suitable filter. Use the clear filtrate.

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: Pulsed amperometric electrochemical detector

Electrode: Glassy carbon

Waveform: See Table 1.

Table 1

Columns

Guard: 4-mm × 5-cm; 8-µm packing L50

Analytical: 4-mm × 25-cm; 8-µm packing L50

Flow rate: 1 mL/min

Injection volume: 10 µL

System suitability

Samples: Standard solution 1 and System suitability solution

[Note—For relative retention times, see Table 2.]

Table 2

Suitability requirements

Resolution: NLT 0.6 between erythromycin related compound N and erythromycin C; NLT 2.5 between erythromycin C and erythromycin A; NLT 2.5 between erythromycin A and erythromycin B, System suitability solution

Tailing factor: NMT 2, Standard solution 1

Relative standard deviation: NMT 3%, Standard solution 1

Analysis

Samples: Standard solution 1, Standard solution 2, and Sample solution

Calculate the percentage of erythromycin A relative to the labeled amount of erythromycin in the portion of Ophthalmic Ointment taken:

Result = (r_u /r_s ) × (C_s /C_u ) × P × F × 100

r_u = peak area of erythromycin A from the Sample solution

r_s = peak area of erythromycin A from Standard solution 1

C_s = concentration of USP Erythromycin RS in Standard solution 1 (mg/mL)

C_u = nominal concentration of erythromycin in the Sample solution (mg/mL)

P = potency of erythromycin A in USP Erythromycin RS (µg/mg)

F = conversion factor, 0.001 mg/µg

Calculate the percentage of erythromycin B relative to the labeled amount of erythromycin in the portion of Ophthalmic Ointment taken:

Result = (r_u /r_s ) × (C_s /C_u ) × P × 100

r_u = peak area of erythromycin B from the Sample solution

r_s = peak area of erythromycin B from Standard solution 2

C_s = concentration of USP Erythromycin B RS in Standard solution 2 (mg/mL)

C_u = nominal concentration of erythromycin in the Sample solution (mg/mL)

P = potency of erythromycin B in USP Erythromycin B RS (mg/mg)

Calculate the percentage of erythromycin C relative to the labeled amount of erythromycin in the portion of Ophthalmic Ointment taken:

Result = (r_u /r_s ) × (C_s /C_u ) × P × 100

r_u = peak area of erythromycin C from the Sample solution

r_s = peak area of erythromycin C from Standard solution 2

C_s = concentration of USP Erythromycin C RS in Standard solution 2 (mg/mL)

C_u = nominal concentration of erythromycin in the Sample solution (mg/mL)

P = potency of erythromycin C in USP Erythromycin C RS (mg/mg)

Calculate the percentage of the labeled amount of erythromycin in the Ophthalmic Ointment by adding the percentages of erythromycin A, erythromycin B, and erythromycin C.

Acceptance criteria: 90.0%–120.0%

4 SPECIFIC TESTS

Sterility Tests 〈71〉: It meets the requirements.

Other Requirements: It meets the requirements for Particulate and Foreign Matter and Container Contents in Ophthalmic Products—Quality Tests 〈771〉, Drug Product Quality, Universal Tests, Particulate and Foreign Matter and Container Contents.

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in collapsible ophthalmic ointment tubes. Store at controlled room temperature.

USP Reference Standards 〈11〉

USP Erythromycin RS

USP Erythromycin B RS

USP Erythromycin C RS

USP Erythromycin Related Compound N RS