Iohexol

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

Iohexol contains NLT 98.0% and NMT 102.0% of iohexol (C₁₉H₂₆I₃N₃O₉), calculated on the anhydrous basis.

2 IDENTIFICATION

A. SPECTROSCOPIC IDENTIFICATION TESTS 〈197〉, Infrared Spectroscopy: 197K

B. The retention times of the two principal peaks of the Sample solution correspond to those of the System suitability solution, as obtained in the test for Organic Impurities.

3 ASSAY

3.1 PROCEDURE

Sample: 500 mg of Iohexol

Sample solution: Transfer the Sample to a glass-stoppered, 125-mL conical flask. Add 25 mL of 1.25 N sodium hydroxide and 500 mg of powdered zinc. Connect the flask to a reflux condenser, and reflux for 1 h. Cool the flask to room temperature, rinse the condenser with 20 mL of water, disconnect the flask from the condenser, and pass the mixture through a filter. Rinse the flask and the filter thoroughly with small portions of water, adding the rinsings to the filtrate. Add 5 mL of glacial acetic acid.

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Direct titration

Titrant: 0.1 N silver nitrate VS

Endpoint detection: Potentiometric

Analysis: Titrate the Sample solution with Titrant.

Calculate the percentage of iohexol (C₁₉H₂₆I₃N₃O₉) in the portion of Iohexol taken:

Result = [(V × N × F)/W] × 100

V = Titrant volume consumed by the Sample (mL)

N = Titrant normality (mEq/mL)

F = equivalent weight of iohexol, 273.7 mg/mEq

W = Sample weight (mg)

Acceptance criteria: 98.0%-102.0% on the anhydrous basis

4 IMPURITIES

4.1 LIMIT OF IONIC COMPOUNDS

Rinse all glassware five times with distilled water.

Standard solution: 0.002 mg/mL of sodium chloride in water

Sample solution: 1 g of Iohexol in 50 mL of water

Analysis

Samples: Standard solution and Sample solution

Acceptance criteria: The specific conductance of the Sample solution is NMT that of the Standard solution (equivalent to 0.01% ionic compounds as sodium chloride).

Change to read:

4.2 LIMIT OF FREE IODIDE

Sample: 5 g of Iohexol

Sample solution: Dissolve the Sample in 20 mL of water.

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Direct titration

Titrant: 0.001 N silver nitrate (USP 1-Dec-2021)

Endpoint detection: Potentiometric

Analysis: Calculate the percentage of free iodide in the portion of Iohexol taken:

Result = [(V × N × F)/W] × 100

V = Titrant volume consumed by the Sample (mL)

N = Titrant normality (mEq/mL)

F = equivalent weight of iodide, 126.9 mg/mEq

W = Sample weight (mg)

Acceptance criteria: NMT 0.001%

4.3 ORGANIC IMPURITIES

Solution A: Acetonitrile

Solution B: Water

Mobile phase: See Table 1.

Table 1

System suitability solution: 1.5 mg/mL of USP Iohexol RS and 0.0075 mg/mL of USP Iohexol Related Compound A RS in water

Sample solution: 1.5 mg/mL of Iohexol

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 254 nm

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

Flow rate: 1 mL/min

Injection volume: 10 µL

System suitability

Sample: System suitability solution

[Note-Iohexol may give two nonresolved peaks due to exo–endo isomerism. In addition, a small peak due to iohexol usually appears at the leading edge of the first principal peak. This small peak has a retention time about 1.2 min less than the first principal peak. The relative retention times for the iohexol related compound A, iohexol endo-isomer, iohexol exo-isomer, and O-alkylated compounds peaks are 0.85, 0.96, 1.0, and 1.1-1.4, respectively.]

Suitability requirements

Resolution: NLT 5.0 between iohexol related compound A and the exo-isomer (the second and greater peak) of iohexol

Analysis

Sample: Sample solution

Calculate the percentage of O-alkylated compounds and any other individual impurity in the portion of Iohexol taken. Exclude peaks with a relative retention time between 0.84 [relative to the endo-isomer of iohexol (first main peak)] and that of the endo-isomer of iohexol. Disregard any peak less than or equal to 0.03% of the principal peaks.

Result = (r_U/r_T) × 100

r_U = peak response of each impurity

r_T = sum of all of the peak responses

Acceptance criteria

O-alkylated compounds: NMT 0.6%

Any individual impurity: NMT 0.1%

Total impurities excluding O-alkylated compounds: NMT 0.3%

Change to read:

4.4 LIMIT OF 2-METHOXYETHANOL

Internal standard solution: 10 µg/mL of secondary butyl alcohol in water

System suitability solution: 5 µg/mL of methanol and 10 µg/mL each of isopropyl alcohol and 2-methoxyethanol in Internal standard solution

Standard stock solution: 10 µg/mL of 2-methoxyethanol in Internal standard solution

Standard solution: Transfer about 0.25 g of USP Iohexol RS and 1.0 mL of Standard stock solution to a headspace vial, and seal the vial with a septum and crimp cap.

Sample solution: Transfer about 0.25 g of Iohexol and 1.0 mL of Internal standard solution to a headspace vial, and seal the vial with a septum and crimp cap.

Blank solution: Transfer about 0.25 g of USP Iohexol RS and 1.0 mL of Internal standard solution to a headspace vial, and seal the vial with a septum and crimp cap.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: GC with suitable headspace autosampler

Detector: Flame ionization

Column: 0.53-mm × 30-m fused-silica; coated with a 1-µm phase G16

Temperatures

Autosampler: 105°

Needle: 130°-140°

Injection port: 150°

Detector: 200°

Column: See Table 2.

Table 2

Carrier gas: Helium

Flow rate: 11 mL/min

Injection volume: 1 mL of the headspace

System suitability

Sample: System suitability solution (USP 1-Dec-2021)

[Note-The typical relative retention times for methanol, isopropyl alcohol, secondary butyl alcohol, and 2-methoxyethanol are 0.5, 0.6, 1.0, and 1.9, respectively.]

Suitability requirements

Resolution: NLT 1.0 between methanol and isopropyl alcohol

Relative standard deviation: NMT 10.0% for the ratio of 2-methoxyethanol to secondary butyl alcohol

Analysis

Samples: Standard solution, Sample solution, and Blank solution (USP 1-Dec-2021)

Calculate the amount of 2-methoxyethanol in the portion of Iohexol taken:

Result = [R_U/(R_S - R_B)] _{(USP 1-Dec-2021)} × (C_S/C_U)

R_U = peak response ratio of 2-methoxyethanol to secondary butyl alcohol _{(USP 1-Dec-2021)} from the Sample solution

R_S = peak response ratio of 2-methoxyethanol to secondary butyl alcohol _{(USP 1-Dec-2021)} from the Standard solution

R_B = peak response ratio of 2-methoxyethanol to secondary butyl alcohol _{(USP 1-Dec-2021)} from the Blank solution

C_S = concentration of 2-methoxyethanol in the Standard solution (µg/mL)

C_U = concentration of Iohexol in the Sample solution (g/mL)

Acceptance criteria: NMT 20 µg/g of 2-methoxyethanol

Change to read:

4.5 LIMIT OF 3-CHLOROPROPANE-1,2-DIOL

Standard solution: 0.025 mg/mL of 3-chloropropane-1,2-diol in ethyl acetate

Sample solution: Transfer 1 g of Iohexol to a separator. Dissolve in 1 mL of water. Extract 4 times with 2 mL of ethyl acetate, and combine the extracts. Dry the combined extracts with anhydrous sodium sulfate. Filter, and wash the filter with a small amount of ethyl acetate. Combine the washings with the filtrate, and concentrate to a volume of 0.7 mL, using a warm water bath and a stream of nitrogen. Dilute with ethyl acetate to 1 _{(USP 1-Dec-2021)} mL.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: GC

Detector: Flame ionization

Column: 0.32-mm × 30-m fused-silica capillary; bonded with a 1-µm layer of phase G46

Temperatures

Injection port: 230°

Detector: 250°

Column: See Table 3.

Table 3

Carrier gas: Helium

Flow rate: 1 mL/min

Injection volume: 2 µL

System suitability

Sample: Standard solution

[Note-The retention time of the 3-chloropropane-1,2-diol peak is about 8 min.]

Suitability requirements

Relative standard deviation: NMT 10.0%

Analysis

Samples: Standard solution and Sample solution

Acceptance criteria: The area of the principal peak from the Sample solution is NMT the area of the principal peak from the Standard solution (NMT 0.0025%).

4.6 LIMIT OF FREE AROMATIC AMINE

Solution A: 3 mg/mL of N-(1-naphthyl)ethylenediamine dihydrochloride in a mixture of propylene glycol and water (70:30)

Standard stock solution: 10 µg/mL of USP Iohexol Related Compound B RS in water

Standard solution: Transfer 5 mL of water and 10.0 mL of the Standard stock solution to a 25-mL volumetric flask.

Sample solution: Transfer 200 mg of Iohexol to a 25-mL volumetric flask, add 15 mL of water, and mix to dissolve.

Blank: Add 15 mL of water to a 25-mL volumetric flask.

Instrumental conditions

Mode: Vis

Analytical wavelength: 495 nm

Cell: 5 cm

Analysis

Samples: Standard solution, Sample solution, and Blank

In conducting the following steps, keep the flasks in iced water and protected as much as possible from light until all of the reagents have been added.

Treat the Samples as follows. Place the flask in an ice bath for 5 min. Add 1.5 mL of 6 N hydrochloric acid, and mix by swirling. Add 1.0 mL of sodium nitrite solution (20 mg/mL), and allow to stand in the ice bath for 4 min. Remove the flask from the ice bath, add 1.0 mL of sulfamic acid solution (40 mg/mL), and swirl gently until gas evolution ceases. [Caution-Considerable pressure is produced.] Add 1.0 mL of Solution A, dilute with water to volume, and allow to stand for 5 min. Measure the absorbance of the Standard solution and Sample solution against the Blank.

Acceptance criteria: The absorbance of the Sample solution is NMT that of the Standard solution (NMT 0.05% of free aromatic amine).

5 SPECIFIC TESTS

5.1 COLOR OF SOLUTION

Sample solution: 647.2 mg/mL

Blank: Water

Instrumental conditions

Mode: UV-Vis

Analytical wavelengths: 400, 420, and 450 nm

Cell: 1 cm

Analysis

Samples: Sample solution and Blank

Pass the Sample solution through a filter of 0.22-µm pore size. Determine the absorbances of the Sample solution against the Blank.

Acceptance criteria: See Table 4.

Table 4

5.2 WATER DETERMINATION 〈921〉, Method I: NMT 4.0%

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in well-closed, light-resistant containers. Store at room temperature.

JSP REFERENCE STANDARD 〈11〉

USP Iohexol RS

USP Iohexol Related Compound A RS

5-(Acetylamino)-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-1,3-benzenedicarboxamide

C₁₆H₂₀I₃N₃O₇ 747.06

USP Iohexol Related Compound B RS

5-Amino-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-1,3-benzenedicarboxamide

C₁₄H₁₈I₃N₃O₆ 705.02