Colistimethate Sodium

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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C₅₈H₁₀₅N₁₆Na₅O₂₈S₅ (colistin A component) 1749.82

C₅₇H₁₀₃N₁₆Na₅O₂₈S₅ (colistin B component) 1735.79

Colistimethate sodium.

Pentasodium colistinmethanesulfonate CAS RN®: 8068-28-8; 21362-08-3 UNII: XW0E5YS77G.

Colistimethate Sodium has a potency equivalent to not less than 390 µg of colistin per mg.

1 Packaging and storage

Preserve as described in Packaging and Storage Requirements 〈659〉, Injection Packaging, Packaging for constitution.

2 Labeling

Where it is intended for use in preparing injectable dosage forms, the label states that it is sterile or must be subjected to further processing during the preparation of injectable dosage forms.

3 USP Reference standards 〈11〉

USP Colistimethate Sodium RS

4 Constituted solution

At the time of use, it meets the requirements for Injections and Implanted Drug Products 〈1〉, Specific Tests, Completeness and clarity of solutions.

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5 Identification

5.1 Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K

5.2 pH 〈791〉

Between 6.5 and 8.5, in a solution containing 10 mg per mL.

5.3 Loss on drying 〈731〉

Dry about 100 mg, accurately weighed, in a capillary-stoppered bottle in vacuum at a pressure not exceeding 5 mm of mercury at 60° for 3 hours: it loses not more than 7.0% of its weight.

6 Free colistin

Dissolve 80 mg in 3 mL of water, and add 0.05 mL of silicotungstic acid solution (1 in 10): no immediate precipitate is formed.

7 Other requirements

Where the label states that Colistimethate Sodium is sterile, it meets the requirements for Sterility and Bacterial endotoxins under Colistimethate for Injection. Where the label states that Colistimethate Sodium must be subjected to further processing during the preparation of injectable dosage forms, it meets the requirements for Bacterial endotoxins under Colistimethate for Injection.

8 Assay

Assay preparation-Dissolve a suitable quantity of Colistimethate Sodium, accurately weighed, in 2.0 mL of water, add a sufficient accurately measured volume of Buffer B.6 to obtain a solution having a convenient concentration.

Procedure-Proceed as directed for Colistimethate Sodium under Antibiotics—Microbial Assays 〈81〉, using an accurately measured volume of Assay preparation diluted quantitatively with Buffer B.6 to yield a Test Dilution having a concentration assumed to be equal to the median dose level of the Standard.