Selamectin

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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C₄₃H₆₃NO₁₁ 769.96 

25-Cyclohexyl-4′-O-de(2,6-dideoxy-3-O-methyl-α-l-arabino-hexopyranosyl)-5-demethoxy-25-de(1-methylpropyl)-22,23-dihydro-5-(hydroxyimino)- avermectin A1a; 

(2aE,4E,5′S,6S,6′S,7S,8E,11R,13R,15S,17aR,20aR,20bS)-6′-Cyclohexyl-7-[(2,6-dideoxy-3-O-methyl-α-l-arabinohexopyranosyl)oxy]-3′,4′,5′,6,6′,7,10,11,14,15,20a,20b-dodecahydro-20b-hydroxy-5′,6,8,19-tetramethylspiro[11,15-methano-2H,13H,17H-furo[4,3,2- pq][2,6]benzodioxacyclooctadecin-13,2′-[2H]pyran]-17,20(17aH)-dione 20-oxime CAS RN®: 220119-17-5. 

1 DEFINITION 

Selamectin contains NLT 96.0% and NMT 102.0% of selamectin (C₄₃H₆₃NO₁₁), calculated on the anhydrous and solvent-free basis.

2 IDENTIFICATION 

Change to read: 

A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197A, 197K, or 197M

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 

Mobile phase: Acetonitrile and water (80:20) 

Standard solution: 0.2 mg/mL of USP Selamectin RS in Mobile phase 

Sample solution: 0.2 mg/mL of Selamectin in Mobile phase 

Chromatographic system 

(See Chromatography 〈621〉, System Suitability.) 

Mode: LC 

Detector: UV 243 nm 

Column: 3.9-mm × 15-cm; 4-µm packing L1 

Column temperature: 30° 

Flow rate: 1.0 mL/min 

Injection volume: 20 µL 

System suitability 

Sample: Standard solution 

Suitability requirements 

Tailing factor: NMT 1.5 

Relative standard deviation: NMT 0.73% 

Samples: Standard solution and Sample solution 

Calculate the percentage of selamectin (C₄₃H₆₃NO₁₁) in the portion of Selamectin taken: 

Result = (r_U/r_S) × (C_S/C_U) × 100 

r_U = peak response from the Sample solution 

r_S = peak response from the Standard solution  

C_S = concentration of USP Selamectin RS in the Standard solution (µg/mL)  

C_U = concentration of Selamectin in the Sample solution (µg/mL)

Acceptance criteria: 96.0%–102.0% on the anhydrous and solvent-free basis 

4 IMPURITIES 

Residue on Ignition 〈281〉 

Sample: 1.0 g 

Acceptance criteria: NMT 0.1% 

Organic Impurities 

Solution A: Water 

Solution B: Acetonitrile 

Mobile phase: See Table 1. Return to original conditions and re-equilibrate the system. 

Table 1 

Diluent: Acetonitrile and water (60:40) 

Standard solution: 2.5 µg/mL of USP Selamectin RS in Diluent 

Sample solution: 500 µg/mL of Selamectin in Diluent 

Chromatographic system 

(See Chromatography 〈621〉, System Suitability.) 

Mode: LC 

Detector: UV 243 nm 

Column: 3.9-mm × 15-cm; 4-µm packing L1 

Column temperature: 30° 

Flow rate: 2.0 mL/min 

Injection volume: 20 µL 

System suitability 

Sample: Standard solution 

Suitability requirements 

Tailing factor: NMT 1.6 for the selamectin peak 

Analysis 

Samples: Standard solution and Sample solution 

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

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

r_U = peak response of each impurity from the Sample solution 

r_S = peak response of selamectin from the Standard solution  

C_S = concentration of USP Selamectin RS in the Standard solution (µg/mL)  

C_U = concentration of Selamectin in the Sample solution (µg/mL) 

F = relative response factor (see Table 2) 

Acceptance criteria: See Table 2. The reporting threshold for impurities is 0.2%. 

Table 2 

a (2aE,2′R,4E,5′S,6S,6′S,7S,8E,11R,13R,15S,17aR,20aR,20bS)-6′-Cyclohexyl-7-[(2,6-dideoxy-3-O-methyl-α-l-arabino hexopyranosyl)oxy]-3′,4′,5′,6,6′,7,10,11,14,15,20a,20b-dodecahydro-4′,20b-dihydroxy-5′,6,8,19-tetramethylspiro[11,15-methano-2H,13H,17H furo[4,3,2-pq][2,6]benzodioxacyclooctadecin-13,2′-[2H]pyran]-17,20(17aH)-dione 20-oxime. 

b (2aE,4E,5′S,6S,6′S,7S,8E,11R,13S,15S,17aR,20aR,20bS)-6′-Cyclohexyl-7-[(2,6-dideoxy-3-O-methyl-α-l-arabino hexopyranosyl)oxy]-5′,6,6′,7,10,11,14,15,20a,20b-decahydro-20b-hydroxy-5′,6,8,19-tetramethylspiro[11,15-methano-2H,13H,17H-furo[4,3,2-pq] [2,6]benzodioxacyclooctadecin-13,2′-[2H]pyran]-17,20(17aH)-dione 20-oxime. 

c (2aE,4E,5′S,6S,6′S,7S,8E,11R,13S,15S,17aR,20aR,20bS)-6′-Cyclohexyl-5′,6,6′,7,10,11,14,15,20a,20b-decahydro-7,20b-dihydroxy-5′,6,8,19- tetramethylspiro[11,15-methano-2H,13H,17H-furo[4,3,2-pq][2,6]benzodioxacyclooctadecin-13,2′-[2H]pyran]-17,20(17aH)-dione 20-oxime. d(2aE,4E,5′S,6S,6′S,7S,8E,11R,13R,15S,17aR,20aR,20bS)-6′-Cyclohexyl-7-[(4-O-(2,6-dideoxy-3-O-methyl-α-l-arabino-hexopyranosyl)-2,6- dideoxy-3-O-methyl-α-l-arabino-hexopyranosyl)oxy]-3′,4′,5′,6,6′,7,10,11,14,15,20a,20b-dodecahydro-20b-hydroxy-5′,6,8,19- tetramethylspiro[11,15-methano-2H,13H,17H-furo[4,3,2-pq][2,6]benzodioxacyclooctadecin-13,2′-[2H]pyran]-17,20(17aH)-dione 20-oxime. 

5 SPECIFIC TESTS 

Water Determination 〈921〉, Method I, Method Ia 

Sample: 0.20 g 

Acceptance criteria: NMT 7.0% 

6 ADDITIONAL REQUIREMENTS 

Packaging and Storage: Preserve in a tight container. 

Labeling: Label it to indicate that it is for veterinary use only. 

USP Reference Standards 〈11〉 

USP Selamectin RS