Selamectin

(Selamectin for Veterinary Use, Ph. Eur. monograph 2268)

C₄₃H₆₃NO₁₁             770               220119-17-5

DEFINITION

(1^3aS,1⁴R,1⁷E,1^7aR,2′R,4 R,4²S,5′S,6E,6′S,8S,9S,10E,12E)-6′-Cyclohexyl-8-[(2,6-dideoxy-3-O-methyl-α-L-arabino- hexopyranosyl)oxy]-1^3a-hydroxy-1⁷-(hydroxyimino)-1⁶ ,5′,7,9-tetramethyl-1^3a,1⁴,1⁷,1^7a -tetrahydro-1² H-3-oxa-1(4,3)-[1]benzofurana-4(4,2)-oxanaspiro[cyclododecaphane-6,10,12(1 )-trien-4 ,2′-oxan]-2-one ((5Z,21R,25S)-25-cyclohexyl-25- de(butan-2-yl)-4′-O-de(2,6-dideoxy-3-O-methyl-α-L-arabino-hexopyranosyl)-5-demethoxy-5-(hydroxyimino)-22,23-
dihydroavermectin A_1a).

Semi-synthetic product derived from a fermentation product.

Content

96.0 per cent to 102.0 per cent (anhydrous substance).

CHARACTERS

Appearance

White or almost white, hygroscopic powder.

Solubility

Practically insoluble in water, freely soluble in isopropyl alcohol, soluble in acetone and in methylene chloride, sparingly soluble in methanol.

IDENTIFICATION

Infrared absorption spectrophotometry (2.2.24).

Comparison: selamectin CRS.

TESTS

Related substances

Liquid chromatography (2.2.29).

Solvent mixture: water R, acetonitrile R (40:60 V/V).

Test solution: Dissolve 25 mg of the substance to be examined in the solvent mixture and dilute to 50 mL with the solvent mixture.

Reference solution (a): Dilute 1.0 mL of the test solution to 100.0 mL with the solvent mixture.

Reference solution (b): Dissolve 2.5 mg of selamectin for peak identification CRS (containing impurities A, C and D) in the solvent mixture and dilute to 5 mL with the solvent mixture.

Reference solution (c): Dissolve the contents of a vial of selamectin impurity B CRS in 1 mL of reference solution (b).

Column:

— size: l = 0.15 m, Ø = 3.9 mm;

— stationary phase: end-capped octadecylsilyl silica gel for chromatography R (4 μm);

— temperature: 30 °C.\

Mobile phase:

— mobile phase A: water for chromatography R;

— mobile phase B: acetonitrile for chromatography R;

Time (min)	Mobile phase A (per cent V/V)	Mobile phase B (per cent V/V)
0 – 28	40	60
28 – 45	40 → 20	60 → 80

Flow rate: 2.0 mL/min.

Detection: Spectrophotometer at 243 nm.

Injection: 20 μL.

Identification of impurities: Use the chromatogram supplied with selamectin for peak identification CRS and the chromatogram obtained with reference solution (c) to identify the peaks due to impurities A, B, C and D.

Relative retention: With reference to selamectin (retention time = about 22 min): impurity A = about 0.2; impurity B = about 0.4; impurity C = about 0.5; impurity D = about 1.7.

System suitability: Reference solution (c):

— resolution: minimum 2.5 between the peaks due to impurities B and C.

Limits:

— correction factor: for the calculation of content, multiply the peak area of impurity D by 1.5;

— impurities A, B: for each impurity, not more than twice the area of the principal peak in the chromatogram obtained with reference solution (a) (2.0 per cent);

— impurities C, D: for each impurity, not more than 1.5 times the area of the principal peak in the chromatogram obtained with reference solution (a) (1.5 per cent);

— any other impurity: for each impurity, not more than the area of the principal peak in the chromatogram obtained with reference solution (a) (1.0 per cent);

— total: not more than 4 times the area of the principal peak in the chromatogram obtained with reference solution (a) (4.0 per cent);

— disregard limit: 0.2 times the area of the principal peak in the chromatogram obtained with reference solution (a) (0.2 per cent).

Water (2.5.12, Method A)

Maximum 7.0 per cent, determined on 0.200 g.

Sulfated ash (2.4.14)

Maximum 0.1 per cent, determined on 1.0 g.

ASSAY

Liquid chromatography (2.2.29).

Test solution: Dissolve 50.0 mg of the substance to be examined in the mobile phase and dilute to 250.0 mL with the mobile phase.

Reference solution: Dissolve 50.0 mg of selamectin CRS in the mobile phase and dilute to 250.0 mL with the mobile phase.

Column:

— size: l = 0.15 m, Ø = 3.9 mm;

— stationary phase: end-capped octadecylsilyl silica gel for chromatography R (4 μm);

— temperature: 30 °C.

Mobile phase: water for chromatography R, acetonitrile for chromatography R (20:80 V/V).

Flow rate: 1.0 mL/min.

Detection: Spectrophotometer at 243 nm.

Injection: 20 μL.

Run time: Twice the retention time of selamectin.

Retention time: Selamectin = about 9 min.

Calculate the percentage content of C₄₃H₆₃NO₁₁ taking into account the assigned content of selamectin CRS.

STORAGE

In an airtight container.

IMPURITIES

Specified impurities A, B, C, D.

A. (5Z,21R,23S,25R)-25-cyclohexyl-25-de(butan-2-yl)-4′-O-de(2,6-dideoxy-3-O-methyl-α-L-arabino-hexopyranosyl)-5- demethoxy-23-hydroxy-5-(hydroxyimino)-22,23-dihydroavermectin A_1a,

B. (5Z,25R)-25-cyclohexyl-25-de(butan-2-yl)-4′-O-de(2,6-dideoxy-3-O-methyl-α-L-arabino-hexopyranosyl)-5-demethoxy-5-(hydroxyimino)avermectin A_1a,

C. (5Z,21R,25S)-25-cyclohexyl-25-de(butan-2-yl)-13-O-de[2,6-dideoxy-4-O-(2,6-dideoxy-3-O-methyl-α-L-arabino- hexopyranosyl)-3-O-methyl-α-L-arabino-hexopyranosyl]-5-demethoxy-5-(hydroxyimino)-22,23-dihydroavermectin A_1a,

D. (5Z,21R,25S)-25-cyclohexyl-25-de(butan-2-yl)-5-demethoxy-5-(hydroxyimino)-22,23-dihydroavermectin A_1a.