Gentamicin Ointment

Aminoglycoside antibacterial.

Gentamicin Ointment is a dispersion of Gentamicin Sulphate in microfine powder in White Soft Paraffin or other suitable anhydrous greasy basis.

The ointment complies with the requirements stated under Topical Semi-solid Preparations and with the following requirements.

A.  Carry out the method for thin-layer chromatography , Appendix III A, using a silica gel precoated plate (Merck silica gel 60 plates are suitable) and as the mobile phase the lower layer obtained by shaking together equal volumes of 13.5m ammonia , chloroform and methanol and allowing to separate. Apply separately to the plate 20 µl of each of the following solutions. For solution (1) disperse a quantity of the ointment containing the equivalent of 7.5  mg of gentamicin in 20  ml of chloroform , extract with 10  ml of water and use the aqueous layer. Solution (2) is a solution of gentamicin sulphate BPCRS in water containing the equivalent of 0.075% w/v of gentamicin. After removal of the plate, allow it to dry in air, spray with ninhydrin solution R1 and heat at 105° for 2  minutes. The three principal spots in the chromatogram obtained with solution (1) correspond to the three principal spots in the chromatogram obtained with solution (2).

B.  In the test for Composition of gentamicin sulphate, the retention times of the four principal peaks in the chromatogram obtained with solution (2) correspond to those of the four principal peaks in the chromatogram obtained with solution (1).

Complies with the test described under Gentamicin Injection but preparing solution (2) in the following manner. Disperse a quantity of the ointment containing the equivalent of 20  mg of gentamicin in 10  ml of chloroform , add 20  ml of a 0.25% w/v solution of sodium tetraborate , shake vigorously, centrifuge and separate the aqueous layer. Filter and dilute to 50  ml with water. To 10  ml add 5  ml of methanol , swirl, add 4  ml of phthalaldehyde reagent , mix, add sufficient methanol to produce 25  ml, heat in a water bath at 60° for 15  minutes and cool.

Dissolve as completely as possible a quantity containing the equivalent of 4  mg of gentamicin in 50  ml of chloroform , extract with three 20  ml quantities of phosphate buffer pH 8.0 and dilute the combined extracts to 100  ml with phosphate buffer pH 8.0 . Dilute 10  ml of the resulting solution to 50  ml with phosphate buffer pH 8.0 . Carry out the microbiological assay of antibiotics , Appendix XIV A. The precision of the assay is such that the fiducial limits of error are not less than 95% and not more than 105% of the estimated potency.

Calculate the content of gentamicin in the ointment, taking each 1000  IU found to be equivalent to 1  mg of gentamicin. The upper fiducial limit of error is not less than 90.0% and the lower fiducial limit of error is not more than 120.0% of the stated content.

The quantity of active ingredient is stated in terms of the equivalent amount of gentamicin.