(Ph. Eur. 11.6 update)
Action and use
Local anaesthetic; Class I antiarrhythmic.
DEFINITION
Lidocaine Ointment contains Lidocaine in a suitable hydrophilic basis.
The ointment complies with the requirements stated under Topical Semi-solid Preparations and with the following requirements.
Content of lidocaine, C14H22N2O
95.0 to 105.0% of the stated amount.
IDENTIFICATION
Warm a quantity of the ointment containing about 25 mg of Lidocaine until the basis has melted, add 1 mL of saturated sodium chloride solution and 0.2 mL of 1M sodium hydroxide and cool. Add 5 mL of ether, shake vigorously for 1 minute and allow the layers to separate. Filter the ether layer through anhydrous sodium sulfate and evaporate the ether to dryness. Dissolve the residue in the minimum volume of chloroform IR, apply the solution directly to a sodium chloride disc and allow the solvent to evaporate. The infrared absorption spectrum of the resulting thin film, Appendix II A, is concordant with the reference spectrum of lidocaine (RS 405).
2,6-Dimethylaniline
Carry out the method for liquid chromatography, Appendix III D, using the following solutions.
(1) Dissolve a quantity of the ointment containing 50 mg of Lidocaine in the mobile phase, dilute to 50 mL with the mobile phase and dilute 1 volume of this solution to 10 volumes with the mobile phase.
(2) Dilute a 0.1% w/v solution of 2,6-dimethylaniline in methanol with the mobile phase to produce a solution containing 0.04 μg per mL of 2,6-dimethylaniline.
(3) Mix equal volumes of a 0.01% w/v solution of lidocaine BPCRS in the mobile phase with a 0.005% w/v solution of 2,6-dimethylaniline in the mobile phase.
CHROMATOGRAPHIC CONDITIONS
(a) Use a stainless steel column (15 cm × 4.6 mm) packed with octadecylsilyl silica gel for chromatography (5 μm) (Apex ODS 2 is suitable).
(b) Use isocratic elution and the mobile phase described below.
(c) Use a flow rate of 0.8 mL per minute.
(d) Use an ambient column temperature.
(e) Use a detection wavelength of 230 nm.
(f) Inject 20 μL of each solution.
MOBILE PHASE
35 volumes of a phosphate buffer pH 8.0 prepared by adding a 0.408% w/v solution of potassium dihydrogen orthophosphate to a suitable volume of a 0.685% w/v solution of dipotassium hydrogen orthophosphate until pH 8.0 is attained and 65 volumes of methanol.
The chromatogram obtained with solution (3) shows a peak due to lidocaine and a peak due to 2,6-dimethylaniline with a retention time relative to lidocaine of about 0.5.
LIMITS
In the chromatogram obtained with solution (1), the area of any peak corresponding to 2,6-dimethylaniline is not greater than the area of the peak in the chromatogram obtained with solution (2) (400 ppm).
ASSAY
Carry out the method for liquid chromatography, Appendix III D, using the following solutions.
(1) Dissolve a quantity of the ointment containing 50 mg of Lidocaine in the mobile phase, dilute to 50 mL with the mobile phase and dilute 1 volume of this solution to 10 volumes with the mobile phase.
(2) 0.010% w/v of lidocaine BPCRS in the mobile phase.
CHROMATOGRAPHIC CONDITIONS
The chromatographic conditions described under 2,6-Dimethylaniline may be used.
DETERMINATION OF CONTENT
Calculate the content of C14H22N2O in the ointment using the declared content of C14H22N2O in lidocaine BPCRS.
