| Objective:Compound lidocaine emulsion is widely used in local skin anesthesia and various medical procedures that related to pain.However,due to the barrier effect of the skin stratum corneum,the compound lidocaine emulsion has the defects of slow onset and short action time,which cannot meet the clinical needs.Microemulsions can promote the penetration of drugs through the skin,but the safety hazards caused by the largeamount use of surfactants and co-surfactants（i.e.,short or medium chain alcohols）limit the development of microemulsion products.Based on the previous research,this article intends to design an alcohol-free,low-surfactant microemulsion carrier with olive oil（OL）,isopropyl myristate（IPM）and vitamin E succinate（VES）as the oil phase and further use the carrier to prepare a safe compound lidocaine microemulsion gel with fast onset.Methods:1.In the pre-prescription research,high performance liquid chromatography（HPLC）method was established for simultaneous measuring lidocaine and prilocaine in vitro,and then the methodology was validated.By investigating the appearance,solubility and melting point of the lidocaine base and prilocaine base,the preliminary quality evaluation of the raw materials was carried out.Differential Scanning Calorimetry（DSC）and Fourier Transform Infrared Spectroscopy（FTIR）were used to investigate the eutectic mixtures of lidocaine and prilocaine.In addition,the solubility of lidocaine and prilocaine in the different oil phases and the solutions were measured for selecting the oil phase of the microemulsion formulations and the receptors in vitro transdermal penetration experiments,respectively.2.The ternary phase diagram method was used to investigate the influence of different co-surfactants and auxiliary oil phases on the OL-IPM microemulsion area.In order to screen the optimal prescription of a blank microemulsion carrier with no cosurfactant and low surfactant content,the particle size,polydispersity index（PDI）,Zeta potential,viscosity,turbidity,pH and conductivity of the blank microemulsion were characterized.The centrifugation,heating-cooling cycle and long-term experiments were conducted for investigating the stability of the blank microemulsion carrier.3.The mixture of lidocaine and prilocaine with a mass ratio of 1:1 was dissolved into the oil phase,and the compound lidocaine microemulsion was prepared by a water titration method.On the basis of quality evaluation,the drug loading,the drug steadystate penetration rate through the isolated abdominal skin of guinea pigs and the drug intradermal retention were used as evaluation indicators to optimize the formulation.Subsequently,the best microemulsion gel prescription was determined by screening the types and the dosages of the gel materials and the penetration enhancers.The drug loading,particle size,PDI,Zeta potential,viscosity and pH were used as the main indicators to characterize the physical and chemical properties of the formulation.The stability of the formulation was evaluated through low temperature,high temperature and long-term experiments.4.Rabbit was used as the model animal,and the skin irritation grade score combined with hematoxylin-eosin（HE）stained skin pathological tissue slice were used to evaluate the safety of the compound lidocaine microemulsion gel.Using the drug steady-state penetration rate and intradermal retention as indicators to investigate the permeation activity of the compound lidocaine microemulsion gel through the skin of guinea pigs in vitro.The guinea pig was selected as the experiment animal,commercially compound lidocaine emulsion and commercially compound lidocaine gel were used as the active control groups,and the saline was used as a negative control,the preliminary pharmacodynamics of compound lidocaine microemulsion gel was evaluated by local acupuncture method.Results:1.The established HPLC analysis conditions: the two drugs were separated in the Inert Sustain? C18 column（4.6 × 250 mm,5 （?）m）with the wavelength as 254 nm and the column temperature as 35 ℃.The mobile phase was methanol-0.5% ammonium dihydrogen phosphate aqueous solution（80:20,V/V）with flow rate of 1.0 m L/min,using triethylamine to adjust the pH as 7.0.Besides,the injection volume was 20 （?）L.After investigation,the established method met the methodological requirements.The measurement results of the appearance,solubility,and melting point of lidocaine base and prilocaine base showed that the quality of the two drugs was qualified and they could be used for further research.The melting point of the eutectic mixture of lidocaine and prilocaine was lower than 20 ℃ and the two drugs had no chemical interaction.In the all tested single oil phase,IPM had the maximum solubility for lidocaine and prilocaine,followed by OL.The solubility of the two drugs were 367.89 and 488.29mg/m L,respectively,which reached the largest in the OL-IPM mixture with mass ratio of 1:1.The solubility of lidocaine and prilocaine in 20% ethanol-saline solution reached the maximum,which were 12.92 and 9.90 mg/m L,respectively,so it was selected as the in vitro transdermal receiving solution.2.Using the mixture of OL,IPM and VES with mass ratio of 3:3:1 as the oil phase,ethoxylated hydrogenated castor oil 40（RH40）and Span 80（Span80）mixed at a weight ratio of 5:1 as the surfactant,the largest mass ratio of the oil phase to the surfactant（O/S）as 4:6 and 80% of water to prepare a blank microemulsion.The blank microemulsion had a particle size of 36.24 nm,PDI value of 0.25,Zeta potential of 13.6m V,viscosity of 17 m Pa·s,turbidity of 71.54 NTU and pH of 6.08,which all met the topical requirements of particle size less than 100 nm,PDI < 0.3,and low viscosity.The blank microemulsion carrier had centrifugation,heating-cooling cycle and long-term stability,indicating that the blank microemulsion can be used as a compound lidocaine microemulsion gel carrier.With O/S as 3.2:6.8,75% of water,the drug was to preeutectic mixed and then blending with other components to prepare the drug-loaded microemulsion.After screening,the optimal gel matrix material was 0.6% carbomer980,and the penetration enhancer was 3% of ethanol.Therefore,the final compound lidocaine microemulsion gel prescription was: 2.5% lidocaine,2.5% prilocaine,2.25%OL,2.25% IPM,0.75% VES,9.29% RH40,1.86% Span80,0.6% carbomer,3% ethanol,75% water.The particle size of the microemulsion gel was 52.15 nm and the Zeta potential was-36.2 m V.The viscosity value was 77.87 Pa·s and the pH value was 6.75,indicated that the microemulsion gel is a suitable transdermal drug delivery system.Besides,the stability results showed that the compound lidocaine microemulsion gel had low temperature,high temperature and long-term stability.3.After smeared with the compound lidocaine microemulsion gel and the blank microemulsion gel,no obvious irritation in the normal rabbit skin was observed.The results of the skin pathological tissue slices showed that after the application of the compound lidocaine microemulsion gel,the structure of each layer of the skin was intact,and no obvious inflammatory histopathological changes were observed.4.The results of the in vitro guinea pigs skin permeation experiments showed that the drug steady-state permeation rate of the compound lidocaine microemulsion gel was better than that of the two control groups.The onset time of compound lidocaine microemulsion gel（50% of the painless response rate）was 24.84 min,the maximum painless response rate（MPR）was 92.5%,and the analgesic effects duration time was245.16 min.The onset time,MPR and the analgesic effects duration time of the commercially compound lidocaine gel and the commercially compound lidocaine emulsion were 45.45 min,51.62 min,86%,80%,201.53 min and 186.80 min,respectively.After statistical analysis,the drug steady-state penetration rate,onset time,analgesic effects maintenance time and the MPR of the compound lidocaine microemulsion gel and the two commercially preparations were statistically different（p < 0.05）.The above results indicated that the self-made compound lidocaine microemulsion gel was quickly onset,and the analgesic effect was better than the two similar commercial preparations.Conclusion:In this work,the prepared compound lidocaine microemulsion gel does not require the co-surfactant,and the usage of surfactants is only 11.15%.The compound lidocaine microemulsion gel is safe,stable with fast onset,and the analgesic effect is better than commercial preparations. |
