Technical Field

This invention is related to preparation of microemulsion based drug carrier system which its viscosity improved by adding gelling agents for the anesthetic purposes. This invention is especially related to preparation of microemulsion formulations involving benzocaine as a local anesthetic active ingredient and latter, to achievement of microemulsion based hydrogels by converting microemulsions into gels using gelling agents.

Previous Technical

Dermal drug delivery has been used for a long time since skin is easy to access, has a large surface area with vast exposure to the circulatory and lymphatic networks and the route is noninvasive. Transdermal and topical administration of drugs has major advantages compare to classical oral administration of the drugs. The advantages of dermal systems are suitability, enhanced patient compliance and elimination of hepatic first-pass effect.

Local anesthetics are a class of drugs able to induce pain relief by virtue of their ability to bind to the sodium channel of excitable membranes, thus blocking the influx of sodium ions and the conduction of the nervous impulse. Benzocaine, or paraamino benzoic acid ethyl ester (molecular formula C9H11 NO2, molecular weight 165.189 g/mol), is a local anesthetic used primarily to relieve pain or irritations on the skin and mucosal surface, it is one of the ester types of long -acting local anesthetics, which has been used for the relief of local pain.

In current technology, several different techniques were developed providing absorption of drugs through skin. Some of the techniques being used today are formulations which are being administered over the skin surface such as creams and gels. During treatment, high viscosity systems such as creams and gels are localized longer over the skin surface. However, they need to interact with skin for the therapeutic effect at least for 1 hour. In addition, the administration site should be covered after application. These systems are not preferred due to unwanted conditions such as the risk of late therapeutic effect of the formulation and of contagion of the formulation to the patients' outfit. Formulations in solution form are more administrable rather than these systems but preparing solution formulation of benzocaine is hard due to its low water solubility. The promising technique among currently available ones is microemulsions.

Microemulsions are therrnodynamicaiiy stable, isotropicaily clear drug delivery systems that have a droplet size smaller than 0.15 Mm. It is very well known that microemulsions carry more drugs through skin layers faster than the other drug carrier systems such as solutions and creams. Microemulsions consist of oil, surfactant, cosurfactant and water. They have several pharmaceutical advantages, such as ease of preparation, transparency and potentials for soiubilizing variety of drugs. In addition to these, they are suitable vehicles for dermal and transdermal delivery of drugs. The components of microemulsions reduce the diffusion barrier effect of skin by interacting with stratum corneum. They provide increased concentration and thermodynamic activity of drugs at the site of application, and hydration effect on the stratum corneum. The most important disadvantage of microemulsions which have numerous advantages is hardness of their stabilization at the application site because of their low viscosity..

In current technique, the other drug delivery system under use is hydrogels. These systems can easily remove the disadvantages of microemulsions and localize on the skin surface longer due to their high viscosity. Although this advantage of these systems, the main disadvantage of hydrogels are their inability to go through the skin layers

The applications that we came across during our literature search are:

In the state of technique no: 6,074,674 in American Patent document, local anesthetic formulation of continued-prolonged released microcrystal and its preparation method by using wax and non-ionic polymers is being expressed.

The other one of these techniques is the application no. WO2014123100 of which describes the development of lidocaine and benzocaine loaded local anesthetic patches. The other one of these techniques is the application no. JP2004149544. In this document, preparation of some anesthetic substances loaded microemulsions by using ingredients such Lutrol F68 and Lutrol F127 and preparation methodology of formulations which converts into gels at the administration site by adding thermosensitive gelling agents into these microemulsions have been explained. The thermosensitive gel formulation described in this document is convenient for intraoral usage but administration of this formulation on skin surface is not a proper way. Because prepared formulations are liquids and they gels at body temperature. Due to these formulations would not stay on skin surface for enough time, preparing thermosensitive gel would not be beneficial for dermal application.

Similarly the other application is no: CA2627021 A1 . This application is about gel formulations in which benzocaine and various herbs such as Hamamelis virginiana are combined. In several researches, gel formulations of benzocaine studied in order to extend the localization time of benzocaine on skin surface. However, stratum corneum which localized at the first layer of the skin behaves as barrier and limits the drug transportation through skin's layers. Therefore it decreases the bioavailability of the drug.

In another study which is titled as 'Preparation and evaluation of bioadhesive benzocaine gels for enhanced local anesthetic effects' (2003) written by Shin SC, Lee JW, Yang KH, Lee CH, benzocaine loaded bioadhesive gel formulations were prepared by using hydroxypropyl methylcellulose (1 ).

In the article 'Mucoadhesive and Drug Release Properties of Benzocaine Gel' (2006) written by Jelvehgari M, Rashidi MR, Samadi H, (2) the preparation of benzocaine loaded gel formulations by using Carbopol was explained. Once these articles were read, it can easily be seen that both gel formulations which are prepared by using either Cabopol or HPMC (hydroxypropyl methylcellulose) are prolonging the duration of active substance on the skin surface but could not provide enough amount of active substance and long duration of permeation through skin layers. This exhibits a disadvantage situation.

In another study titled as New "drug-in cyclodextrin-in deformable liposomes" formulations to improve the therapeutic efficacy of local anesthetics' (3) performed by Maestrelli F, Gonzalez-Rodriguez ML, Rabasco AM, Ghelardini C, Mura P.; it is explained that preparation of benzocaine loaded liposomal gel formulation by using ΗΡβΟϋ (hydroxypropyl-β- cyclodextrine). The expense of liposome technology and their stability problems play main disadvantage role for prepared liposomal gel formulations.

The other similar one of these is American patent application no. US8652088. This application is about a preparation method of some drug substances loaded microemulsions by using microdialysis technique. In this study, microemulsion is the only one formulation prepared. In terms of administering microemulsion to skin, low viscosity of microemulsions plays a disadvantageous role. In previous technique another practice (as stated in the article 'Development, characterization and in vivo evaluation of benzocaine-loaded liposomes' written by Mura P, Maestrelli F, Gonzalez-Rodriguez ML, Michelacci I, Ghelardini C, Rabasco AM) explains the preparation method of benzocaine loaded liposomal gel formulation (4). The main disadvantages of these types of formulations are expensive ingredients and a need for special devices in terms of preparing liposomal formulations.

As conclusion, due to problematic situations mentioned above and inadequate solutions to these problematic conditions, development in technical field for drug delivery systems is necessitated.

The Aim of the Invention

Current invention is about benzocaine loaded microemulsion based hydrogels for the purpose of treatment and their preparation method which meets all the requirements and removes all the disadvantageous situations and provides advantageous conditions. .

Prior aim of the invention is preparation of local anesthetic benzocaine loaded microemulsion formulations, afterwards, increasing viscosity of microemulsion formulations by adding gelling agents in terms of having prolonged release of these formulations and converting them into microemulsion based hydrogels.

One of the purposes of this invention is preparation microemulsion based hydrogel systems which combine microemulsion systems and hydrogel systems in order to achieve extended localization on skin due to their high viscosity and to transfer drugs through skin's layers.

The other aim of this invention is to regulate the release rate of benzocaine and prolong its anesthetic effect by using microemulsion based hydrogel system. By this means, duration of localization on skin surface is prolonged and systemic toxicity is decreased. Another purpose of the invention is to increase drug permeation by virtue of the fact that microemulsion based hydrogel systems decreases the barrier effect of skin. Because of this feature of microemulsion based hydrogels, these systems are more than appropriate in terms of administering drug transdermal or dermal. Similar aim of this invention is to add gelling agents into microemulsion formulations for prolonging the duration of the stay on the skin surface and therefore providing long therapeutic effect of drug substance. In order to fulfil the purposes mentioned above, steps of preparation method of microemulsion based hydrogels are listed: a) Preparation of microemulsion formulations,

• Mixing the ratio in between 0,5-5% of benzocaine with the ratio of 1 -25% of liquid lipid which is isopropyl myristate

• Adding 5-30% of individual or combination of surfactant(s) Span 20, Polisorbate 80, Tween 20, Cremophor EL and Cremophor RH40.

· Adding ethanol into this system as cosurfactant with the ratio of 5-30% and then stirring.

• Adding the ratio of 5-80% of distilled water into mixture and stirring.

• Stirring prepared microemulsions,

b) Adding the ratio in between 0,1 -15 % of Carbopol into microemulsion formulations as gelling agent and then stirring.

Figures Ease the Understandability of Invention

Figure 1 : Production flow diagram of invention which is microemulsion based hyrogels. Explanation of References in Figure

10. Production method of microemulsion based hydrogels

11. Preparation of microemulsion formulations

111. Mixing anesthetic ingredient with liquid lipid

112. Adding surfactants into previous mixture mentioned above

113. Adding co-surfactant ingredient into previous mixture and then stirring

114. Adding distilled water into previous mixture and stirring

115. Stirring microemulsion which is recently formed

12. Adding gelling agents into prepared microemulsion formulations

13. Adding triethanolamine into this mixture in terms of formation of gels.

Detailed description of the invention

In this detailed description, the invention which is benzocaine loaded microemulsion based hydrogel formulations and the preferred structure of preparation method of these formulations will be described with no limitations for better understanding of the subject. The invention is about preparation of microemulsion formulations involving benzocaine which is local anesthetic drug substance and latter, obtainment microemulsion based hydrogels by converting these microemulsions into gels with help of added gelling agent. Microemulsion based hydrogel which is the invention subject is a topically administrable therapeutic gel involving microemulsion structure that is convenient for industrial application, able to decrease the release rate of active ingredient and thus able to localize longer on/in skin surface and its layers. Benzocaine is one of the anesthetic drugs which is used in terms of removing unwanted pain and relieving skin burnsand injuries. This is the reason why benzocaine needs to be rapidly acted and localize at the administration site longer in order to prolong the duration of its strength. Microemulsion based hydrogel formulations decrease the barrier effect of the skin and thus increase the permeation of the drug through skin layers. This is the reason why these formulations are very convenient structures for dermal and transdermal administration of the drug. The substances forming the formulations are harmless and rapid acting agents at the administration site because of being chosen as proper for dermal usage. In addition, according to microscopic studies, it has been shown that they were transported effectively through skin layers in an hour.

Therefore, in current technique, benzocaine loaded microemulsion based hydrogel systems provide longer stay on skin surface.

The invention subject benzocaine loaded microemulsion based hydrogel formulations and their preparation method involves the steps below:

• Preparation of microemulsion formulations (1 1 ),

- mixing anesthetic substance with liquid lipid (1 1 1 ),

- Adding surfactant substances into previous mixture (1 12),

- adding cosurfactant substance into previous mixture and stirring (1 13),

- adding distilled water into previous mixture and stirring (1 14),

- Stirring prepared microemulsions for a significant time (in current invention, it is 5 minutes) (1 15), • Adding gelling agents into prepared microemulsions and stirring in order to obtain microemulsion based hydrogel (12),

• Adding triethanolamine into the prepared mixture in order to make the formulation gel (13),

In the method of invention (10), at first glance, microemulsions which provide nano scale structure for final products are prepared (1 1 ). For this, anesthetic substance is being mixed with liquid lipid (1 1 1 ). In the preferred application of the invention, isopropyl myristate is used as liquid lipid and benzocaine is used as anesthetic substance. The isopropyl myristate is a strong permeation enhancer and increase the diffusion coefficient of skin.

In addition, the pseudoternary-phase diagrams are constructed by using high concentration of surfactant and co-surfactant for the preparation of microemulsions. The pseudoternary- phase diagrams are necessary in order to obtain the microemulsion area. In this invention, five different surfactants are used. Surfactants are added into this mixture at the same temperature (1 12). In the preferred application of this invention, Span 20, polysorbate 80 (Tween 80), Tween 20, Cremophor EL and Cremophor RH40 is used as surfactants individually or with a combination. The combination ratios of surfactants are; 1 :1 , 1 :2, 1 :3, 1 :4, 1 :5, 1 :6, 1 :7, 1 :8, 1 :9, 2:1 , 3:1 , 4:1 , 5:1 , 6:1 , 7:1 , 8:1 , 9:1 .

Afterwards, cosurfactant substances are added into these mixtures (1 13). In the preferred application of invention, ethanol is used as cosurfactant. As final step, distilled water is added into the mixture and is stirred (1 14). In preferred application of the invention, distilled water is used. Afterwards, microemulsions are stirred for five minutes (1 15).

The ratios by weight / volume of materials used in preparation of benzocaine loaded microemulsions (1 1 ) are: 1 - 25% liquid lipid, 0.5 - 5% anesthetic substance, 5 - 30% surfactant, 5 - 30% cosurfactant and 5 - 80% distilled water to complete 100%.

In the next step, gelling agents are added into the prepared microemulsions (12). In the preferred application of the invention, gelling agent Carbopol is added as 0.1 - 15 % of the mixture (12). By doing so, microemulsion based hydrogels are obtained.

The invention subject microemulsion based hydrogel are obtained by stirring the mixture. The biocompatible gelling agent extends the residence time of the invention subject microemulsion based hydrogel formulations on the skin surface and thus increases the efficacy of the treatment.

The invention is very suitable for industrial manufacturing. It can be produced by the pharmaceutical companies which manufacture the semi solid systems. Materials and equipments for preparation procedure are very easy to access. The devices in use for preparation of formulations are simple stirrers which every company already possesses.

REFERENCES Shin SC, Lee JW, Yang KH, Lee CH, Preparation and evaluation of bioadhesive benzocaine gels for enhanced local anesthetic effects, International Journal of Pharmaceutics, 260, 77-81 , 2003. Jelvehgari M, Rashidi MR, Samadi H, Mucoadhesive and Drug Release Properties of Benzocaine Gel, Iranian Journal of Pharmaceutical Sciences, 2(4), 185-194, 2006. Maestrelli F, Gonzalez-Rodriguez ML, Rabasco AM, Ghelardini C, Mura P, New "drug-in cyclodextrin-in deformable liposomes" formulations to improve the therapeutic efficacy of local anaesthetics, International Journal of Pharmaceutics, 395, 222-231 , 2010. Mura P, Maestrelli F, Gonzalez-Rodriguez ML, Michelacci I, Ghelardini C, Rabasco AM, Development, characterization and in vivo evaluation of benzocaine-loaded liposomes

European Journal of Pharmaceutics and Biopharmaceutics. 67(1 ):86-95, 2007.