US3136691A

US3136691A - Aqueous solution of local anesthetic maintained under pressure

Info

Publication number: US3136691A
Application number: US177172A
Authority: US
Inventors: Nordstrom George; Aldo P Truant
Original assignee: Astra Pharmaceutical Products Inc
Current assignee: Astra USA Inc
Priority date: 1962-03-05
Filing date: 1962-03-05
Publication date: 1964-06-09
Anticipated expiration: 1981-06-09
Also published as: DE1220967B; SE315689B; DK109053C; GB1035005A; CH465767A

Description

United States Patent 3,136,691 AQUEOUS SDLUTION OF LOCAL ANESTHETIC MAINTAINED UNDER PRESSURE George Nordstrom and Aldo P. Truant, Worcester, Mass., assignors to Astra Pharmaceutical Products, Inc., Worcester, Mass, a corporation of New York No Drawing. Filed Mar. 5, 1962, Ser. No. 177,172 15 Claims. (Cl. 167-52) The present invention relates to new and valuable pharmaceutical preparations, and more particularly to local anesthetic preparations of improved 'anesthetizing activity which can be applied to pressure dispensation, and to a process of making and using same.

When applying local anesthetics in the form of spraying solutions, solvents such as alcohol, glycerol, polyethylene glycol, and others are used in order to dissolve the local anesthetic in sufficient amounts. However, such spraying solutions have the disadvantage that they irritate mucous membranes and the broken skin. They cause, for instance, sloughing of the mucous membranes, i.e., the surface layer of the mucous membrane is separated from the deeper layers in the form of dead matter at the site of application of the local anesthetic.

Injection of aqueous solutions of water-soluble salts of the local anesthetic agents such as their hydrochlorides introduces relatively large amounts of hydrochloric acid into the tissue, which acid must be neutralized or bufiered and thus removed therefrom. Due to such neutralization and buifering the onset of anesthesia is relatively slow, and rather concentrated solutions must be administered to produce the desired anesthetic eifect.

It is one object of the present invention to provide solutions of local anesthetics which are substantially free of the disadvantages of the known preparations and which are distinguished therefrom by a relatively rapid onset of anesthesia as well as by more extensive and deeper anesthesia.

Another object of the present invention is to provide highly eifective anesthetic preparations which can be pressure-dispensed and which have a much higher anesthetic effect than heretofore attained by conventional preparations of this type.

A further object of the present invention is to provide highly effective and non-irritating injectable preparations containing a local anesthetic agent.

Still another object of the present invention is to provide a simple and effective process of making such improved local anesthetic preparations. I

A further object of the present invention is to provide a highly effective method of locally and topically anesthetizing mucous membranes and other parts of the animal and human body.

Other objects of the present invention and advantageous features thereof will become apparent as the description proceeds.

In principle, these objects of the present invention are achieved by producing solutions of a local anesthetic base of the acid amide type which contain an excess of carbon dioxide over the stoichiometrically required amount. For this purpose carbon dioxide is introduced into a suspension of the local anesthetic base, preferably under pressure, and at a low temperature, and the resulting solution is poured into the dispensing containers such as vials, ampoules, pressure resistant dispensers, and the like.

Although the present invention is useful in the preparation of local anesthetics of the acid amide type, it has proved of special value in the preparation of solutions of the local anesthetic known as lidocaine. On introducing carbon dioxide into a suspension of finely divided lidocaine base, the base dissolves gradually and slowlyand a clear solution is obtained. When filling such a solution into a pressure resistant atomizer or dispenser and adding thereto additional pharmaceutically acceptable driving agent or propellant, for instance, excess carbon dioxide, nitrogen, propellants of the halogenated hydrocarbon type which contain one or more fluorine atoms in addition to other halogens in their molecule, such as dichloro difluoro methane, chloro trifluoro methane, chloro difluoro methane, sym. dichloro tetrafluoro ethane, and others, or a mixture of such agents, pressure dispensable local anesthetic solutions are obtained which can readily be applied topically to the body of animals and humans.

According to another method of preparing such solutions of local anesthetics of the acid amide type, a pharmaceutically acceptable acid addition salt of the local anesthetic, and preferably its hydrochloride, is dissolved in water. About equimolecular amounts of a water soluble, pharmaceutically acceptable hydrogen carbonate, preferably of sodium bicarbonate, are added thereto and the resulting solution is filled into pressure resistant atomizers or dispensers. Carbon dioxide, or other propellants or driving agents as mentioned, preferably mixtures of carbon dioxide with such otherdriving agents, are introduced into the pressure resistant container which is then ready for use.

According to a further embodiment of the present invention, it is also possible to produce injectable solutions of the local anesthetic of the acid amide type by proceeding as pointed out hereinabove, either by suspending the base of the local anesthetic in water and solubilizing the same by the addition of carbon dioxide, or by dissolving the water soluble, pharmaceutically acceptable, acid addition salt of the local anesthetic and adding thereto a water soluble, pharmaceutically acceptable hydrogen carbonate. Preferably, the resulting solutions are then filled into arnpoules or other containers for application by injection and are kept therein under a certain carbon dioxide pressure. 7

This process may be modified by using a suspension of the local anesthetic base in a solution of its hydrochloride or other water soluble, pharmaceutically acceptable acid addition salt and introducing carbon dioxide and an amount of a pharmaceutically acceptable, water soluble hydrogen carbonate into the suspension, said hydrogen carbonate being added in anamonut sufl'lcient to neutralize the acid moiety of the acid addition salt.

Sodium bicarbonate which is the preferred hydrogen carbonate used in this process may be replaced by equimolecular amounts of other neutralizing agents, such as sodium carbonate, sodium hydroxide, and others. Addition of sodium bicarbonate is of advantage because it yields additional amounts of carbon dioxide as is evident from the following equation:

When reacting the hydrochloride of the local anesthetic Patented June 9, 1964 base with sodium bicarbonate, the further advantage is achieved that first the base is precipitated in finely divided form which is solubilized more readily and rapidly by the addition of carbon dioxide than when suspending the base as such in water and then introducing carbon dioxide into the resulting suspension.

It is also possible to change the order in which the hydrogen carbonate and the acid addition salt of the local anesthetic base are reacted with each other. Instead of adding the hydrogen carbonate to the solution of the acid addition salt, the salt solution may be added to the hydrogen carbonate solution, or the two solutions may be introduced simultaneously in equimolecular amounts into the reaction vessel.

Furthermore, the process of reacting the base with carbon dioxide and the process of reacting the acid addition salt of the base with sodium bicarbonate or the like compounds may be combined and carbon dioxide may be introduced into the mixture of base, acid addition salt, and water soluble hydrogen carbonate.

Surprisingly it was found that preparations according to the present invention, i.e., preparations which contain, in solution, the reaction product of carbon dioxide and lidocaine or other local anesthetic bases of the acid amide type in the presence of an excess of carbon dioxide, have a much higher local anesthetic potency than the heretofore used preparations which contain acid addition salts and especially the hydrochlorides of such bases in solution. The potency of solutions according to the present invention is increased about two to three times over that of hydrochloride solutions as can be demonstrated by the rabbit cornea test. When the isolated nerve action potential technique is employed, a fivefold to tenfold increase in potency over that of solutions of the hydrochloride is observed.

Another very important advantage of preparations according to the present invention over known preparations consists in the possibility of preparing solutions of the local anesthetic agent which are highly effective, even at a pH-value below 6.0. While, for instance, lidocaine hydrochloride solutions are substantially ineffective at a pH-value below 6.0, the lidocaine-carbon dioxide solu tion according to the present invention has a very substantial potency even at a pH of about 5.0 as can be demonstrated by the high lidocaine concentration in the isolated nerve.

A further advantage of preparations according to the present invention is their reduced toxicity. This is due to the precipitation of the base from its carbon dioxidecontaining solution upon release of the carbon dioxide on administration. The precipitated base is more slowly absorbed than, for instance, the hydrochloride. Thus, for instance, in certain anesthetic procedures solutions of a lower concentration of lidocaine can be used than heretofore possible. Spraying solutions containing solvents as they were heretofore employed conventionally, produced concentrations of the local anesthetic in excess of 20% at the site of application.

In contrast thereto, preparations according to the present invention produce effective concentrations of only about 4% and less of the local anesthetic after release of the driving agent. In addition to the more economical use of the local anesthetic, it is evident that the hazard of toxic effects is considerably reduced. The higher concentrations of the local anesthetic at the site of application as produced heretofore is, of course, more apt to cause toxic manifestations and side reactions than the low concentration produced by preparations according to the present invention.

The following examples serve to illustrate the present invention without, however, limiting the same thereto.

EXAMPLE 1 40 g. of lidocaine base are placed into an Erlenmeyer side-arm pressure flask provided with a glass tube extending to the bottom of the fiask. Distilled water is added thereto to make up to 1000 cc. Carbon dioxide is introduced into the solution at a pressure of 10 p.s.i. As soon as absorption of carbon dioxide ceases, the mixture is cooled to about 4 C. Charging with carbon dioxide at said pressure and temperature is repeated until all of the lidocaine is dissolved. The time required to effect solution depends on the frequency of charging. If charged three times during an eight hour working day, solution is effected within three to four days.

The resulting solution is filled into pressure spray containers using either nitrogen at a pressure of p.s.i. or a mixture of 20% of dichloro difiuoro methane and 80% of sym. dichloro tetrafluoro ethane as propellant. The solution may also be filled into glass ampoules, if desired, after addition of epinephrine.

EXAMPLE 2 24.64 g. of lidocaine hydrochloride (with one mole of water of crystallization) are dissolved in 350 cc. of distilled water. A solution of 7.2 g. of sodium bicarbonate dissolved in cc. of distilled water is added slowly and with stirring to said lidocaine hydrochloride solution. No etfervescence is observed but lidocaine base is precipitated in finely divided form. The resulting suspension is diluted with distilled water to a volume of 500 cc. The pH of the resulting suspension is 7.15. It is placed into a 2 liter tubulated Erlenmeyer flask. Carbon dioxide is introduced through a tube extending to the bottom of the flask at a pressure of 10 p.s.i. and room temperature. The precipitated lidocaine base goes into solution within four days of repeated charging of carbon dioxide at a pressure of 10 p.s.i. and room temperature. The pH of the resulting solution is 6.72. The solution contains 4.0% of lidocaine base and about 1.0% of sodium chloride formed in the reaction. It is filled in dispensing containers as described in Example 1.

EXAMPLE 3 24.64 g. of lidocaine hydrochloride (with one mole of water of crystallization) are dissolved in 500 cc. of distilled water. The pH of the solution is 4.08. A solution of 7.2 g. of sodium bicarbonate in 200 cc. of distilled water of a pH of 7.86 is added to the lidocaine hydrochloride solution slowly and with stirring. No effervescence is observed. The pH of the resulting suspension is 7.1. It is placed into a calibrated 2 l. tubulated Erlenmeyer flask. 20.0 g. of lidocaine base, 5.01 g. of sodium chloride, and 200 cc. of distilled water are added thereto. Carbon dioxide is introduced into the mixture at a pressure of 10 p.s.i. and at room temperature. The carbon dioxide pressure is maintained until all the lidocaine base is dissolved. Usually 5 days are required to effect complete solution. The solution is then diluted to a volume of 1,000 cc. Its pH is 6.3 and it contains 4.0% of lidocaine base and 1.0% of sodium chloride, one-half of which was formed in the reaction.

EXAMPLE 4 Distilled water is first charged at 4 C. with carbon dioxide at a pressure of 10 p.s.i. until saturated with carbon dioxide. Its pH is then 3.4.

20.21 g. of lidocaine hydrochloride (with one mole of water of crystallization) are dissolved in 500 cc. of said carbon dioxide-saturated water of 4 C. The resulting solution has a pH of 3.5. 5.87 g. of sodium bicarbonate are dissolved at 4 C. in 200 cc. of carbon dioxide-saturated water to yield a solution of a pH of 6.8. Said sodium bicarbonate solution is added to the lidocaine hydrochloride solution slowly and with stirring while maintaining the temperature at 4 C. No effervescence is observed. The resulting suspension is then diluted to a volume of 1000 cc. with said carbon dioxidesaturated water and is placed in a 2 liter tubulated Erlenmeyer flask. Carbon dioxide is introduced into the suspension at 4 C. and a pressure of p.s.i. until all of the lidocaine base is dissolved. The resulting solution has a pH of 6.4. It is filled into 5 cc. ampoules. When heating the sealed ampoules in a sterilizing autoclave at a temperature of 121 C. for 20 minutes, the lidocaine base precipitates but goes again into solution on standing at room temperature.

EXAMPLE 5 The water used in the preparation of the following lidocaine solution is prepared by saturating distilled water at 25 C. with carbon dioxide at a pressure of 10 p.s.i.

10.105 g. of lidocaine hydrochloride (with one mole of water of crystallization) are dissolved in 350 cc. of said carbon dioxide-saturated water. The resulting solution has a pH of 3.75. A solution of 2.935 g. of sodium bicarbonate in 125 cc. of said carbon dioxidesaturated water having a pH of 7.03 is added slowly and with stirring to the lidocaine hydrochloride solu tion. No eifervescence is observed. The resulting suspension is diluted with carbon dioxide-saturated water to a volume of 500 cc., is then placed in a tubulated Erlenmeyer flask, and is charged at room temperature with carbon dioxide at a pressure of 10 p.s.i. Its pH is 6.65. The pressure is released after one hour. 0.25 g. of sodium metabisulfite and 8.0 mg. of epinephrine are added thereto. Charging and pressurizing of the mixture with carbon dioixde to a pressure of 10 p.s.i. is continued until the lidocaine is completely dissolved. The pH of the resulting solution is 6.61. The solution is filled into 5 cc. ampoules and sealed. The ampoules are sterilized by autoclaving at 121 C. for 20 minutes.

EXAMPLE 6 300 g. of lidocaine base are added to 2.5 liters of distilled water in a 4 liter tubulated Erlenmeyer filter flask. A Teflon coated magnet is also placed in the flask. The flask is equipped with a #12 one hole rubber stopper and with a piece of glass tubing extending through the hole in the stopper and reaching almost to the bottom of the flask. The other end of the tubing has a 90 bend with a piece of rubber tubing fitted thereover and extending about 10 beyond its end. The tubulated side arm has a piece of rubber. tubing over it that extends 2" beyond its end. The stopper is tied in securely and then the rubber tubing from the long glass tube is connected to a tub from a tank of carbon dioxide. Carbon dioxide is allowed to bubble through the Water slowly for about three minutes whereafter the rubber tubing on the tubulated side arm is closed by means of a Hoifman clamp. The carbon dioxide pressure is then raised to 10 p.s.i., and the rubber tubing connected to the carbon dioxide source is closed with a Hoffman clamp. The flask is kept refrigerated at 5 .C. for two hours. Thereafter, more carbon dioxide is introduced through the long tube to bring the pressure up to 10 p.s.i. The flask is then placed on a magnetic stirrer and stirred for about one hour, after which it is gassed again and then refrigerated. This procedure, pressurizing with carbon dioxide, cooling, and stirring, is repeated several times daily. It caused all the lidocaine to dissolve after approximately one week. When the lidocaine is completely dissolved, the side arm is opened to vent the pressure and the stopper is untied and removed. Distilled water is added to bring the volume to exactly three liters, Whereafter the stopper is retied, carbon dioxide is bubbled through the solution, the side vent is clamped, and the pressure of carbon dioxide is raised to 10* p.s.i. The flask is then returned to the refrigerator, ready for further use as a 10% solution of lidocaine base.

Equal volumes of said lidocaine-carbon dioxide solution (10%) and a 2.6% solution of cal-boxy methyl cellulose are mixed with each other. The mixture is placed in a tubulated Erlenmeyer filter flask as used to make the initial solution. Carbon dioxide is bubbled 6 slowly through the mixture. Then the side vent is clamped and the pressure of carbon dioxide is raised to 10 p.s.i.

In this manner a viscous lidocaine-carbon dioxide solution is obtained which contains, in solution, 5% of lidocaine base and 1.3% of carboxy methyl cellulose.

.EXAMPLE 7 17.3 g. of lidocaine base are mixed with about 800 cc. of water. The mixture is placed in an Erlenmeyer flask as described in Example 6. 6.0 g. of sodium chloride are added thereto and the mixture is filled up to a volume of 1000 cc. The flask is then charged with carbon dioxide at a pressure of 10 p.s.i. and cooled to 4 C. Charging with carbon dioxide is repeated until solution is effected. The pressure is released, the stopper is removed, and 0.5 g. of sodium metabisulfite and 0.01 g. of epinephrine are added quickly. The stopper is placed back and secured in the flask. The flask is immediately charged with carbon dioxide at a pressure of 10 p.s.i. and charging is repeated. Complete solution is eifected after several hours. The solution is filled into ampoules which are sealed. It contains lidocaine base in an amount equivalent to that of a 2% lidocaine hydrochloride solution. The epinephrine concentration is 0.01 mg./cc.

Although the process according to the present invention has proved of special advantage in preparing solutions of the local anesthetic lidocaine, it can also be applied to other local anesthetics possessing amino groups and especially to local anesthetics of the acid amide type. It has been found, for instance, that it is of great value in the preparation of solutions of mepivacaine, i.e., dl-N- methyl pipecolyl-2,6-xylidide of the following formula Preparations with said local anesthetic base, for instance, are produced according to the present invention as described hereinafter.

EXAMPLE 8 22.58 g. of mepivacaine hydrochloride and 5.94 g. of sodium bicarbonate in about 475 cc. of distilled water are treated with carbon dioxide as described in Example 2. The resulting solution is made up with distilled water to 500 cc.

EXAMPLE 9 20.0 g. of mepivacaine base, 22.58 g. of mepivacaine hydrochloride, 5.94 g. of sodium bicarbonate, 4.13 g. of sodium chloride, and 900 cc. of distilled water are treated with carbon dioxide as described in Example 3. The resulting solution is made up with distilled water to 1000 cc.

EXAMPLE 10 22.37 g. of mepivacaine hydrochloride, 5.87 g. of sodium bicarbonate, and 700 cc. of carbon dioxide-saturated water are treated at 4 C. with carbon dioxide as described in Example 4. The resulting solution is made up with carbon dioxide-saturated water to 1000 cc.

EXAMPLE 11 11.19 g. of mepivacaine hydrochloride, 2.94 g. of sodium bicarbonate, and'475 cc. of carbon dioxide-saturated water are treated with carbon dioxide at room temperature as described in Example 5. The resulting solution is made up to 500 cc. with carbon dioxide-saturated water. 0.25 g. of sodium metabisulfite and 8 mg. of epinephrine are added thereto, whereafter treatment with 7 carbon dioxide is continued as described in Example 5. Another local anesthetic of the acid amide type, which may also be used for producing valuable local anesthetic preparations according to the present invention is the a-(n propylamino) propionyl toluidide-(Z) of the following formula which, in contrast to lidocaine and mepivacaine, contains a secondary amino group.

The following example serves to illustrate the preparation of solutions of said local anesthetic agent according to the present invention.

EXAMPLE 12 3.4 g. of e-(n-propylamino) propionyl toluidide-(Z) base are placed into an Erlenmeyer side-arm pressure flask provided with a glass tube extending to the bottom of the flask. Distilled Water is added thereto to malte up to 200 cc. Carbon dioxide is introduced thereinto at a pressure of psi. As soon as absorption of carbon dioxide ceases, the mixture is cooled to about 4 C. Charging with carbon dioxide at said pressure and temperature is repeated until the base is completely dissolved. The time required to effect solution depends on the frequency of charging. If charged three times during an eight hour working day, solution is effected within three to four days. The resulting solution is filled into 30 cc. ampoules. It contains 1.71% of the base corresponding to 2% of its hydrochloride.

In place of lidocaine, mepivacaine, and ot-(n-propylamino) propionyl toluidide-(2) which are the preferred local anesthetics to be used as starting materials in the production of preparations according to the present invention, there may be employed other local anesthetic agents of the acid amide type. Such acid amide local anesthetics correspond in principle to the following formula wherein R represents lower alkyl, especially methyl;

R represents halogen, when R is lower alkyl and R is hydrogen, or lower alkoxy, when R and R are lower alkyl, or hydrogen or lower alkyl, especially methyl;

R represents a carballtoxy group, when R is lower alkyl, or halogen, when R is lower alkyl, or hydrogen, or lower alkyl, especially methyl;

R and R represent hydrogen or lower alkyl or, together with the nitrogen and the -CH-group to which they are attached, forming a piperidine ring; and

R represents lower alkyl.

This group of acid amides as is evident comprises not only lidocaine but also mepivacaine which is a piperidino compound wherein R and R of the above given formula represent butylene-CH -CH CH -CH forming a piperidine ring with the nitrogen atom and the neighboring CH-group, and the secondary amino compound a-(n-propylamino) propionyl toluidide-(Z) of the preceding examples.

Other acid amides of the above given formula which are useful components of local anesthetic compositions according to the present invention are, for instance, Ot-(l'lbutylamino) acetyl-2-methyl-6-chloro anilides, a-diethyl- & amino acetyl-2,6-dimethyl-4-butoxy anilide, and the like compounds.

The following pharmacological tests were carried out with lidocaine preparations according to the present invention. They clearly demonstrate the superiority of such preparations over the heretofore used solutions of lidocaine hydrochloride.

I. The topical anesthetic effects of a 4% lidocaine hydrochloride solution and of a 2% lidocaine hydrochloride solution and of carbon dioxide-treated lidocaine base solutions of equivalent concentration as they are obtained by proceeding according to Example 1 were compared on the cornea of female, white rabbits according to the method described by S. Wieding in Acta pharmacol. et toxicoL, vol. 8, pages 117-133 (1952), 0.5 cc. of said solutions were applied to the conjunctival sac for 30 seconds whereby the lidocaine hydrochloride solutions were placed in one eye and the corresponding lidocaine basecarbon dioxide solutions were placed in the other eye. The solutions were alternated between the left and right eyes. The onset of anesthesia was immediate in each case and the duration of anesthesia was recorded whereby a graphite point was used as stimulator. The following Table I illustrates the results achieved.

Table I Lidocaine solution No. 01 Increase Duration, expcriin acminutes ments tivity, HOl Base+CO pII percent 4percont 6.65 17 4 4percent 6.50 35 4 106 2 percent .45 21 3 2perccnt. 6.65 33 3 57 II. The recovery of lidocaine from the tongue of normal, female Wistar rats, pretreated with 25 mg./kg. of sodium pentobarbital intraperitoneally, was determined. 0.05 cc. of the lidocaine solutions given in the following Table H were injected. The animals were sacrificed 10 minutes following the injection. Table II illustrates the results.

they are highly effective at a low pH-value. Thus such solutions can be used, for instance, in infected areas of low pH-value where heretofore local anesthetics did not work properly such as for application to the ear. The test method consists in the introduction of a sciatic nerve sensory block in the frog. For this purpose the sciatic nerve preparation of a green frog was immersed in the test solutions of varying molarity of 10 minutes. The preparation was then rinsed for 40 seconds in Ringers solution. After homogenization, the amount of lidocaine absorbed by the sciatic nerve was determined. The results are given in Table III. Solution a is a lidocaine base solution treated with carbon dioxide under a pressure of 15 p.s.i., i.e., a solution according to the present invention of a pH between 5.68 and 6.23. Solution b is an equimolecular lidocaine hydrochloride solution in Ringers solution, the pH of which was adjusted to a pH of 7.20, i.e., to optimum pH-conditions. Solution 0 is an equimolecular lidocaine hydrochloride solution in Ringers solution of a pH of 6.00.

1 Not determined.

It is evident that the carbon dioxide-treated lidocaine base solution is far better absorbed by the sciatic nerve tissue than lidocaine hydrochloride and that this absorption at a pH of about 6.0 considerably exceeds even the absorption of lidocaine from a solution of a pH of about 7.2, i.e., under conditions which, heretofore, were considered to be optimum conditions. It may be pointed out that such superior results are not achieved when replacing carbon dioxide by other weak acids of about the same dissociation constant, such as lactic acid, tartaric acid, citric acid, and the like, but are apparently due to the administration of the local anesthetic base in the form of its hydrogen carbonate in the presence of an excess of carbon dioxide. The theory is advanced, although the reaction of the basic acid amide is by no means limited to such a theory, that the acid amides having tertiary amino groups form addition salts with carbonic acid while the acid amides having secondary amino groups apparently form carbamic acid compounds wherein the secondary amino group is converted into the carbamate group of the formula Clinical tests were carried out with about 31 patients. n epidural administration of a 2% lidocaine hydrochloride solution and a lidocaine base solution treated with carbon dioxide as described in Example 1 of equivalent lidocaine concentration, the following results were U achieved.

Table IV Lidocaiue solution Initial Complete onset of segmental anesthesia Spread in H01 Base-H30; in min. minutes 2 percent 7. 2 16. 5 2 percent.-. '5. 4 11. 8

Clinical experiences have shown that, for instance, carbon dioxide-treated lidocaine solutions are more effective in lower concentrations than lidocaine hydrochloride solutions, that, as stated above, onset of anesthesia is faster, that anesthesia is more profound, and that recovery is faster due to a shorter duration. Addition of epinephrine to the preparations according to the present invention does not impair the fast onset and the extent of anesthesia but increases its duration.

As pointed out above, the preparations according to the present invention can also be applied to the ear. Preparations as heretofore available did not produce satisfactory anesthesia in the ear.

It Was found that it is even possible to reestablish anesthesia after the anesthetic effect of lidocaine base has worn off by simply administering a carbonated saline solution equilibrated with carbon dioxide at 10 psi. Such a solution was injected up the epidural catheter in 16 patients after regression of blockade and return of sensation. The volume equalled or slightly exceeded the volume of the main analgesic dose previously administered in order to insure that the carbonated solution would travel the same distance in the extradural space and reach the same areas which had been formerly bathed with analgesic solution. The carbonated solution was injected 10 minutes to 30 minutes after disappearance of skin analgesia. Plain carbonated normal saline solution was used in.6 cases and in 10 cases epinephrine was added. The following Table V shows the results achieved by such a treatment. In said table the recall of blockade is expressed in percent of the original blockade and is calculated by the follovsu'ng equation:

Recall of blockade in percent= Original segments Table V Number of cases treated with- Recall of blockade Plain Carbonated carbonated saline solusaline tion with solution epinephrine percent 1 percent 1 percent 1 percent 1 percent 1 1 percent l 1 percent 1 1 percent 1 percent. l percent 1 4 These results show that partial reappearance of blockade resulted in every case, but that the extent of recall varied both in spread and intensity. In some cases a barely perceptible hypoalgesia developed in a few dermatomes while in others the block recovered to its full segmental extent with analgesia to pin prick and relief of postoperative pain. The restitution of blockade was transient and weak and the intensity never extended to a return of motor Weakness. Blunting of sensation in the area of preceding blockade began about five minutes after injecting the carbon dioxide solution and then receded again five minutes to ten minutes later.

The addition of epinephrine augmented the recall of analgesia in both the number of segments and the intensity of blockade.

These test results clearly prove the surprisingly advantageous effect of carbon dioxide treatment of lidocaine and, generally, local anesthetics of the basic acid amide type.

As stated hereinabove, solutions of lidocaine, mepivacaine, and other local anesthetics of the acid amide type according to the present invention can be pressure-dispensed by spraying or can be parenterally administered for infiltration and nerve block anesthesia, for peridural and spinal anesthesia.

It is also possible to prepare jellies, ointments, and creams containing such solutions which have been adjusted to the desired consistency with gelling compounds, such as carboxy methyl cellulose and the like. Jellies of this type are advantageously used for providing profound anesthesia of accessible mucous membranes, particularly in the male and female urethra, and in the ear, nose, and throat.

Pharmaceutical preparations according to the present invention are prepared in accordance with the examples given hereinabove and in a conventional manner.

Of course, many changes and variations in the local anesthetics used, in the manner of combining the same with carbon dioxide and of introducing excess carbon dioxide into their solutions, in the mode of administration of such preparations, and the like may be made by those skilled in the art in accordance with the principles set forth herein and in the claims annexed hereto.

We claim:

1. In a process of increasing the anesthetizing effect of aqueous solutions of lidocaine, the step which consists in introducing carbon dioxide under pressure into a suspension of the lidocaine base at a temperature not substantially exceeding room temperature until the lidocaine base is dissolved.

2. In a process of increasing the anesthetizing effect of aqueous solutions of lidocaine, the steps which consist in adding a pharmaceutically acceptable neutralizing agent to a solution of a pharmaceutically acceptable acid addition salt of lidocaine in an amount equimolecular to the acid moiety of said acid addition salt and introducing carbon dioxide under pressure into the resulting mixture at a temperature not substantially exceeding room temperature until solution is achieved.

3. The process according to claim 2, wherein the pharmaceutically acceptable neutralizing agent is sodium bicarbonate.

4. In a process of increasing the anesthetizing effect of aqueous solutions of mepivacaine, the step which consists in introducing carbon dioxide under pressure into a suspension of the mepivacaine base at a temperature not substantially exceeding room temperature until the mepivacaine base is dissolved.

5. In a process of increasing the anesthetizing effect of aqueous solutions of mepivacaine, the steps which consist in adding a pharmaceutically acceptable neutralizing agent to a solution of a pharmaceutically acceptable acid addition salt of mepivacaine in an amount equimolecular to the acid moiety of said acid addition salt, and introducing carbon dioxide under pressure into the resulting mixture at a temperature not substantially exceeding room temperature until solution is achieved.

6. The process according to claim 5, wherein the pharmaceutically acceptable neutralizing agent is sodium bicarbonate.

7. In a process of increasing the anesthetizing effect of aqueous solutions of a local anesthetic agent of the basically substituted acid amide type of the formula wherein R indicates lower alkyl;

R indicates a member selected from the group consisting of halogen, when R; is lower alkyl and R is hydrogen; lower alkoxy when R and R are lower alkyl; hydrogen, and lower alkyl;

R indicates a member selected from the group consisting of carbo-lower alkoxy, when R is lower alkyl; halogen, when R is lower alkyl; hydrogen, and lower alkyl;

R; and R indicate members selected from the group consisting of hydrogen, lower alkyl, and, R and R together with the nitrogen atom and the -CH-group to which they are attached, forming a piperidine ring; and

R indicates lower alkyl,

the step which consists in introducing carbon dioxide under pressure into a suspension of said local anesthetic base of the basically substituted acid amide type at a temperature not substantially exceeding room temperature until the base is dissolved.

8. In a process of increasing the anesthetiziug effect of aqueous solutions of a local anesthetic agent of the basically substituted acid amide type of the formula wherein R indicates lower alkyl;

R indicates a member selected from the group consisting of halogen, when R is lower alkyl and R is hydrogen; lower alkoxy when R and R are lower alkyl; hydrogen, and lower alkyl;

R indicates lower alkyl, the steps which consist in adding a pharmaceutically acceptable neutralizing agent to a solution of a pharmaceutically acceptable acid addition salt of said basically substituted acid amide in an amount equimolecular to the acid moiety of said acid addition salt, and introducing carbon dioxide under pressure into the resulting mixture at a temperature not substantially exceeding room temperature until solution is achieved.

9. The process according to claim 8, wherein the pharmaceutically acceptable neutralizing agent is sodium bicarbonate.

10. A pharmaceutical preparation comprising a solution of lidocaine base in water saturated with carbon dioxide under pressure, said preparation being kept in pressure resistant containers.

11. A pharmaceutical preparation comprising a solution of mepivacaine base in water saturated with carbon dioxide under pressure, said preparation being kept in pressure resistant containers.

12. A pharmaceutical preparation comprising a solution of a local anesthetic base of the basically substituted acid amide type of the formula wherein R indicates lower alkyl;

R and R indicate members selected from the group consisting of hydrogen, lower alkyl, and, R and R together with the nitrogen atom and the CH-group to which they are attached, forming a pipcridine ring; and

R indicates lower alkyl, in water saturated with carbon dioxide under pressure, said preparation being kept in pressure resistant containers.

13. The process of improving the local anesthetic effectiveness of lidocaine, which process consists in applying a solution of lidocaine base in water saturated with carbon dioxide under pressure to the part of the body to be anesthetized.

14. The process of improving the local anesthetic effectiveness of mepivacaine, which process consists in applying a solution of mepivacaine base in water saturated with carbon dioxide under pressure to the part of the body to be anesthetized.

15. The process of improving the local anesthetic eflecwherein R indicates lower alkyl;

R indicates a member selected from the group consisting of carbo-lower alkoxy, when R; is lower alkyl; halogen, when R1 is lower alkyl; hydrogen, and lower alkyl;

R, and R indicate members selected from the group consisting of hydrogen, lower alkyl, and, R and R together with the nitrogen atom and the CH-group 14 to which they are attached, forming a piperidine ring; and R indicates lower alkyl, which process consists in applying a solution of said local anesthetic agent in water saturated with carbon dioxide under pressure to the part of the body to be anesthetized.

References Cited in the file of this patent UNITED STATES PATENTS Prussin Sept. 20, 1960 OTHER REFERENCES