Background Of The Invention

A large variety of N-alkyl-pipecolic acid amides have been synthesized. A number of these compounds have found use as local anesthetics, such as Mepivacaine, namely the racemate of N-methylpipecolic--acid-2,6-xylidide:

and Bupivacaine, namely the racemate of N-butylpipecolic- acid-2,6-xylidide:

References disclosing homologs of this series of compounds include U.S. Patent 2,799,679; British Patent 775,749; British Patent 775,750; British Patent 800,565; British Patent 824,542; British Patent 869,978; British Patent 949,729; U.S. Patent 4,110,331; and U.S. Patent 4,302,465.

There is a summary paper dealing with these types of anesthetics, and related compounds in a paper in Acta Che ica Scandinavica 11, (1957) No. 7 pp. 1183-1190 by Bo Thuresson af Ekenstam et al.

There is a discussion of the ^" effect of optical isomers in related compounds in J. Med. Chem., 14 (1971) pp. 891-892

entitled "Optical Isomers Of Mepivacaine And Bupivacaine" by Benjamin F. Tullar; Acta Pha m. Suecica, 8 (1971) pp. 361- 364 entitled "Some Physicochemical Properties Of The Racemates And The Optically Active Isomers Of Two Local Anaesthetic Compounds", by . Friberger et al -.; Acta Pharmacol et Toxicol, 31 (1972). pp. 273-286 entitled "Toxicological And Local Anaesthetic Effects Of Optically Active Isomers Of Two Local Anaesthetic Compounds", by G. Aberg; Annual Review Of Pharmacology, 9 (1969) pp. 5Q3-520 entitled "Duration Of Local Anaesthesia", by F.P. Luduena and Acta Pharmacol, et Toxicol, 41 (1977). pp. 432-443 entitled "Studies On The Duration Of Local Anaesthesia: Structure/Activity Relationships In A Series Of Homologous Local Anaesthetics", by G. Aberg et al.

Summary Of The Invention

We have found that L-N-n-propylpipecolic acid-2,6-xylidide is markedly superior as a local anesthetic for mammals inclu¬ ding humans to other known homologues of these compounds including the commercially exploited local anesthetics

Mepivacaine, which is the N-methyl homolog, and Bupivacaine, which is the N-n-butyl homolog. This is surprising because the analgesic activity of the racemic n-propyl compound (R- N-n-propylpipecolic acid-2,6-xylididel is so far below that of the racemic Bupivacaine (R-N-n-butylpipecolic acid-2,6- xylidide) as to stultify any further research on the n-propyl compound. Furthermore, research on the optical isomers of Mepivacaine and Bupivacaine reveal that the differences in potency compared to the racemate were not of such magnitude as to justify the commercial production of the optical isomers.

The effect of modifying the alkyl group on the nitrogen atom in the piperidine ring of this family of homologs is not completely understood. However, it is clear that any alkyl group of five or more atoms on this nitrogen atom is too toxic to function as a local anesthetic_

We have discovered that the laevo optical stereo-isomer

N-n-propylpipecolic acid-2 , 6-xylidide has optimal properties as a local anesthetic .

Our method Of preparing L-N-n-propylpipecolic acid-2 , 6-xyli- dide comprises resolving pipecolic acid to isolate the laevo optical stereoisomer . The laevo pipecolic acid hydrochloride is then chlorinated to form a laevo pipecolic acid chloride hydrochloride :

EC1

The acid chloride hydrochloride is then reacted with 2 , 6- xylidine to form the L-pipecolic acid-2 , 6-xylidide hydro¬ chloride :

H_l

L-pipecolic acid-2,6-xylidide is then propylated to yield the L-N-n-propylpipecolic acid-2, -xylidide.

Detailed Description Of The Preferred Embodiment

The following illustrative process described in four steps may be used to prepare the L-N-n-propylpipecolic acid-^-2,6- xylidide:

lfUR£4 * ^{* ■} O PI

Example 1 - Resolution of pipecolic acid

130 grams of pipecolic acid and 158.6 grams of Laevo (+)-tartaric acid are dissolved under stirring in 2,000 ml 95% ethyl alcohol and 125 ml water at a temperature of approximately 80°C. The solution is allowed to cool to room temperature and after two days the crystallized D-pipecolic-tartrat is separated. The L-pipecolic-tar- trate remains in solution.

The filtrate is evaporated and dissolved in 5% acetic acid. Finally the solution is treated with Amberlite IR 45 in an ion exchanger. The eluate thus obtained is evaporated and the resulting crystalline residue is dried with potassium hydroxide in vacuo. The product obtained consists of L-pipecolic acid { _° ~-) 2_4. -26 ^• .•2o (C=5*#, H-Ol.

Example 2 - Preparation of L-pipecolic acid chloride hydrochloride

The chlorination.of the L-pipecolic acid hydrochloride is achieved by adding during a time period of 15-20 minutes four grams of phosphorus pentachloride to a suspension of four grams of L-pipecolic acid hydrochloride in 40 ml acetyl- chloride. The initial reaction is effected at a tempera¬ ture of about 35 C under stirring for a time duration of two hours. The chlorination is completed by adding during . a time period of about 10 minutes an additional two grams of phosphorus pentachloride and stirring over a further period of four hours while maintaining the supension at a temperature of about 35°C.

The resulting L-pipecolic acid chloride hydrochloride is filtered and washed with toluene and acetone. The crystal-

*

A commercial weakly basic polystyrene-polyamine anion exchange resin sold by Rohm & Haas Company of Philadelphia, Pa. designed primarily to remove strong acids.

C means concentration in grams per 1Q0 ml of solution.

line residue is then dried in vacuo. The product starts to sinter at 140°C (using a microscope) then decomposes at 150°C and all is melted at 155°C.

Example 3 - Preparation of L-pipecolic acid-2,6-xylidide

A mixture of 2.7 ml 2,6*-xylidine, 4 ml acetone, and 4 ml N-methylpyrrolido e is gradually added under stirring for two hours at a temperature of approximately 70°C to a suspension of 4 grams of L-pipecolic acid chloride hydrochlo¬ ride in 30 ml of dry toluene. This yields a crystalline product, which is filtered, washed with ^' acetone and dried. This crystalline product is then dissolved in water and the base is precipitated by the addition of ammonia.

The base is then extracted by the use of toluene and is recovered by evaporation. The base is recrystallized from a mixture of hexane and ethanol to yield L-pipecolic acid-2,6-xylidide. The melting point of this compound is 129-130°C, (* *) ² ^ + 46.4° (C=2, 1 M HC1) .

Example 4 - Preparation of L-N-n-propylpipecolic acid - . 2,6-xylidide

7.9 ml n-propylbromide and 6.8 grams of potassium carbonate are added to a solution of 17 grams of L-pipe¬ colic acid-2,6-xylidide dissolved in 60 ml of isopropyl alcohol. Thereafter, 5 ml of water is added to the mixture and the reaction is carried out for a period of about four hours at approximately 72°C.

To complete the reaction, a further 0.8 ml n-propyl¬ bromide are added under continuous stirring and heating for 4 hours. Thereafter the solvent is carefully evaporated off under reduced pressure (20-25 mm Hg) . The residue is treated with a mixture of 250 ml toluene and an equal amount of water under gentle heating at approximately 50°C. The toluene layer is separated and washed three times with 100 ml warm water (40°C1. A 175 ml portion of the toluene is removed by evaporation and the remainder is stored in a

refrigerator at +5°C for 6 hours to achieve crude crystal¬ line L-N-n-propylpipecolic acid-2,6-xylidide.

The crystalline product is separated by filtration, washed with some cooled toluene and dried at 70 C.

Approximately 16 grams of crude L-N-n-propylpipecolic acid- 2,6-xylidide are obtained. Recrystallization from toluene gives approximately 14 grams of the pure product, m.p. 144-146°C, ) ²⁵ -82.0°(C=2, MeOH) .

This product is dissolved in 100 ml ethanol and neutralized with concentrated hydrochloric acid.

Ethanol is removed by evaporation and the hydrochloride product obtained is vacuum dried. Finally the latter is recrystallized from 75 ml isopropyl alcohol. The yield is approximately 12 grams of L-N-n-propylpipecolic acid-2,6-xylidide hydrochloride, m.p. 260-262°C, MJ - 6.6°(C=2, H ₂ 0) .

The reactions for achieving the compound of the present invention may be shown diagrammatically (commencing with L-pipecolic acid hydrochloride) :

coci + POCI ₃ + e-zi

/ propyat on

Useful background information concerning the synthesis of the homologs of N-alkyl-pipecolic acid-2,6-xylidide is contained in British Patents Nos. 775,750; 775,749; 824,542; 800,565 and 949,729; and in the article by Ekenstam et al. in Acta Chem. Scand. 11 (1957) No. 7, pp. 1183-1190, the disclosures of which are incorporated by reference.

A toxicological-pharmacological evaluation of the compound of the present invention, and related homologs is set forth below.

The toxicity of the homogolous series of N-alkyl-pipecolic acid-2,6-xylidides increases step wise and quickly with the lengthening of the alkyl chain.This is manifest from Table 1 set forth below, which compares racemic mixtures of alkyl homologues having from 1 to 5 carbon atoms.

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O PI

TABLE 1 - TOXICOLOGICAL DATA

Tissue * π ₅₀ toxicity I.v.

Molecular limits (intravenous)

No . Structure (Racemate) weight in % πig kg/mouse

1 N-methylpipecolic acϊd- 246.46 3.5 40.3 2,6-xylidide

2 N-ethylpipecolic acid- 260.17 3.Q 21.0 2,6-xylidide

3 N-n-propylpipecόlic acid- 2, 6-xylidide 274.18 1.5 13.6

4 N-n-butylpipecolic acid- 2, 6-xylidide 288.19 0.75 7.8

5 N-n-pentylpipecolic acid- 2, 6-xylidide 302.20 slightly soluble in water and tissue irritating***

Trypan blue test; Hbppe et al. (1950) , 39, 147-151 "Use

Of Trypan Blue And Rabbit Eye Tests For Irritation",

J. Amer. Pharm. Ass. (Sclent. Ed.), the values are approximate threshold values of tissue irritancy.

-#*

LD "5 _ς ,0,, i.e. 50% mortality of mice.

Studies On The Duration Of Local Anaesthesia: Structure/Activity Relationships In A Series Of Homologous Local Anaesthetics, by G. Aberg, et al., Acta pharmacol. et toxicol., 41 (1977) pp. 432-443, Table 1 at p. 435.

From Table 1, it can be seen that the toxicological effects of the subject homologous series increases step- wise and quickly when the N-alkyl chain is lengthened, both with regards to tissue toxicity limits as well as intra- venous toxicity.

The optical stereoisomers of the substances Nos. 1,

2 and 4 in Table 1 have earlier been prepared in pure form.

These products have also earlier been shown to give insigni- ficant differences with regards to the parameters which are of vital importance for the clinical use of an anesthetic product. See study which involves the discussed substances according to Table 1, by G. Aberg et al., "Studies on the Duration of Local Anesthesia: Structure/Activity Relation- ship in a Series of Homologous Local Anesthetics", Acta Pharmacol, et Toxicol. 41 (19771 pp. 432-443.

Some of the pure stereoisomers, whose structures are refer¬ red to in Table 2, have, on repeated occasions, been studied with regards to possible important differences of their practical aneεthesiological importance. Among others, substance No. 1 in Table 1, has been studied by G. Aberg: Studies on Mepivacaine and its Optically Active Isomers with Special Reference to Vasoactive Properties, Dept. of Pharmacology, School of Medicine, Linkoping, Sweden, 1972.

According to this work, no great differences in effect can be expected between the different steroisomers in the homologous group starting with Mepivacaine (the N-methyl compound I .

The stereoisomers of the substances Nos. 1, 2 and 4, (Table 2) have been prepared earlier in a pure form but have shown no significant clinical differences in humans with respect to their use as local anesthetics. According to the invention, the L-isomer of substance No. 3 has now been prepared in a pure form and the results of studies on intracutaneous wheals as well as finger blockades in humans

have shown that this compound has surprisingly good properties.

By use of intracutaneous wheals it is possible to measure the analgesic effect of an anesthetic on the peri¬ pheral nerve endings of the epidermis.

This technique gives information about: (1) The duration of anesthesia. (2) Epidermal diffusion. (3) Vascular effects.

TABLE 2 - INTRACUTANEOUS WHEALS

Molecular Analgesia- Substance weight Melting points Opt, activity- duration No. Form* base HCl-salts °C Systen (-*•-•) 'D r ,° cone. % minutes

246.16 293-95 C=5 ^• -63 0.5 68+11

R 260-62 MeOH 0 II 65+9

D 293-95 +63 II 53+8

260.17 255 C=2 -70 0.25 76+11

R 252-54 EtOH 0 II 63+13

255 +69.5 II 58+11

274.18 260-62 C=2 -82 0.25 220+23***

R 260.5 MeOH .0 84+15

D 260-62 +82 81+14***

288.19 255-57 G=2 -80.9 0.25 76+10

R 255-56 MeOH Q II 118+15

D 255-57 +80.9 II 75+11

Not usable as slightly

R soluble in water, irritating

D

Infiltration anesthesia on forearms of man. The test solutions were injected intracutaneously on the dorsum of the forearms of 12 healthy volunteers, 20-25 years old. Three wheals were made on each arm in a rotating system in a double-blind fashion. The local anesthetic effect was tested using a pin-prick technique. The "duration of anesthesia" was defined as the time during which none out of six pin-pricks into the wheal was felt. The method has been described by Dhuner et al. (1972) . Dhuner, K.G., D.H. Lewis, A. Nyquist, D. Selander &. E. Stig: Vascular effects of the isαners of mepivacaine, Acta Anaest. Scand. 1972, Suppl. 48, 45-52.

* ** L=Iaevo, R=Racatιic, D=Dextro

C equals grams per 100 ml of solution.

New isolated stereoismers

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TABLE 3 - RESULTS OF THE FINGER BLOCKADE^

Analgesia-

Substance Time of onset duration No. Form Concentration % minutes* minutes Frequency

1.0 7 + 3 124 + 21 11Λ2 " + 2 102 + 22 13/13

D " + 2 71 + 13 9/13

0.5 9 + 2 -141 + 18 11/13

R 10 + 1 123 + 19 12/12

D 9 + 3 112 + 21 10/13

0.25 12+ 2 739 + 46 6/7 10 + 1 263. + 31 9Λ0

D 14 + 3 175 + 21 7/8

0.25 13 + 3 259 + 37 13/20

R 9+ 2 388 + 50 15/20

D 15 + 4 243 + 38 14/20

Not usable, too irritating against tissue

Digital nerve blocks in man. The test solutions were injected to block the ulnar volar nerves on the 2nd and 4th fingers of the same volunteers who has participated in the intracutaneous wheal tests described above. The tests were carried out in a double blind fashion in a rotating system, so that each solution was used with equal frequency on each of the blocke fingers. A total of four injections, 1.0 ml per injection, was done in each subject. One week later the procedure was repeated in the same subjects using the remaining test solutions. This method has been des¬ cribed by Dhuner et al. (1972) (for citation see Table 2 footnote 1) .

To measure the efficacy and duration of the blocks, the individual fingers vere pricked rapidly 10 times with a needle algesimeter and the number of painful pricks rioted. Each finger was tested every five minutes until complete recovery. The time of complete anesthesia has been called "duration of anesthesia". i.e. - the time frcm injection to complete analgesia

***** i.e. - the number of successful anesthesias of total number of anesthesias performed (no anesthesia is possible if the ulnar volar nerves are not contacted by the anesthetic) new isolated stereoisαners

When comparing the results of intracutaneous wheal (Table 2) and finger-blockade (Table 3) it can be seen that of the 12 optically active and inactive structures, sub¬ stance No. 3, namely L-N-n-propylpipecolic acid-2,6-xylidide, gives the optimal increase of anesthetic effects.

This unexpected increase in effect runs contra to what would be expected from the activity of prior stereoisomers.

Until now, racemic Bupivacaine, namely No. 4 R, has been the compound having the largest clinical use in this series. During the past 15 years Bupivacaine has had a world wide increase in clinical use. As the above data discloses the L-N-n-propylpipecolic acid-2,6-xylidide is strikingly superior to Bupivacaine, being both a far better anesthetic and much less toxic.

In order to give a better illustration of the dif¬ ferences in effect, a series of various intracutaneous wheals has been investigated with regards to the skin tem- perature within the wheals compared to the skin temperature around the outside of the wheals during anesthesia. Based on this investigation, interesting differences in the tem¬ peratures can be observed.

Equipment used for the temperature measurements (experimental)

The given values have been registered 10 minutes after the intracutaneous injection, and thereafter the con¬ trol values were immediately determined.

The mean value of 6 wheals for each substance forms the basis for the values. The control values, however, represent the mean value obtained from 4 different sites measured outside the wheals 90 degrees apart and 2 centi¬ meters from the wheal's center.

In the temperature measurements, an Oriel tempera- ture apparatu has been used. This is battery driven and

_B U E

A widely used apparatus to measure skin temperature, OMPI

is connected to a detector (diameter, 2,5 mm) and has an accuracy of + 0.1 C. The temperature of the skin is measured pressing the detector against the skin with a pressure of 100 grams. It is retained against the skin until a constant temperature is obtained.

In Table 4, T-, is the measurement obtained at the center of the wheal, and T ₂ is the average of four measure¬ ments taken 90 degrees apart outside of the wheal and with- in 2 centimeters of the wheal's center.

TABLE 4 - TEMPERATURE MEASUREMENTS , ACCURACY: + 0. 1°C

• i OC

Substance Intracutaneous T ₂ °C °C

No. as Hl-salt Cone. % Wheal Controls Difference

1 Lidocaine 1.0 30.2 29.3 + 0.9

2 Lidocaine + 1.0 27.8 28.9 - 1.1 adrenaline* 1:200000

3 R-Bupivacaine 0.25 31.5 32.1 - Q.6

4 L-N-n-propyl¬ 0.25 30.0 31.2 - 1.2 pipecolic acid- 2,6-xylidide

5 R-N-n-propyl- 0.25 33.1 33.2 - Q.l pipecolic acid-

2 , 6-xylidide

6 R-N-2-hydroxy- 1.0 31.2 30.7 + Q.5 ethyl-pipecolic acid-2,6-xylidide

*

Adrenaline (epinephrine). is a vasoconstrictor which reduces the cross-section of the wheal region's blood vessels and retains the Lidocaine in the wheal region.

From Table 4 it is seen that example No. 4, namely L-N-n-propylpipecolic acid-2,6-xylidide surprisingly, lowers the temperature over its wheal to approximately the same extent as the combination of lidocaine with ^' drenaline 5 (Table 4, example No. 2). This may be due to an effect on the blood vessels within the wheal similar to that of adrenalin. The racemic compound of example No. 5 showed practically no change in temperature.

10 Tables 1-4 demonstrate the connection between

L-N-n-propylpipecolic acid-2,6-xylidide and the most im¬ portant parameters in each table, e.g. optical stero- isomer, duration of analgesia in intracutaneous wheals, the analgesic duration of fingerblocks, and the maximiza--

15 tion of temperature decrease. It is manifest that L-N-n- propylpipecolic acid-2,6-xylidide is the optimal anesthetic within this homologous series.

L-N-n-propylpipecolic acid-2,6-xylidide may be used as an 20 injectable local anesthetic in the form of a water soluble salt. It may be used as the base in suppositories, or it may be used as a topical anesthetic by being blended into conventional solvents and carriers including thixo- tropic mixtures which form gels, or in a suspension, or 25 it may be tableted in conjunction with conventional tableting materials.

Salts of L-N-n-propylpipecolic" acid-2,6-xylidide may be made with the common mineral acids, aliphatic car-

30 boxylic acids, aromatic carboxylic acids and amino acids.

^" Conventional safeguards must, of course, be used in respect to L-N-n-propylpipecolic acid-2,6-xylidide, such as the use of isotonic solutions when the anesthetic is employed as an injectable. Such isotonic solutions may be prepared

35. from suitable salts, such as the water soluble chlorides of sodium, potassium, calcium and magnesium respectively, or the water soluble sulphates of sodium, potassium and magnesium.

"BUR-S

Of course, the concentration of L-N-n-propylpipecolic acid-2,6-xylidide when administered as a local anesthetic will be regulated to avoid tissue irritation or toxic reaction effects. The regulation of the concentration of this anesthetic can be achieved by following conventional toxicity tests and protocols heretofore established for local anesthetics.

The present invention may be embodied in other spe- cific forms without departing from the spirit or essen¬ tial attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the fore¬ going specification, as indicating the scope of the invention.

OMPI