IE55803B1 - L-n-n-propylpipecolic acid-2,6-xylidide and method for preparing the same - Google Patents

Description

This invention relates to L-N-n-propylpipecolic acid2,6-xylidide and method for preparing the same.

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-xylidides and Bupivacaine, namely the racemate of N-butylpipecolicacid-2,6-xylidide: I )—1 (CH2)3 CH3 CH3 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 Chemica Scandinavica 11, (1957) No. 7 pp. 1183-1190 by Bo ThureSson a£ Ekenstam et al.

There is a discussion of the effect of optical isomers in related compounds in J, Hed. Chem., 14 (1971) pp. 891-892 entitled Optical Isomers Of Mepivacaine And Bupivacaine by Benjamin F. Tullary Acta Pharm. Suecica, 8 (1971) pp. 361-364 entitled Some Physicochemical Properties Of The Racemates And The Optically Active Iscaners Of TVjo Local Anaesthetic Compounds, by P. 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. 503— 520 entitled Duration Of Local Anaesthesia, by F.P.

Luduena and Acta Pharmacol, et Tpxicol, 41 (1977) pp. 432.443 entitled Studies On The Duration Of Local Anaesthesia: Strueture/Activity Relationships In A Series Of Homologous Local Anaesthetics, by. G. Aberg et al.

We have found that L-N-n-propylpipecolic acid-2,6xylidide is markedly superior as a local anesthetic for mammals including 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-xylidide) is so far below that of the racemic Bupivacaine (R-N-nbutylpipecolic acid-2,6-xylidid§) as to stultify any further research on the n-propyl compound. Furthermore, research on the optical isomers of Mepivacaine and Bupivacaine reveal that thp 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 nitro gen 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 stereoisomer N-n-propylpipecolic acid-2,6-xylidide haa optimal properties as a local anesthetic.

Our method of preparing L-N-n-propylpipecolic acid — 10 2,6-xylidide 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: HCl The acid chloride hydrochloride is then reacted with 2,6-xylidine to form the L-pipecolic acid-2,6-xylidide hydrochloride: L-pipecolic acid-2,6-xylidide is then propylated to 20 yield the L-N-n-propylpipecolic acid-2,6-xylidide.

The following illustrative process described in four steps may be used to prepare the L-N-n-propylpipecolic acid-2,6-xylidide: Example 1 - Resolution of pipecolic acid 130 grams of pipecolic acid and 158.6 grains 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-tartrate is separated. The L-pipecolic-tartrate 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 9 consists of L-pipecolic acid [X] D -26.2(05**, H2O).

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 acetylchloride. The initial reaction is effected at a temperature 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 suspension at a temperature of about 35°C.

The resulting L-pipecolic acid chloritfe 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 100 ml of solution. line residue is then dried in vacuo. The product starts Lo sinter at. 14O'*C (using a microscope) then decomposes at 15()eC and all is melted at 155°C.

Example 3 - Preparation of L-pipecolic acid-2,6-xylidide 5 A mixture of 2.7 ml 2,6-xylidine, 4 ml acetone, and ml N-methylpyrrolidone 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 hydrochloride 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, [<*]25d +46.4(02, 1 M HCI).

Example 4 - Preparation of L-N-n-propylpipecolic acid— 2,6-xylidide 7.9 ml n-propylbromide and 6.8 grains of potassium carbonate are added to a solution of 17 grams of L-pipecolic 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-propylbromide 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°C). 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 crystalline 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, [o<]2□ -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 hydro10 chloride 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. 260-262eC, [«. 125O -6.6*02, H2O).

The reactions for achieving the compound of the present invention may be shown diagrammatically (commencing with L-pipecolic acid hydrochloride); •3H7 ‘3 Useful background information concerning the synthesis of the homologs of N-alkyl-pipecolic acid-2,6xylidide 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.

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

The toxicity of the homologous series of N-alkylpipecolic 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. < TABLE 1 - TOXICOLOGICAL DATA NO. Structure (Racemate) Molecular weiqht Tissue* toxicity limits in % I.v. (intravenous) mq/kq/mouse 1 Nmethylpipecolic acid2,6-xylidide 246.46 3.5 40.3 2 3 N-ethylpipeoolic acid2,6-xylidide 260.17 274.18 3,0 1.5 21.0 13.6 N-n-propyipipeconc acia— 2,6-xylidide 4 N-n-butylpipecolic acid2,6-xylidide 288.19 0.75 7.8 5 N-n-pentylpipecolic acid2,6-xylidide 302.20 slightly soluble in water and tissue irritating*** * Trypan blue test? Hoppe et al. (1950), 39, 147-151 Use Of TTypan Blue And Rabbit Eye Tests For Irritation, J. Amer. Pham. Ass. (Sclent. Ed.), the values are approximate threshold values of tissue irritancy.

** U35Q, i.e. 50% mortality of mice.

*** Studies On The Duration Of Local Anaesthesia: Structure/ftctivity Relationships In A Series Of Homologous Local Anaesthetics, by G. Aberg, et al·, Acta pharmacol. et toxiool., 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 stepwise and quickly when the N-alkyl chain is lengthened, both with regards to tissue toxicity limits as well as intravenous toxicity. < The optical stereoisomers of the substances Nos. 1, and 4 in Table 1 have earlier been prepared in a pure form. These products have also earlier been shown to give insignificant 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 Relationship in a Series of Homologous Local Anesthetics, Acta Pharmacol, et Toxicol. 41 (1977) pp. 432-443.

Some of the pure stereoisomers, whose structures are referred to in Table 2, have, on repeated occasions, been studied with regards to possible important differences of their practical anesthesiological 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 stereoisomers in the homologous group starting with Mepivacaine (the Nmethyl compound).

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 c 1 have shown that this compound has surprisingly good properties.

By use o£ 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.

TABUS 2 - DOTACJIRNBOUS WHEALS1 Substance Molecular weight base Meltings points HCl-salts °C Opt. activity System [«n? Analgesia- duration °conc. % minutes No. Form* I L 246.16 293-95 05** -63 0.5 68+11 R . 260-62 MeOH 0 w 65? 9 D 293-95 +63 It 53*8 2 L 260.17 255 02 -70 0.25 76+11 R 252-54 EtCH 0 n 63+13 D 255 +69.5 It 58+11 3 L 274.18 260-62 02 -82 0.25 220+23*** R 260.5 MeOH 0 It 84+15 D 260-62 +82 H 81+14*** 4 L 288.19 255-57 02 -80.9 0.25 76+10 R 255-56 MeOH 0 • 118+15 D 255-57 +80.9 It 75+11 5 L Not usable as slightly R D soluble in water, irritating 1 Infiltration anesthesia on forearns of man. The test solutions were injected intracutaneously on the dorsum ofthe 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 Ohuner et al. (1972). Ehuner, K.G., D.H. Lewis, A. Nyquist, D. Selander & E. Stig: Vascular effects of the iscmets of mepivacaine, Acta Anaest. Scand. 1972, Suppl. 48, 45-52.

* OLaevo, R=»Racemic, D=»Dextro ** C equals grace per 100 ml of solution.

*** New isolated stereoisomers r TABLE 3 - RESULTS OF THE FINGER BLOCKADE2 Substance Time of onset minutes * Analgesia- duration minutes Frequency** NO. Form Concentration % 1 L 1.0 7+3 124 + 21 11/12 R w + 2 102 + 22 13/13 D It + -2 71 + 13 9/13 2 L 0.5 9 + 2 141 + 18 11/13 R It 10+1 123 + 19 12/12 0 H 9 + 3 112 + 21 10/13 3 L 0.25 12 + 2 739 + 46*** 6/7 R « 10 + 1 263 + 31 9/10 D N 14 + 3 175 + 21*** 7/8 4 L 0.25 13 + 3 259 + 37 13/20 R It 9+2 388 + 50 15/20 D N 15 + 4 243 + 38 14/20 5 Not usable. too irritating against tissue 2 Digital nerve blocksin jnan. The test solutions were injected to block the ulnar νοϋ nerves on the~2nd and 4th fingers of the sane volunteers who had 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 blocked 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 described by Ehuner et al. (1972) (for citation see Thble 2 footnote 1).

Tb measure the efficacy and duration of the blocks, the individual fingers were pricked rapidly 10 times with a needle algesimeter and the number of painful pricks noted. 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 fran injection to complete analgesia ** i.e. - the number of successful anesthesias of total number of anesthesias performed (no anesthesia is passible if the ulnar volar nerves are not contacted by the anesthetic) *** new isolated stereoisomers When comparing the results of intracutaneous wheal (Table 2) and finger-blockade (Table 3) it can be seen that C of the 12 optically active and inactive structures, substance No. 3, namely L-N-n-propylpipecolic acid-2,6xylidide, 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 differences in effect, a series of various* intracutaneous wheals has been investigated with regards to the skin temperature within the wheals compared to the skin temperature around the outside of the wheals during anesthesia.. Based on this investigation, interesting differences in the temperatures 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 control 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 centimeters from the wheal's center· In the temperature measurements, an Oriel temperature apparatus* has been used. This is battery driven and * A widely used apparatus to measure skin temperature. is connected to a detector (diaineter, 2.5 mm) and has an accuracy of + 0.1°C. The temperature of the skin is measured by 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, Τχ is the measurement obtained at the center of the wheal, and T2 is the average of four measurements taken 90 degrees apart outside of the wheal and within 2 centimeters of the wheal's center. > > & TABLE 4 - TEMFERATORg MEASCREMENIS, ACCURACY: ± 0.1°C NO. Substance as BCl-salt Gone. % Tf^C Xntracutaneous Wheal T?2 eC Controls °C Difference C 5 1 Lidocaine 1.0 30.2 29.3 + 0.9 c 2 Lidocaine + adrenaline* 1:200000 1.0 27.8 28,9 - 1.1 3 ft-Bupivacaine 0.25 31.5 32.1 - 0.6 10 4 L-N-n-propylpipeoolic acid2,6-xylidide 0.25 30.0 31.2 - 1.2 15 5 R-N-n-propylpipecolic acid2,6-xylidide 0.25 33.1 33.2 - 0.1 6 R-N—2—hydroxy— 1.0 31.2 30.7 + 0.5 ethyl-pipecolic acid· 2,6-xylidide * Adrenaline (epinephrine) is a vasoconstrictor which reduces the 20 cross-section of the wheal region’s blood vessels and retains the Lidocaine in the wheal region. r Prom 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 game extent as the combination of lidocaine with adrenaline (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.

Tables 1-4 demonstrate the connection between L-N-n-propylpipecolic acid-2,6-xylidide and the most important parameters in each table, e.g. optical stereoisomer, duration of analgesia in intracutaneous wheals, the analgesic duration of fingerblocks, and the maximization 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 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 thixotropic mixtures which form gels, or in a suspension, or 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 carboxylic 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 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.

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 5 this anesthetic can be achieved by following conventional toxicity tests and protocols heretofore established for local anesthetics.

Claims (9)

CLAIMS:

1. Therapeutically active N-alkyl-pipecolic acid

2. 6-xylidide or a water soluble salt thereof, which is the L-isomer of N-n-propylpipecolic acid-2,6-xylidide. 5 2. A method of preparing a compound of claim 1, comprising reacting the L-isomer of pipecolic acid-2,6-xylidide with a propylating agent.

3. A method according to claim 2, comprising reacting L-pipecolic acid chloride hydrochloride with 2,βίο xylidine and reacting the thus obtained L-pipecolic acid-2,6-xylidide with a propylating agent.

4. A method in accordance with claim 2 or 3, wherein as propylating agent there is used a propyl halogenide.

5. A method in accordance with claim 4, wherein the 15 halogenating agent is propyl bromide.

6. The L-isomer of N-n-propylpipecolic acid-2,6xylidide or a water soluble salt thereof for use as a local anaesthetic.

7. A compound according to claim 1, substantially as 20 hereinbefore described with reference to the accompanying Examples.

8. A method of preparing a compound according to claim 1, substantially as hereinbefore described with reference to the accompanying Examples. »

9. A compound according to claim 1, whenever prepared by a method claimed in a preceding claim.