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Long acting local anaesthetics - a study in rats

Dyhre, Henrik LU (2006)
Abstract
Local anaesthetics provide excellent pain relief after central and peripheral nerve blocks. However, in many clinical situations there is an insufficient duration of action to allow effective treatment of long-term pain with single injection techniques. This problem became the focus of the present studies. The natives in South America used to chew the leaves from Erythroxylon Coca and were well aware of the numbness in the mouth after chewing it, as well as the centrally stimulating effect.



Niemann described the purification process and named it cocaine in 1860. The eye surgeon Koller described in 1884 cocaine as a topical local anaesthetic of the eye. The toxic effects of cocaine were soon identified since they had... (More)
Local anaesthetics provide excellent pain relief after central and peripheral nerve blocks. However, in many clinical situations there is an insufficient duration of action to allow effective treatment of long-term pain with single injection techniques. This problem became the focus of the present studies. The natives in South America used to chew the leaves from Erythroxylon Coca and were well aware of the numbness in the mouth after chewing it, as well as the centrally stimulating effect.



Niemann described the purification process and named it cocaine in 1860. The eye surgeon Koller described in 1884 cocaine as a topical local anaesthetic of the eye. The toxic effects of cocaine were soon identified since they had resulted in many deaths among both patients and addicted persons.



Development via modern organic chemistry led to new local anaesthetics. The nerve impulses flow along the nerve by small electrical changes. The two most important ions in this process are sodium (Na+) and potassium (K+). Local anaesthetics inhibit the sodium-influx (by blocking the Na+-channels). In a local anaesthetic solution two forms of the drug are in equilibrium with each other, the ionised (BH+) and the non-ionised (B) form. Both forms are necessary for the action; the base form (B) penetrates the membrane of the nerve axon and the protonated form (BH+) is the active part and blocks the Na+- channels intracellularly. The concentration relationship between these two forms depends on the pKa of the drug and the tissue (and solution) pH. It has been stated that an increase of pH (alkalinization) increases the non-ionised form that gives a higher penetration rate and leads to a prolonged duration of action in a local anaesthetic block. In Paper I, this could not be concluded in the infraorbital nerve block (IONB) in rats after pH modulation by dissolving lidocaine or pethidine in normal saline and varying their pH by adding HCl or NaCO3. However, after dissolving the tested drugs in a buffer (THAM) at physiological pH the duration of nerve block was prolonged. This was presumable due to the inherent low buffer capacity in the original solution. Many local anaesthetics exist as two enantiomers, i.e. as two molecules that are mirror images of each other. According to their geometrical configuration these are named S (sinister) and R (rectus). Another description is how the enantiomer rotates the plane of polarized light; l or (-) as in levo and d or (+) as in dextro. When applied intradermally, the S(-)-enantiomers levobupivacaine and ropivacaine show vasoconstrictive properties over a wide range of concentrations, whereas similar concentrations of the racemate (R,S)- bupivacaine and its R(+)-enantiomer induce vasodilatation. Braun showed already in 1903 that vasoconstrictive properties (for example inclusion of adrenaline) of local anaesthetic solutions enhanced the duration of the nerve block. Still today adrenaline is used for this purpose. In Paper II the hypothesis was that the S(-)-enantiomers could produce long durations of blocks. However, similar duration of blocks were produced by S-enantiomers and the racemate (R,S)-bupivacaine. This could be because of injections into a presumably richly vascularized area where the vasoconstriction cannot affect the local anaesthetics. The two used blocks (IONB and SNB) are more comparable to a peripheral nerve block than what an intradermal weal test is.



Inclusion of local anaesthetics into various formulations has been tried in the attempt to prolong the duration of block. In this Thesis two different lipid systems were used with the aim to make a slow-release formulation with prolonged nerve blocking properties without toxic side effects. In Paper III the local anaesthetic lidocaine, at maximum of 20%, was dissolved into a polar lipid system, SPC/GD. In SNB in rats the 20% lidocaine in lipid formulation prolonged the duration of the block three times compared to clinically used 2% lidocaine HCl aqueous solution. The in vitro release of lidocaine from the 200 mg/ml (20%) lidocaine in SPC/GD showed a sustained release. Blood concentrations of lidocaine in arterial blood in rats after SNB with 0.1 ml of 20% lidocaine in lipid formulation or 2% lidocaine HCl aqueous solution were measured. The AUC (area under the curve) of the blood concentrations was 8.8 times higher after the tenfold higher dose but the Cmax did not like AUC increase in proportion to the tenfold difference in dose.



Another lipid formulation (non-polar) was used in Papers IV and V, namely MCT (medium-chain triglycerides). MCT are already used in other available medications, i.e. the injectable depot neuroleptic drug flupentixol decanoate (Fluanxol® Depot, Lundbeck). The preparation has been in clinical use for decades.



The well-known eutectic mixture of local anaesthetics (EMLA®; developed at AstraZeneca Inc.), which consists of a 1:1 mixture of lidocaine and prilocaine, proved to be freely soluble in MCT at any concentration, Paper IV. The duration of SNB was prolonged three times with 20% and about 180 times with 60% formulation, in comparison to 2% aqueous HCl solution. Formulations 60% and ethanol produced neurotoxic signs as shown by light microscopy. There is a possibility to use the high-concentration formulations as an alternative to ethanol for neurolytic long-term nerve blocks. The mean Cmax values of the local anaesthetics in blood were only about twice as high after 20% formulation in comparison with 2% aqueous HCl solution in spite of the tenfold higher dose. The terminal half-lives and the calculated times for 50% release into the circulation attested to the depot characteristics of this formulation.



In the attempt to further prolong nerve block, the long acting bupivacaine was mixed with lidocaine in MCT. The function of lidocaine in the mixture was to increase the solubility of bupivacaine in MCT. Concentrations between 2 and 64% were prepared. The duration of nerve block was prolonged almost four times with the 32% and 13 times with the 64% in comparison to the clinically used 0.5% aqueous bupivacaine HCl solution. Slight to moderate signs of neurotoxicity were seen only after administration of the 64% formulation. The findings in this Thesis suggest that MCT as carrier of local anaesthetics has advantageous pharmaceutical and pharmacological properties. A favourable balance between effects and toxicity may conceivably be obtained. After supplemental testing in more sensitive models for toxicity such formulations could be candidates for clinical trials. (Less)
Abstract (Swedish)
Popular Abstract in Swedish

Lokalbedövningsmedel ger en god smärtlindring efter centrala och perifera nervblockader. I många kliniska situationer hade det dock varit önskvärt med en längre duration av nervblockaden. De olika delarbetena i denna avhandling fokuserade på detta problem.



Indianerna i Syd Amerika har länge känt till att om man tuggar bladen ifrån Erythroxylon Coca så upplev's en bedövningskänsla i munnen samt en central- stimulerande effekt. Processen att renframställa kokain beskrev's 1860 och första gången kokain använde's som ett bedövningsmedel var 1884. Tidigt noterade's att kokain gav allvarliga biverkningar, företrädesvi's ifrån hjärtat och nervsystemet. Utvecklingen av den moderna... (More)
Popular Abstract in Swedish

Lokalbedövningsmedel ger en god smärtlindring efter centrala och perifera nervblockader. I många kliniska situationer hade det dock varit önskvärt med en längre duration av nervblockaden. De olika delarbetena i denna avhandling fokuserade på detta problem.



Indianerna i Syd Amerika har länge känt till att om man tuggar bladen ifrån Erythroxylon Coca så upplev's en bedövningskänsla i munnen samt en central- stimulerande effekt. Processen att renframställa kokain beskrev's 1860 och första gången kokain använde's som ett bedövningsmedel var 1884. Tidigt noterade's att kokain gav allvarliga biverkningar, företrädesvi's ifrån hjärtat och nervsystemet. Utvecklingen av den moderna kemin ledde till nya lokalbedövningsmedel. En nervimpul's sprid's snabbt läng's en nervtråd genom förändringar i den elektriska laddningen som i huvudsak åstadkomm's av natrium (Na+)- och kaliumjoner (K+). Lokalbedövningsmedel blockerar inflödet av natriumjoner (genom att blockera Na+-kanaler).



I en lösning som innehåller lokalbedövningsmedel finn's det två olika former som är i balan's med varandra. Dessa former, joniserad och icke joniserad, är bägge nödvändiga för nervblockaden. Den icke joniserade passerar genom nervmembranet och den joniserade formen blockerar inne i Na+-kanalen.



Förhållandet mellan de två olika formerna är beroende av pKa (surhetsgraden) för ämnet samt vävnaden's (och lösningen's) pH. När pH ökar (alkalinisering) så ökar den icke joniserade formen vilket medför en högre penetration av ämnet över nervmembranet och därmed en förlängd nervblockad. I Arbete I kunde detta inte visa's efter blockad av Nervu's Infraorbitali's (IONB) på råtta efter pH förändringar av lidokain- eller pethidinlösning genom tillsat's av HCl eller NaCO3. Efter tillförsel av en buffert (THAM) till lidokain eller pethidin vid fysiologiskt pH (7.4) så förlängde's dock durationen av nervblockad. Slutsatsen bedömde's vara att lösningarna i det första fallet hade en låg naturlig buffertkapacitet. En injektion i ett trångt område såsom vid IONB framkallar sannolikt en vis's sänkning av pH och därför förändra's en lösning omedelbart vid injektionen till omgivningen's låga pH. Lösningar som innehåller buffert kan däremot stå emot pH sänkningen så att den icke joniserade formen kan penetrera över nerven's membran.



Många lokalbedövningsmedel består av två enantiomerer, vilket innebär en blandning i förhållandet 1:1 av två molekyler som är varandra's spegelbilder. Dessa benämn's efter sin geometriska form S (sinister) respektive R (rectu's). En annan beskrivning är hur de vrider planpolariserat lju's; l eller (-) som i levo och d eller (+) som i dextro. De två S(-)-enantiomererna levobupivakain och ropivakain har efter injektion ytligt på huden (intradermalt) visat sig vara kärlsammandragande. Däremot visade's att R(+)-enantiomeren och vanligt bupivakain (racemat, dv's 1:1-blandningen av R och S) gav kärlvidgning. Braun påvisade redan 1903 att kärlsammandragande egenskaper, t ex genom tillsat's av adrenalin till lokalbedövningsmedel, kunde förlänga durationen av en nervblockad. Än idag använd's adrenalin i detta syfte. Hypotesen i Arbete II var att S(-)-enantiomererna levobupivakain och ropivakain skulle kunna ge långa durationer av nervblock. Dock var blockad durationerna liknande den som noterade's med blockad av bupivakain (R,S). Orsaken till detta kan vara att de två använda blockaderna, IONB samt blockad av Nervu's Ischiadicu's (SNB), sker i ett kärlrikt område där en kärlsammandragande effekt inte påverkar lokalbedövningsmedlet. De två använda nervblockadteknikerna liknar dock mer en perifer nervblockad än vad en ytlig injektion på huden gör.



Lokalbedövningsmedel har blandat's med olika sorter's beredningar med målsättningen att förlänga durationen av en nervblockad. I Arbete III ? V använde's två olika lipidsystem med målet att få fram långsamt frisättande beredningar med egenskapen att förlänga nervblockad utan toxiska biverkningar. I Arbete III löste's lidokain, till max 20% koncentration, i ett polärt lipidsystem, SPC/GD. Efter SNB såg's en trefaldig förlängning med 20% lidokain i SPC/GD jämfört med 2% lidokain HCl i vattenlösning. Frisättningen av lidokain ifrån 20% lidokain i SPC/GD visade en utdragen frisättning. Blodkoncentrationerna av lidokain i artärblod från råtta efter SNB av både 0.1 ml av 20% lidokain i SPC/GD samt 2% lidokain HCl i vattenlösning mätte's. Arean under kurvan (AUC) av uppmätta koncentrationer avsatta mot tid var 8.8 gånger högre efter den tiofaldiga ökningen av do's medan högsta uppmätta blod koncentrationen (Cmax) endast var trefaldigt ökad.



Ett annat lipidsystem (av opolära lipider) använde's i Arbete IV ? V, bestående av medellånga triglycerider (MCT). MCT använd's redan som bärare till läkemedel, nämligen flupentixol decanoate (Fluanxol® Depot, Lundbeck) som är ett neurolepticum avsett för intramuskulärt bruk. Detta preparat har använt's i decennier.



EMLA® (utvecklat av AstraZeneca Inc.) består av en 1:1 blandning av lidokain och prilokain. Denna blandning visade sig vara helt löslig i MCT, Arbete IV. Efter nervblockad (SNB) visade det sig att 20% i MCT gav en trefaldig förlängning medan 60% i MCT visade en förlängning med 180 gånger i jämförelse med 2% i vattenlösning. Beredningar på 60% och alkohol visade tecken på nervskada vid ljusmikroskopering. Det kan finna's en möjlighet till att använda beredningar med höga koncentrationer som ett alternativ till alkohol vid neurolytisk nervblockad. Blodkoncentrationerna bestämde's här på liknande sätt som i Arbete III och Cmax visade endast en tvåfaldig ökning trot's en tiofaldigt högre do's, 20% i MCT jämfört med 2% i vattenlösning.



Med syftet att ytterligare förlänga durationen efter nervblockad blandade's det långverkande lokalbedövningsmedlet bupivakain med lidokain i MCT. Syftet med att blanda i lidokain var att öka lösligheten av bupivakain i MCT. Blandningar mellan 2 och 64% gjorde's. Förlängning av nervblockad noterade's ungefär med fyra gånger för den 32% och 13 gånger för den 64% i jämförelse med 0.5% bupivakain HCl i vattenlösning. Här såg's endast mindre tecken till nervskada och endast efter injektion av den 64% beredningen.



Sammanfattningsvi's kan konstatera's att MCT som bärare av lokalbedövningsmedel har fördelaktiga egenskaper. Sådana beredningar borde, efter ytterliggare undersökningar med mer känsliga tekniker för nervskada, kunna bli föremål för kliniska prövningar. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Docent Irestedt, Lars, Anestesi - intensivvårdskliniken Karolinska sjukhuset, Stockholm
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Medicin (människa och djur), Medicine (human and vertebrates), in vivo and effect kinetics, Comparison of in vitro, Neuro toxicity, Infraorbital nerve block, Sciatic nerve block, Lipid depot formulation
pages
108 pages
publisher
Department of Anaesthesiology and Intensive Care, Lund
defense location
CRC ing 72, plan 1, Hörsal (Aula) Universitetssjukhuset UMAS
defense date
2006-05-30 13:00:00
ISBN
91-85481-87-4
language
English
LU publication?
yes
additional info
id
9144507e-932a-4e2e-9cdf-17ba2dbde058 (old id 546820)
date added to LUP
2016-04-01 15:55:11
date last changed
2018-11-21 20:37:22
@phdthesis{9144507e-932a-4e2e-9cdf-17ba2dbde058,
  abstract     = {Local anaesthetics provide excellent pain relief after central and peripheral nerve blocks. However, in many clinical situations there is an insufficient duration of action to allow effective treatment of long-term pain with single injection techniques. This problem became the focus of the present studies. The natives in South America used to chew the leaves from Erythroxylon Coca and were well aware of the numbness in the mouth after chewing it, as well as the centrally stimulating effect.<br/><br>
<br/><br>
Niemann described the purification process and named it cocaine in 1860. The eye surgeon Koller described in 1884 cocaine as a topical local anaesthetic of the eye. The toxic effects of cocaine were soon identified since they had resulted in many deaths among both patients and addicted persons.<br/><br>
<br/><br>
Development via modern organic chemistry led to new local anaesthetics. The nerve impulses flow along the nerve by small electrical changes. The two most important ions in this process are sodium (Na+) and potassium (K+). Local anaesthetics inhibit the sodium-influx (by blocking the Na+-channels). In a local anaesthetic solution two forms of the drug are in equilibrium with each other, the ionised (BH+) and the non-ionised (B) form. Both forms are necessary for the action; the base form (B) penetrates the membrane of the nerve axon and the protonated form (BH+) is the active part and blocks the Na+- channels intracellularly. The concentration relationship between these two forms depends on the pKa of the drug and the tissue (and solution) pH. It has been stated that an increase of pH (alkalinization) increases the non-ionised form that gives a higher penetration rate and leads to a prolonged duration of action in a local anaesthetic block. In Paper I, this could not be concluded in the infraorbital nerve block (IONB) in rats after pH modulation by dissolving lidocaine or pethidine in normal saline and varying their pH by adding HCl or NaCO3. However, after dissolving the tested drugs in a buffer (THAM) at physiological pH the duration of nerve block was prolonged. This was presumable due to the inherent low buffer capacity in the original solution. Many local anaesthetics exist as two enantiomers, i.e. as two molecules that are mirror images of each other. According to their geometrical configuration these are named S (sinister) and R (rectus). Another description is how the enantiomer rotates the plane of polarized light; l or (-) as in levo and d or (+) as in dextro. When applied intradermally, the S(-)-enantiomers levobupivacaine and ropivacaine show vasoconstrictive properties over a wide range of concentrations, whereas similar concentrations of the racemate (R,S)- bupivacaine and its R(+)-enantiomer induce vasodilatation. Braun showed already in 1903 that vasoconstrictive properties (for example inclusion of adrenaline) of local anaesthetic solutions enhanced the duration of the nerve block. Still today adrenaline is used for this purpose. In Paper II the hypothesis was that the S(-)-enantiomers could produce long durations of blocks. However, similar duration of blocks were produced by S-enantiomers and the racemate (R,S)-bupivacaine. This could be because of injections into a presumably richly vascularized area where the vasoconstriction cannot affect the local anaesthetics. The two used blocks (IONB and SNB) are more comparable to a peripheral nerve block than what an intradermal weal test is.<br/><br>
<br/><br>
Inclusion of local anaesthetics into various formulations has been tried in the attempt to prolong the duration of block. In this Thesis two different lipid systems were used with the aim to make a slow-release formulation with prolonged nerve blocking properties without toxic side effects. In Paper III the local anaesthetic lidocaine, at maximum of 20%, was dissolved into a polar lipid system, SPC/GD. In SNB in rats the 20% lidocaine in lipid formulation prolonged the duration of the block three times compared to clinically used 2% lidocaine HCl aqueous solution. The in vitro release of lidocaine from the 200 mg/ml (20%) lidocaine in SPC/GD showed a sustained release. Blood concentrations of lidocaine in arterial blood in rats after SNB with 0.1 ml of 20% lidocaine in lipid formulation or 2% lidocaine HCl aqueous solution were measured. The AUC (area under the curve) of the blood concentrations was 8.8 times higher after the tenfold higher dose but the Cmax did not like AUC increase in proportion to the tenfold difference in dose.<br/><br>
<br/><br>
Another lipid formulation (non-polar) was used in Papers IV and V, namely MCT (medium-chain triglycerides). MCT are already used in other available medications, i.e. the injectable depot neuroleptic drug flupentixol decanoate (Fluanxol® Depot, Lundbeck). The preparation has been in clinical use for decades.<br/><br>
<br/><br>
The well-known eutectic mixture of local anaesthetics (EMLA®; developed at AstraZeneca Inc.), which consists of a 1:1 mixture of lidocaine and prilocaine, proved to be freely soluble in MCT at any concentration, Paper IV. The duration of SNB was prolonged three times with 20% and about 180 times with 60% formulation, in comparison to 2% aqueous HCl solution. Formulations 60% and ethanol produced neurotoxic signs as shown by light microscopy. There is a possibility to use the high-concentration formulations as an alternative to ethanol for neurolytic long-term nerve blocks. The mean Cmax values of the local anaesthetics in blood were only about twice as high after 20% formulation in comparison with 2% aqueous HCl solution in spite of the tenfold higher dose. The terminal half-lives and the calculated times for 50% release into the circulation attested to the depot characteristics of this formulation.<br/><br>
<br/><br>
In the attempt to further prolong nerve block, the long acting bupivacaine was mixed with lidocaine in MCT. The function of lidocaine in the mixture was to increase the solubility of bupivacaine in MCT. Concentrations between 2 and 64% were prepared. The duration of nerve block was prolonged almost four times with the 32% and 13 times with the 64% in comparison to the clinically used 0.5% aqueous bupivacaine HCl solution. Slight to moderate signs of neurotoxicity were seen only after administration of the 64% formulation. The findings in this Thesis suggest that MCT as carrier of local anaesthetics has advantageous pharmaceutical and pharmacological properties. A favourable balance between effects and toxicity may conceivably be obtained. After supplemental testing in more sensitive models for toxicity such formulations could be candidates for clinical trials.},
  author       = {Dyhre, Henrik},
  isbn         = {91-85481-87-4},
  language     = {eng},
  publisher    = {Department of Anaesthesiology and Intensive Care, Lund},
  school       = {Lund University},
  title        = {Long acting local anaesthetics - a study in rats},
  year         = {2006},
}