Long acting local anaesthetics - a study in rats

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 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.