Prolonged analgesic effect of amitriptyline base on thermal hyperalgesia in an animal model of neuropathic pain.

The best analgesic drugs for neuropathic pain have a long duration of action, can be given via multiple routes, and can be used preemptively. We evaluated the antinociceptive effects and duration of action of subcutaneously injected amitriptyline base (AMT-Base) (in oil). A plantar test in a spinal nerve ligation (SNL) model of neuropathic pain in rats showed that typical amitriptyline HCl (AMT-HCl) (in saline) and AMT-Base had a significant dose-dependent antinociceptive effect: the antinociceptive duration of a single 100 µmol/kg injection of AMT-HCl was 5 h and of AMT-Base was 24 h when given 7 days after a SNL, and of a single 200 µmol/kg injection of AMT-Base was 39 days when given 1 h before and 4 days when given 7 days after a SNL. The post-ligation antinociceptive duration of AMT-Base was 4.8 times that of AMT-HCl, but the duration of preemptive (pre-ligation) AMT-Base treatment was 9.7 times that of AMT-Base. We can conclude that preemptive amitriptyline base provides long-lasting antinociception for neuropathic pain experimentally.

Class I antiarrhythmic drugs produced a spinal anesthetic effect in rats.

Class I antiarrhythmic drugs are commonly used to treat cardiac rhythm disorders. Some of those drugs were recently reported to have both a cutaneous analgesic and a neural blocking effect. We evaluated whether these drugs have a spinal anesthetic effect. Three Class I antiarrhythmic drugs (class IA: quinidine, IB: mexiletine, and IC: flecainide) were tested. After they had been intrathecally injected in rats, the potencies and durations of these drugs on spinal anesthesia were recorded. Bupivacaine, a commonly used local anesthetic, and 5% dextrose solution were used as controls. Bupivacaine, flecainide, quinidine, and mexiletine produced a dose-related spinal blockade of motor function, proprioception, and nociception, but dextrose solution produced no spinal anesthetic effect. The descending order of potency was bupivacaine>flecainide>quinidine>mexiletine (p<0.05 for all differences). On an equipotent basis, flecainide, quinidine, and bupivacaine produced similar durations of action, all of which were significantly longer than that of mexiletine (p<0.05). In conclusion, intrathecal injections of Class I antiarrhythmic drugs produced a dose-related spinal anesthetic effect. These drugs may be potential candidates for developing new local anesthetics.

The mechanism investigation in substitution of 21-bromo-3alpha-hydroxyl-3beta-methoxymethyl-5alpha-pregnan-20-one with nucleophiles.

A mechanistic study on the nucleophilic substitution of a strictly geometric 21-bromo-3alpha-hydroxyl-3beta-methoxymethyl-5alpha-pregnan-20-one was described. Reaction of the alpha-bromoketone with excess lithium imidazole followed by the addition of extra bases including n-butyllithium, methyllithium, lithium piperidine, and lithium pyrrolidine provided unexpected alpha-nucleophilic carbonyl adducts that derived from strong base. Data from HPLC and proton NMR suggested an epoxide as the intermediate. Two possible reaction pathways were proposed for the nucleophilic substitution reaction. One pathway is the normal SN2 substitution reaction, directly provided the imidazoly product without the formation of the unexpected alpha-substituted products. The other pathway went through an epoxide intermediate, in which imidazole anion or the strong bases added would attack from the less hindered site of the epoxide to give the substitution product.

Synthesis of 3alpha-hydroxy-21-(1'-imidazolyl)-3beta-methoxyl-methyl-5alpha-pregnan-20-one via lithium imidazole with 17alpha-acetylbromopregnanone.

The synthesis of biologically active 3alpha-hydroxyl-21-(1'-imidazolyl)-3beta-methoxymethyl-5alpha-pregnan-20-one was accomplished in six steps. The key steps were the improvement of stereoselectivity for acetyl isomers in C-17 and the introduction of imidazole into the core structure by use of lithium imidazole. This latter key step provided the desired product in 82% yield without the formation of 1,3-disubstituted imidazolium salt as impurity, which is generally observed in traditional method.