Lack of stereoselectivity for the antiallodynic effect of mexiletine in spinally injured rats

Resolution of Racemic Aryloxy-Propan-2-yl Acetates via Lipase-Catalyzed Hydrolysis: Preparation of Enantiomerically Pure/Enantioenriched Mexiletine Intermediates and Analogs

The lipase kinetic resolution (KR) of aryloxy-propan-2-yl acetates, via hydrolysis, produced enantiomerically pure/enantioenriched mexiletine intermediates and analogs. Racemic acetates rac-1-(2,6-dimethylphenoxy)propan-2-yl acetate (rac-5a), rac-1-(2,4-dimethylphenoxy)propan-2-yl acetate (rac-5b), rac-1-(o-tolyloxy)propan-2-yl acetate (rac-5c) and rac-1-(naphthalen-1-yloxy)propan-2-yl acetate (rac-5d) were used as substrates. A preliminary screening (24 h, phosphate buffer pH 7.0 with 20% acetonitrile as co-solvent, 30 °C and enzyme:substrate ratio of 2:1, m:m) was carried out with twelve lipases using acetate 5a as substrate. Two enzymes stood out in the KR of 5a, the Amano AK lipase from Pseudomonas fluorescens and lipase from Thermomyces lanuginosus (TLL) immobilized on Immobead 150. Under these conditions, both the (R)-1-(2,6-dimethylphenoxy)propan-2-ol [(R)-4a] and the remaining (S)-1-(2,6-dimethylphenoxy)propan-2-yl acetate [(S)-5a] were obtained with enantiomeric excess (ee) > 99%, 50% conversion and enantiomeric ratio (E) > 200. The KR study was expanded to racemic acetates 5b-d, leading to the corresponding chiral remaining acetates with ≥95% ee, and the alcohols 4b-d with ≥98% ee, and conversion values close to 50%. The best conditions for KRs of rac-5b-d involved the use of lipase from P. fluorescens or TLL immobilized on Immobead 150, 24 or 48 h and 30 °C. These intermediates had their absolute configurations determined using 1H NMR spectroscopy (Mosher’s method), showing that the KRs of these acetates obeyed the Kazlauskas’ rule. Molecular docking studies corroborated the experimental results, indicating a preference for the hydrolysis of (R)-5a-d.

Design and assessment of a potent sodium channel blocking derivative of mexiletine for minimizing experimental neuropathic pain in several rat models

Physical or chemical damage to peripheral nerves can result in neuropathic pain which is not easily alleviated by conventional analgesic drugs. Substantial evidence has demonstrated that voltage-gated Na+ channels in the membrane of damaged nerves play a key role in the establishment and maintenance of pathological neuronal excitability not only of these peripheral nerves but also in the second- and third-order neurons in the pain pathway to the cerebral cortex. Na+ channel blocking drugs have been used in treating neuropathic pain with limited success mainly because of a preponderance of side-effects. We have developed an analogue of mexiletine which is approximately 80 times more potent than mexiletine in competing with the radioligand, 3H-batrachotoxinin for binding to Na+ channels in rat brain membranes and also it is much more lipophilic than mexiletine which should enhance its uptake into the brain to block the increased expression of Na+ channels on second- and third-order neurons of the pain pathway. This analogue, HFI-1, has been tested in three different rat models of neuropathic pain (formalin paw model, ligated spinal nerve model and contusive spinal cord injury model) and found to be more effective in reducing pain behaviours than mexiletine.