Mexiletine Selective Membrane Electrode and its Pharmaceutical Applications

S,S,S-Tris(2-ethylhexyl) phosphorotrithioate as an effective solvent mediator for a mexiletine-sensitive membrane electrode

S,S,S-tris(2-ethylhexyl) phosphorotrithioate proved to be an effective solvent mediator for constructing a mexiletine-sensitive membrane electrode in combination with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate. Among a series of phosphorus compounds containing phosphoryl (P=O) groups, this solvent mediator showed the highest sensitivity to mexiletine in phosphate-buffered physiological saline containing 0.15 mol L-1 NaCl and 0.01 mol L-1 NaH2PO4/Na2HPO4 (pH 7.4), giving a detection limit of 2x10(-6) mol L-1 with a slope of 58.8 mV decade-1. This is the best reported detection limit of any mexiletine-sensitive electrode developed to date. Owing to its high selectivity toward inorganic cations, the electrode was used to determine the level of mexiletine in saliva, the monitoring of which is quite effective for controlling the dose of this drug noninvasively. The mexiletine concentrations determined with the mexiletine-sensitive electrode compared favorably with those determined by high-performance liquid chromatography.

Mexiletine-sensitive membrane electrode for medical application

Response characteristics of mexiletine-sensitive membrane electrodes based on crown ether and ion-exchanger were examined in a physiological saline in order to find an electrode suitable for determining concentrations of this drug under physiological conditions. Among various crown ethers screened, 4',4"(5")-di-tert-butyldicyclohexano-18-crown6 showed the highest sensitivity to mexiletine in physiological saline containing 0.15 M NaCl and 5 mM 4-(2-hydroxyethyl)-2-piperazineethanesulfonic acid (Hepes) NaOH (pH 7.4). However, the detection limit of 30 microM was 10 times higher than that of 3 microM observed with the electrode based on an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate. Having high selectivity against inorganic cations such as Na+ or K+, the electrode using the ion-exchanger enabled us to determine the level of mexiletine in saliva, the monitoring of which is quite effective for controlling the dose of this drug noninvasively. The mexiletine concentrations determined with the mexiletine electrode compared favourably with those determined by high-performance liquid chromatography which requires an additional procedure to extract mexiletine from saliva.