Voltammetric study of drugs

Voltammetric Study of Drugs at Liquid-Liquid Interfaces Mitsugi Senda, Yuko Kubota, and Hajime Katano... 

Voltammetric Study of Drugs at Liquid-Liquid Interfaces... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Differential pulse (DP) voltammetry, a voltammetric technique with high sensitivity, is normally performed and the equipment as well as the electrochemical procedures used for the voltammetric studies of DNA-drug interaction are described (see Procedure 29 in CD accompanying this book). 

A recent study by Xi et al. (1998) utilized single carbon fiber instead of multiple fiber electrodes and a fast-scan voltammetry instead of chronoamperometry. An additional difference was that the rats were first trained to acquire stable rates of heroin selfadministration and then were implanted with the electrodes. Under these conditions 60% of the rats responded to self-injection of 0.06A).l mg/kg of heroin with a monophasic increase, 20% with a biphasic increase/decrease and 20% with a biphasic decrease/ increase of the electrochemical signal (Xi et al., 1998). Only monophasic increases were observed with doses of 0.2 mg/kg of heroin. Tonic monophasic increases in signal were observed also by chronoamperometry with stearate-coated electrodes in a comparative study of cocaine and amphetamine self-administration in parallel with microdialyis (Di Ciano et al., 1995). From this study, however, it appears that, although the effect of drug self-injection on dialysate DA and on the tonic voltammetric signal is qualitatively... 

In this study low concentration of timolol are determined by adsorptive stripping voltammetry. The preconcentration of the analyte is done by adsorption of the drug on a surface of the electrode and the adsorbed species are determined by application of a voltammetric scan on the electrode. This technique has recently been employed by different authors for the determination of a number of drugs. 

Drugs such as 2-thiobarbituric acid are oxidized in the presence of iodide mediators. In aqueous media, an electroio-dination process occurs and the resulting voltammetric signal allows quantitative determination [190]. A similar methodology with a limit of detection at ca. 40 gM level has been developed for barbiturates [191]. Also, catechin as a model system for polyphenols was studied in the presence of an iodide redox mediator [192]. 

Voltammetric techniques are quite useful for the direct measurement of the stability of some pharmaceuticals in aqueous solution. Depending on the compounds involved, voltammetry can be used to monitor either the decomposition of a drug or the formation of a decomposition product. This type of study is typically performed by applying a potential to the working electrode that is on the voltammetric plateau for the starting material or end product to be monitored. The resulting current versus time data provide the desired kinetic information. 

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