Journal of Pharmaceutical and Biomedical Analysis 50 (2009) 718–723 Contents lists available at ScienceDirect Journal of Pharmaceutical and Biomedical Analysis journal homepage: www.elsevier.com/locate/jpba NMR analysis, protonation equilibria and decomposition kinetics of tolperisone Gábor Orgován a , Károly Tihanyi b , Béla Noszál a,∗ a Department of Pharmaceutical Chemistry, Semmelweis University, Research Group of Drugs of Abuse and Doping Agents, Hungarian Academy of Sciences, Budapest H-1092, H˝ ogyes Endre u. 9, Hungary b Gedeon Richter Plc, Department of Pharmacology and Drug Safety, H-1103, Gyömr˝ oiút 19-21, Budapest, Hungary article info Article history: Received 26 March 2009 Received in revised form 28 May 2009 Accepted 29 May 2009 Available online 6 June 2009 Keywords: Tolperisone Decomposition kinetics Protonation constants Tautomerization Tautomerization Mannich base abstract The rate constants of spontaneous and hydroxide-catalyzed decomposition and the tautomer-specific protonation constants of tolperisone, a classical muscle relaxant were determined. A solution NMR method without any separation techniques was elaborated to quantitate the progress of decomposition. All the rate and equilibrium constants were determined at four different temperatures and the activation parameters were calculated. The molecular mechanism of decomposition is proposed. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Tolperisone, 2-methyl-1-(4-methylphenyl)-3-piperidin-1- ylpropan-1-one hydrochloride is a centrally acting, classical muscle relaxant [1] for extrapiramidal movement disorders. Phar- macological studies on aminoketones revealed that these types of compounds cause muscle weakness with no sedation [2]. Further, clinical studies on tolperisone focused on its therapeutic value in movement disorders [3], rheumatic diseases [4], painful muscle spasms [5,6] and peripheric vasodilation [7]. Tolperisone is used as racemate, its enantiomers, however, have different pharmacologic properties: (+)-tolperisone has greater muscle relaxant activity than the (-) enantiomer, whereas (-)-tolperisone has higher broncho- and vasodilator effect [8,9]. Although it has been used for more than 40 years in therapy, its mechanism of action is still unknown. Tolperisone has a local anesthetic effect, like lidocain [10], as it blocks the voltage gated sodium channels [11,12].A comparative assessment study shows that tolperisone is still one of the best centrally acting muscle relaxants in therapy [13]. It has no serious side effects, though anaphylactic reactions can occur [14]. Recent reviews with several pharmacological details [15,16] are available. A definite shortcoming of tolperisone is its propensity to decompose in aqueous solution to piperidine and a vinylke- ∗ Corresponding author. Tel.: +36 1 217 0891; fax: +36 1 217 0891. E-mail address: nosbel@gytk.sote.hu (B. Noszál). tone (2-methyl-1-(4-methylphenyl)prop-2-en-1-one) (Fig. 1). The decomposition is faster at higher pH [17]. Decomposition has so far been characterized at a descriptive level [18,19] with no exact kinetic parameters. Attempts have been made to stabilize tolperi- sione, e.g. by -cyclodextrin [20]. The degradation product of eperisone(2-methyl-1-(4-ethylphenyl)-3-piperidin-1-ylpropan-1- one hydrochloride), the ethyl analogue of tolperisone has been fully characterized by MS, NMR, UV and IR spectroscopy [21]. Obvious reasons why its decomposition kinetics has not been quantitated were the lack of an appropriate solution analytical method to monitor the progress of decomposition, and also, the lack of input acid–base equilibrium parameters of the reactants and products. Here we report (a) an 1 H NMR method for the simultaneous determination of tolperisone and its decomposition products; (b) the related acid–base properties at four temperatures; (c) the kinetic constants including the activation parameters; and (d) the proposed mechanism of decomposition. 2. Experimental methods All experiments were performed at thermostated temperature (288, 298, 308 and 323 K), the ionic strength was held constant at 0.15M using KCl as auxiliary electrolyte. 0731-7085/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpba.2009.05.036