Electrochemical transfer of the first electron to cytotoxic radiosensitizer etanidazole (ETN) and ETN radical anion formation in the alkaline buffered aqueous solution were studied by means of electrochemical impedance spectroscopy (EIS). The heterogeneous electron transfer rate constant for the first reduction of ETN (radical anion production), k0, was calculated. The value of k0 from EIS measurements and the previously obtained value of so-called E71 potential indicate that ETN, compared with other possible chemical radiosensitizers, requires lower energy to accept the first electron during the metabolic pathway. However, the necessity of the alkaline pH may decrease the ability of ETN to act as hypoxic radiosensitizer in the human body.

J Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic v v i Dolejškova 3 182 23 Prague 8 Czech Republic and Dipartimento di Scienze dell'Ambiente e della Vita Universita degli Studi del Piemonte Orientale 'A Avogadro' Viale T Michel 11 15 100 Alessandria Italy

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