To derive accurately the thermodynamic parameters governing the hydrolysis of the lactone ring at physiological pH, a derivative spectrophotometric technique was used for the simultaneous estimation of lactone and carboxylate forms of the 10-hydroxy-camptothecin (10-HC). Validation of the analytical method was done with respect to reproducibility, percent recovery, and level of detection. Hydrolysis of the lactone ring of 10-HC followed a 1st order decay with a rate constant equal to (0.0281 +/- 0.001) min(-1) in PBS at pH 7.4 and at a temperature of 310 K, The activation energy for the hydrolysis reaction as calculated from the Arrhenius equation was (79.41 +/- 0.92) kJ . mol(-1), whereas the enthalpy and entropy of hydrolysis of 10-hydroxy-camptothecin were on average 12.45 kJ . mol(-1) and 52.37J . K-1 . mol(-1), respectively. The positive enthalpy and entropy values of the 10-HC-lactone hydrolysis indicate that the reaction is endothermic and entropically driven. (C) 2008 Elsevier Ltd. All rights reserved.