Abstract
First derivative fluorescence spectroscopy was used to monitor the kinetics of SN38 hydrolysis in the presence and absence of serum albumins at physiological pH. In plain phosphate-buffered saline (PBS) pH 7.4, it was found that SN38-lactone hydrolysis exhibited a positive Enthalpy (12.08 ± 3.8 kJ · mol-1) and Entropy (54.40 ± 12.8 J · K-1 · mol-1) indicating that SN38-lactone hydrolysis is endothermic and entropically driven. Hydrolysis of SN38 observed at multiple temperatures in the presence of serum albumin revealed that despite the similar values of activation energy involved in all the reaction media, SN38-lactone hydrolysis was prevented to a greater extent by the presence of human serum albumin (HSA), compared to the absence or in presence of bovine serum albumin (BSA). Taken together with the association constants KA, as estimated from static fluorescence quenching experiments and steady-state emission anisotropy, it is suggested that the greater stabilization of SN38-lactone in the presence of HSA is due to the significantly higher binding affinity of the SN38-lactone species and the weaker binding of the SN38-carboxylate moiety as opposed to the binding of the respective species to BSA.
Keywords: 7-ethyl-10-hydroxycamptothecin, SN38, thermodynamics, kinetics, hydrolysis, Gibbs free energy, enthalpy, entropy, energy of activation, association constants, scatchard analysis, Hill analysis, HSA, BSA, Camptothecin (CPT), Albumin
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