Mechanisms and kinetics of thiotepa and tepa hydrolysis: DFT study.

N,N',N″-triethylenethiophosphoramide (Thiotepa) and its oxo analogue (Tepa) as the major metabolite are trifunctional alkylating agents with a broad spectrum of antitumor activity. In vivo and vitro studies show alkylation of DNA by Thiotepa and Tepa can follow two pathways, but it remains unclear which pathway represents the precise mechanism of action. In pathway 1, these agents are capable of forming cross-links with DNA molecules via two different mechanisms. In the first mechanism, the ring opening reaction is initiated by protonating the aziridine, which then becomes the primary target of nucleophilic attack by the N7-Guanine. The second one is a direct nucleophilic ring opening of aziridyl group. Thiotepa and Tepa in pathway 2, act as a cell penetrating carrier for aziridine, which is released via hydrolysis. The released aziridine can form a cross-link with N7-Guanine. In this study, we calculated the activation free energy and kinetic rate constant for hydrolysis of these agents and explored interaction of aziridine with Guanine to predict the most probable mechanism by applying density functional theory (DFT) using B3LYP method. In addition, solvent effect was introduced using the conductor-like polarizable continuum model (CPCM) in water, THF and diethylether. Hyperconjugation stabilization factors that have an effect on stability of generated transition state were investigated by natural bond order (NBO) analysis. Furthermore, quantum theory of atoms in molecules (QTAIM) analysis was performed to extract the bond critical points (BCP) properties, because the electron densities can be considered as a good description of the strength of different types of interactions.