Mechanistic study of the hydrolytic degradation and protonation of temozolomide

Abstract

Temozolomide, an anticancer and chemotherapy prodrug, undergoes pH dependent ring-opening under both acidic and alkaline conditions. While the rate of degradation accelerates with an increase in pH, it never comes to a complete halt under acidic conditions. Herein, the ring-opening of temozolomide is investigated, in both neutral and acidic conditions, to find out the energy differences and the effects of an acidic environment on its activation energy. Two possible and different pathways have been considered for the ring-opening reaction. When compared to path-2, the rate-determining step (first TS) for path-1 is about 15 kcal mol⁻¹ more favourable. However, the second path led to a more stable product. It seems that the energy-favoured overall mechanism is a combination of the two paths (tautomerization may occur during the process). Moreover, in order to elucidate the role of the acidic conditions on the increased stability and mechanism of the drug, the protonation of all possible sites was examined. This predicts that the protonation of the oxygen of the amide group is the preferable site for protonation that would stabilize the system by about 1.23 kcal mol⁻¹ more than the next favourable protonation site (protonated nitrogen in the imidazole ring). The low energy barrier (6 kcal mol⁻¹) for proton exchange in these two sites results in the simultaneous existence of both N-protonated and O-protonated structures. In addition, the pK_a values were calculated and averaged out at −2.625. This value indicates the extremely acidic feature of the protonated temozolomide that does not protonate or deprotonate over the normal pH range. The protonation and degradation mechanism were treated using density functional theory (B3LYP) and by employing the complete basis set (CBS-4M) method. Moreover, the high-level G3MP2 level was used on some occasions.