Pulse radiolysis study of daunorubicin redox reactions: redox cycles or glycosidic cleavage?

Two aspects of daunorubicin reactivity were investigated by pulse radiolysis. The reactions of O2 and O2- with the semiquinone and the hydroquinone transients of daunorubicin were determined and their rate constants measured. Although O2- can reduce the drug and its semiquinone form, it is a more powerful oxidant towards the two reduced transients. The hydroquinone daunorubicin glycosidic cleavage in aqueous solution was studied. Three intermediates were seen and characterized by their absorption spectra, their formation and decay kinetics. The competition between these two main processes was evaluated in the conditions of pulse radiolysis. Even under low O2 partial pressures the redox cycles are much more rapid than the glycosidic cleavage and a relatively high O2- steady state is settled. Biological implications are discussed.