Structural Photodynamic Behavior of Topotecan, a Potent Anticancer Drug, in Aqueous Solutions at Different pHs

Abstract

In this work, we report on photophysical studies of the anticancer drug topotecan (TPT) in aqueous solutions at different pHs. We used steady-state (UV-visible absorption and emission) and time-resolved picosecond (ps) emission spectroscopies to investigate the role of the H-bonding interactions as well as the proton concentration (pH = 0.48-7.40) on the behavior of topotecan (TPT) in its ground- (S0) and electronically first (S1) excited-states. At physiological conditions (pH = 7.40), the drug exists at S0 in equilibrium between the enol (E), cation (C), and zwitterion (Z) forms. The photoformation of Z* (τZ = 5.83 ns) occurs from directly excited (λexc = 371 nm) E as the two-step reaction: E*→C*→Z*. In this process, a very fast (less than 10 ps) protonation of E* leads to C*, which subsequently undergoes fast (580 ps) deprotonation to give Z* as the final photoproduct. At higher proton concentrations (pH = 0.48-1.31), a ground-state equilibrium exists between three different cationic species (C1, C2, and C3). The proton motion from the acidic solution to the C forms of TPT to give the reactions C1*→C2*→C3* is governed by the proton diffusion. In these conditions, both dynamic and static quenching occurs. The rate constant values k*(DPT1) and k*(DPT2) for the direct protonation of C1* and C2*, respectively, depend on the pH of the surrounding. The number of protons implicated in the reaction changes from ∼2 (pH = 0.48-0.78) to ∼1 (pH = 0.78-1.31), thus indicating the existence of two different reactions and proton-transfer dynamics. These results evidence the conformational, structural, and dynamical changes of aqueous solutions of TPT with the pH of the environment. They should help to understand the molecular structure/activity of TPT at cellular level.