Research Article: A Switching Mechanism in Doxorubicin Bioactivation Can Be Exploited to Control Doxorubicin Toxicity

Date Published: September 15, 2011

Publisher: Public Library of Science

Author(s): Nnenna A. Finn, Harry W. Findley, Melissa L. Kemp, Thomas Lengauer

Abstract: Although doxorubicin toxicity in cancer cells is multifactorial, the enzymatic bioactivation of the drug can significantly contribute to its cytotoxicity. Previous research has identified most of the components that comprise the doxorubicin bioactivation network; however, adaptation of the network to changes in doxorubicin treatment or to patient-specific changes in network components is much less understood. To investigate the properties of the coupled reduction/oxidation reactions of the doxorubicin bioactivation network, we analyzed metabolic differences between two patient-derived acute lymphoblastic leukemia (ALL) cell lines exhibiting varied doxorubicin sensitivities. We developed computational models that accurately predicted doxorubicin bioactivation in both ALL cell lines at high and low doxorubicin concentrations. Oxygen-dependent redox cycling promoted superoxide accumulation while NADPH-dependent reductive conversion promoted semiquinone doxorubicin. This fundamental switch in control is observed between doxorubicin sensitive and insensitive ALL cells and between high and low doxorubicin concentrations. We demonstrate that pharmacological intervention strategies can be employed to either enhance or impede doxorubicin cytotoxicity in ALL cells due to the switching that occurs between oxygen-dependent superoxide generation and NADPH-dependent doxorubicin semiquinone formation.

Partial Text: Doxorubicin (Adriamycin, Dox) is an antibiotic anthracycline that is used frequently in chemotherapy for a variety of solid tumors and leukemias [1], [2], [3]. The efficacy of doxorubicin treatment is limited by drug resistance mechanisms [4], [5], [6]. Although the underlying mechanism of doxorubicin resistance is not fully understood, researchers have determined several factors that influence cellular doxorubicin toxicity, most notably the expression of membrane transporters P-glycoprotein/MDR1 (Pgp) [3], [7], [8], [9] and the generation of reactive oxygen species (ROS) and free radicals via doxorubicin redox cycling [10]. Because the modulation of Pgp activity in vivo[8], [9] and the use of antioxidants [11], [12] have failed to demonstrate any long term disease-free survival, alternative mechanisms have been proposed to describe the antitumor effects of doxorubicin and thereby offer plausible explanations for why some cancers are sensitive to doxorubicin treatment while others are not.

Although the anthracycline drug doxorubicin is used clinically for the treatment of leukemias and solid tumors [1], [2], [3], the efficacy of doxorubicin treatment is limited by the development of drug resistance [4], [5], [6]. Evidence points to the reductive conversion of doxorubicin as an important ‘first step’ in the regulation of doxorubicin toxicity [2], [3], [4], [5], [13]. While the doxorubicin bioactivation network has been studied extensively, with the overall network structure for cytosolic doxorubicin bioactivation having been deciphered and believed to be conserved across different cell types [4], [20], [21], the adaptation of the bioactivation network to changes in the levels of system components or changes in doxorubicin concentration is much less well understood. Here we show that the doxorubicin bioactivation network is a dynamic system that is sensitive to network component levels and doxorubicin concentrations. Moreover, we illustrate that the intracellular doxorubicin bioactivation network is capable of executing multiple modes of doxorubicin metabolism; the network contains toxicity-generating and ROS-generating reactions that control doxorubicin metabolism via reductive conversion or redox cycling. We illustrate how these reactions can be modulated by pharmacological intervention strategies to either enhance or hinder doxorubicin toxicity in a concentration-dependent manner.