Archazolid-B Provides Alternative Therapy for Trastuzumab-Resistant ErbB2 Positive Breast Cancer

Abstract

Trastuzumab is the most commonly used humanized monoclonal antibody for specific therapy of breast cancer. Even though its clinical introduction was a breakthrough, a large fraction of the treated tumors are or become resistant. It is well known that receptor internalization and recycling are crucial for ErbB2 mediated signal transduction. Accordingly, inhibition of its molecular machinery could provide alternatives for tumor treatment. Vacuolar H+-ATPase (V-ATPase) is involved in the regulation of endocytotic/recycling pathways. Archazolid-B was described as a potent V-ATPase inhibitor, which induces apoptosis and impairs migration of tumor cells. Based on these observations we investigated the effect of Archazolid on trastuzumab-resistant Jimt-1 cells in vitro and in vivo. 10nM of Archazolid caused decreased membrane ErbB2 expression, which was accompanied by the reduced relative phosphorylation on Y1248 and intracellular accumulation of the receptors. As an in vivo model, SCID mice were transplanted with Jimt-1 xenografts and treated with Archazolid. Following administration of the V-ATPase inhibitor there was a significant decrease in tumor growth compared to control tumors. Confocal microscopic images of tumor sections showed heterogeneous distribution of the proliferation marker Ki67 positivity. Tumor areas with low Ki67 nuclear expression showed intracellular accumulation of ErbB2 molecules similarly to the in vitro experiments, compared to high Ki67 expressing areas with prominent membrane ErbB2 expression. This heterogeneity might be due to diverse tumor-vascularization. Our results demonstrate that Archazolid can be used for in vitro and in vivo tumor growth inhibition based on its interference with ErbB2 recycling.