Methotrexate, paclitaxel, and doxorubicin radiosensitize HER2-amplified human breast cancer cells to the auger electron-emitting radiotherapeutic agent 111In-NLS-trastuzumab

Abstract

Our goal in this study was to elucidate the mechanisms by which methotrexate radiosensitizes HER2-positive human breast cancer cells to the Auger electron emitter 111In-trastuzumab modified with nuclear-localization sequence peptides (111In-NLS-trastuzumab) and to compare these mechanisms with the potential sensitizing effects of paclitaxel and doxorubicin when combined with this radiopharmaceutical. Methods: Experiments were performed inMDA-MB-231 human breast cancer cells, their HER2-transfected subclones (231-H2N), and 2 trastuzumabresistant variants (trastuzumab-resistant-1 and -2 [TrR1 and TrR2]). Effects of coexposure of these cells to 111In-NLS-trastuzumab and low-dose, radiosensitizing methotrexate, paclitaxel, or doxorubicin were assessed by clonogenic cell-survival assay. Quantification of residual DNA damage was measured by the γH2AX-immunofluorescence assay, and cell cycle distribution wasmeasured by fluorescence-activated cell sorting analysis. The radiation-enhancement ratio was calculated as the ratio of the surviving fraction (SF) of cells treated with 111In-NLStrastuzumab alone to that of cells treated concurrently with 111In-NLS-trastuzumab and methotrexate, paclitaxel, or doxorubicin. Results: A reduction in the SF in HER2-positive 231-H2N (55.7% ± 1.3%) and TrR1 (62.6% ± 6.5%) cells was demonstrated after exposure to 111In-NLS-trastuzumab (∼0.2 MBq/μg, 100 nmol/L) but not in MDA-MB-231 or TrR2 cells expressing low levels of HER2 (SF > 90%, P > 0.05). Coadministration of methotrexate, paclitaxel, or doxorubicin enhanced the cytotoxicity of 111In-NLS-trastuzumab toward 231-H2N and TrR1 cells but not toward MDA-MB-231 or TrR2 cells. The radiationenhancement ratios for methotrexate, paclitaxel, and doxorubicin for 231-H2N or TrR1 cells were 2.0-2.2, 1.6-1.8, and 2.7-2.8, respectively. Methotrexate or doxorubicin combined with 111In-NLS-trastuzumab, compared to treatment with 111In-NLS-trastuzumab alone, significantly increased residual γH2AX foci in 231-H2N and TrR1 cells but not in MDA-MB-231 or TrR2 cells or in any cell line treated concurrently with paclitaxel and 111In-NLS-trastuzumab.Cells exposed to low-dosemethotrexate accumulated in the G1/S phase of the cell cycle, whereas low-dose paclitaxel or doxorubicin caused cells to arrest in the G2/M phase. Conclusion: Low-dose methotrexate, paclitaxel, or doxorubicin potently sensitized HER2-overexpressing human breast cancer cells, with and without acquired trastuzumab-resistance, to the Auger electron emissions from 111In-NLS-trastuzumab through cell cycle distribution changes and in part through the inhibitory effects of these agents on DNA damage repair. Copyright © 2010 by the Society of Nuclear Medicine, Inc.