In vitro cytotoxicity of 211At-labeled trastuzumab in human breast cancer cell lines:: effect of specific activity and HER2 receptor heterogeneity on survival fraction

Introduction: Radioimmunotherapy with anti-HER2 monoclonal antibodies (mAbs) such as trastuzumab is a promising strategy for treating HER2-positive breast and ovarian carcinoma patients. The objective of this study was to determine the cytotoxic effectiveness of trastuzumab labeled with the 7.2-h half-life a-particle emitter At-211. Methods: Experiments were performed on SKBr-3, BT-474 and the transfected MCF7/HER2-18 human breast carcinoma cell lines. Intrinsic radiosensitivity was determined after exposure to external beam irradiation. The cytotoxicity Of (211) At-labeled trastuzumab was measured by clonogenic assays. The distribution of HER2 receptor expression oil the cell lines was measured using fluorescence-activated cell sorting. A pharmacokinetic (PK)microdosimetric model was established to assess the effects of specific activity (SA), HER2 receptor expression and absorbed dose on Survival fraction (SF). Results: With external beam irradiation, the 2-Gy SF for BT-474, SKBr-3 and MCF7/FIER2-18 cells was 0.78, 0.53 and 0.64 Gy, respectively. Heterogeneous HER2 expression was observed, with a subpopulation of cells lacking measurable receptor (14.5%, SKBr-3,0.34%, MCF-7/HER2; 1.73%, BT-474). When plotted as a function of activity concentration, SF curves were biphasic and inversely proportional to SA; however, when the model was applied and absorbed doses calculated, the SF curve was monoexponential independent of SA. Thus, the PK model was able to demonstrate the effects of competition between cold and labeled mAb. These studies showed that the relative biological effectiveness of At-211-labeled trastuzaumab was about 10 times higher than that of external beam therapy. Conclusion: These in vitro Studies showed that At-211-labeled trastuzamab mAb is an effective cytotoxic agent for the treatment of HER2-positive tumor cells. The SA of the labeled mAb and the homogeneity of HER2 receptor expression are important variables influencing the efficiency of cell killing. (c) 2006 Elsevier Inc. All rights reserved.