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To determine whether the dual EGFR/HER2 kinase inhibitor lapatinib can radiosensitize EGFR+ breast cancer in vivo, mice bearing xenografts of basal-like/EGFR+ SUM149 breast cancer cells were treated with lapatinib and fractionated radiation therapy over a 10 d period and tumor growth inhibition determined and correlated with changes in activation of downstream signaling to ERK1/2 and AKT by immunohistochemistry. Basal-like/EGFR+ SUM149 breast cancer tumors were completely resistant to treatment with lapatinib alone but highly growth impaired with lapatinib + radiation exhibiting an enhancement ratio average of 2.75 and a fractional tumor product ratio average of 2.06 over the duration of the study. Immunohistochemical analysis demonstrated that radiosensitization by lapatinib correlated with inhibition of ERK1/2 but not AKT. In vitro analysis of other basal breast cancers consistently demonstrated a loss of radiation induced MEK1/2 inhibition with lapatinib treatment. Treatment of the SUM102 basal-like/EGFR+ cell line with the MEK inhibitor CI1040 resulted in radiosensitization at levels similar to that seen with lapatinib alone. Furthermore, constitutive activation of RAF in SUM102 cells abolished the ability of lapatinib to radiosensitize these cells and resulted in enhanced radioresistance relative to parental cells. Collectively, this work demonstrates that lapatinib radiosensitizes EGFR+/basal-like breast cancer both in vitro and in vivo in a MEK/ERK dependent manner, and suggests that lapatinib in combination with fractionated radiation therapy may provide an effective treatment option for breast cancers of this subtype. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B73.

We describe here the ultrastructural localization of Giardia cyst antigens in the filaments associated with the outer portion of intact cysts and on developing cyst wall filaments in encysting trophozoites. Post-embedding immunogold labeling of thin sections of intact Giardia cysts with polyclonal and monoclonal antibodies specific for cyst wall antigens (major protein bands of approximately 29, 75, 88, and 102 KD on Western blots) showed strong labeling of the filamentous cyst wall, whereas no labeling was seen on the membranous portion. High-resolution field emission scanning electron microscopy (FESEM) of Giardia cysts revealed that the cyst wall-specific polyclonal rabbit antisera and monoclonal mouse antibody produced gold labeling of 20-nm filaments in the cyst wall as detected with secondary electron imaging (SEI) and backscatter electron imaging (BEI) at 10 kV, despite coating of the cells with platinum by ion sputtering. FESEM studies of encysting Giardia trophozoites demonstrated that immunostaining with antibodies to cyst wall antigens produced colloidal gold labeling of developing cyst wall filaments on the cell surface; however, the intervening membrane domains were unlabeled. Substitution of normal serum for cyst wall-specific antibodies, or preabsorption of specific antibodies with Giardia cysts, eliminated immunolabeling of the filaments.