Development of preclinical models of human BT474 and MDA-MB-361 breast cancer that endogenously overexpress HER2 and that respond to trastuzumab.

e11061 Background: The anti-Her-2 antibody trastuzumab can be effectively combined with chemotherapy to treat Her-2 positive metastatic breast cancers. However, Her-2 positive breast cancers readily develop resistance to trastuzumab based therapies, and new models are needed to study such resistance, and to find new therapeutic options. Methods: We began the in vivo selection of human Her-2 positive breast cancer cell lines, BT474 and MDA-MB-361, in Severely Compromised Immunodeficient (SCID) mice. Tumor cells were implanted orthotopically into the mammary fat pad of SCID mice, and resulting tumors were serially passaged into new hosts over a 3 year period. Results: The resulting variants grow rapidly in SCID mice. Variants of both BT474 and MDA-MB-361 respond in vivo to trastuzumab monotherapy (20mg/kg twice weekly, i.p.) compared to controls (p<0.05). After 2-6 months of trastuzumab monotherapy, all variants developed resistance to trastuzumab; such resistant variants were adapted to tissue culture. BT474 variants produced a spontaneous lymph node metastasis in SCID mice, 10 months after the surgical resection of the orthotopically implanted primary tumor; this metastatic variant was also adapted to tissue culture. BT474 variants were tagged with luciferase and then implanted intracranially into SCID mice; the progressive growth of the tumors cells in the mouse brain was thereafter monitored for 3 months by bioluminescence imaging, which provides ample opportunity to test new treatments for brain metastases. MDA-MB-361 variants growing orthotopically responded (i.e. p<0.05 vs. controls) to the combination therapy of trastuzumab (20mg/kg twice weekly, i.p.) plus chemotherapy; chemotherapy consisted of a bolus of cyclophosphamide (CTX, 100mg/kg, i.p) followed by a maintenance metronomic CTX (20mg/kg/day, p.o.) regimen. Conclusions: We developed two preclinical models of human Her-2 positive breast cancer which may be used to study trastuzumab based therapies, Her-positive metastatic breast cancer, the growth of Her-2 positive breast cancer at a secondary site (i.e. the brain), as well as the evolution of tumor resistance to trastuzumab based therapies.