Trastuzumab resistant cells remain sensitive to lapatinib. Fulvestrant inhibits lapatinib and combination resistant cell growth. (A) Growth curves of UACC-812 and BT474 parental and resistant cells treated with different target therapies/regimens for nine days: trastuzumab (T) (10 μg/ml), lapatinib (L) (1 μM), trastuzumab plus lapatinib (L + T), or endocrine therapy, fulvestrant (F) (10-7M); media of parental cells (C). Cell numbers were quantified by absorbance at 655 nm after staining with methylene blue. Conditions were repeated in quadruplicate. Significance between groups was determined by multiple comparisons using the Sidak method (*P < 0.0001, UACC-812 TR:T versus TR:T + L, LR:L versus LR:L + F, or LTR:L + T versus LTR:L + T + F; BT474 TR:T versus TR:T + L, TR:T versus TR:T + F, LR:L versus LR:L + F, or LTR:L + T versus LTR:L + T + F). (B) UACC-812 parental, lapatinib resistant, and combination resistant cells were treated with fulvestrant for 24, 48, 72 h and whole-cell extracts were analyzed by Western blot with the indicated antibodies.