Impact of PEG-LPrA2 treatment on growth of estrogen receptor positive and negative breast cancer xenografts. (a) Growth of established MCF-7 breast cancer (BC) xenografts. (b) Tumor mass of established MCF-7 BC xenografts. (c) Growth of established MDA-MB231 BC xenografts. (d) Tumor mass of established MDA-MB231 BC xenografts. Female SCID mice were orthotopically inoculated into the mammary glands with human estrogen-receptor-positive MCF-7 (for ovariectomized mice) or estrogen-receptor-negative MDA-MB231 cells (2 × 106). The mice were treated with pegylated leptin peptide receptor antagonist 2 (PEG-LPrA2) (n = 10/each cell type) or with pegylated scrambled peptide (control; n = 10/each cell type). Ovariectomized mice were supplemented with a subdermal estradiol capsule. Tumor growth was determined using a caliper (tumor volume = π/6 × width2 × length). Data expressed as mean ± standard error. *P < 0.05 and **P < 0.01, significant differences with respect to control mice.