Background: Breast cancer is an important cause of cancerrelated death in women. In this pathological condition, arginase plays a role by providing ornithine as a substrate for the biosynthesis of polyamines which are important in tumor progression. The aim of this work was to determine the arginase activity in the plasma and tumors of patients with breast cancer; also, we investigated the relationship between this activity and the presence of the estrogen receptor. Patients and Methods: We evaluated the plasma arginase activity levels in 80 women with breast cancer and 42 healthy control subjects. We also measured the arginase levels in 42 breast cancer biopsies and 42 control tissues. Results: The mean activity of arginase in plasma was higher in breast cancer patients (0.78 nM/min/mg protein ± 0.04; p = 0.001) than in healthy volunteers (0.53 nM/ min/mg protein ± 0.04); however, this difference was indicative of patients in the advanced stages of the disease (n = 38, stage III; p < 0.0001). In addition, we did not find a relationship between the estrogen receptor and arginase activity. Conclusion: Our results show a higher arginase activity in the plasma of patients in the advanced stages of the disease, suggesting that arginase activity could serve as a possible biological marker of breast cancer progression.

3085 Background: Patients with metastatic breast cancer may be initially responsive to treatment, but a significant number develop refractory disease. There is a critical unmet need to develop effective therapeutic approaches given the unique metabolism of tumor cells. Methods: We examined the cytotoxic properties of a novel peptide, CT20p, derived from the C-terminus of Bax. For delivery to cells, the amphipathic nature of CT20p allowed it to be encapsulated in polymeric nanoparticles (NPs). NPs were made using aliphatic hyperbranched polyester (HBPE) that incorporated surface carboxylic groups and interior hydrophobic cavities for encapsulation of CT20p. To examine the cytotoxic potential and targeting capacity of CT20p-NP-HBPE, we treated MDA-MB-231 and MCF-10A breast cancer cell lines with the combination and measured changes in mitochondrial function, cell metabolism and induction of cell death. The ability of CT20p-NP-HBPE to cause tumor regression was examined by subcutaneously implanting MDA-MB-231 cells in nude mice. Results: Initial results showed that CT20p caused the release of calcein from mitochondrial-like lipid vesicles, without disrupting vesicle integrity, and, when expressed as a fusion protein in cells, localized to mitochondria. While the peptide alone had little effect upon intact cells, when encapsulated and delivered by nanoparticles, CT20p-HBPE-NPs proved an effective killer of breast cancer cells. CT20p-NP-HBPE initiated non-apoptotic cell death within 3 hours of treatment by targeting mitochondria and deregulating cellular metabolism. Nanoparticles alone or nanoparticles encapsulating a control peptide had minimal effects. The cytotoxicity of CT20p-NP-HPBE was most pronounced in breast cancer cells, sparing normal, epithelial cells. In implanted breast tumors, CT20p-NP-HBPE accumulated in tumors within 24 hours and reduced tumor burden by 50-80%. Conclusions: These results reveal the innovative features of CT20p that allow nanoparticle-mediated delivery to tumors and the potential application in combination therapies that target the unique metabolism of cancer cells.