Phase II of Naltrexone in Hormone-Refractory Metastatic Breast Cancer

Inclusion Criteria: * Metastatic, hormone-receptor positive breast cancer * Disease that has progressed despite previous systemic hormonal therapy. Hormone therapy must be terminated at least 2 weeks prior to study enrollment. * Prior chemotherapy, immunotherapy, or biological therapy is allowed if at least 3 weeks since last treatment. Patient must recover from the acute toxic effects of the treatment prior to study enrollment. * Measurable disease as defined by solid tumor response (RECIST) criteria or non-measurable bone disease that is Positron-emission tomography (PET) avid * Karnofsky performance status \>70% * Female, age 18 years or older * Adequate organ function within 14 days of study enrollment including the following: * Adequate bone marrow reserve: absolute neutrophil (segmented and bands) count (ANC) ≥ 1.5 x 10\^9/L, platelets \>75 x 10\^9/L, and hemoglobin \> 8 g/dL * Hepatic: bilirubin ≤ 2 times the upper limit of normal (× ULN), aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 2 × ULN. (AST and ALT ≤ 5 × ULN is acceptable if liver has tumor involvement) * Renal: creatinine ≤ 2 times the upper limit of normal * Women of childbearing potential are required to use an effective method of contraception (ie, a hormonal contraceptive, intrauterine device, diaphragm with spermicide, condom with spermicide, or abstinence) during the study and for 3 months after the last dose of study drug. * Voluntary written informed consent before performance of any study-related procedure not part of normal medical care, with the understanding that consent may be withdrawn by the subject at any time without prejudice to future medical care. Exclusion criteria: * Brain metastases unless stable for 1 month or more following radiation therapy. * Pregnant or lactating women. PET-CT is not approved during pregnancy. A negative urine or serum pregnancy test is required for all females of child bearing potential within 7 days prior to study entry. Pregnancy testing is not required for post-menopausal or surgically sterilized women. * Use of any short-acting or long-acting opioid medication (including morphine, meperidine, oxycodone, hydromorphone, hydrocodone, fentanyl, tramadol) within 10 days prior to study enrollment * Pain uncontrolled with the use of non-narcotic drugs (acetaminophen or non-steroidal medications) * History of sensitivity to naltrexone * Acute hepatitis or liver failure * Immunosuppressive therapy for patients with autoimmune diseases, organ transplant, or other indications