Out of 13 patients we enrolled, 8 patients had serial PET-CT scans that were evaluable for response. Of these 8 patients, 5 had stable or decreased SUVmax values at 4 weeks and 3 had clinical or imaging progression. Median time to progression was short at 7 weeks. Naltrexone was well tolerated. There were no discontinuations due to toxicity and no grade 3 or 4 toxicities were noted. Naltrexone showed modest activity in this short study suggesting the contribution of opioid receptors in ER-positive breast cancer. Our data do not support further de...