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177Lu-labeled ethylenediamine-N,N,N′,N′-tetrakis methylene phosphonic acid (177Lu-EDTMP), was used to palliate metastatic bone pain as a new bone-seeking radiopharmaceutical. In this phase II study, we assessed the efficacy and safety of 177Lu-EDTMP for bone pain palliation in patients with breast cancer and hormone refractory prostate cancer with bone metastases.Sixteen patients were enrolled in the trial and were subsequently divided into 2 groups, the low-dose group (1295 MBq) and the high dose group (2590 MBq) to determine differences in toxicities and response rates. Pain scores, Karnofsky indices, mobility scores, and requirement of analgesic administration were assessed at 0, 2, 4, 6, 8, and 12 weeks after injection of 177Lu-EDTMP. Toxicity was assessed by analyzing hemoglobin, leukocyte, and platelet counts.An obvious reduction in the mean pain score was observed at 2 to 6 weeks after the administration of 177Lu-EDTMP. The rate of complete responses in bone pain palliation was 55% in group 1 and 80% in group 2 at 6 weeks after treatment. Of the 5 patients who required additional analgesics, all were able to reduce or completely stop taking these medications by 4 weeks after therapy. Mean (SD) Karnofsky indices of 58.18 (9.82) (range, 50–70) and 56.00 (8.94) (range, 50–70) at baseline increased to 82.73 (9.05) (range, 60–90) at 6 weeks after 177Lu-EDTMP treatment in group 1 and 85.00 (5.77) (range, 80–90) at 8 weeks after injection in group 2, respectively. Mobility scores decreased from 2.91 (1.04) (range, 1–4) and 2.80 (0.84) (range, 2–4) at baseline to 1.00 (0.67) (range, 0–2) and 0.50 (0.58) (range, 0–1) at 8 weeks after administration of 177Lu-EDTMP in groups 1 and 2, respectively, primarily owing to improved mobility. In group 1, 1 patient experienced grade III toxicity in both hemoglobin and platelet counts. No grade IV toxicities were observed. In group 2, there were no grade III or IV toxicities found in hemoglobin, platelets, or leukocytes counts. Moreover, no clinically significant adverse effects were observed, and no significant differences in either efficacy or safety were detected between the 2 dose levels.This study indicated that 177Lu-EDTMP was an effective and safe treatment for palliation of metastatic bone pain in patients with prostate or breast cancer. A dose of 1295 MBq (35 mCi) was sufficient for bone pain palliation therapy, and doses as high as 2590 MBq (70 mCi) were well tolerated.