A phase 1 study of ¹⁵³Sm-EDTMP with fixed high-dose melphalan as a peripheral blood stem cell conditioning regimen in patients with multiple myeloma

Despite response rates of 30% after high-dose chemotherapy with autologous hematopoietic stem cell transplant, patients with multiple myeloma are not cured. ¹⁵³Samarium ethylenediaminetetramethylenephosphonate (¹⁵³Sm-EDTMP; Quadramet®) is a short-range, β-emitting therapeutic radiopharmaceutical with avid skeletal uptake. In total, 12 patients were treated with escalating doses of ¹⁵³Sm-EDTMP (N=3/group; 6, 12, 19.8, and 30 mCi/kg) and a fixed dose of melphalan (200 mg/m²). No dose limiting toxicity was seen. To better standardize the marrow compartment radiation dose, the study was modified such that an additional six patients were treated at a targeted absorbed radiation dose to the red marrow of 40 Gy based on a trace labeled infusion 1 week prior to the therapy. Despite rapid elimination of unbound radiopharmaceutical via kidneys and bladder, no episodes of nephrotoxicity, hemorrhagic cystitis, or delayed radiation nephritis were observed with a median follow-up of 31 months (range 8.5-44). Median times to ANC>0.5 and platelet >20 × 10⁶/I were 12 and 11 days, respectively, with no graft failures. Overall response rate was 94% including seven very good artial responses and five complete responses. Addition of ¹⁵³Sm EDTMP to melphalan conditioning appears to be safe, well-tolerated and worthy of further study.