Phase I evaluation of CycloSam^® (Sm-153-DOTMP) bone seeking radiopharmaceutical in dogs with spontaneous appendicular osteosarcoma

¹⁵³Sm-DOTMP (CycloSam^®) is a newly-patented radiopharmaceutical for bone tumor treatment. DOTMP (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene-phosphonate) is a macrocyclic chelating agent with superior binding properties to ¹⁵³Sm when compared with EDTMP (Quadramet™, used for palliative treatment of bone cancer). CycloSam^® was administered at 1 mCi/kg (37 MBq/kg) in a prospective pilot study to seven dogs with bone cancer resulting in no myelosuppression. Then, 13 dogs were enrolled in a prospective clinical trial study using traditional 3+3 dose escalation and starting at 1.5 mCi/kg. Baseline evaluation included hematologic and biochemical testing, diagnosis confirmation, thoracic and limb radiographs, technetium-99 m-HDP bone scintigraphy, and ¹⁸F-FDG PET scan (SUVmax). Toxicity (primary endpoint) was assessed through weekly blood counts and adverse events. Dogs received 1.5 mCi/kg (n = 4), 1.75 mCi/kg (n = 6), and 2 mCi/kg (n = 3) of ¹⁵³Sm-DOTMP. Dose-limiting neutropenia and thrombocytopenia were seen at 2 mCi/kg. No dose-limiting nonhematologic toxicities occurred. Efficacy (secondary endpoint) was assessed by objective lameness measurement (body-mounted inertial sensors), owner quality-of-life (QoL) questionnaire, and repeat PET scan. Objective lameness measurement improved in four dogs (53%–60% decrease) was equivocal in three dogs, and worsened in four dogs (66%–115% increase); two dogs were not evaluable. Repeat ¹⁸F-FDG PET scan results varied and change in lameness did not consistently correlate with SUVmax changes. QoL score worsened (n = 5) or was improved/stable (n = 7). Carboplatin chemotherapy (300 mg/m2 IV every 3 weeks ×4) started 4 weeks after ¹⁵³Sm-DOTMP injection. No dog died of chemotherapy-related complications. All dogs completed study monitoring. The recommended dose for CycloSam^® in dogs is 1.75 mCi/kg, which resulted in some pain control with minimal toxicity and was safely combined with chemotherapy.