Biodistribution and Radiation Dosimetry of Intraperitoneally Administered ¹²⁴I-Omburtamab in Patients with Desmoplastic Small Round Cell Tumors.

Abstract

The aim of this study was to assess the pharmacokinetics, biodistribution, and radiation dosimetry of ¹²⁴I-omburtamab administered intraperitoneally in patients with desmoplastic small round cell tumor. <b>Methods:</b> Eligible patients diagnosed with desmoplastic small round cell tumor with peritoneal involvement were enrolled in a phase I trial of intraperitoneal radioimmunotherapy with ¹³¹I-omburtamab. After thyroid blockade and before radioimmunotherapy, patients received approximately 74 MBq of ¹²⁴I-omburtamab intraperitoneally. Five serial PET/CT scans were obtained up to 144 h after injection. Multiple blood samples were obtained up to 120 h after injection. Organ-absorbed doses were calculated with OLINDA/EXM. <b>Results:</b> Thirty-one patients were studied. Blood pharmacokinetics exhibited a biphasic pattern consisting of an initial rising phase with a median half-time (±SD) of 23 ± 15 h and a subsequent falling phase with a median half-time of 56 ± 34 h. Peritoneal distribution was heterogeneous and diffuse in most patients. Self-dose to the peritoneal cavity was 0.58 ± 0.19 mGy/MBq. Systemic distribution and activity in major organs were low. The median absorbed doses were 0.72 ± 0.23 mGy/MBq for liver, 0.48 ± 0.17 mGy/MBq for spleen, and 0.57 ± 0.12 mGy/MBq for kidneys. The mean effective dose was 0.31 ± 0.10 mSv/MBq. Whole-body and peritoneal cavity biologic half-times were 45 ± 9 and 24 ± 5 h, respectively. <b>Conclusion:</b> PET/CT imaging with intraperitoneally administered ¹²⁴I-omburtamab enables assessment of intraperitoneal distribution and estimation of absorbed dose to peritoneal space and normal organs before therapy.