Biodistribution and radiation dosimetry of intraperitoneally administered 124I-omburtamab in patients with desmoplastic small round cell tumors.

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 (DSRCT). Methods: Eligible patients diagnosed with DSRCT with peritoneal involvement were enrolled in a phase I trial of intraperitoneal radioimmunotherapy with 131I-omburtamab (NCT01099644). After thyroid blockade and prior to radioimmunotherapy, patients received ~74 MBq 124I-omburtamab intraperitoneally. Five serial PET/CT scans were performed up to 144 h post-injection. Multiple blood samples were obtained up to 120 h post-injection. Organ absorbed doses were calculated with OLINDA/EXM. Results: Thirty-one patients were studied. Blood pharmacokinetics exhibited a biphasic pattern consisting of an initial rising phase with a median half-time (± standard deviation) of 23±15 h and a subsequent falling phase with a median half-time of 56±34 h. Peritoneal distribution was heterogenous but diffuse in most patients. Self-dose to the peritoneal cavity was 0.58±0.19 mGy/MBq. Systemic distribution and activity noted in major organs was 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 biological half-times were 45±9 h and 24±5 h, respectively. Conclusion: PET/CT imaging with intraperitoneally administered 124I-omburtamab enables assessment of intraperitoneal distribution and estimation of absorbed dose to peritoneal space and normal organs prior to therapy.