Theranostic Intratumoral Convection-Enhanced Delivery of124I-Omburtamab in Patients with Diffuse Intrinsic Pontine Glioma: Pharmacokinetics and Lesion Dosimetry

Diffuse intrinsic pontine glioma (DIPG) is a rare childhood malignancy with poor prognosis. There are no effective treatment options other than external beam therapy. We conducted a pilot, first-in-human study using ¹²⁴I-omburtamab imaging and theranostics as a therapeutic approach using a localized convection-enhanced delivery (CED) technique for administering radiolabeled antibody. We report the detailed pharmacokinetics and dosimetry results of intratumoral delivery of ¹²⁴I-omburtamab. Methods: Forty-five DIPG patients who received 9.0–370.7 MBq of ¹²⁴I-omburtamab intratumorally via CED underwent serial brain and whole-body PET/CT imaging at 3–5 time points after injection within 4, 24–48, 72–96, 120–144, and 168–240 h from the end of infusion. Serial blood samples were obtained for kinetic analysis. Whole-body, blood, lesion, and normal-tissue activities were measured, kinetic parameters (uptake and clearance half-life times) estimated, and radiation-absorbed doses calculated using the OLINDA software program. Results: All patients showed prominent activity within the lesion that was retained over several days and was detectable up to the last time point of imaging, with a mean ¹²⁴I residence time in the lesion of 24.9 h and dose equivalent of 353 ± 181 mSv/MBq. Whole-body doses were low, with a dose equivalent of 0.69 ± 0.28 mSv/MBq. Systemic distribution and activities in normal organs and blood were low. Radiation dose to blood was very low, with a mean value of 0.27 ± 0.21 mGy/MBq. Whole-body clearance was monoexponential with a mean biologic half-life of 62.7 h and an effective half-life of 37.9 h. Blood clearance was biexponential, with a mean biologic half-life of 22.2 h for the rapid α phase and 155 h for the slower β phase. Conclusion: Intratumoral CED of ¹²⁴I-omburtamab is a novel theranostics approach in DIPG. It allows for delivery of high radiation doses to the DIPG lesions, with high lesion activities and low systemic activities and high tumor–to–normal-tissue ratios and achieving a wide safety margin. Imaging of the actual therapeutic administration of ¹²⁴I-omburtamab allows for direct estimation of the therapeutic lesion and normal-tissue–absorbed doses.