Preclinical Evaluation of (89)Zr-Panitumumab for Biology-Guided Radiation Therapy.

PURPOSE: Biology-guided radiation therapy (BgRT) uses real-time line-of-response data from on-board positron emission tomography (PET) detectors to guide beamlet delivery during therapeutic radiation. The current workflow requires (18)F-fluorodeoxyglucose (FDG) administration daily before each treatment fraction. However, there are advantages to reducing the number of tracer injections by using a PET tracer with a longer decay time. In this context, we investigated (89)Zr-panitumumab ((89)Zr-Pan), an antibody PET tracer with a half-life of 78 hours that can be imaged for up to 9 days using PET. METHODS AND MATERIALS: The BgRT workflow was evaluated preclinically in mouse colorectal cancer xenografts (HCT116) using small-animal positron emission tomography/computed tomography (PET/CT) for imaging and image-guided kilovoltage conformal irradiation for therapy. Mice (n = 5 per group) received 7 MBq of (89)Zr-Pan as a single dose 2 weeks after tumor induction, with or without fractionated radiation therapy (RT; 6 × 6.6 Gy) to the tumor region. The mice were imaged longitudinally to assess the kinetics of the tracer over 9 days. PET images were then analyzed to determine the stability of the PET signal in irradiated tumors over time. RESULTS: Mice in the treatment group experienced complete tumor regression, whereas those in the control group were killed because of tumor burden. PET imaging of (89)Zr-Pan showed well-delineated tumors with minimal background in both groups. On day 9 postinjection, tumor uptake of (89)Zr-Pan was 7.2 ± 1.7 in the control group versus 5.2 ± 0.5 in the treatment group (mean percentage of injected dose per gram of tissue [%ID/g] ± SD; P = .07), both significantly higher than FDG uptake (1.1 ± 0.5 %ID/g) 1 hour postinjection. To assess BgRT feasibility, the clinical eligibility criteria was computed using human-equivalent uptake values that were extrapolated from preclinical PET data. Based on this semiquantitative analysis, BgRT may be feasible for 5 consecutive days after a single 740-MBq injection of (89)Zr-Pan. CONCLUSIONS: This study indicates the potential of long-lived antibody-based PET tracers for guiding clinical BgRT.