Purpose: Immuno^positron emission tomography (PET), the combination of PET with monoclonal antibodies (mAb), is an attractive option to improve tumor detection and to guide mAb-based therapy. The long-lived positron emitter zirconium-89 ( 89 Zr) has ideal physical characteristics for immuno-PET with intact mAbs but has never been used in a clinical setting. In the present feasibility study, we aimed to evaluate the diagnostic imaging performance of immuno-PET with 89 Zr-labeled-chimeric mAb (cmAb) U36 in patients with squamous cell carcinoma of the head and neck (HNSCC), who were at high risk of having neck lymph node metastases. Experimental Design: Twenty HNSCC patients, scheduled to undergo neck dissection with or without resection of the primary tumor, received 75 MBq 89 Zr coupled to the anti-CD44v6 cmAb U36 (10 mg). All patients were examined by computed tomography (CT) and/or magnetic resonance imaging (MRI) and immuno-PET before surgery. Six patients also underwent PET with 18 F-fluoro-2-deoxy-D-glucose. Immuno-PETscans were acquired up to 144 hours after injection. Diagnostic findings were recorded per neck side (left or right) as well as per lymph node level (six levels per side), and compared with histopathologic outcome. For this purpose, the CT/MRI scores were combined and the best of both scores was used for analysis. Results: Immuno-PETdetected all primary tumors (n = 17) as well as lymph node metastases in 18 of 25 positive levels (sensitivity 72%) and in 11of 15 positive sides (sensitivity 73%). Interpretation of immuno-PET was correct in 112 of 121 operated levels (accuracy 93%) and in 19 of 25 operated sides (accuracy 76%). For CT/MRI, sensitivities of 60% and 73% and accuracies of 90% and 80% were found per level and side, respectively. In the six patients with seven tumorinvolved neck levels and sides, immuno-PETand 18 F-fluoro-2-deoxy-D-glucose PETgave comparable diagnostic results. Conclusion: In this study, immuno-PET with 89 Zr-cmAb U36 performed at least as good as CT/MRI for detection of HNSCC lymph node metastases.

Selection of the right drug for the right patient is a promising approach to increase clinical benefit of targeted therapy with monoclonal antibodies (mAbs). Assessment of in vivo biodistribution and tumor targeting of mAbs to predict toxicity and efficacy is expected to guide individualized treatment and drug development. Molecular imaging with positron emission tomography (PET) using zirconium-89 (89Zr)-labeled monoclonal antibodies also known as 89Zr-immuno-PET, visualizes and quantifies uptake of radiolabeled mAbs. This technique provides a potential imaging biomarker to assess target expression, as well as tumor targeting of mAbs. In this review we summarize results from initial clinical trials with 89Zr-immuno-PET in oncology and discuss technical aspects of trial design. In clinical trials with 89Zr-immuno-PET two requirements should be met for each 89Zr-labeled mAb to realize its full potential. One requirement is that the biodistribution of the 89Zr-labeled mAb (imaging dose) reflects the biodistribution of the drug during treatment (therapeutic dose). Another requirement is that tumor uptake of 89Zr-mAb on PET is primarily driven by specific, antigen-mediated, tumor targeting. Initial trials have contributed toward the development of 89Zr-immuno-PET as an imaging biomarker by showing correlation between uptake of 89Zr-labeled mAbs on PET and target expression levels in biopsies. These results indicate that 89Zr-immuno-PET reflects specific, antigen-mediated binding. 89Zr-immuno-PET was shown to predict toxicity of RIT, but thus far results indicating that toxicity of mAbs or mAb-drug conjugate treatment can be predicted are lacking. So far, one study has shown that molecular imaging combined with early response assessment is able to predict response to treatment with the antibody-drug conjugate trastuzumab-emtansine, in patients with human epithelial growth factor-2 (HER2)-positive breast cancer. Future studies would benefit from a standardized criterion to define positive tumor uptake, possibly supported by quantitative analysis, and validated by linking imaging data with corresponding clinical outcome. Taken together, these results encourage further studies to develop 89Zr-immuno-PET as a predictive imaging biomarker to guide individualized treatment, as well as for potential application in drug development.

Immuno-PET is an appealing concept in the detection of tumors and planning of antibody-based therapy. For this purpose, the long-lived positron emitter 89 Zr (half-life, 78.4 h) recently became available. The aim of the present first-in-humans 89 Zr immuno-PET study was to assess safety, biodistribution, radiation dose, and quantification of the 89 Zr-labeled chimeric monoclonal antibody (cmAb) U36 in patients with head and neck squamous cell carcinoma (HNSCC). In addition, the performance of immuno-PET for detecting lymph node metastases was evaluated, as described previously. Methods: Twenty HNSCC patients, scheduled to undergo surgical tumor resection, received 75 MBq of 89 Zr-cmAb U36 (10 mg). Immuno-PET scans were acquired at 1, 24, 72, or 144 h after injection. The biodistribution of the radioimmunoconjugate was evaluated by ex vivo radioactivity measurement in blood and in biopsies from the surgical specimen obtained at 168 h after injection. Uptake levels and residence times in blood, tumors, and organs of interest were derived from quantitative immuno-PET studies, and absorbed doses were calculated using OLINDA/EXM 1.0. The red marrow dose was calculated using the residence time for blood. Results: 89 Zr-cmAb U36 was well tolerated by all subjects. PET quantification of blood-pool activity in the left ventricle of the heart showed a good agreement with sampled blood activity (difference equals 0.2% 6 16.9% [mean 6 SD]) except for heavyweight patients (.100 kg). A good agreement was also found for the assessment of mAb uptake in primary tumors (mean deviation, 28.4% 6 34.5%). The mean absorbed red marrow dose was 0.07 6 0.02 mSv/MBq and 0.09 6 0.01 mSv/MBq in men and women, respectively. The normal organ with the highest absorbed dose was the liver (mean dose, 1.25 6 0.27 mSv/MBq in men and 1.35 6 0.21 mSv/MBq in women), thereafter followed by kidneys, thyroid, lungs, and spleen. The mean effective dose was 0.53 6 0.03 mSv/MBq in men and 0.66 6 0.03 mSv/MBq in women. Measured excretion via the urinary tract was less than 3% during the first 72 h. Conclusion: 89 Zr immuno-PET can be safely used to quantitatively assess biodistribution, uptake, organ residence times, and radiation dose, justifying its further clinical exploitation in the detection of tumors and planning of mAbbased therapy. Monocl onal antibodies (mAbs) have been approved for use as diagnostics and therapeutics in a broad range of medical indications, but especially in oncology (1). Immuno-PET, the tracking and quantification of mAbs with PET in vivo, is an exciting novel option to improve diagnostic imaging and to guide mAb-based therapy and has been described previously (2-6).To enable PET of mAbs, an appropriate positron emitterwith a half-life (t 1/2 ) that is compatible with the time needed to achieve optimal tumor-to-nontumor ratios (typically 2-4 d for intact mAbs)-has to be securely coupled to the targeting molecule. 124 I (t 1/2 , 100.3 h) and 89 Zr (t 1/2 , 78.4 h) are particularly suitable in combination with intact mAbs, becaus...

89 Zr-immuno-PET is a promising noninvasive clinical tool that measures target engagement of monoclonal antibodies (mAbs) to predict toxicity in normal tissues and efficacy in tumors. Quantification of 89 Zr-immuno-PET will need to move beyond SUVs, since total uptake may contain a significant non-target-specific contribution. Nonspecific uptake is reversible (e.g., blood volume) or irreversible (due to 89 Zr-residualization after mAb degradation). The aim of this study was to assess nonspecific uptake in normal tissues as a first critical step toward quantification of target engagement in normal tissues and tumors using 89 Zr-immuno-PET. Methods: Data from clinical studies with 4 89 Zr-labeled intact IgG1 antibodies were collected, resulting in a total of 128 PET scans (1-7 d after injection from 36 patients: 89 Zr-obinutuzumab [n 5 9], 89 Zr-cetuximab [n 5 7], 89 Zr-huJ591 [n 5 10], and 89 Zr-trastuzumab [n 5 10] [denoted as 89 Zr-anti-CD20, 89 Zr-anti-EGFR, 89 Zr-anti-PSMA and 89 Zr-anti-HER2, respectively]). Nonspecific uptake was defined as uptake measured in tissues without known target expression. Patlak graphical evaluation of transfer constants was used to estimate the reversible (V t ) and irreversible (K i ) contributions to the total measured uptake for the kidney, liver, lung, and spleen. Baseline values were calculated per tissue combining all mAbs without target expression (kidney: 89 Zr-anti-CD20, 89 Zr-anti-EGFR, and 89 Zr-anti-HER2; liver: 89 Zr-anti-CD20; lung: 89 Zr-anti-CD20, 89 Zr-anti-EGFR, and 89 Zranti-PSMA; spleen: 89 Zr-anti-EGFR and 89 Zr-anti-HER2). Results: For the kidney, liver, lung, and spleen, baseline V t was 0.20, 0.24, 0.09, and 0.24 mL⋅cm −3 , respectively, and baseline K i was 0.7, 1.1, 0.2 and 0.5 μL⋅g −1 ⋅h −1 , respectively. For 89 Zr-anti-PSMA, a 4-fold higher K i was observed for the kidney, indicating target engagement. In this case, nonspecific uptake accounted for 66%, 34%, and 22% of the total signal in the kidney at 1, 3, and 7 d after injection, respectively. Conclusion: This study shows that nonspecific uptake of mAbs for tissues without target expression can be quantified using 89 Zr-immuno-PET at multiple time points. These results form a crucial base for measurement of target engagement by therapeutic antibodies in vivo with 89 Zr-immuno-PET. For future studies, a pilot phase including at least 3 scans at 1 or more days after injection is required to assess nonspecific uptake as a function of time, to optimize study design for detection of target engagement.