# These authors contributed equally to this work. AbstractPurpose-We evaluated biodistribution and tumor targeting of 89 Zr-lumretuzumab before and during treatment with lumretuzumab, a human epidermal growth factor receptor 3 (HER3)-targeting monoclonal antibody.Experimental design-20 patients with histologically confirmed HER3-expressing tumors received 89 Zr-lumretuzumab and underwent Positron Emission Tomography (PET). In Part A, 89 Zr-lumretuzumab was given with additional, escalating doses of unlabeled lumretuzumab and scans were performed 2, 4 and 7 days postinjection to determine optimal imaging conditions. In Part B, patients were scanned following tracer injection before (baseline) and after a pharmacodynamic (PD)-active lumretuzumab dose for saturation analysis. HER3 expression was determined immunohistochemically in skin biopsies. Tracer uptake was calculated as standardized uptake value (SUV).Results-Optimal PET conditions were found to be 4 and 7 days after administration of 89 Zrlumretuzumab with 100 mg unlabeled lumretuzumab. At baseline using 100 mg unlabeled lumretuzumab, the tumor SUVmax was 3.4 (±1.9) at 4 days postinjection. SUVmean for normal blood, liver, lung and brain tissues were 4.9, 6.4, 0.9 and 0.2, respectively. Saturation analysis (n=7) showed that 4 days after lumretuzumab administration, tumor uptake decreased by 11.9% (±8.2), 10.0% (±16.5) and 24.6% (±20.9) at PD-active doses of 400, 800 and 1600 mg, respectively, when compared to baseline. Membranous HER3 was completely downregulated in paired skin biopsies already at and above 400 mg lumretuzumab.Conclusions-PET imaging showed biodistribution and tumor specific 89 Zr-lumretuzumab uptake. Although, PD-active lumretuzumab doses decreased 89 Zr-lumretuzumab uptake, there was no clear evidence of tumor saturation by PET imaging as the tumor SUV did not plateau with increasing doses.

Rationale: Knowledge on monoclonal antibody biodistribution in healthy tissues in humans can support clinical drug development. Molecular imaging with positron emission tomography (PET) can yield information in this setting. However, recent imaging studies have analyzed the behavior of single antibodies only, neglecting comparison across different antibodies.Methods: We compared the distribution of four 89Zr-labeled antibodies in healthy tissue in a retrospective analysis based on the recently published harmonization protocol for 89Zr-tracers and our delineation protocol.Results: The biodistribution patterns of 89Zr-lumretuzumab, 89Zr-MMOT0530A, 89Zr-bevacizumab and 89Zr-trastuzumab on day 4 after tracer injection were largely similar. The highest tracer concentration was seen in healthy liver, spleen, kidney and intestines. About one-third of the injected tracer dose was found in the circulation, up to 15% in the liver and only 4% in the spleen and kidney. Lower tracer concentration was seen in bone marrow, lung, compact bone, muscle, fat and the brain. Despite low tracer accumulation per gram of tissue, large-volume tissues, especially fat, can influence overall distribution: On average, 5-7% of the injected tracer dose accumulated in fat, with a peak of 19% in a patient with morbid obesity.Conclusion: The similar biodistribution of the four antibodies is probably based on their similar molecular structure, binding characteristics and similar metabolic pathways. These data provide a basis for a prospectively growing, online accessible warehouse of molecular imaging data, which enables researchers to increase and exchange knowledge on whole body drug distribution and potentially supports drug development decisions.

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<div>Abstract<p><b>Purpose:</b> We evaluated biodistribution and tumor targeting of ⁸⁹Zr-lumretuzumab before and during treatment with lumretuzumab, a human epidermal growth factor receptor 3 (HER3)–targeting monoclonal antibody.</p><p><b>Experimental Design:</b> Twenty patients with histologically confirmed HER3-expressing tumors received ⁸⁹Zr-lumretuzumab and underwent positron emission tomography (PET). In part A, ⁸⁹Zr-lumretuzumab was given with additional, escalating doses of unlabeled lumretuzumab, and scans were performed 2, 4, and 7 days after injection to determine optimal imaging conditions. In part B, patients were scanned following tracer injection before (baseline) and after a pharmacodynamic (PD)-active lumretuzumab dose for saturation analysis. HER3 expression was determined immunohistochemically in skin biopsies. Tracer uptake was calculated as standardized uptake value (SUV).</p><p><b>Results:</b> Optimal PET conditions were found to be 4 and 7 days after administration of ⁸⁹Zr-lumretuzumab with 100-mg unlabeled lumretuzumab. At baseline using 100-mg unlabeled lumretuzumab, the tumor SUVmax was 3.4 (±1.9) at 4 days after injection. SUVmean values for normal blood, liver, lung, and brain tissues were 4.9, 6.4, 0.9 and 0.2, respectively. Saturation analysis (<i>n</i> = 7) showed that 4 days after lumretuzumab administration, tumor uptake decreased by 11.9% (±8.2), 10.0% (±16.5), and 24.6% (±20.9) at PD-active doses of 400, 800, and 1,600 mg, respectively, when compared with baseline. Membranous HER3 was completely downregulated in paired skin biopsies already at and above 400-mg lumretuzumab.</p><p><b>Conclusions:</b> PET imaging showed biodistribution and tumor-specific ⁸⁹Zr-lumretuzumab uptake. Although, PD-active lumretuzumab doses decreased ⁸⁹Zr-lumretuzumab uptake, there was no clear evidence of tumor saturation by PET imaging as the tumor SUV did not plateau with increasing doses. <i>Clin Cancer Res; 23(20); 6128–37. ©2017 AACR</i>.</p></div>