TY - JOUR T1 - Pretargeted immunoPET of pancreatic cancer: overcoming circulating antigen and antibody internalization to reduce radiation doses JF - Journal of Nuclear Medicine JO - J Nucl Med DO - 10.2967/jnumed.115.163824 SP - jnumed.115.163824 AU - Jacob Lee Houghton AU - Brian Matthew Zeglis AU - Dalya Abdel-atti AU - Ritsuko Sawada AU - Wolfgang W Scholz AU - Jason S. Lewis Y1 - 2015/10/01 UR - http://jnm.snmjournals.org/content/early/2015/10/14/jnumed.115.163824.abstract N2 - 5B1 is a fully-human, monoclonal antibody that has shown promise for the PET imaging of cancers expressing CA19.9 — a carbohydrate prevalent in cells with aberrant glycosylation and an established effector of metastasis. The long physiological half-life of the antibody and interference from circulating CA19.9 may increase the time required to generate quality images as well as the risk of radiation exposure to healthy tissues during repeated PET imaging. Pretargeting methodologies are an effective approach to expeditiously acquire PET images, but in this case, the pretargeting approach is complicated by the internalization of 5B1 by CA19.9-expressing cells. We sought to adapt and optimize a pretargeting strategy that exploits the bioorthogonal reaction between transcyclooctene (TCO) and tetrazine (Tz) to overcome these complications. Methods: 5B1 was modified with TCO, and a novel NOTA-PEG7-Tz radioligand was synthesized with the goal of improving upon a previously reported analog. BxPC3 and Capan-2 cells were evaluated for their ability to internalize anti-CA19.9 antibodies using a fluorometric assay, and xenografts of the same lines were used for in vivo studies. The pretargeting approach was optimized, and the two radioligands compared using biodistribution and PET imaging in murine models of pancreatic cancer. Results: BxPC3 and Capan-2 cells were shown to rapidly internalize anti-CA19.9 mAbs, including 5B1. 64Cu-NOTA-PEG7-Tz showed improved in vivo pharmacokinetics relative to 64Cu-NOTA-Tz using 5B1-TCO as the targeting vector. PET imaging and biodistribution studies showed that injecting the radioligand 72h after the administration of 5B1-TCO resulted in the best uptake (8.2 ± 1.7 %ID/g at 20 h post injection) and tumor-to-background activity concentration ratios. Dosimetry calculations revealed that the pretargeting system produced a >25-fold reduction in total body radiation exposure relative to 89Zr-DFO-5B1. PET/CT imaging in an orthotopic Capan-2 xenograft model — which secretes large amounts of CA19.9 and more rapidly internalizes anti-CA19.9 antibodies — showed that this approach is viable even in the difficult circumstances presented by a circulating antigen and internalized targeting vector. Conclusion: The 5B1-TCO and 64Cu-NOTA-PEG7-Tz system evaluated in these studies can delineate CA19.9-positive xenografts in murine models of pancreatic cancer despite the challenges posed by the combination of circulating antigen and internalization of the 5B1-TCO. ER -