TY - JOUR T1 - In Vivo Imaging of Antileukemic Drug Asparaginase Reveals a Rapid Macrophage-Mediated Clearance from the Bone Marrow JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 214 LP - 220 DO - 10.2967/jnumed.116.177741 VL - 58 IS - 2 AU - Laurens T. van der Meer AU - Samantha Y.A. Terry AU - Dorette S. van Ingen Schenau AU - Kiki C. Andree AU - Gerben M. Franssen AU - Debbie M. Roeleveld AU - Josbert M. Metselaar AU - Thomas Reinheckel AU - Peter M. Hoogerbrugge AU - Otto C. Boerman AU - Frank N. van Leeuwen Y1 - 2017/02/01 UR - http://jnm.snmjournals.org/content/58/2/214.abstract N2 - The antileukemic drug asparaginase, a key component in the treatment of acute lymphoblastic leukemia, acts by depleting asparagine from the blood. However, little is known about its pharmacokinetics, and mechanisms of therapy resistance are poorly understood. Here, we explored the in vivo biodistribution of radiolabeled asparaginase, using a combination of imaging and biochemical techniques, and provide evidence for tissue-specific clearance mechanisms, which could reduce the effectiveness of the drug at these specific sites. Methods: In vivo localization of 111In-labeled Escherichia coli asparaginase was performed in C57BL/6 mice by both small-animal SPECT/CT and ex vivo biodistribution studies. Mice were treated with liposomal clodronate to investigate the effect of macrophage depletion on tracer localization and drug clearance in vivo. Moreover, macrophage cell line models RAW264.7 and THP-1, as well as knockout mice, were used to identify the cellular and molecular components controlling asparaginase pharmacokinetics. Results: In vivo imaging and biodistribution studies showed a rapid accumulation of asparaginase in macrophage-rich tissues such as the liver, spleen, and in particular bone marrow. Clodronate-mediated depletion of phagocytic cells markedly prolonged the serum half-life of asparaginase in vivo and decreased drug uptake in these macrophage-rich organs. Immunohistochemistry and in vitro binding assays confirmed the involvement of macrophagelike cells in the uptake of asparaginase. We identified the activity of the lysosomal protease cathepsin B in macrophages as a rate-limiting factor in degrading asparaginase both in vitro and in vivo. Conclusion: We showed that asparaginase is rapidly cleared from the serum by liver-, spleen-, and bone marrow–resident phagocytic cells. As a consequence of this efficient uptake and protease-mediated degradation, particularly bone marrow–resident macrophages may provide a protective niche to leukemic cells. ER -