18F-labeled anti-human CD20 cys-diabody for same-day immunoPET in a model of aggressive B cell lymphoma in human CD20 transgenic mice
Metabolic imaging using [18F]FDG is the current standard for clinical PET; however, some malignancies (e.g., indolent lymphomas) show low avidity for FDG. The majority of B cell lymphomas express CD20, making it a valuable target both for antibody-based therapy and imaging. We previously developed PET tracers based on the humanised anti-CD20 antibody obinutuzumab (GA101). Preclinical studies showed that the smallest bivalent fragment, the cys-diabody (GAcDb, 54.5 kDa) with a peak uptake at 1–2 h post-injection and a biological half-life of 2–5 h, is compatible with short-lived positron emitters such as fluorine-18 (18F, t1/2 110 min), enabling same-day imaging.
GAcDb was radiolabeled using amine-reactive N-succinimidyl 4-[18F]-fluorobenzoate ([18F]SFB), or thiol-reactive N-[2-(4-[18F]-fluorobenzamido)ethyl]maleimide ([18F]FBEM) for site-specific conjugation to C-terminal cysteine residues. Both tracers were used for immunoPET imaging of the B cell compartment in human CD20 transgenic mice (hCD20TM). [18F]FB-GAcDb immunoPET was further evaluated in a disseminated lymphoma (A20-hCD20) syngeneic for hCD20TM and compared to [18F]FDG PET. Tracer uptake was confirmed by ex vivo biodistribution.
The GAcDb was successfully 18F-radiolabeled using two different conjugation methods resulting in similar specific activities and without impairing immunoreactivity. Both tracers ([18F]FB-GAcDb and [18F]FBEM-GAcDb) specifically target human CD20-expressing B cells in transgenic mice. Fast blood clearance results in high contrast PET images as early as 1 h post injection enabling same-day imaging. [18F]FB-GAcDb immunoPET detects disseminated lymphoma disease in the context of normal tissue expression of hCD20, with comparable sensitivity as [18F]FDG PET but with added specificity for the therapeutic target.
[18F]FB-GAcDb and [18F]FBEM-GAcDb could monitor normal B cells and B cell malignancies non-invasively and quantitatively in vivo. In contrast to [18F]FDG PET, immunoPET provides not only information about the extent of disease but also about presence and localisation of the therapeutic target.
KeywordsAnti-CD20 cys-diabody Antibody fragments ImmunoPET 18F B cell lymphoma Same-day imaging
The authors thank Felix B Salazar for technical support and the Preclinical Imaging Center (UCLA) for help with small-animal PET scans.
This work was supported by NIH grant CA149254, NIH grant CA212718 and by a generous gift from Ralph and Marjorie Crump to the Crump Institute for Molecular Imaging. Small-animal imaging and flow cytometry were funded in part by the UCLA Jonsson Comprehensive Cancer Center (JCCC) Support Grant (NIH CA016042). AM Wu, JM Timmerman and RM van Dam are members of the JCCC.
Compliance with ethical standards
Conflict of interest
JM Timmerman reports receiving commercial research grants from Bristol-Myers Squibb, Kite Pharma, and Valor Biotherapeutics, and is a consultant/ advisory board member for Celgene and Seattle Genetics. AM Wu holds ownership interest in and is a consultant/advisory board member for ImaginAb, Inc. No potential conflicts of interest were disclosed by the other authors.
All procedures performed in studies involving animals were in accordance with the ethical standards of the University of California Los Angeles (UCLA) Animal Research Committee. This article does not contain any studies with human participants performed by any of the authors.
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