F-labeled anti-human CD20 cys-diabody for same-day immunoPET in a model of aggressive B cell lymphoma in human CD20 transgenic mice.

Purpose: Metabolic imaging using [F]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 (F, t 110 min), enabling same-day imaging.

Methods: GAcDb was radiolabeled using amine-reactive N-succinimidyl 4-[F]-fluorobenzoate ([F]SFB), or thiol-reactive N-[2-(4-[F]-fluorobenzamido)ethyl]maleimide ([F]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). [F]FB-GAcDb immunoPET was further evaluated in a disseminated lymphoma (A20-hCD20) syngeneic for hCD20TM and compared to [F]FDG PET. Tracer uptake was confirmed by ex vivo biodistribution.

Results: The GAcDb was successfully F-radiolabeled using two different conjugation methods resulting in similar specific activities and without impairing immunoreactivity. Both tracers ([F]FB-GAcDb and [F]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. [F]FB-GAcDb immunoPET detects disseminated lymphoma disease in the context of normal tissue expression of hCD20, with comparable sensitivity as [F]FDG PET but with added specificity for the therapeutic target.

Conclusions: [F]FB-GAcDb and [F]FBEM-GAcDb could monitor normal B cells and B cell malignancies non-invasively and quantitatively in vivo. In contrast to [F]FDG PET, immunoPET provides not only information about the extent of disease but also about presence and localisation of the therapeutic target.