Mass spectrometric characterization of intact desferal-conjugated monoclonal antibodies for immuno-PET imaging.

Rationale: Immuno-PET imaging may prove to be a diagnostic and progression/intervention biomarker for Alzheimer's disease (AD) with improved sensitivity and specificity. Immuno-PET imaging is based on the coupling of an antibody with a chelator that captures a radioisotope thus serving as an in vivo PET ligand. A robust and quality controlled process for linking the chelator to the antibody is fundamental for the success of this approach. Methods: The structural integrities of two monoclonal antibodies (trastuzumab and JRF/A beta N/25) and the quantity of desferal-based chelator attached following modification of the antibodies were assessed by online desalting and intact mass analysis. Enzymatic steps for the deglycosylation and removal of C-terminal lysine was performed sequentially and in a single tube to improve intact mass data. Results: Intact mass analysis demonstrated that inclusion of enzymatic processing was critical to correctly derive the quantity of chelator linked to the monoclonal antibodies. For trastuzumab, enzymatic cleaving of the glycans was sufficient, whilst additional removal of the C-terminal lysine was necessary for JRF/A beta N/25 to ensure reproducible assessment of the relatively low amount of the attached chelator. Conclusions: An efficient intact mass analysis based process was developed to reproducibly determine the integrity of monoclonal antibodies and the quantity of the attached chelator. This technique could serve as an essential quality control approach for the development and production of immuno-PET tracers.