HIGH CONTRAST TUMOR IMAGING WITH RADIO-LABELED ANTIBODY FAB FRAGMENTS TAILORED FOR OPTIMIZED PHARMACOKINETICS VIA PASYLATION
Claudia T Mendler, Lars Friedrich, Iina Laitinen, Martin Schlapschy, Markus Schwaiger, Hans-Juergen Wester and Arne SkerraIEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 33, NO. 7, JULY 2014, 2014, DOI 10.1109/TMI.2014.2313405
Research Area A
Although antigen-binding fragments (Fabs) of antibodies constitute established tracers for in vivo radiodiagnostics,their functionality is hampered by a very short circulation half-life. PASylation, the genetic fusion with a long, 2conformationally disordered amino acid chain comprising Pro, Ala and Ser, provides a convenient way to expand protein size and, consequently, retard renal ﬁ ltration. Humanized a HER2 and a CD20 Fabs were systematically fused with 100 to 600 PAS residues and produced in E. coli . Cyto ﬂ uorimetric titration analysis on tumor cell lines con ﬁ rmed that antigen-binding activities of the parental antibodies were retained. The radio-iodinated PASylated Fabs were studied by positron emission tomography (PET) imaging and biodistribution analysis in mouse tumor xenograft models. While the unmodi ﬁ ed a HER2 and a CD20 Fabs showed weak tumor uptake (0.8% and 0.2% ID/g, respectively; 24 h p.i.) tumor-associated radioactivity was boosted with increasing PAS length (up to 9 and 26-fold, respectively), approaching an optimum for Fab-PAS 400 . Remarkably, 6- and 5-fold higher tumor-to-blood ratios compared with the unmodi ﬁ ed Fabs were measured in the biodistribution analysis (48 h p.i.) for a HER2 Fab-PAS 100 and Fab-PAS 200 , respectively. These ﬁ ndings were con ﬁ rmed by PET studies, showing high imaging contrast in line with tumor-to-blood ratios of 12.2 and 5.7 (24 h p.i.) for a HER2 Fab-PAS 100 and Fab-PAS 200 . Even stronger tumor signals were obtained with the corresponding a CD20 Fabs, both in PET imaging and biodistribution analysis, with an uptake of 2.8% ID/g for Fab-PAS 100 vs . 0.24% ID/ g for the unmodi ﬁ ed Fab. Hence, by engineering Fabs via PASylation, plasma half-life can be tailored to signi ﬁ cantly improve tracer uptake and tumor contrast, thus optimally matching reagent/target interactions.