Cationization increases brain distribution of an amyloid-beta protofibril selective F(ab')₂ fragment

Antibodies and fragments thereof are, because of high selectivity for their targets, considered as potential therapeutics and biomarkers for several neurological disorders. However, due to their large molecular size, antibodies/fragments do not easily penetrate into the brain. The aim of the present study was to improve the brain distribution via adsorptive-mediated transcytosis of an amyloid-beta (Aβ) protofibril selective F(ab')₂ fragment (F(ab')₂-h158). F(ab')₂-h158 was cationized to different extents and the specific and unspecific binding was studied in vitro. Next, cationized F(ab')₂-h158 was labelled with iodine-125 and its brain distribution and pharmacokinetics was studied in mice. Cationization did not alter the in vitro affinity to Aβ protofibrils, but increased the unspecific binding somewhat. Ex vivo experiments revealed a doubling of brain concentrations compared with unmodified F(ab')₂-h158 and in vivo imaging with single photon emission computed tomography (SPECT) showed that the cationized F(ab')₂-h158, but not the unmodified F(ab')₂-h158 could be visualized in the brain. To conclude, cationization is a means to increase brain concentrations of therapeutic antibodies or fragments and may facilitate the use of antibodies/fragments as imaging biomarkers in the brain.