Abstract LB211: Discovery of RenNano®-derived human heavy-chain-only antibodies that cross the blood-brain barrier

Therapeutic antibodies have been successfully used to treat several diseases, such as cancers and autoimmune diseases. However, the utility of conventional antibodies for neurological conditions is limited by the blood-brain barrier (BBB). Several strategies to address this issue have been reported, including receptor-mediated transcytosis (RMT) of antibodies using transferrin receptors. We hypothesize that this strategy could be further improved by the use of single-domain antibodies (sdAbs), such as the variable domain of heavy-chain-only antibodies (HCAbs), or variable new antigen receptors (VNARs), which are significantly smaller, and therefore could be used to more efficiently transport drugs of interest across the BBB. To this end, we developed anti-transferrin receptor 1 (TFR1) HCAbs utilizing our fully human heavy-chain-only antibody mice (RenNano®). We immunized RenNano® mice with recombinant TFR1 proteins, isolated the B cells from spleen and lymph nodes, and performed single B cell antibody screening using the Beacon® Optofluidic system. Most of the antibodies tested were cross-reactive to human and monkey TFR1. Furthermore, even though the antigen specificity relies on the VHH domain instead of a conventional antibody variable domain, the affinity of these HCAbs can reach 10−8-10−9 (KD). Of the 7 HCAbs tested, 6 were internalized into the human brain microvascular endothelial cell line, hCMEC/D3. To assess brain penetration of these antibodies in vivo, mice expressing human TFR1 (hTFR1 mice) received a tail vein injection with either isotype control, positive control pabinafusp alfa (a BBB penetrating anti-TFR1 monoclonal antibody enzyme conjugate) analog or RenNano®-derived HCAbs. After 0.5, 6, 24, and 72 h of exposure, mice brains were dissected for the quantification of HCAbs and for immunohistochemical analyses. The levels of anti-TFR1 HCAbs in the parenchyma was significantly higher than isotype controls and pabinafusp alfa analog. In brain sections, HCAbs were clearly observed in the parenchyma, and were colocalized with TFR1-expressing cells. These results demonstrate that HCAbs developed from RenNano® mice are able to penetrate the BBB. Taken together, these data highlight the tremendous potential for HCAbs and its variable domain sdAbs for transporting cargo across the BBB. Due to their smaller size and simpler structure, sdAbs could ultimately provide therapeutic benefit for neurodegenerative diseases, and offer promising potential fo