Serotransferrin enhances transferrin receptor-mediated brain uptake of antibodies

The propensity of antibody-based therapies to systemically enter the brain interstitium and ameliorate pathology associated with numerous neurological maladies is precluded by the presence of the blood-brain barrier (BBB). Through distinct mechanisms, the BBB has evolved to regulate transport of essential ions, minerals, certain peptides and cells between the blood and the brain, but very restrictive otherwise. Hijacking receptor-mediated transport pathways of the BBB has proved fruitful in developing "Trojan Horse" therapeutic approaches to deliver antibody-based therapies to the brain milieu. The transferrin receptor (TfR)-mediated transcytosis pathway (RMT) is one such example where large recombinant molecules have been designed to bind to the TfR, which in turn activates the RMT pathway, resulting in delivery across the BBB into the brain milieu. Based on these findings, we here investigated whether the addition of serotransferrin could trigger the endogenous TfR-mediated RMT pathway and hence be used to enhance the uptake of TfR binding antibodies. By using an in vitro model of a mouse BBB we could test whether co-administration of mouse serotransferrin with mouse and human-based monoclonal antibodies enhanced brain uptake. In all cases tested, no matter if the monoclonal antibodies were designed to bind the TfR in a monovalent, partially monovalent/bivalent or entirely bivalent fashion, with high or low affinity or avidity, the addition of mouse serotransferrin significantly improved transport across the artificial BBB. This was also true for TfR binding antibodies that on their own passes the BBB poorly. These results were subsequently confirmed using a human in vitro BBB model, along with human serotransferrin and human TfR-binding antibody. To corroborate the in vitro results further, we conducted two pilot in vivo brain uptake study in wildtype mice, by intravenously co-administering a monoclonal TfR-binding antibody in the presence or absence of mouse serotransferrin as a proof-of-concept. In a similar outcome to the in vitro studies, we observed a significant almost two-fold increase in uptake of two different TfR binding antibodies in the brain when it was co-administered with mouse serotransferrin. These findings show for the first time that serotransferrin supplementation can significantly improve the ability of TfR-binding antibodies to traverse the BBB, which provides a realistic therapeutic opportunity for improving the delivery of therapeutic antibodies to the brain.