dc.description.abstract
The blood-brain barrier is a well-coordinated and highly selective barrier whose main function is to regulate brain homeostasis and the transport of endogenous and exogenous compounds between the blood and the brain. It permits the selective brain uptake of nutrients and impedes the entrance of potentially harmful substances and pathogenic organisms into the brain. Due to this restrictive nature of the blood-brain barrier, the transport of neurotherapeutics from the blood to the brain results extremely difficult, and has become a major pharmaceutical challenge in recent decades. Only lipophilic molecules with a molecular weight under 400-600 Da are able to cross the blood-brain barrier. For this reason, 98% of all small molecule drugs and almost 100% of large molecule drugs are unable to cross the blood-brain barrier, and hence, most neurological and neurodegenerative diseases currently have few or no treatment options.
During the last decades, numerous strategies have been proposed to overcome the blood-brain barrier and efficiently deliver therapeutic agents to the brain. One of these strategies consists in linking the pharmacologically active substance to a molecular vector that acts as a molecular Trojan Horse and is capable of crossing the blood-brain barrier using a receptor-mediated transcellular transport system of the brain capillary endothelial cells. These molecular vectors can be natural ligands, peptides or monoclonal antibodies (mAbs) that bind to a particular receptor and trigger endocytosis or transcytosis processes. Several mAbs directed against the transferrin receptor (TfR), which is abundant in brain capillaries, have been extensively studied. However, there is still no consensus regarding their transcytotic capacity and their ability to transport substances across the blood-brain barrier. Moreover, the intracellular mechanisms that these antibodies or constructs undergo inside the endothelial cells remain unclear.
To gain insight on this strategy, this thesis aimed to study the potential of the 8D3 monoclonal antibody, directed against the murine TfR, to transport substances across the blood-brain barrier in mice.
On this basis, a series of experiments were performed where the 8D3 antibody was conjugated to different cargoes, the resulting constructs were administered in vivo to mice, and the distribution and intracellular mechanisms that these constructs undergo at the blood-brain barrier were studied.
Overall results suggest, firstly, that the 8D3 antibody is able to bind to the TfR and trigger endocytosis of the 8D3-TfR complex, but does not complete transcytosis except in rare occasions. Secondly, the 8D3 antibody is capable of internalizing the conjugated cargo inside the brain capillary endothelial cells through a clathrin-dependent endocytosis process, and hence, of overcoming the first obstacle in the transport across these cells. However, once inside the endothelial cells, the constructs tend to progressively accumulate in mature endosomal structures of large size and great complexity. These results can be mainly explained by the high affinity of the peptidomimetic mAbs for their receptors and their consequent difficulty to dissociate from them. Only a small percentage of the endocytic vesicles fuse with the abluminal membrane and open up to the basal lamina. Thus, in these cases, the constructs complete transcytosis. Nevertheless, they remain attached to the abluminal membrane and never reach the brain parenchyma, probably, in this case as well, because of the high affinity of the 8D3 for the TfR.
Based on this premise, different aspects can be approached regarding the design of the constructs. Nanocarrier functionalization, development of acid-cleavable linkages between mAb and cargo, and reducing mAb’s affinity for the target receptor are some of the ideas that are currently under development for the optimization of this receptor-mediated transport based strategy to overcome the blood-brain barrier. These aspects are today main focus of research and starting point towards new perspectives.
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