Glioblastoma multiforme (GBM) is the most prevalent and aggressive brain tumour. GBM is the second cause of death from neurological diseases and accounts for 4% of cancer associated deaths. Despite surgical tumour resection followed by chemotherapy and radiotherapy, life expectancy is still limited to 15-20 months due to tumour relapse. Therefore, there is a great need to find more efficient therapies. New treatments need to overcome two main challenges: 1. to reach the tumour margins, where the blood-brain barrier (BBB) is intact, and 2. to eradicate glioma stem cells (GSCs), which are held responsible for tumour relapse after current treatments. Recent advancements at the interface between biotechnology and chemistry provide attractive novel treatment possibilities for GBM. The main objective of the project developed during this MSCA was to engineer a masked antibody derivative with the capacity to overcome the blood-brain barrier (BBB) and selectively kill the remaining glioblastoma stem cells (GSC).
GSCs represent a subpopulation of cells within glioblastoma that are characterized by increased resistance to chemotherapy and radiotherapy. There are several antibody-drug conjugates being developed against GSCs but none has reached clinical approval due to the difficulty in selectively targeting them without affecting other healthy cells expressing the same receptors. A common strategy to direct therapies to GSCs is targeting cell surface makers, such as CD133, a transmembrane glycoprotein. However, this receptor is also expressed in other noncancer cells such as hematopoietic stem cells and epithelial cells with microvilli. Thus, therapies targeting this receptor can result in severe on-target off-site effects. In the last decade, the field of activatable antibodies has emerged, enabling the engagement of antigens that were previously considered undruggable. Therefore, to avoid such deleterious effects, we are developing targeting molecules that are only activated in response to tumour microenvironmental cues, especially enzymatic activity. In this MSCA we have contribute to developing a masked CD133-specific antibody-derivative that is only activated in the tumour environment.
Anti-CD133 masked antibodies may revolutionize the treatment of GBM only if they are capable of reaching all GSCs. Since GSCs are also found in the periphery of the tumour, these biotherapeutics need to cross the BBB. This is mainly formed by brain capillary endothelial cells, which establish tight junctions that hinder paracellular transport. BBB-shuttle peptides are molecules capable of transporting cargoes into the brain parenchyma without disrupting the BBB. Although we and others have developed brain shuttles in the past, there is still a need to discover new and improved shuttles with the capacity to resist protease degradation and transport bigger molecules into the brain. During the course of this action, we have produced a new family of protease-resistant brain shuttles that are able to enhance the transport of antibodies into the brain, which should dramatically increase its efficacy.
The concept of GSC-targeting with activatable antibodies in combination with brain shuttles may change the current paradigm for the treatment of brain tumours. Furthermore, our workflow may be applied to dramatically enhance the efficacy of other biotherapeutics with on-target off-site dose-limiting effects and diseases affecting the brain.
