Highly Specific Blood-Brain Barrier Transmigrating Single-Domain Antibodies Selected by an In Vivo Phage Display Screening

A major bottleneck in the successful development of central nervous system (CNS) drugs is the discovery and design of molecules that can cross the blood-brain barrier (BBB). Nano-delivery strategies are a promising approach that take advantage of natural portals of entry into the brain such as monoclonal antibodies (mAbs) targeting endogenous BBB receptors. However, the main selected mAbs rely on targeting broadly expressed receptors, such as the transferrin and insulin receptors, and in selection processes that do not fully mimic the native receptor conformation, leading to mistargeting and a low fraction of the administered dose effectively reaching the brain. Thus, there is an urgent need to identify new BBB receptors and explore novel antibody selection approaches that can allow a more selective delivery into the brain. Considering that in vitro models fail to completely mimic brain structure complexity, we explored an in vivo cell immunization approach to construct a rabbit derived single-domain antibody (sdAb) library towards BBB endothelial cell receptors. The sdAb antibody library was used in an in vivo phage display screening as a functional selection of novel BBB targeting antibodies. Following three rounds of selections, next generation sequencing analysis, in vitro brain endothelial barrier (BEB) model screenings and in vivo biodistribution studies, five potential sdAbs were identified, three of which reaching >0.6% ID/g in the brain. To validate the brain drug delivery proof-of-concept, the most promising sdAb, namely RG3, was conjugated at the surface of liposomes encapsulated with a model drug, the pan-histone deacetylase inhibitor panobinostat (PAN). The translocation efficiency and activity of the conjugate liposome was determined in a dual functional in vitro BEB-glioblastoma model. The RG3 conjugated PAN liposomes enabled an efficient BEB translocation and presented a potent antitumoral activity against LN229 glioblastoma cells without influencing BEB integrity. In conclusion, our in vivo screening approach allowed the selection of highly specific nano-antibody scaffolds with promising properties for brain targeting and drug delivery.

STEM-02. IN VIVO PHAGE DISPLAY IDENTIFIES PEPTIDE TARGETING N-CADHERIN ON GLIOMA STEM CELLS

Glioblastoma (GBM) is the most aggressive primary brain tumor with high mortality rates and resistance to conventional therapy. Glioma stem cells (GSCs) comprise a sub-population of glioma tumor cells with the ability of self-renewal and tumor recapitulation, and may be responsible for GBM’s treatment resistant properties. Identification of surface receptors that are novel and specific to GSCs may be the key to the development of effective therapeutic strategies. We have selected a GSC specific targeting peptide isolated through in vitro and in vivo phage display biopanning. This screening technique allowed us to determine a peptide (GBM-IC2) which binds specifically to GSCs in vitro, and to GBM tissue in vivo. Although this screening process allows for isolation of cell specific targeting peptides, it does so without identification of the cellular binding partner. Given the specificity of the peptide, identification of the cellular receptor may allow for discovery of novel markers to identify GSCs. To identify the peptide binding partner of GBM-IC2, the biotinylated peptide was incubated with GSC protein lysate. The peptide, along with its binding partner, was isolated using streptavidin agarose resin. The binding partner protein was then identified using mass spectroscopy. This revealed N-cadherin (CDH2) as a potential binding partner for the GBM-IC2 peptide. GBM-IC2 demonstrated specificity for targeting CDH2 compared to control peptide using ELISA. Lentiviral induced overexpression of CDH2 in HEK293 cells allowed for GBM-IC2 peptide binding. Competition assay was performed by applying anti-CDH2 antibody to GBM-IC2 peptide and GSCs in culture. Application of anti-CDH2 antibody decreased peptide binding to GSCs, confirming CDH2 as the binding partner for GBM-IC2. These results demonstrate that cell specific targeting peptides isolated through phage display may lead to the isolation of novel cell specific proteins through immunoprecipitation isolation and mass spectroscopy analysis.

STEM-23. METASTATIC BRAIN TUMOR IMAGING THROUGH TARGETED PEPTIDES ISOLATED VIA PHAGE DISPLAY BIOPANNING AGAINST BRAIN METASTASIS-INITIATING CELLS

To effectively target metastatic brain tumors (MBTs), the paradigm of initiating treatment against MBTs following detection on clinical imaging needs to be shifted to an understanding of the mechanisms that drive the formation and maintenance of brain metastasis-initiating cells (BMICs). Targeting this tumor sub-population, which may form as a result of activation of the epithelial-mesenchymal transition, may allow for more effective means of understanding and targeting brain metastases. In order to isolate BMICs, we have harvested cells from patient derived MBTs originating from lung cancer and cultured the cells using stem cell media conditions. We then performed in vitro and in vivo phage display biopanning to isolate 12-amino acid length peptides that specifically target BMICs. Several peptides were isolated from both in vitro and in vivo biopanning strategies. Of the peptides recovered, one peptide, LBM4, demonstrated specific binding to MBT cells over primary lung cancer cells in vitro through flow cytometry analysis and immunocytochemistry. Fluorescent tagged LBM4 intravenously injected into mice harboring intracranial brain metastases demonstrated peptide localization to the tumor within the intracranial cavity visualized with live animal imaging. Peptide imaging of tumor corresponded to MRI imaging confirming that the peptides could serve as an alternative to tumor imaging, with the potential for greater sensitivity resulting from the cellular targeting of MBTs. Our results demonstrate that we can use a combination of in vitro and in vivo phage display biopanning to isolate cell specific targeting peptides. MBT targeting peptides can potentially result in a shifting of the clinical treatment paradigm of brain metastases, through the development of more effective targeted therapeutics aimed at BMICs, as well as improving detection of MBT cells which may result in earlier tumor visualization, as well as delineation of tumor recurrence versus radiation effects.

BSCI-15. METASTATIC BRAIN TUMOR TARGETING PEPTIDES ISOLATED THROUGH PHAGE DISPLAY BIOPANNING AGAINST BRAIN METASTASIS-INITIATING CELLS

To effectively target metastatic brain tumors (MBTs), the paradigm of treating MBTs after visualization on clinical imaging needs to be shifted to an understanding of the mechanisms that drive the formation and maintenance of brain metastasis-initiating cells (BMICs). Targeting this tumor sub-population, which may form as a result from activation of epithelial-mesenchymal transition, may allow for more effective means of isolating and targeting brain metastasis. In order to isolate BMICs, we have harvested cells from patient derived MBTs originating from lung cancer and cultured the cells using serum-free media conditions. In vivo phage display biopanning was used to isolate 12-amino acid length peptides that specifically target BMICs. Of the peptides recovered, one peptide, LBM4, was tested for specificity of binding to MBTs through in vitro and in vivo binding assays. When comparing patient derived metastatic brain tumors cells against brain metastasis cell lines, we found that both types of cells demonstrated similar morphology when grown in serum media conditions, but when grown in serum-free media, both demonstrated a tumor sphere morphology, similar to a stem cell-like state. LBM4 demonstrated specific binding to MBT cells over primary lung cancer cells in vitro through flow cytometry analysis and immunocytochemistry. Fluorescent tagged LBM4 intravenously injected into mice harboring intracranial BM demonstrated peptide localization to the tumor within the intracranial cavity visualized with live animal imaging. In vivo phage display biopanning is an effective tool that is able to isolate cell specific targeting peptides. MBT targeting peptides can potentially result in a shifting of the clinical treatment paradigm of brain metastases, through the development of more effective targeted therapeutics aimed at BMICs, as well as improving detection of MBT cells which may result in earlier tumor visualization as well as delineation of tumor recurrence versus radiation effects.