*1.4.3.2 Selectin-targeted nanocarriers*

Selectins are single-chain transmembrane proteins including E, L, and P selectins, which are involved in cell adhesion via binding of sugar polymers. Selectin has distinct role in tumor inflammation and progression. Tumor cells exhibit cell tethering and rolling via selectin-dependent recognition of carbohydrates ligands to enhance distance during migration. An overexpression of E-selectin is observed in endothelial cells of high-grade glioma, although their definite role is not well established yet. Recently, Ferber et al. have reported that p-selectin is overexpressed not only in tumor endothelial cells but also in glioblastoma cells [116]. Using a dendritic polyglycerol sulfate (dPGS) nanocarrier, this group has delivered paclitaxel (PTX) in combination with a peptidomimetic of the anti-angiogenic protein thrombospondin-1 (TSP-1 PM) to inhibit the tumor growth *in vivo* using both murine and human orthotopic GB mouse models.

#### *1.4.3.3 Connexin-targeted nanocarriers*

Connexins, four-transmembrane glycoproteins, are major constituents of gap junction channels. Six connexin subunits assemble to form a hemi channel in the plasma membrane that docks with another such hemi channels of the adjacent cells to assemble the tight junction and mediate cell-cell interaction. A membrane protein connexin 43 (Cx43) is preferentially expressed in brain tumor and peritumoral area. To achieve Cx43-targeted drug delivery, Nukolova et al. have developed Cx43 mAb-conjugated nanogels loaded with cisplatin and using a C6 glioma model, the authors have shown that these nanogels can effectively inhibit tumor growth and significantly enhance the survival of animals while reducing the systemic toxicity of cisplatin [117]. In addition, the same group also functionalized this Cx43-targeting nanogels with another antibody of brain-specific anion transporter (BSAT1) to achieve additional tumor growth inhibition efficacy via dual targeting [118].

*Crossing Blood-Brain Barrier with Nano-drug Carriers for Treatment of Brain Tumors… DOI: http://dx.doi.org/10.5772/intechopen.101925*

#### *1.4.4 Other receptor and transporter-mediated targeting systems*

Beyond these receptors mentioned above, there are many other receptors or transporters protein such as insulin receptor, acetylcholine receptor, glucose transporter (GLUT), large amino acid transporter-1, organic cation transporter OCT3 and OCTN2, etc., are expressed on BBB or on tumor cells that are explored for nanocarrier-mediated drug delivery to brain tumor. For example, Zhang et al. reported PEGylated immunoliposomes (PILs) modified with 83-14 mAb to the human insulin receptor to target gene that EGFR gene (which plays a major role in brain tumor progression) in U87 cancer cells [119], and later the same group modified the nanocarrioers with additional transferrin receptor for achieving RNAi in mice intracranially xenografted with human U87 glioma [120]. Nicotine acetylcholine receptors (nAChRs) expressed on BCEC are also targeted for delivering chemotherapeutics to brain tumor. To this end, Saha et al., from our group, have developed a nicotinylated liposomes to deliver small-molecule STAT-3 inhibitor WP-1066 for combating mouse glioblastoma [121]. The same group has also developed another BBB-crossing liposomes grafted with amphetamine at their exo-surface and using this liposome they deliver combination of paclitaxel (PTX) and PD-L1siRNA (RNAi agent for immune checkpoint inhibitor) to the glioblastoma-bearing mouse brain. This combination therapy is reported to enhance the median survival of mouse till 45 days while the untreated control mice died at 17 days [122]. Among transporters, glucose transporters (GLUTs) and large amino acid transporters (LAT-1) are widely used for nanocarrier-mediated drug delivery to brain tumor. During tumor progression, tumor cells continuously need supply of nutrients such as glucose and amino acids, which leads to overexpression of such transporter in glioma cells as well as BBB [123, 124]. Recently, Anraku et al. have developed a self-assembled supramolecular ~30nm nanocarrier containing multiple glucose molecules via association of oppositely charged pairs of polyethylene glycol (PEG)-based block ionomers. A remarkable enhancement in brain accumulation of the micelle post ~15 min administration is observed, which is much higher than that for other nanoparticles [125]. Bhunia et al. from our group have reported a LAT-1-targeting liposomes containing L-DOPA on their exo-surface (Amphi-DOPA liposome) for delivering small-molecule STAT-3 inhibitor WP-1066 to glioblastoma-bearing mouse brain. A significant tumor growth inhibition is observed when mice are treated with WP-1066-loaded Amphi-DOPA liposomes compared with the untreated or non-targeting controltreated mice [124].

#### **1.5 Conclusion and future perspectives**

During the past decades, significant progressed has been made in developing nanocarriers for glioma therapy. Major focus in this research area has been implementation of different ligands or targeting different receptors and transporters overexpressed on BBB and brain tumor cells for delivering the payload to brain tumor tissue. However, less is known about the key critical design parameters of the nanoparticles facilitating BBB crossing. For example, it has been observed that nanocarriers with size 20–30 nm are most effective in BBB crossing while among the different shapes, nanorod is most efficient in BBB crossing followed by spherical nanocarrier. More detail information is needed in future regarding the role of surface potential, formulation or composition, drug loading method, etc., in facilitating transport across BBB. In addition, the factors influencing pharmacokinetic behaviors of the nanocarriers should be well studied and evaluated, which is very crucial for developing an effective brain-targeting drug carrier. The poor prognosis of GBM has also prompted to develop many new therapeutic strategies exploiting

inherent physical properties of the nanomaterials such as photodynamic or photothermal therapies and hyperthermia. However, biodegradability and nanotoxicity of such newly developed materials should be studied in detail. In this regard, liposomal or lipid-based nanocarriers exhibit reasonable safety profiles. Furthermore, as brain tumor cells are highly infiltrating, nanocarriers that only deliver the payload to tumor core via leaky BBB are not sufficient, rather an image-guided delivery of therapeutics has attracted significant attention in recent years indicating the need of developing theragnostic nanoparticles. Significant attention should also be paid in enhancing targeting efficiency of the nanocarrier, which is far away from satisfactory yet, either by increasing number of targeting ligand or using high-affinity ligands with optimum ratio.

In conclusion, the following aspects should be considered on designing efficient brain tumor targeting nanocarrier in future:

1. The small nanocarriers with multiple functionalities on the surface and with high fluorescence. The multiple anchoring site can facilitate conjugation with a greater number of same ligands or different ligands specific to multiple receptors and loading of more drug molecules. The fluorescence can facilitate the bioimaging. 2. Biocompatibility of the nanocarriers to eliminate scope of nanotoxicity. 3. Optimum circulation stability and biodegradability of the nanocarriers. 4. Accessibility by noninvasive advanced imaging technique such as magnetic resonance imaging and real-time in vivo microscopy to avoid unnecessary sacrifice of the animal. 5. Application of multiple approaches to develop multimodal nanocarriers for effective BBB penetration followed by chemotherapy and bioimaging.
