br Targeting Specific Cell Populations br Cellular
2.1 Targeting Specific Cell Populations
Cellular targeting has been extensively studied as a solution to improve intratumoral and cell type-specific NP delivery. This can be accomplished by the incorporation of specific molecular tags on the particle surface that can interact with extracellular proteins on endothelial, tumor, or immune cell membranes to facilitate targeted uptake. Numerous candidate cell-surface markers have been examined preclinically as targets for NP delivery (Table 1). Many of these are cell surface receptors that are preferentially expressed by tumors and not normal tissues to improve tumoral uptake. Targeting of well-known tumor markers including HER-2, EGFR, or PSMA can improve tumoral localization and uptake of NPs in vivo26-35. Attempts have been made to improve the efficacy of tumor-targeted NPs by targeting the most relevant cell populations capable of repopulation and tumor propagation. CD44 is a multifunctional cell surface antigen protein involved in cell function related to proliferation, migration, and signaling that is enriched in cancer initiating cells (CIC). Wei et al. recently demonstrated that the efficacy of salinomycin-encapsulated lipid-polymer NPs can be markedly enhanced against CD44+ subpopulations of prostate tumors with the incorporation of specific CD44 antibodies on the NP surface36. CD44 also strongly interacts with the anionic polysaccharide hyaluronic ATP (HA) and surface decoration with HA has been found to improve tumor targeting in CD44 overexpressing cell lines37-40. The ability to selectively target specific cell populations while excluding similar cells in close proximity may be especially important for cancer immunotherapy. Several important immunogenic cell populations in the TME including regulatory T-cells (T-regs) and TAMs exist as closely related pro- and anti-immunogenic subpopulations. Concomitant eradication of pro-immunogenic populations with non-selective delivery systems could offset the potential therapeutic advantages gained by removing their immunosuppressive counterparts. Fortunately, targetable cell surface markers have been identified for
many relevant populations of immune cells in the TME and the preclinical studies described below provide proof-of-principle for the selective modulation of specific subpopulations using targeted NPs.
It is worth noting that some therapeutic interventions, including ionizing radiation, can modulate the expression of cell surface receptors in ways that may be important for cancer immunotherapy. P-selectin is an endothelial integrin which is overexpressed in many human tumors. Shamay and colleagues confirmed that P-selectin-targeted fucoidan NPs improved intracellular delivery and tumor control in P-selectin overexpressing tumors41. They further demonstrated that focal radiotherapy (6 Gy) stimulated P-selectin overexpression in Lewis Lung xenografts that expressed very low levels at baseline. Interestingly, unilateral radiotherapy not only enhanced P-selectin expression in radiated tumors, but also stimulated an abscopal-like effect in which P-selectin expression was enhanced in unirradiated tumors using bilateral flank xenografts. As predicted, focal radiotherapy significantly improved the efficacy of targeted compared to untargeted liposomes. Previous studies with folate-targeted hapten immunotherapy also observed improved tumor targeting in unirradiated tumor deposits following focal radiotherapy42. The ability to modulate the expression of targetable cell surface receptors in the TME of distant foci of disease using focal radiotherapy could be used to enhance abscopal responses to cancer immunotherapies. Other potential radiation-inducible targets include various integrins, the glutathione receptor, and class 1 MHCs (among others).
2.2 Enhancing Endosomal Escape
One major obstacle facing intracellular payload delivery is degradation in the late endosome. The interaction of NPs with cell surface receptors stimulates receptor-mediated endocytic translocation of the NP from the cell surface into the cytoplasm. Molecules taken up by this process are generally degraded in the acidic environment of the late endosome. Without additional escape mechanisms, most NP payloads are degraded in the endosome along with their NP carrier. This is particularly problematic for macromolecules such as nucleic acids that can’t easily diffuse across the endosomal membrane once released from the degrading NP. Several preclinical methods of improving endosomal escape have been identified.
Perhaps the most simplistic among these is the use of amphiphilic NP carriers which are stabilized by acid-labile linkers. These carriers can take advantage of the acidic environment to stimulate rapid release of drug payload within the endosome. Hydrophobic small molecule drugs can then readily diffuse from the endosome into the cytoplasm to exert their antitumor effects. One recent example of this approach was provided by Yang and colleagues43. They synthesized paclitaxel-loaded PEG-BHyd-dC12 micelles stabilized by an acid-labile hydrazone bond. Using Cou-6 encapsulated NPs, they confirmed endosomal localization of the particles following receptor-mediated uptake. The labeled payload remained trapped in the endosomes of NPs lacking the acid-labile bonds whereas it quickly diffused into the cytoplasm of the functionalized particles. Acid-labile NPs were significantly more cytotoxic in several lung cancer lines than non-labile micelles or free drug. While these kinds of delivery systems may suffice for delivery of hydrophobic small-molecule drugs, more sophisticated platforms are necessary for macromolecular payloads.