Table 2.
Selected examples of NM targeting CAMs for drug delivery.
| Class of targeting ligand | Targeting ligand | CAM | NM carrier | Payload | Site of delivery/purpose | Main result | References |
|---|---|---|---|---|---|---|---|
| Carbohydrate-based | Sialyl Lewisx (sLex) | E-selectin, P-selectin | Liposomes, NPs | Various cytotoxic and anti-inflammatory drugs | Activated endothelium, cancer or inflamed tissues | Preferential drug delivery to activated endothelium | (63–70) |
| 3'-(1-carboxy)ethyl (3'-CE) | E-selectin | Liposomes | / | Inflamed endothelium (HUVEC) | Higher internalization by huvecs | (73) | |
| Quinic acid (Qa) based sLex mimetic | E- and P-selectin | HPMA copolymer | / | Inflamed endothelium | Selective binding and higher internalization by IVECS | (74) | |
| Fucoidan | P-selectin | dextran sulfate-based NPs | BYL719 (PI3Kα inhibitor) | Tumor microenvironment | Tumor growth suppression, Reduction of systemic adverse effects of BYL719 | (75) | |
| mAbs | Anti-E-selectin Ab | E-selectin | Liposomes | DEX | Inflamed kidney | Reduced inflammation in vivo | (77) |
| anti-VCAM-1 | VCAM-1 | Liposomes based formulation-LipoCardium | PGA2 | Atherosclerotic plaque in LDL receptor knock-out mice | Reduction in plaque progression, reduced death due to MI | (80) | |
| Anti-ICAM-1 Ab | ICAM-1 | Liposomes | LOP | Inflamed rat paw | Better pain control, in vivo | (81) | |
| DEX | Lung endothelium | Reduced lung inflammation | (82) | ||||
| Peptides (primary sequence)* |
DITWDQLWDLMK | E-selectin | HPMA-based polymer | DOX, D(KLAKLAK)2 or DEX | Tumor vasculature, atherosclerotic plaque in ApoE-/- mice | Tumor growth reduction, inhibition of metastases, plaque stabilization | (85–87) |
| BSA-based NPs | DEX | Acute lung injury | Better accumulation in the inflamed region | (88) | |||
| HA-PTX micelles | PTX | Breast cancer lung metastasis | Inhibition of tumor growth and metastasis, and decreased systemic toxicity | (89) | |||
| IELLQAR | E-selectin | Self-assembled NPs | SN38-metabolite of irinotecan | Tumor endothelium | Inhibition of tumor growth, “drug-free” NP reduced the metastases formation |
(92) | |
| VHPKQHR | VCAM-1 | Liposomes, lipoparticles, micelles, lipid nanoemulsions | CCR2 Antagonist, flavonoids |
Inflamed (tumor) endothelium, atherosclerotic plaque in ApoE-/- mice | Inhibition of metastases in mice models, reduced transmigration of monocytes, atherosclerosis inhibition | (12, 100–102) | |
| Cyclic FLDVRK (mZD7349 peptide) | VCAM-1 | PLGA NPs | SIM | Activated HUVEC | Improved uptake into HUVEC, decreased phosphorylation of eNOS | (103) | |
| EWVDV | P-selectin | Magnetic Fe3O4 NPs | / | Binding to platelets that can accumulate in breast cancer tumors | Platelets targeting was successful in breast cancer, but not in pancreatic cancer models | (97) | |
| Micelles | PTX | Targeting circulating platelets, for delivery to CTC and primary tumor | Suppression of lung metastases in TNBC model | (95) | |||
| Lipid NPs | Ticagrelor and celecoxib | Targeting platelets | Accumulation in tumor via capturing platelets, antimetastatic effects | (96) | |||
| LVSVLDLEPLDAAWL | P-selectin | Lipid nanoemulsions | DEX | Inflamed lungs | Accumulation of NM in the inflamed lungs, reduction of inflammation (cytokines expression) | (98) | |
| Cyclo(1,12) PenITDGEATDSGC (cLABL) | ICAM-1 | PLGA NPs | DOX | Binding to activated HUVEC, Lung epithelial cells | Delivery of DOX to ICAM-1 expressing cells | (99) | |
| However, high “antibody-like” binding affinity can also be achieved with short targeting peptides. As NM ligands, peptides possess several advantages, mainly owing to their small size - low immunogenicity, stability, easy manufacturing and low cost (83). For example, our group utilized a short high-affinity E-selectin binding peptide (Esbp, DITWDQLWDLMK), identified in (84)) for targeted drug delivery of a cytotoxic drug (Doxorubicin, DOX) or a pro-apoptotic peptide (D(KLAKLAK)2, KLAK) to tumor vasculature. The hydrophilic HPMA-based copolymer bearing multiple copies of Esbp showed high binding affinity (at low nanomolar range) and selectivity to activated endothelial cells (85). The polymer-DOX conjugate (P-Esbp-DOX) demonstrated selective cytotoxicity toward E-selectin-expressing vascular endothelial cells that was 150-fold higher compared to a non-targeted polymer (85). In vivo, the E-selectin targeted polymer-drug conjugates (P-Esbp-DOX and P-Esbp-KLAK) decreased the rate of tumor growth and prolonged the survival of mice bearing primary Lewis lung carcinoma, or established melanoma (B16-F10) lung metastases (86). P-Esbp-DOX was also proven to be safe and highly efficacious in treating mice with established colorectal cancer liver metastases [unpublished data]. In addition to tumor targeting, this system was employed by our group for targeted delivery of an anti-inflammatory drug (DEX) to inflamed atherosclerotic plaques, to prevent cardiac remodeling and atherosclerosis (87). Esbp-modified bovine serum albumin (BSA) nanoparticles or Esbp-hyaluronic acid-paclitaxel (Esbp-HA-PTX) micelles were used by other groups to target DEX or PTX to acute lung injury or to inhibit breast cancer metastasis, respectively (88, 89). Other sequences such as IELLQAR (shown to bind to E-selectin and with much lower affinity to P- and L-selectin) (90–92), YRNWFGRW and YRNWDGRW (93) have been proposed as well for selective targeting of E-selectin. Interestingly, a study by Fernandes et al. directly compared all of these peptides for their ability to bind E-selectin. Their results suggest that Esbp is the ideal candidate for NM development, as it binds E-selectin better than the IELLQAR sequence and with better specificity when compared to the other two sequences (94). Other short, high-affinity peptides were incorporated into NMs for targeting VCAM-1 [sequences VHPKQHR(GGSKGC) and cyclic FLDVRK (cyclo(MePhe-Leu-Asp-Val-D-Arg-D-Lys)) reviewed in (12)], P-selectin [sequences EWVDV (95–97) and LVSVLDLEPLDAAWL (98)] and ICAM-1 (cyclo(1,12)PenITDGEATDSGC (99) (Table 2). Similar to short peptides, short sequences of nucleotides, commonly referred to as DNA or RNA aptamers were also developed for targeting CAMs. A thioaptamer targeting E-selectin (ESTA-1) (109) demonstrated selective binding with nanomolar binding affinity, and was able to target porous silicon particles to the tumor vasculature in a breast cancer xenograft model, endothelium of bone marrow or atherosclerotic plaque (104–106). Conversely, other groups reported that they couldn't confirm ESTA-1 binding to human E-selectin in their assays (110) and identified a new high affinity E- and P-selectin binding aptamer (SDA). These discrepancies could serve as a warning of difficulties in the translation of these ligands among different systems, laboratory facilities, and types of NM. CAM-targeting of NM can also be achieved through usage of cell-membrane fragments of cells known to engage with CAMs. Liang et al. (107) used the presence of α4 integrin on the surface of macrophages to generate membrane-coated liposomes that could bind to VCAM-1 and effectively deliver drug payload to lung metastases. Recently, a new approach for generating targeting ligands on NMs emerged from the advances in genetical engineering. Park et al. (108) developed VCAM-1 targeted NM by coating poly(lactic-co-glycolic acid) (PLGA)-based NPs with membrane fragments of cells expressing the ligand for VCAM-1. They chose a cell line which expresses β1 integrin, and modified it to express integrin α4, which together form a complex, VLA-4, a ligand for VCAM-1. This NM was loaded with DEX and used for drug delivery to inflamed lungs and suppressing inflammation. Aptamers | ESTA-1 | E-selectin | Porous silicon microparticles | PTX, microRNA | Bone marrow, xenograft breast tumors, atherosclerotic plaque | Bone marrow accumulation, breast cancer targeting. Reduced endothelial inflammation | (104–106) |
| Cell-membrane fragments | Macrophage-derived cell membranes | VCAM-1 | Liposomes | DOX | Lung metastases | Improved lung metastases imaging and tumor growth inhibition | (107) |
| Genetically engineered cell-membrane fragments | VCAM-1 | PLGA NPs | DEX | Inflamed lungs | Reduction of inflammation (cytokine levels) | (108) |
DOX-doxorubicin (hydrochloride) DEX-dexamethasone PTX-paclitaxel LOP-loperamide SIM-simvastatin HUVEC-human umbilical vein endothelial cells MI-myocardium infarction.
eNOS- endothelial NO synthase.
*
Peptide sequences are presented in the primary binding form, without spacers and amino acids used for conjugation to NM.