Nanotechnology has the potential to revolutionize cancer diagnosis and therapy. Advances in protein engineering and materials science have contributed to novel nanoscale targeting approaches that may bring new hope to cancer patients. Several therapeutic nanocarriers have been approved for clinical use. However, to date, there are only a few clinically approved nanocarriers that incorporate molecules to selectively bind and target cancer cells. This review examines some of the approved formulations and discusses the challenges in translating basic research to the clinic. We detail the arsenal of nanocarriers and molecules available for selective tumour targeting, and emphasize the challenges in cancer treatment.

Nanocarriers as an emerging platform for cancer therapy

The rapid recognition of intravenously injected colloidal carriers, such as liposomes and polymeric nanospheres from the blood by Kupffer cells, has initiated a surge of development for "Kupffer cell-evading" or long-circulating particles. Such carriers have applications in vascular drug delivery and release, site-specific targeting (passive as well as active targeting), as well as transfusion medicine. In this article we have critically reviewed and assessed the rational approaches in the design as well as the biological performance of such constructs. For engineering and design of long-circulating carriers, we have taken a lead from nature. Here, we have explored the surface mechanisms, which affords red blood cells long-circulatory lives and the ability of specific microorganisms to evade macrophage recognition. Our analysis is then centered where such strategies have been translated and fabricated to design a wide range of particulate carriers (e.g., nanospheres, liposomes, micelles, oil-in-water emulsions) with prolonged circulation and/or target specificity. With regard to the targeting issues, attention is particularly focused on the importance of physiological barriers and disease states.

Long-Circulating and Target-Specific Nanoparticles: Theory to Practice

Protein and peptide drugs hold great promise as therapeutic agents. However, many are degraded by proteolytic enzymes, can be rapidly cleared by the kidneys, generate neutralizing antibodies and have a short circulating half-life. Pegylation, the process by which polyethylene glycol chains are attached to protein and peptide drugs, can overcome these and other shortcomings. By increasing the molecular mass of proteins and peptides and shielding them from proteolytic enzymes, pegylation improves pharmacokinetics. This article will review how PEGylation can result in drugs that are often more effective and safer, and which show improved patient convenience and compliance.

Effect of pegylation on pharmaceuticals

https://www.farm.ucl.ac.be/Full-texts-FARM/Danhier-2010-2.pdf

To exploit the tumor microenvironment: Passive and active tumor targeting of nanocarriers for anti-cancer drug delivery

A review of stimuli-responsive nanocarriers for drug and gene delivery

Chemistry of polyethylene glycol conjugates with biologically active molecules

Targeting Folate Receptor with Folate Linked to Extremities of Poly(ethylene glycol)-Grafted Liposomes: In Vitro Studies

Attachment of drugs to polyethylene glycols

Folic acid, attached to polyethyleneglycol-derivatized, distearoyl-phosphatidylethanolamine, was used to target in vitro liposomes to folate receptor (FR)-overexpressing tumor cells. Confocal fluorescence microscopic observations demonstrated binding and subsequent internalization of rhodamine-labeled liposomes by a high FR-expressing, murine lung carcinoma line (M109-HiFR cells), with inhibition by free folic acid. Additional experiments tracking doxorubicin (DOX) fluorescence with DOX-loaded, folate-targeted liposomes (FTLs) indicate that liposomal DOX is rapidly internalized, released in the cytoplasmic compartment, and, shortly thereafter, detected in the nucleus, the entire process lasting 1-2 h. FR-mediated cell uptake of targeted liposomal DOX into a multidrug-resistant subline of M109-HiFR cells (M109R-HiFR) was unaffected by P-glycoprotein-mediated drug efflux, in sharp contrast to uptake of free DOX, based on verapamil-blockade experiments with quantitation of cell-associated DOX and flow cytometry analysis. Delivery of DOX by FTLs to M109R-HiFR cells increased continuously with time of exposure, reaching higher drug concentrations in whole cells and nuclei compared with exposure to free DOX. The in vitro cytotoxic activity obtained with DOX-loaded FTLs was 10-fold greater than that of the nontargeted liposome formulation, but was not improved over that of free DOX despite the higher cellular drug levels obtained with the targeted liposomes in M109R-HiFR cells. However, if M109R-HiFR cells were exposed to drugs in vitro and tested in an in vivo adoptive assay for tumor growth in syngeneic mice along a 5-week time span, FTL DOX was significantly more tumor inhibitory than free DOX. It is suggested that the biological activity of liposomal DOX released inside the cellular compartment is reduced in vitro due to the aggregated state of DOX, resulting from the liposome drug-loading process, and requires a long period of time and/or an in vivo environment for full expression.

https://clincancerres.aacrjournals.org/content/clincanres/6/5/1949.full.pdf

Nuclear delivery of doxorubicin via folate-targeted liposomes with bypass of multidrug-resistance efflux pump.

Purpose: To compare the in vivo tissue distribution of folate-targeted liposomes (FTLs) injected i.v. in mice bearing folate receptor (FR)-overexpressing tumors (mouse M109 and human KB carcinomas, and mouse J6456 lymphoma) to that of nontargeted liposomes (NTLs) of similar composition.

Experimental Design: A small fraction of a folate-polyethylene-glycol (PEG)-distearoyl-phosphatidylethanolamine conjugate was incorporated in FTLs. Both FTLs and NTLs were PEGylated with a PEG-distearoyl-phosphatidylethanolamine conjugate to prolong circulation time. Liposomes were labeled with [3H]cholesterol hexadecyl ether with or without doxorubicin loading. Liposome levels in plasma, tissues, or ascites were assessed by the number of [3H] counts. For doxorubicin-loaded formulations, we also determined the tissue doxorubicin levels by fluorimetry. To estimate the amount of liposomes directly associated with tumor cells in vivo , we determined the [3H]radiolabel counts in washed pellets of ascitic tumor cells using the ascitic J6456 lymphoma

Results: FTLs retained the folate ligand in vivo , as demonstrated by their ability to bind ex vivo to FR-expressing cells after prolonged circulation and extravasation into malignant ascitic fluid. In comparison with NTLs, FTLs were cleared faster from circulation as a result of greater liver uptake. Despite the lower plasma levels, tumor levels of FTL-injected mice were not significantly different from those of NTL-injected mice. When NTLs and FTLs were loaded with doxorubicin, liver uptake decreased because of liver blockade, and uptake by spleen and tumor increased. When tumor-to-tissue liposome uptake ratios were analyzed, the targeting profile of FTLs was characterized by higher tumor:skin, and tumor:kidney ratios but lower tumor:liver ratio than NTLs. After a concomitant dose of free folic acid, FTLs (but not NTLs) plasma clearance and liver uptake were inhibited, indicating that accelerated clearance was mediated by the folate ligand. Surprisingly tumor uptake was not significantly affected by a codose of folic acid. In the J6456 ascitic tumor model, tumor cell-associated liposome levels were significantly greater for FTL-injected mice than for NTL-injected mice, despite slightly higher levels of the latter in whole ascites.

Conclusions: Whereas folate targeting does not enhance overall liposome deposition in tumors, the targeting profile of tumor versus other tissues is substantially different and intratumor liposome distribution in ascitic tumors is affected favorably with a selective shift toward liposome association with FR-expressing cells.

https://clincancerres.aacrjournals.org/content/clincanres/9/17/6551.full.pdf

Samuel Zalipsky

Papers

Chemistry of polyethylene glycol conjugates with biologically active molecules

Targeting Folate Receptor with Folate Linked to Extremities of Poly(ethylene glycol)-Grafted Liposomes: In Vitro Studies

Attachment of drugs to polyethylene glycols

Nuclear delivery of doxorubicin via folate-targeted liposomes with bypass of multidrug-resistance efflux pump.

In Vivo Fate of Folate-Targeted Polyethylene-Glycol Liposomes in Tumor-Bearing Mice