DOX, a chemotherapy drug that is used in many patients because of its powerful anti-cancer activity. Yet its therapeutic potential is constricted by serious side-effects such as cardiotoxicity and broad adsorption to healthy tissue. The solution to these problems has been to put doxorubicin in liposomes, which have become a powerful approach. Liposomes, nanocarriers made of phospholipids, can wrap doxorubicin in a film that will prevent it from being destroyed prematurely to facilitate more selective drug delivery. This enhances the therapeutic index of the drug by increasing its absorption into the tumor and decreasing the amount that is spread to healthy tissues. The second innovation in liposomal delivery is to use functional groups for additional specificity and stability of the drug delivery system. The clickable group that allows liposomes to connect to many types of targeting moieties via click chemistry is a form of this functional group, called the DBCO (dibenzocyclooctyne) lipid. The DBCO-modified liposomes are special because they can be linked to specific ligands like monoclonal antibodies or peptides that can be directed against tumour cells or other diseases. Such targeted delivery is essential for off-target elimination and enhanced doxorubicin therapy. Through targeted delivery of doxorubicin to cancer cells, DBCO-based liposomes can pump more drug into the tumour for better tumour inhibition without harming healthy tissue. Also because of the adaptability of DBCO chemistry, it is possible to design multiple-use drug delivery platforms whereby the liposomes are combined with chemotherapy, targeting ligands, and even diagnostic or imaging molecules in order to form a therapy-to-monitoring device. A second, fundamental modification to liposomes other than functionalising them with DBCO lipids is liposomal PEGylation. PEGylation is the bonding of polyethylene glycol (PEG) chains to the surface of liposomes. PEGylation is also a standard procedure in the delivery industry to optimise pharmacokinetics and biodistribution of liposomal drug. PEG chains form a "stealth" shield around the liposomes to avoid detection and clearance by the immune system as it's also called the "RES effect" (reticuloendothelial system). This stealth property extends the time that liposomes circulate in the bloodstream, allowing more of them to accumulate at the tumor site via enhanced permeability and retention (EPR). Further, PEGylation can prevent immunogenicity and forming anti-liposomal antibodies that may delay clearance of the drug delivery vehicle.
Figure 1. Schematic structure of doxorubicin encapsulated in PEGylated liposome. (Taher M, et al.; 2023)
Doxorubicin is an anthracycline antibiotic and one of the most common chemotherapy drugs used for cancers, such as breast cancer, lymphoma and leukemia. But it is restricted in its clinical application by its systemic toxicology – especially its cardiotoxicity. Creating liposomal doxorubicin solutions has had a very promising record of breaking these limitations. By packaging doxorubicin into liposomes, the drug is not only kept out of the hands of an accelerated metabolism and degradation, it's also released at the tumor site more selectively. Liposomes are made of lipid bilayers, which can store hydrophilic or hydrophobic drugs in the centre or bilayer (depending on your definition). This caplet reduces doxorubicin's toxicity by stopping it from interfering with healthy tissue in the bloodstream. In addition, liposomes can be useful for passive tumor targeting as they are engineered with the EPR effect — where nanoparticles cling more strongly to tumor tissues through the open-veins. This passive targeting is one of the advantages of liposomal drug delivery systems, which deliver extremely high concentrations of the chemotherapeutic agent to tumor cells. But to make liposomal doxorubicin still more selective against tumor tissues and less off-target, scientists have been focusing on targeting other target sites in liposomal formulations. Such modifications usually involve binding ligands to the liposome surface, and then directly targeting the cancer cells. This can be done through a number of surface-functionalization strategies, like binding antibodies or peptides that target tumor antigens. For example, one such solution is to add DBCO-functionalized lipids that enable exact coupling to targeting ligands using click chemistry.
DBCO (dibenzocyclooctyne) lipids are relatively new liposomal drug delivery systems. This DBCO functional group is a reactive handle which allows targeting moieties to be specifically and efficiently attached to the surface of liposomes using the familiar click chemistry mechanism with azide-functionalized molecules. There are some advantages of this approach to conjugation over traditional conjugation, since it is highly specific, bioorthogonal, and goes ahead in relatively benign conditions with little chance of an adverse reaction or degraded liposomal drug delivery system. Incorporating DBCO lipids into liposomes makes it easy to develop drug delivery platforms with multiple functionalities. When antibodies, peptides or other ligands are attached to the liposomal surface, they can create drug delivery systems that target individual receptors on cancer cells or other cancerous tissues. Such active targeting, combined with passive targeting aided by the EPR effect, significantly increases drug storage at the site of the tumour, and thus its efficacy. The first is that the PEG chains act as a hydrophilic protective barrier over the liposomes, so that the immune system does not immediately identify and eliminate the liposomes. This "stealth" property allows the liposomes to travel for longer in the blood stream so they will accumulate at the tumor site by EPR effect. Second, PEGylation eliminates the chance of immunogenicity and anti-liposomal antibodies causing the drug delivery vehicle to be cleared too early. PEGylation has been widely used because it helps to prevent the liposomes from being opsonised by serum proteins (which is normally followed by liver and spleen phagocytosis). Opsonisation is diminished, and PEGylation extends the half-life of liposomal compounds, allowing the drug to circulate longer and have better odds of making it to the target site. What's more, PEGylated liposomes do not evoke immune reactions, which makes them more appropriate for chronic diseases like cancer or autoimmune diseases. PEGylation of Doxorubicin-loaded liposomes then increases both the drug's pharmacokinetic and therapeutic potential by allowing for an improved and longer drug delivery. Doxorubicin-encapsulated liposomes are formulated using DBCO lipids and PEGylated as a multifunctional, potent drug delivery system. The DBCO lipids enable the targeting ligands to be attached precisely so that the drug is delivered only to cancer cells or other tumour targets; PEGylation speeds up the time to circulate and weakens the immune system's response to the liposomes. This combo not only increases drug targeting and efficacy, but also decreases drug's overall toxicity by avoiding off-target interactions with healthy tissues. These novel drug delivery systems could be used to develop more tailored, effective treatments for cancer. Targeted drug delivery could even narrow the therapeutic window for drugs such as doxorubicin to maximize their anti-cancer activity with as few adverse effects as possible. And by incorporating multiple agents – chemotherapy agents, targeting ligands, imaging agents – into a single liposomal carrier, combinations of treatments can be introduced that further boost therapy. After all, Doxorubicin-encapsulated liposomes, DBCO lipids and PEGylation combine to create a promising alternative to the difficulties of current chemotherapies, and could give new hope to patients with a variety of cancers.
Alternate Names:
Doxorubicin-Loaded Liposomes
Doxorubicin Encapsulated Nanoliposomes
Liposome-Encapsulated Doxorubicin
Doxorubicin-Containing Liposomes
Doxorubicin Liposome Formulation
Doxorubicin Liposomal Drug Delivery System
Liposomal Doxorubicin
Liposome-Formulated Doxorubicin
Liposomal Doxorubicin Nanoparticles
Doxorubicin-Encapsulated Nanocarriers
Liposomal Doxorubicin (LD)
Doxorubicin-Loaded Liposomal Nanoparticles
References:
1. Taher M, et al.; PEGylated liposomes enhance the effect of cytotoxic drug: A review. Heliyon. 2023, 9(3): e13823.
Preparation and characterization of doxorubicin liposomes
Methods Mol Biol.
Authors: Niu G, Cogburn B, Hughes J.
Abstract
During nanoparticle system in drug delivery, liposomes were perhaps the best characterized and one of the first to be developed. Stealth liposomes (SLs), containing polyethylene glycol-conjugated lipid, which can form a hydro-layer around liposomes bilayer, have a long circulation time and hence result in enhanced drug efficiency. Doxorubicin (DOX), an effective anticancer drug, can be loaded into liposomes by transmembrane pH gradient method to get high encapsulation efficiency with high drug/lipid ratio. Liposomal doxorubicin is a successful clinical formulation, and also a perfect model drug system for cancer-therapy research. Here we described the preparation of SLs via extrusion, DOX loading by transmembrane pH gradient method, and characterization analysis, including phospholipid concentration, size, transmission electronic microscopy graph, encapsulation efficiency, and in vitro drug release.
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