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Young living diffuser user guide. Anticancer drugs present narrow therapeutic ranges and a high toxicity, a fact that makes imperative to control the delivery of the drug to the target site 11; Parenteral formulations have the additional advantage of circumventing any problem related to drug absorption; however, the preparation of injectable formulations of many hydrophobic drugs is not trivial due to their very low water solubility.
Indeed, intravenous injection of these drugs may cause embolization of blood vessels due to drug aggregation, and ultimately, local toxicity as a result of high drug concentrations at the site of deposition Moreover, most of these drugs need to be solubilized in toxic 42 42 J.
This situation is well illustrated by the antineoplastic family of the taxanes, which have very low water-solubility, and therefore, the use of surfactants and alcohols to administrate these compounds is necessary. Cremophor EL Taxol has been the standard solvent system for paclitaxel, but a great number of pharmacologic and biologic effects related to this drug formulation have been described, including clinically relevant acute hypersensitivity reactions, and peripheral neuropathy Besides, polyvinyl chloride PVC -free equipment for Cremophore EL administration is obligatory, since cremophor EL is known to leach plasticizers from PVC infusion bags and polyethylene-lined tubing sets which can cause severe hepatic toxicity These negative effects have been successfully solved by incorporating the drug into several nanocarriers, and currently a great number of formulations of Cremophor-free taxanes have been developed.
They include glycol chitosan nanoparticles, polyglutamic acidpaclitaxel conjugates, nanoemulsions and liposomes Moreover, 83; 28; paclitaxel-loaded albumin nanoparticles Abraxane or ABI 40; 85 have recently been approved by FDA for the treatment of metastatic breast cancer Increase in blood circulation time The inefficacy of classical chemotherapy has been associated primarily with the inadequate biodistribution of the drugs, which partially accumulate into nontarget tissues, leading to severe side effects.
Changes in drug biodistribution and accumulation in the target tissues can be achieved through the incorporation of these drugs into specific nanocarriers. Classical non-selective nanocarriers are known to be opsonized and rapidly cleared by the mononuclear phagocitic systems MPS , which is predominantly distributed in liver, lungs, spleen, and bone marrow.
This uptake can be very advantageous for the chemotherapeutic treatment of MPS-localized tumors like hepatocarcinoma or hepatic metastasis. This therapeutical benefit has been observed with doxorubicin in a murine hepatic metastases model, when this drug was incorporated into biodegradable poly alkylcyanoacrylate nanoparticles Apart from these specific cases, nanocarriers should avoid uptake by the MPS.
Presently, it is know that in order to prevent this uptake, nanocarriers should be small and provided of a neutral and hydrophilic surface coating The technology most frequently applied to prevent the uptake by the MPS has been the modification of the nanocarrier surface with polyethylene glycol This technology is usually referred as PEGylation, and the 43 Article I 43 surface-modified carriers as Stealth or long-circulating PEGylation works by preventing the opsonization of the nanocarriers through a combination of mechanisms that include: 1 the shielding of the nanocarrier charged surface, 2 the increase of its hydrophilicity, 3 the enhancement of the repulsive interaction between nanocarrier and blood components, 4 and the formation of a polymeric layer around the particle s surface, which makes it impermeable to other solutes PEGylation has been applied to enhance the plasmatic half-life of several nanocarriers, including liposomes 36, nanoparticles 89 and micelles Passive tumor targeting Healthy tissues have tight, continuous blood vessel walls with pores that are approximately 9 nm in diameter.
Therefore, the size of blood vessel pores restricts the extravasation of large molecules and nanomedicines In contrast, the blood vessels formed in solid tumors are irregular and dilated, the endothelial cells are poorly aligned and disorganized, there is an irregular or an absence of a basal lamina, an there are a large number of pores with sizes up to nm depending on the specific tumor. This particular blood vessel structure results from the rapid formation of a vascular network in solid tumors, which is necessary to provide oxygen and nutrients for its fast growing mass This differentiated structure of the tumor blood vessels results in a facilitated access of macromolecules and nanomedicines.
Indeed, nanosystems are too large to penetrate through the vascular wall of healthy tissues, while they can easily penetrate to solid tumors Besides, tumor tissues lack a functional lymphatic system for draining lipophilic and polymeric materials 97; 98, thus making the elimination of the nanocarriers from the tumor very difficult. This phenomenon of facilitated intake and prolonged retention of nanometric materials was first identified by Maeda et al.
The EPR allows the passive targeting of nanomedicines with a suitable size to tumoral tissues, and it is the reason behind the enhanced activity and reduced toxicity of many anticancer nanomedicines in comparison to free drugs.
There are numerous examples of the application of this concept to anticancer drugs. A recent one was from Constantinides et al. This research group attributed this effect to the higher tumor accumulation of the drug delivered in the nanoemulsion. Similar enhancements in therapeutic efficacy of antitumor agents have been obtained with doxorubicin in liposomes 99, chitosan nanoparticles , and block copolymer micelles All of 44 44 J. Active targeting Since Paul Ehrlich envisioned the concept of the magic bullet, many efforts have been directed towards the design of therapeutic agents with the capacity to actively target cancer cells.
Active targeting of cancer cells based on specific molecular recognition interactions between a nanometric carrier and the target cells has been researched profusely The ultimate goal is to achieve a drug delivery system that selectively accumulates in cancer tissues, where a loaded cytotoxic agent can exert its effect, while avoiding undesirable side-effects. To achieve molecular recognition of cancer cells, different kind of ligands have been conjugated to drug delivery platforms: 1 ligands for receptors overexpressed in cancer cells e.
Some considerations need to be taken into account with actively targeted nanomedicines. First, the incorporation of ligands may increase the complexity and the particle size of the nanomedicines, hindering their preparation and enhancing the risk of biological side-effects 3. Moreover, it is essential that the targeting agents used to functionalize the nanocarriers bind with high selectivity to its receptor in the cancer tissue, and that this receptor is either uniquely expressed on cancer cells, or at least overexpressed to a great extent in these.
Finally, once the functionalized nanosystems interact with the target receptors, other processes might be necessary to achieve the full benefit from this active targeting: e.
This situation is overcome by the formation of new blood vessels that will provide the necessary exchange of molecules between the blood and the tumor tissue, thus ensuring a continuous tumor growth. For that reason, the angiogenic process is a promising target for the development of new 45 Article I 45 therapeutic agents that control tumor expansion The vascular network is highly accessible to parenterally delivered therapeutic agents, and therefore, nanosystems can easily target proliferating tumor vessels.
Moreover, due to the reduced area of the tumor vasculature compared to tumor interstitium, lower doses are needed to achieve therapeutic responses Angiogenesis is a complex process which is regulated by many molecular mediators. These mediators bind to cell receptors that are frequently overexpressed in cancer vasculature.
Therefore, these receptors can be used as specific receptors to target functionalized nanosystems. One of the most prominent proangiogenic regulators is epidermal growth factor EGF , which has been used to derivatize silicon nanoparticles loaded with the pore-forming protein melittin that lyses the cell membranes of tumor endothelium This specific ligand has been coupled to cationic nanoparticles for gene therapy directed to angiogenic blood vessels. Systemic injection of the nanosystem into mice led to sustained regression of tumors due to the apoptosis of the tumor-associated endothelium Similarly positive results have been obtained with other type of nanocarriers that are not covered in detail in this review, e.
Active targeting to the cancer cells Nanomedicines designed to actively target cancer cells have complex design requirements. On one hand, in order to achieve high tumor specificity, the targeting moiety should have a high binding affinity for its receptor on cancer cells. On the other hand, it is known that in the case of solid tumors, the use of very high affinity ligands may impede the penetration of the nanocarriers into the inner tumor mass Another choice that must be made when designing an actively-targeted nanomedicine is the cell receptor, which may help or not internalization upon binding.
For many drugs, intracellular delivery is necessary , as we discuss in more detail in the following section. For other drugs with receptors on the plasmatic membrane, or for those that penetrate freely into cells, binding to a non-internalizing receptor might be more adequate In the next paragraphs we discuss some of the most widely used targeting ligands. Monoclonal antibodies were the first targeting ligands able to bind to specific tumor antigens.
At present, there are several formulations comprising antibodies approved or undergoing clinical trials. For example, Mylotarg, a 46 46 J.
Besides, there are a significant number of animal studies in the literature that support the efficacy of antibody targeted nanocarriers. For instance, anti-her-2 immunoliposomes with doxorubicin were more efficient against breast cancer than PEGylated liposomes In a different work, a scfv antibody fragment was used to deliver small interfering RNA sirna to lymphocytes, achieving a 10,fold increase in affinity for the target receptors compared to the control Another impacting work is that from MacDiarmid et al.
Actively targeted minicells resulted in very marked antitumor effects, and in an enhanced efficacy even compared to other less sophisticated nanomedicines i. Doxil Despite this recognized efficacy, the incorporation of antibodies to nanomedicines is a complex issue, because the number of associated molecules should be enough for receptor recognition, but not too high to activate the MPS. Aptamers are short single-stranded DNA or RNA oligonucleotides selected in vitro to bind to a wide variety of targets, like receptors on cancer cells 3.
For instance, docetaxel loaded poly d, L-lactic-co-glycolic acid -block-poly ethylene glycol PLGA-b-PEG were functionalized by aptamers and a complete regression of the tumor was fulfilled see Figure 3. A prototypical example is the consensus sequence arginine-glycine-aspartic acid RGD , which binds to integrin receptors and has been used to target nanomedicines to tumor neovasculature However, the lack of selectivity of RGD for cancer-related integrins limits the interest of this system.
Transferrin is a very useful ligand that presents the advantage of being intracellularly internalized by its own receptor Transferrin receptors are overexpressed on cell surfaces when metabolic processes are increased.
Therefore, cancer cells like pancreatic, colon, lung and bladder cancer- will present a higher density of this receptor. Unfortunately, other fast-growing but healthy cells might also overexpress transferrin receptors, reducing the effectiveness of this targeting moiety Currently there are two formulations of transferrin-modified nanosystems in clinical trials: MBP, liposomes containing oxaliplatin that are in phase I , and CALAA, a polymer-sirna conjugate, with transferrin receptor triggered drug release.
This formulation has just begun phase I clinical trials in Folate has been used as well as targeting ligand due to the up-regulation of folate receptors on the tumor cell surface. Liu et al. In this work higher tumor accumulation of the drug was achieved with folate-conjugated micelles compared to non-conjugated micelles Carbohydrates like mannose and galactose have also been used as targeting ligands. Nevertheless, these ligands are limited because of their broad distribution on the healthy cells.
For example, PK-2 a polymer-doxorubicin conjugate was stopped at phase I clinical trials due to the accumulation of the nanosystems in the healthy hepatocytes Other targeting molecules like epidermal growth factor EGF , heparin sulphate, chondroitin sulphate, hyaluronan, vitamin B 12 or wheat germ agglutinin have also been investigated for cancer therapy Tumor extravasation and distribution Tumors are characterized by heterogeneous blood flow in non-necrotic regions, and for slow and unpredictable blood flow in necrotic and semi-necrotic regions.
Moreover, unlike most normal tissues, tumor interstitium has high 48 48 J. As the homogenous distribution of nanosystems throughout the tumor is crucial for an optimal response, some drug delivery strategies have been developed to overcome this issue. If not, the nanomedicine will have to make its way through to the intracellular space. For some drugs, this access to the intracellular space is a considerable challenge. Indeed, internalization through the cell membrane and trafficking to the correct cellular compartment represents a critical challenge Prototypical drugs that show restricted cell internalization and inefficient transport to its target cell compartment are gene medicines.
For these, it is imperative to ensure their penetration through the cell membrane, and their stability from degradation in lysosomes. RNA-based therapeutics need to be addressed to the cytosol, while DNA-based therapeutics need to reach the cell nucleus to become effective. P-gp or multidrug resistance protein MRP. Cell Internalization Nanomedicines are internalized into eukariotic cells through endocytic pathways; the best characterized are clathrin-mediated endocytosis, caveolaemediated endocytosis, and macropinocytosis.
All of these endocytic mechanisms share a common feature: they require the adhesion to the cell membrane of the nanostructure to be internalized. Despite this, these mechanisms have several differences regarding their mechanism and characteristics, which are reviewed in more detail elsewhere ; Depending on the size of the internalizing vesicles we could distinguish between macropinocytosis, which leads to the largest 49 Article I 49 vesicles 0.
Cell penetration of the nanoparticles by endocytosis can be triggered either by non-specific adhesion to the cell surface or through the binding to specific cell receptors on the cell membrane. Endocytosis of non-bioconjugated nanocarriers has been described for carriers with very different biomaterial compositions.
For instance, nanocapsules can enter cells though an endocytic pathway, and this can lead to a more effective intracellular delivery of the drug carboplatin Endocytic transport of nanoparticles from the hydrophobic polyester PLGA alone or forming blends with the hydrophilic polymer poloxamer have been described ; Similarly, endocytic uptake was described for mesoporous silica nanoparticles in HeLa cells, allowing the delivery of a membrane-impermeable protein to the cytosol Studies have shown that size is a critical parameter for the internalization of these carriers: up to a fold increase in nanoparticle internalization was found for PLGA nanoparticles with a particle size below nm compared to PLGA nanoparticles with a particle size above nm A particular case of non-specific binding and internalization of nanocarriers is that from cationic polyaminoacids and some cell-penetrating peptides.
These polypeptides are internalized after association to anionic glycosaminoglycans present on the plasmatic membrane. The complex between these polycations and the glycosaminoglycans trigger the endocytic process Initially, these materials were thought to cross the plasmatic membrane through clathrinmediated endocytosis , but some later evidence has shown that, at least some of them e.
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