Cancer nanotechnology, challenges and achievements

Cancer is one of the world’s most life threatening diseases, with millions of new cases every year. The war against this disease is going on strong. Some battles have been won and others lost, but the weapons that this disease uses are really powerful. They are heterogenity, adaption and resistance. The scientific community has been actively researching on the development of new and improved weapons to overcome and finally win the battle against cancer. Here we will talk about the current approaches against this life threatening disease. 

It is widely known that current cancer treatments include: 

1. Surgical intervention 

2. Radiation 

3. Chemotherapeutic drugs, which often also kill healthy cells and cause toxicity to the patient. 

Unfortunately the effectiveness of current cancer treatments depends on the early diagnosis and the type of cancer. It is therefore needed to develop chemotherapeutics that can either passively or actively target cancerous cells and eliminate them effectively. 

In summary 'passive targeting' investigates the characteristic features of tumur biology that allows nanocarriers to accumulate in the tumor site by the Enhanced Permeability and Retention (EPR) effect. 

The EPR effect is a unique phenomenon of solid tumors related to their anatomical and pathophysiological differences from healthy tissues. Angiogenesis, which is a physiological process involving the growth of new blood vessels from pre-existing vessels, leads to high vascular density in solid tumors. Large gaps exist between endothelial cells in tumor blood vessels, and tumor tissues show selective extravasation and retention of macromolecular drugs, which is the desired effect in order to see the therapeutic efficacy and the shrinkage or elimination of tumors.

Impaired reticuloendothelial/ lymphatic clearance of macromolecules from tumor, or lack of such clearance, is another unique characteristic of tumors, resulting in intratumor retention of macromolecular drugs thus delivered. 

It has been found that the effective pore size in the endothelial lining of blood vessels in most peripheral human tumors ranges from 200 to 600 nm in diameter, and the EPR effect allows for passive targeting to tumors based on the cut-off size of leaky vasculature. 

But on the other hand there are a lot of limitations to the passive targeting, and one way to overcome these limitations is to program the nanocarriers so that they actively bind to specific cells after extravasation. 

This binding may be achieved by attaching targeting agents such as ligands, which are molecules that bind to specific receptors on the cell surface, to the surface of the nanocarrier by a variety of conjugation chemistries. 

Nanocarriers will recognize and bind to target cells through ligand–receptor interactions, and bound carriers are internalized before the drug is released inside the cell. In general, when using a targeting agent to deliver nanocarriers to cancer cells, it is imperative that the agent binds with high selectivity to molecules that are uniquely expressed on the cell surface, so that it minimizes the undesired and strong side effects of the cancer therapy. This is otherwise known as active targeting. 

But what are nanocarriers? Why are they important in cancer diagnosis and therapy? Why not just use the traditional chemotherapeutic drugs? What are their advantages over the current approved cancer treatments? How many types of nanocarriers have been discovered and what is the future perspective? If you want to know more specifics on nanocarriers you can read my other post on this topic entitled 'Nanocarriers for cancer therapy'. 

To simply define, a nanocarrier is nanomaterial composite, used as a transport mean for another substance, such as a drug or an imaging agent, which can be monitored by a specific machine. Such carriers should be targeted to the pathological area to provide maximum therapeutic efficacy while also providing diagnostic imaging. 

The family of nanocarriers includes polymer conjugates, polymeric nanoparticles, lipid-based carriers such as liposomes and micelles, dendrimers, carbon nanotubes, and gold nanoparticles, including nanoshells and nanocages. 

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. Cancer has been the most often investigated among the many potential targets for these nanocarriers. 

Integration of diagnostic imaging capability with therapy may be key to overcoming the challenges of cancer heterogeneity and adaption. In addition, codelievery of imaging contrast agent and chemotherapeutic drugs can provide real-time validation of the targeting strategy, resulting in an another step forward for individual-based therapy. 

If molecular targets became unavailable, imaging can be used to map out alternative targets. The advantage of this approach is that it can provide early feedback of therapeutic efficacy before detection by means of traditional diagnosis, such as tumor shrinkage. 

That is why “theranostic” was originally used as a term to describe a treatment platform that combines a diagnostic test with targeted therapy and which monitors response to therapy. We will talk more in deep about 'theranostics' in another post, because it is a fascinating and a very promising topic. 

In theranostic treatments imaging can be used to track nanoparticles systemically, prevalidate appropriate targeting, and track the expression pattern of surface markers for adaptive targeting, as well as provide real-time information on tumor response. It is very important to see how the therapy is going, because the canccer therapy has pretty strong side effects, that may be lethal and if it is controlled that the chemotherapeutic drug is not arriving at its tumore target, the treatment regime can be urgently changed, before the damage happens and shows the symptoms in the organism. The surface properties of the polymeric nanoparticulate drug delivery systems play a key role on the biological behavior that is shown in the organism by the drug delivery system.

Surface modification can be defined as the improvement and replacement of the surface properties of nano-sized drug delivery systems. In the field of pharmaceutical technology, surface modification provides several advantages to improve the physicochemical properties and pharmaceutical activities of many nanosized carriers particularly polymeric nanoparticles. 

By the modification circulation times are prolonged, especially the accumulation in the tumor tissues is improved to higher levels.

On the other hand lipid-based carriers pose several challenges, which represent general issues in the use of other targeted nanocarriers such as polymeric nanoparticles. For example, upon intravenous injection, particles are rapidly cleared from the bloodstream by the reticuloendothelial defence mechanism, regardless of particle composition. 

Moreover, instability of the carrier and burst drug release, as well as non-specific uptake by the mononuclear phagocytic system (MPS), provides additional challenges for translating these carriers to the clinic. 

Several anticancer drugs enter the cells in our body through diffusion method. But there are some integral proteins in the cell membrane that are known as MDR transporters, which transport a variety of anticancer drugs out of the cancer cell and produce resistance against chemotherapy. As a solution to this challenge the delivery of drugs through targeted nanocarriers that are internalized by cells, can provide an alternative route to diffusion of drugs into cells. 

It is very important not to forget that cancer drug resistance is very complex and has been linked to elevated levels of enzymes that can neutralize chemotherapeutic drugs. However, it is more frequently due to the overexpression of MDR transporters that actively pump chemotherapeutic drugs out of the cell and reduce the intracellular drug doses below lethal threshold levels. 

Since fortunately not all cancer cells express the MDR transporters, chemotherapy will kill only drug-sensitive cells that do not or only mildly express MDR transporters, while leaving behind a small population of drug resistant cells that highly express MDR transporters. But with new forming tumors, chemotherapy may fail because residual drug-resistant cells can dominate over the normal cells, resulting in a more aggressive tumor mass.

Among the MDR transporters, the most widely investigated proteins are: P-glycoprotein, the multidrug resistance associated proteins and the breast cancer resistance protein. These proteins have different structures, but they share a similar function of expelling chemotherapy drugs from the cells. 

Combination treatments with targeted nanocarriers for selective delivery of drugs and MDR pump inhibitors will likely address some of the problems posed by resistant tumors in the future.

Do not forget that it is always better to prevent than to cure, so do not neglect the regular check ups at the doctor and if you have familiar history with the cancer disease double them up to twice a year.

Stay well!

References:

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2. Alonso, M.J. Nanomedicines for overcoming biological barriers. Biomed. Pharmacother. 58,168–172 (2004).

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Nanocarriers for cancer therapy

Nanotechnology has the potential to revolutionize cancer diagnosis and therapy. Advances in protein engineering and materials science have contributed to new nanoscale targeting approaches that may bring new hope to cancer patients. 

Active approaches achieve this by conjugating nanocarriers containing chemotherapeutics with molecules that bind to overexpressed antigens or receptors on the target cells. 

Nanocarriers can offer many advantages over free drugs. They: 

• protect the drug from premature degradation; 

• prevent drugs from prematurely interacting with the biological environment; 

• enhance absorption of the drugs into a selected tissue (for example, solid tumour); 

• control the pharmacokinetic and drug tissue distribution profile; 

• improve intracellular penetration. 

For rapid and effective clinical translation, the nanocarrier should: 

• be made from a material that is biocompatible, well characterized, and easily functionalized; 

• exhibit high differential uptake efficiency in the target cells over normal cells (or tissue); 

• be either soluble or colloidal under aqueous conditions for increased effectiveness; 

• have an extended circulating half-life, a low rate of aggregation, and a long shelf life. 

The use of different pharmaceutical nanocarriers has become one of the most important areas of nanomedicine. Nanocarriers first reached clinical trials in the mid-1980s, and the first products, based on liposomes and polymer–protein conjugates, were marketed in the mid-1990s. Later, therapeutic nanocarriers based on this strategy were approved for wider use and methods of further enhancing targeting of drugs to cancer cells were investigated. For a summary of this methods you can read the post entitled 'Cancer nanotechnology, the most important challenges and achievements'. 

Nanocarriers are nanosized materials (diameter 1–100 nm) that can carry multiple drugs and/or imaging agents. Nanocarriers can also be used to increase local drug concentration by carrying the drug within and control-releasing it when bound to the targets. 

Nanocarriers encounter numerous barriers en route to their target, such as mucosal barriers and non-specific uptake. To address the challenges of targeting tumors with nanotechnology, it is necessary to combine the rational design of nanocarriers with the fundamental understanding of tumor biology. 

Despite the variety of novel drug targets and sophisticated chemistries available, only four drugs (doxorubicin, camptothecin, paclitaxel, and platinate) and four polymers have been repeatedly used to develop polymer–drug conjugates. 

Lipid-based carriers have attractive biological properties, including general biocompatibility, biodegradability, isolation of drugs from the surrounding environment, and the ability to entrap both hydrophilic and hydrophobic drugs. Through the addition of agents to the lipid membrane or by the alteration of the surface chemistry, properties of lipid-based carriers, such as their size, charge, and surface functionality, can easily be modified. 

Liposomes, polymersomes, and micelles represent a class of amphiphile-based particles. Liposomes are spherical, self-closed structures formed by one or several concentric lipid bilayers with inner aqueous phases. They vary in size from 50 to 1,000 nm and can be loaded with a variety of hydrophilic drugs into their inner aqueous compartment and, sometimes, even with water insoluble drugs into the hydrophobic compartment of the phospholipid bilayer. 

They are biologically inert and biocompatible. Drugs included in liposomes are protected from the destructive action of external medium. Liposomal vehicles are very effective candidates for noninvasive imagining and targeted drug delivery. I have done my scientific master thesis on Pegilated liposomes for tumor diagnosis and treatment.

If you are interested to research further you can find my publication, along with other interesting researches, at the Journal of Liposome Research, entitled 'In vitro studies on 5-florouracil-loaded DTPA-PE containing nanosized pegylated liposomes for diagnosis and treatment of tumor'. Given their long history, liposome-based carriers serve as a classic example of the challenges encountered in the development of nanocarriers and the strategies that have been tried. For example, PEG has been used to improve circulation time by stabilizing and protecting micelles and liposomes from opsonization, which is a plasma protein deposition process that signals Kupffer cells in the liver to remove the carriers from circulation.

In addition to rapid clearance, another challenge is the fast burst release of the chemotherapeutic drugs from the liposomes. To overcome this phenomenon, doxorubicin, for example, may be encapsulated in the liposomal aqueous phase by an ammonium sulphate gradient. 

This method achieves a stable drug entrapment with negligible drug leakage during circulation, even after prolonged residence in the blood stream. 

In clinical practice, liposomal systems have shown preferential accumulation in tumours, via the EPR effect, and reduced toxicity of their cargo. I will not go into detail on the EPR effect, because we talked about it in the article 'Cancer nanotechnology, the most important challenges and achievements'. If you want to learn more on the EPR effect please read it. 

However, long-circulating liposomes may lead to extravasation of the drug in unexpected sites. The most commonly experienced clinical toxic effect from the PEGylated liposomal doxorubicin is the hand-foot syndrome, but that can be addressed by changing the dosing and scheduling of the treatment. 

Other challenges facing the use of liposomes in the clinic include the high production cost, fast oxidation of some phospholipids, and lack of controlled-release properties of encapsulated drugs. 

Polymersomes on the other hand have an architecture similar to that of liposomes, but they are composed of synthetic polymer amphiphiles. However, as with polymer therapeutics, there are still no clinically approved strategies that use active cellular targeting for lipid-based carriers. 

Micelles, which are self-assembling closed lipid monolayers with a hydrophobic core and hydrophilic shell, have been successfully used as pharmaceutical carriers for water-insoluble drugs. 

Organic nanoparticles include dendrimers, viral capsids and nanostructures made from biological building blocks such as proteins. 

Dendrimers are synthetic, branched macromolecules that form a tree-like structure whose synthesis represents a relatively new field in polymer chemistry. Polyamidoamine dendrimers have shown promise for biomedical applications because they can be easily conjugated with targeting molecules, imaging agents, and drugs, have high water solubility and well-defined chemical structures, are biocompatible, and are rapidly cleared from the blood through the kidneys, made possible by their small size (<5nm), which eliminates the need for biodegradability. 

In vivo delivery of dendrimer–methotrexate conjugates using multivalent targeting results in a tenfold reduction in tumour size compared with that achieved with the same molar concentration of free systemic methotrexate. This work provided motivation for further pre-clinical development, and a variety of dendrimers are now under investigation for cancer treatment. 

Although promising, dendrimers are more expensive than other nanoparticles and require many repetitive steps for synthesis, posing a challenge for large-scale production. 

Inorganic nanoparticles are primarily metal based and have the potential to be produced with near monodispersity. Inorganic materials have been extensively studied for magnetic resonance imaging and high-resolution superconducting quantum interference devices. Inorganic particles may also be functionalized to introduce targeting molecules and drugs. Specific types of recently developed inorganic nanoparticles include nanoshells and gold nanoparticles. 

Nanoshells (100–200 nm) may use the same carrier for both imaging and therapy. They are composed of a silica core and a metallic outer layer. Nanoshells have optical resonances that can be adjusted to absorb or scatter essentially anywhere in the electromagnetic spectrum, including the near infrared region, where transmission of light through tissue is optimal. Absorbing nanoshells are suitable for hyperthermia-based therapeutics, where the nanoshells absorb radiation and heat up the surrounding cancer tissue. 

Scattering nanoshells, on the other hand, are desirable as contrast agents for imaging applications. Recently, a cancer therapy was developed based on absorption of NIR light by nanoshells, resulting in rapid localized heating to selectively kill tumours implanted in mice. Tissues heated above the thermal damage threshold displayed coagulation, cell shrinkage and loss of nuclear staining, which are indicators of irreversible thermal damage, whereas control tissues appeared undamaged. 

A similar approach involves gold nanocages which are smaller (<50 nm) than the nanoshells. These gold nanocages can be constructed to generate heat in response to NIR light and thus may also be useful in hyperthermia-based therapeutics. Unlike nanoshells and nanocages, pure gold nanoparticles are relatively easy to synthesize and manipulate. 

Non-specific interactions that cause toxicity in healthy tissues may impede the use of many types of nanoparticles, but using inorganic particles for photo-ablation significantly limits non-specific toxicity because light is locally directed. However, inorganic particles may not provide advantages over other types of nanoparticles for systemic targeting of individual cancer cells because they are not biodegradable or small enough to be cleared easily, resulting in potential accumulation in the body, which may cause long-term toxicity. 

The choice of an appropriate nanocarrier is not easy, and the few existing comparative studies are difficult to interpret because several factors may simultaneously affect biodistribution and targeting. In addition, developing suitable screening methodologies for determining optimal characteristics of nanocarriers remains a challenge. Therefore, successful targeting strategies must be determined experimentally on a case-by-case basis, which is a pretty hard work. 

Systemic therapies using nanocarriers require methods that can overcome non-specific uptake by mononuclear phagocytic cells and by non-targeted cells. It is also not clear to what extent this is possible without substantially increasing the complexity of the nanocarrier and without influencing commercial scale-up. 

Improved therapeutic efficacy of targeted nanocarriers has been established in multiple animal models of cancer, and currently more than 120 clinical trials are underway with various antibody-containing nanocarrier formulations. 

For the doctor, in addition to enhancing confidence through the ability to image the type and location of the tumor, it is undoubtedly needed to construct appropriate therapeutic regimens. Similar to combination drug strategies that may be personalized to optimize treatment regimens, oncologists in the near future may be presented with the ability to choose specific nanocarrier/targeting molecule combinations which could lead to improved therapeutic outcomes and reduced costs. 

To further investigate how cancer develop and the pathways of cancer development you can read my post 'How does cancer develop?' 

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