Scholarly article on topic 'Nanotechnology in the war against cancer: new arms against an old enemy – a clinical view'

Nanotechnology in the war against cancer: new arms against an old enemy – a clinical view Academic research paper on "Nano-technology"

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Academic research paper on topic "Nanotechnology in the war against cancer: new arms against an old enemy – a clinical view"

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Nanotechnology in the war against cancer: new arms against an old enemy - a clinical view

Future

ONCOLOGY

Sten Friberg1 & Andreas M Nyström*2

ABSTRACT Clinical oncology is facing a paradigm shift. A new treatment philosophy is emerging and new targets are appearing that require new active agents. The medical use of nanotechnology - nanomedicine - holds several promising possibilities in the war against cancer. Some of these include: new formats for old drugs, that is, increasing efficacy while diminishing side effects; and new administration routes - that is, dermal, oral and pulmonary. In this overview, we describe some nanoparticles and their medical uses as well as highlight advantages of nanoparticles compared with conventional pharmaceuticals. We also point to some of the many technical challenges and potential risks with using nanotechnology for oncological applications.

Introduction & historical background

The scientific giant and prolific researcher Paul Ehrlich (1854-1915, Nobel Laureate in Physiology or Medicine in 1908) envisioned a 'Magic Bullet' against microbes and tumors as early as the late 19th century. This bullet would eradicate all bacterial intruders or malignant cells in the human body without affecting normal cells [1]. Ehrlich himself developed the first bullet: the drug Salvarsan against syphilis. This achievement set the foundation for ensuing discoveries of numerous antibiotics against various infections. In this field, development was fast. Ehrlich's fundamental discoveries in immunology, including the antibody-antigen interaction and ligand-receptor function among many others, also showed promise for a magic bullet against cancer. In this area, however, progress has been much slower than for infectious diseases. The first targeting bullets in oncology were hormones, estrogen against cancer of the prostate and testosterone against cancer of the breast (Charles B Huggins, 1901-1967, Nobel Laureate in Physiology or Medicine in 1966) [2].

Not until the last two decades have we witnessed the next step forward in targeted oncologic therapy, monoclonal antibodies [3]. These have resulted in a number of molecularly targeted inhibitors that prove that Ehrlich's principle of targeting was correct and feasible. However, these inhibitors have only improved the prognosis for a few malignant diseases, and the overall mortality in cancer still remains high [4]. Thus, the need for better oncologic therapies is still imminent and it is in this regard that nanotechnology might provide a number of new possibilities.

The biomedical use of nanotechnology began more than 50 years ago, well before the research community coined the terminology 'nano'. Liposomes were discovered in the mid-1960s and can be regarded as the first nanomedicine [5]. Liposomes were soon found to be useful as a vector or formulation aid for pharmaceuticals [6,7]. Such applications were further investigated and expanded upon in the late 1970s with the use of macromolecular amphiphilic materials such as polymers that are capable of forming micelles [8-10]. Liposomes were approved as an anthracycline formulation

KEYWORDS

• cancer • clinical oncology • nanomedicine

• nanoparticles • therapy

'Swedish Medical Nanoscience Center, Department of Neuroscience, Retzius väg 8, Karolinska Instituted SE-171 77 Stockholm, Sweden institute of Environmental Medicine, Nobels väg 13, Karolinska Institutet, SE-171 77 Stockholm, Sweden

*Author for correspondence: And reas.nystrom@ki.se

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in the mid-1990s mainly for their lower incidence of cardiotoxicity compared with normal intravenously administrated anthracylines such as doxorubicin [11].

Medical background

Today, around 50% of all humans diagnosed with a malignant tumor die from their disease. In the year 2012, the actual figures for 40 European countries were: 3.45 million new cases of various cancers and 1.75 million deaths from malignancies [4]. In spite of major scientific discoveries, numerous ingenious constructions and enormous financial investments during the last decades, mortality from cancer still remains high.

Nanotechnology is a multidisciplinary field bringing together chemists, physicists, biologists, pharmacologists, physicians, clinicians, veterinarians and many other specialists. This broad array of research fields is expected to yield new paradigms in the arsenal of tools used in clinical oncology [12]. The battlefield in the war against cancer must be broadened, and tomorrow's oncologic therapy is likely to be very personalized, very complex and very expensive. The medical applications of nanotechnology have expanded rapidly over the past few decades, and we believe that nanomedicine will have a profound impact on future therapies [13].

In this article, we will briefly describe some of the possibilities that nanotechnology has to offer. We will avoid results based only on in vitro studies because their clinical relevance can be questioned [14].

Definitions

Nanos is the Greek word for dwarf. Nanotechnology refers to the handling and control of matter with at least 1D between 1 and 100 nanometers (nm). One nm is 10-9 m, or 1/1,000,000 mm. An illustration ofnanomateri-als undergoing clinical phase testing is given in Figure 1. There is no unanimous definition of the word 'nanomedicine,' but The European Science Foundation gives the following: 'Nanomedicine uses nanosized tools for the diagnosis, prevention and treatment of disease and to gain increased understanding of the complex underlying pathophysiology of disease. The ultimate goal is improved quality of life.' [15] We concur with this definition. In this review, we will focus on the therapeutic aspects and leave diagnostic aspects out of the present publication simply due to the sheer volume of literature on the subject.

Nanomedical diagnostics, including in vitro assays, lab-on-a-chip applications, theranostic systems and other radiological uses of NP-based contrast agents is a large and important area of nanomedicine research that has, and will in the future, reform other medical disciplines. There are several high-quality reviews written in area of clinical nanomedicine and we would recommend the interested reader to look at these as well [12,16-25].

If the definition of nanomedicine is not unanimous, then neither is there consensus in the literature on the nomenclature of the various particles. The word 'nanoparticle (NP)' has appeared in the literature as nanovehicle, nanovector, nanostructure, nanoconstruct and others. We have chosen the term 'NP' to refer to all of these types of constructs. We do not include antibodies in our definition of a NP, but we do refer to them because they are indicative of the targeting principle and are paving the way for the clinical applications of nanomedicine [3]. Polymer-drug conjugates are mentioned in this review, as some of them do form assemblies postinjection or have sizes in the nanosize range. We believe that the two medical specialties that are likely to benefit sooner than others from nanotechnology are diagnostic radiology and clinical oncology.

The unique properties of NPs depend largely on their surface to volume ratio. To give an illustration, assume a piece of material measuring 1 cm x 1 cm x 1 cm. This cube weighs 1 g and has a volume of 1 cm3 and a surface area of 6 cm2. If this cube is cut into smaller ones, measuring 100 nm x 100 nm x 100 nm, the result is a large number (1015) of cubes. The total volume of these numerous cubes, however, will remain 1 cm3 and the weight will likewise remain 1 g. By contrast, the total surface area will increase to 6 x 1014 cm2 or 60 m2.

What are nanoparticles?

NPs are solid particles that come in a plethora of sizes, compositions and characteristics. Some of the better-known NPs are shown in Figure 1.

NPs can be loaded with drugs, bioactive agents and diagnostic tools that can be absorbed on the surface, entrapped inside or dissolved within the matrix of the NP [26]. Thus armed, the NPs can serve as vectors inside a living body. NPs are usually made of lipids, metal or metal oxide crystals, silicates, proteins or polymers. Nanomedicine

— which already exists as a medical specialty

— focuses on achieving better diagnostics,

Lipid-based nanocarriers

Polymer-based nanocarriers

Viral nanopoarticles

Figure 1. Nanomaterials in clinical practice.

Reproduced with permission from [23], © Elsevier (2015).

increased disease prevention, enhanced treatments, fewer side effects and improved quality of life for patients. A schematic model of one type of oncologic therapy utilizing nanotechnology is given in Figure 2.

NPs are small, and thus more difficult to analyze, compared with molecular pharmaceuticals

in respect to characterize their chemical composition and properties. For example, measurements of sizes require that several methods of analysis are employed both in solution and in the solid phase. Moreover, after in vivo administration NPs can differ considerably in size compared the situation in vitro. In vivo, the particle's surface

Figure 2. Picture of a person with a tumor on one forearm. The patient is weighing 70 kg. The

tumor measures 2 cm in diameter, and it weighs 1 g. If the patient were to be treated with general chemotherapy, then 70,000 g of his body are exposed to the drug, intended for only the malignant 1 g. 99.9999% of the total number of cells in the patient's body would - in wanton - be exposed to toxicity (except the cells in the CNS. With 'targeted therapy' the situation could be completely reversed. The following is what the procedure might be: the drug is loaded into a nanoparticle system (e.g., liposomes). The load also includes magnetic particles (e.g., iron based). The vectors are constructed in such a way that they are dissolved by temperatures exceeding +42°C. The loaded nanoparticles are given intravenously to the patient. A magnet is attached to the skin near the tumor. In some time (hours), the majority of the nanoparticles will have accumulated in the tumor. Employing microwaves, the temperature in the tumor is then elevated to above +42°C, causing dissolution of the nanoparticles. The active drug is released, exposing the tumor cells to high concentrations of the drug. Normal cells are spared, and side effects from the bone marrow, mucous membranes and the skin are avoided. There is a synergistic effect in utilizing hyperthermia to dissolve the vector: Most malignant tumor cells are more susceptible to elevated temperatures than most normal cells. At +43°C, the majority of malignant cells are lethally injured, whereas most of the normal cells can recover. It all sounds simple. But in reality, there are numerous obstacles and pitfalls on the road. In this publication, we will point to some of them.

• Magnetic nanoparticle

• Chemotherapeutic

1 Polyethylene glycol)

it.« ifrv

Blood vessel

Magnetic localization

Release activiation via oscillating magnetic field or ultrasound

can be covered by lipids and proteins (a so-called 'biocorona') depending on the particle's material, electrical charge and hydrophilicity, among others [27]. The size of the particle governs to a large extent the half-life, biological distribution, pharmacokinetics and elimination route. More recent works have also indicated that although size is an important parameter for determining the final fate of the NP, shape and NP flexibility also influence the glomerular filtration and we point to a recent well written review on the

topic by Toy et al. [28]. A drawback is that the majority of the methods used to try to predict the behavior of NPs in vivo are inevitably based on analytical techniques applicable only under in vitro conditions. Therefore, toxicity might occur unexpectedly under in vivo conditions. These problems are extremely important for both the industrial use of NPs as well as for the environmental aspects and biomedical applications [29]. Significant research efforts are currently focused on understanding the in vivo fate of NPs, their

excretion profiles and their life cycle as well as the parameters that govern their effects such as the biocorona interface with blood proteins and lipids. Some of today's more commonly used NPs in medicine are discussed below.

Liposomes

Liposomes are vesicular bodies of biocompatible phospholipid bilayers 100-500 nm in diameter and represent the first materials that can be seen as true nanomedical drug delivery agents. Their history can be traced to research in the mid-1960s [5,6], and they are well suited for carrying drugs, genes, or bioactive factors such as vaccines or viruses. Once in the circulation of the patient, the liposome must be protected from the body's defenses in order to extend its halflife in circulation. To achieve this, liposomes are often coated with a polymer layer to create what is known as a stealth liposome [30]. One of the frequently used protective layers is poly(ethylene glycol) (PEG) (Figure 3). The first liposome to be approved by the US FDA was Doxil® in 1995, which is doxorubicin encapsulated in a liposome for use against ovarian cancer. The liposomal formulation of doxorubicin has the same anti-neoplastic effect as the native drug but considerably fewer side effects, especially with regard to cardiac toxicity [11,17]. Liposomes can be loaded in several ways. For example, hydrophobic drugs can be sequestered on the lipid bilayer and more hydrophilic drugs such as Paclitaxel can be sequestered within the aqueous interior of the lipid. Lipids can also be constructed to contain more than one drug so as to enable synergistic drug combinations or to add a contrasting agent for combined diagnostic imaging and therapy (so-called 'theranostics'). Magnetic NPs incorporated into liposomes can also be used for magnetic localization (as shown in Figure 2) [31], and the surface of the liposome can be constructed to bear monoclonal antibodies for targeting [16]. Release of the active components can be from passive diffusion or via external triggers such as heat and light or via physiological changes such as pH (see below), which is a prerequisite for biologic effect, sine qua non.

Liposomes can be immunogenic and can cause keratopathy, mucositis and hand-foot syndrome (palmar-plantar erythrodysesthesia) as depicted in Figure 4. The hand-foot syndrome is an effect of the much extended plasma halflife of doxorubicin encapsulated in a liposome that allows the drug to reach distant capillaries.

This side effect is usually mild and transient, but it can be serious and even fatal, (termed Complement Activated Pseudo Allergy) [33]. Similar side effects are to be expected for other NPs that are designed to have long half-lives in plasma. At present (April 2015) there are more than 20 liposomes undergoing clinical trials against various types of malignancies (Table 1), please also see a comprehensive list of nanomedi-cines [23]. The latest NP to be approved of by the FDA was Marqibo® in 2012. This liposome carries the 'old' chemotherapeutic Vincristine [16], and the liposome vector protects its cargo from the patient's antibodies, macrophages and other defense mechanisms.

Micelles

Micelles are lipid-based or polymer-based (plastic) constructs with a hydrophobic/hydrophilic core-shell morphology [18,49]. They are smaller than liposomes and can, therefore, infiltrate tissues where liposomes are excluded. Polymer-based micelles are very versatile drug delivery carriers because they can be tailored into almost infinite variations of size, electric charge, hydro-phobicity, material characteristics, degradabil-ity, etc. Polymers such as PEG have also been used to construct polymer therapeutics such as PEGasys® and PEGIntron® that are protein-polymer conjugates of interferon that protect the interferon from rapid breakdown in vivo which reduces the number of injections for the patient

Figure 3. Computer simulation of long poly(ethylene glycol) chains on the surface of a nanoparticle and (top) and short poly(ethylene glycol) chains (bottom).

Reproduced with permission from [32], © Elsevier (2006).

Figure 4. Hand-foot syndrome. This patient suffered from acute myeloic leukemia and was treated with Depocyt® 1500 mg/m2 during 12 h, with 12 regimens planned. Depocyt is a liposomal formulation of cytarabine. After the ninth regimen the patient developed red and painful feet and hands (palmar-plantar erytrodysestesia). The cytostatic treatment was stopped. The following days the patient developed sharply delimited erytematose patches and vesicles including scolding in palms and foot pads. After 20 days the skin disorders had spontaneously disappeared. Reproduced with permission from [34], © Massachusetts Medical Society (2011).

and increases compliance [38,50]. Similarly to liposomes, polymer micelles can be constructed to be triggered via the same mechanism of pH, temperature and light. At present, there are several micelle-based drugs in clinical trials, but none have yet been approved of by the FDA. Micelles are particularly well suited for exploiting the enhanced permeability and retention effect (EPR, see side-box).

Polymers

Polymers (poly = the Greek word for many) are among the most investigated nanomedicines. Polymers are light and flexible, and with modern polymer chemistry they can be produced with customized composition, molecular weight and topology [24,51]. Polymers can serve as vectors or for formulating or protecting the drug [10,52-53]. Examples are poly(glutamic acid) coupled to Paclitaxel® (Xyotax/Opaxio in Phase III trials for use against ovarian cancer) and PEG coupled to L-asparaginase (Oncaspar® for use against acute lymphatic leukemia).

Dendrimers

Dendrimers are symmetrically branched polymers with a single molecular weight. Due to their exact size and composition, they offer precise control over pharmacokinetics and biodistribution. Dendrimers have multiple handles that are capable of simultaneously carrying

drugs, imaging agents and targeting ligands. Dendrimers are very expensive to produce, and this has limited their use in clinical applications. Currently, there is regulatory testing of dendrimers for use as antivirals in vaginal anti-HIV creams and as a taxane formulation system [54].

Inorganic nanoparticles

Many of the different classes of inorganic NPs, such as gold, have been known since Roman times [26]. Inorganic NPs (Figure 1) offer numerous possibilities, and their electronic, optical and magnetic properties can be utilized in a variety of situations. So far, iron - usually in the form of iron oxide - and gold - usually in colloidal form - have been the focus of research [55-59]. These particles are usually 20-150 nm in diameter. One of their greatest clinical advantages is that they can be controlled from outside the patient via magnets in the case of magnetic NPs (Figure 2) or via light for gold NPs [60-63].

Supraparamagnetic iron oxide NPs are already an established tool in diagnostic radiology and serve as contrast agents in MRI. They can also be used as triggers and for hyperthermia-based tumor ablation. Gold NPs are similarly used with lasers to achieve localized thermal ablation. Coupling of gold particles to targeting ligands may allow for localization to cell-surface structures. If the gold particles are also coupled to an isotope, the conjugate can be used as a biological

tracer, and if the gold particle is coupled to a cytotoxic drug it can act as a theranostic.

A major step forward in clinical oncology was recently published by a group from Germany. Maier-Hauff et al. [45] treated 59 patients with recurrent malignant glioblastoma. No curative treatments currently exist for this malignancy, and the overall survival for these patients is around 6 months. Maier-Hauff's et al. patients were given NanoTherm® intravenously (an inorganic NP) (Figure 2) (the further therapeutic steps are described in the legend to that figure). The patients also received radiotherapy with 30 Gy (2 Gy/day, 5 days a week). No severe side effects were reported, and the overall survival of this group of patients increased by more than 13 months compared with historical controls. These results might not seem impressive, but this is a major step forward for an otherwise doomed group of patients. It should be noted that there were no cytotoxic drugs involved in the treatment. Another example of inorganic NPs utilized mainly for imaging applications is quantum dots. Quantum dots are small crystalline

NPs, typically in the size range of 5-50 nm in diameter, that are composed of semiconductor materials that have unique optical properties. These are often constructed from cadmium selenide, and used in diagnostic radiology. However the use of cadmium or other toxic heavy metal materials raises a safety concern for human use. In our opinion, we think that the reward of an early diagnosis must be weighed in a risk versus reward analysis before these materials are further used in the clinical setting and then focused on diagnostic applications.

Targeting of NPs

Some of the pioneers in nano-oncology equipped their NPs with 'targeting' devices trying to direct the therapy to the malignant cells. A number of ligands have been utilized, including antibodies, aptamers [64] and lectins among others [65-73]. Receptors on the cell surface of the cancer cells have included transferrin receptors, folic acid receptors, somatostatin receptors and receptors for melanocyte stimulating hormones among others. All initial ligand-receptor binding experiments

Table 1. Examples of some nanoparticles in clinical practice.

Nanoparticle Name Indication Drug Comment Ref.

Lipid-based Caelyx® Breast and liver DOX [11]

nanocarriers cancer

Doxil® Breast and ovarian cancer Doxorubicin (DOX) First approved nanomedicine, 1995

Marqibo Solid tumors Vincristine Approved 2012 [35]

Myocet® Solid tumors DOX

SGT 53 Gene (p53) [36]

ThermoDox® DOX Thermal dissolution [37]

Micelles Genexol-PM Breast, lung, pancreatic and ovarian cancer Paclitaxel Phases III and IV [38]

Polymers CALAA-01 Solid tumors siRNA [39]

Eligard Prostate cancer Antihormone Controlled release [40]

Oncaspar All Asparagine [41]

Opaxio® Esophageal cancer Paclitaxel [42]

Xyotax/Opaxio® Ovarian cancer Paclitaxel Phase III [43]

Inorganic Aurimmune™ Ear, nose and throat cancer Gold/TNF [44]

NanoTherm® Glioblastoma, prostate cancer among others Iron Thermal dissolution [45]

Protein Abraxane® Breast and non- Albumin [46]

particles small-cell lung cancer formulation of paclitaxel

1 Some examples of NPs in clinical practice, registered or in clinical phase testing (currently ca. 125 NP systems are in clinical ph ase

testing for oncological indications) [20].

Dox: Doxorubicin; NP: Nanoparticle.

Data taken from [47,48].

were performed in vitro where viscosity was low (all work was done in a fluid medium) and where mechanical obstacles were absent. In vivo, however, the situation is much more complex and includes high viscosity, rigid barriers, stiff matrices and long distances. There are fundamental laws of physics and chemistry governing in vivo diffusion, adsorption, adhesion, hemodynamics and metabolism that cannot be avoided. First, the forces attracting the ligand to its target are utterly weak (van der Waal's forces and hydrogen bonds), and the targeting ligand and the receptor must be very close (<0.5 nm) before any interaction can occur [74]. In practice, this means that targeting is very difficult if not impossible in solid tumors in humans. Second, passive diffusion is a time-consuming process. The time required for a NP to traverse one single normal cell is estimated to be a few hours [74]. If the distance from the afferent blood capillary to the tumor target is a few cell layers, it might take a day or two for the NP to diffuse toward the nearest cancer cell. It is not likely that the ligand will remain intact after such a journey. Even stealth liposomes have been shown to still remain within 50 nm of the blood vessel two days after intravenous injection [75]. Third, the more multifunctional (complicated) the NP becomes, the less effective it will become [76], and the production costs will increase steeply. Fourth, the ligand cannot distinguish between the receptor on a malignant cell from the same receptor on a normal cell. In the patient's body there might be a thousand times as many receptors on normal cells compared the malignant cell population, and there is a good chance that the therapeutic will be hijacked by normal structures on its way to its target [73,77]. Fifth, ligand functionalized NPs lose their targeting capacities when a biocorona is formed [78]. Sixth, tumor cells in the center are shielded by their dead but still protective siblings. Finally, the ligand on the NP is directed against receptors considered to be 'overexpressed' by proliferating tumor cells. However, the stem cells in the tumor population are not proliferating and are, therefore, not 'overexpressing' the receptors. The stem cells will evade the therapeutic and can cause a recurrence of the cancer.

Results based on in vitro experiments have not always proven to be transferrable to in vivo situations and they have created false promises based on false premises [79]. Reaching cancer cells in humans with targeted NPs has not been very efficient and binding fractions as low as 0.01% of the total dose administered have been recorded.

This means that all particles that reach the target (e.g., tumor cells) do so via passive diffusion, and adding ligands to the particles does not typically increase the amount that reaches the target [47].

There are still no targeted NPs on the market for oncologic treatment, but several are under clinical phase testing. One such example is BIND-014, a polymer NP system that delivers docetaxel utilizing prostate-specific membrane antigen as a targeting agent (PSMA) [80].

One of the reasons why results based on studies in experimental animals have not been transferrable to humans is that the growth conditions for experimental malignancies are very different from those in humans. Experimental cancers have been selected for fast growth rates (in order to obtain fast and thus less expensive results). However, fast-growing cancers create a phenomenon termed the enhanced permeation and retention effect (EPR; see side-box EPR), which may not exist in all human tumors [47].

• Sidebox on EPR

In fast-growing tumors in experimental animal models the vascular beds are often malfunctioning. They are leaky and allow molecules in the circulation to diffuse into the tumor [47-48,81]. The enhanced permeability of tumor vasculature combined with the lack of adequate lymphatic drainage leads to the prolonged half-life (retention) of a drug in the tumor bed and, therefore, prolonged exposure of the tumor cells to the anti-neoplastic drug. The EPR effect has been utilized by polymer conjugates, micelles and other NPs, usually smaller than 200 nm in diameter [82]. However, the vessels in laboratory animals are different from vessels in humans where most tumors are slow-growing and often require years to reach a size of 1 cm3 [83]. The vessels in these slow-growing tumors develop slowly and in good anatomical order unlike those in the experimental animal models. Thus, the existence of EPR in humans solid cancers is debated [84-87].

• End of sidebox on EPR

From the point of administration to its intended destination (a cancer cell), the NP is facing numerous soluble and cellular hurdles in the circulation. Upon enduring this trip and reaching its target, the NP must traverse the vascular wall. The vascular distribution in a human malignant tumor is heterogeneous, and this means that the distribution of NPs in a tumor is also heterogeneous. No matter where the NPs are extravasated in a

malignant tumor, they will not reach all regions of that tumor [88,89]. The biodistribution of any medical drug in the human body is governed by three pharmacokinetic steps: vascular transport, transvascular transport and interstitial transport. The third barrier affects the distribution of molecules as small as oxygen. If penetration of small molecules in the interstitial tissue is poor, then the passive diffusion of considerably larger molecules such as NPs will be even poorer. Limited penetration and uneven distribution of drugs represent monumental barriers to their efficiency. Even small molecules (like doxorubicin and pacli-taxel) never penetrate far from the vicinity of the afferent blood vessels [90,91].

Some possibilities offered by nanoparticles

Despite the limitations mentioned above, NPs may provide several advantages, including:

• New drug formulations;

• New administrative routes;

• Incorporation of new therapeutics;

• Revival of previously discarded drugs;

• New targets in the patient's body — for example, the CNS.

• New drug formulation systems

NPs open up new possibilities to control the pharmacokinetics and biodistribution in a very different way than has previously been available. Controlled release of effectors and drugs are now possible. For example, Doxorubicin® in its native form has a half-life in plasma of 0.2 h. In its lipo-somal formulation, the half-life increases to 55 h resulting in a very different clinical effect [11]. Another important aspect is a decrease in the side effects of 'old' drugs. For example, the cardio toxicity of doxorubicin is diminished when incorporated in liposomes without diminishing its antitumor effect [11]. Another example is the liposomal formulation ofVincristine, Marqibo®, and the depot type of delivery in which controlled releases over long periods of time can be achieved for drugs like the luteinizing hormone-releasing hormone agonist Eligard®.

• New routes of administration

Most of today's cytotoxic drugs are insoluble in water. This necessitates special solvents (which can be toxic in themselves) and only intravenous administration routes in most cases (e.g., Taxol®).

Utilizing nanotechnology, several of these drugs can be converted to oral formulations or manipulated into nanocrystals that can be suspended in water and thereby make it possible to administer them orally [92]. This would make taking the drugs easier for patients and simultaneously decrease the costs for both patients and the healthcare systems [93]. The milling of crystalline drugs to nanosized crystals of the active component might be a route to increase solubility and bioavailability for some drugs where the stability of the drug permits [17]. The intranasal route has been suggested as a non-invasive route for delivery of drugs past the blood—brain barrier (BBB). This route works, but it is not always sufficient to reach tumors because the distance from the supplying axon to the tumor might be too long for the NP to remain intact during transport.

• New drugs/therapeutics

NP-based carriers can also be envisioned for the delivery of gene therapy or siRNA-based therapy. siRNAs, for example, have too short of a biological half-life and need shielding to survive the harsh conditions in vivo. Such NPs are typically positively charged to form nanosized polyplexes with siRNA or DNA. The therapeutic possibilities of siRNA-based therapeutics are enormous if the shortcomings of low stability and localization can be avoided. A huge array of literature exist on this topic and we have highlighted a few for the interested reader [39,41,50,55,71,94-108].

• Revival of discarded therapeutics

Numerous antineoplastic agents have been discarded during preclinical investigations, often due to solubility or bioavailability problems. Using nanotechnology, previously discarded drugs can be revived as pharmaceuticals for clinical use, which may have a tremendous effect in the pharmaceutical industry [109]. Marqibo® is one such example of an old drug in a new bottle where the 50-year-old plant extract Vincristine experienced a revival via NP formulation. Another example is the radiosen-sitizer Wortmannin, which suffered from low solubility and high toxicity. Incorporation of a NP increases its solubility, enhances its stability and renews its clinical potential [110-112].

• New targets for oncologic treatment in a patient's organs

The CNS is the most protected organ in the body due to the effectiveness of the BBB, and there

are very few possibilities for delivering therapies intrathecally. The expectations for NPs have been high, but most NPs developed to date have not been successful in crossing the BBB. A promising exception are small (<10 nm) supraparamagnetic iron oxide NPs, which can be focused to a desired area as shown in Figure 2. A well written discussion regarding the barriers for intrathecal deliver is given in the review of Biddelston-Thorpe et al [113].

Releasing the therapeutic cargo

The currently approved nanomedical systems that aim to deliver a therapeutic cargo, and are in clinical use, are mostly based on controlling the release of the pharmaceutically active component via passive diffusion. These drug delivery systems are constructed to impose a diffusion barrier through their physical construction, such as the core-shell morphology of micelles and the lipid bilayer of liposomes, and sometimes with the aid of secondary interactions for further diffusion control. In the research world, there is a great interest in further advancing these systems so that the active components can be released on cue at a specific location. Most systems utilize the following principles:

• pH

Tumors usually have a lower pH (<7.0) than normal tissue (pH 7.2-7.4). This can be taken advantage of if the NP is constructed in such a manner that it dissolves in acidic environments [114,115]. To achieve such a behavior, polymers are often used to construct the NPs.

• Temperature

The vector can be constructed in such a way that it dissolves at temperatures above normal (i.e., at 43°C). Such a locally elevated temperature can be achieved by either oscillating magnetic fields (for magnetic particles) or plasmon resonance (for gold NPs) [37]. One example of this technique is shown in Figure 2.

• Light

The particle can designed to be dissolved by light (photodegradation) [116]. In this way, only NPs in the exposed volume/area subjected to that wavelength will dissolve. This has been used with laser therapy for visible tumors that are seen by the naked eye and are accessible (ENT, esophagus, urinary bladder and colon, among others). For deeper lying tissues, the use of near-infrared light is being investigated [117].

Regulatory considerations

Nanomedicines must be evaluated according to the same standards as small molecule and biological pharmaceuticals. However, NPs are more complex in terms of setting up pharmacokinet-ics and pharmacodynamics assays, determining excretion pathways of the nanocarrier and performing assays necessary for the pharmaceutically active component. The excretion profiles and circulation times for small molecule drugs can change dramatically depending on the parameters of the nanocarriers, for example, clearance can change from renal to hepatic. Ideally the NPs should be nontoxic, and they should be made of biodegradable and excretable materials. Likewise, immune activation and charged species should be avoided. Prior to any clinical testing, the method of producing the NP must be studied and Good Manufacturing Practice must be employed. The addition of imaging agents, targeting ligands, etc., and the inherent variation of their concentration in the construct, will add several layers of complexity to the evaluation of NP homogeneity. This is a major barrier to translating results from the bench to the clinic. Other technical aspects to consider in the early research phase include the stability of the constructs in solution, their stability in storage, aggregation phenomena, and associated changes in size (size alone is a major parameter determining half-life in circulation and liver and spleen uptake) [20,41,103,118].

The demands on a nanomedicine are considerably different and more numerous than for a conventional 'small molecule' drug [119]. Biodistribution following any administration route (oral, dermal, intravenous, rectal, nasal, etc.) must be known in detail and can be analyzed via radiolabeling techniques. Methods to detect the NPs in vivo over time must be available. Due to their small size, native NPs can be difficult to analyze and separate in a complex blood sample and this requires fluorescent or radionuclide labeling. If the tracer is radioactive, it will not be sufficient to prove that the radioactivity is eliminated from the body because the conjugate can be split into its two components with the isotope being excreted and the NP remaining in the patient. At the same time, it is necessary to also evaluate the pharmacokinetics and pharmacodynamics of the drug that the NP is delivering compared those of the native drug. This is often possible, but not in all cases since some novel drugs indeed are too hydrophobic for native administration and require a formulation system [120,121].

The FDA has begun to assemble relevant approval standards for not only the original nan-oproduct but also for its metabolites and future generic copies. Thus, present standards for characterization of the biological effects — and side effects — of NPs might be inadequate. To further these important aspects and others of nanomedicines, the FDA is working with the Nanotechnology Characterization Lab of the National Cancer Institute. Also, conventional animal models might be insufficient to predict the effect these molecules will have in humans. Any regulatory authority approving new drugs must ensure that every pharmacological candidate is excreted in humans by normal organs or degraded by normal metabolic pathways. Moreover, even if the intact NP is non-toxic it must also be shown that its metabolites are harmless. Once outside of the human body, the effects of the NP on the environment must be determined. Finally, like radioactivity, there is the question how to handle the waste.

Conclusion

A careful review of the nanomedical field reveals a rich, complex, dynamic and promising specialty that is very likely to improve healthcare for humans in the future. The road to success, however, will be long and full of hurdles and pitfalls. If we were asked to condense the characteristics of nanomedicine into one single word, it would be VERSATILITY. Targeting of NPs to cancer cells still has yet to show its value, but there are so many other areas where nano-oncology might offer improvements, including:

• New drug formulations;

• New administrative routes;

• Applications of new therapeutics;

• Revival of previously discarded drugs;

• New targets in the patient's body.

EXECUTIVE SUMMARY

• Nanoparticles (NPs) for medical use are based on liposomal research from 1960 and onward.

• Liposomal doxorubicin was approved for clinical use in the mid-1990s. Medical background

• A majority of nanomedical systems are directed against cancer therapy.

• Significant research funding have been spent on developing new nanomedicines but few have reached the clinic. Definitions

• A nanomaterial has at least 1D between 1 and 100 nm. What are NPs?

• There is no such thing as one type of NP - they vary in size, composition and distribution.

• Liposomes and protein particles are in clinical use, as well as polymer therapeutics, and iron oxide NPs.

• Advanced multifunctional NP systems are in preclinical evaluation. Conclusion

• Bringing nanomedicines to the clinic is as complicated as for small molecular traditional drugs.

• Main benefit of nanotherapy have been reduced side effects of traditional treatment regimens. Targeting of NPs

• Targeted NPs aim to deliver the carrier of a drug to a specific tissue.

• Targeted therapy with NPs is complex due to the presence of serum proteins in vivo, the high shear forces in circulating blood and the existence of numerous barriers.

Regulatory considerations

• The road to the clinic for nanomedicines are as challenging as for small molecular therapeutics.

• Complexity increases with additional function - multifunctional nanomedicines 'theranostics' require very consistent batch production.

If only a small fraction of the possibilities mentioned in this publication become reality in the future, then several improvements in the war against cancer will have been achieved.

Future perspective

The versatility of nanotechnology offers clinical medicine numerous therapeutic possibilities such as new formulations for old drugs, new administrative routes and the utilization of RNA-based therapeutic agents. However, several obstacles remain, and the road from bench to bedside may be long. The seven challenges for nanomedicine which were described 7 years ago still remain [119].

Financial & competing interests disclosure

AM Nystrom is Chief Medical Officer of Polymer Factory Sweden AB and is a shareholder in the company that is

marketing dendrimers. Financial support has been provided by The Royal Swedish Academy of Sciences, Percy Falks Foundation, Swedish Research Council, Carl Bennet AB, Karolinska Institutet, and VINNOVA -Swedish Governmental Agency for Innovation Systems. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Open access

This work is licensed under the Creative Commons Attribution-NonCommercial 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/ licenses/by-nc-nd/3.0/

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