Abstract

NDDS longevity and targeting

One of the most common uses of NDDSs is to combine prolonged circulation times with targetabilty. Such NDDSs are particularly useful for tumour targeting because tumours (as well as other inflammation zones) usually have increased vascular permeability as well as poor lymphatic drainage^6,7. This enables long-circulating NDDSs to accumulate in tumours through the EPR effect, which forms the basis for passive targeting⁸ . Nevertheless, EPR-based drug delivery strategies face several challenges. First, tumours — especially large, solid tumours — are pathophysiologically heterogeneous. Some parts of such tumours are not vascularized, do not exhibit the EPR effect, may have sizeable necrotic areas^9,10 and have varied microvascular permeability¹⁰. In addition, the increased interstitial pressure that exists within tumours may limit the EPR-mediated accumulation of NDDSs even if the vasculature is leaky¹¹.

NDDSs that are used for passive targeting and/or spontaneous accumulation must have long circulation times to ensure that sufficient drug is delivered to the target tissue. The usual approach to obtain long-circulating NDDSs is to coat them with hydrophilic and flexible polymers, such as poly(ethylene glycol) (PEG), as was first suggested for liposomes¹². The pegylation of NDDSs prevents their interaction with opsonins and impedes their capture and clearance by the mononuclear phagocyte system. However, pegylated NDDSs can induce the production of antibodies that can accelerate the blood clearance of nanoparticles (as was demonstrated for clinically used pegylated liposomes), particularly after repetitive administration; this is the so-called accelerated blood clearance phenomenon^13,14.

Although PEG is still the gold-standard polymer that is used to prepare long-circulating NDDSs, other hydrophilic polymers that are used include poly[N-(2-hydroxypropyl) methacrylamide]¹⁵, poly(acryloyl morpholine), poly-N-vinylpyrrolidones¹⁶ and polyvinyl alcohol¹⁷. In general, the concept of pegylation, as well as the use of alternative polymers for longevity, is well established and reviewed; thus, in the next section only some of the key developments in this area are briefly mentioned. Importantly, the shape of NDDSs can also influence their pharmacokinetics and biodistribution^18,19.

Liposomal long-circulating NDDSs are the most frequently studied type of NDDS; however, synthetic amphiphilic polymers have also been used to sterically stabilize several other types of NDDS to alter their bio-distribution. For example, pegylation of gold nanoparticles reduced their uptake by the mononuclear phagocyte system and their subsequent clearance from the body²⁰. One potential application of pegylated gold nanoparticles is their use in photothermal tumour therapy (known as ablation) following their accumulation in the tumour²¹. Pegylated methotrexate-conjugated poly-l-lysine dendrimers also accumulated efficiently in solid tumours in rats and mice via the EPR affect²², and pegylation reduced the toxicity of positive charge-bearing dendrimers in cell-culture experiments²³. Quantum dots can also be modified by pegylation. In mice, pegylated quantum dots had prolonged circulation times, which were attrib-utable to their decreased uptake in organs (such as the spleen and liver) in which the mononuclear phagocyte system is active²⁴.

Active targeting of NDDSs can be achieved by attaching targeting ligands, such as monoclonal antibodies, transferrin, various peptides, folate, aptamers (single-stranded oligonucleotides) or certain sugar moieties, onto their surface^25,26. To prevent steric hindrance between the targeting moiety and the protective polymer (such as PEG), the targeting ligand is usually attached to the chemically activated distal end of the NDDS-grafted polymeric chain²⁷. An example of such active targeting of NDDSs is pegylated doxorubicin-loaded liposomes that have human epidermal receptor 2 (HER2; also known as ERBB2)-specific antibodies attached; these were suc-cessfully used to target HER2-overexpressing SK-BR3 cells in mice²⁸. A nucleosome-specific monoclonal antibody (mAb 2C5) that recognized multiple types of tumour cells via tumour cell-surface-bound nucleosomes improved the targeting of doxorubicin-loaded liposomes to tumour cells and increased cancer cell cytotoxicity in in vitro and in vivo models²⁹ , including nude mice xeno-grafted with the U-87 cell line, which was derived from a human glioblastoma³⁰. In another example of active targeting, pegylated gold nanoparticles conjugated to monoclonal F19 antibodies were used as targeted label-ling agents for human pancreatic carcinoma tissues³¹. Lactoferrin-conjugated PEG–polylactic-acid nanoparticles improved the delivery of the conjugated particles to the brain in an experimental murine model³².

An interesting approach to increase targeting centres on self-assembling polyalkylcyanoacrylate-based nanoparticles. These types of nanoparticles could serve as pharmaceutical carriers for various drugs and could be produced in pegylated and ‘activated’ forms, enabling various ligands to be easily attached³³. Polyisohexyl-cyanoacrylate nanoparticles loaded with doxorubicin produced by BioAlliance Pharma as Livatag (doxorubicin Transdrug) are currently in a Phase III trial in Europe and in the United States as a second-line treatment of hepatocellular carcinoma after sorafenib, at a stage with no available approved treatment (Livatag press release; see Further information).

Aptamers that impart affinity and specificity by electrostatic, hydrogen or hydrophobic bonding, but not via the base pairing, have also been used as stable and efficient targeting moieties for NDDSs. For example, liposomal NDDSs modified with specific aptamers have been used to target leukaemia cells³⁴.

Targetability can also be applied to stimuli-sensitive NDDSs. Gemcitabine-loaded pegylated pH-sensitive liposomes modified with an epidermal growth factor receptor (EGFR)-specific antibody efficiently inhibited tumour growth in mice³⁵. Micellar NDDSs that were composed of amphiphilic conjugates of PEG and a co-polymer of poly(ε-caprolactone) and poly(malic acid) loaded with doxorubicin conjugated to poly(malic acid) via a pH-sensitive bond were modified with folate for enhanced cellular uptake³⁶. This preparation provided an enhanced release of doxorubicin at lowered pH values inside cancer cells³⁶. NDDSs with similar properties based on pH-sensitive polymethacrylate (PMA)-grafted poly(amidoamine) nanocarriers and additionally pegylated and modified with a folate moiety efficiently delivered paclitaxel to tumours and inhibited tumour growth in mice³⁷.

Clearly, the separation of long-circulating NDDSs into those that achieve active targeting and passive targeting is conditional. That is, both phenomena are closely connected because NDDSs will accumulate in the target area via the EPR effect (passive targeting) before ligand-mediated interaction with target cells (active targeting) occurs.

Another interesting issue is associated with the use of targeting ligands that are also naturally present (in their free form) in the circulation, such as folate, transferrin or certain peptides. Although one may expect competition between the ligands attached to NDDSs and the native ligands for the binding sites, the success of folate-and transferrin-targeted NDDSs (see below) demonstrates that this potential problem can be overcome. This is probably achieved through multipoint interactions of ligand-modified NDDSs with the target and/or because of the rapid recirculation of the cognate receptors, which provides sufficient opportunities for an interaction of the NDDS with the receptor.

Disease applications for NDDS-based therapies

Cardiovascular diseases

NDDSs hold promise as therapeutic, diagnostic and theranostic agents for cardio-vascular diseases³⁸, particularly atherosclerosis³⁹ (FIG. 2). Multifunctional micellar NDDSs that combined a targeting pentapeptide, a fluorophore and a drug targeting atherosclerotic plaques by specifically binding to clotted plasma proteins were used to visualize atherosclerotic lesions in a mouse model. This preparation was shown to increase the amount of the anticoagulant agent biva-lirudin that was delivered to the lesion⁴⁰. Because the increased uptake of low-density lipoproteins (LDLs) by resident macrophages in plaques is associated with the progression of atherosclerosis, targeting plaque-associated macrophages could be a method of inhibiting LDL uptake⁴¹. Indeed, targeted polymeric NDDSs loaded with pravastatin specifically and dramatically inhibited the phagocytic activity of macrophages without affecting non-target cells⁴².

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Figure 2

Multifunctional and stimuli-sensitive NDDSs in cardiovascular pathologies and infectious diseases

a | Nanoparticulate pharmaceutical drug delivery systems (NDDSs) can be engineered to target activated macrophages in atherosclerotic plaques or proteins present in a clot. These nanoparticles can thereby reduce plaque size through these two distinct mechanisms. b | NDDSs can be used to deliver metals to bacteria, where they can generate reactive oxygen species (ROS) to induce membrane blebbing and DNA damage. Antibiotics can also be delivered intracellularly using NDDSs.

Multimodal diagnostic micellar NDDSs that were fluorescent and paramagnetic and targeted to macrophage scavenger receptors using specific antibodies have been used to visualize lesions in the abdominal aortas of mice with atherosclerosis⁴³. Furthermore, polymeric NDDSs made of polyketals that were used to deliver small-molecule drugs, such as the p38 mitogen-activated protein kinase (MAPK) inhibitor SB239063, and anti-oxidant proteins, such as superoxide dismutase, to the area of the myocardial infarction in a rat model significantly improved cardiac function⁴⁴.

Liposome-based systems have also been frequently used to study the potential use of NDDSs in cardiovascular diseases⁴⁵. For example, liposomes loaded with ATP or co-enzyme Q accumulated well in the infarcted areas of the myocardium and improved cardiac parameters in rat and rabbit models of myocardial infarct^46,47. In a rat model of heart transplant, organ rejection was simultaneously imaged and treated using multifunctional polymeric NDDSs co-loaded with superparamagnetic iron oxide nanoparticles (SPIONs) and plasmid DNA⁴⁸. The SPION acted as a magnetic resonance imaging (MRI) contrast agent, whereas the plasmid DNA suppressed the local immune response⁴⁸. NDDSs for targeted thrombolytic therapy were developed by conjugating a thrombolytic agent (recombinant tissue plasminogen activator) to dextran-coated iron oxide nanoparticles, which were additionally modified with a thrombus-targeted peptide that was sensitive to activated factor XIII⁴⁹. These NDDSs had increased binding to the margins of intravascular thrombi and good thrombolytic activity in a mouse model of pulmonary embolism.

The translation of these NDDSs and others identified in similar studies into clinical practice remains the primary challenge to advancing the field and will be addressed in the closing section of this article.

Stimuli-sensitive NDDSs

The use of NDDSs that respond to different types of stimuli to control the properties and behaviour of the NDDS is an important and growing area of research⁷⁷ (FIG. 3; TABLE 2). The stimuli can be internal and intrinsic to the target tissue (such as changes in pH, temperature, redox condition or the activity of certain enzymes) or external and artificially applied (such as a magnetic field, ultrasound and various types of irradiation) (FIG. 4).

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Figure 3

Penetration of NDDSs into pathological tissue and interaction with target cells

Moieties attached to nanoparticulate pharmaceutical drug delivery systems (NDDSs) can enhance their penetration into the target tissue or cell. Externally applied stimuli (such as heat) or intrinsic stimuli (such as a change in pH in the target tissue) can cause the NDDS to release its cargo in the vicinity of the target tissue (such as a tumour). Intracellular proteins specific to the target tissue can be used to ensure cargo release in the correct cell type. EPR, enhanced permeability and retention.

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Figure 4

Methods for targeted drug release

Various external stimuli can be applied to facilitate intracellular delivery (part a), and various local stimuli, such as enzymes present in the tumour environment, can be used to eliminate the protective poly(ethylene glycol) (PEG) layer from a nanoparticulate pharmaceutical drug delivery system (NDDS) to facilitate its interaction with target cells (part b). MRI, magnetic resonance imaging.

Intracellular delivery and organelle targeting

After reaching the target, NDDSs may still need to cross the barrier of the cell membrane to deliver their drug load into specific organelles within the cytoplasm. Several strategies to prevent lysosomal degradation following the cellular internalization of NDDSs have been described¹⁰⁶, including the use of targeting and cell-penetrating moieties as well as agents that can destabilize the lysosomal membrane, such as fusogenic lipids and pore-forming proteins¹⁰⁷.

Cell-penetrating peptides are able to internalize nanosized particles, including liposomes, into cells^108–110. Complexes of HIV TAT peptide (TATp)–liposomes and a plasmid encoding green fluorescence protein (pGFP) were used for successful in vitro transfection of several tumour and normal cells, as well as for the in vivo transfection of Lewis lung carcinoma tumour cells in mice¹¹¹. Octa-arginine-modified, bleomycin- or doxorubicin-loaded liposomes demonstrated good intracellular penetration and strong inhibition of tumour growth in murine models^112,113.

To provide intracellular drug delivery, the cell-penetrating function should be shielded — for example, under a PEG coating — until the NDDS is inside the target cell (where PEG is cleaved by local conditions)^5,86. NDDSs such as targeted long-circulating pegylated liposomes and PEG–phosphatidylethanolamine-based micelles that possess several functionalities^86,114 have been described. A pegylated TATp-modified pGFP-loaded liposomal preparation that had a PEG coating that detached at a lowered pH provided highly efficient transfection of tumour cells in vivo¹¹⁵. The potential therapeutic use of this approach was demonstrated in mice bearing ovarian cancer xenografts using a liposomal system that exposed a hidden cell-penetrating peptide (TATp) upon removal of PEG by the lower pH environment inside the tumour^116,117.

Other micellar-based NDDSs were constructed using degradable ε-caprolactone-based co-polymers; the func-tional group on the co-polymer was used to incorporate siRNA against multidrug resistance protein 1 (MDR1; also known as ABCB1) or doxorubicin¹¹⁸. These micelles were also modified with the integrin-specific RGD4C peptide for cancer targeting and with the cell-penetrating TATp. Fluorescently labelled micelles were effectively taken up by cells and they improved drug delivery into cell nuclei, resulting in enhanced cytotoxicity in a mouse tumour model¹¹⁸.

However, delivering drugs or drug-loaded NDDSs inside cells may not be sufficient to achieve the maximum therapeutic response, and can even result in increased off-target effects. As such, the delivery of drug-loaded NDDSs directly to the organelle of interest could maximize the intended effect. This requires developing NDDSs that combine longevity, targetability and organelle recognition.

As mitochondrial dysfunction has been linked to many diseases, including cancer¹¹⁹, several strategies including the incorporation of the lipophilic cation rhodamine 123 to target NDDSs to mitochondria have been described¹²⁰. Cell lysosomes have been targeted with NDDSs modified with octadecyl-rhodamine B¹²¹. Cytotoxic ceramides have been delivered to lysosomes of cancer cells using transferrin-modified liposomes, which resulted in reduced tumour growth in mice bearing an ovarian carcinoma xenograft⁶⁶. Rhodamine B-modified fusogenic liposomes have also been used to deliver glucocerebrosidase into the mitochondria of Gaucher’s-type cells with low glucocerebrosidase activity¹²². Glucocerebrosidase is an enzyme that is deficient in patients with Gaucher’s disease, and so this type of NDDS is of particular interest for the treatment of lysosomal storage diseases¹²².

NDDSs for multimodal imaging and theranostics

Currently used medical imaging modalities include γ-scintigraphy, MRI, computed tomography and ultra-sonography, together with some modifications of these methods. Because each modality has its own advantages and limitations, it is highly desirable to have more than one type of reporter group associated with the same NDDS. Multifunctional NDDSs have been constructed for multimodal imaging, which could overcome various problems associated with individual imaging modalities, such as insufficient sensitivity or resolution¹²³. With this in mind, several NDDSs that combine optical imaging with computed tomography or positron emission tomography (PET), or combine PET with computed tomography have been developed; these systems enable higher resolution and can be based on traditional NDDSs, such as liposomes or polymeric micelles, or they can use template nanoparticles with intrinsic contrast properties (such as gold or iron oxide nanoparticles, or quantum dots), which could be additionally modified with contrast moieties for other imaging modalities.

Thus, iron-oxide-based nanoparticles have been modified using click chemistry with a near-infrared dye to yield a dual MRI–near-infrared contrast agent¹²⁴. Pegylated liposomes co-loaded with iohexol (a computed tomography imaging agent) and gadoteridol (a Gd-containing agent for MRI) had a long circulation lifetime and enabled simultaneous enhancement of computed tomography and magnetic resonance signals in mice and rabbits¹²⁵. Dual-mode computed tomography–MRI imaging in rats and mice was achieved using multifunctional Gd-loaded dendrimer-entrapped gold nanoparticles¹²⁶. Simultaneous optical imaging, computed tomography imaging and MRI was made possible using liposomes labelled with a fluorophore tracer, ^99mTc, ¹¹¹In or ⁶⁴Cu and Gd^127,128 in mouse models. Moreover, the addition of a radionuclide¹²⁹ enables the imaging system to become a therapeutic. The use of polychelating amphiphilic polymers, which enable the attachment of many reporter metal atoms and can be incorporated or absorbed onto the nanocarrier, enabled a marked increase in the number of bound reporter metal atoms per particle and image-signal intensity¹³⁰. In mouse models, such preparations can be targeted to tumours with a cancer-specific mAb with rapid and specific tumour accumulation and could serve as effective contrast agents for MRI of tumours¹³¹.

A relatively novel development in the area of multi-functional NDDSs is the use of so-called theranostics; that is, the simultaneous use of pharmaceutical NDDSs for diagnostic and/or imaging purposes together with therapeutic use. Through the use of contrast reporter moieties, theranostics enable real-time monitoring of the biodistribution and target accumulation of NDDSs. Theranostics often use liposomes, polymeric micelles, dendrimers and iron oxide nanoparticles as the carrier component of NDDSs, and heavy metal atoms (such as ¹¹¹ In, ^99m Tc, Mn and Gd) as contrast moieties for γ-scintigraphy and MRI^132,133. For example, liposomes co-loaded with quantum dots and the dopamine agonist apomorphine had increased uptake into the brains of mice (apomorphine accumulation in the brain was increased by 2.4-fold) and enabled fluorescent brain imaging¹³⁴. Multifunctional receptor-targeted nano-complexes containing a neurotensin-targeting peptide, DNA, a fluorophore and Gd have been used for brain imaging combined with DNA delivery in rats¹³⁵.

Multifunctional core-shell polymeric nanoparticles composed of a poly(lactic-co-glycolic acid) core and a positively charged glycol-chitosan shell and co-loaded with DNA and quantum dots enabled transdermal DNA delivery and transfection of epidermal Langerhans cells in mice, and allowed the migration of target cells to be monitored¹³⁶. Long-circulating pegylated liposomes loaded with doxorubicin and additionally decorated with a tumour-specific antibody and contrast moieties^137,138 had increased therapeutic activity in vivo, and their target accumulation could be easily followed by γ-scintigraphy or MRI.

Iron-oxide-based NDDSs, which combine magnetic drug targeting and MRI, have attracted attention in recent years. SPIONs (which are popular MRI imaging agents) are frequently combined with therapeutic agents¹³⁹ and PEG for longevity. For example, chlorotoxin-conjugated iron oxide nanoparticles delivered therapeutic methotrexate and an MRI contrast agent to 9L glioma xenografts in mice¹⁴⁰. Magnetic particles modified with a recombinant peptide containing the amino-terminal fragment of urokinase-type plasminogen activator (uPA) can target tumours overexpressing uPA receptors, which enabled enhanced imaging and delivery in mice¹⁴¹. A targeted theranostic NDDS was prepared by covalent attachment of a near-infrared imaging agent and a translocation peptide to magnetic nanoparticles co-loaded with siRNA and was used for multimodal tumour imaging and to monitor the silencing activity of siRNA in tumour models^142,143. Furthermore, hydrophobic photosensitizers (such as protoporphyrin IX) were loaded into pH-sensitive PEG–poly(β-amino ester)block co-polymer micelles for simultaneous tumour imaging and photodynamic therapy in mice¹⁴⁴. A pegylated, siRNA-containing, magnetic-resonance-contrast liposome-based NDDS that contained fluorescent labels for histology downregulated the expression of survivin and inhibited tumour growth in xenografted tumours in mice¹⁴⁵. In addition, pegylated liposomes that were loaded with Gd(III) for MRI, siRNA against survivin for tumour therapy, and additionally labelled with rhodamine–PE (phosphatidylethanolamine) for fluorescent detection have been studied in mice¹⁴⁵.

Targeting of theranostic NDDSs provides an additional advantage, as was shown for the co-delivery of Gd and doxorubicin in liposomes targeted with a peptide specific for a neural cell adhesion molecule, which resulted in higher drug concentration in tumour tissues, increased inhibition of tumour growth and enhanced MRI visualization of Kaposi’s tumours in mice¹⁴⁶. Antibody-mediated targeting of liposomes co-loaded with iron oxide and doxorubicin to pancreatic tumours in mice enhanced both the antitumour activity and the tumour magnetic resonance signal compared with untargeted liposomes¹⁴⁷. The combination of a chemotherapeutic drug and a contrast agent on the same NDDS facilitates the development of personalized nanotherapeutic approaches because the efficiency of nanocarrier accumulation in the tumour (which is determined by the leakiness of the blood vessels and traced by a technique that uses contrast, such as mammography) can assist in the determination of the correct dose of the therapeutic agent for the treatment of each individual tumour^148,149.

NDDSs for combination therapy

Drug combinations that can be delivered by NDDSs may be useful to combat drug resistance in patients with cancer¹⁵⁰. A targeted liposome-polycation-hyaluronic acid NDDS that co-delivered three different siRNAs and one miRNA (to downregulate proliferation pathways, angio-genesis and the cellular defence against apoptosis) into tumours resulted in an 80% decrease in tumour size in a mouse model of melanoma¹⁵¹. Pegylated liposomes modified with trilysinoyl oleyamide have been used to co-deliver myeloid cell leukaemia 1 (MCL1)-specific siRNA and the histone deacetylase inhibitor suberoxy-lanilide hydroxamic acid into tumours, which resulted in a greater inhibition of tumour growth in mice bearing KB (human cervical cancer) xenografts¹⁵².

Similarly, liposomes co-loaded with doxorubicin and DNA encoding a dominant mutant form of survivin and targeted to tumours had enhanced antitumour activity in a murine model of Lewis lung carcinoma^153,154. Peptide-targeted liposomes loaded with doxorubicin or cisplatin together with oligonucleotides against the molecules involved in two main drug resistance mechanisms, B cell lymphoma 2 (BCL-2) and MDR1 (multidrug-resistance protein 1), had high efficacy in human ovarian cancer xenografts¹⁵⁵. Moreover, a combination of the colorectal cancer drug irinotecan and 5-fluoroorotic acid in pegylated liposomes (at a 5:1 ratio) had synergistic effects and had higher therapeutic efficacy than did the free drug mixtures in animal models, including KB-tumour-bearing mice^156,157. For example, in a Phase I clinical trial, liposomes loaded with irinotecan and floxuridine liposomes (in a 1:1 molar ratio) were well tolerated and had antitumour activity in patients with advanced colorectal cancer, whereby 13 out of 15 patients responded to therapy¹⁵⁸. The drug is currently in Phase II (TABLE 1).

Challenges in developing multifunctional NDDSs

When developing multifunctional and stimuli-sensitive NDDSs, one has to keep in mind that the different NDDS-attached functions may affect the biological activity of those NDDSs differently. Thus, although targeted ligands can result in improved cellular uptake, the surface properties, size and shape of the NDDSs will define their pharmacokinetics and biodistribution. Interestingly, some studies^159,160 have demonstrated that the use of targeting ligands (that is, antibodies) does not necessarily increase the accumulation of NDDSs in the target area (with tumours as an example) when compared with EPR-mediated accumulation. Instead, the targeting ligand may enhance intracellular drug delivery in situations in which an internalizable ligand was used^159,160, which could be beneficial for drugs that act intracellularly. It is also important to bear in mind that many surface-attached functions (most notably targeting moieties) could compromise the increased longevity of NDDSs that is imparted by pegylation; the surface density of such ligands should be closely monitored to provide a desirable targeting effect without overly compromising longevity.

I have mainly considered findings from early preclinical studies in this Review. The question then arises as to why, with so many interesting and impressive data, there still seems to be a long and arduous journey to bring these promising preclinical results into clinical practice? Potential answers to this question have been analysed in several recent publications^161–164. Based on these discussions, one can first of all note that many data obtained in animal models cannot be easily translated into humans. This is particularly important in cancer research, in which subcutaneous tumour xenografts still represent the most frequently used model for targeted cancer therapy. Effective tumour targeting requires a sufficient number of productive contacts to be made between the targeted NDDSs and the target cells, which, in turn, requires effective preliminary accumulation of these NDDSs in the tumour via the EPR effect. In humans, this process is much more heterogeneous than in animal models because of a high heterogeneity in tumour blood flow. It is particularly problematic for animal models of subcutaneous tumours, in which the access of the blood to the tumour interstitium is greater than it is in solid tumours in patients. Unfortunately, the situation is even less clear with models of other non-cancer diseases.

In addition, there is ongoing discussion as to whether passive or active targeting will be more beneficial for various potential therapeutic indications. Nevertheless, the current thinking suggests that passive spontaneous accumulation of long-circulating NDDSs in tumours should be sufficient for drug delivery into the tumour mass, whereas intracellular drug delivery could be more effective if NDDSs are supplemented with specific targeting moieties that recognize cell-surface ligands to promote the internalization of drug-loaded NDDSs. Moreover, many experimental models of various human diseases are not uniformly standardized, which often makes the comparison of results difficult. From a technological point of view, many of the effective and experimentally elegant NDDSs are difficult to reproducibly scale up in the industrial setting to produce NDDSs on a commercial scale because of their complexity, which will also add substantially to the final price of the product.

Assuming the problems described above are eventu-ally resolved, the translation of the current knowledge into clinical applications will require the active involvement of pharmaceutical and biotechnology companies and an increased understanding of the benefits of NDDSs by the broad clinical community. Once more clinical data become available, multifunctional, stimuli-sensitive and theranostic NDDSs will continue to be developed and improved. To become a successful product, such NDDSs should be reasonably simple. That is, their preparation should not require complex multistep processes, should not include uncommon or expensive constituents, and they should preferably be made of components that are generally recognized as safe. In the future, the scalability of NDDSs should not represent a problem and be preferably based on known and accepted processes and protocols. The final product should also be sufficiently stable under normal storage conditions and easy to use; that is, it should not require complex administration protocols or regimens. If these requirements are met, multifunctional, stimuli-sensitive and theranostic NDDSs could become an important component of personalized therapy in the near future.

Acknowledgements

Glossary

Footnotes

References