Nanomaterials for Drug Delivery

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Science  20 Jul 2012:
Vol. 337, Issue 6092, pp. 303-305
DOI: 10.1126/science.1219657

All drugs face several transport barriers on their tortuous journey from their site of introduction to their molecular site of action. Critical barriers include rapid filtration in the kidney and clearance via the reticulo-endothelial system (RES)—particularly for drugs that spend a lot of time in the bloodstream—as well as transport from the bloodstream to target cells within tissues. At the tissue or cellular target, the drug must cross the plasma membrane, and within the cell, it must escape the harsh acidic environment of endolysosomes, within which biomolecular drugs such as proteins and oligonucleotides may be inactivated or degraded. Other barriers are the nuclear membrane and the multiple drug resistance mechanisms that pathological cells can develop. Recent studies illustrate some particularly promising ways in which nanomaterials as drug or vaccine carriers can assist in navigating these barriers, with a particular focus on administration by injection.

For drugs administered through injection, the vasculature provides both a road to the destination—the site of disease—and many detours for the drug to be lost in transit. Transport in the vasculature and in the tissues that those vessels perfuse depends on convection in the circulation as well as diffusion and convection in the tissue interstitium (i.e., between the blood capillaries and the lymphatic capillaries) (1). One challenge has been to attenuate the filtration rate of molecules from the bloodstream, especially into the kidney, and to avoid premature clearance by the RES.

To address this, the hydrodynamic radius of protein drugs has been increased to ∼10 nm by grafting the hydrophilic polymer poly(ethylene glycol). This technology, referred to as PEGylation, is based on the twin observations that larger hydrophilic molecules are filtered by the kidney more slowly than smaller ones and that PEGylated proteins more successfully evade premature clearance through the RES. Gao et al. recently created site-specific, PEG-like conjugates by polymerizing very long polymer chains from one of the protein termini. This approach increased the hydrodynamic radius from 3 nm for the native protein to >20 nm for the conjugate (2). In a similar, genetically encoded approach, Schellenberger et al. expressed therapeutic proteins as fusions with a long, unstructured, hydrophilic polypeptide that seems to share many of the valuable attributes of PEG. Such fusions led to projected extension of circulation half-lives of exenatide, an important peptide drug for type 2 diabetes, from 2 hours to >100 hours (3).

The tumor vasculature is much leakier to colloidal objects than the healthy vasculature, allowing micelles and associated drugs to accumulate there. For tumor-targeting protein drugs, the approaches described above can lead to enhancement in circulation lifetime by a factor of 10 and improved accumulation in tumors relative to the native protein (2).

Even larger self-assembled polymer micelles are being developed to target specific tissues and even subcellular compartments in tumors. In one such approach, MacKay et al. (4) conjugated a hydrophobic cancer drug via an acid labile linker to an elastin-like polypeptide. Driven by the hydrophobicity of the conjugated drug, these polymers self-assemble into micelles <50 nm in diameter. These micelles then localize to tumors. In the acidic environment of the endolysosomes of the tumor cells, the drug is cleaved to reach its nuclear target, leading to a potent therapeutic response in an animal model.

In a remarkable feat of multifunctional polymer design, Murakami et al. have developed micelles that form a complex with a platinum-based DNA-targeting anticancer drug in a pH- and Cl-dependent manner, with drug release and activation favored at acidic pH and high Cl (5). The latter conditions are unique to late endosomes (just prior to endosomal fusion with lysosomes) and lysosomes. Late endosomes are localized near the cell nucleus, allowing directed delivery of the drug close to its DNA target, thus escaping the cytoplasmic resistance mechanisms that often inactivate the free drug.

Other micelle approaches abound (6, 7). Notable among these is an optimization of micelle shape for highly prolonged circulation (8). Polymer micelles for use in drug delivery are typically formed by self-assembly of block copolymers with a hydrophobic domain (which drives polymer association in the presence of water) and a hydrophilic domain (which restricts this association to the nanometer dimension). The relative volume of the two polymer segments controls the shape of the resulting micelle. Christian et al. have shown that polymer architectures that lead to cylindrical micelles enable highly prolonged circulation in the blood stream, thereby reducing off-target toxicity and increasing efficacy in tumor killing with drugs such as paclitaxel (8).

An example of nanomaterials design.

A complex structure helps siRNA to be carried to the tumor cell surface, internalized by endocytosis, and then released from the endolysosome into the cytoplasm. (A) Chains of positively charged cyclodextrins (light blue) form electrostatic nanoparticles with the negatively charged siRNA (purple). These complexes are administered by injection. (B) In the bloodstream, nanoparticle clearance is blocked by PEG chains terminated with hydrophobic moieties that bind as guests in the hydrophobic host center of the cyclodextrin. The nanoparticles are further functionalized with transferrin (green) as a tumor-targeting moiety (many tumor cells overexpress the transferrin receptor), using the PEG as a linker. (C) In the tumor cell, the particle dissociates to release the siRNA into the cytoplasm.


Other features of tumors have been used for targeting to amplify a signal at the tumor site. For example, von Maltzahn et al. have targeted nanomaterials to molecular features unique to the tumor endothelium to induce coagulation there; the resulting coagulum was then targeted with drugs that bind biochemically to the nascent clot (9). Because coagulation is autocatalytic, substantial signal amplification occurs between the initial tumor targeting step and the final binding of the drug to the coagulum. Here, biomolecular recognition was used in the first step to localize the procoagulant nanoparticles to the tumor endothelium through binding of a nanoparticle-conjugated domain of tissue factor receptor, targeting the tissue factor that is naturally up-regulated on the pathological tumor endothelium (9).

Annexin 1 is also highly expressed on the tumor endothelium. A short peptide sequence has recently been discovered that serves as a ligand for this marker, providing efficient drug localization to the tumor endothelium (10). This ligand can presumably be used to target micelles to tumor vasculature, as has been done with other peptides to target sites of atherosclerotic plaque after intravascular injection (11).

Biomacromolecular drugs typically cannot permeate through membranes to access targets in the cytoplasm. An exciting example of nanomaterials to penetrate these barriers is the delivery of small interfering RNA (siRNA) using cationic nanocarriers. As the cell attempts to neutralize the basic charge of the nanomaterials, the resulting osmotic imbalance destabilizes the endolysosomal membranes and releases the drug into the cytoplasm (12). Additional features to promote stability and targeting have been designed into this Lego-like nanomaterial (see the figure). Initial evidence for gene knockdown in tumors in patients supports the function of this nanomaterial design (12).

Vaccines are another area in which size matters. In the interstitium of most tissues, fluid flows between the blood capillaries and the draining lymphatics. After injection (e.g., intradermally), nanomaterials that are sufficiently small (sub-100 nm) to avoid entrapment in the tissue interstitium are efficiently transported by this interstitial flow into the draining lymphatics and lymph nodes. Here they can be collected by lymph node–resident dendritic cells—the first key cellular player in generating an immune response (13).

Particulate antigen from outside the cell is most frequently processed and presented on class II major histocompatibility complex (MHC) molecules, which results in humoral immunity. The resulting antibodies can be useful for preventing disease, for example by binding and neutralizing a virus, but are less useful for killing aberrant cells, such as cells already infected by the virus. To achieve this, the antigen must be presented on MHC class I molecules, leading to the induction of lymphocytes that can kill virally infected cells and even tumor cells.

Antigen presented on MHC class I usually derives from protein within the cell, such as viral proteins. However, under some circumstances, antigen from extracellular sources can be shuttled to MHC class I. This “antigen cross-presentation” is particularly effective when antigen is conjugated to nanoparticles by a reducible disulfide bond (14), which is cleaved in the reductive environment within the endosome, resulting in strong cellular immunity that can kill aberrant cells. Such small nanoparticles can also very effectively target antigen-presenting cells after pulmonary administration to induce a potent immune response in the lung and at other mucosal surfaces, to protect from viral infections such as influenza (15). Other nanomaterial designs have also been shown to lead to strong cross-presentation, including multilamellar vesicles, stabilized by interlamellar cross-linking, carrying antigen as well as adjuvant molecules (16).

The level of sophistication of designing nanocarriers to be functionally responsive has recently been taken to new levels with compelling demonstrations in vitro. Douglas et al. have used DNA origami—intricately folded nanoscale DNA structures—to develop hollow nanocontainers with DNA aptamer–based locks (17). These aptamers also bind to particular biomolecular signals; when the aptamer detects the binding antigen key, the nanocontainer is opened and the payload is released.

These examples show how highly functional multicomponent polymeric nanostructures can guide drugs to tissues, coax them through the biological barriers at the surfaces of and within cells, and escape drug clearance and drug resistance. However, in translation to human clinical trials, the devil is often in the details: Materials must be developed that maintain their stability, size distribution, and targeting specificity in the complex and concentrated protein environment of the body, that can be eliminated from the body at the same rate as planned administration to avoid accumulation, and that can be processed by the body without formation of untoward metabolites.


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