3.8.3 Particles for Drug Delivery
Carriers for Drug Delivery
Nanostructures and nanoparticles can be used for drug delivery purposes, either as the drug formulation itself or as the drug delivery carrier (Cascone et al 2002, Baran et al 2002, Duncan 2003, Kipp 2004). Current research focuses on cancer therapy, diagnostics and imaging, although many challenges still need to be solved (Ferrari 2005). In addition, nanostructures are being investigated for gene delivery purposes (Kneuer et al 2000, Salem et al 2003, Ravi Kumar et al 2004, Gemeinhart et al 2005, Yoo et al 2005, Roy et al 2005).
Many different formulations involving nanoparticles have been used for drug delivery purposes, including albumin (Damascelli et al 2003), poly(D,L-lactic-co-glycolide)acid (PLGA) (Panyam et al 2002, Weissenbock et al 2004), solid lipid formulations (Muller et al 1997, 2000, Wissing et al 2004), cetyl alcohol/polysorbate nanoparticles (Koziara et al 2004), hydrogels (Gupta and Gupta 2004), gold (Hainfeld et al 2004, Paciotti et al 2004), polyalkylcyanoacrylate composites (Cruz et al 1997, Olivier et al 1999, Kreuter et al 2003), magnetic iron oxide (Gupta and Gupta 2005), methoxy poly(ethylene glycol)/poly(ε-caprolactone) (Kim et al 2003), and gelatin (Cascone et al 2002). Albumin nanoparticles are already the subject of clinical studies for anticancer drug delivery purposes (Damascelli et al 2003).
Not all of the ‘nanoparticle formulations’ mentioned are strictly or solely nanoparticulate in the sense that their size is not always below 100 nm and some drug delivery systems include particles up to several hundreds of nanometres. In many cases, the technology to produce very small particles did not exist in the early stages of development, but now there is an increasing refinement in their size and it is relevant to this Opinion to discuss this group of products together. The route of administration may be oral, parental (subcutaneous, intramuscular, intra-arterial, intravenous) and via the skin.
The aims for nanoparticle entrapment of drugs are either enhanced delivery and uptake by cells and/or the reduction in toxicity of the free drug to non target organs. For intravenous administration, long circulating and target- specific nanoparticles are needed. One of the problems is evasion of the entrapment of nanoparticles in the mononuclear phagocytic system, as present in liver and spleen (Gibaud et al 1996, Moghimi et al 2001). Surface modification with poly(ethyleneglycol), (PEG) resulted in a prolonged presence in the circulation by avoiding recognition and phagocytosis by the mononuclear phagocytic system (Bazile et al 1995). Besides reduction of therapeutic efficacy, liver entrapment also may have an effect on liver function. For cyanoacrylate and polystyrene nanoparticles (around 214 nm and 128 nm, respectively) transient liver alterations were observed after a single and chronic intravenous administration (Fernandez-Urrusuno et al 1995, 1997). Inflammatory responses were characterized by secretion of acute phase protein α1-acid glycoprotein by hepatocytes. In addition, antioxidant defences of hepatocytes were depleted, probably as a result of local release of oxidative species. Although nanoscale formulation is aimed at enhancing drug delivery without loss of drug activity, a study comparing insulin-chitosan nanoparticles to chitosan solution and chitosan powder formulations showed that the insulin-chitosan nanoparticles were less effective in terms of bioavailability and lowering blood glucose level in both a rat and sheep model (Dyer et al 2002).
After oral administration, only 10% of 60 nm polystyrene particles were recovered from the tissue of the gastrointestinal tract. Most of these particles were present in lymphoid tissue such as Peyer’s Patches and lymphoid aggregates in the large intestine (Hillery et al 1994).
After dermal administration, negatively charged nanoparticles of about 50 and 500 nm were found to permeate the skin, while positively charged and neutral particles of all sizes did not. It was suggested that particle size was less important than the total charge, explaining why both 50 and 500 nm sized latex particles showed permeation and 100 or 200 nm negatively charged particles did not (Kohli and Alpar 2004). A greater concentration of charge was suggested to be responsible for overcoming the skin barrier, explained for the 50 nm particles as being the small size and large surface area, and for the 500 nm explained by the high number of charged groups. It can be expected that penetration of damaged skin will be easier. Thus, when nanoparticles are used in ointments there should be particular consideration of skin permeation.
Encapsulation in sub 130 nm size poly(lactic acid – glycolic acid), PLGA, particles increased cellular uptake of a photosensitizer, resulting in enhanced cytotoxicity in vitro (Konan et al 2003). Toxicity of free nanoparticles was not determined in this study. Chemical characteristics such as surface charge may determine the fate of nanoparticles in cells. PLGA nanoparticles were found to be ingested by cells by endocytosis (Panyam et al 2002, Konan et al 2003). The escape from these endosomes into the cellular cytoplasm was suggested to be caused by a change in surface charge from negative to positive, resulting in cytoplasmic delivery of the incorporated drug. The hypothesis concerning the influence of the positive surface charge for escaping the endosomes was supported by data obtained with negatively charged polystyrene nanoparticles which remained in the endosomal compartment of the smooth muscle cells used in this study.
Nanoparticles may be used for gene delivery, applications including plasmid DNA administration for vaccination (Salem et al 2003, Cui and Mumper 2002, 2003, Zhang et al 2005) and cancer therapy (Ramesh et al 2004, Gordon and Hall 2005). Gene transfer was accomplished in vitro and in vivo using various types of nanoparticles. With silica nanoparticles of about 42 nm, gene transfer was obtained with very low cell toxicity (Ravi Kumar et al 2004). A clinical trial with gene therapy aimed at determining safety and tolerability was performed in cystic fibrosis patients (Konstan et al 2004).
Specific targeting to retinal epithelium cells in the eye is possible (Bourges et al 2003). For very small quantum dots (<10 nm) specific targeting of peptide coated quantum dots to the vasculature of lungs and tumours has been reported (Åkerman et al 2002). PEG coating abrogated uptake by the reticuloendothelial system of liver and spleen. In contrast, about 40-50 nm magnetic nanoparticles coated with PEG were quite well taken up by endocytosis (Gupta and Curtis 2004)
For indomethacin loaded nanospheres (size below 200 nm) composed of methoxy poly(ethylene glycol)/poly(ε-caprolactone) polymers, the in vitro cytotoxicity was reduced when compared to free indomethacin, although some minor toxicity of 15-20% growth reduction was still present (Kim et al 2003). In vivo acute toxicity studies found a LD50 value of 1.47 g.kg-1 , and 50% of this LD50 value administered for 7 days did not induce acute toxicity in heart, lung, liver and kidney. It was concluded that these methoxy poly(ethylene glycol)/poly(ε-caprolactone) polymer nanospheres were non- toxic.
Surface modifications of nanoparticles offer possibilities for medical applications such as drug targeting in terms of cellular adhesion and invasion and transcellular transport. Carbohydrate binding ligands on the surface of biodegradable PLGA nanospheres were found to associate at higher rates with cell membranes (Weissenböck et al 2004). Such increased adherence may lead to an enhanced activity of the drug presented as or incorporated in nanoparticles. For solid lipid nanoparticles, (SLN), in vitro cytotoxicity was dependent on the surfactant used for stabilization of the nanoparticles with one of the investigated surfactants inducing cytotoxicity (Muller et al 1997, Olbrich et al 2004). The stabilizing surfactants showed the largest differences in toxicity, although toxicity could be markedly reduced by binding to the nanoparticles. SLNs of various composition were investigated for their use in skin application (Santos Maia et al 2002). In vitro studies showed an increased drug (glucocorticoid) penetration of skin and epidermal localization. For nanoparticles in the size of 200 – 400 nm, the composition of the lipid matrix was shown to have an impact on the cytotoxicity of SLN (Schöler et al 2002). Coupling specific proteins such as antibodies to the nanoparticle surface may enable a more specific immune directed targeting of the particles to certain cells or organs (Nobs et al 2004).
Organ Specific Targeting
One of the advantages of the use of nanoparticles for pharmaceutical formulations is the potential to cross the blood brain barrier (BBB). However, this also may be the major drawback for systemic administration of nanoparticles in terms of potential brain toxicity. Such passage was suggested to be possible by the toxic effect of nanoparticles (about 200nm) on cerebral endothelial cells (Olivier et al 1999), although for similar nanoparticles (about 300nm) this was contradicted and not found for a different type of nanoparticle (Lockman et al 2003). Physical association of the drug to the nanoparticles was necessary for drug delivery to occur into the brain (Kreuter et al 2003). When nanoparticles with different surface characteristics were evaluated, neutral nanoparticles and low concentrations of anionic nanoparticles were found to have no effect on BBB integrity, whereas high concentrations of anionic nanoparticles and cationic nanoparticles were toxic for the BBB. The brain uptake rates of anionic nanoparticles at lower concentrations were superior to neutral or cationic formulations at the same concentrations. Therefore, nanoparticle surface charge must be considered for toxicity and brain distribution profiles (Lockman et al 2004).
Specific migration into draining lymph node is of importance for both treatment and diagnostic purposes. Nanoparticle formulations of polyisobutylcyanoacrylate (Nishioka and Yoshino 2001) and fluorescent quantum dots (Kim et al 2004, Soltesz et al 2005) were shown to localize into such draining lymph nodes. Also uptake by endothelial cells can be used for diagnostic as well as therapeutic (prevention of cardiovascular restenosis) purposes (Davda and Labhasetwa 2002, Uwatoka et al 2003, Westedt et al 2004).
The use of nanoparticles as drug carriers may reduce the toxicity of the incorporated drug (Kim et al 2003), although discrimination between the drug and the nanoparticle toxicity cannot always be made. The structure and properties of gold nanoparticles make them useful for a wide array of biological applications. Toxicity, however, has been observed at high concentrations using these systems. Goodman et al (2004) demonstrated that for 2 nm gold particles cationic particles were moderately toxic, whereas anionic particles were relatively non-toxic. Such very small sized gold nanoparticles were found to be non toxic when administered to mice for tumour therapy (Hainfeld et al 2004).
For thiol derivatized PEG - colloidal gold nanoparticles with tumour necrosis factor (TNF) an enhanced anti-tumour activity was reported when compared to free TNF (Paciotti et al 2004). Topoisomerase inhibitors when formulated in lipid containing nanoparticles showed increased anti-tumour activity in an in vivo nude mouse xenograft human tumour model (Williams et al 2003). Although phagocytosis by macrophages does not seem to be necessary for the uptake of nanoparticles, the immune system is not totally inactive when dealing with nanoparticles. For 100 nm polystyrene particles, an IgE adjuvant activity was observed in an animal model system of ovalbumin allergy (Nygaard et al 2005). Antibodies against fullerenes could be induced after the intraperitoneal injection of C60 conjugated with serum proteins.