Materials used in Nanomedicine Applications

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1 2 Polymer Materials used in Nanomedicine Applications 2.1 Types of Polymer Linear Polymers A linear homopolymer has the simplest polymer architecture. Linear polymers are long chains without branches or crosslinked structures (Figure 2.1). Both synthetic and natural linear polymers are used in nanomedical applications: one example of such polymers are polypeptides, which are linear polymer chains of amino acids covalently bound to each other by amide bonds between the carboxyl group and the amino group of adjacent amino acid residues. Figure 2.1 Schematic representation of a linear polymer showing the random chain conformation. 2012, Nyström group The basic building blocks of polymers are called monomers, which are typically polymerised by radical-forming or ring-opening mechanisms to form a linear polymer. Polymers are macromolecular chains, and entanglement of such chains gives the 7

2 Update on Polymer Based Nanomedicine polymers many of their properties, such as a propensity to form fibres and elasticity. Such properties can never be achieved with small molecules. The properties of a polymer are intimately related to its molecular structure. Figure 2.2 shows some examples of polymers commonly used for biomedical and nanomedical applications [1]. Figure 2.2 Polymers commonly used for biomedical and nanomedical applications. DXO - 1,5-dioxepan-2-one, PDXO - poly(1,5-dioxepan-2-one) Reproduced with permission from A.C. Albertsson, and I.K. Varma, Biomacromolecules, 2003, 4, 6, , ACS Polymers with a linear, random-coil structure have been used in polymer therapeutics. These include the synthetic polymers polyethylene glycol (PEG), poly(n-(2- hydroxypropyl) methacrylamide) (PHPMA), polyvinylpyrrolidone, polyethyleneimine (PEI) and linear polyamidoamines, and the natural polymers (dextran (α-1,6 polyglucose), dextrin (α-1,4 polyglucose), hyaluronic acid and chitosans [2]. Cationic linear polymers such as PEI have been widely explored as non-viral vectors in vitro and in vivo. It has become clear that linear polymers with molecular weights greater than 22 kda are best able to overcome the nuclear barrier [3], and achieve the highest gene transfection rates [4]. The rates of liver and spleen uptake of such cationic polymers are generally too high, and, thus, these polymers are often not the best alternative for in vivo applications. 8

3 Polymer Materials used in Nanomedicine Applications Many polymers have been proposed as drug delivery carriers, but only a few of them (mainly the ones with linear architecture) have been accepted into clinical practice. PEG was first introduced into clinical use in the early 1990s [5]. PEG can enhance the plasma stability and solubility of the drug, while reducing immunogenicity. Several PEGylated drugs are now used in clinical practice. Examples include Adagen (PEGadenosine deaminase), which is used to treat immunodeficiency disease, Macugen (PEG-anti-vascular endothelial growth factor aptamer), which is used to treat agerelated macular degeneration, Pegasys (PEG-α-interferon 2a), which is used to treat hepatitis B and hepatitis C, and Oncaspar (PEG-L-asparaginase), which is used to treat acute lymphoblastic leukaemia. CRLX101 (formerly IT-101), a linear cyclodextrin polymer-based nanoparticle containing camptothecin (CPT), is in Phase IIa clinical trials for the treatment of cancer, Figure 2.3 [6, 7]. Other linear polymers, such as polyglutamic acid, polysaccharide, and polyallylamine hydrochloride, are used as polymeric drug delivery carriers. Figure 2.3 Schematic diagram of CRLX101 [8], a linear cyclodextrin-polyethylene glycol copolymer conjugated with camptothecin (CPT). Reproduced with permission from J. Cheng, K.T. Khin, and M.E. Davis, Molecular Pharmaceutics, 2004, 1, 3, , ACS 9

4 Update on Polymer Based Nanomedicine Block Copolymers A block copolymer contains two or more separate blocks of monomers to form a polymer with the structure poly(a-b-b) [9], where A and B are monomers and b denotes that it is a block. Copolymers can be classified according to how the monomers are arranged in the polymer structure. Linear copolymers consist of a single main chain, whereas branched copolymers consist of a single main chain with one or more polymeric side chains. Examples of branched copolymers are graft copolymers, brush copolymers and comb copolymers [10]. Block copolymers with two, three or four distinct blocks are called diblock copolymers, triblock copolymers and tetrablock copolymers, respectively, (Figure 2.4) [10]. Figure 2.4 Common block copolymers in nanomedical applications [11]. PTMC - poly(1,3-trimethylene carbonate), P2VP poly-2-vinyl pyridine, PCL - polycaprolactone Reproduced with permission from F.H. Meng, Z.Y. Zhong, and J. Feijen, Biomacromolecules, 2009, 10, 2, ACS 10

5 Polymer Materials used in Nanomedicine Applications Block copolymers possess more degrees of freedom concerning such properties as monomer selection, molecular weights of the blocks, and the balance between hydrophobicity and hydrophilicity, and, thus, this polymer architecture has been thoroughly explored for the formation of polymer-based micelles through selfassembly due to hydrophobic interactions between non-water-soluble blocks. Polylactide acid (PLA), for example, is a very attractive biomaterial for pharmaceutical and medical applications due to its non-toxic, biocompatible and biodegradable properties. In the same way, PEG has well-established properties of non-toxicity, flexibility, hydrophilicity, and biocompatibility. Much research, therefore, has been focused onto developing block copolymers and nanoparticles of PEG and PLA [12]. Diblock copolymers PLA-b-PEG, based on monomethoxypoly(ethylene glycol), and triblock copolymers (PLA-b-PEG-b-PLA), based on PEG, have been synthesised and used as polymer materials [13]. Recent progress in polymerisation techniques allows the synthesis of a broad variety of amphiphilic block copolymers that form core-shell micellar structures in solution [2]. Clinical trials of polymeric micelle drug carriers started in the 2000s, and R&D in field has increased steadily since then [14]. Several clinical trials of block copolymer drug carrier systems, such as Genexol-PM and NK105, are in progress [15, 16]. The primary objective of these carrier systems is tumour targeting. However, these systems can also solubilise water-insoluble drugs, which is another desirable property. Genexol- PM is a PEG-b-poly(DL-lactide) block copolymer, which incorporates paclitaxel in a micelle form. Genexol-PM may be superior to the conventional formulation (Taxol) in terms of the safe solubilisation of water-insoluble paclitaxel, without the use of the commonly employed excipient Cremophor EL, which can cause severe side effects and toxicity [15]. Another block copolymer, NK105, has also shown solubilisationrelated advantages over the conventional paclitaxel formulation in clinical trials [16] Graft Copolymers A graft copolymer is a special type of branched copolymer in which the side chains are structurally or configurationally distinct from the main chain. Both graft copolymers and block copolymers are composed of several segments, and they differ at the inter-segment linkage site: block copolymers are composed of terminally connected structures, while graft polymers have comb-type structures [10]. The individual chains of a graft copolymer may be homopolymers or copolymers. An antisense agent (Fomivirsen) has been approved by the FDA for clinical use, demonstrating the potential for such polymers in disease management [17]. A great deal of work has been invested in attempts to mediate antisense therapy through the use of viral vectors, cationic liposomes, polymeric micelles or peptides 11

6 Update on Polymer Based Nanomedicine as carriers to enhance the intracellular delivery of antisense oligonucleotides, but relatively few carriers have reached Phase III trials. This is primarily due to the low efficiency and high cytotoxicity of most carriers [18], where a major problem has been that some antisense therapeutics suffer from unfavourable interactions with serum proteins in the bloodstream. Improvement in the systemic and intracellular delivery of antisense agents is, therefore, needed [19]. It was reported in 2009 that a graft copolymer can enhance the in vitro delivery of antisense oligonucleotides in the presence of serum [20]. Polypropyl acrylic acid has been modified by grafting onto it either PEO or a more hydrophobic analogue, polyoxyalkylene amine, known as Jeffamine. The carrier system with the Jeffamine graft copolymer effectively mediates specific gene silencing in the presence of serum, while the system with the PEO graft copolymer fails to display any significant antisense activity. These results suggest a new approach for the controlled therapeutic delivery of antisense oligonucleotides. Graft copolymers have confined and compact structures, and, thus, the high density of non-absorbing side chains can assemble to form a brush layer that functions as an efficient protein-resistant layer. The grafting ratio may also affect the protein-adsorption performance [21] Dendritic Polymers Dendritic polymers have unique architectures that include dendrons, dendrimers, and dendronised polymers. Dendrimers and dendrons are symmetrically branched and exact structures, and this makes them a unique polymer material suitable for use in biomedical applications. Dendrimers, when properly synthesised, have one single molecular weight, in the same way as a protein does. They typically have diameters of 1-3 nm, and are synthesised by iterative steps of activation and coupling to build the structure layer-by-layer. Each layer of monomers is called a generation, and a pie segment of a dendrimer is called a dendron (Figure 2.5). The precise and symmetrical structure of the dendrimer gives it several advantages over other nanostructured materials: the placement of ligands or active drug molecules, for example, can be exactly tailored within the dendritic framework, while materials with no batch-tobatch variation can be prepared from dendrimers. Other advantages are the increased solubility and lower viscosity of dendrimers compared to linear analogues, which result from the high number of surface groups available for conjugation. These properties have led to many of the potential applications of dendrimers being sought in biomedical applications [22, 23]. The main drawback of dendrimers is that the synthesis of dendritic material is complex, which has limited their use to specialised dendrimer research groups over the last decade. Recently, however, several classes of dendrimers have become commercially available. 12

7 Polymer Materials used in Nanomedicine Applications Common Dendrimer Types Dendrimers are symmetrically branched polymers formed from layers of monomers, and, thus, the monomer units of dendrimers are based on the general structure AB x, where x is 2. There are two main methods of synthesising a dendrimer: the convergent method starts from the surface groups and the dendrimer is formed inwards, with a final reaction occurring with the multifunctional core, while the divergent method is the opposite, with growth starting from the core and moving outwards (Figure 2.6). Organic protection/activation reactions are used, followed by chromatography purification at each step, to give full architectural control over the structure, size and purity of the product, and to generate in this way an exact structure. Dendrimers are typically synthesised up to the fifth or sixth generation, which is a practical limit imposed by the complexity of forcing such reactions to completion in the face of steric congestion. The analytical techniques required to verify the structural integrity impose a further limitation. For example, size exclusion chromatography cannot resolve structural imperfections in a dendrimer in which a couple of monomer units are missing. Mass spectrometry is generally the most straightforward method, but it can only be used if the structures are ionisable. Dendrimers are typically produced in small amounts (a few grams) and commercial dendrimers are expensive. Other dendrimer architectures that are also highly branched but much less well defined include hyperbranched polymers, dendrigrafts and linear dendritic structures. Figure 2.5 Schematic drawing of a G2 dendrimer showing its characteristic treelike branching architecture, in which each monomer unit has been added at a branching point to yield a spherical polymer with a large number of surface groups. Each successive layer of branching units constitutes a new generation (G), with a specific number in the dendrimer series [24]. Reproduced with permission from S.H. Medina, and M.E.H. El-Sayed, Chemical Reviews, 2009, 109, 7, ACS 13

8 Update on Polymer Based Nanomedicine Figure 2.6 Convergent and divergent growth of dendrimers [24]. Reproduced with permission from S.H. Medina, and M.E.H. El-Sayed, Chemical Reviews, 2009, 109, 7, ACS Two components are needed to construct a dendrimer: a multifunctional core and an ABx monomer unit around which to build up the structure. The most commonly used types are: polyamidoamine dendrimers (PAMAM), poly 2,2-bis(methylol)propionic acid (PBisMPA) dendrimers, polybenzyl ether dendrimers (PBzE), polymelamine (triazine) dendrimers, polypropyleneimine dendrimers (PPI), and polylysine dendrimers (PLL). PAMAM, PPI and PBisMPA are commercially available. PAMAM (left), PBisMPA (middle) and PPI (right) in Figure 2.7 are the most commonly employed structures in therapeutic applications: these have been well characterised both in vitro and in vivo, and their toxicological properties have been evaluated. 14

9 Polymer Materials used in Nanomedicine Applications Figure 2.7 Structures of PAMAM, PBisMPA and PPI dendrimers Nanomedical Uses of Dendrimers The main biomedical applications of dendritic polymers are their use as drug delivery systems (mainly for cancer) and as imaging agents (mainly for magnetic resonance imaging). Their exact size and the precise placement of functional groups on the surface of dendrimers make them highly suitable vehicles for such applications. Dendrimers, in contrast to other polymer assemblies, have a size distribution that is introduced solely by the error of the instrument used for measurement. This means that variations in biodistribution and pharmacokinetics are much smaller than those that arise when polymer nanoparticles and micelles are used [25], and this is a major regulatory advantage. The number of available functional groups on the surface of dendrimers is high, enabling a high, active component concentration to be reached and minimising the volume that it is necessary to inject to reach a specific therapeutic concentration in the bloodstream. The high number of functional surface groups also allows researchers to explore multi-valent targeting as a method to achieve high tissue specificity. Finally, dendrimers are covalent constructs and can be considered to be unimolecular micelles, which means that dendrimers do not have a critical micelle concentration (CMC) that would otherwise limit their usefulness for drug delivery. Polymer micelles or liposomes that are injected in the blood pool are typically highly diluted by the blood. If the concentration of polymers falls below the CMC, the structure falls apart, resulting in a burst release of the therapeutic agent. Unimolecular micelles do not have a CMC, and thus burst release can be avoided [26-28]. There are several drawbacks of using dendrimers in drug-delivery applications. Dendrimers are very expensive and difficult to produce on a large scale. Further, they are typically very small, with a diameter of a few nanometres, which results in them being rapidly cleared from the blood stream by kidney filtration (Figure 2.8) [29]. 15