Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)

acts as the interface between the core and the exterior environment, and the hydrophobic core serves as the reservoir for the incorporation of lipophilic drugs (Figure 17.1).

In drug delivery, biodegradable and biocompatible copolymers are chosen as a result of their degradation into non-toxic oligomers or monomers that are eventually absorbed in the body and/or eliminated. Many of the amphiphilic block copolymers used in drug delivery contain either a polyester, polyether, or a poly(amino acid) derivative as the hydrophobic block. A number of these hydrophobic polymers that have been used as the core of a micelle are listed in Table 17.1. Polyesters such as poly(D,L-lactide) (PDLLA), poly(glycolide) (PGA), and poly(s-caprolactone) (PCL) have been chosen as the core block because these polymers are biocompatible, biodegradable, and FDA approved for use in medical devices. Poly(propylene oxide) (PPO) is a polyether that has been used as the hydrophobic block in the well-studied Pluronic® family, the triblock copolymers of poly(ethylene glycol)-b1ock-poly(propylene oxide)-b1ock-poly(ethylene glycol) (PEG-b-PPO-b-PEG).7 Finally, poly(amino acid)s such as poly(aspartic acid) (PAsp) and poly (b-benzyl-L-aspartate) (PBLA) have also been widely explored as materials for the hydrophobic block because they can be easily modified synthetically. The wide range of materials available for selection as the hydrophobic block of the copolymer enables the preparation of micelles' having a variety of microenvironments within their cores.

In contrast, the hydrophilic block is important for stabilizing the micelle in its aqueous environment. The criteria for the selection of the hydrophilic block is that the polymer should be both uncharged and water soluble (e.g., poly(ethylene glycol), polyacrylamide, polyhydroxyethylmetha-crylate, poly(N-vinyl-pyrrolidone) (PVP), and poly(vinyl alcohol)).8 Poly(ethylene glycol) (PEG) is most commonly used as the hydrophilic segment in micelles. The molecular weights of PEG used in block copolymers typically range between 1000 and 12,000 g/mol with a chain length that is typically equal to or greater than the length of the core-forming block.4 PEG can also be referred to as poly(ethylene oxide) (PEO), a polymer with a molecular weight that is greater than 20,000 g/ mol, whereas PEG refers to polymers with molecular weights below this value.9 Because of its high water solubility and large excluded volume, PEG can stabilize the micelles by sterically excluding other polymers from the surface of the micelles.1,2,10 In addition, PEG can inhibit the surface adsorption of biological components such as proteins, improve the residence time of the micelles in the circulation, and limit the micelle clearance by the MPS.10-13

The selection of building blocks for the micelle system includes the type of polymers and the length of the core- and corona-forming polymer blocks; these will influence the physico-chemical properties of the micelles and also affect the overall therapeutic efficacy of the delivery system.

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