Recent Progress in Polymer Therapeutics as Nanomedicines - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 100

in PEGylation is clearly demonstrated by the number of PEG-protein
conjugates reaching the market or under clinical trials. Today,
polymer-protein conjugates are used routinely as anti-cancer
therapeutics, as an adjunct to chemotherapy, and are also being
developed as components for combination therapy [132].

4.5 Defined-Shape Polymeric Nanocarriers

4.5.1 Self-Assembled Polymers

Self-assembling aggregates, such as micelles and vesicles, can be
used as delivery vehicles for therapeutics. Most self-assembling
structures are composed of amphiphilic molecules containing
hydrophobic and hydrophilic domains. In aqueous environment, the
amphiphilic molecules associate through non-covalent interactions
forming supra-molecular structures. The hydrophilic domains of the
amphiphilic molecules face the aqueous solvent, and the hydrophobic
domains are shielded from the solvent in a bulk hydrophobic
volume core. Usually drugs are encapsulated in the hydrophobic
region of the aggregate and released by passive diff usion, or on the
disassembly or the degradation of the aggregate. Self-assembled
aggregates can be designed to release their payload in response to
environmental changes in pH, temperature, and redox potential, as
reviewed by Branco and Schneider [133]. The self-assembly process
is spontaneous, and when conducted in the presence of small drugs,
easy loading is achieved [133, 134].
Lipids were the first self-assembled materials used for drug
delivery, mainly in the form of micelles and liposomes (lipid-
based vesicles). Such systems have been extensively investigated,
bearing fruit in a significant number of commercially available drug
formulations, and many undergoing clinical trials. Self-assembling
polymers have been used in a similar manner to lipid systems holding
few advantages over them. Self-assembled polymeric vehicles
have enhanced stability compared with lipid aggregates, leading
to a better control over drug release [134]. In addition, polymeric
aggregates' size and shape can be tailor-made by suitable choice of
monomers type and sequence and molecular weight of the polymer.
Moreover, using polymers, bigger micelles can be achieved than with
low-molecular-weight lipids and surfactants (usually 20-200 nm for