# Polymers and Polymer Nanocomposites for Cancer Therapy +
## 6 Polymersomes and Liposomes  
Natural cells possess membranes formed by amphiphilic macromolecules. In an effort to mimic and improve these structures and exploit them for research diagnostic and therapeutic purposes, polymersome technology was born. Polymersomes are vesicle membranes made of amphiphilic synthetic block copolymers. They are self-assembled structures. Polymersomes have attracted much attention as versatile carriers due to their tunable membrane properties and ability to encapsulate or integrate a broad range of drugs. A long blood circulation time can be accomplished when PEG is used as a component of the block copolymer membrane, which contributes to good responses to various internal or external stimuli [119]. A recent documented review underlined the fact that the fundamental importance of cell membranes has incited scientists to not only study them but also to create synthetic analogs and models (such as bilayer structures based on natural or synthetic components ranging from lipids to surfactant) and amphiphilic block copolymers. Liposomes (phospholipid vehicles) can also be used as carriers for the administration of drugs [120]. Compared to liposomes, polymersomes have broader possibilities for tuning their characteristics, such as biodegradability, biocompatibility, and other possibilities. Polymer vesicles (besides the differences already mentioned) have been proven to be more robust and less water-permeable compared to liposomes. Among the biomedical applications, drug/gene delivery holds the most promise due to low critical aggregation, concentration, and robustness in the bilayer. Ideal polymersomes should release the drug at the target site in a controllable manner. Hence, the polymersome building blocks have to be responsive to external or internal stimuli [121]. Polymersomes are increasingly being used as platforms for simultaneous drug delivery and imaging and are therefore becoming popular theranostic products. Examples of polymersome block copolymers are poly(acrylic acid-co-distearate acrylate), the poly(trimethylene carbonate)-co-poly(L-glutamic acid) copolymer, polybutadiene-b-PEO (PB-b-PEO), poly(ethyl ethylene)-b-PEO (PEE-b-PEO), and PS-b-PEO [119; 122]. The use of nanoparticles as tumor-targeting agents is steadily increasing, and the influence of their characteristics (size, stealth) has been established. However, not much is known about the impact of tumor presence on nanocarric circulation times. A study reported on the influence of tumor presence on in vivo circulation time and the biodistribution of polybutadiene-co-poly (ethylene oxide) polymersomes. In the study, the polymersomes were loaded with gamma emitter and administeredintravenously, followed by timed ex vivo biodistribution. A large reduction in circulation time was observed in tumor-bearing mice. Pretreatment with chlodronate liposomes depleted macrophages in the spleen and liver and restored the circulation time of the polymersomes, with no significant difference in circulation time between healthy mice and tumor-bearing mice pretreated with clodronate liposomes [123]. Polymersomes are able to encapsulate large amounts of multiple drugs with both hydrophilic and hydrophobic properties simultaneously, and they also offer a mechanism to combat multidrug-resistant cancers and poor patient tolerance of the cytotoxic compounds used. Researchers have reported the formulation and evaluation of a novel electroneutral polymersome capable of high encapsulation efficacies in multiple drugs (DOX, 5-FU, and Leucvorin). In vivo biodistribution of the polymersomes was established and was found to accumulate largely in tumor tissue. The encapsulation of the three drugs was assessed both in vitro and in vivo and compared to the same concentration of the three drugs in solution. A higher efficacy and higher maximum tolerance dose for the combination drug-loaded polymersomes was reported [124].