Poly(ethylene oxide)-poly(propylene oxide) block copolymer micelles and gels in drug delivery: State-of-the-art and future perspectives

Amphiphilic poly(ethylene oxide)-poly(propylene oxide) block copolymers (PEO-PPO) are thermo-sensitive biomaterials that display a unique concentration-depending behaviour in aqueous medium. They are commercially available in a broad variety of compositions and molecular weights. At relatively low concentrations, above the critical micellar concentration (CMC), they form stable aggregates known as polymeric micelles. Due to the presence of a hydrophobic core, these micelles are useful in the solubilization and stabilization of poorly water-soluble drugs. When higher concentrations are attained some of the PEO-PPO materials show a transition from a liquid solution to a viscoelastic gel upon heating. This temperature-governed gelation motivated the interest in these copolymers for the development of injectable drug delivery implants. The present chapter presents a thorough overview of the most important developments comprising the application of PEO-PPO block copolymers in the solubilization, stabilization and sustained delivery of both hydrophobic and hydrophilic drugs. In addition to pristine copolymers, the advances on structural modification through chain extension using blocks of the same or different nature and on cross-linking approaches are described. Hybrid formulations in which the PEO-PPO copolymers are combined with other functional polymers are also presented. Specific examples of the application of each type of PEO-PPO-based system are shown and their potential for administering drugs through different routes and improving drug bioavailability and therapeutic effect is discussed.