Polymeric nanocarriers and nanoreactors: a survey of possible therapeutic applications

There is, today, great need for new systems and strategies for therapeutic applications that will lead to improvements in patient conditions and prognoses, especially in complex diseases such as neurodegenerative diseases and cancer. Recently, polymer nanocarriers have been developed to protect and transport active compounds to pathological sites more efficiently than free compounds in terms of stability, amount required, localization and efficacy. There are two strategies to deliver active compounds: conventional drug delivery systems based on transport and release of active compounds in biological compartments and nanoreactors that transport active compounds and permit them to act in situ, behaving like rudimentary artificial organelles. Here, we present both strategies with their advantages and limitations, and indicate how they can contribute to therapeutic improvement. We focus on presenting the design and development of polymer nanocarriers and nanoreactors as an essential stage of conceiving therapeutic approaches. The properties of the polymer carrier and its behavior under biological conditions dramatically influence the efficacy of the active compound, and thus of the treatment scheme. The key contributions that nanocarriers and nanoreactors could make include protecting active compounds from degradation in biological compartments other than those desired, and concentrating such compounds within their assemblage to allow for multiple deliveries in one single polymer assembly. To efficiently cope with the challenges of complex pathological conditions it is necessary to go one step beyond conventional drug delivery systems by designing and developing nanocarriers that mimic organelles, by combining various active molecules in a single carrier, and even by combining therapeutic agents along with agents for detection, as in a theragnostic approach.