**3. Conclusion and future prospects**

The main application of NPs as DDSs and the challenges, regarding to this, lies on the efficient administration of these carriers. Along with all the benefits that these DDSs provide, the choice of route of administration is essential to their performance. The difficulties are related to their absorption at the proper organs, tissues and cells.

Unconditionally, the use of NPs as DDSs may solve most of the problems of drug delivery, mainly related with low bioavailability in the target tissues. Mucoadhesive, mucus-penetrating NPs or 'nano-in-micro-particle' DDSs may significantly increase the retention time on the mucous surface by enhancing the drug absorption and thereby improving drug bioavailability. Nanotechnologies are able to overcome the physiological barriers of the different tissues regarding the route of administration. Polymeric carriers have a stabilizing role on the drug included. They protect it from the unfavourable impact of the environment or the biological fluids. The synthetic-based polymer nano-carriers can assure a controlled release in the target tissue and minimize the side effects. The advantages that these could provide unconditionally improve the drug bioavailability, reduce the administration frequency and dose and enhance the patient compliance. Still not well assessed, the problem with the eventual toxicity and adverse effects of these drug carriers must draw our attention. Although polymeric particles may be biodegradable, their degradation rate must be analysed and toxicity profiles must be assessed in various *in vitro*, *ex vivo* and *in vivo* models. The development and application of strict safety rules are needed to create an effective and safe drug formulation.

In the future, the main emphasis of investigations will be put on the achievement of a noninvasive drug administration, aiming targeted and controlled release of API with a minimal effective dose. The comprehensive exploration of the problems, associated with the route of drug administration within its complexity, the tissues under normal and pathological conditions, and the multi-compartment pharmacokinetics, will significantly accelerate the further progress in this field.
