# Polymeric Nanoparticles--Tools in a Drug Delivery System in Selected Cancer Therapies
## 1 Introduction  
Cancer is undoubtedly one of the most common causes of death among people worldwide [1]. Environmental factors, inappropriate lifestyles (i.e., lack of physical activity, inadequate diet) and genetic predisposition increase the possibility of neoplastic transformation, whereby cells acquire unique characteristics that promote its further development [2, 3]. The ability to proliferate unlimitedly, evade the immune response and create new blood vessels to facilitate metastasis are just some of the many characteristics of cancer cells [2, 4].  
At present, the treatment process is mainly based on debilitating and burdensome chemotherapy, which is also not inert to normal cells and is often insufficient due to emerging cell resistance to the chemotherapeutic agent in question [3]. For this reason, combined methods are used in practice, involving the surgical removal of the tumor combination with pharmacotherapy or radiotherapy [3, 5]. Such methods, although they have their advantages, also have limitations, which include the adverse destruction of normal cells, the short half-life of chemotherapeutic agents in the circulation or the difficult access to the medicinal substance in certain tumor types, e.g., glioblastoma multiforme [2, 5].  
Polymeric nanoparticles (PNPs) are increasingly being used as a drug delivery system (DDS) to solve some of the problems resulting from, among other things, difficulties in delivering the drug related to the location of tumor cells, as well as to reduce the risk of effects on neighboring unchanged cells [6].  
Polymeric nanoparticles have small sizes, mainly ranging from 1 nm to 1000 nm [7]. Depending on the method of their formulation, they can take two different structural forms--nanocapsules and nanospheres. In the first case, the medicinal substance is entrapped in a core and surrounded by a polymer layer, but it can also be the core itself.  
Nanospheres, however, take the form of a cross-linked polymer inside which the drug is loaded (Figure 1). The medicinal substance in either case can also be absorbed on the surface of the nanoparticle [7,8].  
Biodegradable and biocompatible polymers are mainly used to produce PNPs. These include synthetic polymers, i.e., poly(lactide) (PLA) or poly(\(\varepsilon\)-caprolactone) (\(\varepsilon\)-PCL), as well as \(co\)-polymers such as poly(lactide-\(o\)-glycolide) (PLGA), which are currently approved by the Food and Drug Administration (FDA) [6, 7, 8, 9]. Polymeric drug delivery systems also use biopolymers, mainly chitosan or collagen, due to their nonimmunogenic properties. This way, once the drug is released, the matrix is hydrolyzed into harmless products.  
Another major advantage of PNPs is that it is possible to manipulate the properties of a given polymeric nanoparticle by appropriate modification of the polymer, as a result of which it is possible to control the release of the drug from the nanoparticle at a precise location in the body and also to make the carried water-soluble drug [7,9]. A short half-life, on the other hand, can be compensated for by encapsulating the drug into the nanoparticle [7,9].  
Anticancer therapy using polymeric nanoparticles is increasingly being used in research, due to the fact that it is possible to target the therapeutic effect only on cancer cells. The small size undoubtedly mainly plays a role in cancer therapy, where access to the medicinal substance is limited. An example is brain diseases, where the main obstacle is the blood-brain barrier (BBB), which can be abolished in most cases, due to the ability of drug-loaded PNPs to interact on a ligand-receptor basis [5]. Polymeric nanoparticles are also being used in the treatment of colorectal cancer and women's cancers, and there have been in vitro attempts to use them as carriers for genome-editing tools. Therefore, the aim of this review is to present the latest information on the use of polymeric nanoparticles as a drug delivery system in selected cancer therapies and gene therapy.