# Polymeric Nanoparticles--Tools in a Drug Delivery System in Selected Cancer Therapies
## 2 Polymeric Nanoparticles--From Shape to Application  
In the controlled delivery of a drug for cancer therapy, the properties of a specific polymeric nanoparticle are crucial. Its size, shape, molar mass, polymer molar ratio or zeta potential and stability in a biological system are highly dependent on the method used to produce PNPs [6,7,10, 11, 12].  
Figure 1: Types of structural forms of polymeric nanoparticles.  
The size of the nanoparticle should be suitable for the site of application so that it may act efficiently and penetrate biological barriers, but on the other hand, nanoparticles that are too small may be neutralized by the phagocytic system [7; 13]. The most effective diameter for a polymeric nanoparticle is assumed to be around 100 nm for delivery to vascularized tumors, due to the ability of nanoparticles this size to pass through fenestrations in the endothelium of a blood vessel [6]. However, in the case of cancerous tumors in the brain's periphery, smaller diameters of polymeric nanoparticles of around 70 nm seem to be more effective, as confirmed by a study conducted by [PERSON] et al. [14] in an animal model [14]. The shape of the PNPs is mainly responsible for their function and pharmacology; therefore, it is highly desirable to obtain PNPs with a spherical structure due to the fact of better uptake of a particular nanoparticle by the target cells [7; 13]. In order to prolong the residence time of PNPs in the circulation and to prevent undesirable interaction with plasma proteins, polyethylene glycol (PEG) coating is used. Such modification is directly related to the increase in the half-life in the bloodstream and the improvement of the stability of the nanoparticles [7]. In addition to prolonging the residence time of the particles in the body, PEG coating also increases the hydrophilicity of the particles, and is used to encapsulate hydrophobic and hydrophilic drugs to achieve controlled release of the medicinal substance [7; 13].  
The most common polymers that are PEGylated are poly(D,L-lacide-\(co\)-glicolide) (D,L-PLGA) and PLA nanoparticles, which gives them the properties described earlier. Interestingly, studies report that the coating also reduces the effects of toxicity to normal cells [7; 13]. In order to enhance selectivity, polymeric nanoparticles are functionalized with, e.g., folic acid, or conjugated with antibodies directed against surface receptors of cancer cells overexpressing a particular receptor [7].  
Standard methods for obtaining nanoparticles can be divided into two different groups: top-down and bottom-up techniques (Figure 2).  
Bottom-up techniques form nanoparticles from single monomers through polymers. This method includes emulsion polymerization and recombination technology, which is mainly used to produce PNPs for gene therapy (Table 1) [6; 10; 11; 12].  
The top-down technique can be distinguished by the formulation of nanoscale particles by: nanoprecipitation, solvent evaporation, salting out, dialysis or supercritical fluid technology (Table 2). The main idea of this method is based on reducing the dimensions of the starting material to a scale corresponding to nanostructures.  
Figure 2: Methods of formulation nanoparticles.