# Polymeric Nanoparticles--Tools in a Drug Delivery System in Selected Cancer Therapies
## 7 PNPs as a Drug Delivery System in Cancer-Associated Gene Therapy  
The main advantages of using a drug delivery system in cancer treatment are the possibility of reducing side effects of the drug, better acceptance by the patient, the use of a lower effective dose of the drug and the possibility of controlling the rate of drug release at a precise location, while increasing the stability and efficacy of the active substance [28]. Due to these features, as well as advances in genetic engineering and biotechnology, PNPs are increasingly being used as DDSs used to introduce molecules into cells [104; 105]. Biopolymers such as chitosan, dextran or pullulan are of particular importance due to their easy production process and nonimmunogenicity, biocompatibility and biodegradability [104]. Synthetic polymers such as poly (beta-amino ester) (PBAE) or d,l-PLGA are also used [104; 106]. Ease of interaction with DNA molecules is also provided by the use of cationic polymers, which are capable of interacting with anionic groups in nucleic acid [11; 105]. The mechanism of action of polymeric nanoparticles as gene carriers in anticancer therapy works by overcoming both extracellular and intracellular barriers [107; 108]. A PNP loaded with a molecule such as DNA, oligonucleotides (ONs), etc., binds to the surface-specific receptor of a cancer cell, undergoes endocytosis and in the form of an endosome is transported into the cytoplasm [107; 108]. The degrading nanoparticle releases its contents. In the case of a DNA molecule, the particle is transported to the cell nucleus, where it affects the process of gene transcription [107]. In the case of mRNA, protein synthetase is disrupted, while in the case of siRNA or miRNA, it binds to the RISC protein complex, leading to silencing of gene expression [107]. Each of the events leads to inhibition of proliferation of the neovascular cells and ultimately their death (Figure 7) [108]. Figure 7: Mechanism of action of PNPs as gene delivery systems for cancer treatment. Reprinted with permission from Ref. [108]. Copyright 2021 MDPI. In order to better eradicate the cancer tumor, immunotherapy is also widely used, focusing in particular on the activation of cells of the immune system, mainly T lymphocytes and cells with a CD8+ surface antigen, to eliminate cancer cells of a specific tumor type [109]. The most commonly used technique is the so-called adoptive T-cell immunotherapy, which is based on the use of patient-derived cells, their modification and reintroduction into the patient's body [110]. Inherent in the course of neoplastic diseases, as mentioned earlier, is the presence of TME, which consists of many cell types that produce both growth factors, chemotactic factors and cytokines. Due to this, a chronic presentation of the neoplastic antigen is produced, resulting in the acquisition of tolerance by T cells, which is associated with a decrease in the level of activation and proliferation of T cells involved in the eradication of neoplastic cells through receptor-related disorders, i.e., programmed death receptor 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) [109; 110]. Therefore, a promising approach seems to be the use of methods based on antibodies that block receptors contributing to PD-1 inhibition and simultaneously activate T cells. One solution is to use nanoparticles that target both of these elements and, at the same time, avoid nonspecific binding to the receptors. In a study conducted by [PERSON] et al. [111] on C57 BL/6 mice, the effect of mPEG-PLGA nanoparticles conjugated with PD-1 and OX40 antibodies obtained by nanoprecipitation on the ability to activate T lymphocytes was evaluated. The results of the study showed that the use of such a combination allows better activation of the mentioned cells, and thus increased therapeutic efficacy compared to antibodies administered in free form.