# Polymeric Nanoparticles--Tools in a Drug Delivery System in Selected Cancer Therapies
## 8 Conclusions  
Cancer is one of the most common causes of death worldwide, and current treatments are not fully effective and have many negative side effects. Because of this, alternative drug delivery methods are currently being sought to help overcome the limitations of cancer therapy, reduce systemic toxicity and prevent multidrug resistant on cytostatic agents. The use of polymeric nanoparticles in cancer therapy appears promising, especially those derived from d,l-PLGA, a copolymer approved by the FDA for use in active pharmaceutical ingredient delivery systems. These carriers act in an active or passive mechanism and have been proven effective against a variety of cells. The results of numerous studies show great potential for the use of PNPs as chemotherapeutic delivery systems in colorectal cancer, breast cancer, ovarian cancer and glioblastoma multiforme. In addition, PNPs are currently being investigated as carriers in gene therapy. Additionally, worthy of much attention is the fact that some of the PNPs are being evaluated in clinical trials, and some are approved by the FDA for treatment.  
Based on a review of the literature, it should be noted that there are a growing number of studies on improving the effectiveness of polymeric nanoparticles. A considerable amount of research suggests that by manipulating the structure and coating of compounds, i.e., PEG, it is possible to significantly control the residence time in the system and reduce undesirable interactions with plasma proteins. This is an important aspect, as it can be concluded that it has a significant impact on the process of drug delivery. Depending on the type of tumor, the location and target site of nanoparticle delivery should also be taken into account. In the case of gastrointestinal cancers, the main limitation is the varying environment depending on the gastrointestinal tract. Therefore, it is suggested that in such a situation it is best to use pH-sensitive nanoparticles coated with an enteric coating to protect the PNPs from early degradation. In addition, to improve the efficacy of nanoparticles, it is necessary to take into account individual patient factors arising from variability in the number and expression of surface receptors of neoplastic cells, as well as individual morphological and structural features of the entire tumor microenvironment. In this aspect, it is undoubtedly important to assess the occurrence of MDR, which, as studies indicate, can be abolished by using codelivery of drugs with synergistic effects. Therefore, in order to improve the overall quality and efficacy of treatment with polymeric nanoparticles, it is crucial to precisely analyze the patient's profile, tumor type and genetic status, as well as to select the appropriate polymer and manufacturing method depending on the encapsulated drug. However, the problem in this aspect is still the high individual variability of tumor heterogeneity, as well as difficulties at the production stage related mainly to the low encapsulation efficiency of some chemotherapeutics.  
Nevertheless, monotherapy is a rapidly developing scientific field, and the results of published research in recent years demonstrate that PNPs can be used as carriers for anticancer drugs, giving hope for the development of more effective treatment methods and regimens for cancer patients.  
Conceptualization, B.S.-M. and M.M.; literature review, M.M. and N.K.; writing--original draft preparation and review and editing, all the authors; supervision, B.S.-M.; funding acquisition, B.S.-M. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.