# Biomedical Application of Cyclodextrin Polymers Cross-Linked via Dianhydrides of carboxylic Acids
## 5 Molecular Drug Targeting  
The first studies on PMDA-NS for molecular targeting of cancer were conducted by [PERSON] and others in 2016. The team obtained a stimuli-sensitive NS in a simple reaction of \(\beta\)-cyclodextrinrin with pyromellitic dianhydride and 2-hydroxyethyl disulfide. When the disulfide bridge reacts with the thiol group of glutathione (GSH) (Figure 7), the concentration is higher in chemoresistant tumor cells than in the healthy cells. To determine the capacity of GSH-NS and the response to GSH at intracellular concentrations, doxorubicin was selected as the model anticancer drug. The obtained polymer can absorb water and shows a tendency to be cleaved in an aqueous solution of GSH due to the reducing agent. The in vitro drug release was very slow and prolonged over time. In addition, doxorubicin release was found to be proportional to the glutathione concentration, and a greater number of disulfide bridges increased the proportional release of the drug. Then, the biological tests were carried out on HCT-15 (colon tumor cell line), HepG-2 (hepatocyte tumor cell line), and A2780 (ovarian tumor cell line). Based on the results, the increased inhibition was confirmed for GSH-NS. The use of NS tested may be an interesting strategy in molecular targeted therapy. In this way, the in vivo side effects and the therapeutic dose can be reduced [42]. Later, [PERSON] et al. in 2016, developed studies on the complex of doxorubicin (Figure 8) with GSH-NS. In vitro tests on cytotoxicity, DNA damage, and in vivo biodistribution, pharmacokinetics, and growth of cancer cells of rats were performed. The first test showed increased toxicity of the drug,  
Figure 6: Structure of trans-resveratrol. Figure 7: Structure of glutathione (GSH). which was related to cells with a high concentration of glutathione. Microscopic and cytofluorometric studies showed an increased penetration of the glutathione sensitive polymer in relation to a pure chemotherapeutic one. The exposure of the cells indicated increased effectiveness of the drug at low doses. DU145 (prostate cancer cells) and HCT-116 (colon cancer cells) showed increased efficiency for the drug NS; however, PC-3 (prostate cancer cells) and HT-29 (colon cancer cells) showed no significant differences for the pure drug or the complex. Increased DNA damage in cells with high levels of GSH has been confirmed for the complexed drug by the comet assay [43]. It is assumed the increase in cell deaths is caused due to the formation of DNA adduct as well as topoisomerase II poisoning. In vivo tests indicated an accumulation of pure doxorubicin and complex in the liver, spleen and, heart, little was found in the lungs and the prostate. Additionally, tumor growth was found to be limited and the proliferation of positive cells reduced for GSH-NS. Based on the results, it can be said that GSH-NS enhanced the chemotherapeutic effect of doxorubicin in cells with high glutathione concentration and no significant toxicity to organs. This type of drug delivery seems to be effective in targeting chemoresistant tumors. The effective dose of the drug and its systemic side effects were found to be reduced [18]. In 2018, [PERSON]'s team decided to study further GHS-NS and its effect on erlotinib (Figure 9). The drug is used to treat lung cancer but its oral bioavailability is limited due to the poor solubility, instability in the gastrointestinal environment, and extensive first-pass metabolism. Therefore, the complexed drug and standard formulation were examined to determine the effect of GSH-NS. The tests were carried out for stability, in vitro drug release, in vitro, cytotoxicity on A549 (human lung carcinoma cells), pharmacokinetics, and biodistribution in mice (in vivo). The stability of the complexed drug was increased, and the release was found to be slow and prolonged over time. The concentration of the released drug was found to be proportional to the concentration of glutathione.