# The Influence of Ionizing Radiation on Paclitaxel-Loaded Nanoparticles Based on PLGA
## 3 Results and Discussion
### Synthesis and Characterization of Polymers  
The copolymer of L-lactide and glycolide with monomer composition L-LA:GA = 0.85:0.15 was synthesized using a bismuth catalyst system, consisting of the catalyst, BiOct3, and the co-initiator, PEG200, leading to the formation of a linear polyester. Importantly, the polymeric matrix obtained was neither cyto- nor genotoxic, which is critical in biomedical applications. In order to assess the cytotoxicity of the polymer, the NRU test was performed. It was observed that for the highest concentrations of the extracts tested (1 mg mL\({}^{-1}\)), the cell survival rate was 106 \(\pm\) 6% when compared to untreated cells (Table 1). These results confirmed the lack of cytotoxic effect of the PLGA sample tested. The umu-test with and without metabolic activation was used to assess the genotoxicity of the polymer. At the highest concentration tested (1 mg mL\({}^{-1}\)), the PLGA samples did not inhibit the growth of Salmonella typhimurium cells (G > 0.5). This, together with the induction ratio (IR) being lower than 1.5, indicates the lack of genotoxicity of the polymer (Table 1). The results of the copolymerization of l-lactide with glycolide are presented in Table 2. The polymerization reached 94.5% conversion of l-LA and the near-complete conversion of GA. The synthesized polymer was irradiated using \(\gamma\)-rays and EB with a dose of 25 kGy. Molecular masses and dispersity coefficients of irradiated and non-irradiated PLGA samples were determined by GPC in triplicate. The resulting polymer was characterized by a low dispersity index (\(D\) = 1.83), and the average molecular mass \(M_{\text{n}}\) = 13.8 kDa (Table 2), which was in line with the calculations based on the monomer feed ratio (\(M_{\text{n}}\) \(\sim\)14.5 kDa). As it is shown in Table 2, a considerable decrease in \(M_{\text{n}}\) was observed for the copolymer subjected to both \(\gamma\)-rays and EB (13.8% and 14.1% of \(M_{\text{n}}\) decrease for PLGA samples irradiated with \(\gamma\)-rays and EB, respectively) along with an increase in dispersity. Interestingly, no significant difference between the influence of \(\gamma\)-rays and EB on the \(M_{\text{n}}\) of PLGA was observed. In comparison, in our previous research [26] describing the influence of irradiation on PTX-loaded nanoparticles of different polymer matrices (poly(l-lactide)--PLA, poly(\(\varepsilon\)-caprolactone)--PCL, the copolymer of l-LA and \(\varepsilon\)-caprolactone--PLACL, and the copolymer of GA and \(\varepsilon\)-caprolactone--PCLGA), a relatively higher decrease in \(M_{\text{n}}\) was observed for \(\gamma\)-irradiated copolymers (\(\Delta M_{\text{nPLACL}}=-12.7\%\), \(\Delta M_{\text{nPLGA}}=-28.2\%\)) compared to the copolymers irradiated with EB (\(\Delta M_{\text{nPLACL}}=+5.5\%\), \(\Delta M_{\text{nPLGA}}=+3.2\%\)) [26]. The overall decrease in the molecular mass of PLGA observed after the irradiation process may suggest that the chain scission was a leading mechanism, taking place during the sterilization of the polymer by both \(\gamma\)-rays and EB. This is consistent with the literature, according to which the dominant mechanism for PLGA irradiated with low doses of \(\gamma\)-rays and EB is chain scission [27; 28]. During the irradiation, PLGA is excited, and active species, such as free radicals, are formed. The active species initiate further reactions, causing bond cleavage among the polymer chain. As a result, the molecular mass of the polymer is lowered [28]. The microstructure of the polymers was investigated using \({}^{13}\)C NMR spectroscopy. The characteristic signals were assigned to corresponding co-monomeric sequences by analogy according to the literature [19]. As seen in Figure 1, the signal for consecutive -LLL