br metastases derived from lung
metastases derived from lung cancer or even other PTX-sensitive tu-mors. Other PLGA NPs loaded with carboplatin  or loperamide  have demonstrated their ability to cross the blood-brain barrier.
On the other hand, we performed a specific analysis in DRGs to determine the presence of PTX. The primary sensory Dexmedetomidine in DRGs play an essential role in pain transmission and are directly involved in the development of peripheral neuropathy. In fact, the presence of high concentrations of PTX in DRGs modifies neuron membrane properties, which has been associated with pain, allodynia and hyperalgesia [56,57]. These side eﬀects may lead to treatment discontinuation or reduced drug posology with the consequent decrease in antineoplastic eﬃcacy . Therefore, if less PTX enters the dorsal root ganglia, it could prevent the development of neuropathy. Some clinically used formulations such as Abraxane® have not been able to overcome this limitation and may induce an even more severe level of peripheral
neuropathy than Taxol® . Surprisingly, as shown in Fig. 10C, a significantly lower PTX con-centration (< 1 ng/6 DRGs) was detected in DRGs when PLGA-PTX NPs was used in comparison to free PTX, suggesting that NPs could decrease the drug entry into DRGs. Since the alteration of DRGs by PTX has been associated with the development of peripheral neuropathy in oncology patients, the use of our PLGA-NPs could represent a means of avoiding this side eﬀect. However, further experiments are necessary in order to demonstrate this benefit.
Analysis of tumor tissue showed that free PTX induced a higher PTX concentration after the first hour of administration (˜1100 ng/g) than PLGA-PTX NPs (˜200 ng/g). The concentration of PTX in tumor reached a similar level with both PLGA-PTX NPs and free PTX treatments after 24 h (Fig. 10D). Nevertheless, despite a non-prominent PTX accumu-lation in tumor tissue, modulation of the tumor volume by PLGA NPs
PTX (see in vivo studies) suggested that the nanoformulation required greater than 24 h to penetrate and accumulate in the subcutaneous tumor, which could be verified by performing these studies over longer periods. Interestingly, the high accumulation of PTX in lung tissue shortly after PLGA-PTX NP treatment (1 h) could represent improved antitumor eﬃcacy in the primary lung tumor, although more studies are necessary to support this observation. What is more, a greater an-titumor eﬀect was observed after the third dose of treatment despite a reduction of tumor PTX concentration, suggesting that the NPs enhance the entry of the drug into tumor cells, as observed in the internalisation assays. In fact, Adesina et al.  showed that a polylactide-based NP loaded with a fluorophore was undetectable in tumor tissue up to 48–72 h after administration. Accumulation of these NPs was also ob-served mainly in the lungs, followed by tissues where the re-ticuloendothelial system is located, such as the spleen, liver or kidneys, as occurred with our NPs. Klippstein et al.  also observed an ac-cumulation of the DiR-labelled PEGylated PLGA NPs in lung tissue after 24 days treating colon cancer murine tumor models. In our case, ac-cumulation of PTX in this tissue may represent an advantage in the treatment of lung cancer. Finally, pharmacokinetic studies in blood revealed that PLGA-PTX NP treatment maintained significantly higher levels of PTX in the latter periods of the assay (12 and 24 h) in com-parison with free PTX, although free PTX induced a higher concentra-tion of drug in plasma after the first hour of exposure (Fig. 10E). These results represent a significant improvement in drug bioavailability in comparison with free PTX administration where no PTX could be de-tected in blood after 24 h, which has also been described with other polymer and lipid NPs [61–64].