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Acute respiratory distress syndrome (ARDS) is a life-threatening condition characterized by the rapid onset of lung inflammation Therefore, monitoring the spatial distribution of the drug directly administered to heterogeneously damaged lungs is desirable. In this work, we focus on optimizing the drug <i>N</i>-acetylcysteine (NAC) adsorption on poly-<span style="font-variant: small-caps;">l</span>-lysine-modified magnetic nanoparticles (PLLMNPs) to monitor the drug spatial distribution in the lungs using magnetic resonance imaging (MRI) techniques. The physicochemical characterizations of the samples were conducted in terms of morphology, particle size distributions, surface charge, and magnetic properties followed by the thermogravimetric quantification of NAC coating and cytotoxicity experiments. The sample with the theoretical NAC loading concentration of 0.25 mg/mL was selected as an optimum due to the hydrodynamic nanoparticle size of 154 nm, the surface charge of +32 mV, good stability, and no cytotoxicity. Finally, MRI relaxometry confirmed the suitability of the sample to study the spatial distribution of the drug in vivo using MRI protocols. We showed the prevailing transverse relaxation with high transverse relaxivity values and a high <i>r</i>₂⁽*⁾/<i>r</i>₁ ratio, causing visible hypointensity in the final MRI signal. Furthermore, NAC adsorption significantly affects the relaxation properties of PLLMNPs, which can help monitor drug release in vitro/in vivo.