Annealing as a tool for the optimization of lyophilization and ensuring of the stability of protein-loaded PLGA nanoparticles

The aim of this work was to use annealing as a tool to optimize the lyophilization cycle by decreasing its duration time, and simultaneously preserve the stability of poly(lactic-co-glycolic acid) nanoparticles and with upmost importance, maintain the structural stability of loaded insulin, used as model therapeutic protein. The contribution of a cryoprotectant in this preservation process was also evaluated. Insulin-loaded nanoparticles co-encapsulated with and without trehalose as cryoprotectant were produced, resulting in a particle size of about 250–300 nm, a PdI around 0.25 and a zeta potential in the range of −20 to −24 mV. The insulin association efficiency was higher than 90%, and the loading capacity was of 11–12%. The use of annealing allowed the decrease of duration time of primary drying in about 38%, representing a global decrease of lyophilization time of around 26%. The residual moisture content of all lyophilizates was around 1%, and the reconstitution of lyophilizates obtained using annealing was even faster than those without annealing. The co-encapsulated trehalose better preserved the nanoparticle size throughout the lyophilization process using annealing, compared to formulation containing no cryoprotectant. Fourier-transform infrared spectroscopy showed that the trehalose-containing nanoparticles presented higher insulin structural maintenance, compared to nanoparticles without cryoprotectant, presenting an insulin structural maintenance of 85.3 ± 0.7% and 86.0 ± 1.0% for annealing and no annealing, respectively. This formulation also presented the closest structural similarity with native insulin. Interestingly, the structural features of insulin loaded into nanoparticles upon lyophilization with and without annealing were practically identical, showing that annealing had no detrimental effect in insulin structure. Circular dichroism and fluorescence spectroscopy confirmed these results. Overall, this work gave rise to the importance of annealing in decreasing the duration time of lyophilization of protein-loaded poly(lactic-co-glycolic acid) nanoparticles, and simultaneously ensuring the stability of the carrier and loaded protein.

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