Formation of ion pairing as an alternative to improve encapsulation and anticancer activity of all-trans retinoic acid loaded in solid lipid nanoparticles

Guilherme Carneiro,¹ Elton Luiz Silva,¹ Layssa Alves Pacheco,¹ Elaine Maria de Souza-Fagundes,² Natássia Caroline Resende Corrêa,³ Alfredo Miranda de Goes,³ Mônica Cristina de Oliveira,¹ Lucas Antônio Miranda Ferreira<sup>1

1</sup>Department of Pharmaceutics, Faculty of Pharmacy, Federal University of Minas Gerais, Belo Horizonte, Brazil; ²Department of Physiology and Biophysics, Institute of Biological Sciences, Federal University of Minas Gerais, Belo Horizonte, Brazil; ³Department of Biochemistry and Immunology, Institute of Biological Sciences, Federal University of Minas Gerais, Belo Horizonte, Brazil

Abstract: This work aims to develop solid lipid nanoparticles (SLNs) loaded with retinoic acid (RA) to evaluate the influence of two lipophilic amines, stearylamine (SA) and benethamine (BA), and one hydrophilic, triethylamine (TA), on drug-encapsulation efficiency (EE) and cytotoxicity in cancer cell lines. The SLNs were characterized for EE, size, and zeta potential. The mean particle size decreased from 155 ± 1 nm (SLNs without amine) to 104 ± 4, 95 ± 1, and 96 ± 1 nm for SLNs prepared with SA, BA, and TA, respectively. SA-RA-loaded SLNs resulted in positively charged particles, whereas those with TA and BA were negatively charged. The EEs were significantly improved with the addition of the amines, and they increased from 36% ± 6% (without amine) to 97% ± 2%, 90% ± 2%, and 100% ± 1% for SA, TA, and BA, respectively. However, stability studies showed higher EE for BA-RA-loaded SLNs than TA-RA-loaded SLNs after 30 days. The formulations containing SA loaded or unloaded (blank SLNs) with RA were cytotoxic in normal and cancer cell lines. In contrast, the blank SLNs containing TA or BA did not show cytotoxicity in human breast adenocarcinoma cells (MCF-7), while RA-loaded SLNs with the respective amines were significantly more cytotoxic than free RA. Furthermore, the cytotoxicity of BA-RA-loaded SLNs was significantly higher than TA-RA-loaded SLNs. These findings are in agreement with the data obtained in the evaluation of subdiploid DNA content and cell-cycle analysis, which showed better anticancer activity for BA-RA-loaded SLNs than TA-RA-loaded SLNs and free RA. Taken together, these findings suggest that the BA-RA-loaded SLN formulation is a promising alternative for the intravenous administration of RA in the treatment of cancer.

Keywords: solid lipid nanoparticles, all-trans retinoic acid, cancer, treatment, antitumor activity, ion pairing