53
With a view to the oral administration of deep eutectic solvent-based formulations, Santos et al. tested the cytotoxicity of deep eutectic solvents based on citric
acid against the intestinal cell line Caco-2 (Santos et al. 2019). Citric acid itself is
quite toxic due to the acidic pH that it imparts to the culture medium, while the two
active molecules included (ethambutol and L-arginine) were not toxic, with the
deep eutectic solvents showing intermediate values.
Given the interest of deep eutectic solvents for topical applications, it is also
important to determine whether irritation or any other toxicity is observed in the
skin. An investigation was carried out in vitro by Santos de Almeida et al. on a
human keratinocyte cell line, HaCat, using the 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) reduction assay (Santos de Almeida et al.
2017). They compared deep eutectic solvents based on choline combined with glutamine or phenylalanine with ionic liquids based on imidazoles. The choline-based
solvents were found to be less cytotoxic than the imidazole-based ones. The results
for the two choline-based solvents were very similar, with half maximal inhibitory
concentrations for a 24-h exposure of about 0.4% v/v in the culture medium.
Zakrewsky et al. chose a primary cell line, normal human bronchial epithelial
cells to test the in vitro toxicity of ionic liquids destined for transdermal delivery,
and combined this with the antimicrobial activity of the formulations to select the
most promising for in vivo applications (Zakrewsky et al. 2014). Of the 12 formulations tested, the “CAGE” formulation of choline and geranic acid and a cholinehexanoate formulation combined low cytotoxicity with high activity against
biofilms. The potential for skin irritation was assessed by measuring interleukin-α1
secretion from a multilayer human skin model and by Fourier transform infrared
(FTIR) spectroscopy to look at changes in bands characteristic of skin lipids in porcine skin. Although its individual components caused significant interleukin-α1
release 4 h after application, the level observed with the choline:geranic acid solvent
gave a result similar to the negative control. No changes in the band of the Fourier
transform infrared spectrum between 1650 and 1660 cm
−1
were observed for the
choline:geranic acid solvent and two other choline-based deep eutectic solvents,
while the individual components caused significant changes. However, in their
study of protein penetration through skin, they observed stretching of the peaks
between 2850 and 2920 cm
−1
that are characteristic of lipid extraction (Banerjee
et al. 2017). In a later study, the influence of the ratio of choline to geranic acid was
investigated (Tanner et al. 2018). The decrease in the peak, indicating lipid loss, was
proportional to the geranic acid content, that is, the hydrophobicity of the mixture.
However, pure geranic acid did not promote insulin delivery through the skin, probably because it was too hydrophobic to dissolve the protein.
Macário et al. examined the potential skin toxicity of a number of deep eutectic
solvents using two cells lines, (HaCaT, a human keratinocyte line, and MNT-1, a
human melanoma line) with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) reduction as the indicator of cell viability (Macário et al. 2019). In
general, they observed that solvents containing choline chloride or tetramethylammonium chloride as hydrogen bond donor were not toxic to these cell lines, while
2 Deep Eutectic Solvents for Innovative Pharmaceutical Formulations
With a view to the oral administration of deep eutectic solvent-based formulations, Santos et al. tested the cytotoxicity of deep eutectic solvents based on citric
acid against the intestinal cell line Caco-2 (Santos et al. 2019). Citric acid itself is
quite toxic due to the acidic pH that it imparts to the culture medium, while the two
active molecules included (ethambutol and L-arginine) were not toxic, with the
deep eutectic solvents showing intermediate values.
Given the interest of deep eutectic solvents for topical applications, it is also
important to determine whether irritation or any other toxicity is observed in the
skin. An investigation was carried out in vitro by Santos de Almeida et al. on a
human keratinocyte cell line, HaCat, using the 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) reduction assay (Santos de Almeida et al.
2017). They compared deep eutectic solvents based on choline combined with glutamine or phenylalanine with ionic liquids based on imidazoles. The choline-based
solvents were found to be less cytotoxic than the imidazole-based ones. The results
for the two choline-based solvents were very similar, with half maximal inhibitory
concentrations for a 24-h exposure of about 0.4% v/v in the culture medium.
Zakrewsky et al. chose a primary cell line, normal human bronchial epithelial
cells to test the in vitro toxicity of ionic liquids destined for transdermal delivery,
and combined this with the antimicrobial activity of the formulations to select the
most promising for in vivo applications (Zakrewsky et al. 2014). Of the 12 formulations tested, the “CAGE” formulation of choline and geranic acid and a cholinehexanoate formulation combined low cytotoxicity with high activity against
biofilms. The potential for skin irritation was assessed by measuring interleukin-α1
secretion from a multilayer human skin model and by Fourier transform infrared
(FTIR) spectroscopy to look at changes in bands characteristic of skin lipids in porcine skin. Although its individual components caused significant interleukin-α1
release 4 h after application, the level observed with the choline:geranic acid solvent
gave a result similar to the negative control. No changes in the band of the Fourier
transform infrared spectrum between 1650 and 1660 cm
−1
were observed for the
choline:geranic acid solvent and two other choline-based deep eutectic solvents,
while the individual components caused significant changes. However, in their
study of protein penetration through skin, they observed stretching of the peaks
between 2850 and 2920 cm
−1
that are characteristic of lipid extraction (Banerjee
et al. 2017). In a later study, the influence of the ratio of choline to geranic acid was
investigated (Tanner et al. 2018). The decrease in the peak, indicating lipid loss, was
proportional to the geranic acid content, that is, the hydrophobicity of the mixture.
However, pure geranic acid did not promote insulin delivery through the skin, probably because it was too hydrophobic to dissolve the protein.
Macário et al. examined the potential skin toxicity of a number of deep eutectic
solvents using two cells lines, (HaCaT, a human keratinocyte line, and MNT-1, a
human melanoma line) with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) reduction as the indicator of cell viability (Macário et al. 2019). In
general, they observed that solvents containing choline chloride or tetramethylammonium chloride as hydrogen bond donor were not toxic to these cell lines, while
2 Deep Eutectic Solvents for Innovative Pharmaceutical Formulations