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pharmaceutical technology for decades, with all the advantages that this would
bring in terms of patient compliance. However, the high molecular weight, hydrophilic nature, and susceptibility to digestive enzymes of proteins seriously limit its
oral bioavailability. Any formulation must be able both to protect the protein in the
gastrointestinal tract and promote its absorption across the intestinal epithelium.
The stability of insulin in the choline:geranic acid (“CAGE”) deep eutectic solvent was assessed by examining its circular dichroism spectrum after storage at
4 °C and 25 °C. Insulin formulated in choline:geranic acid solvent was also less
susceptible to degradation by trypsin. The protein was found to be stable for 4
months (Banerjee et al. 2018a). The formulation was administered orally to nondiabetic rats in gastro-resistant capsules and showed a dose-dependent glucoselowering effect, which was not observed with insulin in saline. Histological studies
of intestinal samples taken 5 hours after insulin-“CAGE” administration showed no
structural damage (Fig. 2.7). This is in accordance with observations made on
Caco-2 monolayers in vitro. The transport of fluorescently labelled insulin across
these monolayers was also measured and found to be enhanced tenfold by the presence of choline:geranic acid solvent. Since this was accompanied by a reduction in
the transepithelial electrical resistance, it could be concluded that the deep eutectic
solvents could temporarily open tight junctions between the epithelial cells (supplementary material, Banerjee et al. 2018a). Another observation that could shed light
on “CAGE” mechanism(s) of action is that it reduced the viscosity of the intestinal
mucus, which would facilitate contact between the protein and the brush border of
the enterocytes.
The “CAGE” formulation has also been found to exert a biological activity in its
own right. Nurunnabi et al. have reported that the formulation can reduce the absorption of fat from the intestine and could therefore counteract obesity. A model fat
molecule, docosahexaenoic acid, formed particle in the solvent that were too large
to be adsorbed. Rats fed a high-fat diet in conjunction with the “CAGE” formulation
gained less weight than those not given “CAGE” (Nurunnabi et al. 2019).
There are also examples of the use of deep eutectic solvents to improve the oral
bioavailability of small molecules. Rutin, a disaccharide of quercetin, was formulated in a deep eutectic solvent composed of proline and glutamic acid 2:1. A pharmacokinetic study was performed in Balb/c mice, comparing the solvent with a
suspension in water. The solvent allowed a twofold increase in the maximum blood
concentration (C max ) and the area under the curve (AUC) while retarding the time
corresponding to the maximum plasma concentration (T max ) from 15 minutes with
the aqueous suspension to an hour with the deep eutectic solvent (Faggian
et al. 2016).
The same team also formulated the naturally occurring alkaloid berberine in
deep eutectic solvents for oral applications (Sut et al. 2017). After a solubility study,
three deep eutectic solvents were chosen for a pharmacokinetic study: proline:malic
acid 1:2, proline:urea 2:1, and proline:malic acid:lactic acid:water 1:0.2:0.3:0.5. All
three increased the area under the curve compared with an aqueous suspension, the
most effective being the proline:malic acid:lactic acid:water composition (Fig. 2.8).
2 Deep Eutectic Solvents for Innovative Pharmaceutical Formulations
pharmaceutical technology for decades, with all the advantages that this would
bring in terms of patient compliance. However, the high molecular weight, hydrophilic nature, and susceptibility to digestive enzymes of proteins seriously limit its
oral bioavailability. Any formulation must be able both to protect the protein in the
gastrointestinal tract and promote its absorption across the intestinal epithelium.
The stability of insulin in the choline:geranic acid (“CAGE”) deep eutectic solvent was assessed by examining its circular dichroism spectrum after storage at
4 °C and 25 °C. Insulin formulated in choline:geranic acid solvent was also less
susceptible to degradation by trypsin. The protein was found to be stable for 4
months (Banerjee et al. 2018a). The formulation was administered orally to nondiabetic rats in gastro-resistant capsules and showed a dose-dependent glucoselowering effect, which was not observed with insulin in saline. Histological studies
of intestinal samples taken 5 hours after insulin-“CAGE” administration showed no
structural damage (Fig. 2.7). This is in accordance with observations made on
Caco-2 monolayers in vitro. The transport of fluorescently labelled insulin across
these monolayers was also measured and found to be enhanced tenfold by the presence of choline:geranic acid solvent. Since this was accompanied by a reduction in
the transepithelial electrical resistance, it could be concluded that the deep eutectic
solvents could temporarily open tight junctions between the epithelial cells (supplementary material, Banerjee et al. 2018a). Another observation that could shed light
on “CAGE” mechanism(s) of action is that it reduced the viscosity of the intestinal
mucus, which would facilitate contact between the protein and the brush border of
the enterocytes.
The “CAGE” formulation has also been found to exert a biological activity in its
own right. Nurunnabi et al. have reported that the formulation can reduce the absorption of fat from the intestine and could therefore counteract obesity. A model fat
molecule, docosahexaenoic acid, formed particle in the solvent that were too large
to be adsorbed. Rats fed a high-fat diet in conjunction with the “CAGE” formulation
gained less weight than those not given “CAGE” (Nurunnabi et al. 2019).
There are also examples of the use of deep eutectic solvents to improve the oral
bioavailability of small molecules. Rutin, a disaccharide of quercetin, was formulated in a deep eutectic solvent composed of proline and glutamic acid 2:1. A pharmacokinetic study was performed in Balb/c mice, comparing the solvent with a
suspension in water. The solvent allowed a twofold increase in the maximum blood
concentration (C max ) and the area under the curve (AUC) while retarding the time
corresponding to the maximum plasma concentration (T max ) from 15 minutes with
the aqueous suspension to an hour with the deep eutectic solvent (Faggian
et al. 2016).
The same team also formulated the naturally occurring alkaloid berberine in
deep eutectic solvents for oral applications (Sut et al. 2017). After a solubility study,
three deep eutectic solvents were chosen for a pharmacokinetic study: proline:malic
acid 1:2, proline:urea 2:1, and proline:malic acid:lactic acid:water 1:0.2:0.3:0.5. All
three increased the area under the curve compared with an aqueous suspension, the
most effective being the proline:malic acid:lactic acid:water composition (Fig. 2.8).
2 Deep Eutectic Solvents for Innovative Pharmaceutical Formulations