Solubilization of itraconazole by surfactants and phospholipid-surfactant mixtures: interplay of amphiphile structure, pH and electrostatic interactions

Although surfactants are frequently used in enabling formulations of poorly water-soluble drugs, the link between their structure and drug solubilization capacity is still unclear. We studied the solubilization of the “brick-dust” molecule itraconazole by 16 surfactants and 3 phospholipid:surfactant mixtures. NMR spectroscopy was used to study in more details the drug-surfactant interactions. Very high solubility of itraconazole (up to 3.6 g/L) was measured in anionic surfactant micelles at pH = 3, due to electrostatic attraction between the oppositely charged (at this pH) drug and surfactant molecules. 1H NMR spectroscopy showed that itraconazole is ionized at two sites (2+ charge) at these conditions: in the phenoxy-linked piperazine nitrogen and in the dioxolane-linked triazole ring. The increase of amphiphile hydrophobic chain length had a markedly different effect, depending on the amphiphile type: the solubilization capacity of single-chain surfactants increased, whereas a decrease was observed for double-chained surfactants (phosphatidylglycerols). The excellent correlation between the chain melting temperatures of phosphatidylglycerols and itraconazole solubilization illustrated the importance of hydrophobic chain mobility. This study provides rules for selection of itraconazole solubilizers among classical single-chain surfactants and phospholipids. The basic physics underpinning the described effects suggests that these rules should be transferrable to other “brick-dust” molecules. The chain length of surfactants and phospholipids has a qualitatively different effect on drug solubility. [Display omitted] •Itraconazole solubility up to 3.6 g/L is reached in anionic micelles at pH = 3.•Electrostatic attraction between the drug and surfactant molecules drives the effect.•Itraconazole has a charge of 2+ at these conditions (1H NMR results).•Hydrophobic chain length effect direction depends on the type of amphiphile.•Chain melting temperatures of phosphatidylglycerols correlate with drug solubility.