Imiquimod Solubility in Different Solvents: An Interpretative Approach

Area of Science:

• Pharmaceutical Sciences
• Physical Chemistry

Background:

• Imiquimod (IMQ) is typically formulated in lipophilic semi-solid topical preparations.
• Developing novel IMQ formulations, such as powders from spray drying, requires understanding its solubility in diverse solvents.

Purpose of the Study:

• To investigate the solubility of imiquimod (IMQ) in various solvents.
• To assess the impact of temperature, stirring, and contact time on IMQ equilibrium solubility.
• To explore the applicability of solubility parameters and solution models for IMQ.

Main Methods:

• Equilibrium solubility measurements of IMQ in water, ethanol, methanol, acetone, acetonitrile, and dimethyl sulfoxide at different temperatures.
• Evaluation of solid-liquid equilibrium attainment times.
• Analysis of IMQ solutions using principles of solution thermodynamics and spectroscopy.

Main Results:

• IMQ equilibrium solubility is significantly affected by temperature, stirring intensity, and contact time.
• Solid-liquid equilibrium was reached in 13 days from solid IMQ and 2 days from supersaturated solutions.
• No correlation was found between IMQ solubility and solvent solubility parameters.
• IMQ solutions exhibited non-ideal behavior due to intermolecular hydrogen bonding and π-stacking, evidenced by a hyperchromic effect in polar solvents.
• IMQ reacted with acetone, rendering solubility data unreliable.

Conclusions:

• IMQ solubility is complex and influenced by solvent-solute interactions, including hydrogen bonding and π-stacking.
• Standard solution models like Scatchard-Hildebrand are not directly applicable to IMQ solutions.
• Acetone is unsuitable as a solvent for IMQ due to chemical reactivity.
• Findings provide crucial data for developing innovative IMQ powder formulations.