Physicochemical studies of lidocaine-menthol binary systems for enhanced membrane transport.

The melting properties of lidocaine and l-menthol binary systems were studied using differential scanning calorimetry (DSC). A eutectic mixture was obtained for the lidocaine:menthol ratio of 30:70 (w:w) with a eutectic point of 26 degrees C. The binary melt systems formed within a range of 30:70-50:50 (w:w) remained as homogeneous oils at ambient temperature. The solubilities of pure lidocaine and lidocaine from the binary melt systems were determined with and without propylene glycol in pH 8.0 phosphate buffer. Lidocaine from the melt systems was less soluble in the buffers due to the partition of lidocaine between the oil and aqueous phases. The addition of propylene glycol to the buffer significantly increased both the solubility and heat of solubilization of lidocaine. The permeation rates of lidocaine from the binary melt systems across shed snake-skin were concentration dependent and significantly higher than those from the reference solutions.