An investigation into the distribution of lecithins in nanosuspension systems using low frequency dielectric spectroscopy

A range of nanosuspensions comprising a model drug (RMKP 22) with varying c oncentrations of Phospholipon 90 were prepared using high pressure homogeni zation and analyzed using low frequency dielectric spectroscopy as a novel means of characterizing the distribution of Phospholipon 90 within the susp ensions. A corresponding range of aqueous Phospholipon 90 suspensions were also studied for the purpose of comparison. The dielectric responses were i nterpreted using a modification of the Maxwell-Wagner approach, whereby the systems were considered to comprise a high frequency response correspondin g to the bulk layer serially connected to a lower frequency response corres ponding to an electrode barrier layer. The low frequency responses of both the Phospholipon 90 dispersions and the nanosuspensions were found to be es sentially independent of phospholipid concentration, indicating the presenc e of a barrier layer covering the electrode surfaces. In contrast, the high frequency (bulk) loss response was found to increase with Phospholipon 90 concentration for the surfactant suspensions while a maximum in response wa s seen with Phospholipon 90 concentration for the nanosuspensions; this beh aviour was attributed to the presence of impurities within the phospholipid s. Based on this investigation, a model is proposed with which the dielectr ic response may be related to the surface coverage of the suspended drug pa rticles. (C) 1999 Elsevier Science B.V. All rights reserved.