Effect of oil phase lipophilicity on in vitro drug release from O/W microemulsions with low surfactant content

In this work we investigated the effects of oil phase lipophilicity on in vitro drug release from topical o/w microemulsions (MEs) containing low percentages of emulsifiers. Three different lipids, isopropyl myristate (IPM), isopropyl palmitate (IPP) and isopropyl stearate (IPS), whose lipophilicity increased in the order IPM < IPP <IPS, were used as oil phase to prepare o/w MEs containing low amounts (7.7% w/w) of two surfactant/cosurfactant mixtures, isoceteth-20/glyceryl oleate (5: 2) (MEs 1-3) and oleth-20/glyceryl oleate (5: 2) (MEs 4-6). All the MEs were prepared using the phase inversion temperature (PIT) method. Three active compounds (0.5% w/w), Naproxen (NAP), Idebenone (IDE) and Butylmethoxydibenzoylmethane (BMBM) were selected as model drugs and their release rates from PIT microemulsions were evaluated using Franz-type diffusion cells. All the MEs gave a mean droplet diameter ranging from 28 to 44 nm and showed a single peak in size distribution. The addition of IDE to MEs 1-6 did not significantly change ME droplet size. On the contrary an increase of the droplet size beyond the ME limit (150 nm) was observed when isoceteth-20 was used as surfactant to prepare MEs containing NAP or MEs containing BMBM and IPS as oil phase. Pseudo-first order release rates were observed only for NAP from MEs 1-3, while MEs containing IDE showed an initial slow release followed by an increased release of the test compound. The release rate constants were found to be dependent on the ME composition and on the active compound incorporated. The highest release rate was observed from ME 1 containing IPM as oil phase and NAP as drug. As regards BMBM, its release rate was not calculated since no release was observed until 6 h from the beginning of the experiment. The cumulative amount of active compound released after 22 was inversely related to drug lipophilicity (NAP Log P = 2,9; IDE Log P 3,5; BMBM Log P 4,8). These findings could be attributable to a reduced thermodynamic activity of the drugs in the vehicles containing the most lipophilic oil phase due to an increase of drug solubility which could lead to an unfavorable drug partition from the oil phase. The results of this study suggest that the choice of proper combinations of oil phase lipids and emulsifiers may allow achieving drug controlled delivery from topical o/w microemulsions with low emulsifier content.