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Octanol/water partitioning simulation by reversed-phase high performance liquid chromatography for structurally diverse acidic drugs: Effect of n-octanol as mobile phase additive.

Giaginis C, Theocharis S, Tsantili-Kakoulidou A

Department of Pharmaceutical Chemistry, School of Pharmacy, University of Athens, Panepistimiopolis, Zografou, Athens 15771, Greece.

The role of n-octanol as mobile phase additive for the lipophilicity assessment of 45 structurally diverse acidic drugs both at neutral (pH 2.5) and ionized form (pH 7.4) was investigated. Extrapolated retention factors logk(w) were determined on a BDS C18 column using methanol as organic modifier and different amounts of n-octanol as mobile phase additive. For more polar compounds, the effect of n-octanol in retention was found to decrease as their lipophilicity increased. In the case of carboxylic acids and oxicams, the differentiation in retention, in presence and absence of n-octanol, could be further attributed to the attenuation of polar interactions, concerning mainly hydrogen bonding. At pH 2.5, the use of n-octanol saturated buffer, without further addition of n-octanol in the mobile phase, led to 1:1 correlation with logP. At physiological pH, 1:1 correlation was obtained between logD(7.4) and logk(w)(oct) indices upon addition of 0.25% n-octanol, in the case of weak acids. For strongly ionized compounds, a good correlation was also established under the same conditions. The corresponding equation, however, possessed a large negative intercept and a slope lower than unity.

Published 10 September 2007 in J Chromatogr A, 1166(1): 116-25.
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