Journal of Chromatography A, 1166 (2007) 116–125 Octanol/water partitioning simulation by reversed-phase high performance liquid chromatography for structurally diverse acidic drugs: Effect of n-octanol as mobile phase additive Costas Giaginis a,b , Stamatios Theocharis b , Anna Tsantili-Kakoulidou a,∗ a Department of Pharmaceutical Chemistry, School of Pharmacy, University of Athens, Panepistimiopolis, Zografou, Athens 15771, Greece b Department of Forensic Medicine and Toxicology, Medical School, University of Athens, 75 Mikras Asias Street, Athens 11527, Greece Received 25 April 2007; received in revised form 30 July 2007; accepted 3 August 2007 Available online 9 August 2007 Abstract 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 log k 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 log P. At physiological pH, 1:1 correlation was obtained between log D 7.4 and log k oct w 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Acidic drugs; Reversed-phase chromatography; n-Octanol as mobile phase additive; Lipophilicity; Polar interactions 1. Introduction Lipophilicity, widely expressed by the logarithm of n-octanol/water partition coefficient log P (or distribution coef- ficient log D for ionizable compounds), plays an important role in several ADME (absorption, distribution, metabolism and elimination) aspects, as well as in the pharmacodynamic and toxicological profile of drugs [1,2]. A rich arsenal of calculation procedures has been developed for rapid estimation of log P, such as C log P software [3,4]. For complex structures, however and especially in the case of log D, such approaches may provide rather rough estimates and thus experimental techniques remain a prerequisite for accurate drug lipophilicity assessment [4,5].A gradually increasing number of studies to date alternatively use reversed-phase HPLC for lipophilicity assessment instead of the traditional shaking flask method. C18 silanized silica gel consti- ∗ Corresponding author. Tel.: +30 210 7274530; fax: +30 210 7274747. E-mail address: tsantili@pharm.uoa.gr (A. Tsantili-Kakoulidou). tutes the most widely used packing material for reversed-phase columns that are appropriate for the lipophilicity evaluation of drugs. However, the interference of silanophilic interactions in the partition mechanism was recognized long ago as a serious drawback, especially in the case of basic solutes or compounds with strong hydrogen bond acceptor groups [6,7]. The problem is usually faced by the addition of a masking agent in the mobile phase and the selection of a stationary phase with reduced free silanol sites [8,9]. In this regard, base deactivated silica pack- ing material (e.g. BDS C18) is a better choice than octadecyl silica ODS, since a higher degree of silanization is achieved by its manufacture [10,11]. Base deactivation is usually obtained by the use of small alkyls to react with surface silanol sites. In addition, recent technology has led to the development of polar embedded stationary phases, which are considered to be fur- ther deprived from silanophilic effects. In fact, incorporation of a polar embedded functional group at the bottom of the alkyl- bonded chains provides electrostatical shielding to the remaining silanol sites (e.g. LC-ABZ + and Discovery-RP-Amide-C16) and a high degree of orientation for the alkyl chains [12,13]. 0021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2007.08.004