Introduction

Lipophilicity is the one physico-chemical parameter that continually attracts prime interest in QSAR studies, as befits its role as a predominant descriptor of pharmacodynamic, pharmacokinetic and toxic aspects of drug activity. Numerous monographs and reviews treat this topic in adequate detail [1-6]. The partition coefficient P between water (or buffer) and 1-octanol was for a long time used as the preferential experimental expression of the lipophilic properties of a compound. 1-Octanol/water partitioning is losing this role as the method of choice due to methodological drawbacks and limitations, which have been extensively discussed in the literature [7].

Chromatographic approaches (HPLC and TLC) are very important experimental alternatives to 1-octanol/water partitioning. Retention of solutes in chromatography is mainly governed by adsorption and partitioning processes. In order to derive lipophilicity descriptors by chromatographic approaches it is mandatory to limit the influence of adsorption on retention. Adaptation of chromatographic procedures to the experimental determination of lipophilic properties resulted in the development of reversed-phase chromatography, in which the commonly hydrophilic, polar stationary phase is replaced by a hydrophobic, nonpolar phase.

The use of chromatographic approaches in the QSAR field goes back to the early work of Martin and Synge [8] as well as Consden et al. [9], who established relationships between the RF values obtained from partition chromatography and partition coefficients.

The RP value of a compound x is defined as the ratio of the migration distance of the solute x (Zx-Z0) to that of the mobile phase (Zp-Z0):

As linear correlate between chromatographical behavior and chemical structure BateSmith and Westall [10] introduced the RM value:

Combining Eqs. 1 and 2 yields:

Thus, RM represents the logarithmic ratio of the distances between solute spot and solvent front (i.e., a measure of the solute interaction with the lipophilic, stationary phase) on the one hand and the migration distance of the solute (i.e., a measure of the migration of the solute with the hydrophilic, mobile phase) on the other hand.

The less polar a solute x, the stronger will be its interaction with the stationary phase, which is expressed by decreasing RF values and increasing RM values. According to Eq. (2), RM values below 0.5 give positive RM values and vice versa. Thus, RM represents the direct correlate of the lipophilicity of a solute, provided its estimation is based on pure or at least preferential partition chromatography. Correct RM values can only be obtained on the basis of precisely measured Rv values, which is achieved under two experimental prerequisites. First, starting and running points have to be evaluated densitometrically and not by hand. Second, for the determination of the true front, which is not identical with the visible front, front markers have to be used (for details, see [11]).

Biagi and coworkers were the first to use reversed-phase TLC for lipophilicity measurements systematically and to develop them further. The introduction of the RUv value, i.e., Rm extrapolated to modifier-free conditions, is the merit of this group. According to current view, only the i?Mw values exhibit significant comparability with log P. Biagi et al. [12-14] were able to show such interrelations for several pharmacological classes. The comprehensive work of this group is reviewed in several recent papers [15-17].

Physico-chemical conditions of reversed-phase TLC as well as its relevance for QSAR studies have been thoroughly discussed in an excellent review by Tomlinson [18]. He demonstrated the additive, constitutive character of Ru and the analogy of ARM to Jt as a fragmental constant for lipophilic group contributions. In addition, the impact of steric and electronic effects on RM was discussed in detail.

In 1965 Boyce and Milborrow [19] published the first QSAR paper applying Ru values as lipophilicity parameters in correlations with biological activity. In addition, the linear increase of RM with alkyl chain length of congeneric compounds impressively demonstrated the applicability of RM as a substitute of log P.

In the following sections we want to provide the reader with the theoretical and methodological background enabling the practitioner to derive precise and reproducible TLC data. Given up-to-date technology (e.g., densitometry) TLC represents an attractive experimental alternative to both HPLC and octanol/water partitioning.

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