Lipophilicity Assays

Lipophilicity refers to the affinity of compounds for the lipid environment and is an important physicochemical parameter during drug discovery. Lipophilicity affects the transport of drugs through the lipid membranes and the interaction of drugs with target proteins, which can be used to predict the biological, pharmacokinetic, and metabolic properties of drugs. BOC Sciences offers lipophilicity and pK_a assays to help customers design new compounds with the right balance of lipophilicity.

Fig. 1 Characteristics of highly lipophilic compounds (logD 7.4 > 3.5)

Compared with lipophobic compounds, lipophilic compounds generally have higher biomembrane permeability, increasing their oral bioavailability through better absorption in the gastrointestinal tract. Lipophilicity plays an important role in solubility, absorption, membrane penetration, plasma protein binding, distribution, CNS penetration and partitioning into other tissues or organs such as the liver. In addition, lipophilicity plays an important role in ligand recognition for target proteins, CYP450 interactions, HERG binding, and PXR-mediated enzymatic induction. Lipophilicity can be measured by the distribution of the drug between the organic and aqueous phases. In drug development, the lipophilicity of a compound can be expressed by the partition coefficient logP or the partition coefficient logD.

LogD/LogP Assays (Shake-flask Method)

LogP refers to the distribution coefficient of molecules between the aqueous and lipophilic phases, usually octanol and water. The LogP shake flask method typically dissolves solutes in a volume of octanol and water, and shakes for a period of time to measure the concentration of solutes in each solvent. Typically, LogP only correctly describes the distribution coefficient of neutral (uncharged) molecules. The liposolubility of a compound is usually measured by LogD when the drug molecule has an ionizable group or is charged at a physiological pH. LogD assays typically use buffers to adjust the aqueous phase to a specific pH. In addition, since most compounds are ionized at physiological pH (7.4), the liposolubility at pH 7.4 is often determined, i.e. LogD_7.4. BOC Sciences has developed a standard LogD/LogP analysis protocol using the octanol/buffer shake flask method to determine lipophilicity and quantify samples by LC-MS/MS.

Fig. 2 The calculation formulas of lipophilicity

pK_a Assays

The acid dissociation coefficient (pK_a) can predict the ionization degree of the molecule at a specific pH value. Most drugs are weak acids or bases that exist in solutions in an equilibrium form between non-ionized and ionized forms. The pK_a of a compound affects its distribution in solution, which in turn affects its availability for physical, chemical, and biological reactions. In addition, the pK_a of a compound will affect properties such as logD and solubility, as well as absorption, distribution, metabolism, elimination, and potency of the compound. BOC Sciences offers several methods for assessing pK_a:

• pH-metric pKa. The pH-metric method is based on potentiometric acid-base titration. Regardless of whether the sample has a UV chromophore, this method can measure all pK_a between 2 and 12 as long as the sample is dissolved in the solution throughout the experiment.
• UV-metric pKa. The pH range of the UV-metric method is 1.0 to 13.0, which can provide pK_a results for samples with chromophores. The UV absorbance of these chromophores varies with pH.

RP-HPLC Assays

Reversed-phase high performance liquid chromatography (RP-HPLC) has the advantages of rapidity, accuracy, economy of compounds and solvents, and low sample volume for measuring the lipophilicity of compounds. Currently, this analytical technique has been used to quantify the lipophilicity of drug molecules. In RP-HPLC, LogD/LogP values are determined from changes in retention times of compounds and comparisons to a series of reference standards.

BOC Sciences is also committed to the method development of the determination of lipophilicity by thin layer chromatography (TLC). If you are interested in our lipophilicity assay services, please contact us to learn more.