Lipophilicity and Partition Coefficient (LogP)

Lipophilicity expresses a drug's relative affinity for lipid versus aqueous environments and is most commonly quantified as the logarithm of the octanol-water partition coefficient (logP). As a preformulation property, it conditions membrane permeability, solubility and partitioning behaviour, and is a key determinant of a candidate's developability.

Definition

The partition coefficient (P) is the equilibrium ratio of a compound's concentration in an organic phase (conventionally n-octanol) to its concentration in water for the un-ionized species, usually reported as logP; the distribution coefficient (logD) is the analogous pH-dependent ratio that includes ionized species.

Scope

The entry covers the definition and measurement of the partition coefficient (logP) for the un-ionized species, the distribution coefficient (logD) that accounts for ionization at a given pH, the relationship of lipophilicity to permeability and solubility, computational estimation, and its place in developability heuristics. It is reference content rather than a screening protocol.

Core questions

• How does lipophilicity (logP) relate to membrane permeability and aqueous solubility?
• What distinguishes the partition coefficient (logP) from the pH-dependent distribution coefficient (logD)?
• How is lipophilicity measured and estimated, and how does it inform developability?

Key concepts

• Partition coefficient (logP)
• Distribution coefficient (logD)
• Octanol-water system
• Lipophilicity-permeability relationship
• Lipophilicity-solubility trade-off
• Shake-flask and chromatographic measurement
• Computational logP estimation

Mechanisms

Lipophilicity captures how readily a molecule leaves water to enter a lipid-like phase, modelled by partitioning between n-octanol and water. The un-ionized partition coefficient (logP) reflects intrinsic affinity, while the distribution coefficient (logD) folds in the fraction ionized at a given pH and so better represents behaviour under physiological conditions for ionizable drugs. Moderate lipophilicity favours passive permeation across lipid membranes, but increasing logP generally lowers aqueous solubility, producing a trade-off central to developability: too low and a molecule permeates poorly, too high and it dissolves poorly and may bind nonspecifically. Lipophilicity is measured by shake-flask or chromatographic methods and estimated computationally from structure, and it appears as a parameter in widely used drug-likeness heuristics.

Clinical relevance

Because lipophilicity shapes absorption and distribution, it helps explain why structurally similar drugs differ in oral exposure, and it is used to flag developability risks early. This entry describes a physicochemical property for reference and educational purposes and is not a basis for prescribing or individualized therapy.

Evidence & guidelines

Leo, Hansch and Elkins (1971) established the systematic use of partition coefficients in medicinal chemistry, and Lipinski et al. (2001) embedded lipophilicity in developability heuristics relating it to solubility and permeability. Its biopharmaceutical role connects to the permeability dimension of the classification framework of Amidon et al. (1995).

History

The quantitative use of the octanol-water partition coefficient grew out of the structure-activity work of Hansch and Leo, whose 1971 review systematized partition coefficients and their measurement and estimation. Lipinski et al. (2001) later popularized lipophilicity as a developability filter, cementing logP and logD as routine preformulation parameters.

Key figures

• Corwin Hansch
• Albert Leo
• Christopher A. Lipinski

Seminal works

• leo-1971
• lipinski-2001

Frequently asked questions

What is the difference between logP and logD?

LogP is the partition coefficient of the un-ionized form of a compound between octanol and water, whereas logD is the pH-dependent distribution coefficient that accounts for both ionized and un-ionized species at a specified pH.

Why does lipophilicity matter in preformulation?

Lipophilicity influences both membrane permeability and aqueous solubility in opposite directions, so it helps anticipate how well a candidate will be absorbed and whether solubility enhancement may be needed.