Drug-Like Properties: Solubility, Permeability and Metabolic Fate
A potent compound is useless if it does not dissolve, does not cross a membrane or is cleared in minutes. Those three properties trade against each other, and most of the medicinal chemist's work is finding a structure that keeps all of them acceptable.
What a learner can do afterwards
- Predicts how adding a polar group changes solubility and permeability in opposite directions
- Uses the ionisation constant and the pH of a compartment to say what fraction is neutral
- Names the common sites of oxidative metabolism in a structure and proposes a way to block one
- Explains why a compound with excellent potency can still fail on exposure
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Solubility is a tug of war between the crystal lattice holding molecules together and water pulling them apart. Like dissolves like: polar and charged groups welcome water while greasy parts refuse it. Adding a polar group almost always lifts solubility, yet that same polarity makes crossing fatty gut membranes harder. The two properties pull in opposite directions.
Acids donate protons and bases accept them, and each acid travels with a conjugate base partner. Only the neutral form slips through membranes easily. The Henderson Hasselbalch relation turns pH and pKa into the exact neutral fraction, so always state the compartment first: blood near pH 7.4 and the stomach near pH 2 give the same drug wildly different neutral shares.
Metabolism is mostly oxidation: liver enzymes steal electrons or insert oxygen at exposed C-H bonds, amines, and sulfur atoms. Common fixes block the soft site with fluorine, add bulk nearby, or lower the electron richness of the ring. A compound with great potency but a five-minute half life is a metabolism problem, not a potency problem.
Weigh all three properties before you celebrate a binder. Solubility gets it dissolved, permeability gets it across, and slow clearance keeps it around. Buffers hold compartment pH steady, which is why your charge predictions stay put long enough to matter for absorption.
Dissolve it, keep it neutral enough to cross, and shield it from fast oxidation.
More to read
11.3 Solubility
Solubility is a tug of war between the crystal lattice holding molecules together and water pulling them apart. Like dissolves like: polar and charged groups welcome water, greasy parts do not. Adding a polar group almost always lifts solubility, which is why salts of drugs dissolve far better than neutral forms. But that same polarity makes crossing fatty gut membranes harder, so solubility and permeability pull in opposite directions.
14.1 Brønsted-Lowry Acids and Bases
Acids donate protons and bases accept them, and every acid comes with a conjugate base partner. The strength of the acid, captured in its Ka, decides how much sits neutral versus charged at a given pH. Only the neutral form slips through membranes easily, so an acid with a pKa far above gut pH absorbs well while one far below it stays charged and stuck. Sketching the two forms at the compartment pH is the first move in any permeability call.
14.2 pH and pOH
pH is the negative log of hydrogen ion concentration, so each unit is a factor of ten. The Henderson Hasselbalch relation turns pH and pKa into an exact neutral fraction, which is the number permeability cares about. Blood at pH 7.4 and the stomach near pH 2 can give the same drug wildly different neutral fractions. Always state the compartment before predicting charge.
14.6 Buffers
A buffer is a weak acid plus its conjugate base, and it soaks up added acid or base with barely a pH shift. Blood and cells are buffered, which is why a drug meets a steady pH rather than whatever it brings along. Formulation teams use the same trick to keep a drug dissolved in the vial. Buffers explain why compartment pH values stay put long enough to matter for absorption.
17.1 Balancing Oxidation-Reduction Reactions
Metabolism is mostly oxidation: liver enzymes steal electrons or insert oxygen at exposed C-H bonds, amines and sulfur atoms. Balancing redox equations trains the eye to spot who loses electrons, which is exactly the skill for finding metabolic soft spots. Common fixes are blocking the site with fluorine, adding bulk nearby, or lowering the electron richness of the ring. A compound with great potency but a five minute half life is a metabolism problem, not a potency problem.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.