The three gates a drug must pass
ADMET stands for absorption, distribution, metabolism, excretion, and toxicity — the journey a molecule takes through the body. For an oral drug, three early gates dominate optimization: it must dissolve (aqueous solubility), it must cross the gut wall (permeability), and it must survive the liver long enough to work (metabolic stability). Lose any one and the molecule is dead, no matter how potent.
Repairing solubility and permeability
Solubility problems usually trace to molecules that are too flat, too crystalline, or too greasy. The chemist's toolkit:
- Lower lipophilicity by trimming a greasy group or swapping a phenyl for a more polar ring — often the fastest win.
- Break planarity by introducing an sp3 center or a twist, which disrupts tight crystal packing and raises solubility.
- Add an ionizable group (e.g. a basic amine) in a region that tolerates it, dramatically boosting aqueous solubility.
Permeability problems often come from too much polarity or too many hydrogen-bond donors. Here you do the reverse: cap a donor, mask a polar group as a bioisostere, or use a first-pass-aware prodrug. The skill is fixing one gate without breaking the other — you are threading a needle, not turning a single dial.
Blocking metabolic soft spots
Most small molecules are cleared by liver enzymes, especially cytochrome P450. If a molecule is metabolized too fast, it is gone before it can act. The fix begins with identifying the metabolic soft spot — the exact atom the enzyme attacks — usually by running a microsomal assay and characterizing the metabolites.
Once you know the soft spot, you block it: install a fluorine where a hydroxylation occurs, replace a labile methyl, or swap a metabolically fragile ring for a sturdier bioisostere. The classic move is the fluorine block — fluorine is small enough to fit but shuts down oxidation at that position. Done well, you extend half-life without disturbing potency.