One enzyme family, most of the work
The cytochrome P450 enzymes — CYPs — are a family of iron-containing enzymes embedded in the liver (and gut) that carry out the bulk of phase I metabolism. The name comes from a quirk of spectroscopy: when their heme iron binds carbon monoxide, the complex absorbs light strongly at 450 nm. That trivia aside, what they actually do is breathtaking chemistry: they insert a single oxygen atom into otherwise unreactive C–H bonds, using molecular oxygen and an electron supply.
Humans have dozens of CYP genes, but a small handful handle the majority of drugs. The single most important is [[cyp3a4|CYP3A4]], estimated to metabolise roughly half of all marketed small-molecule drugs. Others — CYP2D6, CYP2C9, CYP2C19, CYP1A2 — cover much of the rest. Each has a different pocket shape and substrate preference, which is why two similar molecules can be cleared by completely different routes.
Typical CYP-catalysed phase I reactions: Aromatic hydroxylation: Ar-H -> Ar-OH Aliphatic hydroxylation: R-CH3 -> R-CH2OH N-dealkylation: R2N-CH3 -> R2N-H + HCHO O-dealkylation: R-O-CH3 -> R-OH + HCHO S-oxidation: R-S-R' -> R-S(=O)-R' Epoxidation: C=C -> epoxide Net effect: a new -OH, -NH or other polar handle is exposed, ready for phase II conjugation.
Finding the soft spot
When a CYP attacks a molecule, it goes for the easiest spot — a position that is both electron-rich enough to react and physically accessible in the enzyme's pocket. Medicinal chemists call that vulnerable position a metabolic soft spot. Common soft spots are exposed methyl groups, benzylic positions, electron-rich aromatic rings, and N- or O-methyl groups (which fall off via dealkylation).
- Run the compound in a microsomal assay — liver microsomes plus the cofactor NADPH — and measure how fast it disappears.
- Identify the metabolites by mass spectrometry to see *where* the oxygen landed — that pinpoints the soft spot.
- Block or deactivate that position — add fluorine, swap the ring, remove the vulnerable methyl — and re-test.
- Confirm the change improved metabolic stability without killing potency, and iterate.