CYP3A4

If cytochrome P450 is the body's demolition crew, CYP3A4 is its busiest and most versatile worker. It is the single most abundant drug-metabolizing P450 in the human liver and gut, and it is estimated to be involved in clearing roughly half of all marketed small-molecule drugs.

CYP3A4's standout feature is a large, flexible active site that can accommodate molecules of very different sizes and shapes — from small painkillers to bulky immunosuppressants. This promiscuity makes it the default route by which many structurally unrelated drugs are oxidized, but it also makes CYP3A4 a hotspot for drug–drug interactions, because so many compounds compete for, inhibit, or induce the same enzyme.

Because it lines the wall of the small intestine as well as the liver, CYP3A4 contributes heavily to first-pass metabolism, lowering the oral bioavailability of its substrates before they ever reach systemic circulation. Medicinal chemists routinely check whether a candidate is a CYP3A4 substrate, inhibitor, or inducer, since any of these properties can complicate dosing or co-medication.