Drug Metabolism: Phase I Reactions

Phase I reactions introduce a functionally reactive polar group to a drug molecule. This transformation occurs mainly in the liver, catalyzed by the cytochrome P450 system. If the resulting metabolite is polar enough, it is readily excreted in urine. If nonpolar, it undergoes drug metabolism phase II reactions to form a more excretable conjugate.

Phase I reactions occur predominantly in the liver, facilitated by the cytochrome P450 system of hemoproteins. These hemoproteins are located in the lipophilic endoplasmic reticulum of hepatic cells. This hepatic localization makes the liver the primary site for initial drug oxidation and metabolite formation.

The oxidized P450 form combines with the parent drug to form a binary complex. NADPH–P450 reductase reduces this complex, which then uses molecular oxygen and an electron to form an activated oxygen-P450-substrate complex. The activated oxygen transfers to the drug, producing the oxidized metabolite with a hydroxyl group.

The polarity of the phase I metabolite determines its fate. If the metabolite is sufficiently polar, it is readily excreted in urine due to its water compatibility. If the metabolite remains nonpolar, it must undergo phase II reactions to form a polar conjugate that can be more easily eliminated from the body.

NADPH–P450 reductase uses NADPH to reduce the binary drug-P450 complex during the monooxygenase cycle. This reduction step is essential for generating the activated oxygen-P450-substrate complex. Without this electron donation, the cycle cannot proceed to transfer activated oxygen to the drug substrate.

The hydroxyl group introduced during phase I oxidation increases the drug's polarity and reactivity. This functional modification makes the metabolite more water-soluble and suitable for urinary excretion if sufficiently polar. If still nonpolar, the hydroxyl group provides a site for phase II conjugation reactions.

Phase I reactions introduce polar groups to drugs, but if the resulting metabolite remains nonpolar, it must undergo subsequent phase II reactions. Phase II processes add conjugate groups to enhance water solubility and excretability. Together, these sequential reactions ensure drugs are transformed into forms the body can eliminate efficiently.