The metabolism of N-hydroxyphenacetin in vitro and in vivo

Xenobiotica. 1985 Nov;15(11):915-27. doi: 10.3109/00498258509045045.


N-Hydroxyphenacetin (100 mg/kg) injected i.p. into rats rapidly appeared in the blood and disappeared with a t1/2 of 14 min; phenacetin and 4-acetamidophenol were major metabolites in blood. Ferrihaemoglobin was formed, but 4-nitrosophenetole was not detected in blood. N-Hydroxyphenacetin injected i.p. into rats was excreted in the urine unchanged (partly conjugated 2.1% of the dose, 2% was excreted as phenacetin, 19% as 4-acetamidophenol) and 1.8% as 2-hydroxyphenacetin. In addition, small amounts of 3-hydroxyphenacetin (0.4%) and traces of N-[4-(2-hydroxyethoxy)phenyl]acetamide (beta-HAP) (0.05%) were found. Time-course kinetics have shown that N-hydroxyphenacetin is metabolized in vitro to phenacetin, 2- and 3-hydroxyphenacetin, and 4-acetamidophenol by microsomal and cytosolic preparations of rat and rabbit liver. However, after the initial reaction, the formation of phenacetin and 2- and 3-hydroxyphenacetin did not continue with time, indicating that these products were not formed enzymically. N-Hydroxyphenacetin incubated with rat erythrocytes formed ferrihaemoglobin; the relationship between ferrihaemoglobin, phenacetin and 4-nitrosophenetole concn indicated that N-hydroxyphenacetin was oxidized by oxyhaemoglobin to acetyl 4-ethoxyphenyl nitroxide, which yielded phenacetin and 4-nitrosophenetole spontaneously.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetaminophen / blood
  • Animals
  • Erythrocytes / metabolism
  • Female
  • Guinea Pigs
  • In Vitro Techniques
  • Kinetics
  • Liver / metabolism
  • Male
  • Methemoglobin / metabolism*
  • Phenacetin / analogs & derivatives*
  • Phenacetin / blood
  • Phenacetin / metabolism
  • Phenacetin / urine
  • Rabbits
  • Rats


  • N-hydroxyphenacetin
  • 4-nitrosophenetole
  • Acetaminophen
  • Methemoglobin
  • Phenacetin