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Amphetamine Metabolism
  • Amphetamine is converted by hydroxylation (R-H → R-OH) to 4-hydroxyamphetamine.
  • It is also converted to norephedrine by deamination (R-C-NH2 → R-C=O) to phenylacetone.
  • The enzymes CYP2D6 and flavin-containing monoxygenase are the predominant enzymes involved in metabolism to the primary metabolites.
  • Secondary metabolites are produced by hydroxylation (R-H → R-OH) of 4-hydroxyamphetamine and 4-hydroxynorephedrine or by hydroxylation of norephedrine to 4-hydroxynorephedrine.
  • Phenyactone is converted by hydroxylation to 4-hydroxyphenylacetone or to benzoic acid by oxidation.
  • Benzoic acid is further metabolized to hippuric acid by glycine conjugation.
30 to 40% of amphetamine is excreted in the urine as the unchanged parent compound at normal urinary pH. At higher urine pH, amp is in the unionized free base form. As the free base, the molecule is more lipophilic and less polar, making it less soluble in urine and more prone to being reabsorbed by the renal tubules where it can cycle through the blood stream again. Therefore, there is less secretion of amphetamine when it is in this form. Conversely, in an acidic environment, amphetamine will be in the ionized form, which is more soluble in urine and easily excreted. The point here is that with a variation in pH, the concentration of amphetamine in the urine can be vary greatly.
Amphetamine can be detected in urine for two to three days after ingestion. The half-life of amphetamine is between seven and 32 hours, depending on urinary pH. The half-life increases with higher urinary pH due to free amphetamine reabsorption by the kidney tubules. Lower pH decreases the half-life. The half-life can also increase with repeated use and accumulation of the drug.
Interpretation of positive amphetamine results can be quite challenging due to its similarity in structure to many over-the-counter drugs as well as prescription drugs. Diet formulations and decongestants such as ephedrine, pseudoephedrine, phentermine, and phenylpropanolamine are examples. Amphetamine is prescribed clinically as Adderall.