Nordoxepin Explained

Nordoxepin, also known as N-desmethyldoxepin, is an organic compound. A colorless solid, it attracted attention as the major active metabolite of the tricyclic antidepressant (TCA) doxepin (Sinequan).^[1] It has been found to play a significant role in the antidepressant effects of doxepin.

Nordoxepin is a mixture of (E) and (Z) stereoisomers. Whereas pharmaceutical doxepin is supplied in an approximate 85:15 ratio mixture of (E)- and (Z)-stereoisomers and plasma concentrations of doxepin remain roughly the same as this ratio with treatment, plasma levels of the (E)- and (Z)-stereoisomers of nordoxepin, due to stereoselective metabolism of doxepin by cytochrome P450 enzymes, are approximately 1:1.

Nordoxepin is pharmacologically active similarly to doxepin,^[2] but relative to doxepin, is much more potent and selective as a norepinephrine reuptake inhibitor.^{[3] [4]} In general, the demethylated variants of tertiary amine TCAs like doxepin are much more potent inhibitors of norepinephrine reuptake, less potent inhibitors of serotonin reuptake, and less potent in their antiadrenergic, antihistamine, and anticholinergic activities.^[5]

Nordoxepin is formed from doxepin mainly by CYP2C19 (>50% contribution), while CYP1A2 and CYP2C9 are involved to a lesser extent, and CYP2D6 and CYP3A4 are not involved.^[6] Hydroxylation of doxepin and nordoxepin is mediated mainly by CYP2D6.^[7] Total exposures to both doxepin and nordoxepin differ by almost 10-fold in CYP2D6 ultra-rapid versus poor metabolizers. Both doxepin and nordoxepin are also transformed into glucuronide conjugates. The elimination half-life of nordoxepin is approximately 31 hours, which is almost twice that of doxepin (mean 17 hours).

See also

• Cidoxepin

Notes and References

1. Book: Williams DA . Antidepressant . Lemke TL, Williams DA . Foye's Principles of Medicinal Chemistry. https://books.google.com/books?id=Sd6ot9ul-bUC&pg=PA605. 24 January 2012. Lippincott Williams & Wilkins. 978-1-60913-345-0. 605–.
2. Weber J, Siddiqui MA, Wagstaff AJ, McCormack PL . Low-dose doxepin: in the treatment of insomnia . CNS Drugs . 24 . 8 . 713–720 . August 2010 . 20658801 . 10.2165/11200810-000000000-00000 . 26739281 .
3. Book: Dawkins K, Manji HK, Potter WZ . Pharmacodynamics of Antidepressants . Cutler NR, Sramek JJ, Narang PK . Pharmacodynamics and Drug Development: Perspectives in Clinical Pharmacology. https://books.google.com/books?id=ncRXa8Dq88QC&pg=PA160. 20 September 1994. John Wiley & Sons. 978-0-471-95052-3. 160–.
4. Book: Baumann P, Hiemke C . Central Nervous System Drugs . Anzenbacher P, Zanger UM . Metabolism of Drugs and Other Xenobiotics. https://books.google.com/books?id=f-XHh17NfwgC&pg=PA302. 23 February 2012. John Wiley & Sons. 978-3-527-64632-6. 302–.
5. Book: Nelson JG . Tricyclic and Tetracyclic Drugs. Schatzberg AF, Nemeroff CB . The American Psychiatric Publishing Textbook of Psychopharmacology . https://books.google.com/books?id=Xx7iNGdV25IC&pg=PA264 . 2009. American Psychiatric Pub. 978-1-58562-309-9. 264–.
6. Härtter S, Tybring G, Friedberg T, Weigmann H, Hiemke C . The N-demethylation of the doxepin isomers is mainly catalyzed by the polymorphic CYP2C19 . Pharmaceutical Research . 19 . 7 . 1034–1037 . July 2002 . 12180536 . 10.1023/a:1016478708902 . 8089917 .
7. Kirchheiner J, Henckel HB, Franke L, Meineke I, Tzvetkov M, Uebelhack R, Roots I, Brockmöller J . 6 . Impact of the CYP2D6 ultra-rapid metabolizer genotype on doxepin pharmacokinetics and serotonin in platelets . Pharmacogenetics and Genomics . 15 . 8 . 579–587 . August 2005 . 16007002 . 10.1097/01.fpc.0000167331.30905.9e . 41765748 .