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Dextrorphan (DX) is a pharmacologically active metabolite of dextromethorphan (DM).
Chemistry
It is the result of O-demethylation of the prodrug by several enzymatic systems, although it is chiefly a product of the Cytochrome P450 IID6 (CYP4502D6) pathway.
Pharmacology
Being structurally similar to dextromethorphan, and it has affinity for the same receptors in the central nervous system but with a slight difference in selectivity.
Dextromethorphan is a weak non-competitive NMDA receptor antagonist but dextrorphan is a more potent antagonist*. DX is a strong anti-tussive, but is slightly less effective than DM, and has no metabolites with significant therapeutic activity in such capacity and therefore is not a drug available on the market.
Antagonism of the NDMA receptor is thought to impede the development of tolerance to opiates.
See also
"Dextrorphan"