DXM

Dextromethorphan (DXM or DM) is an antitussive (cough suppressant) drug found in many over-the-counter cold and cough medicines. Dextromethorphan has also found other uses in medicine, ranging from pain relief to psychological applications. Pure dextromethorphan occurs as a powder made up of white crystals, but it is generally administered via syrups, tablets, or lozenges manufactured under several different brand names and generic labels.

Chemistry

Dextromethorphan is the dextrorotatory enantiomer of the methyl ether of levorphanol, a narcotic analgesic. It is also a stereoisomer of levomethorphan, an opioid analgesic. It is named according to IUPAC rules as (+)-3-methoxy-17-methyl-9α,13α,14α-morphinan. As the pure free base, dextromethorphan occurs as an odorless, white to slightly yellow crystalline powder. It is freely soluble in chloroform and essentially insoluble in water. Commercially, dextromethorphan is commonly available as the monohydrated hydrobromide salt, although some newer extended-release formulations contain dextromethorphan bound to an ion exchange resin based on polystyrene sulfonic acid. Dextromethorphan's specific rotation in water is +27.6° (20°C, Sodium D-line).

Indications

The primary use of dextromethorphan is as a cough suppressant, for the temporary relief of cough caused by minor throat and bronchial irritation (as commonly accompanies the common cold), as well as other causes such as inhaled irritants.

Pharmacokinetics

At therapeutic doses, dextromethorphan acts centrally (meaning that it acts on the brain) as opposed to locally (on the respiratory tract). It elevates the threshold for coughing, without inhibiting ciliary activity. Dextromethorphan is rapidly absorbed from the gastrointestinal tract and converted into an active metabolite within 15 to 60 minutes of ingestion. The average dosage necessary for effective antitussive therapy is between 15 mg and 60 mg, depending on age. The duration of action after oral administration is approximately three to eight hours for dextromethorphan-hydrobromide, and ten to twelve hours for dextromethorphan-polistirex.

Because administration of dextromethorphan can trigger a histamine release (an allergic reaction), its use in atopic children is very limited.

Side-effects

Dextromethorphan can also cause other gastrointestinal disturbances. When injected directly into the blood stream, some studies suggest that dextromethorphan has the potential to cause Olney's Lesions.^[7]^[8] In some rare documented cases, dextromethorphan has produced psychological dependence in people who abused it. However, it does not produce physical addiction, according to the WHO Committee on Drug Dependence.

Drug interactions

Contraindications

Because dextromethorphan can trigger a histamine release (allergic reaction), atopic children, who are especially susceptible to allergic reactions, should be administered dextromethorphan only if absolutely necessary, and only under the strict supervision of a health care professional.^[6]

Clinical pharmacology

Following oral administration, dextromethorphan is rapidly absorbed from the gastrointestinal tract, where it enters the bloodstream and crosses the blood-brain barrier. The first-pass through the hepatic portal vein results in some of the drug being metabolized into an active metabolite of dextromethorphan, dextrorphan, the 3-hydroxy derivative of dextromethorphan. The therapeutic activity of dextromethorphan is believed to be caused by both the drug and this metabolite. Dextromethorphan is metabolized by various liver enzymes and subsequently undergoes O-demethylation (producing dextrorphan), N-demethylation, and partial conjugation with glucuronic acid and sulfate ions. Hours after dextromethorphan therapy, (in humans) the metabolites (+)-3-hydroxy-N-methylmorphinan, (+)-3-morphinan, and traces of the unchanged drug are detectable in the urine.^[6]

A major metabolic catalyst involved is the cytochrome P450 enzyme known as 2D6, or CYP2D6. A significant portion of the population has a functional deficiency in this enzyme and are known as poor CYP2D6 metabolizers. As CYP2D6 is a major metabolic pathway in the inactivation of dextromethorphan, the duration of action and effects of dextromethorphan can be increased by as much as three times in such poor metabolizers.^[9]

A large number of medications (including antidepressants) are potent inhibitors of CYP2D6. There exists, therefore, the potential of interactions between dextromethorphan and concomitant medications. Dextromethorphan crosses the blood-brain barrier, and the following pharmacological actions have been reported:

Brand names

Dextromethorphan is an active ingredient in many brand-name cough suppressant preparations, including:

History

Dextromethorphan was first patented under U.S. Patent 2,676,177 . The FDA approved dextromethorphan for over-the-counter sale as a cough suppressant in 1958. This filled the need for a cough suppressant lacking the sedative side-effects, stronger potential for abuse, and physically addictive properties of codeine phosphate, the most widely-used cough medication at the time (now prescription-only in the United States).^[14] As with most cough suppressants, studies show that dextromethorphan's effectiveness is highly debatable,^[15] especially in children.^[16]

During the 1960s and 1970s, dextromethorphan became available in an over-the-counter tablet form by the brand name Romilar. In 1973, Romilar was taken off the shelves after a burst in sales due to frequent abuse, and was replaced by cough syrup in an attempt to cut down on abuse.^[14]

More recently (around 2000) gel capsule forms began reappearing in the form of Robitussin CoughGels as well as several generic forms of that preparation.

Recreational use

Since their introduction, over-the-counter preparations containing dextromethorphan have been used in a manner inconsistent with their labeling, often as a recreational drug.^[14] At doses higher than medically recommended, dextromethorphan is classified as a dissociative hallucinogenic drug, with visible effects that are similar to ketamine and phencyclidine (PCP).

References

^ Plasma profile and pharmacokinetics of dextromethorphan after intravenous and oral administration. Journal of Veterinary Pharmacology and Therapeutics.
^ DEXTROMETHORPHAN (Street Names: DXM, CCC, Triple C, Skittles, Robo, Poor Man’s PCP)
^ Psychology Today's Diagnosis Dictionary: Hallucinogens
^ Brooks B, Thisted R, Appel S, Bradley W, Olney R, Berg J, Pope L, Smith R (2004). "Treatment of pseudobulbar affect in ALS with dextromethorphan/quinidine: a randomized trial.". Neurology 63 (8): 1364-70. PMID 15505150.
^ Cough Drug May Help Fibromyalgia Pain. WebMD.
^ ^a ^b ^c ^d ^e ^f ^g Dextromethorphan. NHTSA.
^ Olney J, Labruyere J, Price M (1989). "Pathological changes induced in cerebrocortical neurons by phencyclidine and related drugs". Science 244 (4910): 1360-2. PMID 2660263.
^ Hargreaves R, Hill R, Iversen L. "Neuroprotective NMDA antagonists: the controversy over their potential for adverse effects on cortical neuronal morphology". Acta Neurochir Suppl (Wien) 60: 15-9. PMID 7976530.
^ Clinical Pharmacology & Therapeutics - Abstract of article: The influence of CYP2D6 polymorphism and quinidine on the disposition and antitussive effect of dextromethorphan in humans[ast. Retrieved on 2007-07-16.
^ British Journal of Pharmacology - The dextromethorphan analog dimemorfan attenuates kainate-induced seizures via [sigma1 receptor activation: comparison with the effects of dextromethorphan]. Retrieved on 2007-07-16.
^ Hernandez SC, Bertolino M, Xiao Y, Pringle KE, Caruso FS, Kellar KJ (2000). "Dextromethorphan and its metabolite dextrorphan block alpha3beta4 neuronal nicotinic receptors". J. Pharmacol. Exp. Ther. 293 (3): 962-7. PMID 10869398.
^ Kamei J, Mori T, Igarashi H, Kasuya Y (1992). "Serotonin release in nucleus of the solitary tract and its modulation by antitussive drugs". Res. Commun. Chem. Pathol. Pharmacol. 76 (3): 371-4. PMID 1636059.
^ Verma A, Moghaddam B (Jan 1996). "NMDA receptor antagonists impair prefrontal cortex function as assessed via spatial delayed alternation performance in rats: modulation by dopamine". Journal of Neuroscience 1: 373-9.
^ ^a ^b ^c Dextromethorphan (DXM) | CESAR
^ Cough medicines "have no benefit" BBC News: Health, Tuesday, July 6, 2004. Accessed July 28, 2007.
^ [1] "Kids' cough medicine no better than placebo" San Francisco Chronicle, July 8, 2004

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