Hydroxybupropion (code name BW 306U), or 6-hydroxybupropion, is the major

active metabolite An active metabolite, or pharmacologically active metabolite is a biologically active metabolite of a xenobiotic substance, such as a drug or environmental chemical. Active metabolites may produce therapeutic effects, as well as harmful effects. ...

of the

antidepressant Antidepressants are a class of medications used to treat major depressive disorder, anxiety disorders, chronic pain, and addiction. Common side effects of antidepressants include Xerostomia, dry mouth, weight gain, dizziness, headaches, akathi ...

and

smoking cessation Smoking cessation, usually called quitting smoking or stopping smoking, is the process of discontinuing tobacco smoking. Tobacco smoke contains nicotine, which is Addiction, addictive and can cause Substance dependence, dependence. As a resu ...

drug

bupropion Bupropion, formerly called amfebutamone, and sold under the brand name Wellbutrin among others, is an atypical antidepressant that is indicated in the treatment of major depressive disorder, seasonal affective disorder, and to support smo ...

. It is formed from bupropion by the

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

CYP2B6 Cytochrome P450 2B6 is an enzyme that in humans is encoded by the ''CYP2B6'' gene. CYP2B6 is a member of the cytochrome P450 group of enzymes. Along with CYP2A6, it is involved with metabolizing nicotine, along with many other substances. Func ...

during

first-pass metabolism The first pass effect (also known as first-pass metabolism or presystemic metabolism) is a phenomenon of drug metabolism at a specific location in the body which leads to a reduction in the concentration of the active drug before it reaches the ...

. With oral bupropion treatment, hydroxybupropion is present in plasma at

area under the curve In mathematics, an integral is the continuous analog of a sum, which is used to calculate areas, volumes, and their generalizations. Integration, the process of computing an integral, is one of the two fundamental operations of calculus,Inte ...

concentrations that are as many as 16 to 20 times greater than those of bupropion itself, demonstrating extensive conversion of bupropion into hydroxybupropion in humans. As such, hydroxybupropion is likely to play a very important role in the effects of oral bupropion, which could accurately be thought of as functioning largely as a

prodrug A prodrug is a pharmacologically inactive medication or compound that, after intake, is metabolized (i.e., converted within the body) into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be ...

to hydroxybupropion. Hydroxybupropion has two

chiral center In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...

s and is a

mixture In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...

of four possible

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s. In humans however, presumably due to

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...

, only (2''R'',3''R'')-hydroxybupropion and (2''S'',3''S'')-hydroxybupropion are formed. Other metabolites of bupropion besides hydroxybupropion include

threohydrobupropion Threohydrobupropion (developmental code names BW 494, BW A494U) is a substituted amphetamine derivative—specifically a β-hydroxyamphetamine—and a major active metabolite of the antidepressant drug bupropion (Wellbutrin). Bupropion is a no ...

and

erythrohydrobupropion Erythrohydrobupropion (developmental codes BW 287, BW 17U) is a substituted amphetamine derivative—specifically a β-hydroxyamphetamine—and a minor active metabolite of the antidepressant drug bupropion (Wellbutrin). Bupropion is a norepine ...

Pharmacology

Pharmacodynamics

Compared to bupropion, hydroxybupropion is similar in its

potency Potency may refer to: * Potency (pharmacology), a measure of the activity of a drug in a biological system * Virility * Cell potency, a measure of the differentiation potential of stem cells * In homeopathic dilutions, potency is a measure of ho ...

as a

norepinephrine reuptake inhibitor A norepinephrine reuptake inhibitor (NRI, NERI) or noradrenaline reuptake inhibitor or adrenergic reuptake inhibitor (ARI), is a type of drug that acts as a reuptake inhibitor for the neurotransmitters norepinephrine (noradrenaline) and epinep ...

( IC₅₀ = 1.7 μM), but is substantially weaker as a

dopamine reuptake inhibitor A dopamine reuptake inhibitor (DRI) is a class of drug which acts as a reuptake inhibitor of the monoamine neurotransmitter dopamine by blocking the action of the dopamine transporter (DAT). Reuptake inhibition is achieved when extracellular dop ...

(IC₅₀ = >10 μM). Like bupropion, hydroxybupropion is also a

non-competitive antagonist A receptor antagonist is a type of Receptor (biochemistry), receptor ligand (biochemistry), ligand or drug that blocks or dampens a biological response by binding to and blocking a Receptor (biochemistry), receptor rather than activating it like ...

nACh receptor Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ner ...

s, such as α₄β₂ and α₃β₄, but is even more potent in comparison.

Pharmacokinetics

Bupropion is extensively and rapidly absorbed in the gastrointestinal tract but experiences extensive first pass metabolism rendering its systemic bioavailability limited. Exact bioavailability has yet to be determined given an intravenous form does not exist. Absorption is suggested to be between 80 and 90%. Its distribution half-life is between 3–4 hours and exhibits moderate human plasma protein binding (between 82 and 88%) with the parent compound and hydroxybupropion displaying the highest affinity. Bupropion is a racemic mixture and is metabolized hepatically primarily via oxidative cleavage of its side chains by CYP2B6. Hydroxybupropion is the most potent of the metabolites. It is formed via the "''hydroxylation of the tert-butyl group''" by CYP2B6 and is excreted renally. C_max values of hydroxybupropion are 4–7 times that of bupropion, while the exposure to hydroxybupropion is "10 fold" that of bupropion. Hydroxybupropion's elimination half-life is roughly 20 hours, give or take 5 hours and will reach steady state concentrations within 8 days.

Chemistry

Hydroxybupropion is a

racemic mixture In chemistry, a racemic mixture or racemate () is a mixture that has equal amounts (50:50) of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as r ...

of (''R'',''R'')-hydroxybupropion and (''S'',''S'')-hydroxybupropion.

Research

Although there are patents proposing uses and formulations of this compound, hydroxybupropion is not currently marketed as a drug in and of itself and is only available for use in non-clinical research. Hydroxybupropion is not a scheduled drug or a controlled substance. One can access GLP (Good Lab Practice) documents detailing assays/techniques to further research and isolate this drug. Otherwise, there is little regulatory data available for hydroxybupropion at this time. Moreover, there is little information to suggest hydroxybupropion has an abuse potential. However, it has been studied as a possible therapeutic for

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

and nicotine use as a codrug. There are few clinical trials or toxicology studies assessing hydroxybupropion alone at this time. There are clinical studies which assess hydroxybupropion in conjunction with bupropion suggesting hydroxybupropion to be the primary form of the compound responsible for its clinical efficacy. Also, transdermal delivery of bupropion and hydroxybupropion has been assessed finding bupropion to be the superior candidate given its elevated diffusion rate through skin samples. There are few toxicology studies assessing hydroxybupropion alone at this time. However, there are some studies which assess this compound in conjunction with others or its parent compound.

References