Terbutaline

Terbutaline (trade names Brethine, Bricanyl, Brethaire, or Terbulin) is a β2-adrenergic receptor agonist.

Terbutaline is currently on the World Anti-Doping Agency's list of prohibited drugs for Olympic athletes, except when administered by inhalation and a Therapeutic Use Exemption (TUE) has been obtained in advance.

Terbutaline is currently used to delay preterm labor for 48 hours to allow for fetal lung maturity through steroid injections. It should not be used to prevent preterm labor or delay labor past 48-72 hours. In February 2011, the Food and Drug Administration has ordered to put a boxed warning on the drug's label. Pregnant women should not be given injections of the drug terbutaline for the prevention of preterm labor or for long-term (beyond 48-72 hours) management of preterm labor, and should not be given oral terbutaline for any type of prevention or treatment of preterm labor "due to of the potential for serious internal heart problems and death."

The American College of Obstetricians and Gynecologists also discourages the use of terbutaline for preventing preterm labor.

Uses
Terbutaline is used as a fast-acting bronchodilator (often used as a short-term asthma treatment) and as a tocolytic to delay premature labor. The inhaled form of terbutaline starts working within 15 minutes and can last up to 6 hours.

Terbutaline as a treatment for premature labor is an off-label use not approved by the FDA. It is a pregnancy category 'B' medication and is routinely prescribed to stop contractions. After successful intravenous tocolysis, little evidence exists that oral terbutaline is effective. However, following uterine inversion in the third stage of pregnancy, Terbutaline (or either Halothane or magnesium sulfate) can be utilized to relax the uterus if necessary prior to uterine replacement.

Structure activity relationships
The tertiary butyl group in terbutaline makes it more selective for β2-receptors. Since there is no hydroxy group on the 4 position of the benzene ring - the molecule is less susceptible to metabolism by COMT enzyme.

Side effects

 * Maternal - tachycardia, nervousness, tremors, headache, hyperglycemia, hypokalemia, and rarely, pulmonary edema.
 * Fetal - tachycardia and hypoglycemia.

Chemistry
It is synthesized by brominating 3,5-dibenzyloxyacetophenone into the appropriate 3,5-dibenzyloxybromoacetophenone, which is reacted with N-benzyl-N-tert-butylamine, giving the aminoketone. Reduction of this product by hydrogen over a palladium catalyst leads to terbutaline.