Brexanolone is used to treat postpartum depression in adult women.
Side effects of brexanolone include sedation (13–21%), dry mouth (3–11%), loss of consciousness (3–5%), and flushing (2–5%). It can also produce euphoria to a degree similar to that of alprazolam (3–13% at infusion doses of 90–270 μg over a one-hour period).
Allopregnanolone acts as a highly potent positive allosteric modulator of the GABAA receptor. While allopregnanolone, like other inhibitory neurosteroids such as THDOC, positively modulates all GABAA receptor isoforms, those isoforms containing δ subunits exhibit the greatest potentiation. Allopregnanolone has also been found to act as a positive allosteric modulator of the GABAA-ρ receptor, though the implications of this action are unclear. In addition to its actions on GABA receptors, allopregnanolone, like progesterone, is known to be a negative allosteric modulator of nACh receptors, and also appears to act as a negative allosteric modulator of the 5-HT3 receptor. Along with the other inhibitory neurosteroids, allopregnanolone appears to have little or no action at other ligand-gated ion channels, including the NMDA, AMPA, kainate, and glycine receptors.
Similarly to many other GABAA receptor positive allosteric modulators, allopregnanolone has been found to act as an inhibitor of L-type voltage-gated calcium channels (L-VGCCs), including α1 subtypesCav1.2 and Cav1.3. However, the threshold concentration of allopregnanolone to inhibit L-VGCCs was determined to be 3 μM (3,000 nM), which is far greater than the concentration of 5 nM that has been estimated to be naturally produced in the human brain. Thus, inhibition of L-VGCCs is unlikely of any actual significance in the effects of endogenous allopregnanolone. Also, allopregnanolone, along with several other neurosteroids, has been found to activate the G protein-coupled bile acid receptor (GPBAR1, or TGR5). However, it is only able to do so at micromolar concentrations, which, similarly to the case of the L-VGCCs, are far greater than the low nanomolar concentrations of allopregnanolone estimated to be present in the brain.
Biphasic actions at the GABAA receptor
Increased levels of allopregnanolone can produce paradoxical effects, including negative mood, anxiety, irritability, and aggression. This appears to be because allopregnanolone possesses biphasic, U-shaped actions at the GABAA receptor – moderate level increases (in the range of 1.5–2 nmol/L total allopregnanolone, which are approximately equivalent to luteal phase levels) inhibit the activity of the receptor, while lower and higher concentration increases stimulate it. This seems to be a common effect of many GABAA receptor positive allosteric modulators. In accordance, acute administration of low doses of micronized progesterone (which reliably elevates allopregnanolone levels) has been found to have negative effects on mood, while higher doses have a neutral effect.
The mechanism by which neurosteroid GABAA receptor PAMs like brexanolone have antidepressant effects is unknown. Other GABAA receptor PAMs, such as benzodiazepines, are not thought of as antidepressants and have no proven efficacy, despite clinicians prescribing Alprazolam for depression in the past. Neurosteroid GABAA receptor PAMs are known to interact with GABAA receptors and sub-populations differently than benzodiazepines. As examples, GABAA receptor-potentiating neurosteroids may preferentially target δ subunit-containing GABAA receptors, and enhance both tonic and phasic inhibition mediated by GABAA receptors. It is also possible that neurosteroids like allopregnanolone may act on other targets, including membrane progesterone receptors, T-type voltage-gated calcium channels, and others, to mediate antidepressant effects.
Allopregnanolone is a pregnane (C21) steroid and is also known as 5α-pregnan-3α-ol-20-one, 5α-Pregnane-3α-ol-20-one, 3α-hydroxy-5α-pregnan-20-one, or 3α,5α-tetrahydroprogesterone (3α,5α-THP). It is closely related structurally to 5-pregnenolone (pregn-5-en-3β-ol-20-dione), progesterone (pregn-4-ene-3,20-dione), the isomers of pregnanedione (5-dihydroprogesterone; 5-pregnane-3,20-dione), the isomers of 4-pregnenolone (3-dihydroprogesterone; pregn-4-en-3-ol-20-one), and the isomers of pregnanediol (5-pregnane-3,20-diol). In addition, allopregnanolone is one of four isomers of pregnanolone (3,5-tetrahydroprogesterone), with the other three isomers being pregnanolone (5β-pregnan-3α-ol-20-one), isopregnanolone (5α-pregnan-3β-ol-20-one), and epipregnanolone (5β-pregnan-3β-ol-20-one).
The biosynthesis of allopregnanolone in the brain starts with the conversion of progesterone into 5α-dihydroprogesterone by 5α-reductase. After that, 3α-hydroxysteroid dehydrogenase converts this intermediate into allopregnanolone. Allopregnanolone in the brain is produced by cortical and hippocampus pyramidal neurons and pyramidal-like neurons of the basolateral amygdala.
A variety of syntheticderivatives and analogues of allopregnanolone with similar activity and effects exist, including alfadolone (3α,21-dihydroxy-5α-pregnane-11,20-dione), alfaxolone (3α-hydroxy-5α-pregnane-11,20-dione), ganaxolone (3α-hydroxy-3β-methyl-5α-pregnan-20-one), hydroxydione (21-hydroxy-5β-pregnane-3,20-dione), minaxolone (11α-(dimethylamino)-2β-ethoxy-3α-hydroxy-5α-pregnan-20-one), Org 20599 (21-chloro-3α-hydroxy-2β-morpholin-4-yl-5β-pregnan-20-one), Org 21465 (2β-(2,2-dimethyl-4-morpholinyl)-3α-hydroxy-11,20-dioxo-5α-pregnan-21-yl methanesulfonate), and renanolone (3α-hydroxy-5β-pregnan-11,20-dione).
The 21-hydroxylated derivative of this compound, tetrahydrodeoxycorticosterone (THDOC), is an endogenous inhibitory neurosteroid with similar properties to those of allopregnanolone, and the 3β-methyl analogue of allopregnanolone, ganaxolone, is under development to treat epilepsy and other conditions, including post-traumatic stress disorder (PTSD).
In March 2019, brexanolone was approved in the United States for the treatment of postpartum depression (PPD) in adult women, the first drug approved by the U.S. Food and Drug Administration (FDA) specifically for PPD.
The efficacy of brexanolone was shown in two clinical studies in participants who received a 60-hour continuous intravenous infusion of brexanolone or placebo and were then followed for four weeks. The FDA approved allopregnanolone based on evidence from three clinical trials, conducted in the United States, (Trial 1/NCT02942004, Trial 3/NCT02614541, Trial 2/ NCT02942017) of 247 women with moderate or severe postpartum depression.
Allopregnanolone is the name of the molecule commonly used in the literature when it is discussed as an endogenous neurosteroid. Brexanolone is both the INN and the USAN in the context of its use as a medication.
In the United States, brexanolone is a Schedule IVcontrolled substance. Allopregnanolone is available only through a restricted program called the Zulresso REMS Program that requires the drug to be administered by a healthcare provider in a certified healthcare facility. The REMS requires that patients be enrolled in the program prior to administration of the drug.
It is given continuously by intravenous infusion over a period of 60 hours (2.5 days). The dosage of brexanolone is progressively adjusted over a range of 30 to 90 μg/kg/hour during this period.
Several warnings and precautions are addressed in a boxed warning in the drug's prescribing information. Because of the risk of serious harm due to the sudden loss of consciousness, patients must be monitored for excessive sedation and sudden loss of consciousness and have continuous pulse oximetry monitoring (monitors oxygen levels in the blood). While receiving the infusion, women must be accompanied during interactions with children. Women should be counseled on the risks of allopregnanolone treatment and instructed that they must be monitored for these effects at a healthcare facility for the entire 60 hours of infusion. Women who have received the treatment should not drive, operate machinery, or do other dangerous activities until feelings of sleepiness from the treatment have completely gone away.
Exogenous progesterone, such as oral progesterone, elevates allopregnanolone levels in the body with good dose-to-serum level correlations. Due to this, it has been suggested that oral progesterone could be described as a prodrug of sorts for allopregnanolone. As a result, there has been some interest in using oral progesterone to treat catamenial epilepsy, as well as other menstrual cycle-related and neurosteroid-associated conditions. In addition to oral progesterone, oral pregnenolone has also been found to act as a prodrug of allopregnanolone, though also of pregnenolone sulfate.
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