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Alfaxalone

Alfaxalone (also spelled alphaxalone) is a synthetic neuroactive steroid of the pregnane class that produces general anesthesia by acting as a positive allosteric modulator (a molecule that amplifies a receptor's response to its own neurotransmitter) at the GABA-A receptor, the principal inhibitory ion channel of the central nervous system. First introduced in 1971 as the main active component of the intravenous anesthetics Althesin (human) and Saffan (veterinary), it was withdrawn from human use in the 1980s because the solubilizing vehicle Cremophor EL provoked anaphylactoid reactions, then reintroduced in a cyclodextrin formulation marketed for veterinary anesthesia as Alfaxan. Because it lacks classical hormonal activity yet retains rapid, non-cumulative central depressant effects, alfaxalone occupies a distinct pharmacological niche among intravenous anesthetics and now serves as a template for a new generation of water-soluble neurosteroid sedatives such as Phaxan.

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Anesthetic

Hydroxydione

Hydroxydione is a synthetic pregnane neuroactive steroid that, introduced by Pfizer in 1955 under the trade name Viadril (also Presuren), became the first steroid used as an intravenous general anesthetic. The parent steroid, 21-hydroxy-5-beta-pregnane-3,20-dione, is almost insoluble in water; esterifying its 21-hydroxyl group with succinic acid and forming the sodium salt yields hydroxydione sodium succinate, a water-soluble prodrug that plasma esterases (enzymes that cleave ester bonds) hydrolyze in vivo to release the active steroid. Like later neurosteroid anesthetics such as alfaxalone, it produces hypnosis by acting as a positive allosteric modulator of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), without meaningful analgesic or classical hormonal activity. Although valued for a wide margin of cardiovascular and respiratory safety, its very slow onset and a high incidence of injection-site thrombophlebitis (painful inflammation and clotting of the vein) led to its abandonment, yet it remains historically pivotal as the origin of the entire field of steroid anesthesia.

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Anesthetic

Minaxolone

Minaxolone is a synthetic water-soluble aminosteroid (a steroid carrying a basic amino group) developed by Glaxo in the late 1970s as an intravenous general anaesthetic and a successor to Althesin. Structurally it is a 2-beta-ethoxy, 11-alpha-(dimethylamino) derivative of the neuroactive pregnane skeleton, and it acts as a potent positive allosteric modulator at the neurosteroid recognition site of the GABA-A receptor (the brain's principal inhibitory chloride ion channel), enhancing inhibitory conductance to produce hypnosis and surgical anaesthesia. Its basic amino group conferred solubility in water, avoiding the Cremophor EL vehicle (a polyethoxylated castor oil solubiliser) that made earlier lipophilic steroid anaesthetics prone to anaphylactoid reactions. After clinical trials conducted between roughly 1979 and 1982, its development was halted over toxicity and genotoxicity concerns arising in long-term rodent studies, and it was never marketed.

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Anesthetic

Org 20599

Org 20599 is a synthetic water-soluble aminosteroid general anaesthetic developed by Organon in the 1990s as a member of the 2-beta-morpholinyl pregnane class. Chemically it is (2beta,3alpha,5alpha)-21-chloro-3-hydroxy-2-(4-morpholinyl)pregnan-20-one, usually handled as its methanesulphonate (mesylate) salt, and it retains the 3-alpha-hydroxy, 5-alpha-reduced pregnane pharmacophore that defines the natural neurosteroid anaesthetics such as alphaxalone and allopregnanolone. Its principal action is positive allosteric modulation of the GABA-A receptor (the brain's main inhibitory chloride ion channel), with additional direct channel activation (GABA-mimetic agonism) at higher concentrations. It produced rapid-onset, short-duration anaesthesia in rodents but was never marketed; its close analogue Org 21465 reached Phase I human trials and caused excitatory involuntary movements in every volunteer, and the class was discontinued.

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Anesthetic

Org 21465

Org 21465 (also written ORG-21465) is a water-soluble aminosteroid intravenous anaesthetic developed by Organon in the 1990s as a companion candidate to the earlier compound Org 20599. It belongs to a series of 2-substituted (C2-morpholinyl) pregnane steroids engineered to combine the favourable receptor pharmacology of neuroactive steroids with the aqueous solubility needed for a practical injectable formulation, acting principally as a positive allosteric modulator (an agent that amplifies a receptor's response without being the primary trigger) of the GABA-A receptor (the brain's main inhibitory chloride ion channel). In computer-controlled infusion studies in healthy male volunteers the compound produced dose-related sedation and reversible loss of consciousness, but it was accompanied by prominent involuntary excitatory movements, injection-site venous pain, and slow equilibration with the effect site, and its clinical development was subsequently abandoned. It survives in the literature as a historical, niche illustration of the difficulty of translating neurosteroid GABA-A pharmacology into a usable anaesthetic.

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Anesthetic

Renanolone

Renanolone (3alpha-hydroxy-5beta-pregnane-11,20-dione) is a synthetic pregnane steroid that was investigated as an intravenous general anaesthetic during the Glaxo steroid-anaesthetic program of the 1960s and 1970s. It is the 5beta-configured epimer of alphaxalone (alfaxalone), differing only in the stereochemistry of the A/B ring fusion, and it belongs to the same 3alpha-hydroxy-pregnane pharmacophore that defines neurosteroid modulators of the GABA-A receptor (the brain's principal inhibitory chloride ion channel). Like other steroid anaesthetics it produces sedation, hypnosis and anaesthesia by acting as a positive allosteric modulator of GABA-A receptors, although modern photoaffinity work shows it occupies the intersubunit neurosteroid pocket roughly twenty times more weakly than its 5alpha counterpart. Never marketed, it survives as a historical and mechanistic reference compound within the neurosteroid anaesthetic class.

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