Alphadolone

A synthetic pregnane GABA-A positive allosteric modulator; the minor, solubilising partner of alfaxalone in Althesin and Saffan that nonetheless carries its own anaesthetic activity and a distinctive non-sedating spinal analgesic action.

In clinical use the principal hazard of alphadolone-containing anaesthetics was not the steroid itself but the solubilising vehicle. Althesin and Saffan were formulated in Cremophor EL (polyethoxylated castor oil), a surfactant that provokes non-IgE histamine release and complement activation; repeated or sensitised exposure produced anaphylactoid reactions, and immunological studies documented complement (C3 and C4) conversion after dosing. Reported reaction rates ranged from roughly 1 in 400 to 1 in 1300 administrations, and this safety signal drove the withdrawal of Althesin from human anaesthesia in 1984.

The pregnane steroids themselves were comparatively cardiostable, producing less fall in blood pressure than barbiturate induction agents, and the antinociceptive doses studied in animals and in a small human trial did not increase sedation, respiratory depression, nausea or vomiting. Involuntary muscle movements, tremor and venous sequelae were reported with steroid anaesthesia, and in cats Saffan can cause Cremophor-related histamine release with oedema of the paws and ears. There is no current licensed human product; alphadolone is now encountered chiefly in veterinary history and in research settings, it has no established recreational or nootropic use, and its safety outside supervised anaesthesia is uncharacterised.

Alphadolone was developed by Glaxo as part of the steroid anaesthetic programme that produced the experimental code CT1341, whose pharmacology in animals was characterised by Child and colleagues in 1971. Marketed from 1971 as Althesin for human intravenous anaesthesia (and as Alfathesin in some countries), the product combined alfaxalone and alphadolone acetate in a 3:1 ratio dissolved in 20 percent Cremophor EL. Althesin was prized for rapid onset, short duration, a wide therapeutic margin and cardiovascular stability, and it became a popular induction and infusion agent through the 1970s; notably, the widely used Ramsay Sedation Scale was introduced in a 1974 study that titrated intensive care sedation with alphaxalone-alphadolone.

Mounting evidence that Cremophor EL caused hypersensitivity reactions led to the withdrawal of Althesin from human use in 1984. The same steroid pair continued in veterinary anaesthesia as Saffan, especially in cats and reptiles. From 2000 onward, Goodchild, Nadeson and co-workers revived interest in alphadolone specifically, publishing a series of studies, and filing patents, that proposed it as a standalone non-sedating analgesic distinct from its historical role as a solubiliser.

Within anaesthetic history alphadolone is usually cast as the junior partner to alfaxalone, remembered mainly as the component added to make alfaxalone soluble and credited with only a fraction of the anaesthetic potency. That framing understates a genuinely distinctive pharmacology: the demonstration that alphadolone, but not alfaxalone, produces spinally mediated analgesia without sedation, potentiates opioids and blunts morphine tolerance has earned it a small but serious research reputation as a template for GABAergic analgesics. In nootropic and general pharmacology circles it remains obscure, and it holds no standing as a supplement or cognitive enhancer.