4-Hydroxyamphetamine

4-Hydroxyamphetamine, usually written p-hydroxyamphetamine, is formed when CYP2D6 hydroxylates amphetamine on the aromatic ring. It is one of the two main routes amphetamine takes out of the body, the other being deamination, and it is subsequently converted by dopamine beta-hydroxylase to 4-hydroxynorephedrine, a false neurotransmitter that can be stored in noradrenergic vesicles in place of the real one.

Pharmacologically it is an indirect sympathomimetic like its parent, releasing catecholamines rather than acting on receptors directly, and it is taken up by brain tissue and releases catecholamines there in preparation ^[3]. The decisive difference in the intact animal is access: the added hydroxyl group makes the molecule considerably more polar, so far less of it crosses the blood brain barrier after peripheral administration, and its effects are correspondingly peripheral. Behavioural work in mice nonetheless shows centrally mediated effects at sufficient exposure ^[4].

Its clinical identity is ophthalmological. Because it releases noradrenaline from intact postganglionic sympathetic nerve endings, it dilates the pupil when those neurons are healthy and fails to when they are not, which distinguishes a third-order Horner's syndrome from first- or second-order lesions at the bedside ^[1]. It was marketed as Paredrine for this purpose and became unavailable, prompting published regret ^[2]. It also raises intraocular pressure in rabbits, which is a relevant caution for any ocular use ^[5].

There is no meaningful contemporary human exposure to discuss, since it is neither prescribed nor sold; almost all exposure is metabolic, produced from amphetamine.

When it was in ophthalmic use its adverse effects were those of a topical sympathomimetic: pupil dilation with the attendant light sensitivity and blurred near vision, and the possibility of precipitating angle closure in a predisposed eye. Work in rabbits found it raises intraocular pressure, which is the specific reason to be careful in a susceptible eye rather than a general caution ^[5].

As a metabolite, its relevance is different and more interesting. Its conversion to 4-hydroxynorephedrine produces a false neurotransmitter that occupies noradrenergic vesicles while being much weaker at the receptor, which is a plausible contributor to the way sustained high-dose amphetamine exposure blunts sympathetic responsiveness. Because formation depends on CYP2D6, and CYP2D6 activity varies widely between individuals for genetic reasons, how much of an amphetamine dose takes this route differs substantially from person to person.

It is not sold, has no established human dose outside its former ophthalmic formulation, and nothing here is a suggestion to seek it out.

4-Hydroxyamphetamine was characterised during the mid-century work on amphetamine metabolism, when the routes by which the body disposes of amphetamine were being mapped, and the hydroxylation and deamination pathways were established alongside each other ^[6].

Its independent career was ophthalmological. Marketed as Paredrine, it became the standard pharmacological test for localising a Horner's syndrome, a use that depends entirely on its releasing noradrenaline from intact postganglionic terminals ^[1]. That test was valued precisely because the alternative, imaging the entire three-neuron sympathetic pathway, is a much larger undertaking than putting a drop in an eye.

Its disappearance was a supply matter rather than a safety one. When it became unavailable, an American Journal of Ophthalmology commentary in 1991 asked directly whether a good drug had been lost, which is an unusual epitaph for a metabolite ^[2].

Research since has been mostly pharmacological rather than clinical: its accumulation and catecholamine-releasing action in brain tissue ^[3], its centrally mediated behavioural effects in mice ^[4], and its effect on intraocular pressure ^[5].

Among ophthalmologists and neuro-ophthalmologists 4-hydroxyamphetamine is remembered fondly and specifically, as the drop that answered a question no other bedside test answers as cleanly. The published reaction to its withdrawal, framed as the loss of a good drug, is not the usual language for a discontinued diagnostic agent ^[2].

In pharmacology it is a teaching example. It is the standard illustration of how a single polar substituent can convert a centrally acting drug into a peripherally acting one, and of the false neurotransmitter concept through its conversion to 4-hydroxynorephedrine.

Among people interested in stimulants it is essentially unknown, and there is nothing to know in a practical sense: it is not sold, it does not reach the brain well, and it has no recreational or nootropic character. The one fact worth carrying is that its formation depends on CYP2D6, whose activity varies widely between people, so the proportion of an amphetamine dose that goes down this route is not the same for everyone.