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Robalzotan was AstraZeneca's attempt to flip the SSRI script; instead of blocking reuptake, it selectively antagonized presynaptic 5-HT1A autoreceptors, a mechanism meant to speed up and amplify serotonin release compared to a standard SSRI. The theory was sound enough to carry it into a 385-patient, paroxetine-controlled Phase II depression trial, plus separate programs for irritable bowel syndrome and overactive bladder. None of it panned out; the depression trial showed no separation from placebo, and the GI and urology programs were quietly closed out by 2005. It is a clean case study in how a mechanistically elegant idea can still fail the only test that matters, a randomized trial.
- none established in humans; MDD trial failed to beat placebo
- PET studies confirmed clean, dose-dependent 5-HT1A receptor occupancy in the brain
- Robalzotan was tested in three unrelated indications, depression, IBS, and overactive bladder, before AstraZeneca gave up on the molecule entirely.
Mechanism
Selective, high-affinity at presynaptic autoreceptors; blocking these receptors was intended to disinhibit release from raphe neurons, theoretically producing faster and stronger antidepressant effects than reuptake inhibition alone.
Safetyrisks and cautions, not medical advice
An irritable bowel trial in 402 patients was stopped early, and not for futility. Eight people on robalzotan reported hallucinations or hallucination-like events against none on placebo, six of them stopped treatment because of it, and the sponsor then terminated the study. Side effects attributed to the drug ran at 33 percent on the 20 mg dose against 13 percent on placebo, and more than half of every withdrawal in the trial was for a central nervous system event. The published conclusion named that profile, alongside the lack of benefit, as the reason clinical development ended.
History
Developed by Astra (later AstraZeneca) through the late 1990s and early 2000s; reached Phase II in a 385-patient MDD trial that failed to separate from placebo even against an active paroxetine arm. Subsequent Phase II programs in IBS and overactive bladder were also discontinued, the latter in July 2005.
Subjective profileweighing the evidence above
A textbook example of a good receptor hypothesis that simply did not translate into clinical benefit.
Resources
This entry is here for reference.
Research
- 1.Robalzotan (AstraZeneca)
- 2.PET-determination of robalzotan (NAD-299) induced 5-HT1A receptor occupancy in the monkey brain
2 listed here; entry last updated July 2026
Reviews
My notesprivate to this device
FAQ
Why did robalzotan fail if the receptor theory made sense?
Occupying the 5-HT1A autoreceptor did not translate into a clinically meaningful mood benefit in the 385-patient trial; the drug did not separate from placebo even where the paroxetine comparator did.
Is robalzotan available anywhere?
No. It never reached the market and all clinical programs, depression, IBS, and overactive bladder, were shut down by the mid-2000s.
Limitations of the evidence
- a 402-patient irritable bowel trial was terminated early after hallucinations or hallucination-like events in 8 patients on drug and none on placebo
- central nervous system adverse events accounted for just over half of all withdrawals, and the published report names the adverse-event profile alongside the lack of efficacy as the reason development stopped