Identity
PACAP — pituitary adenylate cyclase-activating polypeptide — is an endogenous neuropeptide existing in two active forms, PACAP-38 (38 amino acids, the dominant form in tissue) and PACAP-27. It belongs to the VIP/secretin/glucagon superfamily and shares substantial sequence identity with vasoactive intestinal peptide (VIP), with which it shares two of its three receptors. It is encoded by the ADCYAP1 gene and is expressed widely across the central and peripheral nervous system, including the trigeminal system that matters for headache.
It signals through three G-protein-coupled receptors: PAC1, which is PACAP-selective, and VPAC1 and VPAC2, which respond to both PACAP and VIP. That receptor split is central to the drug development story, since selectivity determines which pathway an antibody hits.
Development & history
- 1989: Isolated and characterised by Miyata and colleagues from ovine hypothalamus, named for its extraordinary potency in stimulating adenylate cyclase in pituitary cells — roughly a thousandfold more potent than VIP in that assay.
- 1990s–2000s: Extensive preclinical work established roles in neurodevelopment, neuroprotection, circadian regulation, stress response and vasodilation. This is the literature that research-chemical marketing draws on.
- 2014: Amin et al. (Brain) established in humans that intravenous PACAP-38 induces migraine-like attacks — turning the peptide from a physiological curiosity into a therapeutic target.
- 2021: AMG 301, an antibody against the PAC1 receptor, tested in phase 2 for migraine prevention (Cephalalgia) — the first attempt to drug this pathway.
- 2023: Lu AG09222 shown to block PACAP38-induced vasodilation, heart rate increase and headache in healthy subjects (J Headache Pain) — human proof of target engagement.
- 2024: The HOPE phase 2 trial (NEJM) reported the first positive phase 2 result for PACAP-targeted migraine prevention: Lu AG09222 at 750 mg IV reduced monthly migraine days by 6.2 versus 4.2 on placebo (difference −2.0 days; 95% CI −3.8 to −0.3; P=0.02). This validated PACAP as a druggable target working independently of CGRP.
- 2025: Eli Lilly's PACAP-neutralising antibody LY3451838 reported phase 1 and phase 2 data in treatment-resistant migraine (Cephalalgia). A randomised trial confirmed PACAP38-induced attacks occur independently of CGRP signalling.
- 2026: Phase 2b PROCEED (NCT06323928, ~498 participants) met its primary endpoint for bocunebart (the development name for Lu AG09222); results presented at the American Headache Society meeting, with pivotal phase 3 planned. A Headache review framed the field as "PACAP blockade and the transformation of clinical practice."
Mechanism (as proposed)
PACAP binds PAC1, VPAC1 and VPAC2 receptors, coupling primarily to Gs and driving cAMP production with exceptional potency. In the trigeminovascular system this produces cranial vasodilation and activation and sensitisation of meningeal nociceptors — the sequence that underlies its ability to provoke migraine. Elsewhere in the nervous system, the same signalling supports the neurotrophic, neuroprotective and neurodevelopmental roles documented in cell and animal models, including interactions with BDNF signalling.
Both faces of this mechanism are real. PACAP genuinely is a neurotrophic signalling molecule with protective effects in preclinical models, and administering it to humans genuinely provokes headache and vasodilation. The second fact is the one with human trial data behind it.
Reading the evidence honestly
This entry has an unusual shape, so it is worth stating the structure plainly.
The strongest human evidence about PACAP is that giving it causes migraine. That finding is replicated, randomised, mechanistically characterised, and stable enough to be used as an experimental model for inducing attacks on demand. It is not an obscure side effect; it is the peptide's best-documented human effect.
The strongest therapeutic evidence is for blocking it. Every clinical programme in this space — bocunebart, LY3451838, AMG 301 — is an antibody that neutralises PACAP or its receptor. The A-grade clinical results in this entry belong to those antibodies. Citing "strong clinical evidence for PACAP" without specifying direction inverts what the trials found.
The neuroprotection story is preclinical. Cell and animal data supporting neurotrophic and neuroprotective effects are genuine and interesting. They have never been tested as a human therapy, and any attempt to do so would have to contend with the vascular and nociceptive effects that show up reliably at doses used in human provocation studies.
The overall grade reflects this: a physiologically important, well-characterised, mechanistically fascinating peptide, whose marketed use as a nootropic or neuroprotective agent has no human efficacy evidence and runs directly against the only well-established human findings about administering it.