Corticotropin (ACTH): H.P. Acthar Gel, pituitary stress hormone
A natural hormone from the pituitary gland that tells the adrenal glands to make cortisol; the drug form (H.P. Acthar Gel) is FDA-approved for infantile spasms, multiple sclerosis flares, and nephrotic syndrome.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
Corticotropin — also called ACTH, or adrenocorticotropic hormone — is a 39-amino-acid peptide hormone released by the pituitary gland that tells the adrenal glands to make cortisol, the body's primary stress hormone. It is an endogenous hormone that every human produces around the clock, with a pronounced morning peak that drives the normal cortisol surge on waking. Clinically, it appears in two very different contexts: as a diagnostic tool, and as a prescription anti-inflammatory drug.
As a pharmaceutical, corticotropin exists in two distinct forms. The first is cosyntropin (also known as tetracosactide or tetracosactrin; brand Cortrosyn or Synacthen), a synthetic 24-residue fragment covering the biologically active N-terminus, used exclusively to test whether the adrenal glands are capable of responding — the cosyntropin stimulation test is the gold-standard diagnostic for adrenal insufficiency. The second is repository corticotropin injection (H.P. Acthar Gel), a long-acting gel formulation of the full 39-residue peptide purified from porcine pituitary and used therapeutically for a narrow set of conditions. The stored sequence on this card (SYSMEHFRWGKPVGKKRRPVKVYPDGAEDQLAEAFPLEF) is porcine ACTH 1–39 — the source material for Acthar Gel — which differs from the canonical human sequence at three C-terminal positions (residues 25, 30, and 31); the N-terminal 24 residues, which carry all the adrenocortical activity and are identical in porcine and human ACTH, are what cosyntropin replicates.
History
Adrenocortical-stimulating activity was identified in pituitary extracts in 1933. The full 39-amino-acid sequence of ACTH was determined by Choh Hao Li's group at UC Berkeley in the 1950s, establishing it as one of the first peptide hormones to be fully sequenced. Initial clinical use mirrored the early corticosteroid era: following Philip Hench's demonstration in 1948 that cortisone produced dramatic improvement in rheumatoid arthritis, ACTH was used alongside synthetic corticosteroids as an alternative way to drive endogenous cortisol release. As better-defined synthetic glucocorticoids (prednisone, methylprednisolone, dexamethasone) became available, ACTH retreated from general anti-inflammatory use.
ACTH found a durable clinical niche in infantile spasms (West syndrome) — a catastrophic epilepsy of early infancy characterized by clusters of spasms, a chaotic EEG pattern called hypsarrhythmia, and developmental arrest or regression. Clinical experience from the 1950s through 1970s established that high-dose ACTH therapy could suppress spasms and normalize the EEG. The synthetic 1–24 fragment cosyntropin was developed in the 1960s once it was established that the N-terminus carries full MC2R-binding activity; it quickly displaced full-length ACTH for diagnostic testing because it was cheaper, less immunogenic, and pharmacodynamically equivalent for adrenal stimulation.
Repository corticotropin injection (Acthar Gel) holds its current FDA-approved label largely by virtue of its pre-1962 grandfathered status under the Kefauver-Harris Amendment. Its commercial history — decades of low-cost obscurity followed by a series of ownership changes and extraordinary price increases in the 2000s and 2010s — has become a reference case in discussions of legacy-drug pricing in the United States (Fridmanis and colleagues 2017, Frontiers in Endocrinology).
What it does
ACTH's principal biological job is to tell the adrenal cortex to make cortisol. This happens through a chain: the hypothalamus releases corticotropin-releasing hormone (CRH), CRH stimulates the pituitary to release ACTH, and ACTH travels via the bloodstream to the adrenal glands where it binds the melanocortin 2 receptor (MC2R) and triggers cortisol synthesis. Cortisol, in turn, feeds back to suppress both CRH and ACTH — a closed-loop control system. ACTH secretion follows a circadian pattern with the morning peak (roughly 06:00–08:00) driving the normal morning cortisol surge; stress of any kind — physical, psychological, or inflammatory — can override this rhythm and trigger acute ACTH and cortisol release.
For the cosyntropin stimulation test, a single injection of synthetic ACTH 1–24 is given and blood cortisol is measured before and at 30–60 minutes afterward. A robust cortisol response (typically above 18–20 μg/dL) indicates that the adrenal glands are functioning and rules out primary adrenal insufficiency. A blunted response points to either primary adrenal gland failure (Addison's disease) or inadequate pituitary ACTH signaling (secondary/tertiary adrenal insufficiency).
For therapeutic use, the sustained ACTH stimulus from repository corticotropin drives prolonged cortisol elevation and, separately, may activate melanocortin receptors on immune cells through pathways that go beyond simply raising cortisol. This hypothesized direct melanocortin receptor-mediated anti-inflammatory action — particularly via MC1R and MC3R on leukocytes — is cited by proponents of Acthar Gel as a mechanistic distinction from giving an equivalent synthetic glucocorticoid directly, though whether this produces a clinically measurable difference in most indications remains unresolved (Fridmanis and colleagues 2017).
Evidence
- Human: Strong for the two core uses. As a diagnostic: the cosyntropin stimulation test has been the clinical reference standard for adrenal insufficiency for decades and is validated across extensive literature (Vigevano and Cilio 1997, Epilepsia). As a treatment for infantile spasms: the 2012 American Academy of Neurology and Child Neurology Society practice parameter established ACTH as one of two first-line therapies (alongside vigabatrin), with the best available evidence for short-term EEG normalization particularly in non-tuberous-sclerosis etiology. For MS acute relapses, repository corticotropin has randomized trial evidence supporting relapse resolution comparable to high-dose intravenous methylprednisolone, though the latter is now the default standard of care due to cost, convenience, and predictability. Evidence for other approved indications (nephrotic syndrome, rheumatic and dermatological conditions) ranges from historical case series to limited controlled data.
- Animal: Extensive and largely historical. ACTH physiology was characterized across thousands of animal experiments from the 1950s through the 1980s. More recent preclinical work has focused on MC1R and MC3R-mediated anti-inflammatory signaling in immune cells and on the role of central melanocortin pathways in seizure models.
- In vitro: MC2R pharmacology, the cAMP–PKA–StAR steroidogenesis cascade, and the molecular determinants of ACTH receptor specificity are among the best-characterized signaling axes in endocrinology (Fridmanis and colleagues 2017, Frontiers in Endocrinology).
Myths and misconceptions
- "Corticotropin and corticosteroids are interchangeable" — ACTH stimulates the adrenal gland to produce endogenous cortisol; synthetic corticosteroids (prednisone, methylprednisolone, dexamethasone) bypass the adrenal gland and act directly on glucocorticoid receptors. Clinical effects overlap substantially but are not identical: the cortisol produced by ACTH is the same molecule as in normal physiology, whereas synthetic steroids vary considerably in glucocorticoid potency, mineralocorticoid activity, and receptor binding kinetics. ACTH may also exert direct effects via melanocortin receptors on immune cells that synthetic corticosteroids do not replicate.
- "The ACTH stimulation test uses the same drug as H.P. Acthar Gel" — Cosyntropin is synthetic ACTH 1–24: a 24-residue truncated peptide administered as a rapid IV or IM bolus for a diagnostic snapshot. H.P. Acthar Gel is purified full-length porcine ACTH (39 residues) in a gelatin depot designed for slow subcutaneous absorption. Different molecular length, different origin (synthetic versus porcine-derived), different pharmacokinetics, entirely different indications.
- "High ACTH always means Cushing's disease" — Elevated ACTH has multiple causes: pituitary corticotropinoma (Cushing's disease), ectopic ACTH secretion from a tumor (small-cell lung cancer, carcinoid, thymoma), or — importantly — primary adrenal insufficiency, where the pituitary tries to compensate for low cortisol by secreting more ACTH. The distinction requires CRH stimulation testing, bilateral inferior petrosal sinus sampling, and imaging.
- "ACTH is a biohacker peptide for cortisol support or energy" — ACTH drives sustained cortisol elevation, which is catabolic, immunosuppressive, and unfavorable for body composition, glucose control, and sleep at supraphysiologic levels. There is no legitimate self-administered use case. ACTH does not appear in compounded peptide formularies and is not sold in research-chemical markets as a credible therapeutic product.
- "Cosyntropin is a different drug from ACTH" — Cosyntropin is the first 24 amino acids of ACTH. It binds MC2R and triggers the identical adrenal cortisol response. The C-terminal 15 residues removed in cosyntropin carry the main immunogenic determinants of natural ACTH but contribute nothing to MC2R binding or adrenal stimulation.
Common questions
What is the mechanism of ACTH in infantile spasms beyond cortisol production? The anticonvulsant mechanism is incompletely understood. Cortisol suppresses CRH gene expression in the hypothalamus, and elevated hypothalamic CRH in the developing brain has been hypothesized to be seizure-promoting. Additionally, ACTH may directly activate MC4R receptors in limbic regions independent of adrenal cortisol. Evidence for a cortisol-independent component includes reports of ACTH response in patients with bilateral adrenalectomy and preclinical work showing that synthetic ACTH analogs lacking adrenocortical activity have anticonvulsant properties in rodent seizure models.
Why is H.P. Acthar Gel so expensive, and is the cost justified? Acthar Gel's list price rose dramatically after its acquisition by Questcor (later Mallinckrodt) in 2001, making it one of the most expensive drugs in the US by annual treatment cost. The grandfathered label is broad, but the primary contemporary use is infantile spasms — where randomized trial evidence does support short-course ACTH efficacy, and the consequences of undertreated IS (intellectual disability, refractory epilepsy, autism) give that treatment high value within a short, defined course. For most other approved indications, the pharmacoeconomic case is harder to make given the availability of inexpensive synthetic glucocorticoids.
How does primary adrenal insufficiency differ from secondary in terms of ACTH? In primary adrenal insufficiency (Addison's disease — adrenal gland damaged or destroyed), the pituitary responds to low cortisol with markedly elevated ACTH. In secondary adrenal insufficiency (pituitary failure) or tertiary (hypothalamic CRH deficiency, often from chronic exogenous glucocorticoid use), ACTH is low or inappropriately normal despite low cortisol. This distinction is critical for treatment: primary AI requires both glucocorticoid and mineralocorticoid replacement; secondary and tertiary AI require glucocorticoid replacement only, because aldosterone is regulated by the renin-angiotensin system rather than ACTH.
Known effects
- Cortisol stimulation (adrenal) — FDA basis for both diagnostic (cosyntropin test) and therapeutic (Acthar Gel) uses
- Infantile spasm cessation — Strong; first-line per 2012 AAN/CNS practice parameter
- MS relapse acceleration of recovery — Moderate; FDA-approved; comparable to methylprednisolone in controlled trials but not preferred standard of care
- Adrenal insufficiency diagnosis — Strong; cosyntropin stimulation test is the clinical reference standard
- Nephrotic syndrome, rheumatic indications — Moderate to limited; FDA-approved (grandfathered label); modern evidence base is thin relative to available synthetic glucocorticoids
Safety signals
Safety signals for corticotropin mirror those of systemic glucocorticoid therapy, because the clinical effects are mediated predominantly through the cortisol the drug drives the adrenal glands to produce. Reported effects from published trials and the FDA label include fluid retention and peripheral edema, hyperglycemia and glucose intolerance, hypertension, hypokalemia, mood changes (insomnia, irritability, and rarely psychosis), cushingoid features with prolonged use, and injection-site reactions. Hypersensitivity reactions — including anaphylaxis — have been associated with H.P. Acthar Gel, consistent with its porcine-derived origin. Infectious risk is elevated through the immunosuppressive cortisol effect, as with any glucocorticoid-equivalent exposure. After prolonged therapeutic courses, HPA-axis recovery may require days to weeks, analogous to tapering after systemic steroid therapy. The cosyntropin stimulation test, as a single-administration diagnostic dose, carries a favorable safety profile; the therapeutic sustained-dosing regimen carries the full glucocorticoid side-effect burden.
Regulatory status
- US: Prescription-only. Cosyntropin (Cortrosyn) is FDA-approved as a diagnostic agent for adrenocortical insufficiency. Repository corticotropin injection (H.P. Acthar Gel) is FDA-approved for infantile spasms, acute MS exacerbations, rheumatic disorders, nephrotic syndrome, and select ophthalmic and dermatological inflammatory conditions (grandfathered label, pre-1962 approval status).
- EU/International: Cosyntropin is marketed as Synacthen (tetracosactide) and is an approved prescription agent across the EU, UK, Canada, Australia, Japan, and most major markets. Full-length repository ACTH formulations comparable to Acthar Gel have variable international availability; Acthar Gel itself is largely a US market product.
- WADA: Corticotropins are prohibited. Glucocorticoids and agents that stimulate their production — including ACTH — fall under WADA S9 (Glucocorticoids) when administered by injection, orally, or rectally. Tetracosactide (cosyntropin) is specifically named in WADA technical documents. Therapeutic use exemption is required for athletes with documented medical need.
Mechanism
ACTH is cleaved from the precursor proopiomelanocortin (POMC) by prohormone convertase 1/3 in corticotroph cells of the anterior pituitary. Among the five melanocortin receptors (MC1R–MC5R), ACTH binds exclusively to MC2R under physiological conditions. This selectivity is unique to MC2R: alpha-MSH, beta-MSH, and gamma-MSH peptides do not activate MC2R, while ACTH does not activate MC1R, MC3R, MC4R, or MC5R at circulating concentrations. The molecular basis for this exclusivity involves a distinct extracellular binding pocket on MC2R and the obligate co-expression of MRAP (melanocortin 2 receptor accessory protein 1), which is required for MC2R trafficking to the plasma membrane and functional ACTH signaling; MRAP deficiency causes familial glucocorticoid deficiency type 2 (Fridmanis and colleagues 2017, Frontiers in Endocrinology).
MC2R is a Gs-coupled receptor expressed predominantly on cells of the adrenal zona fasciculata. Agonist binding raises intracellular cAMP, activating protein kinase A (PKA). In the acute phase (minutes), PKA phosphorylates StAR (steroidogenic acute regulatory protein), which shuttles cholesterol across the mitochondrial outer membrane — the rate-limiting step in cortisol biosynthesis. In the chronic phase (hours to days), ACTH drives transcriptional upregulation of steroidogenic enzymes (CYP11A1, CYP17A1, CYP21A2, CYP11B1) and promotes trophic expansion of zona fasciculata cell mass; sustained ACTH deficiency leads to adrenocortical atrophy.
Beyond the adrenal cortex, MC2R is expressed at lower levels in adipose tissue, skin, and immune cells. ACTH also shares its first 13 residues with alpha-MSH, raising the possibility of modest off-target engagement at MC1R and MC3R at supraphysiologic concentrations — a mechanistic basis for the hypothesized direct melanocortin-receptor-mediated immunomodulatory effects of repository corticotropin in autoimmune disease, independent of cortisol (Fridmanis and colleagues 2017).
Open questions
- Whether the therapeutic effect of repository corticotropin in autoimmune indications is meaningfully distinguishable from equivalent synthetic glucocorticoid exposure, or whether the hypothesized MC1R/MC3R-mediated anti-inflammatory mechanism produces clinically separable benefit.
- Optimal cosyntropin stimulation test cut-offs using modern cortisol immunoassays and LC-MS/MS platforms, where historical thresholds were derived from older polyclonal assays.
- Long-term developmental and endocrine outcomes in infantile-spasms cohorts treated with high-dose early-life ACTH.
- Whether selective MC1R or MC3R agonists could, in principle, deliver the non-glucocorticoid anti-inflammatory component without the cortisol-driven toxicity profile of repository corticotropin.
Related peptides
- Alpha-MSH — shares the first 13 residues with ACTH; cleaved from the same POMC precursor; acts at MC1R and MC3R rather than MC2R; involved in pigmentation and energy balance
- Sermorelin — another anterior pituitary regulatory peptide, but targets somatotroph GH release rather than adrenocortical cortisol
- Leuprolide — GnRH agonist acting on the gonadotroph axis; like ACTH, a prescription pituitary-regulatory peptide with defined clinical indications
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Does ACTH work on infant seizures by activating brain receptors directly, rather than just telling the adrenal gland to make cortisol?
If true, it could lead to safer, targeted treatments for infantile spasms that avoid the serious side effects of high-dose steroids, benefiting infants who fail current therapies.
Could ACTH, at low doses that do not trigger cortisol release, still reduce brain inflammation by activating a separate receptor on immune cells in the brain?
If true, it would support developing targeted ACTH analogs for multiple sclerosis and brain injuries that provide anti-inflammatory benefit without the full side-effect profile of steroid therapy, helping patients who cannot tolerate high-dose steroids.
Would replacing the three pig-specific residues in ACTH with their human versions yield a synthetic ACTH that is less immunogenic and easier to manufacture?
If it worked, Acthar Gel could be made synthetically instead of extracted from pig pituitary glands, lowering the risk of immune reactions and giving a more consistent, scalable supply.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.5873808860778809 | boltz-2 |
| ranking score | 0.6995645761489868 | boltz-2 |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | colabfold_local |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-22 |
▸citationbibtex
@peptide{pep04440,
sequence = {SYSMEHFRWGKPVGKKRRPVKVYPDGAEDQLAEAFPLEF},
target = {mc2r},
author = {peptidemodel},
year = {2026},
status = {bioassayed}
}