The fourth axis
Melanocortin control of inflammation
Beyond pigment, cortisol, and appetite, the melanocortin system has a fourth job — calming inflammation. It runs mainly through MC1R and MC3R on immune cells, works without acting as a steroid, and is now the target of a growing pipeline from ulcerative colitis to dry eye.
The system's gift for repair — the same signal that colours skin and sets hunger also tells inflamed tissue when to stand down and heal.
The melanocortin system is best known for the three jobs it advertises on the surface: it sets pigment, drives the cortisol stress axis, and tunes appetite. But it has a quieter fourth career that the newest drugs are built on — it is one of the body’s own brakes oninflammation.[1] The same peptide that darkens skin and signals fullness, α-MSH, is also a potent anti-inflammatory hormone, and it works through both glucocorticoid-dependent and glucocorticoid-independent routes.[2]
A signal that calms
Inflammation is the immune system’s fire alarm — useful in a crisis, damaging when it won’t switch off. α-MSH sits on the "off" side of that balance. It suppresses the master inflammatory switch NF-κB, dampening the production of the cytokines, nitric oxide, and adhesion molecules that recruit and activate immune cells.[1]The effect is broad: across models of gut, skin, joint, brain, and eye inflammation, melanocortin agonists reduce the inflammatory load — which is exactly why a "pigment and appetite" system keeps turning up in immunology.[2]
Which receptors do the work
Crucially, this is a different arm of the system from the one that makes cortisol. The stress axis runs through MC2R on the adrenal gland; the anti-inflammatory arm runs mainly through MC1R and MC3R expressed directly on immune cells — macrophages, monocytes, and neutrophils.[1][5] MC1R broadly lowers inflammatory signalling, while MC3R was singled out early as a distinct anti-inflammatory receptor: synthetic MC3R agonists blunt inflammation in a way that a selective MC4R blocker doesn’t touch.[3]
| Receptor | Where it acts | Anti-inflammatory role | In the clinic |
|---|---|---|---|
| MC1R | Immune cells, skin, gut and ocular surface | Broadly suppresses NF-κB–driven inflammation | PL-8177 (colitis), dersimelagon (systemic sclerosis) |
| MC3R | Macrophages and monocytes | Drives resolution — clearing dead cells, switching the response off | resomelagon (biased MC1R/MC3R agonist) |
| MC5R | Lymphocytes, exocrine tissue | Least defined; implicated in immune modulation | — |
Not just off — toward resolution
The most interesting part is not that melanocortins switch inflammation off, but that they help switch it over. Ending an inflammatory response is an active process — the immune system has to clear the debris and the spent cells, a step calledresolution. Melanocortin signalling promotes it: in one study a melanocortin agonist increased macrophage uptake of dying neutrophils (efferocytosis) and calmed tissue, and that pro-resolving effect was lost in mice without MC3R but preserved without a working MC1R — pinning the resolution job on MC3R specifically.[4] That places the system inside the modern field of "resolution pharmacology," which aims not to blockade the immune system but to help it finish the job cleanly.[5]
The old drug that did this all along
None of this is entirely new to medicine — it has just been hiding in plain sight. ACTH, given as repository corticotropin, has been used against inflammatory disease since the 1950s, and for most of that time the assumption was that it worked purely by telling the adrenal gland to make cortisol. It turns out that a meaningful part of its benefit is steroid-independent: ACTH is itself a melanocortin, and it acts directly on melanocortin receptors on immune cells to calm them.[6]The old workhorse was quietly using the fourth axis the whole time — which is precisely the door the newer, more selective drugs are trying to walk through.
The pipeline betting on it
For decades this was interesting biology with no clean drug behind it. That is changing, and the anti-inflammatory axis now carries much of the melanocortin pipeline:
- PL-8177 — Palatin’s oral MC1R agonist, engineered to act on the colon lining without being absorbed. Its Phase 2 in ulcerative colitis reported clinical remission in a third of treated patients versus none on placebo, in a small study.[7]
- PL-9643 — a melanocortin eye drop for dry eye disease, applying the same tissue-calming biology straight to the ocular surface; it met its co-primary symptom endpoint in a Phase 3 trial.
- Resomelagon (AP1189) — an oral biased MC1R/MC3R agonist designed to trigger anti-inflammatory signalling while sparing the pigment pathway. It is worth being honest here: its Phase 2b in rheumatoid arthritismissed its primary endpoint, with activity seen only in a subgroup with active systemic inflammation — a reminder that the frontier is real but unforgiving.[9]
- Dersimelagon — better known as an oral MC1R agonist for a pigment disorder, but also studied for the inflammation, vascular damage, and fibrosis of systemic sclerosis, where preclinical work showed it hitting all three at once.[8]
Track all of them, dated and sourced, on the pipeline page.
The honest bottom line
The melanocortin anti-inflammatory axis is one of the best examples of what makes this system worth watching: a single family of receptors, reused across organs, that can quiet inflammation without the blunt cost of general immunosuppression — and that can, at least in principle, be separated from the tanning and appetite effects that come bundled with it. The caution is equally real: most of these programmes are early, one flagship arthritis trial has already missed, and "promising in a small study" is not "proven." That is the frontier, stated plainly. For the underlying biology, start with α-MSH and the system overview.
Common questions
Is the melanocortin system anti-inflammatory?
Yes. Alongside its roles in pigment, cortisol, and appetite, the melanocortin system is one of the body’s built-in brakes on inflammation. The peptide α-MSH suppresses inflammatory signalling — most notably the master switch NF-κB — and calms the immune cells that drive and sustain it.
Which melanocortin receptors control inflammation?
Mainly MC1R and MC3R, acting on immune cells such as macrophages, monocytes, and neutrophils. MC1R broadly dampens inflammatory signalling; MC3R is especially tied to resolving inflammation once it has started. This is a separate arm from MC2R, the receptor that drives cortisol.
How is this different from taking a steroid?
A steroid works downstream of cortisol. The melanocortin anti-inflammatory arm acts directly on immune cells through MC1R and MC3R, so it can quiet inflammation without producing a glucocorticoid. That steroid-independent action is why ACTH-based therapy has helped inflammatory disease for decades and why newer melanocortin drugs are being designed to separate the two effects.
What drugs target the melanocortin anti-inflammatory pathway?
Several are in trials: PL-8177, an oral MC1R agonist for ulcerative colitis; PL-9643, a melanocortin eye drop for dry eye disease; resomelagon (AP1189), an oral biased MC1R/MC3R agonist tested in rheumatoid arthritis and kidney disease; and dersimelagon, an MC1R agonist studied in the fibrosis of systemic sclerosis.
References
- 1.Catania A, Gatti S, Colombo G, Lipton JM. Targeting melanocortin receptors as a novel strategy to control inflammation. Pharmacol Rev. 56(1):1–29. 2004. link ↗
- 2.Wang W, Guo DY, Lin YJ, Tao YX. Melanocortin regulation of inflammation. Front Endocrinol (Lausanne). 10:683. 2019. link ↗
- 3.Getting SJ, Allcock GH, Flower R, Perretti M. Natural and synthetic agonists of the melanocortin receptor type 3 possess anti-inflammatory properties. J Leukoc Biol. 69(1):98–104. 2001. link ↗
- 4.Montero-Melendez T, Patel HB, Seed M, Nielsen S, Jonassen TEN, Perretti M. The melanocortin agonist AP214 exerts anti-inflammatory and proresolving properties. Am J Pathol. 179(1):259–269. 2011. link ↗
- 5.Patel HB, Montero-Melendez T, Greco KV, Perretti M. Melanocortin receptors as novel effectors of macrophage responses in inflammation. Front Immunol. 2:41. 2011. link ↗
- 6.Arnason BG, Berkovich R, Catania A, Lisak RP, Zaidi M. Mechanisms of action of adrenocorticotropic hormone and other melanocortins relevant to the clinical management of patients with multiple sclerosis. Mult Scler. 19(2):130–136. 2013. link ↗
- 7.Palatin Technologies. Positive topline results from Phase 2 ulcerative colitis study of oral MC1R agonist PL-8177. Press release — Phase 2 UC topline (NCT05466890). 2025. link ↗
- 8.Kondo M, Suzuki T, Kawano Y, et al. Dersimelagon, a novel oral melanocortin 1 receptor agonist, demonstrates disease-modifying effects in preclinical models of systemic sclerosis. Arthritis Res Ther. 24(1):210. 2022. link ↗
- 9.SynAct Pharma (ACR Convergence). Efficacy and safety of the biased melanocortin receptor agonist AP1189/resomelagon with methotrexate in DMARD-naïve rheumatoid arthritis: the EXPAND trial. Conference abstract — Phase 2b RA (missed primary endpoint; subgroup signal). 2023. link ↗