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Human trialscardiovascular

Apelin-13

An endogenous peptide that makes the heart contract harder while dilating blood vessels, a rare combination that has made it a long-standing heart failure research target.

Also known as Pyr-apelin-13, pyroglutamylated apelin-13, APJ agonist, APLN-13

Human trials — Studied in people, typically early phase or small — promising rather than proven.

Real human data exist but they are small acute haemodynamic studies - infusions in healthy volunteers and chronic heart failure patients showing increased cardiac index and reduced vascular resistance. There are no efficacy or outcome trials, no chronic dosing data, and no approved product. The clinical field has largely moved to stabilised analogues and small-molecule APJ agonists because native apelin-13 is cleared too fast to be a drug.

How it works

Apelin is the endogenous ligand for APJ (APLNR), a G-protein-coupled receptor closely related to the angiotensin AT1 receptor but functionally opposed to it. Apelin-13 and its pyroglutamylated form are the most potent naturally occurring fragments. APJ activation in cardiomyocytes increases contractility through a calcium-sensitisation mechanism that does not raise cyclic AMP or oxygen consumption the way beta-agonists do, and in endothelium it drives nitric oxide release and vasodilatation. It also opposes the renin-angiotensin system and antagonises vasopressin-driven fluid retention. In humans, six-hour infusions of Pyr-apelin-13 raise cardiac index and cut systemic vascular resistance in both healthy volunteers and heart failure patients. The problem has always been pharmacokinetic: native apelin is cleared by ACE2 and other peptidases in minutes, so the entire field has moved toward stabilised analogues and small-molecule APJ agonists rather than the native peptide.

Targets: APJ receptor (APLNR), Nitric oxide / eNOS pathway, Cardiomyocyte calcium sensitivity, ACE2 axis

Dosing

ProtocolDoseFrequencyRoute
Human haemodynamic research infusionGiven under invasive or echocardiographic haemodynamic monitoring.—continuous infusion over minutes to hours in study settingsintravenous
Research-chemical subcutaneous use (no validated basis)Not established.—not establishedsubcutaneous
  • · Published human studies infused Pyr-apelin-13 at roughly 30 to 300 nmol/min, and six-hour infusions were used to demonstrate sustained cardiac index improvement. There is no fixed microgram protocol and no validated dose for use outside a research setting.
  • · Apelin-13 sold on the peptide market has no human dosing evidence behind it and its half-life of minutes makes subcutaneous self-administration pharmacologically dubious. Any number quoted in a forum protocol is invented.

Titration

In research infusions the rate is escalated stepwise against blood pressure and cardiac index. Nothing outside that setting is validated.

Cycling

No established cycle. All meaningful human exposure has been within short investigational infusions.

Work out your exact syringe units →

Pharmacology

Half-life
A few minutes in circulation - among the shortest of any cardiovascular peptide.
Onset
Haemodynamic effects appear within minutes of starting an infusion and disappear almost as fast when it stops.
Routes
intravenous, subcutaneous
Molecule
Endogenous 13-amino-acid peptide (apelin C-terminal fragment)
Sequence length
13 amino acids
Molecular weight
1550.8 Da

Handling

Diluent
Bacteriostatic water
Typical mix
2 or 3 mL
Vial sizes
5, 10 mg
Lyophilised
Freezer at -20 C for long-term storage; fridge is acceptable for short periods.
Reconstituted
Refrigerated and used within about 2 weeks; this is a short, peptidase-sensitive peptide with poor solution stability.
Light sensitive
Yes — keep it out of the light

Mixing

Research-chemical vials are handled like any lyophilised peptide. Add diluent slowly down the vial wall and swirl; never shake.

Side effects

  • commonHypotension and flushing— Directly mechanistic; observed in the human infusion studies.
  • commonHeadache
  • commonUnknown long-term effects— Honestly, no one knows. Human exposure has been limited to hours, and APJ signalling touches angiogenesis and cell proliferation.
  • uncommonReflex tachycardia

Do not use if

  • Hypotension or any preload-dependent circulatory state.
  • Active malignancy - APJ signalling drives angiogenesis and its tumour biology in humans is unstudied.
  • Any use outside a research setting, since no safety-qualified product and no validated dose exist.

Combining it

  • synergyangiotensin-1-7 — Both act on the protective, ACE2-linked counter-regulatory side of the cardiovascular peptide system; the pairing is mechanistically coherent but entirely untested in humans.
  • cautionACE inhibitors — Additive vasodilatation and hypotension.
  • conflictangiotensin II — Functionally opposed receptors - APJ activation directly antagonises AT1-mediated vasoconstriction.

What to monitor

  • · Blood pressure and heart rate, which are the only meaningful acute readouts.
  • · Cardiac index or echocardiographic output measures in a research setting.
  • · There is no established bloodwork panel for apelin use because there is no established use.

Legal status

Not approved anywhere for human use. Sold as a research chemical with no clinical validation for self-administration.

References

  • Japp et al. 2010, acute cardiovascular effects of apelin in humans and patients with chronic heart failure, Circulation (trial)
  • Targeting the apelin system for the treatment of cardiovascular diseases, Cardiovascular Research 2023 review (review)

Mechanism in depth

Apelin acts on APJ (APLNR), a class A GPCR that is a close structural relative of the angiotensin AT1 receptor but does not bind angiotensin II. APJ couples primarily to Gi, lowering cAMP, and also recruits beta-arrestin, and the balance between those two arms turns out to matter: G protein signalling drives the inotropic effect while beta-arrestin recruitment is linked to the hypertrophic remodelling that chronic APJ stimulation can produce, which is why biased agonists have become the focus of drug development. The peripheral effects come from two distinct sites. On endothelium, APJ activation raises intracellular calcium, activates eNOS and produces nitric oxide-dependent vasodilation, which is why the forearm response to apelin is abolished by NOS inhibition. On cardiomyocytes, APJ activation increases contractility by raising myofilament calcium sensitivity through sodium-hydrogen exchanger and sodium-calcium exchanger activity rather than by raising cAMP and cytosolic calcium the way beta-agonists do. That distinction is the reason apelin is interesting: it is a positive inotrope that does not raise myocardial oxygen consumption or provoke arrhythmia the way dobutamine does, and it simultaneously dilates rather than constricts. In the human studies, intracoronary apelin-36 raised coronary blood flow and the maximum rate of rise of left ventricular pressure while lowering peak and end-diastolic left ventricular pressures, and systemic [Pyr1]apelin-13 infusion raised cardiac index while lowering mean arterial pressure and peripheral vascular resistance in both heart failure patients and healthy controls. APJ also acts as a functional antagonist of AT1 signalling, so the apelin system sits in direct opposition to the renin-angiotensin axis. All of that is genuinely exciting cardiovascular pharmacology, and all of it has been demonstrated in acute studies lasting minutes to hours in a few dozen people.

What usually goes wrong

The fundamental problem is that apelin-13 is not a drug and cannot be made into one by injecting it. Its half-life is a few minutes, so a subcutaneous injection produces a brief and unmeasurable exposure followed by nothing, and none of the haemodynamic effects demonstrated in the human studies were produced that way: those required continuous intravenous, intracoronary or intrabrachial infusion under invasive monitoring. Chronic APJ stimulation also carries a theoretical concern about beta-arrestin-mediated cardiac remodelling, which is precisely why the drug development effort moved to biased agonists. Add to that the fact that anything sold as apelin-13 outside a laboratory is an unregulated research chemical of unverified identity and purity, and the honest summary is that the genuine and rather beautiful cardiovascular pharmacology of this peptide has essentially nothing to do with what happens when someone injects it subcutaneously.

Bloodwork worth running

MarkerWhenWhy it matters
Blood pressure and heart rateContinuously during any infusion; there is no context in which this compound should be used without monitoring.Systemic apelin infusion lowers mean arterial pressure and peripheral vascular resistance. In an unmonitored setting this is the only effect anyone would actually notice, and it is the one that could cause harm.Act if: Any symptomatic hypotension means stop; the effect resolves within minutes given the half-life.
NT-proBNPBaseline and after any sustained intervention.The natural biomarker for whether anything haemodynamic has actually changed in a heart failure patient. Endogenous apelin levels themselves are reduced in chronic heart failure and rise after mechanical unloading, which is part of the rationale for supplementation.Act if: No established threshold; there is no chronic dosing regimen for which this could be a monitoring parameter.
Echocardiographic cardiac index or equivalentDuring infusion in a research setting.The demonstrated human effect is an increase in cardiac index with reduced systemic vascular resistance. If that is the reason someone is interested in this peptide, it is the thing to measure.

Pharmacokinetics

Crosses blood-brain barrier
partial
Metabolism
ACE2 cleaves the C-terminal phenylalanine to give apelin-13(1-12), which is largely inactive at APJ; this is the same enzyme that converts angiotensin II to angiotensin-(1-7), and apelin is one of its principal alternative substrates. Neprilysin and other peptidases contribute. The pyroglutamyl form, [Pyr1]apelin-13, is protected against aminopeptidase attack at the N-terminus and is the dominant circulating species in human plasma.
Elimination
Enzymatic degradation in plasma and on endothelial surfaces. Renal excretion is irrelevant at this timescale.

Receptor targets

  • APJ / apelin receptor (APLNR)

    Gi-coupled reduction in cAMP plus beta-arrestin recruitment. On endothelium, eNOS activation and nitric oxide-dependent vasodilation; on cardiomyocytes, increased contractility through myofilament calcium sensitisation rather than cAMP elevation.

  • AT1 receptor (functional opposition, not binding)

    APJ and AT1 heterodimerise and APJ signalling functionally antagonises angiotensin II-driven vasoconstriction and hypertrophy. Apelin does not bind AT1.

  • ACE2 (as substrate, not target)

    Apelin is a major ACE2 substrate and also upregulates ACE2 expression, which ties the apelin and renin-angiotensin systems together in both directions.

Trials

  • Japp 2010 acute cardiovascular effects of apelin in humans Phase 1/2 mechanistic study · n=50 · 2010

    A series of randomised, double-blind, placebo-controlled studies in 18 patients with NYHA class II-III chronic heart failure, 6 patients undergoing diagnostic coronary angiography and 26 healthy volunteers. Intrabrachial [Pyr1]apelin-13 caused forearm vasodilation in both groups; intracoronary apelin-36 raised coronary blood flow and dP/dt max while lowering LV pressures; systemic [Pyr1]apelin-13 at 30 to 300 nmol/min raised cardiac index and lowered mean arterial pressure and peripheral vascular resistance.

  • Japp 2008 vascular effects of apelin in vivo in man Phase 1 mechanistic study · 2008

    First demonstration of apelin-mediated vasodilation in human forearm resistance vessels, establishing nitric oxide dependence.

  • Brame 2015 first-in-human study of a biased apelin receptor agonist (MM07) Phase 1 · 2015

    Design, characterisation and first human vascular study of a G protein-biased APJ agonist, intended to keep the inotropic and vasodilatory benefit while avoiding beta-arrestin-linked remodelling. This is where the field went after native apelin.

What to expect, and when

Vascular and inotropic effects appear within minutes of starting an infusion and disappear within minutes of stopping. There is no cumulative effect, no chronic dosing data, and no timeline beyond the duration of the infusion itself.

Stacking and comparisons

There is no established stack because there is no established use. Mechanistically, apelin opposes AT1 signalling, so it is conceptually complementary to ACE inhibitors and ARBs rather than redundant with them, and ACE2 sits at the junction of both systems as the enzyme that inactivates apelin-13 and generates angiotensin-(1-7). Anything that lowers blood pressure adds to apelin's vasodilator effect. The combinations it is actually sold in on the peptide market, typically alongside other cardiovascular or longevity peptides, have no supporting data of any kind.

Against dobutamine and milrinone, the conventional inotropes: apelin raises contractility without raising myocardial oxygen demand or cAMP, which is exactly the profile those drugs lack and the reason their long-term use increases mortality. That is the whole promise. Against the stabilised analogues and small-molecule APJ agonists now in development, native apelin-13 is the proof of concept that has been left behind because it cannot survive in plasma. Against angiotensin II, which acts on the closely related AT1 receptor: functional opposites, and their receptors physically interact.

Rough cost

No approved product. Research-chemical vials are sold but I did not verify pricing, and pricing an unverified compound is not informative.

Genuinely uncertain

  • No human pharmacokinetic parameters beyond the qualitative statement of a few minutes' half-life were resolvable; volume of distribution, clearance, protein binding and bioavailability are all null.
  • Blood-brain barrier penetration is marked partial based on central apelin actions described in animal work, not verified human data.
  • Participant counts for the 2008 forearm study and the 2015 biased agonist study were not resolved from the abstracts and are left null; the 50 participants recorded for the 2010 Circulation study is the sum of the three cohorts described in its abstract.
  • Whether chronic APJ agonism causes adverse cardiac remodelling in humans is unknown; the concern derives from receptor pharmacology and animal data.

Papers