Apelin

What is Apelin?

Apelin is an endogenous peptide hormone and the originally identified ligand of the APJ receptor (APLNR), a class A G-protein-coupled receptor. It is synthesized as a 77-amino-acid preproprotein (preproapelin) and proteolytically processed into several bioactive C-terminal isoforms — apelin-36, apelin-17, apelin-13, and the pyroglutamated form (Pyr¹)apelin-13, which is the dominant and most stable isoform in human plasma. Apelin and its receptor are widely expressed in the heart, vascular endothelium, adipose tissue, and central nervous system. Apelin is a research and clinical-pharmacology molecule rather than a consumer therapeutic: there is no FDA-approved apelin peptide drug, and human use has been limited to investigational infusions in clinical physiology studies. The therapeutic frontier is currently small-molecule APJ agonists (BMS-986224, AMG 986) being developed for heart failure.

What Apelin Is Investigated For

Apelin's strongest research signal is cardiovascular. It is one of the most potent endogenous positive inotropes known — short IV infusions of (Pyr¹)apelin-13 increase cardiac index and reduce peripheral vascular resistance in both healthy subjects and patients with chronic heart failure, without the arrhythmogenic or calcium-overload liabilities of catecholamines. Apelin and its receptor are downregulated in advanced heart failure, which plausibly contributes to contractile decline. That pharmacology has driven industry programs targeting the APJ receptor with orally bioavailable small molecules (BMS-986224, AMG 986), both of which have now cleared first-in-human studies. The metabolic story is still maturing: apelin is secreted by adipocytes, upregulated by insulin and TNF-alpha, and restores glucose tolerance in obese insulin-resistant mice, but whether pharmacologic apelin signaling translates to clinically meaningful human metabolic benefit is unproven. The longevity / sarcopenia angle — apelin as an exercise-induced exerkine whose decline tracks age-related muscle loss — is provocative but still confined to rodent and observational human data. What apelin is not: an available peptide therapeutic. The native peptide has a vanishing plasma half-life (minutes), is not sold as an approved drug, and the "research-peptide" apelin products marketed online have no validated human dosing, purity, or safety data.

History & Discovery

Apelin was discovered in 1998 by Kazuhiko Tatemoto and colleagues at Takeda Chemical Industries and the University of Tokyo, who isolated it from bovine stomach extracts while systematically hunting for the endogenous ligand of APJ, an orphan G-protein-coupled receptor that had been cloned by O'Dowd in 1993 on the basis of its sequence similarity to the angiotensin II type 1 receptor (the name "APJ" originally stood for "putative receptor protein related to AT1"). Tatemoto's group used extracellular acidification of APJ-expressing CHO cells as a functional readout and tracked the active peptide through successive chromatography steps. Sequencing revealed a 77-amino-acid preproprotein whose C-terminal 36 amino acids encoded the bioactive peptide, with shorter C-terminal fragments (apelin-17 and apelin-13) also retaining full APJ activity. The pyroglutamated isoform (Pyr¹)apelin-13 was subsequently identified as the dominant and most stable form in human plasma, and has become the de facto reference apelin for human pharmacology studies. The following two decades produced a rapid expansion of apelinergic biology: adipokine identity (Boucher 2005), cardiac inotropic effects in humans (Japp 2010), APJ cross-regulation with ACE2 in the renin-angiotensin system, and exerkine / longevity roles (Vinel 2018, Rai 2017). In 2013–2014, two independent groups identified ELABELA / Toddler / APELA as a second, structurally distinct endogenous APJ ligand, resolving a long-standing puzzle about APJ biology in early development. The current era is defined by small-molecule APJ agonist drug programs (BMS-986224, AMG 986) attempting to translate apelin's cardiovascular pharmacology into a heart-failure therapeutic with acceptable pharmacokinetics.

How It Works

Apelin is a hormone your body makes that tells the heart to pump harder while simultaneously relaxing the blood vessels — a combination that could be valuable for a weakened heart. It also appears to help muscles use sugar more efficiently and may play a role in how the heart and body age. It is a heavily studied biology target, but the peptide itself is not an approved drug, and efforts to turn it into a therapy have focused on designing small molecules that mimic its effects.

Apelin signals through APJ (APLNR), a class A G-protein-coupled receptor structurally related to the angiotensin II type 1 receptor. APJ couples primarily through Gαi/o, with additional Gαq and β-arrestin signaling that are important for biased-agonist drug design. In the cardiovascular system, apelin activation of APJ on cardiomyocytes produces positive inotropy via PLC, PKC, and Na+/H+ and Na+/Ca2+ exchanger activity, increasing myofilament calcium sensitivity without raising cytosolic calcium load — a mechanism distinct from β-adrenergic stimulation. On endothelium, apelin activates eNOS to produce NO-mediated vasodilation, and it upregulates ACE2 transcription, counter-regulating the angiotensin II axis (apelin is itself degraded by ACE2, producing a negative-feedback loop). In adipose tissue, apelin is secreted as an adipokine, is upregulated by insulin and TNF-alpha, and in skeletal muscle promotes glucose uptake and mitochondrial biogenesis through AMPK-dependent pathways. In the CNS, APJ is expressed in hypothalamus, cortex, and spinal cord, with roles in fluid homeostasis, cardiovascular regulation, and pain modulation. Apelin isoforms differ in potency and pharmacology: apelin-36 and apelin-17 are more potent at receptor internalization, while (Pyr¹)apelin-13 is the most stable circulating form and has been the principal isoform used in human infusion studies. ELABELA (also called Toddler / APELA) is a structurally distinct second endogenous APJ ligand identified in 2013–2014, with non-overlapping developmental and adult roles.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about Apelin:

• 011. Whether chronic APJ agonism produces durable hemodynamic and clinical benefit in heart failure, or whether receptor desensitization limits efficacy over time.
• 022. Whether biased APJ agonists that preferentially engage G-protein signaling over β-arrestin recruitment can separate inotropic benefit from desensitization and cardiac hypertrophy — a central hypothesis driving current drug-discovery efforts.
• 033. How apelin and ELABELA signaling differ functionally at the same receptor, and whether isoform- or ligand-biased therapies offer differentiated clinical profiles.
• 044. Whether the striking rodent data on apelin, sarcopenia reversal, and healthspan translates at all to human aging, given that no human trial of apelin-pathway agonism for aging endpoints has been conducted.
• 055. Whether pharmacologic apelin elevation produces clinically meaningful metabolic benefit (insulin sensitivity, glycemic control) in humans, or whether the adipokine biology is primarily a physiologic regulator rather than a drug target.
• 066. The long-term safety of sustained APJ agonism, including effects on cardiac remodeling, vascular structure, and renal function.

Forms & Administration

Apelin is not available as an approved therapeutic in any form. Research use of native apelin peptides is essentially limited to investigational IV infusions for acute hemodynamic studies, because the plasma half-life of apelin-13 and (Pyr¹)apelin-13 is measured in minutes. Industry development has moved toward orally bioavailable small-molecule APJ agonists (BMS-986224, AMG 986) that are not peptides. Any peptide labeled "apelin" from research-chemical vendors is unvalidated for human use. All injectable peptides should only be administered under the guidance of a qualified healthcare provider. Never self-administer without clinician oversight.

Common Questions

Who Apelin Is NOT For

Drug & Supplement Interactions

No formal drug-interaction database exists for apelin because it is not an approved therapeutic. Mechanistically, interactions would be anticipated with other agents that lower blood pressure or alter cardiac output — renin-angiotensin system inhibitors (ACE inhibitors, ARBs), direct vasodilators, nitrates, and calcium channel blockers could all plausibly produce additive hypotension with acute apelin exposure. Apelin upregulates ACE2 and interacts with the angiotensin II signaling axis, so pharmacologic cross-talk with RAAS-acting drugs is biologically plausible but not clinically characterized. Apelin is itself degraded by ACE2, neprilysin, and kallikrein; sacubitril/valsartan (an ARB plus neprilysin inhibitor) would be expected to alter apelin pharmacokinetics. These are theoretical considerations, not documented clinical interactions.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions