Apelin Hormone Fails to Boost Urine Output in First Human Trial of Induced Hyponatremia

For decades, physiologists have known that the body’s salt and water balance hinges on a delicate hormonal tug-of-war. Arginine vasopressin, the antidiuretic hormone released by the posterior pituitary, tells the kidneys to hold on to water. Apelin, a peptide hormone secreted by the same gland, appears to push in the opposite direction, antagonising vasopressin’s effects. In hyponatremic rodents, giving apelin analogues reliably increased urine output, raising hopes that apelin-based drugs could one day drain the excess water that accumulates in patients with the syndrome of inappropriate antidiuresis. But a new human trial, published in Nature Communications, has delivered a sobering reality check: in healthy volunteers with artificially induced antidiuresis, intravenous [Pyr¹]apelin-13 did not produce a clinically relevant increase in urine output compared with placebo.

The study, led by Sophie Monnerat and Sven Lustenberger, who contributed equally, under the supervision of Mirjam Christ-Crain at the University Hospital Basel, was a double-blind, randomised, placebo-controlled crossover trial registered as NCT06277336 and funded by the Swiss National Science Foundation. Fifteen healthy adults, seven of them female, with a mean age of 28 years (standard deviation, 8), attended three study visits each. Because each participant received all three treatments in randomised order, the crossover design allowed each person to serve as their own control, reducing variability and increasing statistical power despite the modest sample size.

To recreate the syndrome of inappropriate antidiuresis in the laboratory, the researchers used a combination of desmopressin, a synthetic vasopressin analogue; water loading; and hypotonic saline. Desmopressin suppresses the kidneys’ ability to excrete free water, mimicking the inappropriate vasopressin activity seen in patients. Water loading then floods the system, and hypotonic saline completes the picture, driving serum sodium down into the hyponatremic range. The result is a controlled, reversible model of the biochemical derangement that affects millions of people worldwide, from those with lung cancer to those taking certain antidepressants.

Once the artificial syndrome was established, participants received one of three treatments as a three-hour intravenous infusion: high-dose [Pyr¹]apelin-13 at 1800 nanomoles, low-dose [Pyr¹]apelin-13 at 180 nanomoles, or placebo. [Pyr¹]apelin-13 is the pyroglutamated form of the 13-amino-acid apelin peptide, the predominant circulating isoform in humans and the version most studied for its vasopressin-antagonising properties. The primary endpoint was cumulative urine output from the start of the infusion until one hour after its completion, a window chosen to capture the drug’s full diuretic effect.

The verdict was unambiguous. Neither the high dose nor the low dose of [Pyr¹]apelin-13 produced a clinically relevant difference in urine output compared with placebo. In rodents, apelin analogues had reliably increased urine output in hyponatremia models, but the corresponding human data had been lacking until now. This trial provides those data, and they do not support the translational promise that the animal work suggested.

Why did the peptide fail in humans when it worked in mice? Several explanations are plausible. Rodent models of hyponatremia may not faithfully reproduce the human vasopressin axis, and the apelin receptor system may differ between species in ways that matter for drug response. The artificially induced syndrome, while biochemically convincing, may not capture the full complexity of the clinical condition, where vasopressin activity is chronic and tissue-level adaptations have had time to develop. It is also possible that the infusion doses, though chosen to span a wide range, did not achieve sufficient receptor occupancy in the relevant renal compartments, or that the peptide’s short half-life limited its diuretic effect within the observation window.

The negative result is particularly notable because the trial was designed with rigorous methodology. The double-blind, placebo-controlled crossover design, the use of a validated artificial model of inappropriate antidiuresis, and the pre-specified primary endpoint all reflect a high standard of clinical pharmacology. The study was conducted across multiple institutions, with collaborators at the University of Edinburgh’s Centre for Cardiovascular Science, the Royal Infirmary of Edinburgh, the University of Cambridge, Erasmus Medical Center in Rotterdam, and LMU University Hospital in Munich, bringing together expertise in endocrinology, renal medicine, and cardiovascular pharmacology.

For patients with the syndrome of inappropriate antidiuresis, the current treatment landscape remains limited. Fluid restriction is the mainstay, but it is often poorly tolerated. Vasopressin receptor antagonists exist but carry risks and are not suitable for everyone. The hope that apelin-based therapies could offer a gentler, physiology-mimicking alternative now faces a significant setback. The peptide’s failure to increase urine output in this controlled setting suggests that the vasopressin-antagonising properties observed in animal models do not translate simply to human renal physiology.

Yet the trial should not be read as the end of the apelin story. The study tested only acute intravenous administration in healthy volunteers with an artificially induced condition. Chronic dosing, different formulations, longer-acting analogues, or patient populations with established disease might behave differently. The artificially induced syndrome, while biochemically convincing, may not capture the full complexity of the clinical condition. Future research may need to explore whether apelin’s role in salt and water homoeostasis operates through mechanisms that require longer exposure or different physiological contexts to become clinically relevant.

The study, published on 16 October 2026 in Nature Communications as an open-access article, received support from the University of Basel, the University Hospital of Basel, and the Swiss National Science Foundation. The authors declared no competing interests. The negative finding, while disappointing for the translational agenda, exemplifies the value of rigorous human trials: they prevent the research community from investing further resources in approaches that do not work, and they redirect attention toward mechanisms that might. In the ongoing effort to understand how the posterior pituitary orchestrates the body’s water balance, apelin has revealed itself to be a more complicated character than the rodent data suggested.

Subject of Research: Health sciences

Article Title: Intravenous [Pyr¹]apelin-13 in Healthy Volunteers with Artificially-Induced Syndrome of Inappropriate Antidiuresis – a double-blind, randomised, placebo-controlled crossover study

Article References: Monnerat, S., Lustenberger, S., Bizzozero, C. A., Chapman, F. A., Santos de Jesus, J., Kustos-Toth, E., Vogt, D. R., Bruce, L., Little, R., Davenport, A. P., Maguire, J. J., Refardt, J., Bidlingmaier, M., Dhaun, N., & Christ-Crain, M. (2026). Intravenous [Pyr¹]apelin-13 in Healthy Volunteers with Artificially-Induced Syndrome of Inappropriate Antidiuresis – a double-blind, randomised, placebo-controlled crossover study. Nature Communications. https://doi.org/10.1038/s41467-026-78333-y

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78333-y

Keywords: apelin, hyponatremia, vasopressin, clinical trial, SIADH, desmopressin, water balance, endocrinology, Intravenous, apelin-13, Healthy, Volunteers