NPR1 Agonist Antibody XXB750 Paradoxically Worsens Heart Failure in Phase 2 Trial

Bottom line: Directly activating the natriuretic peptide receptor with a monoclonal antibody backfired — XXB750 worsened heart failure biomarkers and clinical outcomes, underscoring why many peptide-receptor pathways resist simple agonist approaches.

Natriuretic peptides (ANP, BNP, CNP) are endogenous peptide hormones released by the heart in response to volume overload and wall stress. They signal through natriuretic peptide receptor 1 (NPR1), a guanylate cyclase receptor, to promote vasodilation, natriuresis, and cardiac unloading. Enhancing this pathway has long been a therapeutic target in heart failure.

Study Design

This Phase 2 randomized, double-blind trial, published in Nature Medicine on March 30, 2026, enrolled patients with heart failure and a left ventricular ejection fraction < 50%.¹ Participants were randomized to receive XXB750 (a human monoclonal antibody targeting NPR1), sacubitril/valsartan (active comparator), or placebo. The primary endpoint was change in NT-proBNP levels from baseline.

Key Findings

• NT-proBNP increased: Contrary to the hypothesis, XXB750 raised NT-proBNP levels from baseline, while both placebo and sacubitril/valsartan showed expected reductions or stability.¹
• cGMP decreased: The downstream second messenger of NPR1 activation, cyclic GMP, paradoxically fell in the XXB750 arm — the opposite of what receptor activation should produce.¹
• More worsening heart failure events: Patients receiving XXB750 experienced significantly higher rates of clinical worsening (hospitalizations and urgent visits for heart failure) compared with both placebo and sacubitril/valsartan.¹
• Functional antagonism hypothesis: The investigators concluded that XXB750 may displace endogenous natriuretic peptides from NPR1 without effectively activating the receptor's intracellular guanylate cyclase domain, effectively behaving as a competitive antagonist.¹

Why It Matters

This is an instructive negative trial for the peptide therapeutics field. The natriuretic peptide system is a validated target — sacubitril/valsartan (Entresto), which works by blocking neprilysin-mediated degradation of endogenous natriuretic peptides, is a cornerstone of heart failure therapy. But the XXB750 results demonstrate that directly activating a peptide receptor with an antibody does not necessarily replicate the physiological effects of the native peptide ligand.

The finding also reinforces a broader principle: peptide receptor signaling often depends on ligand dynamics, receptor internalization kinetics, and biased agonism that antibodies may not faithfully reproduce.

For background on natriuretic peptides and cardiovascular applications, see our peptide research archive.

Clinical Implication

Clinicians should continue prescribing sacubitril/valsartan for eligible heart failure patients. XXB750 will not advance to Phase 3. This trial does not diminish the importance of natriuretic peptide pathways — rather, it confirms that how you augment them matters as much as whether you augment them.

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¹ The NPR1 agonist antibody XXB750 in heart failure: a phase 2 randomized trial. Nature Medicine. Published March 30, 2026. DOI: 10.1038/s41591-026-04313-w