Diagonal Therapeutics Announces First Patient Dosed in Phase 1/2 DIAMOND Clinical Trial of DIAG723 for Hereditary Hemorrhagic Telangiectasia (HHT)

First-in-class clustering agonist antibody DIAG723 is designed to restore ALK1 signaling, the genetically disrupted pathway that causes abnormal vascular development, bleeding, and arteriovenous malformations (AVMs) in HHT

Preclinical studies of DIAG723 demonstrated prevention and reversal of key HHT manifestations, indicating the potential for disease modification

WATERTOWN, Mass., July 16, 2026 (GLOBE NEWSWIRE) -- Diagonal Therapeutics, a clinical-stage biotechnology company developing disease-modifying clustering antibodies that correct dysregulated signaling in severe diseases, today announced that the first patient has been dosed in DIAMOND, a Phase 1/2 multicenter, randomized, double-blind, placebo-controlled trial evaluating DIAG723 in people living with hereditary hemorrhagic telangiectasia (HHT).

HHT is a rare, serious genetic vascular disorder characterized by abnormal and fragile blood vessels and recurrent bleeding, often leading to chronic anemia, dependence on iron therapy or blood transfusions, and a significant impact on quality of life. Patients are also affected by AVMs, abnormal vascular shunts formed in organs such as the lungs, liver, and brain, which if ruptured, can cause serious and potentially life-threatening complications. There are currently no therapies approved for the treatment of HHT.

“HHT is a lifelong, genetically driven vascular disorder that can affect nearly every aspect of a patient’s life, from recurrent nosebleeds and chronic anemia to serious complications arising from AVMs,” said John Lee, M.D., Ph.D., Chief Medical Officer of Diagonal. “Despite this substantial burden, HHT treatment has historically focused on managing bleeding and its consequences rather than correcting the underlying genetic driver. DIAG723’s mechanism of action, designed to restore normal ALK1 signaling, represents a differentiated approach for the HHT community, and the initiation of DIAMOND is an important step toward understanding its potential for patients.”

DIAG723 is a first-in-class clustering agonist antibody designed to restore normal ALK1 signaling, the pathway impaired by the loss-of-function mutations that cause HHT. By bridging receptors at the cell surface, DIAG723 is designed to reactivate and normalize signaling, addressing the root cause of fragile blood vessels, recurrent bleeding, anemia, and AVM formation. In preclinical studies, this approach prevented and reversed the AVMs that underlie serious organ complications of HHT, indicating the potential to evolve treatment beyond symptomatic management toward disease modification.

About the DIAMOND Trial

DIAMOND (NCT07623525) is a Phase 1/2, multicenter, randomized, double-blind, placebo-controlled trial evaluating the safety, tolerability, pharmacokinetics, target-engagement biomarkers, and preliminary efficacy of subcutaneously administered DIAG723 in adults with HHT. The trial is enrolling up to 93 individuals with HHT across three parts. Part A (Phase 1) evaluates single ascending doses of DIAG723 to characterize safety, tolerability, pharmacokinetics, and effects on target-engagement biomarkers of ALK1 pathway activity. Part B (Phase 2) evaluates multiple ascending doses over a 14-week treatment period to assess safety and preliminary efficacy, including effects on the frequency and severity of epistaxis (nosebleeds), and on hematological parameters. Part C (Phase 2) enrolls a multidose cohort of HHT patients with co-existing pulmonary arterial hypertension (PAH) to assess safety and exploratory effects on HHT- and pulmonary-related measures. To learn more about the DIAMOND trial and HHT, visit diagonaltx.com/patients.

About DIAG723

DIAG723 is a first-in-class clustering agonist antibody designed to restore ALK1 signaling, a pathway impaired in HHT by loss-of-function mutations that disrupt normal vascular development. In preclinical HHT studies, DIAG723 prevented and reversed arteriovenous malformations (AVMs) and improved anemia. AVMs, a hallmark of HHT, can rupture and cause bleeding-related complications in multiple organs. DIAG723 has received orphan drug designation from the U.S. FDA and the EMA for the treatment of HHT. DIAG723 is also being evaluated for its potential in pulmonary arterial hypertension (PAH), where dysregulated ALK1 signaling contributes to vascular hyperproliferation. In preclinical models of PAH, DIAG723 prevented both disease development and cardiac remodeling, and improved hemodynamics.

About Diagonal Therapeutics

Diagonal Therapeutics is a clinical-stage biotechnology company developing disease-modifying clustering antibodies that correct dysregulated signaling in severe diseases. While most antibody drugs work by blocking overactive signals, Diagonal’s clustering antibodies bridge receptors at the cell surface to reactivate normal signaling, reaching diseases not well addressed by blocking antibodies. Our lead program, DIAG723, our first clustering antibody to enter the clinic, is designed to address the underlying biology of both hereditary hemorrhagic telangiectasia (HHT) and pulmonary arterial hypertension (PAH). Our platform pairs computational and experimental discovery to build a pipeline across hematological, cardiovascular, renal, and other diseases, offering the potential to deliver life-changing therapies for patients. For more information, please visit www.diagonaltx.com.

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