<img src="https://ml-eu.globenewswire.com/Resource/Download/bb724119-468d-4b1d-8206-3c2175d07f2b?size=2" nborder="0" align="left" hspace="5" vspace="5" title="logo.png" alt="Idorsia Pharmaceuticals Ltd"/><ul type="disc"><li>High-dose cohort confirms the favorable safety profile and the dose-dependent immune response against Clostridioides difficile bacteria and spores</li></ul> Allschwil, Switzerland – June 1, 2026 Idorsia Ltd (SIX: IDIA) today announced additional positive clinical results for IDOR-1134-2831, its vaccine targeting Clostridioides difficile (C. difficile) in a Phase 1 clinical pharmacology trial. The glycan target of this vaccine is present on both bacteria and spores – the transmissible form of the pathogen responsible for spreading infection – of the most clinically relevant C. difficile strains, including hypervirulent variants. Following initial results in June 2025, which provided the first clinical validation for Idorsia’s synthetic glycan vaccine technology, the company advanced the program with a higher-dose cohort. Results from healthy participants receiving the high dose confirmed that the vaccine is safe and well-tolerated. The vaccine induced a dose-dependent IgG response, with all participants receiving the high dose showing immunogenicity. In further exploratory tests, the dose-dependent response was particularly pronounced for IgG1, a subclass known to play a key role in opsonization, marking the pathogen for rapid identification and destruction by the immune system. Idorsia’s vaccine is intended to be developed for the prevention of C. difficile infection (CDI) in patients at risk in both hospital and community settings, with the aim of providing broad protection against circulating strains. Martine Clozel, MD, Chief Scientific Officer &amp; Head of Research at Idorsia, commented: “These new results from the high-dose cohort mark an important step forward for our synthetic glycan vaccine platform and reinforce our conviction that we are pioneering a fundamentally new approach to vaccination. The dose-dependent responses against the glycan target present on both C. difficile bacteria and spores, underscore the potential of this vaccine to go beyond traditional strategies and target the full life cycle of the pathogen. Beyond C. difficile, this technology has the potential to transform the landscape of vaccines, allowing prophylaxis against multiple bacteria and more. The fact that the antigens are synthesized in a chemistry laboratory offers immense opportunity. To fully realize this potential and accelerate development, we intend to advance this program in partnership.” C. difficile infection C. difficile is a spore-forming and toxin-producing bacterium. It is the leading cause of antibiotic-associated diarrhea in developed countries. It infects the gastrointestinal tract, often following disruption of the gut microbiota, for example due to antibiotic use. CDI symptoms can range from mild diarrhea to pseudomembranous colitis, which in some cases – particularly in the elderly – can lead to death. CDI is a major public health threat, particularly because recurrences are frequent, and is increasing with the aging population. In the US, C. difficile is estimated to cause almost 400,000 infections each year. One in eleven people over 65 diagnosed with healthcare-associated CDI die within one month. C. difficile infections cause more than 25,000 deaths each year in the US alone. CDI places a significant economic burden on the healthcare system. The acute care direct costs of CDI are estimated to be $5–6 billion per year in the US and €3 billion per year in Europe. Standard treatments of CDI are efficacious in clearing primary CDI. However, CDI recurs in up to 25% of treated patients. The C. difficile vaccine identified by Idorsia targets both spores and vegetative C. difficile bacteria and, therefore, could prevent not only colonization but also transmission. Frail patients, elderly persons, persons in nursing homes, receiving antibiotic treatment, who are hospitalized, or expected to be hospitalized, represent target populations for this prophylactic vaccine. A unique approach to vaccination – what is a synthetic glycan vaccine? Pathogenic bacteria like C. difficile express carbohydrates (glycans) on their surface which are recognized by the host’s immune system, leading to the production of glycan-specific antibodies. This makes glycans on the surface of pathogens attractive candidates for vaccine development. The synthetic glycan in Idorsia’s C. difficile vaccine is synthesized in a carefully designed chemical process. This transforms a complex biochemical extraction, which is associated with heavy investment, into a purely chemical process driven by medicinal chemistry. Idorsia’s C. difficile synthetic glycan vaccine has the advantage of being stable, immunogenic, fully characterizable, and the process has the potential to enable scalable and cost-efficient manufacturing. About the Phase 1 trial The Phase 1 clinical pharmacology trial is a double-blind, randomized, placebo-controlled, two-part trial to assess the safety, tolerability, and immunogenicity of up to 4 ascending dose levels of the IDOR-1134-2831 vaccine in healthy participants aged 18 to 49 years. About Idorsia’s vaccine portfolio In addition to the C. difficile vaccine that is in Phase 1 clinical development, Idorsia is also developing synthetic glycan vaccines for infections caused by Klebsiella pneumoniae and Neisseria gonorrhoeae. Notes to the editor About Idorsia The purpose of Idorsia is to discover, develop and commercialize innovative medicines to help more patients. To achieve this, we will develop Idorsia into a leading biopharmaceutical company, with a strong scientific core. Headquartered near Basel, Switzerland – a European biotech hub – Idorsia has a highly experienced team of dedicated professionals, covering all disciplines from bench to bedside; QUVIVIQ™ (daridorexant), a different kind of insomnia treatment with the potential to revolutionize this mounting public health concern; strong partners to maximize the value of our portfolio; a promising in-house development pipeline; and a specialized drug discovery engine focused on small-molecule drugs that can change the treatment paradigm for many patients. Idorsia is listed on the SIX Swiss Exchange (ticker symbol: IDIA). For further information, please contact: Investor &amp; Media Relations Idorsia Pharmaceuticals Ltd, Hegenheimermattweg 91, CH-4123 Allschwil +41 58 844 10 10 investor.relations@idorsia.com – media.relations@idorsia.com – www.idorsia.com The above information contains certain "forward-looking statements", relating to the company's business, which can be identified by the use of forward-looking terminology such as “intend”, "estimates", "believes", "expects", "may", "are expected to", "will", "will continue", "should", "would be", "seeks", "pending" or "anticipates" or similar expressions, or by discussions of strategy, plans or intentions. Such statements include descriptions of the company's investment and research and development programs, business development activities and anticipated expenditures in connection therewith, descriptions of new products expected to be introduced by the company and anticipated customer demand for such products and products in the company's existing portfolio. Such statements reflect the current views of the company with respect to future events and are subject to certain risks, uncertainties and assumptions. Many factors could cause the actual results, performance or achievements of the company to be materially different from any future results, performances or achievements that may be expressed or implied by such forward-looking statements. Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual results may vary materially from those described herein as anticipated, believed, estimated or expected. 
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Idorsia reports new positive results with its breakthrough C. difficile vaccine – underscoring the unique potential of its glycan technology

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