Alderley Park, Cheshire, U.K., May 21, 2026
Infex Therapeutics announced positive Phase IIa clinical trial results for RESP-X (INFEX702) in patients with non-cystic fibrosis bronchiectasis (NCFB) colonised with Pseudomonas aeruginosa (Pa). The data, presented at the 2026 American Thoracic Society (ATS) International Conference in Orlando, Florida, demonstrated that the investigational monoclonal antibody achieved its primary objective of showing excellent safety and tolerability, while also generating promising early efficacy signals in a patient population with significant unmet medical need.
RESP-X is a first-in-class anti-virulence monoclonal antibody designed to target the Type 3 Secretion System (T3SS) of Pseudomonas aeruginosa, specifically the PcrV protein, which plays a key role in bacterial virulence and immune evasion. Unlike conventional antibiotics that directly kill bacteria, RESP-X works by blocking toxin delivery mechanisms, helping restore the body’s immune response against infection. The company believes this mechanism could provide a novel therapeutic strategy for chronic respiratory infections where antibiotic resistance continues to rise globally.
Phase IIa Trial Demonstrated Strong Safety and Quarterly Dosing Potential
The randomized, double-blind, placebo-controlled Phase IIa study evaluated two RESP-X dose levels, 6 mg/kg and 10 mg/kg, in NCFB patients chronically colonised with Pseudomonas aeruginosa. Conducted at the Liverpool University Hospitals NHS Foundation Trust Clinical Research Facility in the United Kingdom, the study assessed safety, pharmacokinetics, immunogenicity, lung deposition, and exploratory efficacy endpoints. According to the company, RESP-X met all primary and secondary study objectives.
Importantly, the therapy demonstrated an excellent safety profile, with no severe or life-threatening treatment-emergent adverse events related to the drug and no discontinuations caused by adverse events. All reported adverse events were mild to moderate, and investigators observed no infusion-related reactions. The study also showed favorable pharmacokinetics, including a 28.8-day half-life, supporting the potential for convenient quarterly dosing. Bronchoscopy and bronchoalveolar lavage confirmed effective drug deposition in lung epithelial lining fluid, while researchers detected no anti-drug antibodies, indicating low immunogenicity risk.
Encouraging Early Efficacy Signals in High-Risk Bronchiectasis Patients
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Beyond safety, RESP-X demonstrated promising biological and clinical activity in patients with chronic Pseudomonas aeruginosa colonisation. Investigators confirmed that all bacterial isolates collected during the trial encoded the targeted PcrV protein, validating complete target coverage across enrolled patients. The study further showed encouraging reductions in disease exacerbations among Pa-positive patients over the 180-day observation period compared with the year before treatment.
The efficacy findings are particularly significant because there are currently no approved therapies specifically designed to prevent infective exacerbations caused by Pseudomonas aeruginosa in NCFB patients. Chronic colonisation with Pa is associated with worsening lung damage, recurrent infections, frequent hospitalizations, and increased mortality risk. Infex Therapeutics stated that the positive efficacy signals, combined with strong safety and dosing convenience, support advancement of RESP-X into the next phase of clinical development. The company plans to engage with regulatory authorities regarding a future efficacy-focused study.
RESP-X Expands Infex Therapeutics’ Anti-Infective Pipeline
RESP-X was in-licensed from Shionogi & Co. Ltd. and is being developed initially for long-term management of bronchiectasis patients colonised with Pseudomonas aeruginosa. However, the program may eventually expand into additional respiratory and infectious disease indications, including cystic fibrosis, COPD, asthma, ventilator-associated pneumonia, bloodstream infections, burns, diabetic foot ulcers, and complicated urinary tract infections caused by Pa.
According to the company, up to six million patients across major global markets suffer from non-cystic fibrosis bronchiectasis, with approximately 30% chronically colonised with Pseudomonas aeruginosa. The absence of effective preventative therapies for these patients has created a substantial unmet clinical need and commercial opportunity. By focusing on bacterial virulence rather than bacterial destruction, Infex Therapeutics aims to provide a differentiated strategy that may help reduce antibiotic resistance pressures while improving long-term patient outcomes.
Source: Infex Therapeutics press release
