Abstract Introduction: Refractory chronic cough (RCC) poses significant diagnostic and therapeutic challenges characterized by persistent coughing that fails to respond to standard treatments. RCC is often associated with conditions like asthma and gastroesophageal reflux disease (GERD), but in many patients it can be unexplained chronic cough (UCC). The underlying mechanism is thought to be cough hypersensitivity syndrome, but it is unclear if this is consistent across subgroups of RCC/UCC and whether this can be identified. Mannitol, a dry powder bronchial challenge agent, induces both airway bronchoconstriction and cough, yet its effects across different RCC/UCC phenotypes remain poorly understood. This study aims to characterize the baseline demographics and clinical profiles of RCC/UCC patients and investigate their responses to mannitol-evoked cough and airway hyper-responsiveness (AHR).Methods: Patients diagnosed with RCC (≥1-year duration) with demonstrated hypersensitivity to inhaled mannitol (cough dose ratio [CDR] ≥ 12 participated in this Phase 2a study assessing a novel TRPA-1 antagonist (GDC6599, NCT05660850). Patients were categorized into RCC-asthma (atopic, n=6; non-atopic, n=8), RCC-GERD (n=12), and UCC (n=15). Assessments included demographics, 24-hour cough frequency, cough severity on a 0-100mm visual analog scale (VAS), Hull Airway Reflux Questionnaire (HARQ) scores, mannitol-evoked cough dose ratio (CDR/100mg mannitol) and airway hyperresponsiveness (AHR; PD15), mannitol response dose ratio (RDR), FEV1, asthma control questionnaire (ACQ-5), fractional exhaled nitric oxide (FeNO), and blood eosinophil count. Spearman correlation between b assessments were explored.Results: Forty-three patients were recruited (age, sex, cough duration, BMI). VAS and HARQ scores indicated significant symptom burden across all subgroups, with the highest HARQ observed in GERD (median 49, IQR: 38.2, 55.8). Median baseline FEV1, FeNO, and blood eosinophil counts were within normal range. The 24-hour awake cough count was high across all subgroups. Mannitol-induced AHR (achieving a PD15) was present in 13 patients (30%), with asthma showing a higher frequency (43%) and lower PD15 dose (median 73; min, max: 7, 345) compared to the overall population. The correlation between 24-hour cough frequency with CDR was weak, and no correlation between cough frequency with markers of inflammation. CDR modestly correlated with RDR (R=0.54 suggesting a possible relationship between mannitol induced airway hyperresponsiveness and cough hypersensitivityConclusions: Spontaneous cough is an independent neuro-phenotype in patients with RCC/UCC. Results should be interpreted cautiously due to the small sample size, but suggest airway and cough hypersensitivity to mannitol are modestly correlated and may contribute to increased cough burden in patients with CC.