Dose Ranging Study of RPL554 in Chronic Obstructive Pulmonary Disease (COPD) Patients

Inclusion Criteria: * Provide informed consent * Male or female aged 40 to 75 years * Meeting specified contraception requirements * 12-lead electrocardiogram with heart rate 50-90 beats per minute, QT interval corrected using Fridericia's formula (QTcF) ≤450 msec (males) or ≤470 msec (females), QRS interval ≤120 msec, PR interval ≤200 msec and no clinically significant abnormalities * Capable of complying with all study restrictions and procedures, including ability to use the study nebulizer correctly. * Body mass index (BMI) 18 to 35 kg/m2 and minimum weight 45 kg. * COPD diagnosis with symptoms compatible with COPD for at least 1 year * Clinically stable COPD in the previous 4 weeks * Ability to perform acceptable and reproducible spirometry. * Post-bronchodilator spirometry at screening must demonstrate FEV1/forced vital capacity (FVC) ratio of ≤0.70 and FEV1 must be ≥40 % to ≤80% of predicted normal * Chest X-ray (posterior-anterior) at screening, or chest X-ray, magnetic resonance imaging (MRI) or computed tomography (CT) scan in the last 12 months, showing no abnormalities which are both clinically significant and unrelated to COPD. * Meet the concomitant medication restrictions and be expected to do so for the rest of the study. * Current and former smokers with a smoking history of ≥10 pack years. * Capable of withdrawing long acting bronchodilators until the end of the treatment period, and short acting bronchodilators for 8 hours prior to administration of study medication. Exclusion Criteria: * A history of life-threatening COPD including Intensive Care Unit admission and requiring intubation. * COPD exacerbation requiring oral steroids in the previous 3 months * A history of one or more hospitalizations for COPD in the previous 6 months * Lower respiratory tract infection treated with antibiotics in the previous 3 months * Evidence of cor pulmonale or clinically significant pulmonary hypertension. * Patients with a current diagnosis of asthma, active tuberculosis, lung cancer, bronchiectasis, sarcoidosis, lung fibrosis, interstitial lung diseases, sleep apnea, known alpha-1 antitrypsin deficiency or other active pulmonary diseases. * Previous lung resection or lung reduction surgery. * Oral therapies for COPD (e.g. oral steroids, theophylline, and roflumilast) in the previous 3 months and throughout the study. * Pulmonary rehabilitation, unless such treatment has been stable from 4 weeks prior to Visit 1) and remains stable during the trial. * A history of, or reason to believe a subject has, drug or alcohol abuse in the previous 3 years. * Received an experimental drug within 30 days or five half-lives of the first dose * Prior exposure to RPL554. * Women who are pregnant or breast-feeding. * Patients with a history of current uncontrolled disease that the Investigator believes are clinically significant. * myocardial infarction in the previous 6 month; congestive heart failure, a history of unstable or uncontrolled hypertension, or has been diagnosed with hypertension in last 3 months. * Use of oral beta blockers. * Major surgery (requiring general anesthesia) in the previous 6 weeks, lack of full recovery from surgery at screening, or planned surgery through the end of the study. * History of malignancy of any organ system within 5 years, with the exception of localized skin cancers (basal or squamous cell). * Clinically significant abnormal values for safety laboratory tests * Significant non-compliance in previous investigational studies or with prescribed medications. * Requirement for oxygen therapy, even on an occasional basis. * Known hypersensitivity to RPL554 or its excipients/components. * Abnormal clinically significant 12 lead Holter findings, * Any other reason that the Investigator considers makes the subject unsuitable to participate.