Comparison of RPL554 With Placebo and Salbutamol in Asthmatic Patients

Inclusion Criteria: * Provided written informed consent * Males agree not to donate sperm and either be abstinent or use adequate contraception. Females to be post-menopausal or surgically sterile * Non-smoker or ex-smoker \>6 months * Diagnosed asthma for at least 6 months * Pre-bronchodilator FEV1 ≥60% and ≤90% of predicted normal value and ≥1.5 L at screening * Increase in FEV1 of 15% within 30 minutes after a 2.5mg dose of nebulised salbutamol * Systolic blood pressure 90 to 145 mmHg, diastolic blood pressure 50 to 90 mmHg and heart rate 45 to 80 beats per minute (bpm) after resting for 5 minutes in a supine position (average from two measurements) * Capable of withdrawing from LABAs, LAMAs and SAMAs before screening and during study and SABAs before screening and for 8 hours before each dose Exclusion Criteria: * Asthma exacerbation in the last 3 months * Any prior life threatening episode of asthma (intensive care admission) * Any clinically significant disease or disorder or clinically relevant screening result * QTcF interval \>450 ms or QT interval \>500 ms or other abnormality in ECG * History of ischemic heart disease or heart failure. History of recurrent or current clinically significant arrhythmia or ECG abnormality as judged by the investigator * Treatment with systemic glucocorticosteroids within 30 days before screening * A suspected/manifested infection according to WHO risk classification 2, 3 or 4