The invention provides a compound for use in treating or preventing a disease or condition selected from cystic fibrosis, chronic obstructive pulmonary disease (COPD), asthma, mild pulmonary disease, bronchitis, bronchiectasis, idiopathic bronchiectasis, allergic bronchopulmonary aspergillosis, sinusitis, rhinosinusitis, CFTR-related metabolic syndrome (CRMS), pancreatitis, idiopathic chronic pancreatitis and Sjörgren's syndrome, or for use in preventing male infertility caused by congenital absence of the vas deferens, in a patient by modulating CFTR activity, which compound is 9,10-dimethoxy-2-(2,4,6-trimethylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3,4,6,7-tetrahydro-2H-pyrimido[6,1-a]isoquinolin-4-one or a pharmaceutically acceptable acid addition salt thereof. The invention also provides a composition comprising (i) 9,10-dimethoxy-2-(2,4,6-trimethylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3,4,6,7-tetrahydro-2H-pyrimido[6,1-a]isoquinolin-4-one or a pharmaceutically acceptable acid addition salt thereof and (ii) a leukotriene receptor antagonist. The invention also provides a composition comprising (i) 9,10-dimethoxy-2-(2,4,6-trimethylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3,4,6,7-tetrahydro-2H-pyrimido[6,1-a]isoquinolin-4-one or a pharmaceutically acceptable acid addition salt thereof and (ii) a CFTR potentiator or a CFTR corrector.