The present invention relates to a pharmaceutically acceptable acid addition salt of : (i) 9, 10-dimethoxy-2-(2, 4, 6-tri-methylphenylimino)-3-(N-carbamoyl-2-aminoethyl)-3, 4, 6, 7-tetrahydro-2H-pyrimido[6, 1-a]isoquinolin-4-one (RPL554); and (ii) ethane-1, 2-disulfonic acid, ethanesulfonic acid, methanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, hydrochloric acid, hydrobromic acid, phosphoric acid or sulfuric acid.