Phthalazine derivatives as phosphodiesterase 4 inhibitors

The present invention relates to phthalazine derivatives, to the pharmaceutical compositions containing them and to their use as phosphodiesterase 4 inhibitors. Phosphodiesterases are a family of isoenzymes which constitute the basis of the main mechanism of cAMP (cyclic adenosine-3′,5′-monophosphate) hydrolytic inactivation. cAMP has been shown to be the second messenger mediating the biologic response to many hormones, neurotransmitters and drugs [Krebs Endocrinology Proceedings of the 4th International Congress Excerpta Medica, 17-29, 1973]. When the suitable agonist binds to the cell surface, the adenylated cyclase activates and turns Mg -ATP into cAMP. cAMP modulates the activity of the majority, if not of all the cells contributing to the pathophysiology of various respiratory diseases, both of allergic origin and not. It follows that an increase of the cAMP concentration yields beneficial effects such as airway smooth muscle relaxation, inhibition of the mast cell mediator release (basophil granulose cells), suppression of the neutrophil and basophil degranulation, inhibition of the monocyte and macrophage activation. Thus, compounds able of activating adenylate cyclase or of inhibiting phosphodiesterases could suppress the undesired activation of the airway smooth muscle and of a great number of inflammatory cells. The present invention provides a compound selected from the group including: N-3-acetyl-1-(3,5-dichloropyridin-4-ylmethyl)-5-cyclopentyloxy-6-methoxy-4H-phthalazine; 6,7-dimethoxy-1-pyridin-4-ylmethyl-4-thiazol-2-yl-phthalazine; 1-(6,7-dimethoxy-4-pyridin-4-ylmethyl-1H-phthalazin-2-yl)ethanone; 2-methanesulphonyl-6,7-dimethoxy-4-pyridin-4-ylmethyl-1,2-dihydrophthalazine; 2-formyl-6,7-dimethoxy-4-pyridin-4-ylmethyl-1,2-dihydrophthalazine; 1-(6,7-dimethoxy-4-pyridin-4-ylmethyl-1H-phthalazin-2-yl)-1-imidazol-1-ylmethanone; 1-(3,5-dichloro-pyridin-4-ylmethyl)-3-methansulphonyl-6-difluoromethoxy-5-(tetrahydro-furan-2-yloxy)-4H-phthalazine; N→O derivatives thereof; and pharmaceutically acceptable salts thereof. The invention also provides a pharmaceutical composition, which contains a therapeutically effective amount of the above compound in admixture with a pharmaceutically acceptable carrier.