In this study, the synthesis and in vitro and in vivo pharmacological investigations of a new series of phthalazinone/pyridazinone hybrids with both PDE3 and PDE4 inhibitory activities are described. These compounds combine the pharmacophores of recently discovered 4a,5,8,8a-tetrahydro-2H-phthalazin-1-one-type inhibitors of PDE4 and the well-known 2H-pyridazin-3-one-type PDE3 inhibitors such as the tetrahydrobenzimidazoles. Most of the synthesized compounds are pharmacologically spoken PDE3/PDE4 hybrids. All hybrids show potent PDE4 inhibitory activity (pIC(50) = 7.0-8.7), whereas the pIC(50) values for inhibition of PDE3 vary from 5.4 to 7.5. In general, analogues with a 5-methyl-4,5-dihydropyridazinone moiety exhibit the highest PDE3 inhibitory activities. The highest in vivo antiinflammatory activity is displayed by phthalazinones 43 and 44 showing, at a dose of 30 micromol/kg po, 46% inhibition of arachidonic acid (AA) induced mouse ear edema. No correlation was found between the in vitro PDE3 and/or PDE4 inhibitory activity and the in vivo antiinflammatory capacity after oral dosing.

A series of 4-aryl-substituted cis-4a,5,8,8a-tetra- and cis-4a,5,6,7,8,8a-hexahydro-2H-phthalazin-1-ones with high inhibitory activity toward cAMP-specific phosphodiesterase (PDE4) was synthesized. To study structure-activity relationships various substituents were introduced to the 2-, 3-, and 4-positions of the 4-phenyl ring. Substitution at the 4-position of the phenyl ring was restricted to a methoxy group, probably due to unfavorable steric interactions of larger groups with the binding site. The introduction of many alkoxy substituents including distinct ring systems and functional groups was allowed to the 3-position. It was found that in general the cis-4a,5,8,8a-tetrahydro-2H-phthalazin-1-ones are more potent than their hexahydrophthalic counterparts, the best activity residing in (4-imidazol-1-yl-phenoxy)butoxy analogue 16o (pIC(50) = 9.7).

The synthesis and biological activities of a series of N-substituted cis-4a,5,6,7,8,8a-hexa- and cis-4a,5,8,8a-tetrahydro-2H-phthalazin-1-ones are described. It was found that compounds bearing a cycloalkyl group at the 2-position exhibit the highest PDE4 inhibitory activities (pIC(50) = 8.6-9.4). The N-cycloheptyl- and N-adamantanyltetrahydrophthalazinones (7h, 8, 10, 11) show high in vivo antiinflammatory activities after oral application. Additionally, some phthalazinones were found to exhibit potent suppression of LPS-induced TNFalpha release and show moderate potency against fMLP-stimulated production of ROS.

Novel Selective PDE4 Inhibitors. Part 2. Synthesis and Structure-Activity Relationships of 4-Aryl-Substituted cis-Tetra-and cis-Hexahydrophthalazinones.-A series of title aryl-substituted, partially hydrogenated phthalazinones (I) -(III) is prepared as previously demonstrated by cyclocondensation of the corresponding γ-keto acids with hydrazine followed by N-alkylation. The influence of aryl substituents on the activity as selective PDE4 inhibitors is tested in detail, and derivative (IIe) is identified as most potent PDE4 inhibitor of the series. -(VAN DER MEY, MARGARETHA; HATZELMANN, ARMIN; VAN KLINK, GERARD P. M.; VAN DER LAAN, IVONNE J.; STERK, GEERT J.; THIBAUT, ULRICH; ULRICH, WOLF R.; TIMMERMAN, HENDRIK;

Recently, we reported that 4-catechol-substituted cis-(+/-)-4a,5,6,7,8,8a-hexa- and cis-(+/-)-4a,5,8,8a-tetrahydro-2H-phthalazin-1-ones show potent inhibition of phosphodiesterase (PDE4) activity, while the corresponding trans racemic mixtures exhibit only weak to moderate activity. To determine the absolute configuration and PDE4 inhibitory activity of the individual cis-enantiomers, several optically active phthalazinones have been synthesized. The enantiomers of the various gamma-keto acids, used as starting materials, were resolved in a classical way by the formation of diastereomeric salts, and each was converted to optically active phthalazinone in an enantioselective manner. The absolute configuration of the (+)-enantiomer of cis-hexahydrophthalazinone (+)-12 was determined by X-ray crystallography. The carbon atoms at the 4a and 8a positions were found to have the S- and R-configuration, respectively. In the present series of hexa- and tetrahydrophthalazinones, stereoselectivity for PDE4 inhibition is observed; the cis-(+)-enantiomers of the phthalazinones display high inhibitory activity, whereas their (-)-counterparts exhibit only weak to moderate activity. It is likely that all cis-(+)-phthalazinones have a (4aS,8aR)-configuration and vice versa for the cis-(-)-analogues. In the current series, the N-adamantan-2-yl analogue (+)-14 shows the most potent inhibition of PDE4 (pIC(50) = 9.3); the corresponding (-)-enantiomer is 250-fold less active. In addition, the N-substituted tetrahydrophthalazinones under study were investigated for their in vivo antiinflammatory activities by examining the suppression of arachidonic acid (AA) induced mouse ear edema formation. In this assay analogues (+)-14 and (+)-15 were found to be potent antiinflammatory agents showing about 50% inhibition at 30 micromol/kg po.