Pyridazinones structure—activity relationship

The most potent among the compounds tested were the pyridazi-nones (28, R1 = Ph, R2, R3, R4, R5 = H, n = 2-5) [101]. Structure-activity relationships have been investigated in detail with this type of compound. Also, thio-analogues of compounds (28) (3-pyridazinethione derivatives) as well as 2-aminoalky]-6-aryl-3(2//)-pyridazinones were claimed in patents as gastric secretion inhibitors or anti-ulcer agents [100, 102, 103]. 

A series of pyrazolo[3,4-, pyridazinones 430 and analogues, potentially useful as peripheral vasodilators, were synthesized and evaluated as inhibitors of PDE5 extracted from human platelets. Several of them showed ICso values in the range 0.14-1.4 pM. A good activity and selectivity profile versus PDE6 was found for compound 430 (6-benzyl-3-methyl-l-isopropyl-4-phenylpyrazolo[3,4-r/]pyridazin-7(6/7)-one). Structure-activity relationship studies demonstrated the essential role played by the benzyl group at position 6 of the pyrazolopyridazine system. Other types of pyridazinones fused with five- and six-membered heterocycles (pyrrole, isoxazole, pyridine, and dihydropyridine), as well as some open-chain models were prepared and evaluated. Besides the pyrazole, the best of the fused systems proved to be isoxazole and pyridine <2002MI227>. 

Structure-activity relationships of potential positive inotropic benzimidazolyl pyridazinones have been studied in detail [93MI(28)129 [93MI(28)141j. There is also a report on the cardiotonic activity of 2,9-dihydro-6-(5-methyl-3-oxo-2,3,... 

The close agreement between the experimental and calculated (Equation 9) ratios of 18 2/18 3 support exclusion of the 4-hydroxylphenyl analogue from the calculations. Examination of Equation 9 shows an interdependence between the biological activity and the hydrophobic properties of the chemical used, commonly found with many QSAR equations. This interdependent relationship is determined by the and terms, respectively. These terms control phenomena of hydrophobic interactions with receptors and phenomena of transport and distribution within the total biological systems. The occurrence of squared terms of the hydrophobic parameter in structure-activity correlations has been explained on the assumption that the compound has to penetrate several lipophilic-hydrophilic barriers or compartments on its way to the site of action (16, 17). This is consistent with the uptake of pyridazinones by roots and sbsequent translocation to the shoots (chloroplast) as the site of action (13). 

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