The same applies also to intermediate formally obtained by e
The same applies also to intermediate , formally obtained by elimination of the R substituent from compound . If compared to the thienopyrimidine derivatives (series , C), the thieno[3,2-]pyridazin-5(4H)-ones – explored herein seem to maintain similar size and shape and also analogue structural characteristics that could be important for the binding at ARs. In particular, in addition to nature and position of the R and R substituents on both the scaffolds, also the hydrogen bond acceptor atoms have been conserved at similar positions. In fact, the 4-oxo function of the thienopyridazinone compounds could be considered to replace the N1 bcr-abl inhibitors of the lead series . While the 5N-ethyl substituted derivatives , – maintain comparable hA binding affinity with respect to , the analogue 5N-amino compounds – do not recognize this AR subtype or fit it little (compare to ). However, considering the close structural analogy between the two series, the most unexpected result was the great difference in binding data for the hAAR since only thienopyridazinones , and bind with low affinity at this subtype, most of them showing a percentage of inhibition below 50%. To further explore the SAR of this new scaffold, the heteroaromatic ring at position 7 was then replaced with an ethyl group in order to evaluate the requirements at this position. Compound was prepared but it was ineffective at all the ARs. An important goal was synthesis of the 2-phenyl-substituted compound in which similarities with the well-known AR antagonists, i.e. 3,5-disubstituted-8-arylxanthine derivatives, can be recognized., This modification was performed with the aim of exploring the versatility of the thienopyridazinone as building block for future modifications at the level of the condensed thiophene ring. Despite the low hAAR affinity of derivative , the result opened the door to rational future modification at position 2 of the present scaffold. A molecular docking investigation was performed to analyze the binding affinity data of the herein reported thienopyridazinone derivatives. For this task, the recently reported crystal structure of the hA receptor covalently bound to the xanthine-based ligand DU172(hA p = 7.4; pdb code: ; 3.2-Å resolution) was refined within MOE software and employed as protein target. For this purpose, the covalently bound ligand DU172 and the engineered segment bRIL were removed. Moreover, the IL3 missing loop was restored and the wild type receptor sequence changed by the presence of the N159A mutation. Docking analyses were performed with CCDC Gold software. The lowest score and most populated kind of docking conformations presents the thienopyridazinone scaffold located in the binding cavity between the Phe171 (EL2) and the Leu250 side chains (see compound , ). The carbonyl group makes a polar interaction with the Asn254 side chain, and this appears as the key ligand-target interaction for this series of compounds. As indicated above, the 4-oxo function is considered to replace the N1 atom of the lead series . Docking results confirm these data as the compounds present a binding arrangement similar to that of the newly synthesized derivatives, with the N1 atom providing a polar contact with the Asn254 side chain. The 5-alkyl group is located between residues of TM3 (Val87 and Leu88), TM5 (Met180), and TM6 (Trp247) helices. Docking results suggest that the hydrophobic TM3-TM5-TM6 sub-pocket, in which the 5-ethyl group of is inserted, was unsuitable for receiving the polar 5-amino of derivative . The 7-substituent gets oriented toward residues of TM2 (Ala66), TM3 (Val87), and TM7 (Thr278) domains. The slightly lower volume of the 7-heteroaryl moiety of compounds and with respect to that of the 7-phenyl (derivative ) and the 7-(2-pyridinyl) group (compound ) appears to provide a better interaction with the receptor cavity. Also the polarity of the 7-heteroaryl group seems to play a positive effect in the receptor-ligand interaction (compare compounds and to and , respectively).