Synthesis of photoactivable inhibitors of osteoclast vacuolar ATPase

Amides of (2 Z,4 E )-5-[(5,6-dichloroindol-2-yl)]-2-methoxy- N -[3-[4-[3-(carboxymethoxy)phenyl)] piperazin-1-yl]propyl]-2,4-pentadienamide ( 1 ) and of 5-(5,6-dichloro-2-indolyl)-2-methoxy-2,4-pentadienoic acid ( 2 ) are strong inhibitors of the vacuolar ATPase located on the plasma membrane of osteoclasts. In order to understand which V-ATPase subunit is involved in the interaction with these novel inhibitors, analogues containing a photoactivable group and an iodine atom were designed. A series of alcohols or amines containing the photoactivable trifluoroaziridinophenyl or benzophenone moiety and an iodine atom were linked to the above acids via an ester or amide group. These compounds could be thereafter used as a radioactive photoprobe to label the protein. Whereas the compounds containing the photoactivable groups maintained good inhibitory activity, the introduction of the bulky iodine atom was generally detrimental, decreasing potency significantly. Better results were obtained by linking 3-(4-aminopiperidinomethyl)-3′-iodobenzophenone to 3-ethoxy-4-(2-(5,6-dichlorobenzimidazolyl))benzoic acid to give the corresponding amide 27, that inhibited vacuolar ATP-ase with a IC 50 =140 nM. The feasibility of introducing a radioactive 125 I atom was ascertained by exchanging the iodine with a tributylstannyl group, that was again substituted by iodine.