COMPARISON OF INDOLIDAN ANALOG BINDING-SITES OF DRUG ANTIBODY AND SARCOPLASMIC-RETICULUM WITH INHIBITION OF CYCLIC-AMP PHOSPHODIESTERASE

Dihydropyridazinone(DHP) derivatives such as indolidan are positive in otropic agents that show inhibition of cyclic AMP phosphodiesterase(PD E) activity. Indolidan inhibition is selective for PDE3 among the seve n PDE gene families. DHP derivatives and related analogs have been use d to define critical regions of the active site of PDE3 isoforms and r adiolabeled analogs have been used to define indolidan sarcoplasmic re ticulum (SR) receptor sites. We report here studies comparing the stru cture-activity relationships (SAR) for PDE3 inhibition with indolidan binding to two types of sites: canine SR and a monoclonal antibody der ived against indolidan conjugated to a hemocyanin. SR and monoclonal a ntibody binding both fit single-site, high affinity models (IC50 = 1.2 and 62 nM) that were near 52 and 360 times that of SR PDES. Indolidan and thirteen analogs showed similar competition with either SR H-3-LY 186126 binding or SR PDE3 inhibition. Antibody binding maintained sele ctivity but showed a different rank order potency for SR binding. Indo le ring C3 methylation increased and DHP ring C4' methylation decrease d indolidan monoclonal antibody binding while both substitutions incre ased SR binding. These studies support the hypothesis that SR PDE3 is a cardiotonic receptor site in myocardial membranes and indicate that models of the structural features of binding sites derived from inhibi tor data alone could produce models with limited topography relative t o the natural ligand.