Inhibition of human cardiac cyclic AMP-phosphodiesterases by R 80122, a new selective cyclic AMP-phosphodiesterase III inhibitor: A comparison with other cardiotonic compounds

Abstract

Four cyclic AMP (cAMP)-phosphodiesterases (PDE) belonging to families I, II, III and IV were identified in homogenates from human failing hearts. On fractionation of cardiac membranes, the cyclic GMP (cGMP)-inhibitable cAMP- PDE III copurified with the sarcoplasmic reticulum. cAMP-PDE activities were separated from the soluble fraction by DEAE-ion exchange chromatography and identified as belonging to the four different families of cAMP-PDEs. Various cAMP-PDE inhibitors, mostly cardiotonic compounds, were tested for their inhibitory potency on the different cAMP-PDEs and their selectivity for the type III isoenzyme was determined. Isobutylmethylxanthine, papaverine, theophylline and dipyridamole inhibited PDE activity in a weak and nonselective manner. Milrinone, enoximone, adibendan, pimobendan, bemoridan and the newly synthesized 1,2,3,5-tetrahydro-2-oxoimidazo[2,1-b]quinazoline derivatives, R 81267 and R 80122 were selective PDE III inhibitors. However, the IC50 values on this enzyme varied from 10 μM for enoximone to 0.036 μM for R 80122. The selectivity of the drugs for PDE III was calculated by division of the IC50 value for PDE I, II or IV by the IC50 value for PDE III. PDE I/PDE III ratio ranged from 95 for enoximone to near 28,000 for R 80122; the PDE II/PDE III ratios ranged from 95 for enoximone to 3,500 for R 80122. Although there was strong variation between the drugs, most of them showed a high selectivity for PDE III in comparison to PDE I and to PDE II. In contrast, PDE IV appeared to be more sensitive to these substances. PDE IV/PDE III ratios ranging from 14 for milrinone to near 2,900 for R 80122 demonstrated this series of drugs to consist of minor and highly selective PDE-inhibitors. Inhibition kinetics for R 80122 on PDE III were determined. K(i) of R 80122 for PDE III was 0.007 μM. In the same set of experiments, K(i) for cGMP and milrinone were 0.13 and 0.3 μM. cGMP and milrinone behaved as competitive inhibitors whereas R 80122 showed a mixed type inhibition. From these data we concluded that, within the series of cardiotonic drugs we tested, major differences in inhibitory potency as well as in selectivity for PDE III are apparent. In view of putative compartmentalized occurrence of PDE III on the sarcoplasmic reticulum, we suggest that high isoenzyme selectivity might be required to evoke a selective PDE subtype inhibition within cardiac cells.