SYNERGISTIC INTERACTIONS BETWEEN SELECTIVE PHARMACOLOGICAL INHIBITORS OF PHOSPHODIESTERASE ISOZYME FAMILIES PDE-III AND PDE-IV TO ATTENUATE PROLIFERATION OF RAT VASCULAR SMOOTH-MUSCLE CELLS

The interaction between selective inhibitors of 3',5'-cyclic-nucleotide phosphodiesterase (PDE) III (cyclic GMP inhibited phosphodiesterase) and selective inhibitors of PDE IV (Ro 20-1724 inhibited phosphodiesterase) to attenuate fetal bovine serum-stimulated incorporation of [H-3]thymidine into DNA and cell proliferation was studied in a line (A10) of vascular smooth muscle cells (VSMC). The nonselective PDE inhibitors 3-isobutyl-1-methylxanthine (IBMX) and papaverine attenuated DNA synthesis with EC(50) values (16 and 18 mu M, respectively) in the same range as their published IC50 values (2-50 and 2-25 mu M, respectively) as PDE inhibitors. The selective PDE III inhibitors CI-930 and cilostamide used alone attenuated DNA synthesis with EC(50) values (>300 and 5.3 mu M, respectively) that were much higher than published IC50 values (0.15-0.46 and 0.005-0.064 mu M, respectively) for inhibition of PDE III. In the presence of the PDE IV inhibitor rolipram (10 mu M), their EC(50) values were shifted (0.66 and 0.16 mu M, respectively) much closer to their respective IC50 values. When the selective PDE IV inhibitors rolipram and Ro 20-1724 were used alone, they attenuated DNA synthesis with EC(50) values (111 and >100 mu M, respectively) much higher than their IC50 values (0.6-2.6 and 2-13 mu M, respectively) as inhibitors of PDE IV, but 10 mu M CI-930 (PDE III inhibitor) shifted their EC(50) values (0.56 and 1.5 mu M, respectively) much closer to their IC50 values. In experiments that assessed VSMC proliferation using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] method, IBMX and papaverine attenuated proliferation with EC(50) values (27 and 58 mu M, respectively) close to their IC50 values. CI-930 and cilostamide used alone did not cause 50% attenuation of proliferation at the highest concentrations tested (100 and 10 mu M, respectively). In the presence of 5 mu M rolipram, however, their effects were enhanced greatly with EC(50) values (0.86 and 0.23 mu M, respectively) that were close to their IC50 values as PDE III inhibitors. Similarly, rolipram and Ro 20-1724 attenuated VSMC proliferation with EC(50) values close to their IC50 values in the presence (2.1 and 4.6 mu M, respectively) but not in the absence (>100 and >10 mu M, respectively) of 2 mu M CI-930. The interactions between PDE III inhibitors and PDE IV inhibitors to attenuate DNA synthesis and VSMC proliferation were synergistic as determined by the combination index. The data demonstrate that the synergistic interactions that attenuate incorporation of [H-3]thymidine into DNA are accompanied by synergistic attenuations of VSMC division. The closeness of the EC(50) values of PDE III inhibitors when PDE IV is blocked, and of the PDE IV inhibitors when PDE III is blocked, to their respective IC50 values as selective PDE inhibitors supports the view that the effects on DNA synthesis and cell division are caused by inhibition of the respective PDE isozymes. A hypothesis is proposed to explain the synergistic interactions.