Inhibitory effects of hesperetin derivatives on guinea pig phosphodiesterases and Their ratios between high- and low-affinity rolipram binding

The phosphodiesterase (PDE)4 molecule exists as two distinct conformers, PDE4_H and PDE4_L, which have high and low affinities, respectively, for the selective PDE4 inhibitor, rolipram. The inhibition of PDE4_H and PDE4_L is associated with adverse responses, such as nausea, vomiting, and gastric hypersecretion, and with anti-inflammatory and bronchodilator effects, respectively. We determined the therapeutic (PDE4_H/PDE4_L) ratios of hesperetin-7-O-methylether, hesperetin-5,7,3′-O-trimethylether (HTME), hesperetin-7-O-acetate, hesperetin-7,3′-O-diacetate, hesperetin-5,7,3′-O-triacetate (HTA), hesperetin-5,7,3′-O-tripropionate, hesperetin-5,7,3′-O-tributyrate, hesperetin-5,7,3′-O-triisobutyrate, and hesperetin-5,7,3′-O-tripivatate, and compared these ratios to those of hesperetin, hesperetin-7,3′-O-dimethylether, hesperidin, and hesperidin-3′-O-methylether to identify derivatives with therapeutic ratios and to characterize the structure-activity relationships among these compounds. The activities of PDE isozymes 1 through 5 were measured using a two-step procedure using [³H]adenosine 3′,5′-cyclic monophosphate or [³H]guanosine 3′,5′-cyclic monophosphate as substrates. The inhibitory concentration (IC₅₀) for 50% of PDE4 inhibition and effective concentration (EC₅₀) for replacing 50% of [³H]rolipram binding on high-affinity rolipram-binding sites was taken as the PDE4_L and PDE4_H value, respectively. The HTME and the HTA dually inhibited PDE3 and PDE4, and displayed PDE4_H/PDE4_L ratios of 18.3 and 20.8, respectively, suggesting that they may be candidate drugs for treating asthma and chronic obstructive pulmonary disease (COPD) because the combined inhibition of PDE3 and PDE4 has synergistically anti-inflammatory and bronchodilator effects in COPD patients.