Inhibitory effects of quercetin derivatives on phosphodiesterase isozymes and high-affinity [³H]-rolipram binding in guinea pig tissues

Rolipram has high (PDE4_H) and low (PDE4_L) affinities for phosphodiesterase (PDE)-4, respectively. In general, it is believed that inhibitions by PDE4_H and PDE4_L are respectively associated with an adverse response and with anti-inflammatory and bronchodilating effects. This has provided a rational basis for designing new compounds with high PDE4_H/PDE4_L ratios. In the present study, we attempted to determine the PDE4_H/PDE4_L ratios of quercetin (1), qercetin-3-O-methylether (3-MQ, 2), quercetin-3,7,4′-O- trimethylether (ayanin, 3), quercetin-3,7,3′,4′-O- tetramethylether (QTME, 4), quercetin-3,5,7,3′,4′-O-petamethylether (QPME, 5), quercetin-3,5,7,3′,4′-O-pentaacetate (QPA, 6), and quercetin-3-O-methyl-5,7,3′,4′-O-tetraacetate (QMTA, 7). The activities of PDE1∼5, which were partially separated from homogenates of guinea pig lungs and hearts, were measured by a two-step procedure using adenosine 3′,5′-cyclic monophosphate (cAMP) with [ ³H]-cAMP or guanosine 3′,5′-cyclic monophosphate (cGMP) with [³H]-cGMP as substrates. The IC₅₀ values of all of these compounds except quercetin (1), 3-MQ (2), and QMTA (7) on PDE1∼5 inhibition were determined. The anti-inflammatory effects of PDE4 inhibitors were reported to be associated with inhibition of PDE4 catalytic activity. Therefore, these IC₅₀ values for PDE4 inhibition were taken as the PDE4_L values. The effective concentration (EC₅₀), at which one half of the [³H]-rolipram bound to high-affinity rolipram binding sites (HARBSs) of brain cell membranes was replaced, was defined as the PDE4_H value. In the present results, the PDE4_H/PDE4 _L ratios of quercetin (1), ayanin (3), and QPME (5) were >30, >19, and 11, respectively (Table 1), which are higher than or equal to that of AWD12-281, the selective PDE4 inhibitor with the greatest potential currently undergoing clinical trials for treating asthma and chronic obstructive pulmonary disease.