We present the in vitro characterization of a novel phosphodiesterase type 4 inhibitor, CDP840 (R-[+]-4-[2-¿3-cyclopentyloxy-4-methoxyphenyl¿-2-phenylethyl]pyridine), which has shown efficacy in a phase II allergen challenge study in asthmatics without adverse effects. CDP840 potently inhibits PDE-4 isoenzymes (IC50 2-30 nM) without any effect on PDE-1, 2, 3, 5, and 7 (IC50 > 100 microM). It exhibited no significant selectivity in inhibiting human recombinant isoenzymes PDE-4A, B, C or D and was equally active against the isoenzymes lacking UCR1 (PDE-4B2 and PDE-4D2). In contrast to rolipram, CDP840 acted as a simple competitive inhibitor of all PDE-4 isoenzymes. Studies with rolipram indicated a heterogeneity within all the preparations of PDE-4 isoenzymes, indicative of rolipram inhibiting the catalytic activity of PDE-4 with both a low or high affinity. These observations were confirmed by the use of a PDE-4A variant, PDE-4A330-886, which rolipram inhibited with low affinity (IC50 = 1022 nM). CDP840 in contrast inhibited this PDE-4A variant with similar potency (IC50 = 3.9 nM), which was in good agreement with the Kd of 4.8 nM obtained from [3H]-CDP840 binding studies. Both CDP840 and rolipram inhibited the high-affinity binding of [3H]-rolipram binding to PDE-4A, B, C, and D with similar Kd app (7-19 nM and 3-5 nM, respectively). Thus, the activity of CDP840 at the [3H]-rolipram binding site was in agreement with the inhibitor's activity at the catalytic site. However, rolipram was approximately 100-fold more potent than CDP840 at inhibiting the binding of [3H]-rolipram to mouse brain in vivo. These data clearly demonstrate that CDP840 is a potent selective inhibitor of all PDE-4 isoenzymes. In contrast to rolipram, CDP840 was well-tolerated in humans. This difference, however, cannot at present be attributed to either isoenzyme selectivity or lack of activity in vitro at the high-affinity rolipram binding site (Sr).