Enantiomer Discrimination Illustrated by the High Resolution Crystal Structures of Type 4 Phosphodiesterase †

Type 4 phosphodiesterase (PDE4) inhibitors are emerging new treatment for a number of disorders including asthma and chronic obstructive pulmonary disease. Here we report the biochemical characterization on the second generation inhibitor (+)-1 (L-869298, IC50 = 0.4 nM) and its enantiomer (−)-1 (L-869299, IC50 = 43 nM), and their co-crystal structures with PDE4D at 2.0 Å resolution. In spite of the 107-fold affinity difference, both enantiomers interact with the same sets of residues in the rigid active site. The weaker (−)-1 adopts an unfavorable conformation in order to preserve the pivotal interactions between the Mg-bound waters and N-oxide of pyridine. These structures support a model in which inhibitors are anchored by the invariant glutamine at one end and the metal-pocket residues at another end. This model provides explanation for most of the observed structure-activity relationship and the metal ion dependency of the catechol-ether based inhibitors and should facilitate their further design