Why make allosteric modulators of PDE4? There are four PDE4 genes in mammals (PDE4A-B) and each gene encodes multiple transcripts. PDE4 splice isoforms are grouped as “long”, “short” and “super-short” based on their N-terminal sequence. All splice isoforms contain the UCR2 regulatory domain as shown in the cartoon.
Previous efforts to develop drug targeting PDE4 focused on the discovery of simple inhibitors that bind in the active site competitively with cAMP. Because there is little amino acid sequence difference in the active site of the catalytic domain between the different PDE4 genes, compounds binding in the active site inhibit all forms of PDE4. Although PDE4 inhibitors were shown to have potent benefit in animal models of inflammatory, neurological and psychiatric diseases, when entered into human clinical trials, the compounds were found to cause nausea and vomiting. Their poor tolerability precluded further clinical development. Therefore another approach to modulating PDE4 activity is needed.
PDE4 exists in cells as a dimer of two identical subunits. Allosteric modulators of PDE4 are able to close UCR2 over the active site by binding to both UCR2 and the active site. UCR2 is colored green in the view below and the catalytic domain is colored blue. Two aromatic arms extending from the allosteric modulator clamp UCR2 (Phe 196) on the one hand while the opposite side of the modulator forms a hydrogen bond to a key active site glutamine (Gln 535). This closes the active site as shown in the surface view. The two UCR2 domains in the PDE4 dimer display negative cooperativity such that only one active site can be closed at a time. This has the consequence that PDE4 enzymatic activity is not completely inhibited. Why is this important? This is the differentiating feature of a PDE4 allosteric modulator as compared to a simple, active-site director inhibitor. The allosteric mechanism provides a physiological ceiling on the maximum inhibition of PDE4. This results in better tolerability as it becomes more difficult to increase cAMP in cells to non-physiological concentrations. See Burgin et al. (2010) Design of phosphodiesterase 4D (PDE4D) allosteric modulators for enhancing cognition with improved safety. Nature Biotechnology 28:63-70.