ReviewThe next generation of PDE4 inhibitors
Introduction
Of the 11 families of phosphodiesterase (PDE) responsible for the hydrolysis of cAMP and cGMP 1., 2., the cAMP-specific PDE4s are encoded by four genes (A–D) (3••). PDE4s are particularly abundant in inflammatory and immune cells and in airway smooth muscles, where inhibition of PDE4 blocks cell trafficking, cell proliferation and attenuates the production of inflammatory mediators, cytokines and reactive oxygen species 4., 5••.. PDE4 inhibitors are generally anti-inflammatory and bronchodilatory in animal models 4., 5••., 6.. A number of them are being evaluated in the clinic and they have shown promising clinical efficacy for asthma 7•., 8•., chronic obstructive pulmonary disease (COPD) 6., 9••. and atopic dermatitis (10). However, PDE4 inhibitors also produce the characteristic dose-limiting side effects of nausea and emesis that have hampered their clinical development 11., 12., 13.. Cilomilast (1, Fig. 1), which recently completed Phase III clinical trials, also suffered from a limited therapeutical index because of nausea and emesis (9••). To complement recent reviews on PDE4 biology 3••., 14., 15., PDE4 inhibitors 16••., 17•. and their therapeutical potential 4., 5••., 18., 19•., this article focuses on recent developments (2000–2001) in understanding PDE4 catalysis, inhibitor binding and their emetic response. In addition, it highlights several PDE4 inhibitors that were reported to have improved pre-clinical profiles during this period.
Section snippets
Complexity and specificity of PDE4-mediated signalling
Each of the four PDE4 genes encodes a number of splice variants that share identical catalytic and carboxy-terminal domains. The total number of PDE4 gene products identified exceeds 17 and their tissue-specific expression is tightly regulated in response to hormones and other stimuli 3••., 14., 20.. Their upstream conserved regions (UCRs) and divergent amino termini contain signals for localisation, protein–protein interaction and activity regulation, thereby allowing the fine-tuning of cAMP
PDE4 catalytic machinery
PDE4 enzymes are divalent cation-dependent hydrolases 36., 37.. The atomic structure of PDE4B (38••) and a detailed binding study (39••) have provided new insights into its catalytic machinery, substrate specificity and inhibitor binding and have clarified the molecular origin of its two conformational states. The active site is formed by hydrophobic and negatively charged amino acids residing on several α helices. Of the 21 residues absolutely conserved across all 11 families of PDEs, 8 are
Molecular origins of the two PDE4 conformers
The reversible metal ion cofactor binding results in the presence of two coexisting PDE4 conformers (apoenzyme and holoenzyme) that bind inhibitors differentially (39••). Because inhibitor binding to either conformer leads to inhibition of catalysis, this causes the inhibitor potency to partition between its apoenzyme and holoenzyme binding affinity. The high affinity rolipram-binding site of PDE4, which was previously thought to be distinct from its catalytic site 43., 44., is in fact the
Emesis response of PDE4 inhibitors
Nausea and emesis remain the major obstacles in developing PDE4 inhibitors 9••., 11., 12., 45., 46.. Because all PDE4 inhibitors are emetic to a certain degree, it suggests that cAMP elevation in emetic centres is coupled with their emetic response. However, whether emesis is mediated via a specific PDE4 subtype remains to be established. Several PDE4 subtypes have been identified in areas of the brain that have been implicated with the emetic response 22., 23., 47., 48.. The recent
Recent advances in the development of PDE4 inhibitors
Cilomilast (1) is the most advanced inhibitor in clinical development. Efficacy in COPD was observed at a dose of 15mg twice daily with nausea and vomiting detected at 20mg twice daily (9••). On October 31 2000, SmithKline Beecham announced that “Phase III clinical trials of Ariflo for the treatment of COPD have been completed and analysed. Although results indicate clinical benefits, an additional confirmatory trial will be required” (55). These reports suggest that the therapeutic index of
Therapeutical potentials of PDE4 inhibitors
PDE4 inhibitors are currently in clinical trials for the treatment of asthma, COPD, atopic dermatitis, rheumatoid arthritis, multiple sclerosis and Crohn's disease. The rationale behind these therapeutic areas have been discussed in detail in several recent reviews 5••., 17•., 18., 73., 74., 75.. Here we highlight new evidence to support their potential utilities in other indications.
Cellular studies of PDE4 inhibitors indicate that their potency is positively coupled to the rate of cAMP
Conclusion
The therapeutic potential of PDE4 inhibitors in the clinic has thus far been hampered by their dose-limiting side effects of nausea and emesis. The recent advances in understanding the molecular interactions between PDE4 and its inhibitors, the specific functions of PDE4 isozymes and the emetic response of PDE4 inhibitors should open new research avenues and modify our approaches to identifying the next generation of PDE4 inhibitors. The atomic structure of PDE4 and the ability to dissect the
Update
The substrate and inhibitor specificities of PDE4 have recently been explored using site-directed mutagenesis 90., 91.. The muscle-selective A-kinase anchoring protein (mAKAP) assembles a PKA/PDE4D3 signaling module (92). The in vitro anti-inflammatory and immunomodulatory potential of Roflumilast and its in vivo efficacy in airway disease models have also been reported 93., 94., 89.
Acknowledgements
The authors thank Rick Friesen, Yves Girard, Dave Percival and Brian Kennedy at Merck Frosst for comments and suggestions during the preparation of the manuscript.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
•of special interest
••of outstanding interest
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