Phosphodiesterase (PDE) inhibitors are currently a widespread and extensively studied group of anti-inflammatory and anti-fibrotic compounds which may find use in the treatment of numerous lung diseases, including asthma and chronic obstructive pulmonary disease. Several PDE inhibitors are currently in clinical development, and some of them, e.g., roflumilast, are already recommended for clinical use. Due to numerous reports indicating that elevated intracellular cAMP levels may contribute to the alleviation of inflammation and airway fibrosis, new and effective PDE inhibitors are constantly being sought. Recently, a group of 7,8-disubstituted purine-2,6-dione derivatives, representing a novel and prominent pan-PDE inhibitors has been synthesized. Some of them were reported to modulate transient receptor potential ankyrin 1 (TRPA1) ion channels as well. In this study, we investigated the effect of selected derivatives (832—a pan-PDE inhibitor, 869—a TRPA1 modulator, and 145—a pan-PDE inhibitor and a weak TRPA1 modulator) on cellular responses related to airway remodeling using MRC-5 human lung fibroblasts. Compound 145 exerted the most considerable effect in limiting fibroblast to myofibroblasts transition (FMT) as well as proliferation, migration, and contraction. The effect of this compound appeared to depend mainly on its strong PDE inhibitory properties, and not on its effects on TRPA1 modulation. The strong anti-remodeling effects of 145 required activation of the cAMP/protein kinase A (PKA)/cAMP response element-binding protein (CREB) pathway leading to inhibition of transforming growth factor type β₁ (TGF-β₁) and Smad-dependent signaling in MRC-5 cells. These data suggest that the TGF-β pathway is a major target for PDE inhibitors leading to inhibitory effects on cell responses involved in airway remodeling. These potent, pan-PDE inhibitors from the group of 7,8-disubstituted purine-2,6-dione derivatives, thus represent promising anti-remodeling drug candidates for further research.

Phosphodiesterase 7 (PDE7) is an intracellular enzyme that specifically hydrolyzes the second messenger, cyclic-3’,5’-adenosine monophosphate (cAMP), into inactive noncyclic nucleotide, 5’-AMP. To date, many structurally diverse compounds with PDE7 inhibitory properties have been described, including selective PDE7 inhibitors, dual PDE4/PDE7, PDE7/PDE8, and PDE7/GSK-3 inhibitors, and non-selective PDE inhibitors with high affinity for PDE7. Inhibitors of PDE7 have provided beneficial effects in animal models of inflammatory and neurological disorders, including Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and many others. This review is a comprehensive summary of the current state-of-the-art in the field of design and synthesis of PDE7 inhibitors, their physicochemical properties, biological evaluation, and structure-activity relationships as well as it highlights the updated evidence for a potential therapeutic utility of these compounds. Moreover, new approaches to obtain more effective and safer PDE7 inhibitors than those available now are presented.