Targeting autoimmunity: Phosphodiesterase inhibitors as immunomodulators—a review of current insights and future direction

Phosphodiesterase (PDE) inhibitors are a class of pharmacological agents that regulate immune and vascular function by controlling the intracellular concentration of key second messengers. By modulating cyclic nucleotide signaling, particularly cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), PDE inhibitors exert pleiotropic effects on vascular tone, immune cell activation, and fibrotic remodeling. Dysregulation of cAMP- and cGMP-dependent pathways contributes to endothelial dysfunction, chronic inflammation, and tissue remodeling in autoimmune, fibrotic, and neurodegenerative diseases. In rheumatology, PDE3, PDE4, and PDE5 isoforms are of particular translational relevance. PDE3 inhibitors improve vasodilation and platelet regulation; PDE5 inhibitors enhance endothelial NO-cGMP signaling and are clinically established in pulmonary arterial hypertension and digital vasculopathy; while PDE4 inhibition suppresses cytokine-driven immune activation by reducing the production of pro-inflammatory mediators, contributing to therapeutic efficacy in conditions such as psoriasis, psoriatic arthritis, and connective tissue disease-associated interstitial lung disease (CTD-ILD). Emerging PDE-targeted agents, including ibudilast and the PDE4B-selective inhibitor nerandomilast, combine immunomodulatory and antifibrotic properties. These compounds have demonstrated clinical benefit and improved tolerability in patients with progressive fibrotic lung disease, highlighting the therapeutic potential of isoform-selective PDE inhibition. Collectively, targeting the NO-cGMP-cAMP signaling axis provides a unifying mechanistic framework for the treatment of vascular, immune, and fibrotic manifestations of rheumatic disease, positioning PDE inhibition as an emerging pillar of modern translational rheumatology.