Trends in Inhibitors, Structural Modifications, and Structure–Function Relationships of Phosphodiesterase 4: A Review

Phosphodiesterase 4 (PDE4) is a key enzyme responsible for the hydrolysis of cyclic adenosine monophosphate (cAMP), thereby regulating essential signaling pathways involved in inflammation and immune modulation. Structural studies have demonstrated a high degree of conservation within the catalytic domains of PDE4 isoforms, accompanied by subtle conformational variations that underlie their selectivity and tissue-specific distribution. Elucidating these structural features has been instrumental in guiding the rational design of PDE4 inhibitors. Although synthetic PDE4 inhibitors such as roflumilast and apremilast exhibit significant therapeutic efficacy, their clinical application is often limited by dose-dependent adverse effects. These effects primarily arise from insufficient isoform selectivity, as current inhibitors tend to target multiple PDE4 subtypes indiscriminately, resulting in off-target pharmacological actions and reduced tolerability. In contrast, natural products—including flavonoids, terpenoids, and related polyphenolic compounds such as curcumin, α-mangostin, and their derivatives—have emerged as promising molecular scaffolds. Their lower toxicity, favorable biocompatibility, and structural diversity enable fine-tuning of potency and selectivity through rational modification. Integrating structural insights derived from crystallographic and computational studies with the optimization of natural compounds offers a sustainable and effective strategy for the development of safer, isoform-selective PDE4-targeted therapies.