The phosphodiesterase 4 (PDE4) family comprises isoenzymes that selectively hydrolyse the second messenger cyclic adenosine monophosphate (cAMP). PDE4s are widely expressed and play key roles in various physiologic paradigms, including immune responses, memory, cognition, and metabolism. Marketed PDE4 inhibitors, such as roflumilast and apremilast, treat chronic obstructive pulmonary disease and psoriasis. Identification and characterisation of PDE4 inhibitors offer a promising strategy for targeting diverse pathological processes. In this study, leoligin, a natural lignan found in the roots of Edelweiss (Leontopodium nivale subsp. alpinum (Cass.) Greuter (Asteraceae)) and 165 analogues were screened for their PDE inhibitory activity using a cAMP accumulation assay employing an exchange protein activated by cAMP-based biosensor. Six compounds, including leoligin itself, were identified to cause a significant accumulation of cAMP, and structure-activity relationships were deduced. One analogue, designated LT-104A, showed a concentration-dependent activity with the highest determined potency (EC50 = 1.9 μM) in the cAMP accumulation assay. LT-104A was further characterised using a CRE-Luciferase assay, showing comparable activity to known PDE4 inhibitors in inducing the anti-inflammatory cAMP-PKA-CREB pathway. The inhibitory activity of LT-104A was also confirmed against recombinant PDE4D3 in a cell-free cAMP hydrolysis assay (IC50 = 9.3 μM). The potential binding mode of LT-104A with the catalytic domain of PDE4D was predicted using induced-fit docking. Lastly, a functional study was conducted in LPS-activated macrophages, where LT-104A reduced nitric oxide release and decreased mRNA expression of Il1b and Nos2. In conclusion, extensive in vitro screening of leoligin analogues led to the identification and characterisation of a novel PDE4 inhibitor, LT-104A, with potential in vitro anti-inflammatory properties.
