RPL554 (Ensifentrine) is a long acting, potent and specific dual inhibitor of PDE 3/4 (phosphodiesterase 3 and 4). RPL554 displays a broad range of both bronchoprotective and anti-inflammatory activities. Also, RPL554 synergizes with anti-muscarinic drugs in inhibition of ASM tone. RPL554 (Ensifentrine) exhibits anti-inflammatory effects through reduced production of the monocyte chemoattractant protein-1 (MCP-1) and granulocyte monocyte colony stimulating factor (GM-CSF) in cystic fibrosis bronchial epithelial cells during challenge with Interleukin-1β.