In vitro activity: Roflumilast displays anti-inflammatory and immunomodulatory in vitro. Roflumilast inhibits LTB4 synthesis in human neutrophil with IC50 of 2 nM. Roflumilast inhibits fMLP-stimulated ROS formation as luminol-enhanced CL in human neutrophils or eosinophils with IC35 of 4 nM, and 7 nM, repectively. Roflumilast inhibits LPS-stimulated TNF-α synthesis in monocytes with IC40 of 21 nM. Roflumilast inhibits TNF-α synthesis in monocyte-derived dendritic cells with IC20 of 5 nM. Rolfumilast inhibits anti-CD3 and anti-CD28 antibody-stimulated proliferation of CD4+ T cells with IC30 of 7 nM. Rolfumilast inhibits anti-CD3 and anti-CD28 antibody-stimulated synthesis of IL-2, IL-4, IL-5, and IFN-v in CD4+ T cells with IC20 of 1 nM, IC30 of 7 nM, IC25 of 13 nM, and IC35 of 8 nM, respectively.

Kinase Assay: PDE activity is determined with some modifications. The assay mixture contain 50 mM Tris (pH 7.4), 5 mM MgCl2, 0.5 μM cAMP or cGMP, and [3H]cAMP or [3H]cGMP (about 30,000 cpm/assay), the indicated concentration of the inhibitor and an aliquot of the enzyme solution at a final assay volume of 200 μL. Stock solutions of the compounds are diluted 1:100 (v/v) in the Tris buffer mentioned above; appropriate dilutions are prepared in 1% (v/v) DMSO/Tris buffer, which are diluted 1:2 (v/v) in the assays to obtain the desired final concentrations of the inhibitors at a DMSO concentration of 0.5% (v/v). DMSO itself affected none of the PDE activities. After preincubation for 5 min at 37°C, the reaction is started by the addition of substrate (cAMP or cGMP) and the assays are incubated for further 15 min at 37°C. Then 50 μL of 0.2 N HCl is added to stop the reaction and the assays are left on ice for about 10 min. Following incubation with 25 μg of 5′-nucleotidase (Crotalus atrox snake venom) for 10 min at 37°C, the assays are loaded on QAE Sephadex A-25 (1 mL of bed volume in Poly-Prep chromatography columns). The columns are eluted with 2 mL of 30 mM ammonium formate (pH 6.0) and the eluate is counted for radioactivity. Results are corrected for blank values (measured in the presence of denatured protein) that are below 5% of total radioactivity. The amount of cyclic nucleotides hydrolyzed did not exceed 30% of the original substrate concentration.

Cell Assay: roflumilast does not affect PDE enzymes apart from PDE4, and is a subnanomolar inhibitor of most PDE4 splicing variants tested. It showed no PDE4 subtype selectivity apart from PDE4C (4C1, IC50=3 nM; 4C2, IC50=4.3 nM), which is inhibited with a slightly lower potency. Roflumilast is a potent and selective PDE4 inhibitor. Roflumilast is a monoselective PDE4 inhibitor since it does not affect other PDE isoenzymes, including PDE1, PDE2, PDE3, and PDE5 up to 10,000-fold higher concentrations. Roflumilast inhibits human neutrophil functions. Roflumilast inhibits TNFα synthesis in monocyte-derived dendritic cells. Rolfumilast inhibits proliferation and cytokine synthesis in CD4+ T cells. Proliferation is inhibited to a maximum of about 60% by Roflumilast with a potency (IC30) of 7 nM.