The effect of endogenous adenosine upon the inhibition of neutrophil degranulation by PDE4 inhibitors in vitro

Boswell-Smith V, Carter L and Page C.P. The Sackler Institute of Pulmonary. Pharmacology, GKT School of Biomedical Sciences, King’s College, London, SE1 9RT.

We have previously shown that selective phosphodiesterase 4 (PDE) inhibitors can suppress neutrophil elastase (NE) release from activated neutrophils (Jones et al., 2005) and it is previously been demonstrated that endogenous adenosine is required for the inhibitory actions of first generation PDE4 inhibitors upon elastase release from activated neutrophils in vitro (Ottonello et al, 1999). As a result the aim of this study was to investigate the role of endogenous adenosine in the inhibition of NE release from fMLP-activated neutrophils by 2nd generation PDE4 inhibitors.

Human neutrophils were isolated from whole blood by density-dependent centrifugation and incubated with PDE4 inhibitor, cytochalasin B (5µg ml-1) and TNF-α (10 ng ml-1) or vehicle control for 30 minutes. Adenosine deaminase ( ADA) (0.5 or 1 U ml-1) was then added immediately prior to activation with fMLP (10-6 M). After 45 minutes the neutrophils were removed by centrifugation and supernatant NE assessed colorimetrically.

ADA significantly augmented fMLP-induced degranulation of neutrophils in the absence (29.2% ± 3.1) of TNF-α (n=6, paired students t-test, *P<0.05). In the presence of ADA roflumilast, roflumilast N-oxide and cilomilast significantly suppressed NE release in both the absence and presence TNF-α . There was no significant difference in the inhibitory potency of the compounds between when ADA was present or absent. The IC50 ± 95% confidence intervals are shown in table 1.

Although ADA significantly augmented degranulation, it did not significantly alter the efficacy of the PDE inhibitors under any of the conditions tested. These data suggest that endogenous adenosine, which elevates cAMP, is not required for the inhibitory actions of PDE4 inhibitors in isolated neutrophils.

Jones et al., 2005 Pulm Pharmacol and Ther. 18 93-101.
Ottonello et al., 1999 Inflamm Res. 48 (12) 637-642.

Victoria Boswell-Smith is the recipient of an MRC case award supported by Altana Ltd.