Received for publication May 6, 2004.
Revised September 13, 2004.
Accepted for publication September 14, 2004.

Discovery of BRL 50481, a Selective Inhibitor of Phosphodiesterase 7: In vitro Studies in Human Monocytes, Lung Macrophages and CD8⁺ T-Lymphocytes

Abstract

The biochemical and pharmacological characteristics in human pro-inflammatory cells of BRL 50481, a novel and selective inhibitor of phosphodiesterase (PDE) 7, is described. BRL 50481 inhibited the activity of hrPDE7A1 expressed in baculovirus-infected Sf9 cells in a competitive manner (Ki = 180nM) and was 416- and 1884-times less potent against PDE3 and 38- and 238-times less potent against PDE4 at a substrate concentration of 1uM and 50nM cAMP respectively. Western blotting identified HSPDE7A1 but not HSPDE7A2 in three human cell types that are implicated in the pathogenesis of chronic obstructive lung disease, namely CD8⁺-T-lymphocytes, monocytes and lung macrophages. BRL 50481 had no effect on the proliferation of CD8⁺-T-lymphocytes, and only marginally (~2-11%) reduced the generation of TNF from blood monocytes and lung macrophages. However, in the presence of BRL 50481 the inhibitory effect of rolipram was enhanced on all three cell types. The expression of HSPDE7A1 was increased in a time-dependent manner in monocytes that were "aged" in culture medium. Under this condition BRL 50481 now inhibited TNF generation in a concentration-dependent manner. In aged monocytes, rolipram, Org 9935 (a PDE3 inhibitor) and PGE2 inhibited TNF generation in a concentration-dependent manner and interacted additively with BRL 50481. These data demonstrate that BRL 50481 is the first fully documented PDE7 inhibitor that has acceptable selectivity for in vitro studies. Furthermore, although BRL 50481 had only a modest inhibitory effect per se on the pro-inflammatory cells studied, it acted at least additively with other cAMP-elevating drugs especially when HSPDE7A1 was up-regulated.

Key words: cAMP, Phosphodiesterases, Leukocytes/Mast cells