IBMX is a widely-used non-specific inhibitor of cyclic AMP and cyclic GMP phosphodiesterases (PDEs) with IC50s from 7 to 50 µM for PDE1-5, PDE7, and PDE11. PDE8A, PDE8B, and PDE9 are insensitive to IBMX. By inhibiting PDEs, IBMX increases cellular cAMP and cGMP levels, activating cyclic-nucleotide-regulated protein kinases. Methylxanthines, including IBMX, caffeine, and theophylline, bind adenosine receptors, typically antagonizing the suppressive effects of natural agonists. Suppresses alpha-adrenoceptor-mediated 5-HT release from neuroendocrine epithelial cells (IC50 = 1.3 µM).