Montelukast Sodium is a potent, selective, anti-inflammatory CysLT1 receptor antagonist (IC50 < 5nM) with no appreciable affecton the CysLT2 receptor. In rat colitis Montelukast Sodium has demonstrated the ability to increase PGE2 (Prostaglandin E2 - sc-201225) production and lower COX-2 expression. Inhibition is competitive with respect to leukotrienes - particularly LTD4, a biologically active mediator that plays an important role in the control of several biochemical pathways involving pulmonary and immune system function. Because of its method of operation, it is not useful for the treatment of acute asthma attacks. Again because of its very specific mechanism of action, it does not interact with other asthma medications such as theophylline. Another leukotriene receptor antagonist is zafirlukast (Accolate), taken twice daily. Zileuton (Zyflo), an asthma drug taken four times per day, blocks leukotriene synthesis by inhibiting 5-lipoxygenase, an enzyme of the eicosanoid synthesis pathway.