MK 571 is a selective, CysLT1 receptor antagonist. It blocks the binding of leukotriene LTD4, but not LTC4, to human and guinea pig lung membranes with Ki values of 0.22 nM and 2.1 nM, respectively. MK 571 effectively blocks LTD4 activation of recombinant human and murine CysLT1 receptors but is ineffective at blocking LTC4 or LTD4activation of the recombinant human or murine CysLT2 receptors. MK-57 also acts as a specific inhibitor of multi-drug resistance protein-1 (MRP1).