MK 571 sodium salt is a potent and selective antagonist of the cysteinyl leukotriene receptor 1 (CysLT1), as well as an inhibitor of multidrug resistance-associated protein 1 (MRP1), a transporter involved in cysteinyl leukotriene efflux. It is widely employed in research settings to study the role of leukotrienes in inflammatory and allergic responses, particularly in conditions such as asthma, allergic rhinitis, and other respiratory disorders. By blocking the CysLT1 receptor and inhibiting MRP1-mediated transport, it helps to inhibit the effects of leukotrienes, which are known to cause bronchoconstriction, inflammation, and increased vascular permeability. This makes it a valuable tool for understanding the mechanisms of leukotriene-mediated diseases and for evaluating potential therapeutic interventions. Additionally, it is used in pharmacological studies to assess the efficacy of new drugs targeting the leukotriene pathway and to investigate drug resistance mechanisms.