The CysLTs, LTC4 and LTD4, are potent mediators of asthma and hypersensitivity. They induce bronchoconstriction, increase microvascular permeability, and are vasoconstrictors of coronary arteries. Their biological effects are transduced by a pair of GPCRs, CysLT1 and CysLT2.{7174,8519,14081} Montelukast (sodium salt) is a potent, selective CysLT1 receptor antagonist sold under the trade name Singulair for the treatment of asthma as well as for the symptoms associated with allergic rhinitis.{6475,6685,14084,14106} It binds to the human CysLT1 receptor with an IC50 of less than 5 nM with no appreciable binding to the CysLT2 receptor.{7174,8519,14081}