Montelukast sodium is the active pharmaceutical ingredient of SINGULAIR®, and is approved for the treatment of asthma and allergic rhinitis. The molecular structure of montelukast is as shown below:

Montelukast sodium is described in U.S. Pat. No. 5,565,473. A crystalline form of montelukast sodium (hereinafter referred to as “Form A”) is described in U.S. Pat. No. 5,614,632.