Kinetic Studies of Transition of Seratrodast Polymorphic Forms by Differential Scanning Calorimetry

Seratrodast [(±)-7-(3, 5, 6-trimethyl-1, 4-benzoquinon-2-yl)-7-phenylheptan-oic acid], a thromboxane A2 antagonist, developed as an antiasthmatic, has two crystalline forms, form I and form II. Form I was a stable form and form II was metastable form since the crystalline form was transformed into form I by heating. By differential scanning calorimetry, form I showed only an endothermic peak at about 130°C with melting but form II additionally showed an exothermic peak at about 90°C. This exothermic peak was the transition point of form II to form I. A method for the determination of the ratio of form II in seratrodast was established from the ratio of the exothermic calorific value to the endothermic calorific value on the differential scanning calorimetric curve. The transition of form II to form I appeared to follow the zero-order mechanism. The activation energy of this transition was 270kJ/mol calculated from the Arrhenius plots.